Sea-weeding: Manual removal of macroalgae facilitates rapid coral recovery

Coral reef ecosystems globally are under threat, leading to declining coral cover and macroalgal proliferation. Manually removing macroalgae (i.e. ‘sea-weeding’) may promote local-scale coral recovery by reducing a biological barrier, though the impact of removal on community composition of benthic reef organisms has not been quantified. In this three-year study (2018–2021), fleshy macroalgae (predominantly Sargassum spp.) were periodically removed from 25 m2 experimental plots on two inshore fringing reefs of Yunbenun (Magnetic Island) in the central Great Barrier Reef. By the end of the study, coral cover in removal plots (n = 12 plots) assessed through in-field transects increased by at least 47% (2019 mean: 25.5%, 2021 mean: 37.4%), and macroalgal cover decreased by more than half. In contrast, in control plots (n = 12 plots), there was no change in macroalgal cover while coral cover remained stable (2019 mean: 16.4%, 2021 mean: 13.6%). Changes in benthic cover were supported by photoquadrat data, with Bayesian probability modelling indicating a 100% likelihood that coral cover more than doubled in removal plots over the study period, compared to only a 29% chance of increased coral cover in control plots. Synthesis and applications. Manual macroalgal removal can provide rapid benefits and enhance inshore coral reef recovery. Through involvement of community groups and citizen scientists, larger scale removal of macroalgae is a low-tech, high-impact, and achievable method for local reef management

Growth anomalies on the coral genera Acropora and Porites are strongly associated with host density and human population size across the Indo-Pacific

Growth anomalies (GAs) are common, tumor-like diseases that can cause significant morbidity and decreased fecundity in the major Indo-Pacific reef-building coral genera, Acropora and Porites. GAs are unusually tractable for testing hypotheses about drivers of coral disease because of their pan-Pacific distributions, relatively high occurrence, and unambiguous ease of identification. We modeled multiple disease-environment associations that may underlie the prevalence of Acropora growth anomalies (AGA) (n = 304 surveys) and Porites growth anomalies (PGA) (n = 602 surveys) from across the Indo-Pacific. Nine predictor variables were modeled, including coral host abundance, human population size, and sea surface temperature and ultra-violet radiation anomalies. Prevalence of both AGAs and PGAs were strongly host density-dependent. PGAs additionally showed strong positive associations with human population size. Although this association has been widely posited, this is one of the first broad-scale studies unambiguously linking a coral disease with human population size. These results emphasize that individual coral diseases can show relatively distinct patterns of association with environmental predictors, even in similar diseases (growth anomalies) found on different host genera (Acropora vs. Porites). As human densities and environmental degradation increase globally, the prevalence of coral diseases like PGAs could increase accordingly, halted only perhaps by declines in host density below thresholds required for disease establishment

Thermal Stress and Coral Cover as Drivers of Coral Disease Outbreaks

Very little is known about how environmental changes such as increasing temperature affect disease dynamics in the ocean, especially at large spatial scales. We asked whether the frequency of warm temperature anomalies is positively related to the frequency of coral disease across 1,500 km of Australia's Great Barrier Reef. We used a new high-resolution satellite dataset of ocean temperature and 6 y of coral disease and coral cover data from annual surveys of 48 reefs to answer this question. We found a highly significant relationship between the frequencies of warm temperature anomalies and of white syndrome, an emergent disease, or potentially, a group of diseases, of Pacific reef-building corals. The effect of temperature was highly dependent on coral cover because white syndrome outbreaks followed warm years, but only on high (>50%) cover reefs, suggesting an important role of host density as a threshold for outbreaks. Our results indicate that the frequency of temperature anomalies, which is predicted to increase in most tropical oceans, can increase the susceptibility of corals to disease, leading to outbreaks where corals are abundant

The role of environment and microorganisms in diseases of corals: overview of DAO Special 5

Coral reefs are in decline worldwide. In the last several decades, bleaching and disease in a warming ocean have emerged as dominant drivers of ecological change on coral reefs. This special issue of DAO presents papers based on presentations from the 11th International Coral Reef Symposium (2008, Fort Lauderdale, Florida, USA). The articles herein document disease outbreaks involving novel hosts, pathogens and/or locations, experimental studies investigating processes and mechanisms underlying pathogen dynamics, and the application of increasingly sophisticated laboratory and modeling approaches to understanding disease epizootiology

