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.