Evaluating diatom succession: some pecularities of the Great Lakes case

Succession of diatom communities in the Laurentian Great Lakes has several unusual aspects related to physical and chemical characteristics peculiar to these large systems and the constraints these conditions impose upon species which inhabit the Great Lakes. In such systems, paleolimnological reconstructions based on strict analogy to succession patterns in smaller lakes may be incomplete and, in some cases, positively misleading. In the Great Lakes, diatom accumulation rates appear to be regulated by interaction between concentration and supply rates of phosphorus and dissolved silicate. As phosphorus loadings increased historically, storage of diatom frustules in sediments was first increased, then limited, as concentrations of dissolved silica were reduced to levels limiting to diatom growth. Qualitative aspects of the diatom flora are also affected. Indigenous species adapted to growth in winter persist, while those with abundance maxima in summer are extirpated, presumably due to silica limitation in the summer epilimnion. Severe silica limitation also results in shifts to dominance by species whose growth strategies are particularly well adapted to sequestering dissolved silica. Because it is necessary to know the historical context in order to correctly interpret current changes in the Great Lakes diatom flora, paleolimnological studies have proven particularly valuable.