The effect of increasing temperature on algae-fish interactions on coral reefs

Alexia Graba-Landry investigated the effect of increasing temperature to algae-fish interactions on coral reefs. She found that algae were more sensitive to increases in temperature than fishes suggesting that algal overgrowth of coral reefs into the future may be less likely than previously assumed

Cross-Shelf Differences in the Response of Herbivorous Fish Assemblages to Severe Environmental Disturbances

Cross-shelf differences in coral reef benthic and fish assemblages are common, yet it is unknown whether these assemblages respond uniformly to environmental disturbances or whether local conditions result in differential responses of assemblages at different shelf positions. Here, we compare changes in the taxonomic and functional composition, and associated traits, of herbivorous reef fish assemblages across a continental shelf, five years before and six months after two severe cyclones and a thermal bleaching event that resulted in substantial and widespread loss of live hard coral cover. Each shelf position maintained a distinct taxonomic assemblage of fishes after disturbances, but the assemblages shared fewer species among shelf positions. There was a substantial loss of species richness following disturbances within each shelf position. Total biomass of the herbivorous fish assemblage increased after disturbances on mid- and outer-shelf reefs, but not on inner-shelf reefs. Using trait-based analyses, we found there was a loss of trait richness at each shelf position, but trait specialisation and originality increased on inner-shelf reefs. This study highlights the pervasiveness of extreme environmental disturbances on ecological assemblages. Whilst distinct cross-shelf assemblages can remain following environmental disturbances, assemblages have reduced richness and are potentially more vulnerable to chronic localised stresses

Amelioration of ocean acidification and warming effects through physiological buffering of a macroalgae

Concurrent anthropogenic global climate change and ocean acidification are expected to have a negative impact on calcifying marine organisms. While knowledge of biological responses of organisms to oceanic stress has emerged from single-species experiments, these do not capture ecologically relevant scenarios where the potential for multi-organism physiological interactions is assessed. Marine algae provide an interesting case study, as their photosynthetic activity elevates pH in the surrounding microenvironment, potentially buffering more acidic conditions for associated epiphytes. We present findings that indicate increased tolerance of an important epiphytic for aminifera, Marginopora vertebralis, to the effects of increased temperature (+/- 3 degrees C) and p CO2(similar to 1,000 mu atm) when associated with its common algal host, Laurencia intricata. Specimens of M. vertebralis were incubated for 15 days in flow-through aquaria simulating current and end-of-century temperature and pH conditions. Physiological measures of growth (change in wet weight), calcification (measured change in total alkalinity in closed bottles), photochemical efficiency (Fv/Fm), total chlorophyll, photosynthesis (oxygen flux), and respiration were determined. When incubated in isolation,M. vertebralis-exhibited reduced growth in end-of-century projections of ocean acidification conditions, while calcification rates were lowest in the high-temperature, low-pH treatment. Interestingly, association with L. intricata ameliorated these stress effects with the growth and calcification rates ofM. vertebralis being similar to those observed in ambient conditions. Total chlorophyll levels in M. vertebral is decreased when in association with L. intricata, while maximum photochemical efficiency increased in ambient conditions. Net production estimates remained similar between M. vertebralis in isolation and in association with L. intricata, although both production and respiration rates ofM. vertebralis were significantly higher when associated with L. intricata. These results indicate that the association with L. intricata increases the resilience ofM. vertebralis to climate change stress, providing one of the first examples of physiological buffering by a marine alga that can ameliorate the negative effects of changing ocean conditions

Ocean warming has greater and more consistent negative effects than ocean acidification on the growth and health of subtropical macroalgae

