by Bopi Biddanda and Anthony Weinke, Annis Water Resources Institute, GVSU
The system of life on this planet is so astoundingly complex that it was a long time before man even realized that it was a system at all and that it wasn't something that was just there.
- Douglas Adam and Mark Carwardine (Last Chance to See, 1990).
Over time and space, the linkages between land and water are more variable than we ever imagined – changes compounded by anthropogenic activities concentrated in the over-crowded land margin ecosystems of the world. The carbon cycle of coastal ecosystems is a dynamic component of the global carbon cycle. As human population and standard of living continue to increase, agricultural runoff from the world’s continents is fueling enhanced phytoplankton production – and likely increased respiration as well - in coastal waters everywhere. Recent observations suggesting that the coastal ocean carbon cycle may have indeed shifted from its traditional role as sources of atmospheric carbon dioxide in pre-industrial period to active sinks for carbon dioxide in more modern times, further emphasize the need for such cross-ecosystem and long-term process measurements.
Freshwater systems cover ~1% of our planet, and provide a commensurate contribution to global primary production but disproportionally contribute to global carbon cycling through their high rates of respiration. Freshwater systems are also characterized by strong linkages to the surrounding land from which they receive significant inputs of terrestrial carbon such as soil carbon and plant material. Freshwater lakes are particularly active sites for the cycling of these organic carbon inputs. Globally, while organic carbon burial in inland waters is comparable to sequestration on the entire ocean floor, carbon emissions are also considerable. Of the estimated 2 Pg (1 Pg=1015 g) terrestrial organic carbon freshwater systems process per year, nearly half is respired by bacteria in inland freshwater and coastal water bodies. In conjunction with soil inorganic carbon out-gassing, this leads to net carbon emissions of the same order of magnitude as the net uptake by all of the the oceans.
In the face of emerging evidence of the disproportionately large role that freshwater ecosystems play in the global carbon cycle, there is increasing concern about the serious lack of production and respiration measurements in Earth’s inland waters. Furthermore, recent models suggest that the world’s coastal zones may be switching or have switched from their pre-industrial role as CO2 sources into CO2 sinks due to the combined influence of anthropogenically-enhanced nutrient runoff and ongoing climate change. Could a similar phenomenon be taking place in the coastal zones of the Laurentian Great Lakes? The Great Lakes contain ~ 20% of Earth’s surface freshwater and drain more than 200,000 square miles of land, resulting in significant terrestrial loadings. The 10,000 miles of Great Lakes’ shoreline are intersected by almost 3,000 tributaries. Many complex processes occur from the headwaters of these tributaries all the way to their river mouths. For example, land-use of upstream watersheds, has marked influence on downstream biogeochemical processing. The Muskegon River, which empties into Muskegon Lake followed by Lake Michigan, is the second longest river in Michigan and drains an area of ~ 6,000 km2.
In this paper, we summarize the results of an 11-year study of plankton metabolism from Muskegon Lake, a Michigan estuary, to Lake Michigan, one of the Laurentian Great Lakes. Our results show that the pelagic waters of the drowned river-mouth of Muskegon Lake are highly productive, and that the levels of planktonic production and respiration both decrease systematically as one goes further out into low-productivity Lake Michigan (Figure 1). However, production decreases more so than respiration along this land-to-lake transect leaving the nearshore waters net autotrophic (CO2 sinks) and the offshore waters net heterotrophic (CO2 sources). Thus, our findings describe the systematically variable planktonic carbon metabolism in a rapidly changing Great Lakes coastal ecosystem and lay the baseline foundation for monitoring changes in the carbon balance in future years.
Figure 1. Land to lake linkages and gradients: Generalized conceptual diagram of systematic variability in planktonic gross production, respiration, and net production along the land-to-lake gradient in aquatic ecosystems (based on the present study) superimposed on an aerial image of the Grand River output into coastal Lake Michigan (looking South). Both the axes scales are relative, and the horizontal line serves as the “zero carbon balance” reference line. From Land-to-Lake, carbon metabolism decreases systematically, as the systems switch from a potential carbon sink inland, to a carbon source offshore Rivers and Estuaries express integrated signals from an entire watershed, contributing relatively concentrated plumes of sediment, carbon and nutrients to coastal Great Lakes and the oceans, but those signals are are diluted by time they get to offshore waters. Knowing how major ecosystem processes change across the coastal ecosystem gradient will enhance our current understanding and capability to predict future changes in such critical land-margin ecosystems. Aerial photo credit: Marge Beaver, Photography Plus.
Such findings are useful to students, researchers and policy makers. Students can use this information in classes to study the systematic changes within and across a gradient of ecosystems and make comparisons to other distant ecosystems. They can also use it to study how actions in one area of an ecosystem can show effects in another area. Researchers can use this data as a baseline in comparison to similar systems in other areas, and to visualize/track changes through time. They can even expand upon the transect, looking at productivity in the upland tributaries in the watershed, and further offshore and into deeper aphotic waters. Policy makers can use such information to influence regulations regarding land-use development and nutrient use in the watershed and to combat emerging issues such as eutrophication, harmful algal blooms and climate change mitigation.
Over the years, this decade-long research effort was supported by a NASA Michigan Space Grant Consortium Seed Grant and an EPA Great lake Restoration Initiative Grant.
Weinke, A.D., S. T. Kendall, D. J. Kroll, E. A. Strickler, M. E. Weinert, T. M. Holcomb, D. K. Dila, A. A. Defore, M. J. Snider, L. C. Gereaux, B.A. Biddanda (2014). Systematically variable planktonic carbon metabolism along a land-to-lake gradient in a Great Lakes coastal zone. Journal of Plankton Research. Advance Access published August 11, 2014, doi: 10.1093/plankt/fbu066. Link to paper.