RII-Track 4: Paleoecological Insights into the Impacts of
Climate Change on Vermont Lakes: Laurie Grigg, Norwich University
University of Wyoming graduate student and collaborator,
Ioana Stefenescu and PI, Laurie Grigg at AGU poster presentation.
The primary goal of this project was to investigate the relationship between algal productivity and climate in a small Vermont lake during the past 11,000 years. The growing occurrence of algal blooms in Vermont lakes threatens the water quality of our freshwater resources and is linked to the increased inflow of nutrients. Recent and projected changes in climate, including increased precipitation are likely to exacerbate this process but there is less known about how changes in temperature will impact algal productivity. By examining fossil and geochemical evidence preserved in a sediment core from Twin Ponds which is located in Brookfield, VT, this project was able to reconstruct centennial- scale changes in algal productivity and the lake environment (water temperature, oxygen-availability, and nutrient influx).
Most of the data collection for this project was done by PI, Dr. Laurie Grigg and research assistant Irene Magdon, who is a recent graduate of the geology program at Norwich University, during a 6-month fellowship at the University of Wyoming. Grigg returned to Vermont and teaching at Norwich University in January, 2019. In the spring of 2019, Grigg and two Norwich undergraduate students began work on the final phase of data collection for this project. Samples were prepared and submitted to Northern Arizona University for biogenic silica analysis and split cores were taken down to the University of Massachusetts for scanning XRF analysis. Grigg and students also returned to Twin Ponds to core the fringing wetland in search of evidence for changes in the connectivity of the two basins. The remainder of 2019 has been spent compiling and analyzing data, which culminated in a poster presentation at the American Geophysical Union in San Francisco in December, 2019.
Changes in algal (phytoplankton) productivity through time were reconstructed by measuring the abundance of the fossil remains of Daphnia (water flea) and diatoms. Daphnia is a zooplankton that consume algae and produce resting eggs or ephippia, while diatoms are a large and diverse type of algae or phytoplankton and produce skeletal remains made of biogenic silica. Changes in the abundance of the Daphnia ephippia and biogenic silica through time show that before 8000 years ago, lake productivity was generally low. After 8000 years ago both fossil types increase and likely reflect a well-established regional shift towards warmer and wetter conditions driven by the final large collapse of the Laurentide Ice Sheet. Increases in both fossil types also occurred around 4500 and 3500 years ago and suggest periods of increased lake productivity separated by a period of reduced productivity around 4200 years ago. Geochemical analysis of the sediment core shows that these periods of higher productivity occurred during times when summer temperatures were warmer and the duration of ice-cover was shorter, as indicated by increased amounts of calcium and manganese.
During the remainder of the record (0-3100 and 5600-7000 years ago), peaks in the abundance of Daphnia and diatom fossils are out of phase with one another and point towards a productivity response mediated by predator-prey relationships. Periods of high Daphnia ephippia and low biogenic silica imply that Daphnia grazing effectively reduced diatom abundance, whereas periods with low Daphnia ephippia and high biogenic silica show that with less grazing pressure from Daphnia, diatoms were more abundant. These results indicate that over centennial timescales, changes in Daphnia are an important control on phytoplankton productivity. The question that remained was what was causing long-term changes in Daphnia abundance? The increased presence of fish fossils (scale and bones) during periods of increased diatoms suggests that top-down predation of Daphnia by fish was the likely biological control on phytoplankton productivity. However, the geochemical results suggest that changes in the availability of oxygen at the bottom of the lake may have ultimately controlled the presence or absence of fish. Times of increased Daphnia and decreased diatoms are correlated with lower amounts of manganese and calcium, suggesting less oxygen-availability at the bottom of the lake (anoxic conditions) which would have been less favorable to fish survival. More anoxic conditions during times of reduced phytoplankton productivity implies the longer duration of ice cover. The geochemical data also shows that increased diatom productivity is correlated with peaks in erosive variables (potassium, %inorganics, and magnetic susceptibility) which suggests that the influx of nutrients is also an important control on phytoplankton productivity.
Further work is planned for the future to link changes in lake productivity, temperature, nutrient influx, and oxygen-availability with independent reconstructions of temperature and precipitation. These preliminary results suggest that increased algal productivity is associated with shorter winters, warmer summers, and increased precipitation. More work will also be done to provide a more quantitative reconstruction of past fish abundance, however, this study is an important example of how food chain interactions driven by changes in the lake environment can impact algal growth. The results of this project provide an important long-term perspective for understanding the relationship between climate, the lake environment, biological interactions, and water quality in smaller lakes and ponds.
Award Abstract #1738748