Nutrient loading patterns and macrophyte community structure in a lake, stream, wetland ecosystem

Temporal patterns of nutrient loading in a chain of lakes displaying different trophic status were investigated. Stream water passing through an extensive wetland area was found to have higher nutrient content, particularly in winter and summer, contributing to high productivity in the upstream lake. Lakes farther downstream display markedly different macrophyte communities and lower nutrient loading. The most dynamic nutrients, in terms of external loading and internal processing, were the inorganic nitrogen constituents. The principal source of nitrogen loading appeared to be atmospheric deposition. The macrophyte community structure in two lakes were compared in two years of different water flow and nutrient loading rates. During a two-year period of unusually low precipitation and reduced stream flow the submersed aquatic plant community of the more eutrophic lake (Shoe Lake) changed qualitatively and quantitatively to one more similar to that of a less productive lake downstream (East Graham Lake). Inorganic nitrogen loading was sharply reduced in this nitrogen limited system. In Shoe Lake the previously dominant non-rooted species declined along with phytoplankton biomass, and the abundance and diversity of submersed rooted plants increased. Also, the free CO{dollar}sb2{dollar} requiring macrophyte species present in Shoe Lake declined in favor of some previously restricted bicarbonate users. These changes appear to have been caused by a decrease in nutrient loading to the lake chain, and by the consequential greater light availability in Shoe Lake.