There are many tropic levels in a lake ecosystem, and the addition or removal of an organism can cause positive and negative cascades of certain species. Typical fresh water lakes have a wide range of interactions occurring between organisms. For most systems fish consume zooplankton as their energy source, algae are preyed upon by the zooplankton, and the algae feed on the protozoans and bacteria (Kim, et. al., 2000).
Lake ecosystems however, are different, and there are many approaches made to confirm if a certain species of fish are influencing the numbers of specific tropic levels (Cummins, 1996). Fish can decrease the body size of zooplankton, to smaller taxa such as eg. Daphnia longispina to D. hyalina. A 250 mm plankton net can be used to capture zooplankton, studies of the different species present in a sample lead to conclusions regarding the amount of algae in the environment. Larger zooplankton such as Daphnia Magna (Bosmina longirostris), have larger mouths, and consume larger algae particles that smaller zooplankton would (Matveev, et. al., 2000). The largest zooplankton compete for largest phytoplankton, but they are also more vulnerable to predation.
Most fish are only zooplanktiverous when young, and hunt larger invertebrates in adult stages, and size selective as fish hunt be sight. We have no herbivorous fish native to Britain, although the presence on aquatic plants is essential to the spawning of most fish species (Begon, et.al., 1996). Zooplankton can be carnivorous or herbivorous, and have a considerable influence over the amount of phytoplankton in the water body (Grahame, 1987) .
It is possible to view trophic interactions by looking at the physiologies of species. The presence of predators induces defensive physiologies of zooplankton such as spines on egg bearing rotifer (Keratella cochlearis) when predatory rotifer (Asplachna priodonta) is present (Begon, et. al., 1996). Direct dissection and observation of the contents of the fish stomach can give an indication of the type of food it eats, i.e. zooplankton or phytoplankton.
Experiments can also be done to view the ingestion rates on zooplankton using 32P labelled yeast cells, or fluorescent spheres, and through measuring chlorophyll a concentrations (equal to the amount of algae present) from mesocosms with alternative numbers of zooplankton. Field work could involve the experimental removal of fish from an environment, with samples of zooplankton taken before and after to see if zooplankton have proliferated.
There is both evidence for and against fish influences upon zooplankton, and therefore algal biomass, for example in New Zealand, the brown trout (Salmo trutta), is predatory to invertebrates, grazing invertebrates and algae, and so the fish didn’t affect herbivorous species. However, Eel river in California contains Roach (Hespeoleucas symmetricus) which reduce fish fry, and invertebrate predators so that the midge larvae (Pseudochironomus richardsoni) reaches high levels which reduces the biomass of Cladophora algae due to grazing (Begon, et. al., 1996). These examples show just how diverse ecosystems can be, and so the relevant field work and experimental work should be done before a fish is introduced as a form of bioremediation.
