Diatom assemblages in a reservoir sediment core track land-use changes in the watershed

Diatom assemblages in a 144-cm sediment core reliably chronicled watershed changes and modifications of a riverine water-source reservoir in northeastern Ohio (USA) between 1932 and 2005. Non-metric Multidimensional Scaling segregated diatom assemblages into six groups that correspond to known anthropogenic modifications in the watershed and reservoir: zone I (filling, 1932–1936), zone II (high farming activity, 1937–1949), zone III (declining farm activity and increased residential development, 1950–1976), zone IV (implementation of sewage bypass, 1977–1982), reduced development activity caused by economic recession, 1983–1993), zone VI (renewed population growth and construction of a dam extension, 1994–2005). Ten watershed and reservoir environmental variables (percent farmland, population, pH, total alkalinity, total hardness, transparency, surface area, air temperature, export of suspended solids, total phosphorus) were significantly correlated with diatom aggregate distribution in ordination space. Changes in species composition, in concert with watershed and reservoir changes, implicated light, alkalinity, and littoral macrophyte development as the primary drivers of diatom aggregate structure and function. Diatom productivity did not track nutrient loading (TP) and was likely driven by factors other than nutrients. Each zone was defined by a distinct set of environmental variables that differed from all other zones. Thus, diatom aggregate structure likely was determined by a dynamic suite of factors in the watershed and reservoir that had differential effects on diatom aggregate structure through time.