Seasonal Production of the Myriophyllum Component of the Littoral of Lake Wingra, Wisconsin

Abstract

The littoral regions of most aquatic ecosystems represent some of the most productive communities of the world (Wetzel 1964) and, as such, warrant intensive investigation. Lake Wingra (Madison, Wisconsin, U.S.A.) contains an extensive littoral zone. Lake Wingra was selected as an intensive study site within the Eastern Deciduous Forest Biome, US-IBP, and is the focus of an interdisciplinary ecosystems analysis study involving more than twenty-two subprojects. The lake is approximately 139-6 ha, with a littoral zone encompassing 31% (43 ha) of the total area. The lake is highly alkaline (80-200 mg CaCO3 equivalent per litre), with high (7-10-3) pH values prevailing within the littoral during midsummer. Most of the phosphorus enters the lake via a storm sewer system from an urban watershed, and dissolved phosphorus concentrations in the littoral range from 5 to 33 jug 1-1 during summer (Kluesener 1972). Phytoplankton production in the pelagial is very high with the annual mean rate of carbon fixation near 2 g C m-2 day (Koonce 1972). The littoral of Lake Wingra is dominated by the submergent macrophyte Myriophyllum spicatum L. (Eurasian water milfoil). The macrophyte vegetation of the lake affords an extensive substratum for colonization by periphytic epiphyton. Apparent net primary production of the community is high and, in addition, a considerable level of extracellular secretion ('excretion') of photosynthate occurs. This excretant, generally about 5% of total net photosynthesis of macrophytes in Lake Wingra, is available for channelling through other trophic levels within the ecosystem. Another important aspect of the macrophyte component of the littoral of Lake Wingra is the dense structure which the macrophytes provide as a habitat for bluegill sunfish (Lepomis macrochiris Raf.). The habitat has the appearance of a dense forest and may serve as a factor in the high numbers and small individual size of fish in the bluegill population of the lake. The macrophytes contribute considerably to the annual detritus pool. Since submergent macrophytes remove nutrients from both the water and sediments (Bristow & Whitcombe 1971), the plants are expected to have a significant role in the nitrogen and phosphorus cycling in the lake.