Asynchronous changes in trophic status of a lake and its watershed inferred from sedimentary diatoms of different habitats

Paleolimnology is one important approach for setting realistic goals for restoration and management decisions for lakes. Two important assumptions for reconstructing total phosphorus in lakes using diatoms are: (1) phosphorus is the key limiting factor for diatom growth; (2) other factors (chemical, biologic, or physical activity) had not significantly affected the natural way the remains of diatoms were buried. Both other lines of evidence, which is evaluated by a form of causal criteria analysis-Eco Evidence, and evidence from Muskegon Lake itself, were analyzed to evaluate whether total phosphorus (TP) is the causal factor for diatom species composition change in Muskegon Lake. Both evidence supported the cause-effect linkage between TP and diatom species composition change. Interestingly, our data analysis on a long sediment core from Muskegon Lake showed benthic and planktonic diatoms responded to changes in phosphorus conditions at two spatial scales. When inferring TP based on species composition of all diatoms, relationships to geochemical proxies and land use were poor. However, when inferring TP using benthic and planktonic diatoms, benthic diatom inferred TP was related most to in-lake conditions and planktonic diatom inferred TP was related most to changes in the Muskegon River, upstream from the lake. Only benthic diatom inferred TP correlated well with geochemical proxies in the lake. With a 23 day residence time of water in Muskegon Lake, species composition of planktonic diatoms in the core was likely more regulated by exogenous (allochthonous) environmental factors and processes, Muskegon River, than benthic diatoms. Therefore, freshwater landscape and other landscape variables, such as water residence time, are as important as biological feature for understanding long term trophic status change for some lakes. Caution should be used when diatom assemblages are more influenced by other factors than in-lake condition.