Fiqure 37. Simplified feeding 

 associations of four dominant fishes--bay 

 anchovy, sand seatrout, Atlantic croaker, 

 spot--and blue crabs in the Apalachicola 

 estuary. Four food compartments are 

 shown: phytopl ankton (P), holoplankton 

 (H), meroplankton and benthos (MB), and 

 sediments (S). Major food items in the 

 compartments are: DE=detritus, 

 8I=bivalves, HC=harpacticoid copepods, 

 NE=nematodes, IN=insects, PO=polychaetes, 

 SH=shrimp, MY=mysids, CR=crabs, FS=fishes, 

 CC=calanoid copepods, OI=diatoms. Numbers 

 indicate dry-weight contribution of 

 particular food items (within boxes) and 

 food contributions of major food 

 compartments (after Laughlin 1979 and 

 Sheridan 1978). 



transfer from upland systems. The 

 planktonic and detrital pathways come 

 together at the sediment level through 

 repackaging of fecal material and the 

 activity of the microorganisms. The 

 microbes transform dissolved nutrients 

 into available particulate matter. Over 

 2% of the dry-weight mass of the sediments 

 is composed of organic carbon, bacterial 

 biomass, and extracellular polysaccharides 

 (D. C. White personal communication). The 

 sediment organic matrix and POM form the 



basis of the benthic (detrital) food webs. 

 The grazing of detritus and its microbial 

 populations enhances nutrient quality for 

 subsequent microbial development by 

 stimulating further microbial productivity 

 and enhancing the nitrogen and phosphorus 

 content of the POM. Physical disturbance, 

 through wind and tidal action and active 

 predation and biological activity, is one 

 of the reasons why the Apalachicola 

 estuary is such a productive system. 



Seasonal relationships among the 

 various physical and biological factors in 

 the bay system have been developed (Figure 

 38). Although the biological response to 

 a given event usually follows a nonlinear 

 or curvilinear pattern, certain relation- 

 ships have become evident after many years 



of observation. Seasonal variations of 

 temperature and the pulsed river flow are 

 usually out of phase. Local rainfall 

 (Florida) peaks during summer months. 

 Salinity in the estuary is highest during 

 summer and fall months. The timing of the 

 river flow, and the resultant loading of 

 nutrients and POM, is critical to the 

 seasonal biological successions in the 

 estuary, especially during winter and 

 early spring. During such periods of low 

 winter temperature and salinity and high 

 river flow and detrital movement into the 

 estuary, benthic infaunal abundance is 

 high. Epibenthic organisms (especially 

 fishes) reach peak levels during late 

 winter as temperature starts to increase 

 and macroinvertebrates available for food 

 are abundant. Benthic omnivores such as 

 spot and the Atlantic croaker are favored 

 by such conditions. Although these 

 sciaenids overlap in their temporal dis- 

 tribution, food size partitioning by these 

 two bottom-feeding fishes results in 

 distinctive differences in prey type and 

 size (Sheridan 1978). A larger apparatus 

 allows croaker to penetrate deeper into 

 the substrate and consume larger poly- 

 chaetes, shrimp, and crabs. Spot tend to 

 exploit smaller organisms, such as nema- 

 todes, harpacticoid copepods, juvenile 

 bivalves, and smaller forms of poly- 

 chaetes. There is enough dietary overlap, 

 however, to allow the potential for 

 competition between these two species. 



Benthic macroinvertebrates occupy an 

 important trophic link between the primary 

 producers (and microbes) and the upper 



85 



