Chlorophyll Response to Nutrients and Non-algal Seston in Missouri Reservoirs and Oxbow Lakes

Abstract

When unaggregated summer chlorophyll data (Chl) from 184 Missouri reservoirs are plotted against total phosphorus (TP) a ‘bow’ in the distribution develops among a group of points with low Chl:TP ratios (averaging <0.05). Low Chl:TP is mostly associated with turbid, nutrient-rich inflows in reservoirs across the entire trophic range. Non-algal seston (NAS) is our best metric of these inflows and is a co-variable in Chl-TP regressions. This influence is most prominent in years of high runoff and is distinctly seasonal, being most prevalent in early summer prior to full stratification. In late summer, inflows typically enter as subsurface density currents, and nitrogen accounts for more variation in Chl-TP than NAS. Neither variable, however, greatly influences the long-term relation between Chl and TP, which is linear, relatively consistent, and matches the global pattern. In several oxbow lakes, high NAS caused by sediment resuspension is a chronic condition; it seems neither light nor flushing greatly influence Chl: TP in these shallow systems, and values approximate the statewide average. Temporal variation in Chl:TP is demonstrated by daily samples (n=1676) from a single reservoir, with average variability, that covers about 94% of the statewide Chl and about a third of the TP range. The Chl-TP pattern in over half of Missouri reservoirs deviates somewhat from predictions based on cross-system regression models. Nonetheless, for 97% of the study reservoirs, long-term Chl is within a factor of two above or below model predictions, and most are within ±25%. Such differences are modest when compared to the temporal variation measured in an intensively studied reservoir.