CHAPTER 6 

 LONG-TERM ECOLOGICAL RELATIONSHIPS 



Although diurnal and seasonal changes 

 in population and community structure in 

 the estuary are relatively well documented 

 (Livingston IQ/Sb, 1977a, lQ77d, 1Q78; 

 Livingston et al. 1974, 1977), the long- 

 term biological relationships, measured in 

 decades, are still under consideration 

 (Livingston unpublished data; Appendix A). 

 Seasonal changes in important physical and 

 chemical factors are relatively stable in 

 terms of timing (Figures 9, 1?); however, 

 there is considerable annual or year-to- 

 year variation of such factors (Figures 

 10, 14, 15, 16, 17). The coupling between 

 climatological features such as river flow 

 and long-term changes in the commercial 

 catches of oysters, shrimp, and crabs 

 (Meeter et al. 11^79) is often complicated 

 by socioeconomic influences on such data 

 (Whitfield and Beaumariage 1977). 



The specific short-term distribution 

 of a given species is often associated 

 with complex habitat variables and the 

 avail ablility of food. At the same time, 

 long-term changes in a given population in 

 the estuary may be influenced by 

 climatological cycles. Thus, the monthly 

 distribution of brief squid ( Lol 1 ingun cula 

 brevis ) depends to a considerable degree 

 on fluctuations of zooplankton abundance, 

 but the timing and annual abundance of 

 this species is also associated with 

 recurrent cycles of salinity and 

 temperature (Fiaure 39; Laugh! in and 

 Livingston 1<582). Spring migration into 

 the estuary has been correlated with 

 specific changes in both temperature and 

 salinity, while the fall emigration 

 largely depends on temperature changes. 

 Timing of the succession of climatological 

 changes is important since a specific 

 temperature has entirely different 

 meanings to a given species in the spring 

 and in the fall. 



Long-term patterns of blue crab 

 ( Callinectes sapidus ) recruitment cannot 

 be determined solely by the physical and 

 chemical environment (Figure 40; Laughlin 

 and Livingston, unpubl.). For any given 

 year, the winter recruitment was inversely 

 related to blue crab population abundance 

 and to summer recruitment levels. The 

 variable size 1 (monthly mean frequencies 

 of crabs of 1-30 mm; Table 24) was 

 inversely correlated with temperature (p  

 0.01) and with variable size 3 (monthly 

 mean frequencies of crabs 61 mm) (p 

 O.OS). No significant correlations were 

 found with river flow or local rainfall, 

 which were associated with peak 

 recruitments at different times of the 

 year. In a multiple regression with 

 variable size 1 as the dependent variable 

 (Table 25A, N = 12 months), temperature, 

 rainfall, and variable size 2 explained 

 about 89°^ of the variability of relative 

 abundance. The variable size 2 was weakly 

 correlated with all other variables (Table 

 26R). In a multiple regression with 

 variable size 3 as the dependent variable, 

 temperature, river flow, size 1 and size 2 

 explained about 70^ of the variability of 

 relative abundance (Table 25C). 



Winter recruitment was below the 

 6-year average (59 crabs/month) in 

 1972-73, 1974-75 and 1975-75. A single 

 high peak, however, occurred in 1<373 and 

 was correlated with the highest peak of 

 river flow of the 6-year period (Figure 

 40). During the winter months of these 

 years, river flow (which largely 

 determines salinity values in the estuary) 

 reached high (1^73), intermediate (1975), 

 and low (1°76) values, whereas water 

 temperatures deviated little {+_ 1° C) from 

 the 6-year temperature mean (14.9° C). By 

 contrast, summer recruitment for each of 

 these years was well above the 6-year 



90 



