RAPID BIOGEOCHEMICAL SILICA DEPLETION IN THE GREAT LAKES: AN ECOSYSTEM RESPONSE TO LOW-LEVEL, ANTHROPOGENIC PHOSPHORUS ENRICHMENT

Biogeochemical silica depletion is a biological consequence of anthropogenic phosphorus enrichment (cultural eutrophication) manifested at the ecosystem level. The mechanism is enhanced diatom growth that is stimulated by phosphorus enrichment and mediated by microscopic planktonic organisms that require silica (Si) for frustule production as biogenic silica (BSi). Phosphorus enrichment stimulates diatom growth, production and sedimentation that are initially limited by phosphorus supplies. This process utilizes Si for diatom production and the resulting increased BSi sedimentation decreases Si supplies in the water column eventually leading to epilimnetic Si depletion and Si-limited diatom growth and production. This process changed the ecosystem so diatom community production switched from phosphorus limitation to Si limitation. This consequence of eutrophication occurred at a total phosphorus (TP) concentration (<10 µg/L, <0.3 µM) in Lake Michigan in the 1960s, or at TP levels not ordinarily associated with cultural eutrophication. It also occurred at low levels in Lake Erie and Lake Ontario in the early to mid 1800s, much earlier than any other known consequence of cultural eutrophication, as the result of phosphorus loading from early European settlement and forest clearance. Additional anthropogenic phosphorus loading from domestic sources (sewering and phosphate detergents) beginning in the 1940s eventually produced water column Si depletion, a serious consequence of eutrophication in Lake Erie and Lake Ontario. Anthropogenic P enrichment coupled with differential rates of recycling for Si and phosphorus lead to rapid biological silica depletion in one or two decades. Relatively low levels of phosphorus enrichment (4-5 µg/L, <0.2 µM) stimulated diatom production in Lake Superior and Lake Huron, but phosphorus loading in neither system exceeded the threshold necessary to produce Si limitation. Several lines of evidence have shown that biogeochemical silica depletion is a very sensitive ecosystem response to increased phosphorus loading in the Great Lakes. Because no other data are available for all the lakes, paleolimnological reconstructions will be used to demonstrate the historical relations between anthropogenic phosphorus loading and imbalances in Si:P stoichiometry in these large lakes.