There is one definite advantage of being over the age of forty. If you were lucky enough as a kid, you had the opportunity to snorkel or dive in a place like the Florida Keys or the Bahamas when coral reefs were healthy and diverse, colorful, and teeming with life. There are very few places on Earth where this is the case today. Tragically, the large majority of reefs are in a profound state of decline, with limited growth of new coral, and overgrowth by slimy green or brown films of algae. Piles of coral debris are commonplace. The water is often muddy, and the plethora of life has departed. Reefs are no longer magical places. There is absolutely no argument about it, the decline in coral reefs is human inflicted. To this point, the largest culprit is probably pollution, but that will change in coming decades. Enter Ocean Acidification, a process that has already begun, tied to our excessive CO2 emissions, and one that will accelerate soon. Corals cannot grow once the pH drops below a certain level, and if we don't act fast, that level will approach by mid-century. Reefs have survived the two largest mass extinctions the Earth has faced, but they may not survive the mass extinction humans are causing.
Ecosystems, such as reefs, are governed by a delicate balance of interactions between animals and plants. Yin and Yang. If that balance is upset even slightly and one part of the ecosystem is favored at the expense of others, havoc can break out. The recent surge in harmful algal blooms along many coasts and the outbreaks of massive jellyfish in the western Pacific Ocean are signals of Yin or Yang, but not both. Our oceans are getting out of whack.
In the latter part of Module 6, we learned about changes in circulation and health of the oceans that are predicted to occur with climate change. In this module and Modules 8-11, we continue to address the impacts of climate change on natural systems. These changes are very much central to the issue of sustainability of the planet and its populations. "Sustainability" has a variety of definitions, and, in particular, the meaning for environmentalists is substantially different from the meaning for businesses. Regardless of where you might be coming from, sustainability means the preservation of society and our way of life. Most directly, we are concerned with maintaining the needs of people today and in the future, and this very much hinges, as we will see in this module, on sustaining the life support systems of the planet.
Global warming and an array of environmental changes resulting from human activities are already causing profound impacts on organisms across the spectrum of the marine food chain. Warming of the ocean and subtle changes in its chemistry are combining with pollution and overfishing to alter the habitat of many marine creatures. In the near future, these habitats look to be further impacted, and potentially destroyed, with possibly devastating biological and economic consequences, including very negative impacts on people. The goal of this module is to learn about three very different but equally significant impacts of climate change and human activity on life in the ocean: ocean acidification, red tides, and blooms of jellyfish.
To set the stage, watch the Award Winning video Sea Change, produced by Craig Welch.
On completing this module, students are expected to be able to:
After completing this module, students should be able to understand the following topics:
Below is an overview of your assignments for this module. The list is intended to prepare you for the module and help you to plan your time.
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Increasing levels of CO2 in the atmosphere are slowly causing the surface of the ocean to become more acidic. This is because the ocean absorbs some of the CO2, forming a weak carbonic acid. At present, the ocean absorbs about a third of fossil fuel emissions, but this amount is likely to increase to 90% in the future. Over the last century, the average pH of the ocean has decreased, and there are hints that the current levels are beginning to impact organisms that make their shells out of the minerals aragonite and calcite (both composed of CaCO3). Aragonite is more susceptible to dissolution in more acidifc waters than calcite. Coral reefs that are made of the mineral aragonite and are particularly vulnerable to ocean acidification. A recent study has found, for example, that the area of coral covering the Great Barrier Reef in Australia has been cut in half since 1985. However, coccolithophores and foraminifera, organisms that serve a vital role at the base of the marine food chain that are composed of calcite, are becoming increasingly susceptible. Moreover, the future appears to be even more bleak; some CO2 projections suggest by the year 2100 there will be a 150% increase in the ocean’s acidity compared to preindustrial times. Here we review the chemical changes in seawater that result from increasing CO2, and then we discuss the impact on reefs and planktonic organisms in the ocean. Finally, we discuss the evidence for acidification in ancient oceans and its impact on life in the past.
The following video provides a thorough overview of the potential impact of acidification on the oceans.
The ocean contains a massive reservoir of dissolved CO2, hundreds of times more than in the atmosphere, and, actually, by contrast, the amount derived from fossil fuel burning is relatively modest. Since the beginning of the industrial revolution, about 340 to 420 petagrams carbon (a petagram or Pg is 1015 grams) in the form of CO2 has been emitted to the atmosphere, with about a third of that amount absorbed by the ocean, approximately 118 Pg. Seawater today may already contain more CO2 than at any time in many millions of years.
As we discussed in Module 5 on the Carbon Cycle, the absorption of CO2 in the ocean forms weak carbonic acid (H2CO3). Some of this acid dissociates in seawater releasing H+ ions, which make the water more acidic, as well as HCO3- (bicarbonate ions) and CO32- (carbonate ions). This reaction is as follows:
CO2(aq) + H2O = H2CO3 = H+ + HCO3- = 2 H+ + CO32-
Going back to your elementary chemistry course, you might remember that a pH of greater than 7 is regarded as alkaline whereas a pH of less than 7 is acidic. Surface ocean waters have a pH of between about 7.9 and 8.3, which means that they are, by definition, alkaline. Anthropogenic CO2 is thought to have decreased the mean pH of the ocean by 0.1 unit since 1800. This may not sound like that much, but more ominous is the projection that if CO2 levels continue to rise unabated (i.e., projections based on SRES A2 “business as usual”, pH levels will drop a further 0.3 by 2100. As we will see below, in parts of the ocean, these levels would be extremely damaging to organisms that build their skeletons out of CaCO3, which is very sensitive to CO2 addition.
