Nutrients:

For algal growth in lakes, nutrients of primary concern are nitrogen and phosphorus, “macronutrients” that establish the upper limit for algae growth in a lake. Factors such as grazing by zooplankton, temperature and available sunlight can prevent algal growth from reaching its maximum potential, but high levels of algal biomass will not occur in the absence of sufficient nutrients.

Algae:

These organisms are the base of the aquatic food web. Lakes receiving excess nutrients from human impacts tend to have too much algae. This abundance causes numerous problems, many of which have been outlined in previous issues of the Water Line.

Zooplankton:

This group of small animals eats algae and provides forage to small planktivorous fish. In some cases, zooplankton can filter the entire volume of water in a lake in just a few days. In the presence of planktivorous fish, the zooplankton tend to be small because bigger zooplankton are easier to see (and therefore easier to eat). In the absence of grazing pressure by zooplanktivores, the zooplankton community will shift toward larger species. It is important to note that zooplankton don’t usually eat blue green algae, filamentous algae or algae attached to rocks or other structures.

Zooplanktivores:

This trophic level includes shad and shiners. These fish will consume the zooplankton that keep algae populations in check. Reducing the numbers of these fish will theoretically increase the number (and size) of zooplankton in the lake and ultimately lead to lower concentrations of algae.

Piscivores:

“Fish eaters”, like largemouth bass or walleye, are at the top of the freshwater aquatic food chain. Piscivores can eat zooplanktivorous fish in great enough quantities to affect the zooplankton population. Thanks to reduced pressure from the zooplanktivores, the zooplankton increase in number and size and can more readily consume algae, resulting in clearer water in some cases.