Influence of phosphorus scavenging by iron in contrasting dimictic lakes

Internal regulatory controls of phosphorus (P) via iron (Fe) scavenging were quantified in four contrasting dimictic Wisconsin lakes: Mendota (eutrophic, calcareous), Fish (mesotrophic, calcareous), Devil’s (mesotrophic, noncalcareous), and Sparkling (oligotrophic, noncalcareous). Hypolimnetic enrichment of P was highest in Mendota and Devil’s and least in Fish and Sparkling. This enrichment was attributed mainly to internal loading in the noncalcareous lakes and regeneration of sedimenting epilimnetic P in the calcareous lakes. Differences in Fe scavenging efficiencies at fall turnover were related to hypolimnetic Fe:P molar ratios as well as Fe availability and its control by sulfate–sulfide chemistries. In the noncalcareous lakes with high hypolimnetic Fe enrichment (Fe:P > 2), 45% of whole-lake total P was removed. P removal was low (<20%) in the two calcareous lakes with minimal Fe enrichment (Fe:P < 2). These differences in hypolimnetic P enrichment and subsequent Fe scavenging at fall turnover help to explain the differences in the amount of P available for subsequent spring and summer primary production as well as the differences in trophic state of the four lakes.