DIEL FORAGING PERIODICITY OF LOTIC MAYFLY (EPHEMEROPTERA) NYMPHS DURING THE SUB-ARCTIC SUMMER

We used a combination of gut content analysis and direct behavioural o bservations to examine the diel periodicity of lotic mayfly nymphs. In early July, close to the summer solstice, feeding of Heptagenia sulph urea was aperiodic. Baetis rhodani fed most actively during the day, y et drift of large nymphs peaked sharply during the short period of dus k around midnight. In August, Heptagenia dalecarlica was strongly noct urnal, whereas only a slight tendency towards nocturnality was observe d in baetid nymphs. All aspects of Baetis feeding behaviour indicated nighttime activity in September. There were some differences between B aetis size-classes, small nymphs being less constrained to feed at nig ht. Activity of heptageniid nymphs was low in September, but diel vari ation in gut fullness implied primarily nocturnal feeding. Diel variat ion in water temperature was not closely associated with mayfly period icity, and we suggest that the tendency to night-time activity is best explained by fish predation risk. Although the presence of fish was n ot manipulated in this study, nymphs may have perceived chemical fish signals from the water used in the laboratory trials. Some behavioural patterns, e.g. strong nocturnality of Heptagenia nymphs in August, ma y be fixed predator avoidance responses of highly vulnerable prey. We emphasize the importance of considering predation risk and prey size i n studies of lotic mayfly periodicity. In high northern areas, the rol e of these factors in affecting the foraging periodicity of aquatic in sects may vary seasonally in concert with varying day-length.