Long‐term increase in mesozooplankton biomass in the Sargasso Sea: Linkage to climate and implications for food web dynamics and biogeochemical cycling

Abstract

Changes in zooplankton biomass and species composition over long time scales can have significant effects on biogeochemical cycling and transfer of energy to higher trophic levels. We analyzed size‐fractionated mesozooplankton biomass (>200μm) from biweekly to monthly day and night tows taken from 1994 to 2010 in the epipelagic zone at the Bermuda Atlantic Time series Study (BATS) site in the oligotrophic North Atlantic subtropical gyre. During this 17‐year period total mesozooplankton biomass increased 61% overall, although a few short‐term downturns occurred over the course of the time series. The overall increase was higher in the nighttime compared to daytime, resulting in an increase in calculated diel vertical migrator biomass. The largest seasonal increase in total biomass was in the late‐winter to spring (February–April). Associated with the larger increase in late‐winter/spring biomass was a shift in the timing of annual peak biomass during the latter half of the time series (from March/April to a distinct March peak for all size fractions combined, and April to March for the 2–5 mm size fractions). Zooplankton biomass was positively correlated with sea‐surface temperature, water column stratification, and primary production, and negatively correlated with mean temperature between 300 and 600 m. Significant correlations exist between multidecadal climate indices–the North Atlantic Oscillation plus three different Pacific Ocean climate indices, and BATS zooplankton biomass, indicating connections between patterns in climate forcing and ecosystem response. Resultant changes in biogeochemical cycling include an increase in the magnitude of both active carbon flux by diel vertical migration and passive carbon flux of fecal pellets as components of the export flux. The most likely mechanism driving the zooplankton biomass increase is bottom‐up control by smaller phytoplankton, which has also increased in biomass and production at BATS, translating up the microbial food web into mesozooplankton. Decreases in top‐down control or expansion of the range of tropical species northward as a result of warming may also play a role.