ESTUARINE OPEN WATER (16) 



The area of estuarine open water 

 habitat in 1978 in the MDPR was 1.9 x 

 10 6 ha (4.7 x 10 6 acres), or 56.08% of 

 the region (Table 5) . This habitat type 

 increased in area between 1955 and 1978, 

 largely at the expense of salt and 

 brackish marshes. 



The criteria that distinguish the 

 estuarine open water habitat (Figure 33) 

 from other water bodies are size and 

 salinity. This habitat is defined as 

 including water bodies greater than 8 ha 

 (20 acres) with salinities between 0.5 

 and 30ppt (Cowardin et al. 1979). Each 

 of these characteristics, as well as the 

 typically shallow depth of this habitat, 

 has ecological implications. 



The estuarine open water habitat is 

 distinguished from the river, stream, 

 and bayou habitat in the MDPR because 

 there are important ecological differ- 

 ences between large and small bodies of 

 water. Small lakes have a proportion- 

 ately higher ratio of shoreline to 

 surface area than large lakes, and the 

 shoreline is the site of high densities 

 of invertebrates and nekton, intense 

 feeding activity by birds, organic 

 inputs from wetlands, as well as cul- 

 tural activities such as fishing that 

 impinge on the habitat. The density and 

 diversity of organisms are typically 

 higher at the shoreline than in the 

 center of a lake, e.g., Roberts (1981). 

 The edge effect, or ecotone principle, 

 proposes that interfaces between two 

 habitats are biologically richer and 

 more active than the central areas of 

 each habitat (Odum 1971) o 



The depth of the estuarine open 

 water habitat is closely related to its 

 function. The effect of wind on bottom 

 disturbance and turbidity in this habi- 

 tat is one example. Shallow lake bot- 

 toms are easily stirred up by wind. 

 Lake Pontchartrain is only about 4.9 m 

 (16 ft) deep and a 15 mph wind will 

 disturb the bottom (Stone 1980a) . Shal- 

 low estuaries in the MDPR have a high 

 bottom surface/volume ratio, which in- 

 creases the interactions between water 



column and sediments. These inter- 

 actions include the various roles of the 

 benthic community, such as nutrient 

 recycling and providing food for nekton. 



Open water also differs from small 

 sheltered water bodies in having enough 

 fetch to be exposed to wind energy, 

 waves, and turbulence. The estuarine 

 open water habitat is usually turbid 

 because of the suspension of bottom 

 sediments by waves. The shallowness and 

 turbulent mixing of the estuarine open 

 water habitat normally preclude vertical 

 stratification. Stratification would 

 result in low oxygen conditions in the 

 bottom waters that would increase the 

 incidence of fish kills in these natu- 

 rally eutrophic waters. 



The estuarine open water and near- 

 shore gulf habitats are not very dis- 

 tinct on the basis of salinity. The 

 estuarine open water habitat includes 

 Lake Pontchartrain, which ranges from 

 to 16 ppt, while nearshore gulf salin- 

 ities may range from 5 to over 30 ppt. 

 Ecological differences between variable 

 salinity coastal water bodies (estua- 

 ries) and stable salinity marine waters 

 include relatively greater osmotic 

 stresses on the organisms that inhabit 

 estuaries (Remane and Schlieper 1971). 

 This difference is not apparent when 

 comparing the two coastal habitats in 

 question here. 



The bottom community in the estua- 

 rine open water habitat in the MDPR is 

 typically populated with macrobenthic 

 animals, such as bivalves and poly- 

 chaetes, as well as with numerous tiny 

 meiobenthic animals, such as nematodes, 

 and microbenthos (bacteria and proto- 

 zoa). Thus, bottom respiration (carbon 

 requirements) are high (58 to 280 g 

 C/m 2 /yr, Roberts 1981). This implies 

 that a large portion of the organic 

 matter that is produced directly in the 

 water column or that washes into open 

 water from adjacent wetlands is pro- 

 cessed by the benthic organisms in the 

 estuarine open water habitat. 



Most aquatic organisms (especially 

 benthic invertebrates) have fairly 



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