2.4 ENERGY FLOW AND FOOD WEB RELATIONSHIPS 



Organic materials in marine ecosys- 

 terris are channeled through two types of 

 food webs: one based on grazing, which 

 starts with the utilization of the pro- 

 ducts of primary production; and another 

 based on the consumption of detrital pate- 

 rial and associated microbial populations. 

 While these two food webs exist in tidal 

 flat habitats, they are not well-defined. 

 The trophic structure of New England tidal 

 flats includes a number of primary food 

 types and an intricately connected food 

 web of generalized feeders. Many organisms 

 interact and feed at different trophic 

 levels at the same time and are able to 

 utilize both living plant and detrital 

 materials. Also, many tidal flat organisms 

 change their trophic status with increas- 

 ing size. Nost fish, for example, begin 

 their lives as planktivores, pass through 

 a detritus-feeding stage, and finally 

 become predaceous as adults. 



Because detrital material is so 

 conspicuous in the guts of many species 

 associated with tidal flats (Whitlatch 

 1S76; Tenore 1977), food webs in these 

 habitats are considered to be detrital ly 

 driven. The grazing food web apparently 

 contributes less to tidal flat energy. One 

 of the more striking examples of the lack 

 of utilization of the products of primiary 

 production is the scarcity of organisms 

 feeding on Ul va and Enteromorpha. While 

 these microphytes may densely carpet por- 

 tions of New England tidal flats, only a 

 few species (e.g., the snail, Littorina , 

 nereid polychaetes, some gammaridean 

 anphipods, and birds) feed upon them 

 directly. Occasionally dense populations 

 of birds or snails deplete these macro- 

 phytes locally, but probably 90% to 95% 

 are consumed after death and entry into 

 the detrital food web (Mann 1972). Grazing 

 on microalcae by herbivorous snails and 

 some tube-dwelling amphipods is more 

 common although to what extent these 

 organisms rely exclusively upon the micro- 

 algae as food has yet to be determined. 



Although detritus appears to be the 

 major food source of n.any tidal flat or- 

 ganisms, there are uncertainties regarding 

 exactly what fractions of the detrital 

 materials are utilized by detritivorcs. 



The microbial portion (the "living" frac- 

 tion) of the detrital particle is easier 

 to digest and is more nutritious than the 

 structural ("non-living") portion. Fungi, 

 bacteria, and protozoans associated with 

 detrital particles are efficiently removed 

 by detritivores (Fenchel 1972; Hylleberg 

 1975; Lopez and Levinton 1978), and stud- 

 ies have shown that these living materials 

 are more easily digested than the non- 

 living fraction (Kofoed 1975; Wetzel 

 1977). When comparing the ingestion rates 

 of various detritivores, Cammen et al. 

 (1978) found that the microbial portion of 

 detritus accounted for only about 10% of 

 their metabolic demands. This apparent 

 contradiction suggests some possibilities 

 about the importance of the living versus 

 the non-living fractions of detritus to 

 detritivores. First, detritivores may be 

 able to derive most of their nutrition 

 from the non-living fraction. Second, 

 energy obtained from other sources, such 

 as dissolved organic materials or small 

 meiofaunal organisms (see section 3.3) may 

 figure significantly in a detritivore's 

 nutritional requirements. Last, organisms 

 may be selectively feeding on the living 

 portion of the detrital particle. Selec- 

 tivity for high organic food items has 

 been shown in several species of detriti- 

 vores (e.g., Whitlatch 1974; Connor 1980) 

 and selective ingestion of microbial ly- 

 enriched fecal material (termed coproph- 

 agy) is common (Johannes and Satomi 1966; 

 Frankenberg and Smith 1967). While more 

 information is needed to test the various 

 alternative explanations, it is becoming 

 increasingly apparent that inshore detri- 

 tal food web dynamics are more complex 

 than previously considered. 



Many ecologists believe that tidal 

 flat ecosystems are "energy subsidized", 

 iving the bulk of their energy from 

 salt n,arshes, seagrass 

 estuaries, and 



adjacent salt n,arshes, seagrass beds, 

 estuaries, and coastal waters as detrital 

 carbon. It has been difficult in actual 

 practice to assign a relative importance 



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