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

Abstract

The quantitative significance of contributions by the picoplankton (,3 mm) 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 mmol O2 m−2 d−1 and 41‐325 mmol O2 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 being 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.