The Chesapeake Bay is the largest and most productive estuary in the United States, containing more than 1,500 square miles of wetlands that provide critical habitat for fish, shellfish and wildlife. This unique ecosystem is in jeopardy due to excessive levels of nutrients such as phosphorus from both man-made activities and natural processes. These nutrients fuel summertime blooms of harmful algae at the surface as well as low-oxygen conditions in deep waters where many bottom-dwelling organisms cannot survive. A recent study identified an unexpected phosphorus-recycling process as the predominant source of phosphorus in the mid-Bay sediment in the Chesapeake Bay.
Restoration of the Chesapeake Bay has been very costly and challenging because there are multiple man-made and natural nutrient sources from sediments, land and air, and because nutrient cycling pathways are complex. By providing a detailed understanding of one way phosphorus is introduced and recycled in the bay, the findings provide new insights into how this nuisance nutrient could support harmful algae growth and sustain low-oxygen conditions. Ultimately, this research could be useful for developing strategies that effectively restore water quality in this vital fishery ecosystem.
Researchers from the University of Delaware, Old Dominion University and EMSL, the Environmental Molecular Sciences Laboratory, analyzed sediment cores from the mid-bay portion of the Chesapeake Bay. They used phosphate oxygen isotope ratios because the stable isotope composition may differ depending on the phosphate source or cycling pathway. To understand mineralogy, particularly the composition and stability of iron-containing minerals, they used Mössbauer spectroscopy and X-ray diffraction at EMSL, a U.S. Department of Energy national scientific user facility.
The researchers found remineralization—regeneration of inorganic phosphorus from organic matter degradation—is the predominant pathway of phosphorus cycling in sediment studied in the Chesapeake Bay. According to the authors, organic debris from dead algae rains down and settles in sediments. Remineralization of this organic matter subsequently causes dissolved phosphorus to release, which may then diffuse up and support new harmful algae blooms at surface waters. These organisms block sunlight to underwater grasses and consume life-supporting oxygen when they die and decompose, turning bottom water into a dead zone for shellfish such as crabs, mussels and oysters.
Surprisingly, iron oxide-bound phosphorus was the largest phosphorus pool in the sediment. Because a significant fraction of this phosphorus pool should have mobilized at the onset of bottom-water hypoxia in early summer, the recalcitrant iron oxide-bound phosphorus pool in the sediment was higher than expected for anoxic sediments. Taken together, the findings shed light on the relative contribution of remineralization versus remobilization in phosphorus cycling in a critical fishery ecosystem. Since phosphorus pollution and low-oxygen conditions are now common in many coastal environments, the findings could have broad implications for strategies to enhance water quality worldwide.
Funding: U.S. Department of Agriculture and Delaware Experimental Program to Stimulate Competitive Research
Publication: Joshi SR, RK Kukkadapu, DJ Burdige, ME Bowden, DL Sparks, and DP Jaisi. 2015. "Organic Matter Remineralization Predominates Phosphorus Cycling in the Mid-Bay Sediments in the Chesapeake Bay." Environmental Science & Technology. doi:10.1021/es5059617