about the relative importance of various types of 

 primary producers in the nutrition of the animal. 

 Differential rates of digestion of those food 

 sources which are actually ingested cause further 

 difficulty in the interpretation of data on gut con- 

 tents (Peterson and Bradley 1978). The most 

 quickly digested items may be greatly diminished 

 or even absent from most gut contents. 



A recent technique has been developed which 

 has the potential to circumvent all of these prob- 

 lems with interpretation of gut content data. Two 

 different stable carbon isotopes exist in nature, 

 '^C and '^C. The ratio of these two isotopes (the 

 so-called 5 '^C ratio) is constant in the atmosphere. 

 However, photosynthesis does not draw its CO2 

 randomly and can enrich the photosynthate (the 

 plant's carbohydrates) in one or the other carbon 

 isotope. Plants which utilize the Hatch-Slack (C4) 

 pathway of photosynthesis have a characteristic 

 5 '^C ratio that differs greatly from the ratio incor- 

 porated by plants which utilize the Calvin (C3) 

 pathway. Grasses such as Sparti7ia are C4 plants, 

 while most other vascular plants are C3 plants. 

 Benthic diatoms have predictably and consistently 

 intermediate S "C ratios (Haines 1976a, b, Thayer 

 et al. 1978). Distinctions can thus be drawn 

 among the major types of primary producers in 

 the estuary. 



Haines and Montague (1979) have done 

 feeding experiments in the laboratory to demon- 

 strate that animals which consume plant material 

 incorporate a 6 '^C ratio that reflects that of their 

 food. This appears to be true even if the plant de- 

 tritus passes first through a microbial interme- 

 diate (Haines 1977, Haines and Montague 1979). 

 As a result, the relative importance of certain 

 major types of plants (marsh grass vs. seagrass vs. 

 algae) in the diet of a detritivore can now be in- 

 ferred by analyzing the detritivore 's 5'^C ratio. 

 This technique has a tremendous advantage over 

 using gut content information in that it provides 

 an integration of the animal's diet over quite a 

 long (but undefined) period of time instead of 

 yielding just an instantaneous picture of the most 

 recent meal. Some results of this work are avail- 

 able (Haines and Montague 1979) which tend to 

 contradict previous assumptions about the impor- 

 tance of marsh plant productivity to some of the 

 detritivores that dominate the fauna of an estu- 

 arine system. The results reveal that algae (phyto- 

 plankton and benthic microalgae combined) are 



far more important than expected in the nutrition 

 of consumers in estuarine systems. Haines (1977) 

 has also shown that the detrital pool of particles 

 available for breakdown and subsequent incorpo- 

 ration by consumers in a Georgia estuary is largely 

 derived from algal sources, not from marsh grasses. 

 If it is true, however, that phytoplankton and 

 benthic microalgae are more important producers 

 of utilizable detritus than are the highly produc- 

 tive marsh macrophytes, a major question remains 

 unanswered. Where does all of that marsh produc- 

 tivity go? Research is necessary to resolve this 

 issue. 



In estuarine systems of North Carolina, Vir- 

 ginia, and Maryland, and, to a lesser degree, else- 

 where along the east coast of the United States, 

 it is clear that the areal extent of the phytoplank- 

 ton habitat is often far greater than the areal ex- 

 tent of the marshes and seagrass beds. Estuarine 

 systems with large expanses of open water, such 

 as are found in North Carolina, would be expected 

 to support high phytoplankton production. 

 Bigelow (1977) has constructed a summary table 

 (Table 1) for the Newport River estuary in North 

 Carolina, which presents the available data on the 

 productivity of each major type of primary pro- 

 ducer. In this table he also lists the areal coverage 

 of each habitat and from these data calculates the 

 relative importance of each type of plant in the 

 total production of the entire estuary. Phyto- 

 plankton account for 49% of this estuary's total 

 productivity, Spartina alterniflora 42%, while 

 benthic microalgae contribute only 7.4%, and 

 Zostera only 1.4% to the total. Bigelow had no 

 data on the production of benthic macrophytes 

 like Ulva, Enteromorpha, and Ectocarpus, so 

 these plants are not included in these percentages. 



There is more suitable phytoplankton habitat 

 and often also more suitable habitat for benthic 

 microalgae than there is marsh area in North 

 Carolina estuaries. Thus, even though the per- 

 unit-area productivity of phytoplankton and ben- 

 thic microalgae may not be as great as the areal 

 productivity of Spartina, the total amount of 

 microalgae produced in the estuary may be 

 higher. A large proportion of this estuarine phyto- 

 plankton production and essentially all of the 

 benthic microalgal production is transformed into 

 benthic invertebrates on intertidal and shallow 

 subtidal flats. The benthic microalgae are prob- 

 ably far more important in this process than their 



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