Cryptic effects of habitat declines: coral-associated fishes avoid coral-seaweed interactions due to visual and chemical cues

Filamentous calcareous alga provides substrate for coral-competitive macroalgae in the degraded lagoon of Dongsha Atoll, Taiwan

Background

The chemically-rich seaweed Galaxaura is not only highly competitive with corals, but also provides substrate for other macroalgae. Its ecology and associated epiphytes remain largely unexplored. To fill this knowledge gap, we undertook an ecological assessment to explore the spatial variation, temporal dynamics, and diversity of epiphytic macroalgae of Galaxaura divaricata on patch reefs in the lagoon of Dongsha Atoll, a shallow coral reef ecosystem in the northern South China Sea that has been repeatedly impacted by mass coral bleaching events.

Methods

Twelve spatially independent patch reefs in the Dongsha lagoon were first surveyed to assess benthic composition in April 2016, and then revisited to determine G. divaricata cover in September 2017, with one additional Galaxaura-dominated reef (site 9). Four surveys over a period of 17 months were then carried out on a degraded patch reef site to assess the temporal variation in G. divaricata cover. Epiphytic macroalgae associated with G. divaricata were quantified and identified through the aid of DNA barcoding at this degraded site.

Results

Patch reefs in the Dongsha lagoon were degraded, exhibiting relatively low coral cover (5–43%), but high proportions of macroalgae (13–58%) and other substrate (rubble and dead corals; 23–69%). The distribution of G. divaricata was heterogeneous across the lagoon, with highest abundance (16–41%) in the southeast area. Temporal surveys showed consistently high covers (mean ± SD = 16.9 ± 1.21%) of G. divaricata for 17 months. Additional photographic evidence suggested that overgrowth of G. divaricata can persist for 3.5 years. Yet, G. divaricata provides substrate to other macroalgae (e.g., Lobophora sp.) that also limit the growth of corals.

Conclusions

Our study demonstrates that an allelopathic seaweed, such as G. divaricata, can overgrow degraded coral reefs for extended periods of time. By providing habitat for other harmful macroalgae, a prolonged Galaxaura overgrowth could further enhance the spread of macroalgae, and strengthen negative feedback loops, decreasing the recovery potential of degraded reefs.

Seabird nutrients are assimilated by corals and enhance coral growth rates

Nutrient subsidies across ecotone boundaries can enhance productivity in the recipient ecosystem, especially if the nutrients are transferred from a nutrient rich to an oligotrophic ecosystem. This study demonstrates that seabird nutrients from islands are assimilated by endosymbionts in corals on fringing reefs and enhance growth of a dominant reef-building species, Acropora formosa. Nitrogen stable isotope ratios (δ¹⁵N) of zooxanthellae were enriched in corals near seabird colonies and decreased linearly with distance from land, suggesting that ornithogenic nutrients were assimilated in corals. In a one-year reciprocal transplant experiment, A. formosa fragments grew up to four times faster near the seabird site than conspecifics grown without the influence of seabird nutrients. The corals influenced by elevated ornithogenic nutrients were located within a marine protected area with abundant herbivorous fish populations, which kept nuisance macroalgae to negligible levels despite high nutrient concentrations. In this pristine setting, seabird nutrients provide a beneficial nutrient subsidy that increases growth of the ecologically important branching corals. The findings highlight the importance of catchment–to–reef management, not only for ameliorating negative impacts from land but also to maintain beneficial nutrient subsidies, in this case seabird guano.

