The Ecology of Tijuana Estuary, California: An Estuarine Profile

Figure 38. The channel benthic community includes a wide variety of invertebrates. Illustrated are muilet in the
water column; surface benthic animals from left to right: the horn snail, sea hare (Aplysia), mud snails (Nassarius),
sand dollars, hermit crabs (Pagurus), egg cockle (Laev~cardium), sea cucumber (Molpadia) and its commensal pea
crab (Pinnixla barnhart!), and the crab (Cancer productus) with attached mussels (Mytilus edulis); and burrowing in
the sediments from left to right: the mud-flat brachiopod (Glottidia aibrda), bent nose clam, Californa jackknife clam
(Tagelus caltforntanus), wavy chione (Chtone undatella), ghost shrimp, and spionid worms. Mclntire collection,
copyright 1986 by Zedler.
Peterson (19751 thought was due to the rarity of
other deep-burrowing competitors. Hydraulic
harvesting of callianassid shrimp was common at
Tijuana Estuary during the 1960's and 1970's;
their removal, according to Peterson, allowed the
clams to expand their populations with little effect
on other dominant species. Manipulative field
experiments supported his interpretation of the
data.
During the winters of 1977-78, 1978-79, and
1979-80, San Dtego County had unusually heavy
rains, culm~nat~ng In the flood of 1980 when
approximately tw~ce the normal precipitatron and
28 times the mean annual streamflow were
recorded The Influx of fresh water lowered
channel sal~ntty to zero ppt, and the increased
streamflow s~gn~f~cantly altered the sed~ment
structure of the channel bottoms
As a result of the flood, sedlment gratn size
decreased at three of flve sampllng srtes and
increased at two (Rehse 1981) If sed~ment
character~st~cs control specfes cornposit~on, then
three predicttons regarding s~lt/clay deposition
would follow from Hosmer's (1977) study (1 The
purple clam should be most affected, because it
was conflned to coarser sed~ments, (2) the
littleneck clam should be less affected because IPS
highest denslty and bromass were at 15%-20%
siltlclay, and (3) the false mya should be least
affected, because their maxlmum b~omass and
densit~es were at 35% siit/clay
In 1980, Hosmer resampled several areas that
were Included In h ~s 1977 thesis work, and Rehse
(1981 compared the two data sets on btvalves and
callianasstd shrimp Flooding caused mass
mortalities of many species Absent from the 1980
collect~ons were the yellow clam (Nonmenbs
obesa), egg cockle (Laevtcard~um substrtatum),
bentnose clam, whlte sand clam (Macoma secta).
California mactra (Macfra cairlornrcai, Washington
clam (Saxidomus nuttali/], Carpenter's tellen
(Tell~na carpenter/?, and Calitanassa grgas The
dominant b~valve before the 1978-80 period was
the purple clam, while the dominant in 1980 was
false rnya
4 1