CaCO3 is the dominant material used by invertebrate organisms to build their skeletons. There are two different minerals made of CaCO3, known as polymorphs: calcite and aragonite. These minerals have the same composition but different crystal lattice structure and thus their properties and behavior in seawater differ, including their ability to dissolve. To understand how CaCO3 dissolves and precipitates, we need to introduce a term Ω that represents the saturation state of the water. Where waters are highly saturated with respect to CaCO3 and Ω is high, calcite and aragonite are less likely to dissolve than where these waters are less saturated or even undersaturated and Ω is low. Likewise, calcite and aragonite are more likely to precipitate under higher Ω values. The dissolution and precipitation reactions are as follows:
Dissolution reaction: CaCO3 (solid) = Ca2+ + CO32-
Precipitation reaction: Ca2+ + CO32- = CaCO3 (solid)
An increase in CO2 from the atmosphere presents a double whammy for skeletons formed from CaCO3, both aragonite and calcite. The H+ ions and carbonate ions (CO32-) that derive from the dissociation of carbonic acid combine to form bicarbonate ions (HCO3-). This rapid reduction in available carbonate ions decreases Ω and limits calcification by organisms with aragonite- and calcite-based skeletons. However, here we need to dispel two myths. The first myth is that the precipitation of CaCO3 is directly controlled by pH. In fact, precipitation is affected principally by the decrease in CO32, which is coincident with the addition of H+ ions, and reduction in pH. The second myth is that precipitation of CaCO3 can occur in any water that is oversaturated with respect to the particular CaCO3 mineral. In fact, both corals and coccolithophores have been shown to have difficulty calcifying in environments when waters were actually oversaturated. Different organisms can calcify at very different Ω values, but for most the decrease in saturation that results from decreasing CO32- content is a direct threat to calcification. Fiinally, it is key to move that aragonite is more susceptible to dissolution than calcite. Thus, shells made of the CaCO3 polymorph aragonite, including the corals, will be the first to dissolve, followed by those made of the polymorph calcite.
The following video explains the threat of ocean acidification to the calcareous plankton.
The saturation of CaCO3 in the oceans is also a function of temperature and pressure. A delicate balance exists between the production of CaCO3 via the formation of skeletons in the shallow part of the ocean and the dissolution of this aragonite and calcite in the colder and deeper realms of the ocean where waters are less saturated. In most parts of the ocean, undersaturation occurs far below the surface. However, recent increase in dissolved CO2 is leading to a shoaling of the saturation horizon of CaCO3, and, in the future, this will impact especially the organisms that live at depth or in colder waters as well as those that make their shells of the mineral aragonite, which is more soluble in seawater than calcite.
Coral reefs have existed for hundreds of millions of years and provided a habitat for some of the richest diversity on the Earth’s surface. They are the marine version of tropical rainforests. Reefs harbor a slice of the marine food chain, all the way from tiny autotrophic protistans (autotrophs fix carbon through photosynthesis) to large, predatory fish. Hundreds of millions of humans live near reefs and receive important resources from them. Reefs host productive fisheries; they also provide protection to low-lying coastal areas from storms and are vital for a number of key habitats, including mangrove forests.
The organisms that have constructed reefs, largely corals, have evolved over time, and with that change so have the locations of reefs and the dynamics of the reef community changed. Over their long history, reefs have had several intervals of crisis; in particular, they almost ceased to exist at the Permian-Triassic boundary, where over 90% of marine species became extinct, and during the Cretaceous about 100 million years ago when giant clams took over these structures for several tens of millions of years. Both of these ancient times were potentially characterized by ocean acidification. However, reefs have been remarkably resilient over geologic time and generally have been able to adapt to environmental change. For example, as we will see in Module 10, they are able to grow fast enough to keep up with very rapid rates of sea level rise.
With this background, recent human activity has placed reefs in as precarious a position as at almost any time in their history. The last fifty years have witnessed an extremely dramatic decline in the health of many of the major reefs around the world, including reefs of the Caribbean, the Bahamas, and the Florida Keys as well as those in the Indian and Pacific Oceans, including the massive Great Barrier Reef of Australia. The outlook for these rich and complex ecosystems is about as bleak as any ecosystem on Earth. As it turns out, ocean acidification is one of several environmental threats to reefs, with warming, pollution, overfishing and physical destruction all exerting major threats to reefs in the future. As we will see, acidification is perhaps the greatest of all of these threats long term.
Before we start, let's consider how reefs grow. The main organism that constructs modern reefs, the coral, includes a number of species belonging to the Cnidaria, a phylum of organisms that uses stinging cells to capture their prey. Modern corals are colonial structures of millions of individual polyps that grow primarily in shallow and clear tropical and subtropical waters, restricted to these areas by light levels and temperatures as well as by nutrients.
Both types of coral reproduce both sexually and asexually. Asexual reproduction involves simple cell division, or budding, and takes place within the colony, whereas sexual reproduction involves the release of gametes into seawater. This is an amazing process that for many species happens once a year, timed by the lunar cycle. The fertilized egg forms a larval planula that settles before forming a new colony.
Large reef structures including fringing and barrier reefs, as well as atolls, represent the growth of these colonies over many thousands or millions of years.
The algae live within the coral polyp and receive CO2 from the polyp that it requires for photosynthesis. Through photosynthesis, the Zooxanthellae convert the CO2 to O2 and provide this vital gas to the coral as well as crucial nutrients. This is important because by removing CO2, the Zooxanthellae drives up saturation which facilitates calcification in the coral skeleton. Reefs are made up of much more than corals and their algal symbionts. Other organisms, including coralline algae (algae that secrete high magnesium calcite and conduct photosynthesis on their own), are active framework builders in modern reefs. Today, sponges, sea anemones, sea urchins, a diverse array of fish, and many other organisms live within reefs, some playing a vital role in building reefs and keeping them healthy and others taking advantage of their decline.
Although, as we will see, ocean acidification is only just beginning to affect reef growth, there is already another very serious affliction in many reefs around the world---coral bleaching. One of the key changes in corals over the last few decades has been an increase in the frequency and severity of bleaching events. As we have discussed, shallow-water coral species live in symbiosis with algae called a dinoflagellate or Zooxanthellae whose colorful pigments give living corals such beautiful colors, purple, brown, green, yellow, and more. As discussed the symbiosis is key to the coral, the Zooxanthellae removed CO2 from the water, boosts the saturation, and facilitates calcification of the coral. When temperatures become too hot, however, the Zooxanthellae can’t tolerate the heat and it temporarily or permanently leaves the coral leading to a bleached white color.
