Lake Sediments + Daphnia
In 2002 I was very fortunate to participate in research with Dr. Peter Schulze of Austin College on the effects of suspended sediments put into the lake by human activities affect the distribution and abundance of Lake Texoma Daphnia. This was one of my first forays into how human activities can influence aquatic ecosystems, which I am still interested in today with my work on endangered Eurycea salamanders. Natural land erosion upstream of lakes release tiny particles of sediment that enter the lake during rainfall events and remain suspended in the water, which can turn very clear water murky – or “turbid”. Construction and agricultural activities of humans exacerbate the speed and magnitude of this erosion, which can lead to increased turbidity in upstream portions of lakes which eventually settle to the bottom in downstream portions.
Daphnia are a type of zooplankton that feed on tiny aquatic phytoplankton species, and lab research shows that suspended sediments may interfere with the ability of some Daphnia species to feed. This means that suspended sediments could make some Daphnia species less competitive in turbid water, effectively excluding them from turbid portions of lakes. Because Daphnia are important food sources for many invertebrate and fish species higher up the food chain, it is very important to understand how suspended sediments affect their distributions.
I spent most of the summer of 2002 in the laboratory counting and identifying different species of zooplankton collected from Lake Texoma. Here is a summary of what we found, and the results of our research were published in 2006 in the journal Freshwater Biology. Click here to download a PDF.
1. We measured the abundance and eggs per female of four Daphnia species in turbid and relatively clear regions of Lake Texoma (Oklahoma-Texas, U.S.A.) on 12 dates over the course of 5 years.
2. Two species, Daphnia lumholtzi and Daphnia parvula, occurred and reproduced in turbid locations, but two other species, Daphnia mendotae and Daphnia pulicaria, occurred almost exclusively in relatively clear conditions.
3. To test the hypothesis that interference with foraging excludes clear-water Daphnia species from turbid locations, we incubated adult D. mendotae at both a clear and a turbid site. In three successive experiments D. mendotae individuals incubated at the turbid site carried as many or more eggs than individuals incubated at the clear site.
In short, our data plus a review of the literature demonstrates that Daphnia have species-specific responses to turbid circumstances; some species routinely occur in turbid conditions but others are restricted to clear-water environments. We were left with two broad questions to address in future research: (i) Why do some Daphnia species, but not others, thrive in turbid circumstances? (ii) To what extent does the composition of suspended material vary in nature, and how do any such differences in that composition affect zooplankton or other aquatic organisms?