Seasonality of phytoplankton cell size and the relation between photosynthesis and respiration in the Ría de Vigo (NW Spain)

The Ría de Vigo is a dynamic and productive upwelling ecosystem. We measured 14C incorporation (TO14CP) and gross primary production (GPP), community respiration (DCR), net production (NCP) and size-fractioned chlorophyll a (chl a) fortnightly from May 2012 to May 2013 in the euphotic layer of the Ría. Our aim was to improve the depiction of plankton metabolism in the Ría and to test the general hypothesis that community structure determines the degree of heterotrophy in planktonic ecosystems. Higher primary production was measured after upwelling episodes and during the spring bloom, when the community was dominated by microphytoplankton (>70% chl a>20μm). Lower primary production was observed during summer stratification periods (~65% chl a>20μm), and during the pico- and nanophytoplankton-dominated winter (~25% chl a>20μm). Coupling between phytoplankton photosynthesis and biomass varied seasonally, mainly driven by environmental conditions. DCR was 3 times lower and 8 times less variable than GPP, and its variability was mainly driven by the changes in chl a. The integrated metabolic balance was autotrophic most of the year, despite the negative NCP rates at depth. There was an inverse relationship between the DCR:GPP ratio and the percentage of microphytoplankton (% chl a>20μm) only in the summer. However, DCR:GPP and DCR:chl a ratios were similar in winter and spring, despite the seasonal differences in primary production and size structure. The similar TO14CP:NCP and chl a:DCR relations in spring (>70% chl a>20μm) and winter (~25% chl a>20μm), and the differences in summer (>70% chl a>20μm) confirm cell size independence in trophic functioning. We conclude that respiration variability is relevant for the metabolic balance in the Ría, and that the degree of heterotrophy is not systematically related to phytoplankton size over the scales of our study.