Bleaching today is common when temperatures increase very slightly (by about 1°C) in summer months and thus is often an annual summer event. Bleaching is accompanied by slower growth and increased coral mortality. The response to bleaching differs considerably between different species. Some species are able to recover normal growth rates quite rapidly, whereas others are more vulnerable and unable to recover. Bleaching also changes the ecology of a reef, promoting the growth of algae that blanket corals, making the recovery of corals more difficult.
A major coral bleaching event took place in the Florida Keys in summer 2023 when water temperatures rose to 101 deg F and there was widespread bleaching. It is too soon to determine the long-term impacts of this bleaching on the reefs in the Keys but the images are dramatic and the future does not look promising.
One of the most significant bleaching events took place in 2016 and 2017 in the Great Barrier Reef. This catastrophic event caused by warmer than average water caused up to 50% coral mortality in some areas. Bleaching was also extreme in 2020 and you can see from the maps below that it extended southward which in the southern hemisphere means it afflicted cooler waters. The extent of bleaching shocked ecologists and is clearly a window into the future. Now it seems every year, bleaching is a threat to the Great Barrier Reef, and with the warm El Nino conditions, the Austral summer of 2023-2024 may be the most severe bleaching event ever. One scientist has remarked that the reef looked like it had been "carpet-bombed".
Not all corals live in shallow water. A very diverse group of corals are able to thrive in the deep realms of the oceans. These corals lack Zooxanthellae and are able to calcify at cooler temperatures and lower levels of saturation than exist in the shallower waters. The number of known deep-water species has increased dramatically with new methods of exploring the deep oceans, including ROVs (remotely operated vehicles) and AUVs (autonomous underwater vehicles).
These organisms exist in isolated patches at depths down to 2000 meters. In these conditions, the corals grow considerably slower than shallow-water reefs. They feed on zooplankton and in some cases use chemicals coming out of the sea floor for a source of nutrition. Like their shallow-water counterparts, deep coral reefs provide a habitat for a diverse array of creatures. However, at the same time, it is clear that deep corals are in a particularly precarious situation, as we will see later on.
The growth rate of corals, known as the rate of calcification, corresponds more closely to aragonite Ω rather than any other variable. Cores of corals from the Great Barrier Reef indicate a 14% decrease in calcification rates from 1990 to 2005. Corals generally maintain a high Ω at the site of calcification, but with decreasing CO32-, precipitation of aragonite requires more energy, hence the decrease in calcification rate. Overall, the net calcification rates of reefs decrease to zero when the Ω of aragonite is <1, and thus we can predict that worldwide reefs will switch from actively growing via calcification to shrinking via dissolution when CO2 doubles to 560 ppm (the year 2050 in emission scenario A1B). The same logic leads to predictions of decreases in calcification rates of 10-50% by 2050. Experiments show that modern coral species have very different abilities to grow in water with lower saturation, with some species able to continue growing while others can't.
Experiments are one of the best ways to forecast the future, but they have significant limitations. In particular, it is impossible to replicate natural growing conditions in the lab, and further, experiments are conducted at intervals that are significantly shorter than the changes that are occurring in nature.
Field studies and experiments often show very different results. Strangely, field results indicate a much higher susceptibility of growth rates to decreasing levels of saturation than do laboratory experiments. These studies suggest that there is much left to learn about calcification and how it will change in the future. At the same time, it is clear that there is significant variability between species; some species having a much greater likelihood of survival in the future. Moreover, even within individual species, it is apparent that some reefs likely have the ability to survive better than others. Factors such as community composition, growth rate, nutrient levels, and local variations in seawater chemistry and sediment composition will also play a vital role in determining which reefs will survive.
Since the coralline algae are more tolerant of colder waters, they have a very widespread distribution in the oceans, extending from the tropics to polar regions. The algae are also able to use low light levels for photosynthesis and therefore live considerably deeper than most corals. Their distribution suggests that they have great potential to adapt to variable environments. As it turns out, however, they may be in a more precarious position than the corals.
The recent decrease in CO32- has also begun to lower calcification rates of the coralline algae. These species are composed of high Mg calcite, which is the most soluble form of CaCO3 (more so than low Mg calcite and aragonite), so they are particularly prone to ocean acidification. Experimental work confirms that calcification of the coralline algae is particularly sensitive to CO2 levels with growth rates slowing significantly, and actually, dissolution beginning at moderately high CO2 contents.
Perhaps the coralline algae will be the “canary in the coal mine” for the dissolution of all framework structures under rising CO2. For the corals, it is very apparent that the threat varies considerably from reef to reef. Deep-water corals are in particular jeopardy because calcification rates are lower in colder waters, because saturation levels at these depths are lower, and because, in the future, saturation stands to decrease more rapidly in deep than in shallow waters. However, the fate of reefs in the tropics is also likely to vary significantly.
Making predictions about the exact impact of ocean acidification in the future of coral reefs of all types is inherently difficult. Acidification will occur in parallel with other deleterious effects such as bleaching and sea level rise. Species that are most susceptible to the effects of bleaching may turn out to also be more susceptible to extinction. For example, by impacting zooxanthellae, bleaching impacts calcification and thus may exacerbate the impact of acidification. However, these same corals also have the ability to recover from bleaching events more quickly, possess shorter generation times, and thus may have the ability to evolve more rapidly to tolerate bleaching in the future. At the same time, nutrient levels in reef environments have and will continue to decrease as stratification of the upper ocean increases, and this will put considerable strain on future reefs. Temperatures, nutrient levels, local rates of sea level rise, and human activities are factors that will likely decide which reefs are the most vulnerable, possibly with acidification pushing the most threatened systems over the edge into extinction.
