The Ecology of Tijuana Estuary, California: An Estuarine Profile

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4.4.1 Nitragen Fluxes in 1977-1 978
Winfield (1980) set out to determine (1) the net
direction of inorganic nitrogen movement of
selected tidal cycles, (2) the relative tmportance of
ammonium, nitrate and nitrite in channel waters,
and (3) the seasonal patterns in inorganrc nitrogen
flux. HIS field data included both flood and
nonflood years, so that comparrsons became
possible and our abil~ty to extrapolate to longer
periods of time was improved.
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Flood Tidal Flow
Ebb Tidal Flow
A
Concentrattons of inorganic n~trogen were
sampled monthly in two t~dal creeks durrng the
flood and ebb cycles of spring tides Water
samples were analyzed for ammonlum, n~trate, and
nitrite, using methods outl~ned In Strrckland and
Parsons (1972). One of the tidal creeks drained an
area dominated by m~xed cordgrass and
p~cklsweed, and the other dralned an area
dominated by succulents The two statfons did not
differ In nitrogen dynamics, despite therr difference
In vascular plant dominance. Therefore, they were
averaged to calculate nitrogen fluxes
Amrnon~um was usually the domlnant form of
nitrogen Only after the January-February 1978
flooding of Tijuana River did nltrate concentrations
exceed those of arnmonfum Ammonrum (as atomrc
N) ranged from O to 168 pg N/I (monthly means),
with higher concentrations In winter and sprlng,
and higher concentratiorls In flood, rather than ebb
tides (Figure 53) A net import was calculated for
tho study period Concentrat~ons of nitrate,
averaged monthly, ranged from 0 2-3 6 pg N/I,
except far the March 1978 postflood sample (25
pg N/I) Nitr~te was lower, at 0 7-1.4 pg N/I, and
was usually hrghly correlated w~th nitrate
concentrations Calculatrons for the study period
rrrdl~ated that both nitrate and nllrite were oxported
in small amounts Overall, however, there was a
net Import of inorganic nitrogen (2 2 g N/m2/yr.
F~gure 54).
Streamflaws d~d not appear to be important lo
the nitrogen cycle except dunfig flood years The
shrft from domrnance of dissolved inorganic
nitrogen by an,monium to dominance by n~trate IS a
good indictor of rrverine influence Sewage rnputs,
on the other hand, are dominated by ammonlum
(Covin 1984). The preponderance of ammonium in
1977-78 channel waters and also in 1985 data of
Rudnicki (1986) and Fong 11986) indrcate longterm
nitrogen subsidles from urban and agricultural
wastewater,
Ftuxes of organic nitrogen have not Seen
measured but can be assumed from Wrnfield's
data, which show a net export of dissolved organic
carbon fSect~on 45.3). ft IS likely that the
inorganic nitrogen that is imported with the
rncoming tide is fixed ~nto organrc matter and
0
Z
A,.- A. -.u
MAY JUN JUL AUG SEP OCT NOV REC JAN FEE MAR APR
I--. -- ---- - ,977 -- -
-- -1- 1978 ---I
figure 53. Concentrations of ammonrum nitrogen in
flood and ebb tidal waters for (a) the succulentdominated
study site and (b) the mixed cordgrasssucculent
study site of Winfield (1 980). Vertical bar is
+ 1 standard deviation. Reprinted with permission from
Winfieid (1 980).
released to channel waters as partrculate and/or
drssolved organtc nttrogen Thus, ~mported
nutrrents could enter the marine food chaln as
amino acrds and detritaf particles become available
to consumers Durrng sewage sprlls,
concentratrons of organtc nitrogen are much
hfgher, and both water quality and estuarine
organisms are severely damaged
4.4.2 Nitrogen Additions to Salt Marsh
Vegetation
We have long been aware of spatla1 and
temporal vartabifity In marsh plant growth,
espectaiiy for cordgrass While soil salinity
reductrons that accompany flooding (Chapter 5 and
Zedler 1983) have been shown to be ~mportant in
controtiing grctwth, flooding does not expiatn att of
the growth dynamics. Furthermore, freshwater
influerrce 1s not independent of nutrient influxes, as
the pfevious sectlon shows When the Tijuana
River flows, there are many changes in total water
chemistry