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Research Interests:
Humans greatly depend on the ecosystem services provided by freshwater resources, and yet anthropogenic impacts on aquatic ecosystems continue to be critical. Moreover, because these impacts range from local to global, direct to indirect, and short-term to long-term, aquatic ecosystem research is a challenging but important task. I am interested in the ecological effects of these human-induced changes as well as non-human-induced stressors on aquatic ecosystems. My research has involved a diversity of projects that include my undergraduate work examining nutrient limitation in reservoirs with varying watershed composition; my doctoral work on the effects of ultraviolet radiation (UV), dissolved organic matter (DOM), and other stressors on zooplankton; my postdoctoral work on the bioenergetics and trophic effects of invasive Asian carp; and my current work on the effects of the exotic water flea Daphnia lumholtzi in central North Carolina. (top)
Ecological Effects of UV and Related Stressors In many ecosystems, ultraviolet radiation (UV) may have important direct and indirect effects on plankton communities. Dissolved organic matter (DOM) is the primary attenuator of UV, and it may alter vertical gradients in UV, light, nutrients, primary production, and plankton communities, among other variables. Understanding UV-DOM dynamics is important because climate change and anthropogenic watershed modification can alter DOM concentration and quality in aquatic ecosystems. For my dissertation research I used multiple approaches, including large-scale in situ mesocosm experiments, to study the effects of UV, DOM, and other variables on zooplankton. I found that UV and DOM can interact to have beneficial or detrimental effects on survival and reproduction, depending on the taxon (Cooke et al., 2006 Hydrobiol.). For example, when DOM concentrations were elevated, Daphnia abundance and egg production were higher than that of calanoid copepods, especially in the presence of UV. DOM can also affect both the day- and nighttime vertical distribution of zooplankton (Cooke et al., in preparation). Other research I have done suggests that UV may lower calanoid reproduction in subalpine systems (Cooke et al., 2006 FWB), but in other systems UV can actually have a positive effect on calanoids, even at low DOM levels, due to indirect effects (Cooke and Williamson, 2006). In addition to UV-DOM interactions, temperature is an important interacting variable because some organisms use the temperature-dependent process of photo-enzymatic repair to repair UV-induced DNA damage. In subalpine systems I found that UV had negative effects on DNA damage and calanoid copepod reproduction at cooler temperatures, but no effect at warmer temperatures (Cooke et al., 2006 FWB). Similarly, in a temperate lake I observed that zooplankton diel vertical migration was influenced by both UV and temperature, and results indicated that UV may constrain some species to depths with cooler than optimal temperatures (Cooke et al., 2008). These findings have important implications for ecosystems where UV and other physical variables are changing due to climate-related factors. (top)
Ecology of Aquatic Invasive Species Determining the impacts of Daphnia lumholtzi on ecosystem functions of water supply reservoirs I am studying the exotic water flea Daphnia lumholtzi in Falls Lake, an impoundment that serves as Raleigh’s water supply and is located on the Neuse River in North Carolina. Native to regions of Africa, Asia, and Australia, D. lumholtzi is a potentially invasive zooplankton species that has spread rapidly throughout the central and southern United States since its accidental introduction to a Texas reservoir in the early 1990s. D. lumholtzi was first reported in western North Carolina in 1996, but it has not been previously reported in central North Carolina, and its complete range of occurrence and ecological impacts in this region are unknown. But research on D. lumholtzi in other parts of the U.S. indicates that this species may alter both fish communities and algal communities, including cyanobacteria blooms, and thus may it impact freshwater ecosystem services differently compared to the native zooplankton displaced by D. lumholtzi. The three main objectives of my research are to (1) examine the extent of D. lumholtzi spread in the Neuse River system and nearby Cape Fear River system; (2) determine the ecological and physio-chemical factors that govern D. lumholtzi abundance; and (3) determine how D. lumholtzi impacts aquatic ecosystem functions in Falls Lake and other regions of central North Carolina where it is present. (top)
Assessing the potential effects of Asian carp as an invasive threat to the Great Lakes ecosystems For my postdoctoral work at the University of Illinois I collaborated with Dr. Walter Hill to study how the bioenergetics and trophic cascade effects of invasive Asian carp may impact their potential establishment in the Laurentian Great Lakes. Asian carp have not yet invaded these ecosystems but have established reproducing populations throughout the Mississippi River basin and are poised to enter Lake Michigan via the Illinois River. From mesocosm experiments I found that these planktivorous fish can substantially alter zooplankton biomass and species composition, but that the carp may lose biomass under oligotrophic conditions (Cooke et al., 2009). Dr. Hill and I recently developed bioenergetics models for bighead and silver carps, two species of Asian carp. We use zooplankton and phytoplankton data from different regions of the Great Lakes to apply these bioenergetic models and predict carp growth based on plankton food availability. We found that due to low plankton availability, modeled carp growth is negative in most open water habitats, but positive growth is predicted in some (but not all) productive regions such as Green Bay and western Lake Erie. These results are important because they suggest that some regions and seasons may be more conducive to the establishment of these detrimental invaders than others, and thus prevention efforts should focus on these situations. Because Asian carp have invaded habitats throughout the world the implications of our results extend beyond the Great Lakes. This work was recently published in Freshwater Biology (Cooke and Hill, 2010). (top)
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