Latitudinal variation in plankton size spectra in the Atlantic Ocean

The size distribution of the pelagic community has the potential to compare ecosystems with different species composition as well as to identify the main functional properties of the system. Plankton size spectra are an effective approach of summarising the size structure of the community and have the potential to indicate the transfer of energy up the trophic food web. However, data on which open ocean plankton biomass-size structure can be constructed are scarce. Here we present the latitudinal distribution in the Atlantic Ocean of normalised biomass-size spectra (NB-S), plankton biomass and abundance, as well as mean zooplankton size between 70°N and 50°S. Samples were collected from three Atlantic Meridional Transect (AMT) cruises during May/June 2003, September/October 2003 and April/June 2004, as well as from a Marine Productivity (MarProd) cruise farther north in the Irminger Sea during spring 2002. The biomass-size distribution covered a body size range from nano- to mesoplankton and was based on a depth range of 50–0 m on the AMT and 120–0 m on the Irminger Sea cruise. The distribution of normalised biomass versus size was linear on a double log plot at all of the 95 stations. The slopes of the NB-S spectra ranged from −0.93 to −1.26 and −1.12 to −1.46 on the AMT and Irminger Sea cruises, respectively. A “dome-shaped” pattern in the slopes of community size spectra was observed in the Atlantic, indicating a decrease in the trophic transfer efficiency of energy with increasing latitude and phytoplankton biomass. Mesozooplankton biomass, abundance, and mean size followed a distribution similar to ecosystem productivity.