MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser Published January 11 Spatial heterogeneity of phytoplankton assemblages in tidepools: effects of abiotic and biotic factors Anna Metaxas*,Robert E. Scheibling Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 451 ABSTRACT: In any ecological system, the factors that regulate the abundance of species vary with spa- tial scale; therefore, the sources of spatial variability should be described. We examined different sources of variability in the spatial distribution of phytoplankton assemblages and biotic (e.g. plank- tonic and benthic micrograzers, mussels) and abiotic (e.g. nutrients, temperature, salinity, pH) factors that may regulate these assemblages in 4 tidepools at each of 3 intertidal zones (mid, high and splash) on a rocky shore in Nova Scotia, Canada, over a period of 15 mo. Stratum (defined as the depth within a pool) was a significant source of variability, particularly for pennate diatoms which were consistently more abundant near the bottom of pools. There was no indication of vertical zonation of the phyto- plankton assemblages along the intertidal gradient, and differences among zones rarely explained more than 30':: of the spatial vanabillty in phytoplankton abundance. Also, among-zone variation was not apparent for the biotic and abiotic factors. We suggest that among-zone variability in these factors does not adequately explain vertical variability in phytoplankton assemblages. All groups of phyto- plankton vaned significantly among pools within intertidal zones on most sampling dates, and differ- ences among pools explained up to 96% of the variability In phytoplankton abundance. Furthermore, there was significant variability among pools within zones for all biotic and abiotic characteristics of the pools on most sampling dates. We detected significant relationships between the density of benthic micrograzers and small mussels, and the concentration of nutrients in individual pools with the abun- dance of pennate diatoms, cryptomonads and chlorophytes Among the abiotic characteristics of the tidepools, there was a significant relationship between flushing rate and temperature of individual pools, with the abundance of cryptomonads and chlorophytes. We suggest that the factors that regulate phytoplankton assemblages in tidepools probably operate more at the scale of the individual pool rather than the intertidal zone. KEY WORDS: Community regulation . Community structure - Intertidal gradient . Phytoplankton . Spatial scales . Tidepools . Zonation INTRODUCTION The importance of spatial variability in ecological processes and community organization has been emphasized in recent studies (Addicott et al. 1987, Wiegert 1988, Wiens 1989). In any ecological system, different patterns of species abundance and commu- nity organization emerge at different spatial scales of 'Present address: Department of Larval Ecology, Harbor Branch Oceanographic Institution, 5600 U.S. 1 North, Fort Pierce, Florida 34946. USA. E-mail: metaxas@hboi.edu investigation and the relative importance of small- scale phenomema versus broader-scale processes indi- cates the 'openness' of the system (Wiens 1989).Levin (1992) recommended that patterns of variability in community organization within and across systems must be described if prediction of community dynam- ics is to be successful. Both the small-scale phenomena and the broad-scale processes that affect an ecological system have to be defined before their relative impor- tance can be assessed. The importance of sampling procedures in examining variability at different spatial scales has been emphasized (see Andrew & Mapstone O Inter-Research 1996 Resale of full artjcle not permitted