No bundles beyond this point: the coral sterile zone

Many species of colonial corals have sterile zones—areas where polyps are sexually immature. While inconspicuous in many species, the sterile zones observed in Acropora glauca in 2019 were striking, likely due to the tabular growth morphology, highly pigmented eggs, and high degree of intracolony spawning synchrony in this species

Coral disease on the Great Barrier Reef

[Extract] Coral disease is one of the most recent in a series of threats that is challenging the resilience of coral reef communities and is of particular concern because it may interact with and augment the impacts of other commonly recognised threats to coral health (e.g. bleaching, over-exploitation of fish stocks, destructive fishing practices and coastal developments). Since the first report of coral disease by Antonius in 1973, the rate of discovery of new diseases has increased dramatically with more than 29 coral diseases now described (Green and Bruckner 2000, Weil, this Vol.).Although coral disease is emerging as one of the major causes of coral reef deterioration in the Caribbean (Hayes and Goreau 1998; Harvell et al. 2002; Weil et al. 2002),at present we know very little about the ecology or pathology of coral disease on Indo- Pacific reefs. The comparatively few reports of coral disease from Indo-Pacific reefs, despite the region encompassing more than 80%of reefs worldwide (Bryant et al. 1998)is in contrast to the high proportion (>65%) of records in the Global Disease Database from the Caribbean reef region, now widely considered to be a coral disease hotspot (Green and Bruckner 2000; Weil, this Vol.). Such comparisons suggest that either disease is genuinely more prevalent in the Caribbean or lack of studies in other reef regions is underestimating its distribution and abundance. Distinguishing between these two alternatives represents an important step in advancing global epizootiological studies. The rising incidence of marine diseases worldwide in the past few decades (Harvell et al. 1999), and particularly of coral diseases in the Caribbean, underscores the need for assessment of the status of disease on a region-by-region basis. Such assessments will help to identify the origins and reservoirs of pathogens and vectors involved in disease transmission. The Great Barrier Reef (GBR) stretches over 2000 km along the eastern coastline of Australia, representing the largest reef tract under management worldwide. Its unique status as one of the few reef systems under government jurisdiction for timescales that have preceded recent increases in the prevalence of coral disease has the potential to provide important insights into factors influencing disease occurrence and the underlying causes of escalating disease incidence. In this chapter, we summarise the current state of knowledge of coral disease on the Great Barrier Reef by (1) describing syndromes and diseases observed in our studies on GBR reefs and interpreted in the light of published literature and (2) presenting the results of a 5-year, large-scale study in conjunction with a regional disease prevalence study that together provide an overview of the current status of disease occurring on reefs extending over 1200 km of the Great Barrier Reef

Larval precompetency and settlement behaviour in 25 Indo-Pacific coral species

Abstract Knowledge of coral larval precompetency periods and maximum competency windows is fundamental to understanding coral population dynamics, informing biogeography and connectivity patterns, and predicting reef recovery following disturbances. Yet for many species, estimates of these early-life history metrics are scarce and vary widely. Furthermore, settlement cues for many taxa are not known despite consequences to habitat selection. Here we performed a comprehensive experimental time-series investigation of larval settlement behaviour, for 25 Indo-Pacific broadcast-spawning species. To investigate the duration of precompetency, improve predictions of the competency windows, and compare settlement responses within and amongst species, we completed replicated and repeated 24-hour assays that exposed larvae to five common settlement cues. Our study revealed that larval competency in some broadcast-spawning species begins as early as two days post fertilization, but that the precompetency period varies within and between species from about two to six days, with consequences for local retention and population connectivity. We also found that larvae of some species are competent to settle beyond 70 days old and display complex temporal settlement behaviour, challenging the assumption that competency gradually wanes over time and adding to the evidence that larval longevity can support genetic connectivity and long-distance dispersal. Using these data, we grouped coral taxa by short, mid and long precompetency periods, and identified their preferred settlement cues. Taken together, these results inform our understanding of larval dynamics across a broad range of coral species and can be applied to investigations of population dynamics, connectivity, and reef recovery

