Tolerance landscapes can be used to predict species-specific responses to climate change beyond the marine heatwave concept: Using tolerance landscape models for an ecologically meaningful classification of extreme climate events

To predict the responses of organisms to changes in intensity and frequency of heatwaves, it is essential to gain a thorough understanding of how organisms respond to temperature exposure. Species-specific curves are more informative, yet more difficult to ascertain, than the generic heatwave definition (five days or more at a temperature above the 90% percentile) when addressing mortality risk and should be included when predicting thermal risks. The thermal tolerance of organisms is dictated by a combination of exposure intensity and duration: the aim of this study was to build a ‘tolerance landscape’ model, based on exposure thresholds to a potentially stressful temperature range, for the commercially important clam Ruditapes philippinarum using ten years of summer temperature time series in four lagoons in the Northern Adriatic, where this species is being commercially farmed. The model is based on a log-linear relationship between LT50 and exposure time. The two model parameters, i.e. the lethal temperature at 1 min exposure (CTmax), and the temperature sensitivity parameter (z) were estimated on the basis of a systematic literature search. Best-fitting values, i.e. CTmax = 54.5 (±2.3) and z = - 5.72 ◦C (±0.66) are within the ranges found for other bivalves. Results show that the mortality threshold was exceeded for most lagoons in summertime in 2015, 2017 and 2018 suggesting that the risk of exceeding the mortality threshold is increasing, due to an increase in frequency and duration of extreme temperature events. Comparisons with the generic ‘marine heatwave’ definition showed that, while in some occasions ‘heatwaves’ occurred that were not risky for R. philippinarum, in one case the model identified a time period of mortality risk that would not have been classified under the generic ‘heatwave’ definition. These mismatches suggest that tolerance curves can be a good addition to productivity and site selection models, incorporating a metric of species-specific risk that can be used to predict the consequences of climate change on fishery and aquaculture, and can find their place in conservation and restoration toolkits for forecasting changes in habitat suitability under future climate scenarios.