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

Document NameImpacts of changing Si/N ratios and phytoplankton species composition
Document AuthorDortch, Q.
Rabalais, N.N.
Turner, R.E.
Qureshi, N.A.
Short Description Pp 37-48 in N. N. Rabalais and R. E. Turner (eds.), Coastal Hypoxia: Consequences for Living Resources and Ecosystems. Coastal and Estuarine Studies 58, American Geophysical Union, Washington, D.C.
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2001

Abstract: While nitrogen (N) and phosphorus (P) inputs from the Mississippi River have increased since the 1950s concomitantly with increasing productivity and hypoxia, silicate (Si) inputs have decreased. As a result, nutrient ratios have changed so that Si can now be limiting, especially in the spring. Si limitation controls size and species compostion of the diatom bloom by selecting species lower Si requirements. Evidence from the Louisiana shelf indicates that phytoplankton sinking, especially of diatoms in the spring, contributes to the vertical carbon flux that causes hypoxia. Most of the sinking phytoplankton are diatoms that are moderately to heavily silicified. Similar results have been obtained in other eutrophic areas. Consequently, the Si/N input ratio may influence the environmental impacts of increasing nutrient inputs through control of phytoplankton species composition. Nutrient control strategies to reduce hypoxia need to consider the consequences of changing nutrient ratios as well as changing nutrient concentrations.