Internal Phosphorus Loading and Macrophytes: an Alternative Hypothesis

ABSTRACT

Rooted, submersed macrophytes, primarily Elodea densa, have apparently served to restrict internal phosphorus loading during the summer in shallow Long Lake, Washington, instead of contributing phosphorus through senescence (process of aging). Internal loading was consistently controlled for five years by treatment by drawdown and alum, and mean, whole-lake macrophyte biomass ranged from about 30 to 250 g/m² during the 12 study years. Instead of the positive relationship between macrophyte biomass and lake phosphorus content during the summer that has been observed by others, the two variables had an inverse relationship. Gross internal loading (before and after alum control), estimated by a nonsteady-state model, was greater during years when low winds (< 10 km/h) were frequently followed by high winds (> 16 km/h). This indicates the importance of stability in creating an anoxic layer over the sediment, followed by wind mixing to entrain it into the water column. Thus, rather than serving as a direct source in summer through senescence, a dense cover of macrophytes in this lake may impede sediment release of phosphorus by creating a barrier to wind mixing. Late fall and winter senescence may, however, facilitate sediment phosphorus release by the iron-redox mechanism through organic enrichment of the sediment surface. The varied role of macrophytes since treatment suggests that partial alum control may have persisted for at least nine years.