Diel, Seasonal, and Interannual Patterns in Mesozooplankton Abundance in the Sargasso Sea

Temporal changes in mesozooplankton community structure are influenced by a combination of environmental factors. Epipelagic mesozooplankton biomass in the Sargasso Sea has increased over the last two decades, with a related increase in zooplankton-mediated carbon export. Unknown, however, are the patterns and variability at different temporal scales (diel, seasonal, and interannual) in abundance of each major zooplankton taxon, and how these patterns relate to physical and other environmental changes. I enumerated major taxa of mesozooplankton collected from monthly day and night net tows in the epipelagic zone at the Bermuda Atlantic Time-series Study (BATS) site in the Sargasso Sea from 1999 to 2010. Abundances of each taxon were determined using a ZooScan optical imaging system and microscopy. Generalized Linear Models (GLMs) were used to determine what environmental parameters best explain abundance of major taxa. I used annual averages to determine broader patterns. Zooplankton taxa with the most pronounced diel vertical migration (i.e., night:day ratio, N:D, » 1 ) included euphausiids (N:D=1.9), Limacina spp. pteropods (1.5), and other thecosome (shelled) pteropods (1.6). Taxa with a pronounced spring abundance peak included euphausiids, larvaceans, and Limacina spp., while harpacticoid copepods peaked in late summer, and calanoid copepods in both spring and summer. There is some evidence of changes in phenology occurring in calanoid copepods and chaetognaths that exhibited spring abundance peaks on average 1-month earlier than reported for the same taxa in the early 1960’s. Many taxa, including all copepod taxa, exhibited a period of highest abundance increase in 2003, coinciding with a 2003 April diatom bloom and the largest primary production peak (April 2003) in the time series. There was also indication of a long-term increase in calanoid and oncaeid copepod abundance. Sub-decadal-scale climate oscillations, long term warming, and ocean acidification may be driving decreases in larvaceans, Limacina spp., and other shelled pteropod densities. Environmental variables affecting abundance differed among taxa. For example, calanoid copepod density was highly influenced by the abundance of a major predatorchaetognaths. Multi-year densities of calanoid copepods and ostracods both increased with increasing Water Column Stratification Index and the Atlantic Multidecadal Oscillation (AMO) index, indicating warmer sea surface temperatures are favorable for these taxa. These patterns in zooplankton community structure have important implications for energy transfer in pelagic food webs and for biogeochemical cycling