Gloeotrichia echinulata is a colonial, nitrogen-fixing cyanobacterium common in eutrophic lakes. G. echinulata  has recently begun appearing in oligotrophic lakes in the northeastern United States, raising concerns about water quality. Since it has a meroplanktonic life cycle, germinating on the sediments and then dividing in the water column, it has the capacity to transport sediment phosphorus into the water column and hence could facilitate eutrophication through both nitrogen and phosphorus additions. We used paleoecological analyses of algal pigments, dead parent colonies (akinete packages) of G. echinulata, pollen, and sediment phosphorus from a surface core from Long Pond, Maine, USA, to test this hypothesis. 

Results/Conclusions

The paleorecord from Long Pond supports the hypothesis that G. echinulata facilitates lake eutrophication. G. echinulata is present throughout the record, beginning prior to land clearance by European settlers. Beginning between 1925 and 1970, the abundance of G. echinulata dead parent colonies in the sediment increases—an event that coincides with increases in the abundance of cryptophytes, chlorophytes, and cyanobacteria. This increase in taxa indicative of eutrophication occurs while indicators of preservation remain relatively constant, suggesting that the change in the algal community is not an artifact of preservation. Although sediment phosphorus retention appears efficient in oligotrophic lakes, surface sediment layers contain labile phosphorus that could either be inactivated into metal oxides in the sediment profile or taken up by G. echinulata and transported to the water column, supporting further primary production. The most dramatic shift in the pigment-inferred algal assemblage occurs in the last 40 years, and records of in-lake phosphorus concentrations over 30 of those years show no significant increase, further supporting the idea that G. echinulata may be leading rather than following a shift in nutrient status.