Zooplankton changes associated with grazing pressure of northern quahogs (Mercenaria mercenaria L.) in experimental mesocosms

Over the last three decades, the abundance of the bivalve Mercenaria mercenaria, a benthic suspension feeder, has declined dramatically in the Great South Bay, Long Island, NY. This decline undoubtedly has had significant impacts on planktonic dynamics in the estuary and may be a contributing factor to the appearance of blooms of Aureococcus anophagefferens (brown tides) that began in 1985. We conducted three, 300-L mesocosm experiments that manipulated clam abundance in seawater containing an inoculum of Aureococcus anophagefferens obtained from bloom water. Within a week, differences in phytoplankton and zooplankton composition emerged between control and experimental tanks. In two of three experiments, biomasses (μg C L−1) of brown tide and copepods, mostly Acartia tonsa, were lower in tanks with clams compared to controls while the opposite was found for ciliates. Redundancy analysis indicated that total clearance rate (L h−1) by Mercenaria mercenaria was the single best predictor of differences in the composition of the planktonic community. The analysis also showed that the reason for increased ciliate biomass associated with clams was lower than average brown tide biomass rather than reduction in predation pressure due to lower than average copepod biomass in these same tanks. And, although food (i.e., diatoms and dinoflagellates) limitation could have contributed to low copepod abundance in tanks with clam competitors, these copepod reductions may also have resulted from direct predatory impacts of clams on early life stages (eggs and nauplii) of Acartia tonsa. Our findings indicate a potentially complex trophic role for Mercenaria mercenaria in estuarine food webs.