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Dr. Megan Dethier

Dr. Susan Williams

University of Washington University of California-Davis
Friday Harbor Laboratories Bodega Marine Laboratories

Intertidal Ecology of Fucus, San Juan Island, Washington

Please read the following page, and our related publications, for more details on the studies.

Extreme temperatures and accompanying desiccation are probably the physiological stresses most likely to damage intertidal organisms and limit their potential production ­ this happens when abiotic factors exceed some threshold for optimal organismal function. We are examining the roles of temperature and other abiotic and biotic parameters on the ecophysiology of the common intertidal alga, Fucus. Our study tests the interactive effects of abiotic stress and herbivory, which can also affect algae (via altering production, distribution, abundance, chemistry and morphology). Little is known about the interactive effects of stress and herbivory on resource allocation to growth versus reproduction or other major resource sinks.

Fucus gardneriFucus is a model organism for our research. It is very common on protected and semiprotected shorelines from Alaska to central California, it forms dense canopies in the intertidal zone, and its broad vertical distribution encompasses a presumed stress gradient (i.e., high to low on the shore).

We performed our observations at 14 field sites around San Juan Island, Washington. This unusually large number of sites was used in an attempt to encompass the range of variation in physical parameters Location of the San Juan Islandsand biotic environments experienced by this algal species. The San Juan Islands are at the northern end of the Puget Trough and the eastern (inner) end of the Strait of Juan de Fuca. All sites are protected from full oceanic conditions. The map to the right illustrates the location of the islands relative to other major landmasses. The map of the sites shows San Juan Island relative to the other islands in the archipelago. Sites on the west side of the island are exposed to more swell and wind waves, while those on the other sides are relatively protected by nearby islands.

Table 1. lists the sites and their basic physical features; they vary not only in wave exposure but in compass heading and local shading, and thus exposure to desiccating conditions. All sites were bedrock shores of 15 to 30 degrees slope. We sampled in the High zone (near the upper limit of the Fucus) and Mid zone (ca. +1.5 and +0.8m) at 12 of the sites, and in 3 zones (ca. +1.5, +1, and +0.5) at the other 2 sites.

Stowaway TidbiT logger in the IntertidalAt each site we gathered detailed temperature data to examine the effect of this environmental variable on the growth and survival of Fucus. Environmental temperature data were gathered using "Stowaway TidbiT" loggers at the two tidal heights at each site. These small, waterproof loggers were attached to pieces of PVC and bolted to the rock; a piece of thick flexible plastic protected the glass optical-readout points but the sensor was exposed to the environment (see Photo). Each logger was programmed to read the temperature (of air or water) every 20 minutes. Algal canopies were regularly cleared away from the loggers, and data were downloaded every 3-4 months.

In the San Juan Islands, at any given point on the rock, the total number of hours per day of exposure to air remains fairly constant throughout the year (ca. 50% of the time at the High level, 22% at the Mid). However, the timing of the lower low tides swings from daytime in the spring and summer to after dark in the fall and winter, and this change is clearly reflected in the TidbiT data.

Sample graphs illustrate a variety of ways of analyzing the data collected; the entire dataset for each site can also be downloaded. Mean maxima through the year are illustrated for the hottest and coolest sites, showing very similar seasonal patterns but substantial differences in degree of heating. Davison Head (DH) was a south-facing site and Point Caution (PC) north-facing. As expected, TidbiTs in the High zone consistently recorded hotter temperatures than in the Mid zone. A few sites (e.g. Hannah Heights) had Mid zones that were as hot or slightly hotter than the adjacent High zone; these data reflected the Mid TidbiT's placement on a more-horizontal or a more-south-facing surface than the High one (See Table 1.). Temperature minima are not illustrated; no significant freezes occurred during the measurement period, and minima were seldom below 2ºC.

Possible ecological and physiological effects of these temperature differences are being analyzed for publication in several papers (in review and in preparation). We will describe observations of natural populations of Fucus, quantifying patterns of growth, reproduction, and phlorotannin levels across intertidal gradients in herbivory and stress. We also describe field and laboratory manipulations of these processes, measurements of physiological parameters and allocation patterns, and demographic consquences. The research provides some of the first data on how algae respond (in ecological time) to simultaneous variation in these two key selective factors.

Publications using these data:

Haring, N., M.N. Dethier and S.L. Williams. In review. Desiccation facilitates wave-induced mortality of the intertidal alga, Fucus gardneri. Marine Ecology Progress Series.

Williams, S.L. and M.N. Dethier. In review. High and dry: variation in net photosynthesis of the intertidal seaweed, Fucus gardneri. Oecologia.

Dethier, M.N. and S.L. Williams. In preparation. The effects of seasonal environmental stresses on intertidal algal growth rates.

Funding Source:
National Science Foundation, Grants OCE 98-98196078, 0196078, and 9901138.


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