estuary if vertical mixing by tides is a dominant factor. In either case, the salinity
along the bottom upstream will rise on flood tides and then fall on the ebbs. Marine
benthic animals with their affinities to seaward are defended in several ways from
reduced salinity when the tide is low. Oysters, mussels, barnacles, and some other
forms living on hard surfaces simply close their shells for the duration, keeping the
low-salinity water away from their tissues. Polychaetes, mud shrimp, and others living
in the sediment are protected by the increase of salinity downward in their mud
sanctuaries. Mixing of fresher water down into sediment and burrows is inhibited by
the density gradient established at the highest tides, when the saltiest, most-dense
water of the cycle overlies the sediment and sinks into burrows. Thus, estuarine
benthos tends to have marine, not freshwater, affinities far inland (e.g. Alexander et
al. 1935).
(^) San Francisco Bay, on the North American West Coast, extends inland from rocky
headlands at the mouth, along an estuary of more than 100 “river km” that drains the
Sacramento and San Joaquin Rivers. In addition to effects of a strong salinity gradient
and substantial pollution, the estuary’s benthic community has been challenged by
large numbers of invasive species since the mid-19th century, and dramatically so in
the late 1980s by the bottom-paving north Asian clam, Corbula amurensis. Peterson
and Vayssieres (2010) evaluated a 27-year time-series of seasonal or more frequent
benthic fauna analyses of 23 × 23 cm grab samples from four monitoring stations
along the estuary (Fig. 14.11). Analyses were combined into mean annual
composition tables, and the whole (27 × 4) = 108 sites vs. species table was assessed
by nMDS. The distance measures were “Bray–Curtis dissimilarity indices computed
from 4th-root transformed [a frequently applied strategy to reduce the effects of
differential species abundance] annual mean invertebrate abundance data”. The
ordination diagram (Plate 14.1) shows a clear upstream–downstream pattern. The
position along the first ordination axis was then plotted vs. salinity (Plate 14.2),
showing an extremely tight relationship. Peterson and Vayssieres also showed that:
(^) “Benthic assemblage composition was more sensitive to mean annual salinity
than other local physical conditions. That is, benthic assemblages were not
geographically static, but shifted with salinity, moving down-estuary in years
with high delta outflow, and up-estuary during years with low delta outflow,
without strong fidelity to physical habitat attributes such as substrate
composition or location in embayment vs. channel habitat.”
(^) [From their abstract.]
Fig. 14.11 Map of the inner estuary adjacent to San Francisco Bay. Sampling stations
were located at the black circles.
(^) (After Peterson & Vayssières 2010.)