An Experimental Test of Nutrient Reduction on Carbon Cycling in Eutrophic Sediment

Amanda Spivak, MC&G


2015 COI Funded Project


Estuaries are biogeochemically dynamic environments that modify the flux and composition of carbon exported to the sea. Yet, the chemistry and biology of estuaries worldwide are being altered by eutrophication. Excess nutrient loading is a primary driver of eutrophication that increases algal biomass, microbial respiration rates, and the likelihood of hypoxic events while reducing water clarity, seagrass extent, and benthic animal abundance and diversity. In shallow estuaries, sediments play a key role in carbon cycling and are acutely sensitive to the effects of eutrophication. In order to reverse the effects of eutrophication, nutrient mitigation measures have been enacted or proposed for impacted estuaries. However, there is uncertainty about how sediment biogeochemical processes will respond to mitigation actions that reduce nutrient concentrations in groundwater and surface waters and whether legacy organic matter will affect rates and trajectories of ecosystem recovery. Since sediment processes influence estuary carbon dynamics and export, it is important to understand how nutrient mitigation will affect sediment biogeochemistry and carbon metabolism.
Here, I propose a mesocosm experiment to test how mitigation strategies that alter surface water (e.g., inlet widening) and pore water (e.g., sewering) nutrient concentrations affect sediment carbon cycling. Specifically, the concentration (high vs. low) and pathway (pore water vs. surface water) of nutrients delivered to intact sediment cores from a local eutrophic estuary (Childs River) will be manipulated in a factorial design. Stable isotope (13C ) tracers will be added to evaluate how changes in nutrient loading and delivery pathway affect incorporation, transformation, and turnover of labile (benthic microalgae) and recalcitrant (Spartina alterniflora) organic matter. Sediment metabolism, pore water chemistry, and organic matter composition will be characterized throughout the experiment. The fate of the isotopic tracers in bulk sediments, algal and bacterial lipids, pore water, and respiratory fluxes will be monitored over the final month of the experiment to evaluate short term storage and recycling. Results will describe how carbon dynamics in eutrophic sediments respond to changes in nutrient loads delivered via surface water and pore water and will provide insight to how nutrient mitigation policies may affect sediment biogeochemistry and, by extension, estuary carbon dynamics. 
Results from this study will be useful in evaluating how nutrient mitigation strategies may affect the rate and trajectory of estuary recovery from eutrophication. Evaluating how the rates and pathways of organic matter decomposition change in response to nutrient mitigation may provide important insight to why recovery from eutrophication has been rapid in some ecosystems but less clear in others.