Kelp in hot water: Direct and indirect effects of warming seawater temperature on kelp in Nova Scotia

Abstract

Recent declines and losses of highly productive and diverse kelp beds have been observed globally and linked to increases in ocean temperature. Warming seawater temperatures may impact kelp populations directly or indirectly, by altering the interactions of kelp with other species. I investigated the direct impacts of 4 temperature treatments (11, 14, 18 and 21 °C) on the dominant kelp species in Nova Scotia: Saccharina latissima, Laminaria digitata and Agarum clathratum. Exposure to 21 °C led to tissue loss at least twice that observed at 11 °C and mortality within the first 2 wk of exposure for all species. Temperature-induced damaged to the blade tissue was evident after 1-wk exposure to temperatures above 11 °C in S. latissima and L. digitata, and resulted in reduced tissue strength and extensibility. Agarum clathratum had little tissue damage and was less susceptible to temperature-induced tissue weakening and loss. Saccharina latissima increased in strength over summer, suggesting possible acclimation to changing temperatures. I also investigated the indirect impacts of warming temperatures on Nova Scotian kelps by examining the potential for temperature to modify their interactions with the gastropod mesograzer Lacuna vincta or the invasive bryozoan Membranipora membranacea. The nutritional quality (C/N) of kelps were unaffected by temperature, and chemical defenses (phlorotannins) were reduced only in A. clathratum after 1-wk exposure to 21 °C. In feeding experiments L. vincta consumed more S. latissima pretreated at 21 °C than that pretreated at 11 °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 did not affect the quality of kelp as a substrate for M. membranacea, as settlement rates were not different between S. latissima pretreated at ambient temperature (9 – 14 °C) or 21 °C. The absence of temperature induced changes in kelp quality suggests that the effects of L. vincta and M. membranacea will act additively with the direct effects of temperature to increase biomass loss from Nova Scotian kelp beds. This thesis provides a mechanism by which rising temperatures could contribute to observed population declines of kelp species.