The Ecology of Tijuana Estuary, California: An Estuarine Profile

Of the five habitats sampled, phytoplankton
were most dense in the tidal creek that had the
lowest tidal circulation. Seasonal peaks in
chlorophyll and cell counts occurred in spring
when weather was warm and tidal action minimal.
Together, the 1984 observations of blooms during
closure and the 1985 field study suggested that
phytoplankton accumulate when water currents are
reduced and when nutrients are plentiful.
Macroalgal growth patterns were more variable.
During 1985, the cover of floating macroalgae
never reached nuisance levels, i.e., the water
column did not become anoxic and there were no
fish kills. Maximum standing crops averaged only
15 g/m2 (n = 12), with a range from 0-185.
Highest values tended to occur in the inland
lagoon. This does not necessarily mean that
productivity was highest there, because
macroalgae are moved by the prevailing westerly
winds. Rudnicki determined that several factors
act together to increase macroalgal biomass in
estuarine channels. Algal establishment coincided
with daytime low tides, which occur primarily in
winter. Germination mainly occurred along the
shallow edge of creeks and channels where
current speeds and light levels were suitable.
Following establishment, growth was stimulated by
nutrient inputs from rain and runoff and by
afternoon exposure. When the cold tidal water
receded, algae became warmer and received more
light. However, macroalgae biomass did not
necessarily accumulate in all areas of high
productivity. Maximum volumes of macroalgae
developed under two conditions, where circulation
was reduced and where prevailing winds moved
the floating mats. Neither Enferomorpha nor
phytoplankton reached peak densities in weliflushed
channels. Any current strong enough to
scour the macroalgae would certainly limit the
accumulation of phytoplankton as well.
The conditions that stimulate macroalgal growth
were further researched by Mary Kentula (Oregon
State University, in prep.) in the salt marsh of
Mission Bay (25 km north of Tijuana Estuary). In
winter, she collected Enteromorpha-dominated
algal mats from soils beneath cordgrass and
measured their light-saturation points. Under a
broad range of temperatures (1 7-33' C), the mats
became light-saturated at 400-600
pEinsteins/m2/s. By comparison, photosynthesis of
the summer algal mats (dominated by blue-green
algae and by mixtures of blue-greens and diatoms)
a,
5
2
r
E
0
-
6{ Max~rnum and Minimum Tide Levels
3 Monthly Rainfall
'
0
5 Macroalgal Cover
= 4
a,
2 3
2 2
1
0
Table 16. Water quality at Tijuana Estuary during
nontidal and tidal conditions (summary data from Fong
1986 and Rudnicki 1986).
Data Nontidal Tidal
Temperature "C 14
Sal~ntty ppt 13
Dissolved oxygen ppm 4 4
Oo cover of macroalgae 0
Chlorophyll a mg/m3 147
Phytoplankton cells/ml 2,000,000
Exfrnctcon coefficient 3 1
Total Kjehldahi nitrogen ppm 4 4
Ammonfurn ppm 0 05
N~trate pprn 0 007
Total phosphorus ppm 17
Phosphate pprn 0 88
60 1 Phytoplankton Chlorophyll
Figure 50. Seasonal dynamics in channels and creeks
sampled monthly during 1985. Data for nutrient
concentrations and algal abundance are presented
according to the timing of maximum values.
Phytoplankton data are from tidal creek 1; all data are
from Fong (4 986) and Rudnicki (1 986).