Variation in Low Intertidal Communities: Submerged vs. Emerged

Abstract

Organisms living in the low rocky intertidal must cope with changing conditions of submersion and emersion on a regular basis. Some mobile organisms engage in small-scale migrations, using the tidal current for passive transport or for aid in active transport while foraging for food and escaping predation. As such, it is expected that at any given intertidal depth, mobile assemblages will vary based on whether they are submerged or emerged. This study used NaGISA (Natural Geography In Shore Areas) sampling protocols at the 1m below MLLW level to examine variation in rocky intertidal mobile faunal assemblages in response to tidal immersion in Prince William Sound, Alaska. Two NaGISA survey transects were conducted at the same tidal stratum: once at high (submerged) and once at low tide (emerged). Caprellids were the only taxon to show a consistent trend of higher abundance at submersion. Amphipods, isopods and nemerteans showed different trends in abundance between sites, while crabs and errant polychaetes were more abundant during emersion. These differences could be due to water cover, species-specific movement, different species engaging in different migration patterns during the tidal cycle, patchy distribution of particular faunal species, taxon and functional group composition of the algal community, as well as the sampling technique using SCUBA at high tide versus intertidal sampling at low tide.