Table 1. Net annual primary productivity of each major source of plant production in the Newport 

 River estuary (from Bigelow 1977). 



Source of 



primary production 



Effective 

 area of 

 habitat (km^ ) 



Productivity (g C/m /yr) 



Per unit area 

 within its 

 own habitat 



Per unit area 

 averaged over all 

 estuarine habitats 



Reference 



Phytoplankton 31 



Spartina altemiflora 6.2 



Zostera marina 0.3 



Zostera epiphytes 0.3 



Benthic microalgae 15.5 



110 



470 



330 

 73 

 33.7 



110 

 94 



3.18 



0.71 

 16.8^ 



Wimams(1966), 

 Thayer (1971) 



Williams and Murdoch 

 (1969) 



Penhale(1977) 



Penhale (1977) 



Bigelow (1977) 



^Based on data from June to December and multiplied by 2 to get annua! production. 



productivity indicates because of their close prox- 

 imity to benthic consumers, making possible a 

 high utilization efficiency on the part of the con- 

 sumers. As will be developed in succeeding chap- 

 ters, the intertidal flat thus serves as the habitat in 

 which primary production from several estuarine 

 habitats is transformed into benthic animals 

 which provide the food resources for all probing 

 shorebirds and many bottom-feeding fishes. 



Although it is currently impossible to state 

 unequivocally which sources of plant production 

 are the most significant suppliers of nutrition to 

 the consumers at the base of estuarine food 

 chains, certain generalizations are possible. Several 

 authors have recognized that the trophic status of 

 an aquatic animal tends to be complex. In marine 

 systems, most animals reproduce by using plank- 

 tonic larval stages. Such larvae are generally small 

 and become a part of the zooplankton. Some 

 larvae feed in the water column (planktotrophic) 

 and some do not feed until after metamorphosis 

 (leicotrophic). In either case, when the new ani- 

 mal first begins to feed it is usually orders of 

 magnitude smaller than the eventual adult size. Its 

 trophic status is likely to be entirely different 

 from that of the adult or even the juvenile. 



A progression of trophic changes through 

 development as a function of increasing body size 

 is common among marine animals. For instance, 

 many predatory fishes of estuarine systems pass 

 first through a planktivorous larval stage and then 

 a dclritivorous juvenile stage before becoming 

 predaceous as adults. Even as adults, estuarine 

 predators often switch to detritus as a supplemen- 



tary food source when prey are scarce (Odum 

 1970a). Consequently, trophic relationships in 

 estuaries and lagoons tend to be dependent upon 

 available alternative food supplies as well as upon 

 stages in the life cycles. Since most larval fishes 

 and crustaceans in estuaries depend upon eating 

 zooplankton during early weeks, the zooplankton 

 of an estarine system and the phytoplankton 

 upon which most of them feed are far more signi- 

 ficant than their small numbers may suggest 

 (Odum 1970a). In North Carolina sounds, Will- 

 iams et al. (1968) found relatively low zoo- 

 plankton abundances and suggested that the high 

 population levels of larval fishes may be partly 

 responsible. 



Another commonly recognized feature of es- 

 tuarine food chains is their narrow base. Very few 

 plant species are major contributors to the pool 

 (jf organic detritus which fuels estuarine food 

 webs. A wide diversity of consumer organisms is 

 dependent upon the relatively simple base of pri- 

 mary production. This characteristic helps to dis- 

 tinguish estuarine food webs from those of terres- 

 trial ecosystems where diversity is usually highest 

 at lower trophic levels. In addition, within the 

 system energy first fixed (i.e., transformed from 

 light energy to chemical energy) by a single plant 

 can flow up the food chain in several possible 

 ways with several possible trophic end-pt)ints. The 

 discussion of the fauna of intertidal flats will fo- 

 cus upon trophic relationships to illustrate the 

 variety of top predators which exist al the ends of 

 largely detrital-based food chains on the mud and 

 sand flats of estuarine systems. 



18 



