Freshening increases the susceptibility to heat stress in intertidal mussels (Mytilus edulis) from the Arctic

Martin B Nielsen,Trine K Vogensen,Jesper G Sørensen,Jakob Thyrring,Mikael K Sejr

doi:10.1111/1365-2656.13472

Abstract

Temperatures in the Arctic are increasing at a faster pace than at lower latitudes resulting in range expansion of boreal species. In Greenland, the warming also drives accelerating melt of the Greenland Ice Sheet resulting in more meltwater entering Greenland fjords in summer. Our aim was to determine if increasing summer temperatures combined with lower salinity can induce the expression of stress-related proteins, for example, heat shock protein, in boreal intertidal mussels in Greenland, and whether low salinity reduces the upper thermal limit at which mortality occurs. We conducted a mortality experiment, using 12 different combinations of salinity and air temperature treatments during a simulated tidal regime, and quantified the change in mRNA levels of five stress-related genes (hsp24, hsp70, hsp90, sod and p38) in surviving mussels to discern the level of sublethal stress. Heat-induced mortality occurred in mussels exposed to an air temperature of 30°C and mortality was higher in treatments with lowered salinity (5 and 15‰), which confirms that low habitat salinity decreases the upper thermal limit of Mytilus edulis. The gene expression analysis supported the mortality results, with the highest gene expression found at combinations of high temperature and low salinity. Combined with seasonal measurements of intertidal temperatures in Greenland, we suggest heat stress occurs in low salinity intertidal area, and that further lowered salinity in coastal water due to increased run-off can make intertidal bivalves more susceptible to summer heat stress. This study thus provides an example of how different impacts of climate warming can work synergistically to stress marine organisms.

Highlights

The latest decade has been the warmest on record, and Arctic temperatures have risen at two times the global average (AMAP, 2019; Wassmann et al, 2011)
Heat-induced mortality occurred in mussels exposed to an air temperature of 30°C and mortality was higher in treatments with lowered salinity (5 and 15‰), which confirms that low habitat salinity decreases the upper thermal limit of Mytilus edulis
We wanted to test if the low salinity typically found in Greenland fjords during summer would make bivalves more susceptible to heat- induced mortality and induce increased expression of stress-related proteins to assess if increasing summer temperatures in Greenland could limit the distribution of mussels in the intertidal zone

Summary

Introduction

The latest decade has been the warmest on record, and Arctic temperatures have risen at two times the global average (AMAP, 2019; Wassmann et al, 2011). The southern distribution edge of Mytilus edulis in North America has shifted northwards by 350 km in the past 50 years due to a high mortality in southern populations as a direct effect of increased temperatures (Jones et al, 2010), and a recent marine heatwave in the northeast Pacific triggered a range expansions of 37 intertidal species towards the Arctic (Sanford et al, 2019). The effect of air temperature on body temperature during low tide is modified by the physical surroundings of the local habitat Sessile intertidal organisms, such as blue mussels (Mytilus sp.), are subjected to varying temperatures, depending on their position in the intertidal zone, the substrate slope, orientation relative to the sun and shading by kelp (Helmuth et al, 2016; Kearney et al, 2014; Seed & Suchanek, 1992; Sejr et al, 2021). The small-scale habitat variability of intertidal shores results in mosaic patterns of heat exposure resulting in local cold- and hotspots (Helmuth et al, 2006)

Objectives

Results

Conclusion