Diel activity patterns of rocky shore macroinvertebrates in the northern Baltic Sea

Marine invertebrates on rocky shores form dynamic communities in time and space. Invertebrates constitute important links in the food chain and in biogeochemical cycles and are thus essential for functioning ecosystems. Species can be diurnal or nocturnal, and adaptions can be explained by predation pressure and resource availability. Contrary to terrestrial species, marine invertebrates are less specialised in single hosts and can, when needed, switch habitats, a trait that favours survival, growth, and reproduction. Variation in movement patterns of macroinvertebrates was studied on the northwestern Åland Islands (Åland Sea) to assess changes in activity on a diel timescale along the summer season. Two methods were used to collect representative samples of both swimming and non-swimming species. The pool of macroinvertebrate taxa in the study area was controlled for by sampling ambient macroalgae-associated fauna. Results show that all swimming and epifaunal mobile macroinvertebrate taxa move actively either in the free water body or along the sea floor within a 24-h period. A preference towards nocturnal activity could be observed as the total abundance of macroinvertebrates was higher during the night. Mean taxonomic richness was somewhat higher at night for the swimming macroinvertebrates. The community structure of mobile macroinvertebrates changed within a 24-h period as, e.g. Palaemon spp. and P. ulvae were more prevalent, and larger individuals were generally more active at night. The methods used were suitable for describing the activity patterns of macroinvertebrates on shallow rocky shores, and the results are essential for understanding the temporal dynamics of complex littoral rocky shore communities.