DISCUSSION 



The Cooper River-Charleston Harbor estuarine system is 

 characterized as mixohaline with gradual changes in faunal 

 assemblages. The most striking differences in species composi- 

 tion occurred between those stations located at or near the 

 mouth of the estuary and those located far upriver. The fish 

 and decapod crustacean species assemblages associated with 

 these two areas were primarily composed of stenohaline 

 marine species and low-salinity resident estuarine species, 

 respectively. Nevertheless, euryhaline species, which extended 

 from the mouth of the estuary into brackish waters, were 

 the dominant faunal component throughout the estuary as a 

 whole. Except for a few freshwater species, resident estuarine 

 species (e.g., Trinectes maculatus, Anchoa mitchilli, 

 Palaemonetes pugio, Ictalurus catus) were found throughout 

 the system, their distributions often overlapping with those of 

 species derived from the marine environment. This distribu- 

 tional pattern is similar to that described by Weinstein et al. 

 (1980) who noted considerable overlap in distributional pat- 

 terns of resident fishes and stenohaline marine transients in 

 the Cape Fear River, N.C. 



The observed overlapping spatial distributional patterns of 

 resident and transient fishes and decapod crustaceans can 

 be related to salinity regimes within the estuary and to the 

 physiological tolerances of component estuarine species to 

 these regimes. In comparison with estuaries of the Middle 

 Atlantic states, such as Chesapeake Bay, South Carolina 

 estuaries are narrower, deeper, and shorter in length (Mathews 

 and Shealy 1978). The physiography of estuaries (Pritchard 

 1954), in addition to other factors such as runoff, tidal action, 

 and current velocity (Mathews and Shealy 1978), affect verti- 

 cal mixing and, consequently, determine salinity regimes as 

 well. The combined effect of these factors in South Carolina 

 estuaries is a compression of the isohalines, with resultant 

 overlap in the distributional patterns of many estuarine spe- 

 cies. Ultimately, however, it is the physiological tolerances of 

 component estuarine species which really determine their 

 distribution. The spatial limits of freshwater species are main- 

 tained through physiological constraints, while other resident 

 estuarine species are able to tolerate a wider range of salinity 

 and apparently are not limited by competition and predation 

 to the lower reaches of the estuary (Weinstein et al. 1980). 

 Physiological tolerances are also important in determining the 

 upestuary limits of species which are numerically dominant in 

 the Cooper River-Charleston Harbor system. For the most 

 part, these species were unable to penetrate into areas where 

 the isohalines were <0.5°/oo and were generally most abun- 

 dant at stations in the mesopolyhaline zone. 



The overlapping spatial distributions of many resident 

 estuarine, stenohaline marine, and numerically dominant 

 euryhaline species in the Cooper River-Charleston Harbor 

 system are reflected in the greater species richness and abun- 

 dance of individuals at stations in the mesopolyhaline zone. 

 Assemblages at these stations were comparatively diverse, 

 consisting of some resident estuarine and euryhaline species 

 and many stenohaline marine species. Seasonal peaks in spe- 

 cies diversity are largely attributable to those stenohaline 

 marine transients which occur sporadically in low densities 

 throughout the lower reaches of the Cooper River-Charleston 

 Harbor system. Biological interactions such as predation and 

 competition for space and food can also contribute to species 



diversity and richness. Weinstein et al. (1980) noted that in- 

 creased predation pressure probably enhanced species diversity 

 in downstream marsh areas of the Cape Fear River, N.C, by 

 preventing dominant competitors from monopolizing the 

 major food and space resource. An alternative explanation is 

 that enough food may be present in the lower reaches of the 

 river to support a high diversity of species. Euryhaline species 

 such as the sciaenids were numerically dominant in the Cooper 

 River-Charleston Harbor system and were most abundant at 

 downriver stations. Juvenile sciaenids feed opportunistically 

 on a variety of infaunal and demersal species (Chao and 

 Musick 1977). Their successful coexistence in higher salinity 

 areas with stenohaline marine and other estuarine species may 

 be attributed to utilization of food resources from different 

 levels of the water column and to the abundant food resources 

 of the estuarine system. In this case, food would not be a 

 limiting resource and intrafamilial or interspecific competition 

 would not be as important a factor (Chao and Musick 1977). 



Temporal distributional patterns were another important 

 aspect of the fish and decapod community of the Cooper 

 River-Charleston Harbor system. Temporal changes in species 

 associations and abundance were related primarily to fluctua- 

 tions in abiotic variables. Bottom-water temperature in the 

 channel of the estuarine system exerted a substantial influence 

 on the abundance of species collected. The most noticeable 

 decreases in abundance of fishes and decapods coincided with 

 annual minimum temperatures, especially those experienced 

 during the extremely harsh winter of 1977. These seasonal 

 trends in abundance were especially evident for the sciaenid 

 fishes and penaeid shrimps. For those species which may over- 

 winter in the estuary, such as Micropogonias unduiatus and 

 Penaeus spp., extremely low winter temperatures can destroy 

 an entire year-class (Massmann 1971; Farmer et al. 1977). 

 Thus, temperature-related mortalities, as well as emigration of 

 juveniles, probably contributed to the low abundance and 

 biomass observed at that time. Similar explanations were 

 suggested by Weinstein (1979) for decreased abundance of 

 Penaeus spp. in the Cape Fear River, N.C. 



Seasonal differences in species assemblages reflected 

 changes in abundance as well as exclusion of some species 

 from the estuary during part of the year. However, most spe- 

 cies remained in the estuary throughout the year, while their 

 abundances changed seasonally. Nevertheless, while faunal af- 

 finities varied throughout the year, as indicated by cluster 

 analysis, the species composition of the estuarine system as a 

 whole was not altered appreciably. Temporal fluctuations in 

 abundance appear to be a means through which more species 

 are able to utilize the estuary simultaneously by a reduction in 

 densities and competition for food and space. 



The importance of abiotic factors in determining the distri- 

 butional patterns of estuarine biota has elicited concern about 

 the effects of rediversion on the integrity of species assem- 

 blages and, more importantly, on interspecific balance (Shealy 

 and Bishop 1979). A restriction of freshwater inflow will prob- 

 ably cause salinities to be higher and, consequently, modify 

 the existing salinity gradient. Additional consequences of a de- 

 crease in flow rate might include a decrease in nutrient and 

 detritus influx, lowering of the water table, reduction in water 

 turbidity, alteration of estuarine circulation, and reduction in 

 the ability of organisms to withstand stresses of normal 

 drought periods (Heald 1970; Keiser and Aldrich 1976). These 

 alterations, should they occur in the Cooper River-Charleston 



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