Have you ever been told "don't eat seafood in months without an "r"? As kids, many of us were told that but never really knew why. Turns out, that the summer months are typical "red tide" months and raw seafood consumption was not a good idea because of PSP (paralytic shellfish poisoning). In fact, California has a state law that allows substitution of punched "ray" fin for scallops in restaurants during those summer months without notifying the consumer for that reason.
Are "red tides" and PSP bad? Well, examine this map and you will see that the incidence of PSP has spread considerably with red tides. Take about a half hour to pore over this fairly self-explanatory slideshow about plankton ecology and HABs (harmful algal blooms). The link will download a file: HABs.ppt. This should give you what you need to understand the food web discussion in Lesson 8, and all about the requirements of photosynthesis in the sea and why nutrients are important and, at times, problematic. Plankton may be responding to global change as indicated in this summary that appeared in Science magazine.
There are more resources on HABs on the last page of this Lesson. Any questions? Go on to Coral Reefs!