The Ecology of Tijuana Estuary, California: An Estuarine Profile

CHAPTER 5
THE ROLE OF DISTURBANCES IN MODIFYING SALT MARSH
COMMUNITY STRUCTURE AND FUNCTION
Recent changes In specles d~strrbutions and
growth rates have been documented In detail for
the salt marsh of Tijuana Estuary A general
understanding of what controls rnvasron and
exlinctron of species at the ecosystem scale has
developed, as well as abrlrty to predrct expanstons
and declrnes at the population scale Catastrophrc
events, though destructive in many ways, have
proved to be scrent~frcally valuable Thelr effects
have been documented through an exterls~ve
monltor~ng program that began as a survey of
cordgrass habrtats in 1979 and has continued to
date This chapter provides extensive ev~dence of
the estuafy's temporal variabil~ty and lndrcates why
no slnglc descrrption of productivcty or bromass
can fully characterize the salt rnarsh ecosystnm
cordgrass, and the monitoring program has helped
to elucidate these and other distributional patterns.
Mcasurements taken at the monitoring statlons
are as follows Elevatrons are periodically
measured relatrve to Army Corps of Engineers
benchmarks Each Aprll, soil salinity IS measured
In samples near all statrons In the iower marsh,
~nterst~tial soil water IS collected by expressing so11
water (from 0 to 10 crn depth) through frlter paper
onto a salinity refractometer In the upper marsh,
the dr~er soil samples (0-10 crn depth) are
collected and taken to the laboratory, where
unrform so11 pastes are made and measured wrth a
conductrvtty meter (Rrchards 1954) In
September, soil salinrty sampling is repeated and
veqetation IS measured Cordqrass IS assessed bv
0 25-m7 ctrcula;
quadrats (or 25"a of that area if densities are too
5.1 MONITORING PROGRAM m&asurlng helghts of stems wih~n
The lower salt rnarsh has been sampled high for complete measurements) Flowering stems
consistently at apgrOxlmately 100 stations along are noted, live and dead stems are counted The
eight transects (Frgure 601 The exact number of percent cover of each of the other species present
stat~ons has varied sirgb)tly wrth our abrlrtv to 1s estrrnated w,thin cover classes (i I",. lo0-5%,,
relocaie station markers and the expansion of the 6",-25",,, 26"~~-50~,,, 51 ?,-75"0. 76uu-1 OOL",)
distributiorl of corclarass The transects were set
up to characterize fhe cordgrass cornmunrty, thus 5.2 PHYSICAL CHANGES FOLLOWING
they extended from upper drstributionai limits ECOSYSTEM-WIDE DISTURBANCES
toward channeis In ail cases. trarisects were
5.2.1 Soif Salinity Changes
named tor the nearest Army Corps of Erlg~neers
beilchmark (e g . TJE-31)
In 1984, the monitoring program was expanded
to rnclude upper marsh habltals An additional 115
stattons were set up lo extend from the upper
dfstrrbutlonal llmlt of cordgrass inland. After
surveying their elevations, we found that many of
the sampling StatlOnS set up to characterrre upper
rrlarsh dtstrtbution were within the :ange oi
eievatlcrns included rn the lower marsh transects
(Table 241 They are actually cordgrass and
noncordgrass transects This emphasrzes that the
distrrbutlon of cordgrass IS not entrrely predtctahie
from eievation Other factors, rnclud~ng competrt~an
by succulent species, ltrnrt the occurrence of
Interstitial soil salinrties, which have been
measured annually In April and September, Indicate
that the lower marsh is usually hypersaline (I e,
more saline than sea water, Figure 61) The
followrng data stand out
a Reductrons in soil salintty (to 15 ppt)
occurred in April 1980, after the "100-year" flood
Because !he f!ood:ng occ3rrcd pr:msrtiy tc Jaituarp
and February, the April measurement may not
represent mrnimum sot! salrniiy Evaporatron was
no doubt hrgh throughout the spring (Sectton 2 21,
arrd t~dal waters influenced the marsh throughout
f 980, thus the perrod of brackish so~ls was shortlived