U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameSeasonal Phosphorus Limitation on the Louisiana Shelf: A Result of Anthropogenic Nitrogen Loading From the Mississippi River?
Document AuthorAmmerman, J.W.
Sylvan, Jason B.
Dortch, Q.
Nelson, David M.
Brown, Alisa Maier
Morrison, Wendy
Short DescriptionEOS Trans. AGU, 84(52), Ocean Sci. Meet. Suppl., Abstract OS21K-08
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2003

Abstract: Seasonal phosphorus limitation of primary production during the spring and early summer was demonstrated in the Mississippi River plume and Louisiana continental shelf during a series of cruises in 2001. Mapping of surface water properties over most of the Louisiana shelf and extensive bioassay experiments combined to make a large spatial and temporal dataset. The chlorophyll a response to additions of orthophosphate (Pi) was much greater than the response to either nitrate or silica additions during the spring and early summer. The Pi turnover time was less than 30 minutes during May and July 2001. Elevated nitrogen (N) to phosphorus (P) ratios were seen over much of the shelf during the spring and early summer months. The mean N:P ratio was 64 in March 2001 and was greater than 380 in both May and July 2001 for the entire Louisiana shelf. High alkaline phosphatase activities were seen during March, May and July 2001, including extremely high activities near the mouth of the Mississippi River. By September, however, all the above indicators indicated that much of the Louisiana shelf was N-limited. The observed seasonal P-limitation coincides with both the Mississippi’s high flow period during the spring and early summer and the period of high productivity responsible for the annual summer hypoxia. Anthropogenic N loading of the Mississippi River has apparently shifted the Louisiana shelf into spring P-limitation. The Federal-State-Tribal Action plan, recently delivered to the Congress, calling for a 30 percent reduction in N-loading over the next 15 years, will be important in reducing the high N:P ratios which lead to P-limitation. At the same time, P input to the watershed should also be managed to reduce the size of the hypoxic zone.

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