Size structure and the production/respiration balance in a coastal plankton community

The quantitative significance of contributions by the picoplankton (<3 <mu>m) to gross primary production (P) and community respiration (R) was investigated seasonally in the plankton community of Chesapeake Bay. Rates of P and R for the total plankton community, integrated over the euphotic zone, ranged from 119-709 mu mol O(2) m(-2) d(-1) and 41-325 mu mol O(2) m(-2) d(-1), respectively. Rates of P and R within the picoplankton community tended to covary with those of the total plankton community, although the strengths of the two relationships were markedly different. The mean proportion of total community R accounted for by the picoplankton averaged 54%, with the two rates beings highly correlated. in contrast, the relative contribution of picoplankton to total P was highly variable, ranging from 1 to 77%, with fluctuations in picoplankton production rates explaining only 29% of the variability in total P. Although P and R exhibited a significant positive relationship over the entire data set, individual P:R ratios varied substantially, ranging from 0.95 to 4.73. Seasonal variations in P : R ratios for the picoplankton were out of phase with those of the total community. When the total plankton community was most autotrophic (P : R > 1), the picoplankton P : R was net heterotrophic (P : R < 1), and as total plankton P : R ratios decreased toward balanced metabolism (P : R = 1), picoplankton P : R ratios increased to become net autotrophic. Seasonal and spatial variations in the contributions of picoplankton P and R to total rates had a strong effect on the P : R ratio of the plankton community as a whole. There was a pronounced inverse relationship between the P: R ratio of the total plankton community and the proportion of P attributable to the picoplankton, such that high net autotrophy occurred only when P was dominated by the larger size fractions. These findings indicate an important linkage between the size distribution of the primary producers and the overall balance of P and R in the plankton community, which in turn regulates the potential for organic matter export.