DIATOM BASED RECONSTRUCTION OF HISTORICAL WATER QUALITY AND PRODUCTIVITY IN LAKE ECOSYSTEMS

Freshwater algae are commonly used to monitor and assess ecological conditions due to their sensitivity to changes in pH, water temperature and nutrient enrichment. Algae also play a significant role in the biogeochemical cycling of nutrients; therefore, shifts in biomass or species composition may indicate a change in environmental conditions. We hypothesize that fossil diatoms archived in lacustrine sediment will reflect natural and anthropogenic changes in biogeochemical phosphorus cycles. Sediments from Muskegon Lake (Michigan), an ecosystem influenced by human activity since the early 1800s, were used to investigate this hypothesis. Cores were collected and sectioned on shore at 1cm resolution, with diatom analyses performed at 4cm resolution. Sediment was also analyzed for vertical profiles of ²¹⁰Pb activity (sedimentation rates, age, focusing), pollen stratigraphy and corresponding geochemistry of a suite of elements including trace metals and nutrients. A comparison of diatom inferred conditions with empirical measurements of nutrient and chemical loadings were used to determine the rate and magnitude of anthropogenic disturbance. Select relationships observed include: 1) Top section (<16cm depth) shows diatom species indicative of high nutrient conditions, specifically, Achnanthidium minutissimum and Amphora pediculus, which correspond to increasing phosphorus concentrations near the surface; 2) The middle section (60 – 90cm depth) reflects an increase in Cocconeis placentula and C. placentula v. euglypta that predates the peak in anthropogenic loading of lead and mercury, after which, both species decline; 3) Bottom of core (>100cm depth) is abundant in Calvinula scutelloides, a benthic diatom species associated with a cooler climate (Little Ice Age). A comparison of the top and bottom sediment samples suggests a shift from benthic to pelagic primary production. This research provides information for historical perspectives of biogeochemical processes in aquatic ecosystems, leading to quantification of eutrophication/oligiotrophication trends. Insight from this study can also contribute to management strategies aimed at improving habitat quality.