shellfish productivity. (4) Estuarine 

 food webs are highly complex; many species 

 are generalistic feeders which can 

 function as herbivores, carnivores, and 

 detritivores. 



At the same time that Haines was 

 challenging the Georgia model of wetland 

 functioning, the California Sea Grant 

 Program provided funds to investigate 

 Tijuana Estuary and Mugu Lagoon. Both 

 studies sought to determine if southern 

 California wetlands conform to the export 

 model originally ascribed to Georgia 

 marshes. Data obtained by Winfield (1980) 

 for Tijuana Estuary included frequent 

 measurements of POC, DOC, and inorganic 

 forms of nitrogen (IN). Onuf's work at 

 Mugu Lagoon (Onuf et al. 1978) included 

 measurements of POC over selected tide 

 cycles in a two-year period. 



At Tijuana Estuary the results 

 contrast with the outwelling model of Teal 

 (1962) and Odum (1971). Inorganic 

 nitrogen was imported from small tidal 

 creeks onto the salt marsh, with most of 

 the flux occurring as imported ammonia. 

 Organic carbon was exported, but in the 

 dissolved form, not as particulates. 

 During periods of flooding witnessed in 

 1978 there appeared to be a measurable 

 export of both POC and DOC, corresponding 

 to the net outflow of water. Yearly 

 fluxes were compared with estimated 

 organic carbon production and nitrogen 

 uptake to determine the relative magnitude 

 of C and N movements. Winfield (1980) 

 suggests that about 6% of the nitrogen 

 required for vascular plant and algal 

 productivity is met by imported nitrogen. 

 Hence, most of the nitrogen must be 

 generated by processes within the marsh, 

 e.g. nitrogen fixation and 

 remineralization. The amount of organic 

 carbon exported was a small fraction (at 

 most 25%) of that produced on the marsh 

 (Winfield 1980). 



The results of Onuf et al. (1978) 

 concur that there is little evidence 

 to support the concept of major 

 POC export in southern California 

 wetlands. In both Winfield 's and Onuf 

 et al.'s studies, however, large pieces 



of floating matter, such as make up the 

 debris deposits at high water mark, were 

 not measured. From observations of the 

 wood and seaweed deposits sometimes seen 

 within coastal marshes, it is clear that 

 some fluxes are missed by studies which 

 are restricted to the water column. 

 Still, these larger materials may be 

 refractory to utilization by 

 mircoorganisms. As Pomeroy et al. (1976) 

 point out, we need much more study of both 

 the quality and quantity of POC and DOC in 

 marsh ecosystems. 



The combined work on nitrogen and 

 organic carbon flux suggests a model of 

 estuary-coastal interaction which bears 

 some similarity to Haines' (1979) 

 alternative hypothesis for the Georgia 

 marshes (Figure 37). However, there is 

 one important difference. Most of the 

 East Coast marshes are next to large bays, 

 and aquatic habitats are more important. 

 Except for San Diego Bay and the 

 artificially enlarged bays of Mission Bay 

 and Los Angeles Harbor, southern 

 California lacks these coastal embayments. 



In the model, flood tides provide a 

 source of nutrients for algae, soils and 

 vascular plants; ebb tides leach dissolved 

 organic carbon from the microbial mats, 

 litter and standing vegetation. At all 

 times, microorganisms and invertebrate 

 larvae incorporate dissolved organic 

 carbon into biomass, which is then 

 available to larger consumers. 

 Decomposition returns nutrients to the 

 water, which then floods the marsh again. 



In this scheme, microorganisms may 

 well be the principal transformers of 

 inorganic to organic materials. By virtue 

 of the rapid growth rates, high surface 

 area-to-volume ratios, and thin cell 

 walls, the algae, bacteria and fungi are 

 capable of rapid exchange rates. The 

 algal mats may be the most important 

 nutrient absorbers, as well as the most 

 important source of dissolved organic 

 carbon. The rate at which inorganic 

 nitrogen is absorbed and DOC is leached 

 from these algal mats is currently being 

 investigated (Pat Dunn, graduate student, 

 San Diego State University 



54 



