Coral reefs benefit from reduced land-sea impacts under ocean warming

Coastal areas, particularly tropical coral reefs, face immense pressure from both local human impacts and climate change. High population densities near shorelines result in significant land-based stressors like wastewater pollution and urban runoff, compounding the effects of sea-based stressors such as overfishing. These stressors, alongside increasingly frequent and intense marine heatwaves causing mass coral bleaching and mortality, are fundamentally altering coral reef ecosystems. While reducing local human impacts is a cornerstone of coral reef conservation, contemporary governance often keeps terrestrial and ocean management efforts siloed. This study addresses the lack of clear guidance for targeted actions by examining the combined effects of land and sea-based human impacts on coral reef resilience and recovery in Hawai'i, a region that experienced an unprecedented marine heatwave in 2015. The goal is to provide unambiguous targets for integrated land-sea management that will best support coral reef persistence under a changing climate. The challenge lies in overcoming the limitations of using proxies for human impacts (e.g., population density) and employing spatially and temporally resolved data to accurately assess the drivers of coral reef change.

Decades of research underscore the importance of reducing human impacts to maintain coral reef ecosystem integrity. Indigenous stewardship practices often utilized a decentralized and integrated approach to resource management, spanning from mountains to the sea. However, modern centralized governance has largely separated terrestrial and marine management. While the benefits of an integrated land-sea approach are often stated, concrete evidence of its superiority over isolated approaches has been lacking. Recent studies have highlighted connections between local conditions and coral reef resistance and recovery after mass bleaching, but spatially resolved, long-term data on local drivers remain scarce. Existing studies often rely on proxies for human impact or composite indices, which hinder identification of specific policy levers and effective management actions. This paper addresses these shortcomings by providing a comprehensive, long-term analysis.

This study utilizes a unique 20-year time series (2000-2019) of land-sea human impacts and environmental factors in the Hawaiian Islands. Human factors included urban runoff, wastewater pollution, nutrient loading, sediment input, and fishing gear restrictions. Environmental factors encompassed rainfall, wave exposure, ocean temperature variability, heat stress, irradiance, and phytoplankton biomass. Multiple fish biomass metrics were also incorporated to account for their crucial role in maintaining reef ecosystem function. This dataset was combined with recurring, permanently marked underwater surveys of coral reef benthic communities from 2003 to 2019. The study reefs spanned significant spatiotemporal gradients in land-sea human impacts and environmental factors, providing a broad range of conditions representative of global coral reef ecosystems. Data analysis involved quantifying drivers of coral reef benthic change in three periods: 12 years before the 2015 marine heatwave (pre-disturbance), during and immediately after the heatwave, and four years post-disturbance. Statistical techniques included PERMANOVA (permutational multivariate analysis of variance), canonical analysis of principal coordinates, generalized additive mixed-modelling (GAMM), and ordinal logistic regression. These were used to determine the influence of various factors on coral reef trajectories, response to heatwaves, and persistence following heat stress.

The study reveals several key findings regarding the influence of land and sea-based human impacts on coral reef health: **Pre-disturbance:** Reefs with positive coral cover trajectories (increased coral cover over 12 years) exhibited significantly higher herbivorous fish biomass and lower levels of wastewater pollution, nutrient loading, and urban runoff compared to reefs with negative trajectories. Interestingly, higher human population density was associated with positive trajectories, indicating that population density alone is not a reliable indicator of negative impacts. **During the heatwave:** The 2015 marine heatwave in Hawai'i was severe, with all reefs experiencing substantial heat stress. However, coral mortality varied widely. Reefs with higher phytoplankton biomass experienced reduced coral mortality, likely due to increased nutritional subsidies to corals, providing greater energetic reserves to withstand stress. Lower levels of urban runoff and sediment input were also associated with reduced coral loss. **Post-disturbance:** Four years after the heatwave, the cover of reef-building organisms (hard corals and crustose coralline algae) varied considerably among reefs. Simultaneous reductions in wastewater pollution and increases in scraper biomass (herbivorous fish that remove algae) were strong predictors of high reef-builder cover. Scenario modeling demonstrated that an integrated land-sea management approach (reducing wastewater pollution and increasing scraper biomass) yielded a three- to sixfold greater probability of high reef-builder cover than managing land or sea in isolation.

The findings demonstrate that a holistic, integrated approach to managing both land and sea-based human impacts is crucial for supporting coral reef persistence in the face of climate change. The specific factors influencing coral reef health varied across time periods, highlighting the dynamic nature of these ecosystems. The strong positive association between herbivorous fish biomass and positive coral trajectories pre-disturbance, and the importance of scraper biomass for recovery post-disturbance, emphasize the critical roles played by fish in maintaining ecosystem function. Land-based impacts consistently emerged as significant negative drivers of coral health. The study provides clear evidence of the synergistic benefits of combining land-sea management, emphasizing that sea-based efforts alone are insufficient to address the full spectrum of local impacts. While extreme heat stress can overwhelm even healthy reefs, mitigating local impacts enhances resilience and recovery potential.

This study underscores the critical importance of integrated land-sea management for coral reef conservation. Simultaneous reduction of land-based pollution (especially wastewater) and enhancement of herbivorous fish populations significantly improves coral reef persistence, both before, during, and after extreme marine heatwaves. The findings highlight the need for coupled land-sea policies that address the multifaceted drivers of reef decline. Future research should focus on understanding the interplay of local management actions with broader socio-economic factors and the potential for adaptation to future climate change scenarios.

While this study provides strong evidence from a well-defined region, the results may not be directly generalizable to all coral reef ecosystems. The specific land-sea human impacts and their relative importance could vary depending on geographical context, local stressors, and species composition. The study focuses on one major heatwave event; long-term monitoring is needed to confirm the persistence of observed trends under future heatwave events and varying conditions. The focus on relatively isolated Hawaiian reefs could limit the generalizability to highly connected or more heavily impacted reef ecosystems.