Showing papers by "Megan N. Dethier published in 1982"

Pattern and Process in Tidepool Algae: Factors Influencing Seasonality and Distribution

Abstract: The distributions and seasonal changes of six algal taxa commonly found in tidepools in Washington state are examined. Distinct seasonal patterns were found in five taxa: Collinsiella tuberculata and benthic diatoms are most abundant in the mntei;Rhodomela larix in high pools is most abundant in the winter whjle that in low pools peaks in the summer; andPrionitis spp. and articulated corallines fluctuate less regularly but suffer considerable losses in the fall and winter. Cladophora sp. shows an aperiodic pattern, with large reductions in abundance occurring unpredictably throughout the year. Although such seasonal patterns have usually been ascribed to changing temperature and light regimes, experimental and observational data suggest that herbivores can play a key role. Seasonal fluctuations in the abundance of herbivorous molluscs are implicated in some of these algal patterns, especially those ofCoi/insiefia, hi$i-poo\\Rhodomela, and diatoms. The fall and winter losses of the more herbivore-resistant/VrömY/.v and articulated corallines are probably due to increased wave drag acting on thalli that have grown unstably large. Cladophora is also affected by waves; it grows in large mats, loosely attached to the substratum, which slough off when they become thick. Physical extremes of temperature and light may set the upper limits of these algae but have a lesser role in seasonal patterns than previously assumed. The factors excluding certain intertidal algae from pools are also examined. Herbivores play a role in this pattern äs well; algae transplanted into pools remain healthy and may grow, but are rapidly consumed by herbivorous molluscs and crustaceans. Possible refuges from herbivory of many of the common tidepool algae are discussed. Even less consideration has been given to the biotic facAlthough distinctive patterns of marine algal distribution tors affecting organisms in intertidal pools. The preand seasonality have been noted for decades, explanations sence or absence of particular species in tidepools has of these patterns have either not been attempted or have always been assumed to result from adaptations (or depended on correlations with physical parameters. For lack thereof) to the peculiar environmental regimes example, descriptions of algal zonation on many of the found in these habitats. Klugh (1924) believed high world's coasts have been published (Stephenson and temperatures in pools to be the limiting factor for Stephenson 1972), with abiotic factors (e.g. exposure several plants and animals. Lami (l931) and Davy de to air, Insolation, desiccation, and temperature stress) inVirville (1935) correlated salinity and pH extremes voked äs determinants of zonal boundaries (e.g. Doty with algal distributions, and believed these to be of 1946). Numerous studies have shown that biotic factors prime importance. Recently, Femino and Mathieson such äs competition and predation set limits to zonation (1980) stated that the abundance of tidepool algae is for rocky intertidal invertebrates (Connell 1961, 1972, determined by physical parameters. Although such Paine 1966, Dayton 1971), but these concepts have factors probably contribute to limiting distributions seldom been experimentally tested with algae. Excepin pools, their role has not been experimentally demontions include studies by Dayton (1975), Hruby (1976), strated. Perhaps more crucially, most studies in pools Raffaelli (1979), Lubchenco (1980), Hodgson (1980), have been done without regard to herbivores, which Slocum (1980), and Underwood (1980), who are often abundant and could play a structuring role. demonstrated Icey biotic interactions between algae or Paine and Vadas (l969) demonstrated such a role for between algae and their consumers. sea urchins in low Washington pools, and Lubchenco 0006-8055/82/0025-0055502.00 © by Walter de Gruyter & Co. · Berlin · New York 56 Dethier: Seasonal and distributionai patterns in tidepool algae (1078 and ms.) identified several important algaeherbivore interactions in New England tidepools. Studies on seasonal Variation in algal abundance and diversity also have been largely descriptive, and the mechanisms behind such patterns have been sought in changes in the physical environment. Conover (1958 and 1964) reviewed studies describing correlations between temperature, light, salinity, and algal seasonal changes, but admitted that the causal mechanisms were not known. Explanations for the timing of maximum algal biomass or diversity have included variable air and seawater temperatures (Murthy et al 1978), temperature/light interactions (Edwards 1969, Hodgson and Waaland 1979, Femino and Mathieson 1980, Thom 1980), mortality due to high water movement (Doty 197h Santelices 1977), or many factors acting together (Littler et al 1979, Ngan and Price 1980, Gunnill 1980). Seasonal shifts in vertical distributions were examined by