Thursday, March 31, 2011

Decline in Diatoms in Great Lakes

Algal productivity in the Great Lakes, the fixation of atmospheric carbon by algae, is largely a result of the size of the springtime bloom of diatoms. Diatoms are algae that pull silica out of the water column to encase themselves in intricate glass coatings, Evans explains. "The amount of silica removed by the bloom has long been used as an indicator of algal production in the Great Lakes," she says.

Mining data on silica concentrations collected over the past 30 years, Evans and her colleagues determined that algal production was about 80% lower in 2008 than in the 1980s and 1990s. The decline began in the mid-1990s after zebra and quagga mussels invaded the lakes. The study estimates that mussels consume up to 74% of new spring algae. When the foundation of a food web is depleted, Evans says, populations of the fish at the top of the web can suffer.

Phytoplankton production is an important factor in determining both ecosystem stability and the provision of ecosystem goods and services. The expansive and economically important North American Great Lakes are subjected to multiple stressors and understanding their responses to those stresses is important for understanding system-wide ecological controls. Here we show gradual increases in spring silica concentration (an indicator of decreasing growth of the dominant diatoms) in all basins of Lakes Michigan and Huron (USA and Canadian waters) between 1983 and 2008. These changes indicate the lakes have undergone gradual oligotrophication coincident with and anticipated by nutrient management implementation. Slow declines in seasonal drawdown of silica (proxy for seasonal phytoplankton production) also occurred, until recent years, when lake-wide responses were punctuated by abrupt decreases, putting them in the range of oligotrophic Lake Superior. The timing of these dramatic production drops is coincident with expansion of populations of invasive dreissenid mussels, particularly quagga mussels, in each basin. The combined effect of nutrient mitigation and invasive species expansion demonstrates the challenges facing large-scale ecosystems and suggest the need for new management regimes for large ecosystems.

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