Date Published: March 14, 2018
Publisher: Public Library of Science
Author(s): Jade M. S. Delevaux, Robert Whittier, Kostantinos A. Stamoulis, Leah L. Bremer, Stacy Jupiter, Alan M. Friedlander, Matthew Poti, Greg Guannel, Natalie Kurashima, Kawika B. Winter, Robert Toonen, Eric Conklin, Chad Wiggins, Anders Knudby, Whitney Goodell, Kimberly Burnett, Susan Yee, Hla Htun, Kirsten L. L. Oleson, Tracy Wiegner, Tamara Ticktin, Chaolun Allen Chen.
Declining natural resources have led to a cultural renaissance across the Pacific that seeks to revive customary ridge-to-reef management approaches to protect freshwater and restore abundant coral reef fisheries. Effective ridge-to-reef management requires improved understanding of land-sea linkages and decision-support tools to simultaneously evaluate the effects of terrestrial and marine drivers on coral reefs, mediated by anthropogenic activities. Although a few applications have linked the effects of land cover to coral reefs, these are too coarse in resolution to inform watershed-scale management for Pacific Islands. To address this gap, we developed a novel linked land-sea modeling framework based on local data, which coupled groundwater and coral reef models at fine spatial resolution, to determine the effects of terrestrial drivers (groundwater and nutrients), mediated by human activities (land cover/use), and marine drivers (waves, geography, and habitat) on coral reefs. We applied this framework in two ‘ridge-to-reef’ systems (Hā‘ena and Ka‘ūpūlehu) subject to different natural disturbance regimes, located in the Hawaiian Archipelago. Our results indicated that coral reefs in Ka‘ūpūlehu are coral-dominated with many grazers and scrapers due to low rainfall and wave power. While coral reefs in Hā‘ena are dominated by crustose coralline algae with many grazers and less scrapers due to high rainfall and wave power. In general, Ka‘ūpūlehu is more vulnerable to land-based nutrients and coral bleaching than Hā‘ena due to high coral cover and limited dilution and mixing from low rainfall and wave power. However, the shallow and wave sheltered back-reef areas of Hā‘ena, which support high coral cover and act as nursery habitat for fishes, are also vulnerable to land-based nutrients and coral bleaching. Anthropogenic sources of nutrients located upstream from these vulnerable areas are relevant locations for nutrient mitigation, such as cesspool upgrades. In this study, we located coral reefs vulnerable to land-based nutrients and linked them to priority areas to manage sources of human-derived nutrients, thereby demonstrating how this framework can inform place-based ridge-to-reef management.
Over the past century, climate change became a global threat to coral reefs as it directly impacts corals through bleaching, ocean acidification, and intensified storms [1–3]. At the local scale, human activities also impact coral reefs through increasing land-based source pollution and fishing pressure [3–5]. These trends have led some coral reefs to shift towards algae dominated phases, causing the decline of important resources upon which human wellbeing depends [6,7]. Thus, managing for coral reef resilience has become a priority for conservation planning . Resilience is the capacity of an ecosystem to cope with its disturbance regime without shifting to an alternative state, while maintaining its functions and delivery of ecosystem service . Ridge-to-reef management has been widely advocated to foster coral reef resilience, though the degree to which managing local drivers can benefit coral reefs varies among places [10,11]. The types of management actions needed to maintain coral reef resilience will differ spatially, depending on the characteristics of each ridge-to-reef system.
To support ridge-to-reef management in high oceanic islands, this study developed a linked land-sea modeling framework that connects land cover/use to coral reefs through groundwater enriched nutrients at fine spatial resolution. We applied this framework in two ahupua‘a subject to different natural disturbance regimes to compare and contrast the effects of terrestrial and marine drivers on coral reefs. Our results indicate that the terrestrial and marine drivers differed between sites due to their natural disturbance regimes and different island age. Hā‘ena is primarily influenced by large-scale drivers (high rainfall and wave power), while Ka‘ūpūlehu is mostly governed by local drivers (habitat and nutrients). Consistent with previous studies [22,130–132], our coral reef models showed that the high disturbance regime of Hā‘ena has shaped a coral reef community dominated by CCA with cover of high turf algae and many grazers, while the low wave disturbance regime of Ka‘ūpūlehu has allowed for the accretion of a coral dominated community with high turf and many grazers and scrapers. Similar to other studies [16,17,26,133–135], our coral reef predictive models showed that land-based nutrients can increase benthic algae, inhibit reef calcifiers (CCA), and decrease the biomass of locally important fishes. This study shows how coral reefs can differ under the influence of different natural disturbance regimes combined with local-scale terrestrial and marine drivers, thereby reinforcing the need for place-based ridge-to-reef management.
Managers need spatially-explicit place-based models to better understand the impact of anthropogenic drivers on coral reefs and manage them more effectively. Empirical data provide point data at the location of the survey, but do not provide a continuous surface to support spatial prioritization of management actions . Tools that provide visualization and quantify potential impacts are needed to better manage coral reefs [11,168]. The linked land-sea modeling framework presented here can help managers evaluate the spatial variation and influence of terrestrial and marine drivers, mediated by anthropogenic activities, on coral reefs, and prioritize management actions accordingly. Although these linked land-sea models were built to understand the land-sea linkages specific to these places, many of the processes, ecological effects, and management actions, we described can be generalized to other oceanic island environments comprised within this spectrum of natural disturbance regimes. Additionally, when calibrated for a place and assuming the fundamental ecological relationships are constant over time, this framework can be used to forecast and assess indicator distributions based on land cover/use change, marine closures, and climate change scenarios.