The Ecology of Tijuana Estuary, California: An Estuarine Profile

a Succulent
A conceptual model was developed by Zedler
10
--u- Flood Tidal Flow
and Onuf (1984) to describe estuarine filtering
during the wet and dry seasons of nonflood years
8 - --*-Ebb Tidal Flow
and during flood years (Figure 59). Nonflood years
are the most commonly occurring condition. During
these times, the estuary is believed to be a sink for
\ nutrients and sediments coming from both the
m m
watershed and the ocean. It is also a sink for salts
n n
that are brought in by tides and accumulated
through evaporation.
2 - 0 0 0
Z
z z
J
O' M ~ J"N Y J~JL a& s b obr ~ baic v JAN F ~ B M am ~ R We have no data on nutrient inputs during
1977 '---I 978-I
natural flood events at Tiiuana Estuary, and can
only speculate that nutrient influxes and
accumulat~on rates are greater during flood than
nonflood years. All studies of filtering functions
during flooding came after hydrological
modifications had changed both the availability of
sediments for transport and the rates and timing of
streamflows. Onuf's comparisons of flood effects
at Mugu Lagoon before and after 1970 indicate
that sedimentation was much less during floods
that preceded watershed development. The
combined floods of 1978 and 1980 filled in the
central bay of Mugu Lagoon and decreased low-
R tide volume by 40°/0 (Onuf, In press).
s 6 i f ?,
' 4 - ng ++in" 2
Figure 57. Mean concentrations of dissolved organic
carbon for flood and ebb tidal waters of (a) the
succulent-dominated and (b) cordgrass-succulent
study sites of Winfield (1980). Reprinted with
permission from Winfield (1 980).
high, it is unwise to set a percentage for materials
lost from the marsh to adjacent tidal creeks.
Suffice it to say that, from these early data, most of
the marsh plant production appears to be used
within the marsh.
4.5.4 Temporal Variability in Filtering Functions
There is little information on year-to-year
differences in nutrient uptake rates, sediment
accretion, and peat formation. Limited studies of
peat accumulation (Scott 1976; Mudle and Byrne
1980) show that marsh elevations have increased
through geologic time, and comparisons of
elevations before and after flooding (Zedler 1983b)
document short-term accretion. Not much can be
satd about the processes that reverse these
"filtering" functions, either. No measurements have
been made of erosion, other than what is obvious
from aerial photos (Chapter 2). Overall, Tijuana
Estuary is accumulating sediments within the
channels and losing area due to shoreline retreat,
but the processes appear to be more catastrophic
than chronic.
Disturbances wlthln the watershed of Tijuana
Estuary have destabilized slopes and made
available large volumes of sediment, just as in the
watershed of Mugu Lagoon. Unlike Mugu Lagoon,
Tijuana Estuary is somewhat protected from
sed~ment deposrtion by dams that regulate 78% of
the watershed. Still, agricultural and urban
developments below Rodriguez Dam disturb soils
that can be mobilized by flooding Aerial photos of
the 1980 flood show major sediment plumes
flowing out of the mouth of Tijuana Estuary. Only a
small portion of the sediment was deposited within
the wetland.
Because the bulk of the filtering process occurs
as sedimentatlon during catastrophic events, it is
not clear how important the vascular plants are in
controlling the processes of accretion and erosion.
In coastal systems worldwide, there are attempts to
stabilize shorelrnes by maintaining good cover of
beach grasses and cordgrass. At Tijuana Estuary,
the ~mportance of beach vegetation in reducing
sed~ment mobilization is clear, but the role of
cordgrass is not. Cordgrass does not occur In the
path of the rlver; thus, ~t cannot reduce erosion
along the river banks. Some areas of pickleweed
~n the estuary and even some woody vegetation
upstream were scoured out by the 1980 flood.
Cordgrass may be effectlve in increasing
sedimentatlon within the salt marsh. Whether this
IS beneficial or detrimental to maintenance of
wetland habitats depends on the comblned rates of
accretion and sea level rise.