Probably the largest uncertainty with respect to the future of reefs is whether corals will be able to adapt to lower saturation levels, increased temperatures, and lower nutrient levels, and if so, how rapidly. As we have seen earlier, corals have shown great resilience in surviving numerous environmental threats in the past. So, why do reefs recovered after times of environmental stress in the past, at times when temperatures were even warmer than they are today, and CO2 levels higher, appear to be in such dangerous territory today? The answer to this question is that in the past, temperature and CO2 perturbations occurred slowly enough that ultimate increases in weathering (remember feedbacks we discussed in Module 3) essentially decreased the rate of CO2 addition and buffered the ocean with CO32- before extinction occurred. What concerns scientists is that warming today and CO2 addition are rapid enough that the weathering feedback will lag the decrease in saturation by several thousand years.
So, we are left with a lot of questions: will the rates of saturation decrease and temperature rise be too rapid for modern species to adapt? Will algae take over the niche of shallow-water corals and dominate the low pH oceans of the future? Or will a few species of coral and possibly coralline algae develop the ability to calcify rapidly enough to survive the current threats and take over the niche of species that do not? Will Zooxanthellae themselves be able to adapt and assist corals in calcifying? Finally, when will feedbacks, largely through weathering, come into play and make conditions more favorable for calcifying organisms? For some of these questions, it’s a matter of wait and see. However, ongoing research should shed light on others. For example, the genetics of coral populations are currently being explored to understand the ability of corals to adapt to environmental change.
At this stage, however, any outcome is possible for corals, ranging from complete extinction by late in the 21st century to the adaptation to a new set of environmental parameters. Ultimately, like many elements of the ecosystem, the fate of reefs may rest on how well we manage CO2 emissions in the future.
The goal of this lab is to:
First, you will be watching videos from the Catlin Seaview Survey as well as photos to learn how to determine the health of reefs. Then you will be looking at before and after photos of reef bleaching events. Finally, you will be looking at the risk to reefs in the future.
Please watch the videos below. It is easiest to watch the videos in your browser (click button top right of Google Earth) in full screen mode. To move around, insert your cursor in the videos to manipulate the camera and stop on particular items of interest or to change direction. The best way to move the camera is with your keyboard arrows (side to side) and cursor for up and down.
The goal of this part of the lab is for you to show you can identify different types of coral as well as overall health of the reef at different locations. Make sure you have read the material on reefs in the module before attempting to complete the lab. Below are the different types of coral for you to identify. In addition, we show pictures of algae that colonize reefs as well as reef damage from storms. Healthy corals show a variety of colors from the different algal symbionts. Unhealthy corals show fewer colors, more algal colonization, more breakage and often are bleached white. Remember, algae are some of the key markers of an unhealthy reef.
The following images show some of the range of morphologies and colors of colonizing algae
ReefsAllFin.kmz [17]
Load the ReefsAllFin.kmz file [18]. The locations of reefs of interest are shown with flags. The videos are shown with diver markers, the still photographs are shown with wave markers. The videos run best if you open them a browser such as Firefox (see Google Earth window, top right). Before submitting your lab, let’s begin with the practice. Make sure you do this part of the lab to get comfortable with the tasks you will be asked to do and to receive feedback about your answers. Watch the videos at the following locations and answer the questions below. Make sure you maneuver up and (especially) down, as well as side-to-side.
In this section, you will be looking at the health of reefs using their color, the presence and abundance of algal overgrowth and the and the presence of bleaching. Go to the Belize reef (click on the flag in Google Earth, then open the address in your browser). Look around the reef and answer the following questions.
In this part of the lab, you will compare photographs from before and after major events that have impacted reefs. Answer the questions about the changes in the abundance of different types of coral, algal overgrowth, or percent bleaching.
Go to Lizard Island. Please look at the two pictures and answer the following questions.
Corals and coralline algae are not the only organisms highly susceptible to ocean acidification. Coccolithophores, foraminifera, pteropods, three very different groups of plankton (a term that refers to organisms that float passively in the upper ocean) are also threatened by increasing atmospheric CO2 levels. Pteropods, often called sea butterflies, are tiny snails made of aragonite that thrive in shallow waters and play a particularly important role in polar ecosystems.
Because of the susceptibility of aragonite to decreasing saturation levels combined with the effect of temperature (calcification requires more energy in cold water) on saturation, pteropods may cease to exist in polar latitudes by the middle of the 21st century. In some polar areas, these organisms account for over 60% of the zooplankton biomass, thus their extinction or migration to warmer regions could have major repercussions to organisms up the food chain.
The coccolithophores and foraminifera are constructed of low-Mg calcite and therefore are more stable than pteropods under conditions of increasing CO2.
As we have learned earlier, foraminifera are groups of zooplankton with habitats both near the surface of the ocean (planktonic foraminifera) and on the ocean bottom (benthic foraminifera).
The planktonics are sensitive to changes in CO2, have shown subtle changes in shell mass during the Pleistocene in response to CO2 fluctuations, and have already begun to get lighter in response to recent CO2 increase. This recent thinning of shells is likely a result of the effect of decreasing CO32- on calcification by foraminifera. Interestingly, many planktonic species also harbor dinoflagellate symbionts that play a role in calcification. Changes in calcification appear to be a threat to the planktonic foraminifera, but we do not yet know how serious this threat is. However, we do know that the foraminifera are also threatened by the potential loss of their dominant source of food, the coccolithophores, as a direct result of CO2 addition.
Coccolithophores are ubiquitous in the oceans, essentially serving as the dominant species of phytoplankton in vast regions of the open ocean that are characterized by lower nutrient levels. For this reason, considerable attention has been devoted to the potential effects of increasing CO2 on the coccolithophores.
Coccolithophores are haptophyte or golden brown algae (similar to diatoms) that produce tiny calcite scales known as coccoliths during certain phases in their life cycle.
