Growth rates of mayflies (Ephemeroptera) reared in the field differed under contrasting temperatures

Abstract

Aquatic insect growth is tightly linked to environmental temperature. Growth rate tends to increase with rising temperatures. Growth rate integrates different factors related to population fitness, being partly responsible for species distribution. We aim to estimate daily growth rates from mayfly nymphs reared under different thermal regimes in the field, during five different periods from 2016 to 2018. Twelve species of mayflies were reared in mesocosms in six streams (from three altitudinal levels with a mean elevation 725, 1069, and 1509 m.a.s.l.). Additionally, we transplanted nymphs between lowest and highest pairs of streams, thus rearing them under a different thermal regime. Temperature and other ambient variables were recorded at regular intervals. Daily growth rate (dgr) of most species resulted lower in the higher pair of streams (colder sites) than in the medium and lower streams (warmer sites). Transplant experiment also clearly showed this tendency: (1) nymphs transplanted to colder thermal regimes grew slower than those reared under their natural (warmer) regime and (2) nymphs transplanted to hotter thermal regimes grew faster than those reared under their original (colder) regime. Nymphs of three species (Americabaetis alphus, Leptohyphes eximius, and Cloeodes penai) did not show differences in growth among treatments. Our findings relating sensitiveness of Ephemeroptera nymphs to small temperature changes suggest that the distribution of some species will modify by increases in temperature derived from climatic change.