ECOLOGICAL ISSUES 



Two particularly important open 

 questions of estuarine ecology are the 

 subject of a lively debate. They are: 

 (1) what are the sources of nutrients 

 that support estuarine primary produc- 

 tivity (Haines 1979; Turner et al . 

 1979a; Nixon 1981), and (2) what is the 

 role of intertidal wetlands in the 

 organic budget of estuaries and coastal 

 waters, especially the role of wetlands 

 in the trophic support of coastal fish- 

 eries (Turner et al. 1979b; Nixon 1980; 

 Odum 1980). 



In this section we address these 

 issues within the context of the MDPR 

 data base. Detailed studies have been 

 done in the Barataria and Pontchartrain 

 basins. Some data are from other areas 

 and a growing amount of information is 

 from ongoing studies in the Atchafalaya 

 Bay area. 



FACTORS AFFECTING AQUATIC 

 PRIMARY PRODUCTIVITY 



measure of the intensity of upland run- 

 off. Evidence suggests that water 

 clarity and depth control primary pro- 

 ductivity in the lower basin (Hopkinson 

 and Day 1979). Offshore, river flow is 

 the dominant factor controlling produc- 

 tivity (Sklar and Turner 1981). Results 

 of aquatic productivity studies from 

 several sites in the Terrebonne basin 

 (Allen 1975) are similar to those from 

 the Barataria basin. 



WETLANDS AND THE ESTUARINE CARBON BUDGET 



Because of the great expanse of 

 periodically flooded marshes and swamps 

 in the MDPR, one might expect these 

 wetlands to play a major role in con- 

 trolling or augmenting the productivity 

 of adjacent freshwater and estuarine 

 ecosystems. Again, the most detailed 

 evidence for this control is the result 

 of studies carried out within the Bara- 

 taria basin. Day et al. (1982) con- 

 structed an annual organic carbon budget 

 for Barataria basin from a combination 

 of direct and indirect measurments of 

 carbon fluxes (Table 19). 



Studies of aquatic primary produc- 

 tivity have been conducted in all major 

 water bodies of the Barataria basin, in 

 Lake Pontchartrain, in several areas of 

 the Terrebonne basin, and in the near- 

 shore Gulf of Mexico. The problem of 

 primary productivity in the Barataria 

 basin serves as a general example of 

 productivity trends throughout the MDPR. 



The factors controlling produc- 

 tivity change from the swamp in the 

 upper Barataria basin to- the salt marsh 

 and open gulf in the lower basin. In 

 the upper basin, nutrient loading from 

 upland runoff seems to be important in 

 controlling both seasonal patterns and 

 magnitude of production. The importance 

 of upland runoff is substantiated by the 

 results of Witzig and Day (1982), who 

 showed a relationship between trophic 

 state index (TSI) and primary production 

 for waters of the Barataria basin and 

 Lake Pontchartrain. Locations with a 

 high TSI had high inputs of nutrients 

 from upland runoff. Gael and Hopkinson 

 (1979) reported that TSI was signifi- 

 cantly correlated with canal density, a 



The Barataria basin carbon budget 

 indicates that all aquatic habitats are 

 strongly dependent on organic matter 

 washed in from adjacent watersheds, and 

 upstream habitats are significant 

 sources of organic matter for downstream 

 habitats. The portion of wetland pri- 

 mary production exported to adjacent 

 water bodies is lowest in the swamp (2%) 

 and greatest in the salt marsh (30%). 

 In situ production plus carbon inputs 

 from other habitats exceeds carbon out- 

 puts from respiration and sedimentation 

 in all aquatic habitats. Upstream pro- 

 duction provides from 9% to 30% of total 

 carbon inputs to the different water 

 bodies . 



The mass balance technique used to 

 calculate the carbon budget depends to 

 some extent on fluxes calculated by 

 difference, so the resulting budget 

 cannot be validated. Considerable data, 

 however, exist to show: (1) that carbon 

 in significant quantities is exported 

 from the estuaries into the gulf, 

 (2) that carbon is exported from wet- 

 lands to adjacent water bodies, and 



1A9 



