The Physical Environment of False Bay
|
|
Fig 1. False Bay at High Tide
|
Fig 2. False Bay at low tide
|
As seen in this dramatic pair of photos, False Bay transitions
once or twice per day from a wide, open bay to an exposed flat of mud,
sand, and shallow pools of water. When the bay is emptied at low tide,
usually during the day in the summertime, organisms living in the bay may
be exposed to extremes of high temperature and/or desiccation. At high
tide and during tidal exchanges, organisms are exposed to an influx of
water and current, as well as nutrients carried by water. This page attempts
to characterize some of the relevant physical parameters of False Bay that
help to determine the composition of the biological community.
|
Fig 3. Temperature measured over three full days
in False Bay. Measurements were taken from a datalogger in a semi-permanent
tidepool in roughly the center of the bay (see map).
|
Low Tide: Temperature Fluctuations
Animals living in shallow, semi-permanent tide pools
in False Bay experience drastic swings in temperature during the course
of a summer day. In addition to measuring temperature in pools, we also
measured temperatures above and just below the surface of exposed (non-submerged)
sandy and muddy areas. Although temperatures out of water are less predictable,
high temperatures in sand were comparable to those in surface water. As
can be seen from figure 3, organisms living on the surface of False Bay
during low tide can experience temperatures as high as 30°C, a change
of 20°C from the normal Puget sound water temperatures experienced
at high tide.
In addition to heat stress, animals left entirely or partially
out of water during low tide risk desiccation, while those left in standing
water on warm days may face fluctuations in salinity due to evaporation.
Those living in habitats with large amounts of algal cover are somewhat
shielded from high temperature, but must cope with increased levels of
anoxia in sediment underneath algae (Woodin 1972). The inhabitants of False
Bay employ a number of different strageties for coping with these environmental
fluctuations, as described on the False Bay protection
page.
High Tide: Water Energy!
Although False Bay is most accessible to people at low
tide, the vast majority of organisms living there are burrowed deep or
are inactive during that time. The real action is at high tide, when cool,
nutrient-laden water sweeps into the bay. We were not able to make any
measurements of the physical environment of False Bay during high tide,
but fortunately there are many clues about water energy and currents to
be found at low tide. As you walk about the bay, you may notice that in
some areas the sediment under your feet is sandy while in others it is
muddy or gravelly. We used sieves
with standard mesh sizes to make rough measurements of sand grain size
for each of the four focus areas.
|
Fig 4. Histogram showing sediment size distributions for
each of the four areas examined.
|
In general, sediment in areas with higher energy flow
has larger particles (sandy or gravelly to the feet) because smaller particles
tend to be more often carried away. The sandbar area near the mouth of
the bay, which experiences a great deal of current as the tide comes in
and out during the day (recall from the photos how much water has to move
in and out!), has the largest grain sizes of the sediments examined. The
other three sites are more protected and muddier. The ulva and mud shrimp
areas, on either side of the mouth of the bay, have roughly similar sediment,
although the distribution of sediment in the ulva area is slightly shifted
toward larger grain sizes. In keeping with this, we observed that the sediment
in the western area feels "sandier" than the sediment in the eastern mud
shrimp area. The Arenicola area, near the access point to the bay
and far from the mouth, has the smallest grain sizes and most likely experiences
the least water energy.
For more information on the differences among these habitats,
and on how physical environments shape the communities of animals that
live in them, go to information about the specific areas: Sand
Bar, Mud Shrimp Area,
Ulva Area, Lugworm
Area.