Zooplankton Research

Trout Lake boasts clear, clean, cold water. It harbours both a cold water and a warm water fish community. These are sustained by a complex food web at the base of which are found tiny microscopic, photosynthesizing plants called phytoplankton. Except for the odd fly that may be gulped by surface feeding fish, by and large all living animals in the lake are directly dependent on phytoplankton, the base of the food chain (first trophic level). These plants are autotrophs, meaning that they can manufacture their own food. Using sunlight as their energy source, chloroplasts deep inside the cell are able to assemble carbon dioxide and water into glucose, the basic building block of all organic life.

Feeding on these microscopic plants, are a number of microscopic animals commonly referred to as zooplankton. These are caught using a sampling device called a limno-net (see photograph). Given that larval fish and minnows and the like target zooplankton, they play a very important role in allowing energy to flow up the food web. Of particular interest is that section of the food web that assembles lake trout biomass. A simplified look at the food web energy flow looks like this: phytoplankton . . . zooplankton . . . lake herring . . . lake trout. Note that lake herring are zooplankton eaters (planktivores), even though they can be quite large (many grow to 30 cm in length).

Diaptomids (herbivorous copepods) dominate the zooplankton community of Trout Lake and are a major food source for the lake herring. Next in zooplankton abundance are cyclopoids, many of which are carnivorous. Also important is the Mysis relicta population, a rather large (>1 cm) carnivorous zooplankter which can be both an important predator of other zooplankton and a major prey of lake herring and young lake trout.

Food chain interactions

Due to overharvesting the lake trout fishery was shut down in 1991. Since that time lake trout have been recovering numerically in Trout Lake. As more lake trout mature, these in turn eat more and more of the lake herring. If the lake herring population is falling then the diaptomids (their most common food source) should be rising in abundance. Also, more spawning lake trout equals more young of the year lake trout, which means less mysids (their preferred food), which also means more diaptomids - the opposite of what the graph is showing in 1996 and 1997. Please write in if you think you know what is going on.