Relationships between phosphorus loading and trophic state in calcareous lakes of southeast Wisconsin

Abstract

The relationships among external phosphorus loading, lake phosphorus concentrations, and indices of lake trophic state are investigated for four calcareous and one noncalcareous lake in southeast Wisconsin. The total P and molybdate‐reactive P concentrations during winter and spring overturn are significantly higher in the calcareous lakes than predicted by models based on regional studies of Canadian Shield (ELA) lakes and the central New York Finger Lakes. The calcareous Wisconsin lakes have relatively low phosphorus retention coefficients and are eutrophic despite their low or only moderate external P loadings. By contrast, noncalcareous Devils Lake is oligotrophic; it is also adequately represented by the ELA loading vs. response equations. The calcareous Wisconsin lakes and edaphically similar Lake Minnetonka, Minnesota, have lower epilimnetic chlorophyll concentrations in summer than predicted by the Dillon and Rigler equation (based on lake mean total P at spring overturn). Because these stratified lakes are frequently phosphorus‐limited in midsummer, the failure of the Dillon‐Rigler model may be related to seasonal cycles of mixed‐layer concentrations of total P (and SRP) and also to the seasonal patterns of loading. From other studies I suggest that these observations may be part of a larger edaphic pattern related to low phosphorus binding tendencies of certain calcareous lake sediments.

Water Quality, Hydrology, and Response to Changes in Phosphorus Loading of Nagawicka Lake, a Calcareous Lake in Waukesha County, Wisconsin

“…The small bias (0.002 mg/L) was a result of equally weighting the results for scenarios 1 and 2, but using only scenario 1 as the base case. The apparent loss of about 50 percent of the phosphorus in the lake (corresponding to the overprediction of the models) is consistent with that expected with coprecipitation of phosphorus with calcite and the reduction of phosphorus release from the deep sediments, as suggested by Grobbelaar and House (1995), and possibly the external phosphorus load which may have abnormally high amounts of nonavailable phosphorus (Stauffer, 1985). The slight decrease in the correction factor from 2.5 for 2003-04 to 1.9 for the 1970s could have been caused by the coprecipitation of phosphorus being overwhelmed by the increased loading of phosphorus and dissolved organic matter in the 1970s that increased productivity in the lake and increased the release of phosphorus from the bottom sediments.…”

“…Therefore, phosphorus in the deep sediments of calcareous lakes may be effectively trapped. Stauffer (1985) reported, however, that various calcareous lakes have unusually high phosphorus concentrations during periods of cooler water temperatures, such as during winter and early spring.…”

“…Given the unique characteristics of calcareous lakes, such as coprecipitation of phosphorus with calcite and reduced internal loading of phosphorus from the deep sediment, the specific water-quality relations in these models may not be valid for calcareous lakes and may be expected to overpredict in-lake phosphorus concentrations. Past applications of this type of model for calcareous lakes, however, have demonstrated that the models significantly underpredict average annual phosphorus concentrations, but predict average summer phosphorus concentrations reasonable well (Stauffer, 1985).…”

“…At that point, eutrophication can proceed relatively rapidly (Wetzel, 1983). In fact, Stauffer (1985) reported that some calcareous lakes can be more eutrophic than noncalcareous lakes, if there is a very high external loading.…”

“…Calcareous lakes have high concentrations of dissolved calcium carbonate and commonly experience marling or deposition of calcium carbonate (also referred to as calcite) in the sediments of the lake. Calcareous lakes commonly have lower total phosphorus concentrations during summer than noncalcareous lakes with similar phosphorus loading because phosphorus loading from watersheds with these soils may have abnormally high amounts of phosphorus that are tightly bound to eroded sediment (as apatite) and not available for biological productivity (Stauffer, 1985). In addition, the naturally sustained inputs of carbonates and other cations can cause the loss or removal of phosphorus from the lake water by coprecipitation with (adsorption on) calcite (Murphy and others, 1983;Kleiner, 1988;Jager and Rohrs, 1990).…”