DOES GLOEOTRICHIA ECHINULATA MEDIATE EARLY LAKE EUTROPHICATION? IT'S COMPLICATED

Gloeotrichia echinulata, a colonial, nitrogen-fixing cyanobacterium, has recently been noticed in low-nutrient lakes in the northeastern United States, raising concerns about water quality. Since it germinates on the sediments and divides in the water column, it has the capacity to transport sediment phosphorus into the water column where it can be released through leakage or when colonies are grazed. Since it also fixes nitrogen, G. echinulata could be an autochthonous driver of eutrophication through both nitrogen fixation and phosphorus additions. Alternatively, the recent increase in G. echinulata could be largely driven by allochthonous nutrient loading due to changes in land use in watersheds. We used paleoecological analyses of algal pigments, dead parent colonies (akinete packages) of G. echinulata, and pollen from surface cores from three lakes in the northeastern United States—Long Pond (Maine), Pleasant Lake (Maine), and Lake Sunapee (New Hampshire)—to examine the evidence supporting each hypothesis.

Paleorecords revealed different histories of watershed disturbance, lake eutrophication, and the abundance of G. echinulata. The record from Long Pond supported the hypothesis that G. echinulata facilitates lake eutrophication. G. echinulata was present throughout the record, including prior to European land clearance, but it increased substantially beginning sometime between 1925 and 1970. This coincided with increases in the abundance of cryptophytes, chlorophytes, and cyanobacteria but appeared, in recent decades of water quality monitoring, decoupled from lake phosphorus concentrations. In Pleasant Lake and Lake Sunapee, G. echinulata was abundant in the middle-1800s, a time of agricultural activity in these watersheds, supporting a hypothesis of allochthonous drivers. However, the maximum G. echinulata abundance in Lake Sunapee preceded European land clearance, and low abundances of other algae provided no support for either hypothesis. Despite apparently coincident modern increases in G. echinulata in lakes regionally, paleoecological analyses do not reveal a single driving mechanism to explain the current abundance of G. echinulata in low-nutrient systems and instead underscore the importance of differences in catchments and their histories.