Enrichment of Lake Wingra, Wisconsin, by Submersed Macrophyte Decay

This study analyzes enrichment of hard—water eutrophic Lake Wingra by decomposition of submersed aquatic vegetation. A model was constructed to predict daily release of dissolved total phosphorus (DTP) and dissolved organic matter (DOM) from decaying macrophyte shoots. Inputs to the model are: daily mortality, the nitrogen and phosphorus contents of senescing tissues, and water temperature. The mechanism are modeled: leaching of phosphorus, decay of coarse particulate organic matter, and respiration of DOM. In field tests, the model predicted decay rates and the phosphorus content of decaying shoots fairly accurately. Decomposition of Myriophyllum spicatum accounts for about half of the observed flux of DTP and DOM from the littoral zone to the pelagial zone of Lake Wingra. During summer and autumn decaying macrophytes are a major source of biologically available DTP and decomposable DOM for the pelagic community of the lake. Therefore, macrophyte decay potentially stimulates pelagic production. Substantial differences in the timing of DTP and DOM release occur among comparably productive stands of different species decomposing in the same lake. Release of dissolved materials by Potamogeton pectinatus stands occurs during summer, and is nearly completed by the time release by Vallisneria americana stands begins to increase. Most of the release from V. americana stands occurs after mid—August. Release of dissolved materials by a typical stand of M. spicatum is more or less continuous during summer and autumn. When Lake Wingra was dominated by V. americana (∞1900) the rate of enrichment by macrophyte decay during summer was probably less than the present rate. During a period of domination by Potamogeton species (∞1955—1965) enrichment by macrophyte decay during summer may have exceeded the rate of internal loading that occurred after the invasion of M. spicatum.