Award: OCE-1637396

Coral reefs collectively provide ecosystem services estimated to be worth more than 10 trillion dollars annually by creating jobs, providing food, protecting coastlines and generating tourism. In US fisheries alone, for example, over half of the species harvested rely on coral reefs for at least a portion of their life cycles, underscoring the global importance of coral reefs in sustaining commercial, artisanal and recreational fisheries. In addition to their economic and cultural significance, coral reefs are also of enormous ecological value, supporting more than 25% of all marine life despite covering less than 1% of the ocean floor. Their societal value notwithstanding, coral reefs are under severe threat from multiple sources, including global change, habitat destruction, overfishing, and pollution. Episodic disturbances that kill coral, such as strong storms or severe marine heat waves, are becoming more frequent and more widespread. In the past several decades, numerous tropical reefs have shifted from largely supporting corals to largely supporting seaweeds or other types of space holding organisms. Such undesired shifts often are triggered by a coral-killing disturbance, and typically result in serious degradation of ecosystem services. The main goals of this study were to (1) better understand what causes some but not all local reefs to buffer large, coral-killing disturbances without shifting from coral to seaweeds, (2) explore how changing conditions in the coming decades may alter the ability of coral populations to recover after a disturbance, and (3) investigate whether coral or seaweeds could dominate a local reef under the same environmental conditions. This deeper level of understanding is needed to develop management practices that foster resilient coral reefs, which is essential to safeguarding their economic, ecological, and cultural benefits for future generations. The project produced three major research findings. First, different local stressors to corals such as nutrient pollution varied greatly across the landscape and played a dominant role in determining whether a local reef continued to support coral or shifted to seaweed. Nutrient pollution, which can help fuel proliferation of seaweeds, varied in space due to differences in run-off from land and in water circulation patterns. Herbivorous fishes, which are important for controlling seaweeds, are a large part of the local reef fishery. Variation in the degree of nutrient pollution and in the intensity of fishing was not correlated in space. The project found that at sites enriched by nutrients, corals decreased and seaweeds increased in abundance through time. The observed spatial pattern in shifts to seaweeds was not related to spatial variation in herbivory by fishes targeted in the fishery. When a reef did shift from coral to seaweeds, the change not only altered the assemblage of fishes present, it had cascading effects on such key ecosystem processes as nutrient cycling and production and movement of organic matter, as well as on the biogeochemistry of the reef. A second major finding was that nutrient pollution played an important role in coral bleaching in that it affected the ability of corals to tolerate marine heat waves without bleaching and dying. Excess nitrogen from nutrient pollution lowered the thermal tolerance of corals such that when a mild heat wave occurred, a coral was far more likely to bleach and die at a nutrient-polluted site. The type of nitrogen was found to matter, with the form in run-off from agricultural fertilizer (e.g., nitrate) causing coral far more harm during a mild heat wave compared to natural sources (e.g., nitrogenous wastes from reef fishes). The third major finding was that once a reef shifted from coral to seaweeds, it could remain trapped indefinitely in that undesired state and be difficult or impractical to reverse through management intervention. A core element of the project was a participatory training component that provided hands-on research experience for 102 graduate students, 131 undergraduate students (including 16 REU students), 31 postdoctoral researchers, 61 University faculty researchers, 2 Community College (ROA) teachers and 7 K-12 (RET) teachers. Undergraduate students worked alongside project researchers, were involved in all research and outreach activities, and participated in the project's annual All-Investigator Meetings. Another element of the project was an LTER Schoolyard education program aimed at K-12 students and teachers to help broaden participation in STEM disciplines. The project continued to co-develop teacher resources with participating RET teachers and partner schools that have large enrollments of underrepresented and/or economically disadvantaged groups. The teachers used curricula based on project research, attended project-led professional development activities, and traveled to the project field site for research experiences. Additional activities involved public outreach during organized events such as annual Earth Day celebrations, and project undergraduates served as docents at the REEF (Research Experience & Education Facility), an interactive marine educational facility at UC Santa Barbara that serves over 10,000 K-12 and public visitors annually. Last Modified: 12/15/2024 Submitted by: RussellJSchmitt