Despite tliis prevailing concept tliat phytcj- 

 plankton contribute an insignificant fraction of 

 total carbon to the estuaries and lagoons, more 

 recent data provide support for a different view- 

 point. Sellncr and Zingmark (1976) found phyto- 

 plankton production as highas350g carbon/m7>'i' 

 in shallow tidal creeks and estuaries of South 

 Carolina. Haines (197 7) has demonstrated that 

 the majority of the dctrital pool in a Georgia estu- 

 ary has a ratio of stable carbon isotopes (termed 

 the ft '^ ratio) appropriate for either phytoplank- 

 ton or benthic microalgac and not for Spartina, 

 Juncus, and scagrasses. This suggests that phyto- 

 plankton and benthic microalgal production in 

 estuarine systems is far greater than past measure- 

 ments indicate. Further research is necessary to 

 resolve this issue. In North Carolina's soimds, and 

 even in its estuaries, the summer turbidity tends 

 to be low, suggesting that phytoplankton produc- 

 tion coidd often be significant. 



Very little work has been done to measure in 

 situ primary production of benthic algae on inter- 

 tidal sand and mud flats. There are no estimates 

 of macrophvte productivity from these habitat 

 types, despite the obvious abundance of macro- 

 phytes like Ulva, Enteromorpha, and Ectocarpus. 

 Some estimates are available, however, for the 

 benthic microalgac. Pomeroy (1959) measured 

 microalgal productivity throughout the year on an 

 intertidal mud flat in Georgia. He found that 

 annual gross producli\ily of benthic microalgac 

 was about 20()g carbon/m'^/yr. Net production 

 (that quantity measiued in macrophyte studies) is 

 at least 90% of this figure. Pomeroy (1959) 

 observed that benthic |)rimary productivity 

 remained nearly constant year-round on this mud 

 Hat. In sinnmer, |)roducti\ity was greatest at high 

 tide, whereas in winter the algae were more pro- 

 ductive when the tide was out. Four other studies 

 have measured benthic diatom production on tidal 

 flats year-round. Pamatmat (1968) foimd micro- 

 algal productivity on an intertidal sand flat in 

 False Bay, Washington, to be about the same as 

 measured on flats in the Danish Waddcn Sea 

 (Gr^ntved 1960), namely about 116 to 178g 

 carbon/m^/yr. In intertidal flats of a southern 

 California lagoon, microalgal productivity was 

 estimated to be about 200g carbon/mVy (Onuf 

 et al. 1980). Leach (1970) found microalgal pro- 

 ductivity to be 31g carbon/m^/yr in the Ythan 

 estuary in Scotland at latitude 57° N. Since lati- 

 tude seems to cxi^lain much of the variability in 

 these observations (lower productivity at high lat- 



itudes where sunlight is more limited), the inter- 

 tidal Hats in North Carolina probably produce 

 close to 200g carbon/m^/yr. The only available 

 value for a North Carolina microalgal community 

 (Bigelow 1977) is far lower (40g carbon/mVyr), 

 but that figure comes from a 6-month study of 

 the Newport River esttuuy where turbidity is 

 [n'obably greater than in sounds and is probably 

 not representative of North Carolina's intertidal 

 flats in general. 



2.6 FOOD CHAINS OF INTFRTIDAL FLATS 



The entire estuarine ecosystem is commonly 

 viewed as a detritus-based system in which the 

 vast majority of consiuner tood chains is based at 

 the bottom level upon the consumption of detri- 

 tus and its associated microflora. (See Nixon and 

 Oviatt 1973 for an excellent and detailed analysis 

 of energy flow in a New England estuary.) This 

 viewpoint is supported by the numerous obser- 

 vations on (1) marshes and marine grass beds 

 which demonstrate little iji situ herbivory despite 

 very high productivity, and (2) gut contents of 

 consumer organisms which are frequently domin- 

 ated by dctrital particles (Teal 1962,Tenore 1977). 

 Classic studies of the Georgia estuaries, where 

 marshes cover a large proportion of the total estu- 

 arine acreage, have emphasized the tremendous 

 importance of marsh plant detritus in the nutrition 

 of the large majority oi shrimp, crabs, and fishes 

 of the estuarine ecosystem (Teal 1962,OdLun and 

 de laCru/. 1967). 



Despite such conclusions about the impor- 

 tance of detritus in the energy tlowot estuarme 

 systems and in the fueling of the consumer food 

 chains, it has remained difficult to confirm the 

 quantitative importance of detritus and its associ- 

 ated microllora in the nutrition of any given 

 species. Gut contents are not especially useful for 

 determining the diets of detritivores and other 

 consiuTiers low in food chains because oi ditficid- 

 ties in interpretation of such information. Often 

 the gut contents cannt)t be identified because of 

 their advanced state of decomposition. Even the 

 basic distinction between marsh plant and diatom 

 or seagrass detritus is usually impossible. Further- 

 more, what is found in the gut is not necessarily 

 what is being digested and assimilated. The major- 

 ity of the gut contents of a detritivore will usually 

 l)ass through undigested. As a result, even the 

 accurate identification of the source of the detri- 

 tus in guts does not permit reasonable inferences 



16 



