The marine biotoxins are basically red tide, a dinoflagellate that has a paralyzing toxin in it. When the clams/oysters/mussels eat them, they accumulate the toxins in their tissues that are eaten and pass them to the eater. These blooms may be increased by high levels of fertilizer type runoff reaching the water.
The pollutants are usually not industrial type pollutants but E. Coli and other bacteria from leaky shoreline septic systems, and often too many geese that hang out in the parks and along the beaches.
Hope this helps.