| Abstrakt: |
We used a combination of gut content analysis and direct behavioural observations to examine the diel periodicity of lotic mayfly nymphs. In early July, close to the summer solstice, feeding of Heptagenia sulphurea was aperiodic. Baetis rhodani fed most actively during the day, yet drift of large nymphs peaked sharply during the short period of dusk around midnight. In August, Heptagenia dalecarlica was strongly nocturnal, whereas only a slight tendency towards nocturnality was observed in baetid nymphs. All aspects of Baetis feeding behaviour indicated nighttime activity in September. There were some differences between Baetis size-classes, small nymphs being less constrained to feed at night. Activity of heptageniid nymphs was low in September, but diel variation in gut fullness implied primarily nocturnal feeding. Diel variation in water temperature was not closely associated with mayfly periodicity, and we suggest that the tendency to night-time activity is best explained by fish predation risk. Although the presence of fish was not 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, may be fixed predator avoidance responses of highly vulnerable prey. We emphasize the importance of considering predation risk and prey size in studies of lotic mayfly periodicity. In high northern areas, the role of these factors in affecting the foraging periodicity of aquatic insects may vary seasonally in concert with varying day-length. |
