Nutrients, seston, and transparency of Missouri reservoirs and oxbow lakes: An analysis of regional limnology

A long-term (1978-2007) summer monitoring study of 167 Missouri reservoirs and floodplain lakes shows wide ranges in the trophic state variables total phosphorus (TP; 6-395 mu g/L for reservoir means), total nitrogen (TN; 200-3290 mu g/L), chlorophyll (Chl; 1-223 mu g/L) and Secchi depth (0.15-4.3 m). There are strong regional contrasts between eutrophic-hypereutrophic water bodies in the predominantly agricultural Osage Plains, Glaciated Plains and Big Rivers sections and the mostly oligotrophic-mesotrophic reservoirs in the largely forested Ozark Highlands. The ecotonal Ozark Border was intermediate. Missouri reservoirs had slightly less TN relative to TP than predicted by global models, but TN: TP (median = 18.4) is typical of North American lakes and was about five-fold lower in the most enriched reservoirs compared to the least enriched ones. Concentrations of seston, measured as total suspended solids (TSS), volatile suspended solids (VSS), nonvolatile suspended solids (NVSS), filterable (small) suspended solids (fTSS) and the sum of all fractions (Sigma TSS), were strongly correlated with TP, TN, and Secchi depth. The proportion of mineral seston (NVSS, fTSS) increased with TSS and was the dominant fraction in most reservoirs, especially in the agricultural regions. Phytoplankton was dominated by small forms (<11 mu m), and Cyanophyta were the most common algae comprising a median 46% of phytoplankton biovolume. Chlorophyll-nutrient regressions showed a dominant effect of TP (r(2) = 0.83 for reservoir means) with a lesser, negative effect of mineral seston (NAS or fTSS, partial r(2) similar to 0.05). Secchi depth was more strongly controlled by mineral seston than by phytoplankton such that Secchi was a better predictor of nutrients (especially TP) than algal biomass. Frequency of algal blooms (Chl >10 mu g/L) and Secchi <1 m increased sigmoidally with TP and TN with midrange nutrient concentrations (TP 20-50 mu g/L, TN 400-700 mu g/L) showing the greatest response to change. Trophic state criteria appropriate for Missouri reservoirs are similar to other north temperate lakes except for Secchi depth for which cutpoints are much lower because of nonalgal turbidity.