Light or tide? Effects on the emergence and recolonization of harpacticoid copepods from sand flats of the Wadden Sea (southern North Sea)

Abstract

The Wadden Sea is a highly productive coastal ecosystem and an important nursery ground for various demersal fishes of the southern North Sea. Copepods are one of the major food sources of the ecosystem, yet little is known about the movement patterns of this benthic meiofaunal group within both sediment and water column in the intertidal environment, where they would be more vulnerable for predation. The diversity and abundance of harpacticoids emerging and recolonizing intertidal sand flats were investigated during two diurnal and two nocturnal high tides, using specifically designed traps. Our study revealed no significant difference in the amount of copepods emerging from the sediment into the water column during the day and the night. Furthermore, the proportion of emerging adults and copepodids did not differ from each other and most individuals persisted in the sediment during high tide. Interestingly, the most abundant species Arenosetella tenuissima and Tachidius discipes showed an opposite emergence behavior, with the former migrating in higher numbers during the night and the latter during the day.Although tidal inundation was the proximate factor initiating emergence, we suggest that sediment reworking by tidal currents, waves and bioturbation as well as water and food availability above and within the sediment may be additional factors controlling emergence of harpacticoids in the Wadden Sea, with light intensity playing a minor role.Therefore, emergence and recolonization patterns of harpacticoids from subtidal ecosystems cannot directly be transferred to intertidal areas as the time for benthic pelagic processes is limited to high tides.