The Ecology of Tijuana Estuary, California: An Estuarine Profile

,1979
1980 (Flood Year)
1981
W
. -
7
t- 1982 (No Data)
1983
(Drought Year)
also vary. However, the combination of severe
environmental stresses, such as drought combined
with estuary closure, is probably rare. Thus, local
extinction is not viewed as a common event for this
estuary. Species richness in the region's wetlands
is high for systems with good tidal flushing and low
for systems that frequently close to tides (Zedler
1982b); the near loss of annual pickleweed and
sea-blite was no doubt due to a combination of
events.
Whether or not either species will recover
depends in part on management of the estuary.
The dredging that preceded reopening of the
estuary mouth in December 1984 appears to have
increased tidal flushing beyond what it was in
1983. If tidal flushing is greatly improved after
pickleweed has become dominant, this may well be
detrimental to recovery of short-lived species.
Without tidal action, soils dry in summer, and
perennial pickleweed grows very well (78% cover
in 1984). With sluggish tides, drainage of the
intertidal marsh is incomplete, soils remain
saturated for long periods of time, and perennial
pickleweed is at a disadvantage. These were the
conditions of 1983, when pickleweed cover was at
a low of 38% (Table 26). With maximal tidal flow,
the marsh is periodically well-drained, and
pickleweed cover may remain high (as in 1985). If
these conditions persist, competitive dominance by
perennial pickleweed is expected to preclude
reestablishment of annual pickleweed and sea-blite
and to reduce the ability of cordgrass to recover.
This hypothesis is being tested with manipulative
experiments at Tijuana Estuary.
Figure 63. Changes in pickleweed distribution along
eight transects (see Figure 60; data are explained in
Figure 62). See Table 26 for changes in frequency and
percent cover.
These hypotheses of reduced seed bank and
competitive effects of pickleweed are being tested
experimentally.
It is likely that expansfon and shrinkage of
spectes drstrrbuttons is the norm for the region's
highly dynamic wetlands. The estuary's history of
variable rainfall and streamflow, fluctuating sea
levels, and alternating conditions of good and
slugg~sh tidal flow, all suggest that the abundance
and distributional limits of marsh specles should
5.3.3 Summary of Compositional Changes
The species characteristic of southern
Californta coastal wetlands do not conform to the
classical separatton of halophytes and glycophytes
on the basrs of tolerance to 05 ppt salrnities
(Waisel 1972) There is a spectrum of tolerances,
and establishment is determined by the degree and
duration of freshwater influence the low-salinlty
gap Salinity determines germination, and durat~on
of the required salinity and soil moisture conditions
determines seedling survrval This is consistent
with the regeneration niche concept of Grubb
(19771, who hypothesized that multlple
characterist~cs of both species and environment
influence establishment Once established, salt
marsh plant populations may persist or go extlnct,
depend~ng on environmental condittons that may
differ greatly from those controlling establrshment
A conceptual model was developed to
surnmarlse how low-sal~nity gaps control invasions
and extreme stresses control extinctions (Figure
64) Cond~tions at both Tijuana Estuary and San
D~ego R~ver were used in comparison wrtn changes
follsw~ng wetter and drier condit~ons (Zedler and
Beare, in press)