Ecological studies show that bottom-up (e.g., nutrient input) and top-down (e.g., grazing/predation) pressures may change the structure of aquatic ecosystems with ‘cascading’ effects throughout the food chain. Recent research supports that zooplankton grazing in high-nutrient waters promotes the growth of larger phytoplankton over smaller species, creating a higher abundance of these larger single celled algae within the food web.
Using samples from three nutrient enriched southeastern Louisiana bayou lakes (Cataouatche, Lac des Allemands, Salvador), researchers with the NCCOS-funded Barataria Bay Multiple Stressors project at Louisiana State University and Louisiana Universities Marine Consortium conducted experiments on nutrient additions with zooplankton grazing. Unlike previous single species laboratory experiments, they used whole water samples with entire phyto-and zooplankton species composition and maintained under natural conditions to test the hypothesis.
The researchers hypothesized that grazing on phytoplankton by the microzooplankton community (e.g., rotifers) is size-dependent; top-down control of phytoplankton by the microzooplankton explains why small phytoplankton become less abundant than large phytoplankton in eutrophic waters. Using the ‘dilution method,’ the experiment tested the whole microzooplankton community and provided solid evidence that microzooplankton grazing on phytoplankton explains why the average size of phytoplankton, whether as isolated cells or as colonies, tends to increase in eutrophic waters.
The research is published on the scientific journal Hydrobiologia .