Lawson (1957), who suggested that desiccation stress mediated these patterns. None of these authors considered the potential effects of seasonal changes in herbivores (numbers, distributions, or feeding rates) on the algal patterns. Only a few studies have examined such a r öle. Bell (1926) suggested that the springtime loss of certain winter annuals was due to the combined effects of post-reproductive senescence and grazing by snails. Castenholz (1961) showed that grazing by littorinid snails is responsible for the summertime absence of diatoms, and Lubchenco and Cubit (1980) found that littorinids and limpets affect some algal seasonal patterns in New England and Oregon intertidal zones, respectively. Raffaelli (1979) experimentally removed all grazers from areas of shore and demonstrated that this prevented the normal seasonal decline of certain algae. Finally, Underwood (1981) noted that algal seasonal changes correlate with changes in physical harshness of the environment, but cautioned that grazers may also play an important role. In this paper l describe the distribution and seasonal changes in abundance of six algae found commonly in intertidal pools in Washington state. For each alga I attempt to determine the processes behind these patterns and consider the relative roles of herbivores and physical parameters. I also report on some experiments testing why certain intertidal algae are not found in pools, and discuss the refuges that tidepool algae have against various sources of mortality. Study Sites and Methods Small intertidal pools (< l m surface area) were studied at two major areas: on the outer coast of Washington state and in the more protected San Juan Islands. On the outer coast there were two sites: Waadah Island (48°23'N, 124°36'W) and ShiShi, 12 km to the south. The protected sites were Pile Point, Cattle Point, and Colin's Cove on San Juan Island (48°30'N, 123°W), and nearby Deadman Island. The outer coast sites are subject to greater wave action but suffer lower extremes of temperature and desiccation than the San Juan Island sites (Dayton 1975). Spring and summer low tides fall near midday in the San Juans, while on the coast they occur earlier, during cooler portions of the day. Since low tides in the fall and winter fall after dark at both sites, early spring and early fall represent important transition periods when the low tides switch to occurring during the day or during the night, respectively. In addition, storms influence both sites mostly in the fall and winter. Tidepools at a variety of heights were censused on the outer coast (N = 81 pools) and in the San Juan Islands (N = 168). Tidal heights (above mean lower low water) were established using a surveying level and pole and referring to a point of known height; this was determined by comparison with predicted tide levels on several day s. Algal percent covers were visually estimated in each pool; this method is rapid, allowing study of a much larger sample of pools than would otherwise have been possible, and assured that species in low abundance would be censused. For encrusting species, cover of the pool substratum (primary cover) was estimated. For species with erect thalli, secondary cover (the proportion of the pool surface occupied by the alga when the pool is viewed directly from above) was taken äs a more meaningful measure of abundance, since such species frequently occupy little primary substratum. Herbivore numbers were counted in replicate 10 X 10 cm quadrats within each pool. The effects of limpets on certain algae were examined using exclusion fences. These were circular explosures, 16—20 cm in diameter, made of 8 cm high plastic mesh (Vexar®) attached to the rock inside large pools with a putty (Sea-Goin* Poxy Putty®, non-toxic after curing). The fences partially shaded some of the enclosed substrata but without Substantive effects; algal changes in fenced areas were the same äs those where limpets were removed without the use of fences, and no differences were seen between the unshaded centers and moreshaded edges of fenced areas. Small numbers of limpets invaded the fences occasionally, but bi-monthly monitoring ensured effective exclusion. Littorinid snails and small crustaceans moved readily over the fences. The effects of limpet exclusion were also studied using 15 X 15 cm slate settling plates (with fences) placed in large pools. The susceptibilities of intertidal algae to a variety of herbivores were examined in two ways. First, algae were transplanted into areas where they do not normally occur by chiseUng off small sections of rock to which Botanica Marina / Vol. XXV /1982 / Fase. 2 Dethier: Seasonal and distributional patterns in tidepool algae 57 they were attached and puttying the rock to the new substratum. These transplants were made both into unfenced areas of pools and into areas from which limpets were excluded. Controls involved moving algae within their normal habitat. Second. algae were fed to different herbivores in the laboratory, either singly or in an algaJ-ch