Sediment records of phosphorus-driven shifts to phytoplankton dominance in shallow Florida lakes

Primary producer community structure (PPCS) in shallow lakes is influenced by phosphorus (P) load and water column P concentration. Theoretically PPCS may shift between phytoplankton and macrophyte states with intermediate P loading, but phytoplankton dominate when P loading exceeds a critical threshold. We analyzed sediment cores from five shallow, eutrophic lakes (size range: 0.6 to 125 km(2)) that are phytoplankton dominated to determine whether the development of the current state was associated stratigraphically with an increase in sediment total P (TP) and a shift in PPCS. We used sponge biogenic silica (BSi(S)ponges) concentrations and total carbon to total nitrogen ratios (TC:TN) as proxies for macrophyte abundance and sediment organic matter source, respectively. Three stratigraphic groups of sediments were identified with k-means cluster analysis. These samples were grouped by increasing TP concentrations and decreasing age and identified as macrophyte, transitional and phytoplankton sediments. Results show that as P loading increased in the late 19th and early 20th centuries, the lakes produced sediments with an increasing contribution from phytoplankton. Four of our lakes may represent a subset of shallow lakes because of their large size (30 to 125 km(2)) and relatively rapid historic P enrichment. In these Florida lakes, PPCS shifted to phytoplankton dominance with no paleolimnological record of lake-wide alternating stable states or of lake-wide phytoplankton dominance before anthropogenic P enrichment.