The plates are several microns in diameter and can remain attached to the cell covering the soft organelles with a protective shield or break off after they are fully grown. Coccolithophores reproduce asexually, and, given the right conditions have the potential to multiply rapidly. When conditions are suitable, coccolithophores can form blooms of millions of cells per liter of seawater. These organisms are consumed by foraminifera and copepods, and transported to the bottom of the ocean as marine snow.
Because of their prolific production and global distribution, coccolithophores are a vital part of the carbon and carbonate cycle of the oceans. Modern coccolithophorids are dominated by the species Emiliania huxleyi, a species with very small (1-2 micron) and delicate coccoliths. Due to their size and ecology, coccolithophores are inherently more difficult to study in the natural environment than are corals. However, samples can be collected using filters and cores of sediments can be studied to determine the effects of past changes of CO2 on coccolithophore morphology. Coccolithophores can also be cultured in the laboratory where CO2, CO32- and pH levels can be altered to observe the effect of these variables on calcification. Although the results of both field and lab studies are by no means simple, it appears that in the case of E. huxleyi, increasing CO2 and decreasing CO32- has the effect of causing thinner coccoliths with smaller masses. In addition, laboratory studies show that such conditions also lead to malformations in E. huxleyi coccoliths, which are a potential sign of difficulty in calcification. Such trends of decreasing mass are by no means universal, however. Some lab experiments and field collections in particular environments show that species other than E. huxleyi actually grow thicker in low CO32- conditions and even E. huxleyi recently has increased in mass in parts of the ocean. More study is required to determine the precise outcome of the coccolithophores in a high CO2 world. However, the current signs generally point to significant reduction in the rate of calcification, which could lead to significant changes both in marine ecosystems and in the carbon cycle.
Coccolithophores have a spectacular 220 million year fossil record. This record allows paleontologists to observe the effects of past climate change, including increasing CO2, on the livelihood of this group of plankton. Ancient global warming events, including those at 120 million years before present and 55 million years before present, have been gleaned for evidence of ocean acidification in the morphology of coccolithophores. The event at 120 million years shows evidence for a decrease in coccolith size, but this change is not apparent in the 55 million year event. As the species existing during these ancient events was entirely different from those at the present time, it could be that ancient species responded in a different fashion from those living today. Alternatively, the changes in ancient plankton assemblages may be in response to environmental variables other than CO32-, for example, temperature and nutrients. The coccolithophores, like the corals, have been able to survive intervals of great ecological upheaval in the past. However, given that the rates of modern environmental change are so rapid compared to ancient events, we cannot assume that these same groups will have the ability to adapt to the changes that are to come. Moreover, while the coccolithophores appear to be less endangered by increasing CO2 than the corals, largely as a result of their mineralogy, the impact of the decline of these vital algae would likely be even more devastating to the oceans.
Red tides are common events in warm and polluted coastal oceans. They form when dinoflagellate algae explode to huge population levels. Because the dinoflagellates have red plastids, the waters literally turn red. Dinoflagellates take advantage of harsh environmental conditions that kill off other organisms. As you will find out in the pages to follow, these tides can be major public health hazards.
I remember that the grouper was delicious. Blackened, spicy, with an ear of corn and some slaw. On a white paper plate. I polished it off with a Heineken. It was a warm early January day, a seafood festival in Florida City. We went canoeing after lunch. I remember waking up the next morning with an intense thirst and extreme nausea. When I tried to get out of bed, I sprawled on the floor, my left side was completely paralyzed! I rolled to the sink, crawled up, and poured myself a glass of water, I gulped it down, and to my horror, it felt boiling hot! The neurological symptoms soon went away, but nausea and fatigue lasted weeks, I lost at least ten pounds. The doctors took that long to diagnose the problem---I had Ciguatera poisoning. That delicious grouper had ingested a lot of toxic dinoflagellates and transmitted the toxins to me. My first experience with harmful algae.
Red tides represent one of the most serious threats to coastal ecosystems today. Most red tides result from the input of an excessive amount of nutrients from fertilizers, sewage, and soils of nearby land areas to bays, estuaries, and shallow seas. These nutrients cause explosive growth of microscopic species of algae, a number of which carry toxins that are harmful or even lethal to other organisms. Red tides can be caused by major storms such as hurricanes, which cause excess runoff from the land and resuspension of the seed stages of the algae.
Red tides occur when dinoflagellates, and rarely diatoms, grow in massive quantities in surface waters. The photosynthetic organelles of these organisms, known as plastids or chloroplasts, are red, or golden brown in the case of diatoms, and the profusion of cells in surface waters imparts a red or brown color. Some of the culprit dinoflagellate and diatom species produce by-products that are highly toxic to many other organisms living in the coastal zone, all the way from fish to turtles to large mammals such as dolphins, manatees, and whales, as well as to humans. In some cases, shellfish or small fish such as sardines that consume the plankton are not harmed by the toxin but concentrate it for organisms that feed on them, a process known as bioaccumulation. Ingestion of the toxins can result in developmental, immunological, neurological, and reproductive damage of the host organism. For this reason, red tides are also known as harmful algal blooms (HABs). In fact, we will use the term HAB here because these events are not associated with ocean tides, because many HABs are not associated with a red color, and because blooms of dinoflagellates are often not harmful.
Harmful algal blooms are a global phenomenon and have increased in frequency in the last thirty years. Part of the increase may result from awareness of the phenomenon, but increasing pollution is also considered responsible. There are a number of reasons why climate change may further increase the occurrence of HABs. For examples, the increase in the frequency of large storms such as hurricanes will lead to greater runoff and input of nutrients from land. In addition, changes in temperature, wind patterns, upwelling, and stratification will alter the distribution of species. Because the degradation of a large amount of cellular material produced in HABs consumes oxygen, HABs can result in hypoxic or anoxic conditions.
Dinoflagellates are a group of microscopic single-celled organisms or protists that are dominantly autotrophic (i.e., primary producers). Interestingly, many species are also mixotrophic, having the ability to ingest their prey as a source of energy. Some species are entirely heterotrophic, lacking chloroplasts or plastids, and have been termed carnivorous. In fact, there have been reports in the scientific literature that some species have the ability to consume fish after having paralyzed them with neurotoxins. These claims are controversial but have given dinoflagellates a near-mythical reputation among the oceanic plankton.
