MODIFICATION OF THE BIOGEOCHEMICAL CYCLE OF SILICA WITH EUTROPHICATION

Nutrient enrichment dnd consequent alteration of nutrient biogeochemic al cycles is a serious problem in both freshwater and marine systems. The response of aquatic systems to additions of N and P is generally t o increase algal biomass. The partitioning of these nutrients into dif ferent functional groups of autotrophic organisms is dependent upon bo th intrinsic and extrinsic factors. A common response to nutrient load ing in northern temperate aquatic ecosystems is an increase in diatom biomass. Because nutrient enrichment generally leads to increases in w ater column concentrations of total N and total P (and not Si) such nu trient loading can lead to transient nutrient limitation of diatom bio mass due to lack of dissolved silicate (DSi). Increased production of diatom biomass can lead to an increased accumulation of biogenic silic a in sediments, ultimately resulting in a decline in the water column reservoir of DSi. Such biogeochemical changes in the silica cycle indu ced by eutrophication were first reported for the North American Laure ntian Great Lakes. However, these changes are not a regional problem c onfined to the Great Lakes, but occur in many freshwater and marine sy stems throughout the world. Here we summarize the effects of anthropog enic modification of silica biogeochemical cycles for the North Americ an Laurentian Great Lakes, describe some of the biogeochemical changes occurring in other systems, and discuss some of the ecological implic ations of a reduction in water column DSi concentrations, including ch anges in species composition, as DSi concentrations become limiting to diatom growth and biomass, changes in food web dynamics, and altered nutrient-recycling processes.