Climate Variability and Change in a Eutrophic Great Lakes Freshwater Embayment: Shifting Hydrodynamics and the Potential for Indirect Impacts on Biogeochemical Processes, Carbon Cycling and Hypoxia
Abstract
Future changes in the climatic regime of the Great Lakes region have the potential to induce a variety of both direct (e.g. thermal) and indirect (e.g. biogeochemical) alterations in ecosystem function. In the case of the later, we have identified a statistically significant shift in wind direction of the average wind field over the Great Lakes basin that is consistent with a southward migration of the dominant summer storm track. In Green Bay (NW Lake Michigan), we have shown that the new wind field has most likely resulted in periods of decreased thermal stratification and an overall decrease in water mass exchange with Lake Michigan. In subsequent studies, aimed at determining the impact of these shifts in the physical climate regime, time series measurements of currents, turbidity, dissolved oxygen, and the Be-7 activity of particulates in bottom sediments, sediment traps, and suspended particulates have been made over a 3 year period. A tracer of short term particle dynamics, Be-7 (half life 53 d) is useful in estimating particle residence times in the water column, along with episodic sediment deposition and erosion rates, and the average number of deposition/erosion cycles a particle experiences prior to permanent burial in the sediments. Be-7 derived estimates of the age of particulate organic carbon cycling between surface sediments and the overlying waters are on the order of months, and are dependent upon resuspension frequency. Remineralization of organic carbon within this actively resuspended pool of material results in estimated decomposition rates for POC ranging 0.08 to 0.04% per day, a rate intermediate between the rapid remineralization of fresh algal material and post-depositional diagenesis. Comparisons between 1989-90 and 2004-06 show a decrease in resuspension frequency, possibly in response to shifts in regional climatic scale dynamics. This appears to result in an increase in the efficiency of trapping of organic matter in the bay and a significant increase in the organic carbon content of suspended particulates within the resuspension reactor. Measured sedimentation rates determined via Pb-210 and Cs-137 also indicate increased sediment accumulation over this interval. Combined with the wind shift induced reduction in water mass exchange, these climatic changes have the potential to increase hypoxia via enhancing benthic oxygen demand - a common feature of lower Green Bay that recent evidence suggests may be exacerbated, triggering severe oxygen depletion and fish kills.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMOS11D1161K
- Keywords:
-
- 0404 Anoxic and hypoxic environments (4802;
- 4834);
- 0428 Carbon cycling (4806);
- 0429 Climate dynamics (1620);
- 4805 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 1615;
- 4912)