The monads are so tiny (ca. 1-2 microns 

 diameter) that identification is challenging; 

 they appear to belong to the Cyanophyta (P. 

 Fong, SDSL), pers. comm.). Many of the 

 phytoplankton species also occur in nearshore 

 habitats, where their densities are lower. 

 From March to June, there may be 

 phytoplankton blooms in the estuary, with 

 cell counts higher than in marine waters by 

 one to two orders of magnitude. 



Both Rudnicki (1986) and Fong (1986) 

 associated algal blooms with reduced tidal 

 flushing. The air photo of March 1984 

 (Figure 2.10) showed high biomass of 

 macroalgae in tidal creeks and along the 

 shores of the abandoned sewage lagoons. 

 Because the 1983 winter storm washed dune 

 sands into the main estuarine channel, tidal 

 flushing became sluggish and algal biomass 

 accumulated. Likewise, algal growth was high 

 during the nontidal period of 1984. Channel 

 waters were green with phytoplankton during 

 the summer (Chapter 4). During the 1985 

 monthly censuses of channel algae, Rudnicki 

 and Fong found the highest biomass of both 

 macroalgae and phytoplankton in small tidal 

 creeks where current speeds were low. 



3.6.2 Benthic Invertebrates 



Studies of invertebrates at Tijuana 

 Estuary have included resource inventories 

 and short-term or species-specific 

 investigations. Several investigators (Bybee 

 1969; Mclllwee 1970; Ford et al. 1971; 

 Smith 1974; Peterson 1975; International 

 Border Water Commission 1976; D. Dexter, 

 SDSU, unpubl. data; and White and 

 Wunderlich, unpubl.) depicted the 

 invertebrate community prior to flooding in 

 1978. More than 75 species of inverte- 

 brates, primarily benthic forms, were 

 identified during this period (Table 3.5). 



Additional studies documented the effect of 

 the 1980 flood (Rehse 1981) and the nontidal 

 period of 1984 (Griswold 1985). Griswold's 

 observations and continuing monitoring by 

 Dexter documented recolonization in 1985 by 

 invertebrate species that died during the 

 1984 warm hypersaline conditions. In early 



1986, however, most of the recolonists died 

 out again under conditions of continual spills 

 of raw sewage from Mexico. The present 

 benthic macrofaunal community is very 

 different from that described in the 1970's. 



Before 1980, the benthic community was 

 dominated by bivalve molluscs, especially the 

 purple clam (Sanquinolaria nuttalli), 

 littleneck clam (Protothaca staminea), false 

 mya (Cryptomya californica), California 

 jackknife clam (Tagelus californianus), and 

 bent-nose clam (Macoma nasuta). Polychaete 

 worms, gastropod molluscs, and decapod 

 crustaceans were also numerically important 

 (Figure 3.20). Bivalve populations were 

 sufficiently dense that hundreds of 

 recreational clammers often took their limit 

 on weekends (Mclllwee 1970). In addition, 

 ghost shrimp (Callianassa californiensis) 

 were hydraulically harvested as bait (Bybee 

 1969). 



The distributions of infauna are strongly 

 influenced by sediment type. Most larval 

 settlement is limited by substrate availability 

 and adult distributions are influenced by grain 

 size, pH, total organics, organic carbon and 

 nitrogen, and dissolved oxygen. Filter 

 feeders, such as the dominant bivalves, are 

 associated with medium-sized grains because 

 finer sediments contain too little organic 

 material in suspension and because coarser 

 sediments are too unstable. Finer sediments 

 may also act to clog the filtering mechanisms. 

 Deposit feeders, such as ghost shrimp and 

 certain polychaetes, often occur on finer 

 sediments, which have higher concentrations 

 of organic carbon and nitrogen, but less 

 dissolved oxygen and lower pH. 



Hosmer (1977) found that distributions 

 were correlated with sediment type for six 

 species of bivalve molluscs that were common 

 at Tijuana Estuary. Overall, there was a 

 significant decrease in biomass of molluscs 

 with smaller grain size, and the largest 

 individuals were found in coarse, well-sorted 

 sediments. Individual species exhibited a 

 variety of patterns. The remaining nine 

 species of bivalve molluscs found by Hosmer 

 (1977) were not encountered frequently 

 enough to characterize their distribution 

 patterns. 



47 



