Search

Coastal ecosystems are disproportionally inhabited by global human population. Consequently, human impacts originating from land and sea combine with climate-driven disturbances to fundamentally restructure nearshore marine ecosystems. These coincident human stressors are especially acute in the tropics where population centres concentrate along shorelines and marine heatwaves increasingly trigger mass coral bleaching and mortality. However, despite decades of research, we lack consensus as to whether local management supports coral reef resilience under climate change. Here we combined recurring reef surveys with a unique time series of land-sea human impacts over a 20-year period that encompassed an unprecedented marine heatwave in Hawaiʻi. Reefs with high fish biomass and reduced land-based impacts such as wastewater pollution had positive coral trajectories pre-disturbance and experienced minimal coral loss following severe heat stress. Concurrent land-sea management resulted in a 3- to 7-fold increase in the probability of a reef having high reef-builder cover four years post-disturbance. International efforts to conserve 30% of Earth’s ecosystems are gaining momentum, but do not intrinsically link land and ocean conservation targets. Our findings suggest coupled land-sea policy measures are required to realise global conservation goals and provide coral reefs with the best opportunity for persistence in our changing climate.

Ocean warming is increasing the incidence, scale, and severity of global-scale coral bleaching and mortality, culminating in the third global coral bleaching event that occurred during record marine heatwaves of 2014-2017. While local effects of these events have been widely reported, the global implications remain unknown. Analysis of 15,066 reef surveys during 2014-2017 revealed that 80% of surveyed reefs experienced significant coral bleaching and 35% experienced significant coral mortality. The global extent of significant coral bleaching and mortality was assessed by extrapolating results from reef surveys using comprehensive remote-sensing data of regional heat stress. This model predicted that 51% of the world’s coral reefs suffered significant bleaching and 15% significant mortality, surpassing damage from any prior global bleaching event. These observations demonstrate that global warming’s widespread damage to coral reefs is accelerating and underscores the threat anthropogenic climate change poses for the irreversible transformation of these essential ecosystems.

The Hawaiian Archipelago experienced a moderate bleaching event in 2019—the third major bleaching event over a 6-year period to impact the islands. In response, the Hawai‘i Coral Bleaching Collaborative (HCBC) conducted 2,177 coral bleaching surveys across the Hawaiian Archipelago. The HCBC was established to coordinate bleaching monitoring efforts across the state between academic institutions, non-governmental organizations, and governmental agencies to facilitate data sharing and provide management recommendations. In 2019, the goals of this unique partnership were to: 1) assess the spatial and temporal patterns of thermal stress; 2) examine taxa-level patterns in bleaching susceptibility; 3) quantify spatial variation in bleaching extent; 4) compare 2019 patterns to those of prior bleaching events; 5) identify predictors of bleaching in 2019; and 6) explore site-specific management strategies to mitigate future bleaching events. Both acute thermal stress and bleaching in 2019 were less severe overall compared to the last major marine heatwave events in 2014 and 2015. Bleaching observed was highly site- and taxon-specific, driven by the susceptibility of remaining coral assemblages whose structure was likely shaped by previous bleaching and subsequent mortality. A suite of environmental and anthropogenic predictors was significantly correlated with observed bleaching in 2019. Acute environmental stressors, such as temperature and surface light, were equally important as previous conditions (e.g. historical thermal stress and historical bleaching) in accounting for variation in bleaching during the 2019 event. We found little evidence for acclimation by reefs to thermal stress in the main Hawaiian Islands. Moreover, our findings illustrate how detrimental effects of local anthropogenic stressors, such as tourism and urban run-off, may be exacerbated under high thermal stress. In light of the forecasted increase in severity and frequency of bleaching events, future mitigation of both local and global stressors is a high priority for the future of corals in Hawai‘i.

While sewage pollution is contributing to the global decline of coral reefs, its offshore extent and direct reef impacts from water column mixing and benthic seeps are poorly documented. We addressed this knowledge gap on a Hawaiian coral reef using sewage indicator and benthic cover measurements, macroalgal bioassays, and a pollution scoring tool. Fecal indicator bacteria (FIB) and nutrient concentrations were spatially variable in surface and benthic waters, with shoreline values being highest. Shoreline macroalgae δ15N and %N indicated high nitrogen loads containing sewage, while offshore surface and benthic values suggested lower nitrogen loads from environmental sources. Coral cover was negatively correlated with FIB, macroalgal δ15N, and nutrient concentrations. Benthic salinity and temperature measurements detected daily tidal groundwater pulses which may explain these associations. While pollution scores revealed that sewage was largely concentrated along the shoreline, results showed some reached the reef and may be contributing to its declining condition.

Sewage pollution is contributing to the global decline of coral reefs. Identifying locations where it is entering waters near reefs is therefore a management priority. Our study documented shoreline sewage pollution hotspots in a coastal community with a fringing coral reef (Puakō, Hawai'i) using dye tracer studies, sewage indicator measurements, and a pollution scoring tool. Sewage reached shoreline waters within 9 h to 3 d. Fecal indicator bacteria concentrations were high and variable, and δ15N macroalgal values were indicative of sewage at many stations. Shoreline nutrient concentrations were two times higher than those in upland groundwater. Pollution hotspots were identified with a scoring tool using three sewage indicators. It confirmed known locations of sewage pollution from dye tracer studies. Our study highlights the need for a multi-indicator approach and scoring tool to identify sewage pollution hotspots. This approach will be useful for other coastal communities grappling with sewage pollution.

Special Protected Areas (SPAs) were established within the Florida Keys NationalMarine Sanctuary in 1997 to address a variety of management goals including coral reef resourceprotection and recovery. Within these areas all consumptive activities are prohibited. To evaluatewhether SPA establishment has resulted in reef assemblage differences on inshore patch reefs, wequantified reef fish abundance and biomass, grazing intensity and benthic cover on abiotically simi-lar protected and reference reefs. Transect surveys indicated that biomass and mean body lengths forseveral common predatory fish species were significantly greater within SPAs. Size classes of herbiv-orous fish also varied with protection; adult herbivorous fish were significantly more abundant, whilejuveniles were significantly less abundant within SPAs. Grazing intensity and benthic cover weresite-specific and did not vary consistently with protection, suggesting that SPA establishment has notimmediately (within 10 yr) driven observable indirect effects on the benthos.KEY WORDS: Marine reserve · Coral reef · Florida Keys · Trophic cascade · Parrotfish · Herbivory