In the world's oceans, understanding how planktonic food webs operate is crucial to predicting the amounts and rates of organic cycling. Among the least understood regulators of plankton dynamics are the soft?bodied zooplankton that reside at midwater depths (30?1000 in). Top?down control via predation by one group, the physonect siphonophores, has been especially difficult to quantify, principally because these polymorphic animals are extremely fragile and readily fragment when collected with plankton nets. This investigation will use a manned submersible (Johnson?Sea?Link) and the tools it carries to quantify the distribution, abundance, behavior, and feeding of the common, but poorly studied, gelatinous colonies of Nanomia cara. Field studies will be conducted in the Gulf of Maine region (Wilkinson Basin, Georges Bank and Oceanographer Canyon) where populations of this physonect siphonophore Nanomia cara are likely to be important regulators of zooplankton prey stocks. Preliminary investigations have indicated that N. cara has the potential to consume a significant portion of the standing stocks of primary consumers, specifically the non?reproducing stages IV?V of the copepod Calanus finmarchicus. High mortality rates of this calanoid species can be detrimental to fisheries on regional and basin scales. Laboratory investigations of carefully collected colonies will determine rates of digestion and respiration (=oxygen consumption and nitrogen excretion) in environments that are similar to in situ conditions (e.g., temperature and darkness). These data in conjunction with field studies will provide a basis for predicting the predatory impact of Nanomia cara. This study of predation mortality at individual and population levels will broaden the work undertaken in the U.S. GLOBEC program in the Gulf of Maine/Georges Bank regime. Results from this research are also likely to be useful for NOAA's COP Georges Bank Predation study, especially if N. cara preys on larval fishes.
