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2.2.17 



POSSIBLE EFFECTS OF OIL AND GAS PRODUCTION ON PLANKTON 

 PROCESSES IN THE PLUME OF THE MISSISSIPPI RIVER 



Dr. Quay Dortch 



Louisiana Universities Marine Consortium 



Chauvin, LA 70830 



The production and transport of oil and gas can release chemicals into the environment which may affect 

 plankton processes (National Research Council 1985; Capuzzo 1987; Spies 1987). In general terms, both 

 inhibitory and stimulatory responses can be observed. Two responses appear to be cited most often; changes 

 in phytoplankton species composition to smaller, often flagellate, species and decreases in zooplankton grazing 

 rates on algae. It has been presumed that the actual impact on the plankton in the coastal and open ocean was 

 minimal because of the short generation times of planktonic organisms, the likelihood of rapid reseeding of 

 impacted populations with unaffected organisms, and the difficulty of detecting impact in populations subject to 

 high natural variability. However, this ignores the possibility that although effects on any one group of organisms 

 may not be observed, processes within the ecosystem and its overall functioning may be impacted. Furthermore, 

 the Mississippi River plume may not be as open a system as many coastal and open ocean areas, and may be 

 impacted as if it were an enclosed estuary. 



As described by Wiseman (section 2.2.13), the flow of the Mississippi River plume is generally to the west 

 between the coastal boundary layer and the TEXLA coast. Some effects of the river input may be observed all 

 the way to the Mexican border. By analogy with upwelling areas (Maclsaac et al. 1985; Wilkerson and Dugdale 

 1987; Dortch and Postel 1989), the river plume can be described as a conveyor belt (Figure 13). At the mouth 

 there is very high nutrient input, but due to turbidity-induced light limitation, phytoplankton biomass remains 

 low. After the sediment drops out, phytoplankton take advantage of the high light and nutrients, growing very 

 rapidly. Down plume, a chlorophyll maximum is usually observed at about 25% salinity, made up almost entirely 

 of large diatoms. At this point nutrients supplied directly by the river are depleted, but regeneration by 

 zooplankton, bacteria, and benthic organisms continues to supply nutrients. In fact, despite high river input of 

 nutrients, this system is uniquely dependent on regeneration for maintaining its high productivity. However, 

 much of the biomass sediments out below the chlorophyll maximum (probably as phytodetritus) and down plume 

 (probably as zooplankton fecal pellets), so that eventually biomass of all organisms, nutrient concentrations, 

 and nutrient regeneration rates decreases to very low levels. This model can be used to assess the potential 

 effects of a decrease in grazing ranges or a change in species composition. Many factors, which cannot be 

 assessed here, would modify these impacts. 



A decrease in zooplankton grazing would increase the flux of carbon to the benthos. This would exacerbate the 

 hypoxia which already exists on the shelf during periods of strong stratification and which may be related to 

 increasing eutrophication. Because nutrients would be sequestered in the sediments or in hypoxic layers, 

 regeneration downstream would be decreased, resulting in lower overall productivity of the system, especially 

 down plume. Finally, there would be decreased transfer of carbon to higher trophic levels in the water column, 

 leading to decreased production of commercially important species depending on planktonic food chains. 



Large diatoms, which unlike most other phytoplankton, require silicon (Si) for growth and are the major 

 component of the algal biomass which develops in response to nutrient input from the river. Furthermore, they 

 play a vital role in the planktonic food web and flux of carbon to the benthos in the plume. Over the last 40 

 years riverine input of Si has decreased, while that of other nutrients has increased (Turner et al. 1987; Turner, 

 section 2.2.11). There is evidence that in the plume in the region of the chlorophyll maximum, concentrations 

 of Si could be growth limiting for diatoms. Thus, eutrophication may already be changing the phytoplankton 

 species composition to smaller, flagellated phytoplankton, which do not require Si. This will certainly affect 

 trophodynamics and flux of carbon to the benthos, but precise predictions are not possible. If substances released 

 by oil and gas production and transport affect phytoplankton species composition in similar ways, major changes 

 in the ecosystem could occur as a result of multiple, human-induced stresses. 



While the foregoing discussion is general and speculative, it does suggest a number of processes which deserve 

 further attention because they are critical to the ecosystem and vulnerable to impact. These include at a 



