WATERSHED CHARACTERISTICS, CLIMATE, AND IN-LAKE DYNAMICS IN RELATIONSHIP TO THE HISTORY OF GLOEOTRICHIA ECHINULATA

Many low-nutrient lakes in northeastern North America have recently experienced blooms of Gloeotrichia echinulata, a colonial, nitrogen-fixing cyanobacterium. It is a large, readily recognizable species that has the capacity to contribute both phosphorus and nitrogen to the water column and to produce toxins. G. echinulata has drawn considerable attention from both recreational and water management communities where questions have been raised about what controls its distribution and abundance and whether it is a new invasive species. Cyanobacteria as a group often respond positively to warmer climates and nutrient loading from the catchment, but hydrology and in-lake dynamics mediate the overall abundance of many cyanobacterial species. To investigate the history of G. echinulata and the potential role of these many factors in its historical abundance, we conducted paleoecological analyses of G. echinulata, algal pigments, and pollen from surface cores from seven lakes in the northeastern United States.

Paleorecords revealed different histories of watershed disturbance, lake eutrophication, and abundance of G. echinulata among lakes. In five lakes, G. echinulata was most abundant between 1750 and 1900, a time of both variable climate and agricultural activity in these watersheds. However, a sixth lake showed 20^th century expansion of G. echinulata, and the seventh had exceedingly low concentrations of G. echinulata in the last 400 years and none in the half century preceding that. Thus, while land clearance and agricultural activity might be implicated as important drivers of G. echinulata abundances in some of the lakes, more eutrophic conditions were associated with lower populations in at least two lakes, perhaps because of light limitation. Despite apparently coincident modern increases in G. echinulata in lakes regionally, paleoecological analyses reveal that this species inhabited these lakes prior to European settlement. Further, no single mechanism can explain the historical patterns of abundance of G. echinulata, underscoring the importance of catchment differences in mediating regional influences such as climate.