Zooplankton are animals that live in freshwater and marine habitats. They range in size from 100 microns (smaller than the dot on an i) to 2 mm/0.08 in in length. Zooplankton are an integral part of the food web, providing essential food for fish, but they also play an important role as ‘grazers’ (like land herbivores on pastures) of the phytoplankton or algae (the pastures of the sea – i.e., microscopic freshwater plants).
Given their critical position in the food web, they are also an important transfer point for contaminants like methylmercury, the organic form of mercury (Hg) that bioaccumulates in organisms, and biomagnifies up the food chain. Sampling the Finger Lakes zooplankton is one of the most important (and interesting) parts of the Finger Lakes Hg Project. This is because zooplankton can give us a lot of insight into how much Hg is really available to be taken up by the food web.
To collect the zooplankton, using a winch we drop a plankton net to within a few metres above the lake bottom (we do not want to collect any sediments in our sample). Then we pull it back up with the winch. This is called a ‘vertical tow’ (You can also tow a net behind a boat to get what is called a ‘horizontal tow’. Other devices exist that allow you to sample a specific depth of water, called a Schindler-Patalas trap).
Once we’ve taken a vertical tow, the sample bottle at the end of the net contains the zooplankton. We rinse the zooplankton into our acid-cleaned Teflon sampling vials (to minimize contamination issues), and voilà! We have a zooplankton sample ready to be processed in the lab for Hg analysis.