The impact of tidal phasing on intertidal heat stresses

Sessile intertidal organisms are exposed to extreme variations in conditions during exposure (e.g., solar heating and desiccation) that can affect their health and development – and cause mass mortality events. Exposure is strongly dependent on the tides, which locally and regionally vary in magnitude, character, and phasing. Using the blue mussel Mytilus edulis as an example species, we hypothesise that organisms at locations that experience the lowest low tides during the middle of the day experience stronger heating than organisms at locations where the lowest tides occur during the early morning and early evening. In order to test this hypothesis, biomimetic loggers were calibrated to estimate mussel thermal characteristics and placed at two macro-tidal shores (North and South Wales, UK) dominated by semi-diurnal tides, which have a tidal phase difference of ~4 hours. At both locations, the highest temperatures were recorded when low tides occurred in the middle of the day; however, significantly higher temperatures were found for South Wales where spring low tides occur in the middle of the day and exposure durations are longer, whereas midday low tides in North Wales coincide with neap tides and shorter exposure durations. Our results suggest that heat stress for intertidal organisms may be more severe in intertidal areas where spring low tides occur in the middle of the day when solar radiation and air temperature are greatest. A global outlook will also be presented depicting potential high-risk zones for mussels and other sessile organisms. These results may be of importance for shoreline management and shellfish cultivation, especially with regards to future changing climate.