Diatoms and dinoflagellates are most common in the coastal oceans but also have the ability to live in freshwater environments and in intermediate salinity environments where fresh and marine waters mix in estuaries. They are the most prolific group of primary producers in the ocean. Dinoflagellates have a highly complex life cycle that consists of an alternation between a motile stage and a resting or cyst stage. In short, dinoflagellates enter the resting stage via sexual reproduction when conditions in the surface ocean are not suitable for them to thrive. They can remain dormant for weeks, months or years before they “excyst,” when surface conditions improve, reproduce vegetatively, and populate the surface ocean. Excystment and repopulation are triggered by changes in temperature, light, or oxygen levels or even resuspension of cysts by storms. Dinoflagellates generally thrive when nutrient levels are elevated, and, under conditions of extremely high nutrient levels, cell division can be so rapid that extremely high cell counts (millions of cells per milliliter of seawater) are reached, resulting in red tides. The cyst stage acts as a very effective mechanism for seeding blooms.
The following video summarizes the life cycle of the dinoflagellates.
Dinoflagellates have a broad range of different ecologies. As we saw earlier in the module, they can be endosymbionts of corals, facilitating calcification in the host colony. They have this same role in foraminifera and radiolarian, a group of siliceous zooplankton.
Diatoms are autotrophic protists that produce a delicate, microscopic test of opaline silica.
They are non-motile, and, for most of their life cycle, they reproduce asexually. Many nearshore diatom species also have a resting stage akin to the dinoflagellates, allowing them to exit the surface zone when conditions are unfavorable for their growth. This may occur in winter when surface waters are cold or at times when nutrients are depleted. The resting spore stage actually may resemble the vegetative stage of dinoflagellates. Like the dinoflagellates, diatoms are able to reproduce extremely rapidly by simple cell division, and this allows them to rapidly dominate the surface ocean when nutrients are readily available.
Species that are harmful belong to the pennate diatoms that are long and thread-like and have the ability to attach to a host, although the relationship is not symbiotic. Both dinoflagellates and diatoms cysts can move around the oceans by currents, storms, dredging of the ocean bottom, and when cysts act as ballast on ships or even higher-level organisms. Toxins are not known in the cyst stage of either group.
Of some 60 or so species that cause red tides, only a handful is known to be toxic. Dominant dinoflagellate HAB genera include Alexandrium, Karenia, and Pfiesteria. The diatom genus most commonly associated with HABs is Pseudo-nitzschia. Each of these genera produces a different toxin and thus has a different role on organisms further up the food chain. Next, we discuss some of the HAB species in detail.
Alexandrium spp. is the dominant taxon in coastal regions of New England and eastern Canada but it is also found from California to Alaska.
It is a heterotrophic dinoflagellate that produces a saxitoxin, one of the most powerful known types of neurotoxins. These toxins destroy the function of nerve cells and can thereby cause paralysis. Saxitoxins are most effectively concentrated by shellfish such as clams, quahogs, mussels, scallops and oysters that filter large volumes of seawater to acquire their nutrition. Although the saxitoxin does not harm these shellfish, even in small quantities, the toxin can be extremely dangerous for humans, resulting in a serious illness known as paralytic shellfish poisoning (PSP). The saxitoxin attacks the human nervous system within 30 minutes of ingestion with symptoms that may include numbness, tingling, weakness, partial paralysis, incoherent speech, and nausea. In severe cases, the toxin can lead to respiratory failure and death within a few hours. Alexandrium spp. toxins have also been harmful to whales, sea otters and birds.
The following videos describe the causes and impacts of red tides as well as possible antidotes for shellfish poisoning.
Karenia is the dominant taxon causing red tides in Florida and Texas, but rarely species of Karenia have also been found up the east coast in North Carolina.
It produces a toxin known as a brevetoxin (named after a species of Karenia, K. brevis). Like saxitoxins, brevetoxins damage nerve cells, leading to disruption of normal neurological processes and causing neurotoxic shellfish poisoning (NSP). In humans, gastrointestinal symptoms and a variety of neurological ailments result, but there are no known fatalities. However, in fish, the brevetoxins attack the central nervous system and cause respiratory failure. Karenia dinoflagellates are responsible for massive fish and bird kills in the Gulf of Mexico. The brevetoxins are colorless, odorless, and heat and acid stable, thus they survive food preparation.
The genus Gambierdiscus lives in tropical waters, usually in reefs, and produces a toxin known as Ciguatoxin that causes gastrointestinal problems followed by mild neurological symptoms. This syndrome is known as Ciguatera fish poisoning. Because the toxin is fat soluble, it gets concentrated up the food chain by bioaccumulation from seaweed to smaller fish than to larger fish. The larger fish, which are the most dangerous to eat because they have the highest toxin concentrations, include commercially available seafood such as grouper, snapper, and barracuda. Ciguatera is responsible for more human illnesses—estimated between 10,000 to 50,000 cases annually—than any other HAB toxin.
Pseudo-nitzschia and the species Nitzschia navis-varingica are common diatom genera in Californian red tides. These taxa produce domoic acid, which is concentrated by filter-feeding shellfish. This neurotoxin can also bioaccumulate in fish such as anchovies that feed directly on the diatoms. Domoic acid causes a variety of gastrointestinal ailments, memory loss and brain damage in humans and is hence referred to as amnesic shellfish poisoning. Rarely, the neurotoxin can be fatal. It can also affect marine mammals, causing seizures.
