Sea surface freshening can suppress the thermal tipping point of marine copepods

Tipping points govern species distributions, which could be impacted by ocean warming (OW) and sea surface freshening (SSF). Interactions between these stressors may affect tipping points. Also, the accumulation of stress over time (degree day) could condition the reaching of tipping points. Yet, these aspects have never been studied. Here, we hypothesize that SSF reduces the thermal tipping point of Calanus spp., key copepods that play a structuring role in marine food webs. Calanus spp. from the Oslofjord were exposed to a temperature gradient from 8 to 20 °C and a salinity range from 35 to 10 PSU. We found that the survival of Calanus spp. tipped and decreased above temperatures of 14-18 °C, which are already reached in the surface waters of the Oslofjord during summer months. At 29 PSU, this corresponded to a tipping point of heat accumulation of 28 °C.d above which survival decreased. Reduced salinity below 29 PSU, prevalent year-round in surface waters, suppressed this tipping point, inducing linear decreases in survival when temperature rises above 8 °C. Thus, the thermal niche of Calanus spp. will decrease with SSF, impacting whole ecosystems. We provide the first statistical determination of heat accumulation tipping points. We also demonstrate that exposure to a secondary stressor can suppress tipping points to a primary stressor, thereby removing the ability of phenotypic plasticity to buffer negative impacts on fitness. This advances our knowledge of physiological tipping points, which are essential in a world where they are increasingly reached by environmental changes.