Linking phytoplankton community size composition with temperature, plankton food web structure and sea–air CO2 flux

Data collected at open water stations (depth>400 m) in all major ocean basins in 2006–2008 are used to examine the relationship between the size structure of the phytoplankton community (determined by size fractionated chlorophyll filtration), temperature and inorganic nutrient availability. A significant relationship (p<0.0005) was found between community size structure and temperature, with the importance of large cells in the community decreasing with increase in temperature. Although weaker than the temperature relationship, significant relationships were also noted between community cell size and DIN (nitrate, nitrite and ammonium: p=0.034) and phosphate (p=0.031) concentrations. When the data were divided into two groups of equal size, representing the samples with the highest and lowest DIN/phosphate concentrations, respectively, no difference could be identified between the slopes of the lines representing the relationship between size structure and temperature. There was, however, a difference in the intercepts between the two groups. If the relationship between size and temperature was only a response to nutrient availability, we would expect that the response would be the strongest in the groups with the lowest nutrient concentrations. These results suggest that, in addition to a nutrient effect, temperature may also directly influence the size structure of phytoplankton communities. The size structure of the phytoplankton community in this study correlated to the magnitude of primary production, export production (determined after Laws et al., 2000) and integrated water column chlorophyll. Significant relationships were also found between mesozooplankton production (determined using the proxy of calanoid+cyclopoid nauplii abundance as a percentage of the total number of these copepods) and both temperature and phytoplankton size, with production being the lowest in the warmest waters where phytoplankton were the smallest. In the North Atlantic, export production and community size structure appear to be related to ocean uptake of CO2 from the atmosphere. The reported results suggest that ocean warming may directly alter plankton community structure. This, in turn, may alter the structure of marine food webs and impact the performance of the open ocean as a natural carbon sink.