The Ecology of Tijuana Estuary, California: An Estuarine Profile

Figure 37. Channel producers include a phytoplankton community (on left) that includes blue-green algal filaments,
dinoflagellates, such as Gymnodinium, and diatoms, such as the sigmoid Pleurosigma and Gyrosigma, and many
species of Navicula. In addition, there is often an attached or floating mat of macroalgae (such as Ulva, shown on
the right). Mclntire collection, copyright 1986 by Zedler.
In general, the distributions of infauna are Purple clams were present ihroughout a wide
primarily influenced by sediment type. Most larval range of sediments. Higher biomass and densities
settlement is limited by substrate availability and were recorded in coarse sand, decreasing with
adult distr~butions are influenced by grain size, pH, finer sediments. This species was absent from
total organics, organic carbon and nitrogen, and sediments with high silt/clay contents. Littleneck
dissolved oxygen. Filter feeders, such as the clams occurred in very coarse to fine sediments
dominant bivalves, are associated with medium- with highest densities in finer sediments. Maximum
sized grains because finer sediments contain too biomass and density were found in 15%-2O0I0
little organic material in suspension and because silt/clay. False mya occurred in all sediment types
coarser sediments are too unstable. Finer from very coarse to very fine sand. Biomass and
sediments may also act to clog the filtering densities were bimodal, with highest values at
mechanisms. Deposit feeders, such as ghost around 35% silt/clay. California jackknife clams
shrimp and certain polychaetes, often occur on were collected in medium to fine sand. Biomass
finer sediments, which have higher concentrations and density increased with a decrease in sediment
of organic carbon and nitrogen, but less dissolved size. Biomass was highest at 5%-10% silt/clay;
oxygen and lower pH.
density peaked at 20°/o-250~~ silt/cfay; and size
tended to decrease with higher silt/clay
Hosmer (1977) found that distributions were percentages. The bent-nose clam was found in
correlated with sediment type for six species of sediments ranging from coarse sand to very fine
bivalve molluscs that were common at Tijuana sand. Biomass and density increased with a
Estuary. Overall, there was a significant decrease decrease in sediment size. Density and biomass
'
biomass of molluscs with smaller grain size, and were optimum af about 15% silt/cfay. The white
e largest individuats were foind in coarse, well- sand clam (Macoma secta) was found only in
rted sediments. Individual species exhibited a medium to fine sandy sediments and had the
variety of patterns. The remaining nine species of narrowest grain size distributional range. Both
bivalve molluscs found by Hosmer (1977) were not biomass and size of white sand clams were fairly
encountered frequently enough to characterize constant with changes in grain size, although
their distribution patterns.
densrty was higher in medium sand.
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