The Ecology of Tijuana Estuary, California: An Estuarine Profile

Water Depth fcmi
Dry 1 3 5 7 9 1115 DfY 1 3 5 7 9 1115
............
3.6.1 Algae
The obvrous plants of tntertrdal and subttdal
areas are the macroalgae Vascular plants such as
eelgrass (Zostera marrna) are absent, perhaps
because the area of shallow water is too small or
too dynamic for rooted plants to become
established However, large populations of
Enferomorpha sp and/or sea lettuce (Ulva sp,
Ftgure 37) somet~mes develop on the channel
bottoms and later float to the water surface
(Rudn!cki 1986) Seasonal dtstributron patterns
are highly variable, but their abundance is usually
greatest In sprlng (Chapter 4)
Figure 35. Comparison of feeding patterns for four
species of birds that feed in the blocked channel of
Tijuana Estuary. Foraging microhabitats (entire
stippled area) were defined as the water depth where
birds were observed feeding and the depth to which
they probed the sediments; dense stippling indicates
where species spent more than 805 of their time
foraging; number in brackets is sample size (reprinted
from Boland 1981 with author's permission).
3.6 ESTUARINE CHANNELS AND TlDAL
CREEKS
The channel habitats are important for a variety
of organtsms including macroalgae, phytoplankton,
invertebrates, ftshes, and birds (Figure 36) The
Caiifornia least tern and other fish-eating species
use deeper-water habitats, while shorebirds feed tn
the intertidal zone at low tide Shorebtrd uses are
d~scussed in the following section
Channels are sublected to a wide range of
envrronmental conditions Tidal flushing is greatest
at the mouth and decreases with distance from the
mouth, th~s general gradient In turn rnfluences
water movement, salinity, temperature, nutrients,
and d~ssolved gases Finer sediments are removed
by hlgher current ~elOCitie~, so that substrates near
the mouth have coarser sediments than in t~dal
creeks Nutrtents brought into the estuary by tidal
flush~ng are more readtly available to organrsms
near the mouth Temperature salin~ty. and
dissolved oxygen are less variable in areas of
deepest water than In tidai creeks These
environmental factors ~nfluence the species
composttton, distributton, and populatfon dynamics
of the channel organisms
Phytoplankton are also vartable in species
composit~on and density Dinoflagellates (e.g..
Gymnod~n~urn spp 1, diatoms, f~lamentous bluegreen
algae (cyanophytes), and unidentified
unicelis or "monads" are all present rn the water
column (Ftgure 37, Fong 1986) Wh~le mast of
these algae are typical planktonic species, the
diatoms are all pennate forms with brlaterat
symmetry and longitudtnal grooves that allow
locomot~on on substrates Fong believes that most
of these are resuspended from the sediments. The
monads are so tiny ica 1-2 microns diameter) that
tdent~fication IS challenging; they appear to belong
to the Cyanophyta (P Fong, SDSU, pers comm.).
Many of the phytoplankton species also occur In
nearshore habttats, where their densities are lower
During March-June blooms In the estuary, cell
counts are higher than in marine waters by one to
two orders of magnitude
Both Rudnickt (1986) and Fong (1986)
associate algal blooms with reduced lrdal flushing
The air photo of March 1984 (F~gure 13) shows
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 matn estuarine channel, tidal
flushing became sluggrsh and algal btornass
accumulated Likewise, algal growth was high
during the nontidal period of 1984 Channel waters
were green with phytoplankton dtirrng the summer
(Chapter 4) During the 1985 monthly censuses of
channel algae, Rudnlcki and Fong found the
highest biomass of both macroalgae and
phytoplankton in small t~dal creeks where current
speeds were low
3.6.2 Benthie invertebrates
Studies of invertebrates at Tijuana Estuary have
included resource inventories and short-term or
species-specrfrc tnvestigatrons, but most were
conducted before the catastrophic disturbances
d~scussed in Chapter 2. Data collected from
precatastrophrc observations by a number of