Controls on the epilimnetic phosphorus concentration in small temperate lakes.

Phosphorus (P) is one of the key limiting nutrients for algal growth in most fresh surface waters. Understanding the determinants of P accumulation in the water column of lakes of interest, and the prediction of its concentration is important to water quality managers and other stakeholders. We hypothesized that lake physicochemical, climate, and watershed land-use attributes control lake P concentration. We collected relevant data from 126 lakes in Maine, USA, to determine the major drivers for summer total epilimnetic P concentrations. Predictive regression-based models featured lake external and internal drivers. The most important land-use driver was the extent of agriculture in the watershed. Lake average depth was the most important physical driver, with shallow lakes being most susceptible to high P concentrations; shallow lakes often stratify weakly and are most subject to internal mixing. The sediment NaOH-extracted aluminum (Al) to bicarbonate/dithionite-extracted P molar ratio was the most important sediment chemical driver; lakes with a high hypolimnetic P release have low ratios. The dissolved organic carbon (DOC) concentration was an important water column chemical driver; lakes having a high DOC concentration generally had higher epilimnetic P concentrations. Precipitation and temperature, two important climate/weather variables, were not significant drivers of epilimnetic P in the predictive models. Because lake depth and sediment quality are fixed in the short-term, the modeling framework serves as a quantitative lake management tool for stakeholders to assess the vulnerability of individual lakes to watershed development, particularly agriculture. The model also enables decisions for sustainable development in the watershed and lake remediation if sediment quality is conducive to internal P release. The findings of this study may be applied to bloom metrics more directly to support lake and watershed management actions.

Predicting anoxia in low-nutrient temperate lakes.

Absence of dissolved oxygen (anoxia) in the hypolimnion of lakes can eliminate habitat for sensitive species and may induce the release of sediment-bound phosphorus. Lake anoxia generally results from decomposition of organic matter, which is exacerbated by high nutrient loads. Total phosphorus (TP) in lakes is regulated by static aspects of the lake's watershed, but lake TP can be readily increased by human activities. In some low-nutrient lakes, basin morphometry may induce naturally occurring anoxia. The occurrence of natural anoxia is especially important to consider in lake water quality assessments that compare observed conditions to expected reference conditions. To investigate the occurrence of natural vs. anthropogenically influenced anoxia, we constructed a logistic regression model to calculate the probability of low-nutrient lakes (TP < 15 µg/L) developing aerial anoxic extent ≥10% by testing the predictive potential of variables related to basin morphometry, depths of lake thermal strata, epilimnetic TP, and dissolved organic carbon (DOC). Maximum lake depth and the proportion of lake area under the top of the metalimnion were the most important variables to predict the likelihood of hypolimnetic anoxia, which correctly predicted anoxic condition in 84% of lakes (Model 1). Adding TP as a third variable to Model 1 produced a significantly improved model (Model 2) but the prediction success rate was comparable (86%). We also present a model for lakes with limited bathymetric data, which predicts anoxia with 81% accuracy based on maximum lake depth and mean thermocline depth at peak stratification. DOC was relatively low (4.3 ± 1.5 mg/L [mean ± SD]) in the study lakes and its inclusion did not improve model performance. In Model 1, lakes with an anoxic extent ≥10% of lake area had significantly higher epilimnetic TP than lakes with oxic hypolimnia, regardless of prediction category or success. Our results indicate that including TP as a variable helps refine models based on morphometry alone, but lake morphometry and stratification dynamics are the most important factors in the development of anoxic extent in low-nutrient temperate lakes. Our approach informs studies concerned with identifying key factors that influence regime shifts in a variety of ecosystems.