| Summary: | Global biodiversity loss is an internationally recognized problem that has consequences for ecosystem functioning and provision of ecosystem services and warrants investigation into drivers of change in biodiversity. In the context of natural variability, multiple anthropogenic stressors, and a changing climate, the identification of drivers of change is a complex issue. Here, an integrated approach (analysis of long-term datasets and experimental simulation) was employed to investigate drivers of change in intertidal mudflat macroinvertebrate diversity, using the Solent, on England's south coast, as the study system. A model was developed to analyze survey datasets from the 1970s-2010s following a review process. Comparisons of the spatio-temporal patterns of change in diversity across an interconnected three-harbour system revealed differences at the harbour and within harbour scales, suggesting the relevance of local conditions for driving change versus dominance by a regional driver. Further, direct relationships were identified between diversity and within-harbour environmental conditions. In the context of a changing climate, temperature was investigated as a driver of change. The absence of a direct relationship between a regionally derived climate index and diversity and identification of the interaction of local seasonal water temperatures with local environmental conditions have highlighted the relevance of local context for predicting the way in which climate change effects may manifest. The results suggest the potential for macroalgal cover to act as a driver in this system, as direct relationships as well as relationships modified by the preceding seasonal temperatures were identified with respect to diversity, though the data available to test these relationships were limited. The effects of discrete temperature events were also investigated as a driver of change by simulating heat waves in a large outdoor mesocosm system designed to preserve natural sediment temperature profiles, solar and tidal cycles, and faunal densities. Community composition effects were not identified overall or for the abundance of shallow dwelling organisms that may be more vulnerable to extreme temperatures at the sediment surface. For the polychaete Alitta virens and the bivalve Cerastoderma edule, which exhibit different burrowing abilities, neither species exhibited higher mortality as a result of the heat wave simulations performed. Changes in energy reserves, however, suggested sublethal effects for both, which has implications for their vulnerability to the increased frequency, intensity, and duration of these events predicted for the future. The findings across these studies highlighted the relevance of local context to the patterns of change, suggesting that this must be accounted for in making predictions for how broad-scale climate change will drive change in biodiversity. For intertidal organisms potentially living close to their physiological limits, minimizing local anthropogenic stressors could benefit the current macroinvertebrate communities in the face of a changing climate.
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