Organic Matter Degradation is Key for Recycling Phosphorus in Chesapeake Bay Sediments
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. Well documented summertime algae blooms deplete the oxygen content of the bay’s water and challenge the survival of benthic invertebrates and macro-organisms (e.g., shellfish), as well as pelagic organisms in the overlying waters. The prevailing theory is that excessive levels of nutrients such as phosphorus (P) are entering the bay from point and nonpoint sources, and that they are the primary culprit in this ecosystem management challenge. In an effort to better understand and constrain the mechanisms and processes involved in P cycling between the bay sediments and overlying waters, a team of researchers from the University of Delaware, Old Dominion University, and the Department of Energy’s Environmental Molecular Sciences Laboratory (EMSL), analyzed sediment cores from the mid-bay portion of the Chesapeake Bay. To understand the mineralogy of the sediments, particularly the composition and stability of iron-containing minerals, they used Mössbauer spectroscopy and X-ray diffraction capabilities at EMSL, and to understand P transport, they used P isotopic techniques. The team found that the degradation of organic matter in the anoxic sediments results in the regeneration of inorganic P, and that, in contrast, the P from terrestrial and atmospheric inputs becomes bound to iron oxide in the sediments and very little is remobilized into the overlying waters. These results indicate that the cycle at the sediment-water interface works as follows: organic debris from dead algae settles in the sediments, and then degradation of the organic debris results in the liberation of inorganic P, which diffuses upward into the overlying water to resupply P to algae. The algae continue to grow and sustain a dead benthic zone that cannot support shellfish. Because high P levels and low-oxygen conditions are now common in many coastal environments, these findings will have important implications for managing these ecosystems.
Joshi, S. R., R. K. Kukkadapu, D. J. Burdige, M. E. Bowden, D. L. Sparks, and D. P. Jaisi. 2015. “Organic Matter Remineralization Predominates Phosphorus Cycling in the Mid-Bay Sediments in the Chesapeake Bay,” Environmental Science and Technology 49(10), 5887–96. DOI: 10.1021/es5059617.