Kelp in hot water: II. Effects of warming seawater temperature on kelp quality as a food source and settlement substrate

Predicting the effect of climate change on communities requires an understanding of the effects of environmental conditions on species and their interactions. We investigated the potential for warming seawater temperature to modify the interactions of the gastropod mesograzer Lacuna vincta and the invasive bryozoan Membranipora membranacea with kelps in Nova Scotia. The nutritional content (C/N) of the kelps Saccharina latissima, Laminaria digitata and Agarum clathratum were unaffected by temperature (11, 18 and 21°C), and chemical defenses (phlorotannins) were reduced only in A. clathratum after 1 wk exposure to 21°C. C/N and phlorotannin content increased over the season in S. latissima collected monthly in summer 2013 and 2014. The effect of temperature-induced changes in kelp on the grazing of L. vincta was assessed using feeding experiments with S. latissima pretreated at 11 or 21°C. Snails consumed more kelp pretreated at 21°C only when grazing rate was high. The quality of S. latissima as a food source for L. vincta was not affected by temperature, as diets of kelp pretreated at 11 and 21°C supported similar growth, reproduction, and survival of snails. Temperature also did not affect the quality of kelp as a substrate for M. membranacea, since settlement rates were not different between S. latissima pretreated at ambient temperature (9 to 14) and 21°C. The absence of temperatureinduced changes in kelp quality suggests that the effects of L. vincta and M. membranacea will act additively with the direct effects of temperature and cause increased biomass loss from kelp beds.