Plankton metabolic balance at the margins of very large lakes: temporal variability and evidence for dominance of autochthonous processes

Abstract

Summary1. Planktonic metabolic balance (PMBm) of the surface mixed layer (SML) was measured as the ratio of areal rates of gross photosynthesis (AGP) to community respiration (AR) to test the idea that previously neglected allochthonous inputs of organic matter may support chronic excess respiration relative to photosynthesis even in very large lakes during the summer (May–October) season. Four Laurentian Great Lakes coastal sites of varying trophic status, physical structure and dissolved organic carbon (DOC) concentration were studied with oxygen light‐and dark bottle and 14C methods, with excess respiration anticipated in the higher DOC sites.2. Planktonic metabolic balance was net autotrophic in 73% of the observations. The calculated mixing depth at which respiration would predominate over photosynthesis was greater than typically observed mixing depths, varying from 11 to 25 m in the more transparent, low DOC (<3 g m−3) sites to 8–15 m in the higher DOC (4–6 g m−3) sites. Biweekly measurements at one higher and one lower DOC site over two successive summer seasons showed that seasonal gross photosynthesis (ΣAGP) exceeded seasonal community respiration (ΣAR). Despite the location of the sites at the periphery of the lakes, where allochthonous influences should be strongest, the measurements indicated prevailing conditions of net autotrophy in the SML.3. Individual measurements of AR from this study and the literature were correlated with AGP but season average values were more tightly correlated, suggesting a tighter coupling of metabolic rates on a larger scale and a looser coupling on a shorter scale. The observed temporal variability was variable in pattern among years, and likely to confound inferences based on limited sampling.4. It is shown that accepted formulations for AGP and AR lead to the conclusion that PMBm should be largely predictable from knowledge of a biological properties ratio (light‐saturated gross photosynthesis to plankton community respiration, Pmax/R) and a physical properties ratio (euphotic to mixing depths, Zeu/Zm) and this prediction was confirmed using data from this study and from the literature. The evident success of this model points to the pre‐eminent importance of plankton biomass and physical conditions in determining metabolic balance. Variation in these fundamental factors appears capable of explaining the diversity of PMBm reported for different Great Lakes.