Inorganic Nutrient Fluxes in Anemone-dominated Tide Pools

Physical and chemical characteristics of seawater in two natural tide pools on Rosario Beach, Fidalgo Island, Washington, were compared during midday low tides in July 1991. One pool contained a mixed assemblage of macroalgae (40% cover) and invertebrates (50% cover). The second pool was dominated (75% cover) by the sea anemone Anthopleura elegantissima (Brandt, 1835), which contains symbiotic algae (zooxanthellae). Temperature, salinity, dissolved oxygen, and pH levels increased in both pools with irradiance and length of emersion. The resident organisms caused changes in the inorganic nutrient levels of the tide-pool seawater. Anthopleura elegantissima released substantial amounts of ammonium; NH4 + in the anemone-dominated pool increased by 33% whereas NH4 + declined in the mixed assemblage pool by an average of 28%. Nitrate and nitrite declined in both pools, whereas phosphate remained constant during the 6-hr sampling periods. NH4 + release by A. elegantissima was confirmed in studies of artificial tide pools, where NH4 + levels increased by an average of 71 % over an 8-hr period. Release of ammonium by A. elegantissima under natural conditions in the field provides a contrast to nutrient fluxes observed for tropical symbiotic associations. THE TIDE POOL IS a dynamic ecosystem, changing with the ebb and flow of the tides. At high tide the pools are submerged, the water is continuously exchanged, and the system remains open. When the tide recedes, the tide pool is suddenly isolated and becomes a closed ecosystem for different periods of time. Although tide-pool organisms remain in an aquatic environment during low tides, they may be subjected to large temperature and salinity changes, as well as fluctuations in dissolved inorganic materials such as ammonium, nitrate, nitrite, phosphate, and oxygen (Klugh 1924, Ganning 1971). The physical I This research was funded by an NSF REU Site award (OCE-9000676) to Shannon Point Marine Center and by OCE-8996242 to a.M.p. Manuscript accepted 7 December 1992. 2 Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Road, Anacortes, Washington 98221. 3 Current address: Department of Biology, University of South Florida, Tampa, Florida 33620-5150. 4 Author to whom requests for reprints should be addressed. and chemical changes ofseawater in tide pools during emersion periods have been examined in numerous studies (Stephenson et al. 1934, Newell 1970, Truchot and Duhamel-Jouve 1980, Morris and Taylor 1983, Huggett and Griffiths 1986). However, with the exception of study of oxygen and carbon dioxide changes associated with differences in primary production in animal and algae tide pools (Stephenson et al. 1934), none have targeted specific tide-pool inhabitants to examine their effect on the seawater in the tide pool. Changes in the inorganic nutrients during emersion periods may provide new information about the tide-pool organisms. Anthopleura elegantissima (Brandt, 1835) is a temperate sea anemone that is abundant in the intertidal zone along the coast of the northeastern Pacific Ocean (Hand 1955). These anemones contain photosynthetic symbiotic algae (zooxantheliae) that are highly productive; gross production by A. elegantissima is comparable with that of intertidal macroalgae on an areal basis (Fitt et al. 1982). Tropical and temperate algal-cnidarian sym-