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2 2.5 



STABLE ISOTOPE TRACERS OF BIOLOGICAL PROCESSES 

 IN THE GULF OF MEXICO 



Dr. Brian Fry 



Ecosystems Center 



Marine Biological Laboratory 



Woods Hole, MA 02543 



The shelf of the northwestern Gulf of Mexico is a region of contrasts. The Louisiana shelf receives large 

 sediment and nutrient inputs from the Mississippi River and the many relatively open bays that line the coast. 

 In contrast, the Texas coast receives little river input, and its bays are generally contained seawards by long 

 barrier islands. The fisheries productivity of both areas is high, but the Louisiana shelf is the more fertile of the 

 two areas. The causes of this high productivity include river inputs, export from shallow bays, and upwelling of 

 deep Gulf water. Tracer studies are needed to show which of these processes are locally most important for 

 fisheries production. 



Geochemists have used stable isotope tracers to study the patterns of river and estuarine influences in the Gulf 

 of Mexico. Carbon isotope del 13 C studies show widespread deposition of terrestrial-derived carbon into 

 sediments of the Gulf of Mexico. For example, an early study in the Mississippi Sound east of the Mississippi 

 River showed that terrestrial carbon measuring about -26 ppt. del 13 C was dominant in nearshore areas, but that 

 carbon derived from plankton and measuring about -21 ppt del 13 C became more important offshore (Sackett and 

 Thompson 1963). Subsequent studies of sediments off the Louisiana and Texas coasts showed that the deposition 

 of terrestrial sediments was linked to river inputs, with del 13 C values close to -26 ppt being found off the 

 Mississippi River, the Atchafalaya River, and Trinity River (Gearing et al. 1977). In general, however, terrestrial 

 carbon inputs decreased rapidly with increasing distance offshore, and were detectable only in a narrow nearshore 

 zone of 2-30 km. Carbon from common salt marsh Spartina plants did not seem to influence shelf sediments, 

 although sediments can be found in certain marsh areas that do have -16 ppt del 13 C values close to those of - 

 13 ppt. Spartina (Chmura et al. 1987). 



Transect sampling has also proven valuable in stable isotope studies of food webs in estuaries and river plumes. 

 One study examined the possible influence of the Mississippi River in fertilizing the offshore Gulf of Mexico. 

 No large isotopic differences were detectable along transects off Southwest Pass during low-discharge winter 

 months (Thayer et al. 1983). The effects of Mississippi River plumes are being reexamined with samples 

 collected at peak river discharge in April 1989. A recent study has shown that algal blooms associated with river 

 plume water can have distinctive stable isotopic compositions (Cai et al. 1988). Large algal blooms were present 

 in Mississippi River plume water in April, and collections of phytoplankton and zooplankton are being analyzed 

 for del 13 C and del 15 N. The goal of the research is to use isotopic tracers to follow the biological importance of 

 the plume for the Louisiana shelf. 



The isotopic tracers have also been used to discern the food web coupling between estuaries and the nearshore 

 shelf, primarily through studies of common fish and shrimp. One survey of benthic shrimp and stomatopods from 

 the northern Gulf of Mexico showed that offshore resident species that do not enter estuaries have fairly constant 

 del 13 C values of -14 to -16ppt (Fry 1983). However, pink, brown, and white shrimp that migrate offshore from 

 estuaries as juveniles showed large regional differences in isotopic compositions, reflecting their estuarine past 

 (Fry 1983). 



It is not known how far seawards the estuarine food web extends along the Louisiana and Texas coasts. The 

 gradients of isotopic change in estuaries are often much sharper in estuaries than offshore (Fry 1983), and need 

 to be defined with close-interval sampling along the Louisiana and Texas coasts. 



In summary, several biological processes important for the shelf of the northwestern Gulf of Mexico have been 

 studied with stable isotope measurements. Transect sampling has shown that estuarine export often declines 

 sharply with distance offshore. Future studies might combine carbon stable isotope measurements with nitrogen 

 and sulfur isotope measurements to obtain a wider food web analysis of important fisheries species (Macko et 

 al. 1984; Peterson et al. 1986; Fry 1988). One could assess the importance of estuarine export and river plumes 



