Our homeschool group had a class on the Hudson River Estuary and its food chains at Beczak Environmental Education Center in Yonkers, NY. First the kids learned about the Hudson -- its source at Lake Tear of the Clouds in the Adirondack Mountains, how it gradually widens downstream, and empties into the Atlantic Ocean, and its tides and changing salinity. The water in our portion of the river is brackish, meaning it is a mixture of salt and fresh water. Dorene, the wonderful educator at Beczak, demonstrated this with a model of the estuary, into which she had the kids pour cream at the Atlantic end to represent salt water, and water up at the north end to represent fresh water, which were mixed by tilting the table to demonstrate tidal action. She also used pepper to show how pollutants affect the river. A sponge "marsh" helped to filter the pepper pollutant, which washed off the land and into the water with precipitation. She talked about PCB contamination, which was graphically illustrated in the next portion of the program on food chains.
The kids started off with a huge cloth sack representing a striped bass, from which they removed an Atlantic Silversides (a smaller fish), from which they removed an Amphipod (or sand flea, a type of small crustacean), from which they removed Zooplankton (tiny animal plankton. Plankton means "drifter"), from which they removed Phytoplankton (plant plankton), which makes its own food via photosynthesis from sun, carbon dioxide and water. They then learned that phytoplankton take in PCBs with the water. This means that the PCBs go right back up the chain through all these creatures, and ultimately, into people, who eat the fish.