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2.2.11 



SOURCES OF LONG-TERM VARIABILITY IN THE NORTHERN 

 GULF OF MEXICO CONTINENTAL SHELF ECOSYSTEM 



Dr. R. Eugene Turner 



Department of Marine Sciences 



Louisiana State University 



Baton Rouge, LA 70803 



Dr. N.N. Rabalais 



Louisiana Universities Marine Consortium 



Chauvin, LA 70344 



Variability in the ecosystem of the northern Gulf of Mexico continental shelf occurs, in part, because of changes 

 in the water quality of the Mississippi River. Water quality has changed considerably this century and these 

 changes are reviewed. The concentration of suspended sediments has declined this century due to flood 

 protection and navigation improvement efforts. Further, channel shallowing below New Orleans and channel 

 deepening in the Atchafalaya Basin have contributed to varying amounts of suspended sediments being delivered 

 to offshore. These changes affect the light regime, thus influencing phytoplankton production rates. 



The various estimates of light transparency using secchi disk measurements are summarized for the northern Gulf 

 of Mexico continental shelf, especially near the Mississippi River delta. These data include results from in situ 

 studies, monthly across shelf transects, plume surveys at the river deltas, and continental shelf surveys from 

 Mobile Bay to Texas. 



Concurrent measurements of light transparency (submarine photometer) and extinction coefficients estimated 

 from secchi disk depths (SDD) are very similar, especially in well-mixed water columns. In addition, the 

 extinction coefficients estimated from secchi disk data and estimated from in situ phytoplankton production are 

 also strongly correlated. SDD varied between 0.2 cm and >30 meters and were, of course, lowest in low salinity 

 waters located near sediment sources. SDD changed slightly in the Mississippi River turbidity maximum and 

 substantially increased around 20 ppt. It appears as if SDD increased from the 1950's to the 1980's. 



Nitrate, silicate, and phosphate concentrations in river water have also changed, probably reflecting the general 

 increased eutrophication of fresh waters through increased fertilizer usages. These changes undoubtedly have 

 resulted in increased phytoplankton production on the continental shelf, perhaps influencing hypoxic conditions, 

 presently widespread arid severe. 



Additional sources of variation are eustatic sea level rise, climatic cycles (many of which operate over periods 

 longer than decades), river discharge, and solar radiation. All of these sources of variation, and others, will cause 

 measurable variability in the aquatic communities offshore, and complicate discernment of impacts due to Outer 

 Continental Shelf activities. 



