Other evidence that herbivores consume 

 large fractions of live plants comes from 

 wetland restoration studies, wherein new 

 cordgrass transplants are often heavily grazed 

 by ground squirrels and rabbits (Zedler 

 1984a), as well as by insects (Johnson 

 1991). Why transplanted marsh show more 

 evidence of mammalian grazing than natural 

 marshes is unclear. The fact remains that 

 live cordgrass can be highly palatable to 

 marsh consumers. 



Additional evidence for in situ 

 consumption of estuarine productivity comes 

 from studies of invertebrate growth rates. 

 Williams (1981) examined growth rates of 

 mussels (Mytilus edulis) at Tijuana Estuary 

 for comparison with laboratory-reared 

 individuals fed diets of cordgrass and 

 pickleweed detritus. In the laboratory, 

 mussels lost weight when fed freshly made 

 detritus but grew slightly when fed aged 

 detritus. The highest growth rates were found 

 with mussels grown in the dredged channel at 

 Tijuana Estuary. Williams (1981) concluded 

 that bacteria and phytoplankton are more 

 important in the funnelling of estuarine 

 productivity to benthic consumers than 

 detritus from vascular plants. It remains to 

 be demonstrated how much the bacteria or 

 phytoplankton use dissolved organic carbon 

 that has been fixed by, and later leached from, 

 marsh vegetation. 



4.5.3 Carbon Fluxes 



At the same time that Winfield (1980) 

 studied nitrogen fluxes in tidal creeks, he 

 sampled organic carbon to determine the net 

 flux of different components: dissolved organic 

 carbon (DOC) and particulate organic carbon 

 (POC). Because POC is the sum of three 

 major components, live phytoplankton, other 

 live plankton, and dead particles, Winfield 

 identified these analytically. Live biomass 

 was determined from measurements of ATP 

 (total live) and the algal portion was 

 calculated from chlorophyll a measurements, 

 so that other plankton could be estimated by 

 subtraction from totals. Dead biomass was 

 calculated by subtracting the live fraction 

 from total POC. The difficult but precise 



analyses were necessary to quantify tidal 

 water composition. Over 1 ,850 samples were 

 processed for carbon analysis in the 2-year 

 study (Winfield 1980). 



Most studies of carbon flux ignore the DOC 

 component. Yet at Tijuana Estuary, this was 

 the major form of carbon export (Figure 

 4.8). Concentrations of DOC in ebb tide water 

 often exceeded concentrations in flood tide 

 water (Figure 4.9). Furthermore, 



concentrations of C in the dissolved form 

 averaged much higher than in the particulate 

 form (Figure 4.10). DOC ranged from 1 to 

 11 mg C/l, while POC ranged from 0.4 to 1.8 

 mg C/l of creek water. Thus, the ultimate 

 removal of organic materials produced in the 

 marsh results from processes such as 

 leaching of DOC from both live and dead plants 

 and animals and excretion of organic 

 molecules by plants and animals. 



Detrital carbon dominated the particulate 

 component in tidal waters, sometimes making 

 up 98% of the POC, and never less than 36% 

 (Winfield 1980). Live organisms made up 

 the majority of the particulate matter only in 

 June 1977 and April 1978. Both times the 

 live POC was largely algae, as indicated by 

 chlorophyll concentrations. In contrast with 

 DOC results, the POC data suggested that the 

 salt marsh entrains particulate materials, 

 although this is the net result of frequent 

 import and occasional export. 



What fraction of marsh productivity is 

 exported to tidal creeks? Based on his 10- 

 month study period, Winfield (1980) 

 estimated an annual export of 40 g C/m 2 /yr 

 as DOC for the area dominated by succulents 

 and 110 g C/m 2 /yr for the area of mixed 

 cordgrass and succulents. A net import of 

 particulate carbon was determined for POC, 

 with annual estimates at 5-6 g C/m 2 /yr for 

 the areas included in his two sampling 

 stations. At most, then, there was a net 

 removal of 105 g C/m 2 /yr, which is well 

 below the net amount produced by vascular 

 plants (approximately 220 g C/m 2 /yr; 

 Section 4.3) and epibenthic algae (185-340 

 g C/m 2 /yr). Additional fluxes of debris 

 probably occurred on the water surface and as 

 wind-borne materials; neither was measured 



94 



