Silicate:nitrate ratios of upwelled waters control the phytoplankton community sustained by mesoscale eddies in sub-tropical North Atlantic and Pacific

摘要

Mesoscale eddies in sub-tropical gyres physically perturbthe water column and can introduce macronutrients to the euphotic zone,stimulating a biological response in which phytoplankton communities canbecome dominated by large phytoplankton. Mesoscale eddies may therefore beimportant in driving export in oligotrophic regions of the modern ocean.However, the character and magnitude of the biological response sustained byeddies is variable. Here we present data from mesoscale eddies in theSargasso Sea (Atlantic) and the waters off Hawai'i (Pacific), alongsidemesoscale events that affected the Bermuda Atlantic Time-Series Study (BATS)over the past decade. From this analysis, we suggest that the phytoplanktoncommunity structure sustained by mesoscale eddies is predetermined by therelative abundance of silicate over nitrate (Si*) in the upwelled waters. Wepresent data that demonstrate that mode-water eddies (MWE) in the SargassoSea upwell locally formed waters with relatively high Si* to the euphoticzone, and that cyclonic eddies in the Sargasso Sea introduce waters withrelatively low Si*, a signature that originated in the iron-limited SouthernOcean. We propose that this phenomenon can explain the observed dominance ofthe phytoplankton community by large-diatom species in MWE and by smallprokaryotic phytoplankton in cyclonic features. In contrast to the Atlantic,North Pacific Intermediate Water (NPIW) with high Si* may influence thecyclonic eddies in waters off Hawai'i, which also appear capable ofsustaining diatom populations. These observations suggest that the structureof phytoplankton communities sustained by eddies may be related to thechemical composition of the upwelled waters in addition to the physicalnature of the eddy.