Spatial and temporal limits of coral-macroalgal competition: the negative impacts of macroalgal density, proximity, and history of contact

Tropical reefs are commonly transitioning from coral- to macroalgal-dominance, producing abrupt, and often lasting, shifts in community composition and ecosystem function. Although negative effects of macroalgae on corals are well documented, whether such effects vary with spatial scale or the density of macroalgae remains inadequately understood, as does the legacy of their impact on coral growth. Using closely adjacent coral- versus macroalgal-dominated areas, we tested effects of macroalgal competition on the Indo-Pacific corals Acropora millepora and Porites cylindrica. When corals were transplanted to areas of: i) macroalgal-dominance, ii) macroalgal-dominance but with nearby macroalgae removed, or iii) coral-dominance lacking macroalgae, coral growth was equivalently high in plots without macroalgae and low (62-90% less) in plots with macroalgae, regardless of location. In a separate experiment, we raised corals above the benthos in each area and exposed them to differing densities of the dominant macroalga Sargassum polycystum. Coral survivorship was high (≥ 93% after 3 months) and did not differ among treatments, whereas the growth of both coral species decreased as a function of Sargassum density. When Sargassum was removed after two months, there was no legacy effect of macroalgal density on coral growth over the next seven months; however, there was no compensation for previously depressed growth. In sum, macroalgal impacts were density dependent, occurred only if macroalgae were in close contact, and coral growth was resilient to prior macroalgal contact. The temporal and spatial constraints of these interactions suggest that corals may be surprisingly resilient to periodic macroalgal competition, which could have important implications for ecosystem trajectories that lead to reef decline or recovery.

Algae associated with coral degradation affects risk assessment in coral reef fishes

Habitat degradation alters the chemical landscape through which information about community dynamics is transmitted. Olfactory information is crucial for risk assessment in aquatic organisms as predators release odours when they capture prey that lead to an alarm response in conspecific prey. Recent studies show some coral reef fishes are unable to use alarm odours when surrounded by dead-degraded coral. Our study examines the spatial and temporal dynamics of this alarm odour-nullifying effect, and which substratum types may be responsible. Field experiments showed that settlement-stage damselfish were not able to detect alarm odours within 2 m downcurrent of degraded coral, and that the antipredator response was re-established 20-40 min after transferral to live coral. Laboratory experiments indicate that the chemicals from common components of the degraded habitats, the cyanobacteria, Okeania sp., and diatom, Pseudo-nitzschia sp.prevented an alarm odour response. The same nullifying effect was found for the common red algae, Galaxauria robusta, suggesting that the problem is of a broader nature than previously realised. Those fish species best able to compensate for a lack of olfactory risk information at key times will be those potentially most resilient to the effects of coral degradation that operate through this mechanism.

Ecological impacts and management implications of reef walking on a tropical reef flat community

Continued growth of tourism has led to concerns about direct and indirect impacts on the ecology of coral reefs and ultimate sustainability of these environments under such pressure. This research assessed impacts of reef walking by tourists on a relatively pristine reef flat community associated with an 'ecoresort' on the Great Barrier Reef, Australia. Heavily walked areas had lower abundances of live hard coral but greater amounts of dead coral and sediment. Abundances of macroalgae were not affected between sites. Coral-associated butterflyfish were less abundant and less diverse in more trampled sites. A manipulative experiment showed handling holothurians on reef walks had lasting negative impacts. This is the first study to show potential impacts of such handling on holothurians. Ecological impacts of reef walking are weighed against sociocultural benefits of a first hand experience in nature.

Chemically cued suppression of coral reef resilience: Where is the tipping point?

Coral reefs worldwide are shifting from high-diversity, coral-dominated communities to low-diversity systems dominated by seaweeds. This shift can impact essential recovery processes such as larval recruitment and ecosystem resilience. Recent evidence suggests that chemical cues from certain corals attract, and from certain seaweeds suppress, recruitment of juvenile fishes, with loss of coral cover and increases in seaweed cover creating negative feedbacks that prevent reef recovery and sustain seaweed dominance. Unfortunately, the level of seaweed increase and coral decline that creates this chemically cued tipping point remains unknown, depriving managers of data-based targets to prevent damaging feedbacks. We conducted flume and field assays that suggest juvenile fishes sense and respond to cues produced by low levels of seaweed cover. However, the herbivore species we tested was more tolerant of degraded reef cues than non-herbivores, possibly providing some degree of resilience if these fishes recruit, consume macroalgae, and diminish negative cues.