Submersed Vegetation: An Internal Factor in Lake Ecosystem Succession

Abstract

The accretion rate of sediments in a lake ecosystem is an index of its rate of aging. In shallow eutrophic Lake Wingra, sedimentation is coupled to a positive feedback system involving littoral vegetation, the phytoplankton, and sediments near enough to the water surface for colonization by macrophytes. Rooted plants enrich the water with nutrients and organic matter above the normal resuspension of sediments, thereby stimulating pelagic production and sedimentation that eventually produce more surface area colonizable by macrophytes. Analysis of a budget for colonizable surface area shows that relatively large changes in the rate of colonizable sediment accrual result from small changes in internal loading from sediments and littoral plants, the efficiency of decomposition, and the rate of sedimentation. On the other hand, the rate of accretion of colonizable sediment is relatively insensitive to changes in the influx of phosphorus and dry matter to the lake from the watershed. Internal mechanisms of enrichment such as those described here accelerate the eutrophication of lakes. This tendency toward autogenic eutrophication is not consistent with the hypotheses of lake development proposed by Odum (1969) and Margalef (1975). Evidently the direction and rate of lake succession depend upon basin morphometry and the abundance of littoral vegetation as well as inputs from the watershed.