species composition of the bloom differed in 

 low-salinity treatments. Blue-greens 



dominated at 10 ppt, just as they had 

 dominated the stagnant channels in April-June 

 1984 when brackish water (12-15 ppt) was 

 impounded. Enteromorpha grew best at 33 

 ppt. It survived 10 ppt best where nutrient 

 concentrations were high. 



The changes in channel algae at Tijuana 

 Estuary are well explained by the 

 experimental results (Table 4.2). Nutrient 

 additions maximize biomass of both primary 

 producer components, but current speeds and 

 temperature have differential effects. Thus, 

 in years of good tidal flushing and fewer 

 sewage spills (i.e., through the 1970's), 

 neither type of algae was likely to develop 

 bloom conditions because of continual dilution 

 and export. With more frequent and larger 

 sewage spills, blooms became possible. When 

 currents became sluggish following the 1983 

 winter storm, macroalgae were able to 

 accumulate in the nutrient-rich, quiet waters 

 of the estuary, but phytoplankton were still 

 prevented from developing high densities. 

 Then, with no tidal flushing in 1984, 

 phytoplankton densities reached nuisance 

 proportions, while warm water, 

 hypersalinity (60 ppt), and possibly 

 competition for nutrients and light limitation 

 at the channel bottom, reduced establishment 

 and growth of Enteromorpha. Further 

 refinement of this conceptual model of algal 

 dynamics is in progress, with experiments 

 varying salinity, nitrogen, and phosphorus 

 concentrations planned for I986. 



4 . 2 PRODUCTIVITY OF EPIBENTHIC 

 ALGAL MATS 



Epibenthic algal mats are characteristic of 

 the salt marsh, although the community is 

 frequently supplemented by species entrained 

 from the channels. For example, 



Enteromorpha extends into the lower salt 

 marsh in winter and spring. Likewise, some 

 salt marsh algae extend into the channels. 

 Filamentous blue-green algae develop patches 

 in the tidal creeks during summer. Thus, 



there is an overlap in species composition. 

 The salt marsh algae comprise a matrix or 

 mat of filaments and associated unicells that is 

 usually bound to the substrate. The algae that 

 float in channels are usually the fronds of one 

 or two green macroalgae. 



Dense algal mats are often found beneath 

 the canopy of salt marsh vegetation. Biweekly 

 field measurements in 1977 suggested that 

 their annual productivity could match or 

 exceed that of the vascular plants (Table 4.3). 

 The highest productivity rates were measured 

 in the most open canopies of frequently 

 inundated areas during the warm season. The 

 lowest rates were measured in winter in areas 

 where moisture was limiting. In 1977, the 

 lowest productivity occurred in April when 

 there was little rainfall and neap tides did not 

 inundate the higher marsh. A hypothesis was 

 developed (Zedler 1980, 1982b) that the 

 saline soils reduced vascular plant biomass, 

 thereby increasing sunlight availability and 

 algal productivity on the soil surface. 



Kentula (unpubl. data) tested the model of 

 algal-vascular plant interactions at Mission 

 Bay Marsh during 1984, when Tijuana 

 Estuary was nontidal and too dry for soil algae 

 to grow. Her focus was on the cordgrass- 

 dominated lower marsh; she manipulated light 

 levels in field experiments (10%, 25%, 

 50%, and 100% of incident light). After 2 or 

 more weeks of exposure to the light 

 conditions, replicate cores of algae were taken 

 to the laboratory, and treatment effects were 

 assessed with measures of chlorophyll 

 concentration and photosynthetic light- 

 response curves. Her experiments were 

 repeated in winter and spring, when 

 Enteromorpha dominated the algal mats, and in 

 summer, when blue-green algae were 

 abundant. The composition of the algal mat 

 communities was similar to those at Tijuana 

 Estuary during periods of good tidal flushing. 



Kentula's results suggest that the 

 hypothesis of Zedler (1980, 1982b) is too 

 simple. Winter algal communities failed to 

 increase productivity in proportion to 

 increased light availability. Algal mats 

 quickly became light-saturated (at 500-600 



82 