The species Pfiesteria piscicida and P. shumwayae have been the most common dinoflagellate species in red tides in estuaries and bays along the east coast of the US from Delaware to Florida. The species occur in environments where freshwater and saltwater mix and have not been reported from freshwater environments or the open ocean. Pfiesteria blooms are restricted to summer months. Species have been associated with massive kills of menhaden and other estuarine fish in the Chesapeake Bay and the Tar-Pamlico and Neuse River Estuaries in North Carolina. The fish in contact with Pfiesteria rapidly develop bleeding lesions and have skin actively flake off them, and it has been proposed that the presence of live fish stimulates the production of toxin in the dinoflagellate. Ultimately the open lesions may destroy gill function and lead to death. Nevertheless, the connection between fish mortality and Pfiesteria is still doubted by some scientists. In fact, the effects of Pfiesteria on fish and human health has been one of the largest and nastiest controversies in marine science over the last 25 years and it does not appear that the conflict is anywhere near over.
The following video explains some of the research on Pfiesteria.
At least part of the debate has been fueled by the popular press, who have focused attention on the organism after studies suggested that it was carnivorous. These studies indicated that Pfiesteria species ingested the skin of fish after it flaked off. In fact, lab studies have shown that when fish were left in tanks with Pfiesteria, the fish died within hours. For this to occur, however, the dinoflagellate and fish must be in direct contact. Without contact, the same studies show that fish suffered no ill effects. Some scientists alternatively point to water molds or fungi including the species Aphanomyces invadans as the pathogen cause of the ulcerative lesions, skin loss, and damage to gills. A. invadans and other fungi are universally present in fish with ulcers and skin loss. Also weakening the case for Pfiesteria, this genus is still known to exist in North Carolina estuaries, but fish kills have become less frequent recently. Moreover, where lesions on the fish menhaden were observed, nearby fish including catfish, perch, and carp were unaffected. These disparities have cast some doubt on whether Pfiesteria is harmful to fish at all. In fact, great differences exist among public health professionals, and warnings from different state agencies are in conflict.
The health impact of Pfiesteria on humans is also uncertain, as is the method of transmission of the potential toxin. Scientists working with Pfiesteria in the laboratory have suffered from long-term neurological symptoms, such as memory loss, fatigue, and dermatological problems, and fisherman in contact with Pfiesteria-related fish kills have also suffered from similar ailments. However, other groups of fishermen who have come in close contact with lesion-covered fish have not reported adverse effects. There are reports that the hypothetical Pfiesteria toxin is transmitted via aerosols.
Until recently, the missing link in the Pfiesteria conundrum is that the toxin produced by this organism has been elusive. However, in 2007 scientists in a government lab claimed the first positive identification of a toxin associated with Pfiesteria. Even with this identification, questions remain; for one, the toxin is unstable in the natural environment, and second, it is not been proven to have adverse health effects. Thus, the controversy about Pfiesteria is far from over. In all reality, a number of factors may result in the fish kills; in particular, the fish in estuaries may have already been under great stress from other biological agents (bacteria, viruses, fungi, parasites), exposure to chemicals (pollutants, toxins), suboptimal water quality, and rapid water temperature change, that have the potential to cause lesions to form.
Far less controversial than the relationship of Pfiesteria with fish kills are studies that directly relate fish kills to low oxygen levels caused by algal blooms. In a number of estuaries along the eastern US, warmer waters in summer combined with increased production by algae, as a result of increased runoff and eutrophication, lead to severely decreased oxygen levels and major fish kills without the involvement of a toxin.
Finally, HABs are not always produced by dinoflagellates and diatoms. Cyanobacteria, or blue-green algae, another group of single-celled organisms, but one that is prokaryotic rather than eukaryotic, is also known to produce extremely potent toxins that can cause illness in fish, birds, and mammals including humans. Because of their potential to be harmful, this group is known as CyanoHABs. Cyanobacteria are some of the oldest species on Earth and are known to tolerate very tough conditions including hot, cold, salty waters and darkness. They live under ice sheets, near hydrothermal vents, and were some of the first organisms to colonize the ocedans after the massive asteroid that killed the dinosaurs.
Although the full scale of health effects of CyanoHABs on humans is not yet determined, the toxins may have gastrointestinal, respiratory, allergic, and neurological responses, and potentially lead to liver damage. In addition, prolific growth of cyanobacteria can block sunlight, which can harm other organisms, and use up oxygen which can lead to hypoxia and anoxia. As in the case of the HAB dinoflagellates, the growth of CyanoHABs may be stimulated by nitrogen loading from agricultural industrial runoff, as well as sewage disposal.
As we have seen, there is a great variety in the biology and ecology of HAB species. However, they share one major thing in common: all of them have the ability to wreak havoc on coastal fisheries. Since the growth of most if not all of the species directly responds to nitrogen loading, limiting the harm on fisheries will require significant changes in agricultural practices combined with modifications of drainage in coastal regions. Such changes will be difficult, if not impossible, to accomplish in the near term. Thus, the best strategies to deal with HABs focus on the integration of highly detailed algal sampling programs, ecological forecasts, and resource management. For example, if HABs can be predicted, then warnings can be issued and areas placed off-limits to fisheries. Such strategies are being employed in areas where HABs are common, including the Gulf of Maine, the Gulf of Mexico, and the Pacific Northwest.
HAB forecasts integrate ecological models, based on the physics, chemistry, and biology of nearshore and offshore regions, with satellite data and in situ measurements of cell counts and toxin levels. For example, models can be used to predict the development and movement of a HAB in the region of interest. Models can be used to identify HAB triggers (i.e., nutrients or temperature), likely areas of cyst seedbeds, likely bloom toxicity based on cell density, and progression of the toxin through the food chain, as well as the ultimate decline of the HAB.
A new threat to fisheries around the world has developed over the last decade---a surge in the number of jellyfish in coastal waters. The most dramatic of these outbreaks is in Japanese waters, where the giant Nomura’s jellyfish has increased significantly, wreaking havoc with fisheries in the Sea of Japan.
Jellyfish populations are normally held in check by fish, mostly because these two groups compete for the same food sources. However, overfishing in many parts of the ocean has led to increasing jellyfish populations. Jellyfish may also be aided by warming ocean temperatures, which favors their development, and by the destruction of habitats of other natural predators such as turtles.