Latitudinal variation in reproductive synchrony in Acropora assemblages: Japan vs. Australia

Abstract The annual mass spawning of scleractinian corals on the Great Barrier Reef (GBR), Australia, is purported to be unprecedented in terms of the taxonomic and geographical scale of spawning synchrony. Here, we compare spawning synchrony both within and among coral species in four regions spanning 10°of latitude on the GBR and compare this with four regions separated by a similar latitudinal range within the Japanese tropical and sub-tropical Archipelago. On the GBR, peak reproductive activity at all latitudes occurred in November whereas there was a clear disjunction in the period of peak reproductive activity among the four regions in Japan, with tropical locations spawning up to 3 months earlier. In Sekesei Lagoon (22°N), a high proportion of Acropora colonies were mature following the full moon in April; at Akajima Island (26°N) peak reproductive activity occurred in May; at Oku (28°N) peak reproductive activity occurred in June and in Amakusa (31°N), reproductive activity likely peaked in July. However, mature colonies of Acropora were found prior to every full moon for at least 5 months at two regions examined in detail (The Whitsundays Islands on the GBR and Akajima Island). While these regions on GBR appear to act more homogeneously than regions over a similar latitudinal range in Japan with respect to the timing of peak reproductive activity, and the proportion of colonies mature was often higher during these peaks, the reproductive season on the GBR is much longer (5 months) than is typically appreciated

Latitudinal variation in reproductive synchrony in Acropora assemblages: Japan vs. Australia

Abstract The annual mass spawning of scleractinian corals on the Great Barrier Reef (GBR), Australia, is purported to be unprecedented in terms of the taxonomic and geographical scale of spawning synchrony. Here, we compare spawning synchrony both within and among coral species in four regions spanning 10°of latitude on the GBR and compare this with four regions separated by a similar latitudinal range within the Japanese tropical and sub-tropical Archipelago. On the GBR, peak reproductive activity at all latitudes occurred in November whereas there was a clear disjunction in the period of peak reproductive activity among the four regions in Japan, with tropical locations spawning up to 3 months earlier. In Sekesei Lagoon (22°N), a high proportion of Acropora colonies were mature following the full moon in April; at Akajima Island (26°N) peak reproductive activity occurred in May; at Oku (28°N) peak reproductive activity occurred in June and in Amakusa (31°N), reproductive activity likely peaked in July. However, mature colonies of Acropora were found prior to every full moon for at least 5 months at two regions examined in detail (The Whitsundays Islands on the GBR and Akajima Island). While these regions on GBR appear to act more homogeneously than regions over a similar latitudinal range in Japan with respect to the timing of peak reproductive activity, and the proportion of colonies mature was often higher during these peaks, the reproductive season on the GBR is much longer (5 months) than is typically appreciated

Influence of marine reserves on coral disease prevalence

Predicted increases in disease with climate warming highlight the need for effective management strategies to mitigate disease effects in coral communities. We examined the role of marine protected areas (MPAs) in reducing disease in corals and the hypothesis that the composition of fish communities can influence coral health, by comparing disease prevalence between MPA and non-protected (control) reefs in Palau. Overall, the prevalence of diseases pooled, as well as the prevalence of skeletal eroding band (SEB), brown band disease (BrB) and growth anomalies (GAs) individually in major disease hosts (families Acroporidae and Poritidae), were not significantly\ud reduced within MPAs. In fact, the prevalence of SEB was 2-fold higher within MPAs overall; however, the 4 studied MPAs were ineffective in enhancing coral assemblage or fish stock health. A negative association between the prevalence of SEB and richness of a fish species targeted by fishers in\ud Palau highlights the potential role that well-managed MPAs could play in reducing SEB. The composition of coral communities and their susceptibility to bleaching also influenced the prevalence of disease on the studied reefs. The prevalence of diseases pooled and SEB were positively associated with the cover of major disease hosts (families Acroporidae and Poritidae), and the prevalence of BrB and\ud bleaching were also positively associated. Although our study did not show positive effects of MPAs on coral heath, we did identify the potential for increased fish diversity within MPAs to reduce coral disease. Our study also highlights the complexity of relationships between fish assemblages, coral community composition and coral health on Indo-Pacific reefs