u.E/m 2 /s for temperatures ranging from 15 

 °C to 33 °C). Chlorophyll concentrations 

 were significantly higher for algae grown in 

 full sunlight, but rates of photosynthesis per 

 unit chlorophyll were not much greater for 

 this winter community. 



Only the summer community responded as 

 expected. The rates of photosynthesis for mats 

 of blue-green algae and diatoms increased 

 substantially with increased light (saturation 

 at 900 u.Einsteins/m 2 /s for temperatures of 

 20 °C to 33 °C). Under conditions of high 

 light, summer algal mats are thus limited by 

 the shade of salt marsh vegetation. With light 

 saturation and ambient temperatures, the 

 summer algal mats are twice as productive as 

 the winter community. The revised 

 hypothesis is that canopy shade is a major 

 limiting factor only for the summer 

 community of epibenthic algae, which 

 dominates the annual productivity. The 

 relatively open vegetation canopies thus allow 

 algae to contribute substantially to salt marsh 

 productivity. 



The Enteromorpha-6om\na\ed algal mats 

 declined in summer, and increasing light 

 availability appeared to limit the duration of 

 the winter community indirectly. Kentula 

 noted that green algae began to decline in late 

 spring, but they did so more rapidly in open 

 areas than under the cordgrass canopy. 

 Higher temperatures and desiccation were 

 likely causes. This suggestion is consistent 

 with Rudnicki's (1986) finding that 

 Enteromorpha declines in warm water and 

 helps explain the predominance of epibenthic 

 green algae only in winter at the Tijuana 

 Estuary salt marsh (Zedler 1982a). In 

 marshes at both Mission Bay and Tijuana 

 Estuary, mats with blue-green algae and 

 diatoms were dominant in summer. 



Subsequent laboratory experiments using 

 shallow-water microcosms were conducted by 

 Fong (1991). Her findings further support 

 the above conclusions. High light (500 

 u.E/m 2 /s) and high temperatures (25° C) 

 favored blue-green algae and phytoplankton, 

 while lower light (150 u.E/m 2 /s) and lower 



temperature (12-20° C) stimulated the 

 green macroalgae. 



4 . 3 VASCULAR PLANT PRODUCTIVITY 

 AND BIOMASS 



The productivity of many salt marshes is 

 high among natural plant communities, 

 rivaling energy-subsidized agriculture in 

 tons of carbon fixed per unit area per year. 

 This generalization originated from studies in 

 Georgia, where smooth cordgrass (Spartina 

 alterniflora) dominates the intertidal zone. 

 There, the widespread occurrence of 

 cordgrass, the high rainfall and long growing 

 season, the abundance of nutrients, and good 

 tidal flushing are all reasons for productivity 

 rates in excess of 3-5 kg/m 2 /yr (Odum 

 1971; Pomeroy and Wiegert 1981; Chalmers 

 1982). 



In Tijuana Estuary and most California 

 estuaries, cordgrass has restricted areas of 

 dominance. In addition, the summer droughts 

 lead to hypersaline soils. Seneca and Blum 

 (1984) demonstrated that the Pacific 

 cordgrass (S. foliosa) has lower 

 photosynthetic potential than its eastern 

 counterpart when grown and measured in the 

 laboratory. Substantial doubt that high 

 productivity would be found in southern 

 California salt marshes led Winfield (1980) 

 and Onuf (1987, Onuf et al. 1978) to assess 

 plant productivity in detail. 



Winfield's work at Tijuana Estuary 

 supported the hypothesis that the marshes of 

 the region are less productive of vascular 

 plant material than those in Georgia and 

 elsewhere along the Atlantic and Gulf of 

 Mexico Coasts. His use of the harvest method 

 and Smalley's (1959) calculation, which 

 sums biomass peaks of individual species, 

 suggested that annual rates were well below 1 

 kg/m 2 /yr net above-ground dry weight 

 (Table 4.4). Winfield measured very low 

 carbon content, however, indicating that ash 

 (i.e., salts and metals) contributed much to 

 the oven-dry weights. The carbon content of 

 11 plant species ranged from 19%-35% of 

 dry weight (Winfield 1980), compared to 



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