The inshore reefs of the Great Barrier Reef (GBR) have undergone significant declines in water quality following European settlement (approx. 1870 AD). However, direct evidence of impacts on coral assemblages is limited by a lack of historical baselines prior to the onset of modern monitoring programmes in the early 1980s. Through palaeoecological reconstructions, we report a previously undocumented historical collapse of Acropora assemblages at Pelorus Island (central GBR). High-precision U-series dating of dead Acropora fragments indicates that this collapse occurred between 1920 and 1955, with few dates obtained after 1980. Prior to this event, our results indicate remarkable long-term stability in coral community structure over centennial scales. We suggest that chronic increases in sediment flux and nutrient loading following European settlement acted as the ultimate cause for the lack of recovery of Acropora assemblages following a series of acute disturbance events (SST anomalies, cyclones and flood events). Evidence for major degradation in reef condition owing to human impacts prior to modern ecological surveys indicates that current monitoring of inshore reefs on the GBR may be predicated on a significantly shifted baseline.

Glomalin, an arbuscular mycorrhizal protein component of soil, can be used as an indicator of terrigenous‐derived carbon. We measured glomalin in sediments using the terrestrial end‐member as a reference in four coastal settings: (1) intertidal seagrass meadows distributed over a rainfall gradient, (2) sediments inshore and offshore from the mouth of a river, (3) coastal coral reefs at various distances from the shore, and (4) intertidal wetlands with varying levels of groundwater influence. Across the rainfall gradient, glomalin in seagrass meadow sediments increased at sites with high mean annual rainfall during the wet season (r² = 0.27; F_1,29 = 5.75; p = 0.029). Glomalin decreased in inshore river sediments (terrestrial) to offshore (marine) sediments (r² = 0.81; F_1,17 = 71.7; p ≤ 0.0001). Furthermore, glomalin in reef sediments decreased with distance from the shore. The high intertidal was rich in glomalin where groundwater flowed directly into the wetland compared with those with little groundwater influences. Our data indicate that rivers and groundwater transport terrestrial material, and that mangroves, salt marsh, seagrass meadows, and coral reefs accumulate it, but the connections vary among sites, within sites, and seasonally. Variations in glomalin concentrations are indicative of links between the terrestrial and marine environment that reflect proximity, filtration services, and the level of subsidies that marine ecosystems derive from terrestrial sources. Assessment of glomalin contributes to evaluating terrestrial–marine connectivity, and thus provides knowledge to improve catchment management for the protection of marine ecosystems.

The combination of thermal stress and ocean acidification (OA) can more negatively affect coral calcification than an individual stressors, but the mechanism behind this interaction is unknown. We used two independent methods (microelectrode and boron geochemistry) to measure calcifying fluid pH (pH_cf) and carbonate chemistry of the corals Pocillopora damicornis and Stylophora pistillata grown under various temperature and pCO₂ conditions. Although these approaches demonstrate that they record pH_cf over different time scales, they reveal that both species can cope with OA under optimal temperatures (28°C) by elevating pH_cf and aragonite saturation state (Ω_cf) in support of calcification. At 31°C, neither species elevated these parameters as they did at 28°C and, likewise, could not maintain substantially positive calcification rates under any pH treatment. These results reveal a previously uncharacterized influence of temperature on coral pH_cf regulation—the apparent mechanism behind the negative interaction between thermal stress and OA on coral calcification.

Corals are globally important calcifiers that exhibit complex responses to anthropogenic warming and acidification. Although coral calcification is supported by high seawater pH, photosynthesis by the algal symbionts of zooxanthellate corals can be promoted by elevated pCO₂. To investigate the mechanisms underlying corals’ complex responses to global change, three species of tropical zooxanthellate corals (Stylophora pistillata, Pocillopora damicornis, and Seriatopora hystrix) and one species of asymbiotic cold-water coral (Desmophyllum pertusum, syn. Lophelia pertusa) were cultured under a range of ocean acidification and warming scenarios. Under control temperatures, all tropical species exhibited increased calcification rates in response to increasing pCO₂. However, the tropical species’ response to increasing pCO₂ flattened when they lost symbionts (i.e., bleached) under the high-temperature treatments—suggesting that the loss of symbionts neutralized the benefit of increased pCO₂ on calcification rate. Notably, the cold-water species that lacks symbionts exhibited a negative calcification response to increasing pCO₂, although this negative response was partially ameliorated under elevated temperature. All four species elevated their calcifying fluid pH relative to seawater pH under all pCO₂ treatments, and the magnitude of this offset (Δ[H⁺]) increased with increasing pCO₂. Furthermore, calcifying fluid pH decreased along with symbiont abundance under thermal stress for the one species in which calcifying fluid pH was measured under both temperature treatments. This observation suggests a mechanistic link between photosymbiont loss (‘bleaching’) and impairment of zooxanthellate corals’ ability to elevate calcifying fluid pH in support of calcification under heat stress. This study supports the assertion that thermally induced loss of photosymbionts impairs tropical zooxanthellate corals’ ability to cope with CO₂-induced ocean acidification.