Carbon cycling in Salt Marsh Ecosystems
Salt marshes are dynamic habitats that provide valuable services to coastal communities. We study carbon cycling in order to understand how these ecosystems function and respond to perturbations. With support from the National Science Foundation, we evaluated the impact of eutrophication on the biogeochemistry of tidal creeks that connect salt marshes to the coastal ocean. We found that nutrients delivered to tidal creeks have little effect on the benthic microalgae that colonize sediments at the bottom of the creeks. Instead, microalgae are tightly coupled to sediment bacteria. A significant fraction of carbon fixed by microalgae is leaked out of the cell and rapidly assimilated by nearby sediment bacteria. In turn, these bacteria break down algal exudates and respire the inorganic carbon back into the environment where it is taken up by benthic algae. Adding inorganic nitrogen to the water overlying benthic microalgae did not enhance rates of algal production or bacterial decomposition. This suggests that benthic microalgae and sediment bacteria in the tidal creeks are largely insensitive to nutrient levels in tidal creek waters. Consequently, it is unlikely that nutrients that escape uptake by marsh grasses are intercepted by benthic microalgae in tidal creeks before they are exported to the coastal ocean. Our findings were recently published in Marine Ecology Progress Series (Spivak & Ossolinski 2016) and our data are archived at BCO-DMO.org.
Small Cannibals? The Role of Ponding in Salt Marshes
Shallow ponds are natural features of salt marshes. Yet, the spatial extent of ponds is predicted to increase in response to certain land management practices and sea level rise. Since ponds can form through biogeochemical mechanisms, expanding ponds could eat away at decades of buried peat and affect the carbon balance of salt marshes. With this in mind, our lab has been working to characterize the ecology, biogeochemistry, and metabolism of several shallow ponds within the Plum Island Ecosystems-Long Term Ecological Research site (Rowley, MA). This work is supported by NSF and our initial data are archived at BCO-DMO.org.
How Effective is Salt Marsh Restoration?
A primary goal of salt marsh restoration is to repair ecosystem functions that have been altered, diminished, or lost as a result of human activities. Such functions can include carbon burial and soil accretion which are economically valuable and important for erosion protection and resilience to sea level rise. Quantifying the extent of recovery and time required to repair ecosystem functions is imperative for evaluating the efficacy of restoration techniques. To address this, we are quantifying changes in plant communities, soil properties and geochemical rates, carbon accumulation, and greenhouse gas fluxes in salt marshes restored over 13 y. Our results will describe how local Cape Cod, MA, salt marshes have changed since restoration and will be useful for predicting when restored sites will be similar to natural marshes in terms of soil biogeochemistry. This project is a collaboration with K. Kroeger and M. Gonneea at USGS and J. Tang and F. Wang at MBL and is supported by MIT Sea Grant.
Food Web Structure and the Geochemistry and Ecology of Coastal Lakes
River herring represent one of the oldest and, formerly, most economically and culturally important fisheries in New England. Because of their anadromous life history—whereby adults live most of their lives in marine waters but inhabit freshwaters during spawning and their early life stages—river herring have wide-ranging economic and ecological importance. Structures, such as dams, that block their migration pathway have reduced herring population levels to ~1% of historic sizes. Recent conservation efforts in Maine have led to the removal or modification of several dams along the state’s major rivers. This has opened up access to lakes and tributaries that have been devoid of anadromous fishes for 100-200 years. In collaboration with J. Llopiz (WHOI) we are evaluating how herring recolonization affects the geochemistry and ecology of 7 Maine lakes.