Microzooplankton Herbivory Reduced By Petroleum Pollutants In A Mesocosm Study

Microzooplankton are major consumers of phytoplankton and essential components in biogeochemical cycling of carbon and nutrients in the oceans. However, their ecological functions in marine ecosystems could be substantially affected by various environmental factors. Crude oil pollution from anthropogenic activities and chronic natural seeps occurs in various regions of the oceans and could cause lethal or sublethal effects to microzooplankton. While on site measurement of the effects on microzooplankton immediately after exposure to environmental perturbation caused by crude oil pollutants is not always possible, a mesocosm study simulating oil spill incident to investigate its effects on grazing impact of microzooplankton on phytoplankton was conducted. A natural plankton community was exposed to chemically dispersed crude oil (DOil) in outdoor mesocosms for 7 days. Dilution experiments were conducted to estimate the grazing rates of microzooplankton on the 2^nd and 6^th days after mesocosm set-up. Results based on chlorophyll-a concentrations revealed that microzooplankton grazing rates ranged 0.4-2.3 d^-1 in control (Ctrl) mesocosms. The estimated percentages of standing stock consumed by microzooplankton in Ctrl mesocosms on the 2^nd and 6^th days were 52% and 78%, respectively. On the other hand, there was a lack of microzooplankton grazing on the 2^nd and 6^th days in DOil mesocosms as reflected by negative grazing rates. Coefficients of phytoplankton growth were negative on the 2^nd Day and averaged 0.7 d^-1 on the 6^th day in DOil mesocosms. Acute effects from petroleum pollutants could kill most microzooplankton in DOil mesocosms and result in negligible herbivory. In implication, reduction in microzooplankton herbivory caused by environmental perturbations like petroleum pollutants could bring instability to the transfer of carbon and nutrients from primary producers to the higher trophic levels.