The Ecology of Tijuana Estuary, California: An Estuarine Profile

CHAPTER 3
ECOLOGICAL COMMUNlTiES AT TDJUANA ESTUARY
The diversity of habitats at Tijuana Estuary
results from the variability in topography, tidal
influence, and streamflow inputs. This chapter
describes the major communities of organisms at
the estuary; additional information on their
relationships to other wetlands in southern
California is provided in Zedler (1 982b).
3.1 PHYSlOGRAPHlC DIVERSITY
Tijuana Estuary includes seven major habitat
types that we have designated as: transition from
upland to wetland, salt marsh, salt panne, brackish
marsh, estuarine channels and tidal creeks,
intertidal flats, and dunes and beach. They are
listed in order of their occurrence as one proceeds
southwest across the estuary ( Table 3). Of the
wetland habitats, the three that cover the largest
area are salt marsh, channels and tidal creeks, and
dunes and beach (Table 3). Most of the estuarine
research has focused on the salt marsh habitat;
regional comparisons of salt marshes appeared in
Zedler (1982b). In this chapter, we characterize
the dominant species of plants and animals for
each habitat and highlight species of special
concern.
Table 3. Areal comparisons of habitat types at Tijuana
Estuary based on computer-assisted image processing
of the 1985 aerial photograph. See Figure 3 for area
included in the comparison.
Habitat t v~e % of Total Hectares Acres
Transition, upper marsh
and upland species
Lower and mid-marsh
areas
Salt pannes
Brackish marsh
Channels and rntertidal
flats
Beach and dunes
Total area considered
>n thrs analysts
--
Small variations in hydrology and elevation are
responsible for the physiographic diversity of
Tijuana Estuary. Unfortunately, detailed surveys
have been carried out only in the salt marsh;
topographic mapping and hydrologic analysis of
the entire estuary are underway (P. Williams, SDSU,
in prog.). Even though the natural physiography
has been substantially altered by catastrophic
events and human disturbances, most of the
habitats present today probably represent
variations on what existed at the turn of the
century. One likely exception IS the brackish
marsh habitat, which appears to be directiy
dependent on urban runoff. Another 1s the
transition zone, which has borne the brunt of urban
and agricultural encroachment. Just how closely ~t
resembles the natural condition will never be
known, because there are no sites where
undisturbed coastal scrub grades into undisturbed
coastal wetland.
With or without human disturbance, none of the
estuarine communities is viewed as statlc In either
species composition or population sizes. Our
knowledge of community change is limited by the
lack of consistent sampling of most habitats.
However, for the most extensive community, the
salt marsh, there is now a 7-year record that allows
discussion of vegetation dynamics (Chapter 5).
This data base, plus the record of variable climatic
conditions and streamflow regimes, makes it clear
that Tijuana Estuary is continually subject to
environmental variatton. The result IS a mosaic of
populations that are constantly shifting in space
and time.
These long-term patterns of habltat change
have been revealed only recently; they are
emphasized here because they are not obvious
upon short-term observation. Other dynamic
aspects of the estuary are more easily
documented. These are the daily and seasonal
fiuctuations in ttdal height, the aiternatlon of we!
and dry seasons that creates pools in winter and
salt pannes in summer, the seasonal patterns of
temperature that stimulate development of
macroalgae in the inland lagoons and tidal creeks,
and the migration of birds that use the intertidal