Zooplankton are tiny crustaceans that live in the lake and feed on algae. Some feed on other zooplankton or tiny ciliates and rotifers. Samples are collected weekly, with with samples taken during the day and the night from the following depths: 0, 3, 5, 10, 15, 20, 25, 30, 32, and a pooled mixed layer (PML), formed from equal amounts from 1, 3, and 5 meters.
A Schindler Trap draws a sample of water from a designated depth, then releases the water to capture the zooplankton. When the samples are counted, the various crestatious species are identified and measured through a microscope. Species of zooplankton observed at Castle Lake include Daphnia, Diaptomus, Holopedium, Bosmina, and others.
The data can be used for several purposes, including a clear profile and density distribution of the various species migration patterns. By measuring their respective sizes, one can calculate their relative biomass, an accurate measure of the lakes phytoplankton growth (tropic level below zooplankton), as well as an indicator of the carrying capacity of lakes fish (the tropic level above).