The influence of climate change scenarios on populations of the mayfly Cloeon dipterum

Populations of the mayfly Cloeon dipterum from 48 ponds (3000 l fibre-glass tanks of 1 m depth) were monitored over the course of 1 year. To simulate possible patterns of climatic change, the ponds were subject to three tempe rature treatments: continuous heating to 3 degreesC above ambient; heating to 3 degreesC above ambient during the summer only; and no heating. Further experimental complexity included enhanced nutrient input into the ponds an d the presence or absence of fish, giving a factorial combination of 3 temp erature regimes x 2 nutrient levels x presence/absence of fish predation. Few nymphs were found in the presence of fish. Where fish were absent, the temperature treatments did not significantly affect nymph abundances, and o nly marginally influenced mean nymph body-lengths. In contrast, the nutrien t treatment had significant effects on both nymph abundance and size, with greater numbers of generally larger nymphs occurring in those fish-free pon ds receiving additional nutrients. Adult emergence began earlier in the yea r from the heated ponds, particularly those ponds receiving additional nutr ients. Adult body-length differed between temperature treatments, but consi stent patterns were difficult to ascertain because of interactions with nut rient treatment and seasonal effects. Our results show that during the short term at least, elevated temperature as a simulation of climate change does not have an overwhelming influence o n either mayfly abundance or size. The influence of temperature is subtle a nd subject to complex interaction with other habitat variables. We therefor e suggest that the direct consequences of small changes in temperature will likely be of little significance to C. dipterum, relative to indirect effe cts operating through interactions with predation and nutrient input.