The massive Nomura’s jellyfish is a great threat to Chinese, Japanese and Korean fisheries. These creatures can grow to two meters diameter (the size of large refrigerators) with a weight of 200 kg.
Because of their size, they consume massive amounts of zooplankton, depleting this vital part of the food chain for other organisms. The key threat of the Nomura’s derives from the fact that this jellyfish reproduces extremely rapidly. A mature jellyfish has the ability to produce billions of eggs at a time, and they can do this when they are attacked. Once fertilized, these eggs develop into a resting polyp stage that also has the ability to multiply rapidly, effectively carpeting areas of the seafloor. When conditions are suitable, the polyp reproduces asexually, developing into the medusa stage, which grows into the mature jellyfish.
Once ideal conditions develop, either by increasing nutrients or warming of the surface ocean, Nomura’s jellyfish populations literally explode and render fishing virtually impossible because nets become filled with jellyfish. These jellyfish can also continue to reduce the number of fish in the oceans by feeding on their eggs. Moreover, there is evidence that jellyfish can tolerate conditions, like hypoxia, that fish cannot.
The following video summarizes the impact of giant jellyfish on Japanese fisheries.
Blooms of other jellyfish species are being reported in many other parts of the ocean. In the Gulf of Mexico, for example, the last thirty years populations of two species of jellyfish, the sea nettle, and the moon jellyfish, have exploded especially in dead zones as these are one of the few organisms that can tolerate hypoxia. Jellyfish in the Gulf now swarm over hundreds and perhaps even thousands of square miles each summer.
Here also, invasive species of jellyfish, including the Australian jellyfish, have been reported. Several other factors besides hypoxia have caused the increase in Gulf of Mexico jellyfish. As in the Sea of Japan, overfishing has reduced one of the main jellyfish competitors. In addition, drilling platforms have provided habitats in which jellyfish polyps can multiply. As in the Sea of Japan, jellyfish in the Gulf of Mexico are impacting the fishing industry.
Jellyfish swarms, too, have plagued other regions; they include northern Australia where the highly venomous box jellyfish has expanded its range, the Black Sea, and the Bering Sea off Alaska. Worldwide, jellyfish are one of the few organisms that can thrive in dead zones. With the spread of such dead zones in the oceans as a consequence of marine pollution and climate change, we could be entering the age of the jellyfish.
In this module, you should have learned the following concepts:
You should have read the contents of this module carefully, completed and submitted any labs, the Yellowdig Entry and Reply, and taken the Module Quiz. If you have not done so already, please do so before moving on to the next module. Incomplete assignments will negatively impact your final grade.
Links
[1] https://www.youtube.com/channel/UCJcoPrmCdVhGqOmPspWy2uA?feature=emb_ch_name_ex
[2] https://www.e-education.psu.edu/earth103/node/869
[3] http://www.mbari.org/
[4] https://www.youtube.com/channel/UCNia_X3a7HLxBSN-z57UGQw?feature=emb_ch_name_ex
[5] https://www.youtube.com/channel/UC-87aDLv5WFJ83fxt21gsEQ?feature=emb_ch_name_ex
[6] https://www.barrierreef.org/
[7] http://www.flickr.com/photos/dkeats/5628135789/
[8] https://creativecommons.org/licenses/by/2.0/
[9] https://www.youtube.com/channel/UCU1QB1a5XJa_nTHD2lzr7Ew?feature=emb_ch_name_ex
[10] http://seasonsinthesea.com/mar/kelp.shtml
[11] https://www.flickr.com/photos/48722974@N07/
[12] http://www.flickr.com/photos/48722974@N07/4479046686/
[13] http://creativecommons.org/licenses/by-nc-sa/2.0/
[14] https://oceanservice.noaa.gov/facts/coral_bleach.html
[15] https://creativecommons.org/licenses/by-nd/4.0/
[16] https://www.youtube.com/watch?v=ostV7YTyqDk
[17] https://www.e-education.psu.edu/earth103/sites/www.e-education.psu.edu.earth103/files/module07/ReefsAllfin_0.kmz
[18] https://www.e-education.psu.edu/earth103/sites/www.e-education.psu.edu.earth103/files/module07/ReefsAllfin.kmz
[19] https://www.flickr.com/photos/noaaphotolib/
[20] https://www.flickr.com/photos/noaaphotolib/5017963451/
[21] https://creativecommons.org/licenses/by/2.0/deed.en
[22] https://www.pmel.noaa.gov/co2/file/Pteropod+shell+experiment
[23] http://www.theresilientearth.com/?q=content/marine-life-survived-8x-current-co2-levels
[24] https://www.whoi.edu/
[25] http://www.whoi.edu/science/B/ecohab/work/ecohab2000.html
[26] http://oceandatacenter.ucsc.edu/PhytoGallery/Diatoms/pseudo%20nitzschia.html
[27] https://www.youtube.com/channel/UC6ZFN9Tx6xh-skXCuRHCDpQ?feature=emb_ch_name_ex
[28] https://www.youtube.com/channel/UC5jcv1eLSxuM5KCoPFq8JZg?feature=emb_ch_name_ex
[29] http://botany.si.edu/references/dinoflagellates/gambierdiscus_to.htm
[30] http://www.vims.edu/research/departments/eaah/programs/pfiesteria_research/resources/images/index.php
[31] https://www.vims.edu/research/units/legacy/pfiesteria_research/resources/images/index.php
[32] https://www.youtube.com/channel/UCBkWMXkD1g8ok7fs5vOAhaQ
[33] https://www.youtube.com/channel/UCU1QB1a5XJa_nTHD2lzr7Ew
[34] https://vancouversun.com/news/jellyfish-thrive-in-adverse-conditions-ubc-study-finds
[35] https://www.youtube.com/channel/UCeW5IMmy5uUzEv3f4ijV2cg
[36] https://creativecommons.org/licenses/by-nc-sa/2.0/