Macroalgae are the major habitat-forming organisms in many coastal temperate and subtropical marine systems. Although climate change has been identified as a major threat to the persistence of macroalgal beds, the combined effects of ocean warming and ocean acidification on algal performance are poorly understood. Here we investigate the effects of increased temperature and acidification on the growth, calcification and nutritional content of 6 common subtropical macroalgae; Sargassum linearifolium, Ulva sp., Amphiroa anceps, Corallina officinalis, Delisea pulchra and Laurencia decussata. Algae were reared in a factorial cross of 3 temperatures (23°C [ambient], 26°C and 28°C) and 3 pH levels (8.1 [ambient], 7.8 and 7.6) for 2 wk. The highest (28°C) temperature decreased the growth of all 6 macroalgal species, irrespective of the pH levels. In contrast, the effect of decreased pH on growth was variable. The growth of Ulva sp. and C. officinalis increased, L. decussata decreased, while the remaining 3 species were unaffected. Interestingly, the differential responses of macroalgae to ocean acidification were unrelated to whether or not a species was a calcifying alga, or their carbon-uptake mechanism—2 processes that are predicted to be sensitive to decreased pH. The growth of the calcifying algae (C. officinalis and A. anceps) was not affected by reduced pH but calcification of these 2 algae was reduced when exposed to a combination of reduced pH and elevated temperature. The 3 species capable of uptake of bicarbonate, S. linearifolium, L. decussata and Ulva sp., displayed positive, negative and neutral changes in growth, respectively, in response to reduced pH. The C:N ratio for 5 of the 6 species was unaffected by either pH or temperature. The consistent and predictable negative effects of temperature on the growth and calcification of subtropical macroalgae suggests that this stressor poses a greater threat to the persistence of subtropical macroalgal populations than ocean acidification under ongoing and future climate change

Larval phenotypic plasticity in the boom-and-bust crown-of-thorns seastar, Acanthaster planci

Many echinoderm larvae exhibit phenotypic plasticity: a change in phenotype in response to environmental food levels. We investigated phenotypic plasticity in the larvae of the crown-of-thorns seastar Acanthaster planci, an opportunistic boom-and-bust species with larvae that have a strong response to food conditions. The increased predation pressure resulting from outbreaks (population explosions) of A. planci is deleterious to coral reefs, but the link between population outbreaks and larval ecology is poorly understood. We hypothesised that the larvae of A. planci would have a different morphological profile in the oligotrophic conditions typical of tropical waters than in the eutrophic conditions associated with increased nutrients. We predicted that larvae reared in low food conditions would increase their ciliated band length to enhance feeding potential. Larvae were fed algal concentrations representing starvation (0 cells ml-1), low food (oligotrophic; 1000 cells ml-1), high food (eutrophic; 10000 cells ml-1) or excessive food (100000 cells ml-1) conditions. A phenotypic response was evident. Larvae in the 2 high food treatments had a shorter ciliated band length relative to body size. Conversely, larvae in the starvation and low food treatments had longer ciliated bands relative to body size, a change that would enhance particle capture capacity and facilitate larval success. This plastic response of the larvae of A. planci could have flow-on effects to adult populations, potentially facilitating population outbreaks

Superstars: assessing nutrient thresholds for enhanced larval success of Acanthaster planci, a review of the evidence

Crown-of-thorns starfish, Acanthaster planci (COTS), predation is a major cause of coral reef decline, but the factors behind their population outbreaks remain unclear. Increased phytoplankton food resulting from eutrophication is suggested to enhance larval survival. We addressed the hypothesis that larval success is associated with particular chl-a levels in tightly controlled larval:algal conditions. We used chl-a conditions found on coral reefs (0.1-5.0μgchl-aL(-1)), including nominal threshold levels for disproportionate larval success (≥1.0μgchl-aL(-1)). High success to the juvenile occurred across an order of magnitude of chl-a concentrations (0.5-5.0μgchl-aL(-1)), suggesting there may not be a narrow value for optimal success. Oligotrophic conditions (0.1μgchl-aL(-1)) appeared to be a critical limit. With a review of the evidence, we suggest that opportunistic COTS larvae may be more resilient to low food levels than previously appreciated. Initiation of outbreak populations need not require eutrophic conditions

Effect of acute seawater temperature increase on the survival of a fish ectoparasite

Extreme warming events that contribute to mass coral bleaching are occurring with increasing regularity, raising questions about their effect on coral reef ecological interactions. However, the effects of such events on parasite-host interactions are largely ignored. Gnathiid isopods are common, highly mobile, external parasites of coral reef fishes, that feed on blood during the juvenile stage. They have direct and indirect impacts on their fish hosts, and are the major food source for cleaner fishes. However, how these interactions might be impacted by increased temperatures is unknown. We examined the effects of acute temperature increases, similar to those observed during mass bleaching events, on survivorship of gnathiid isopod juveniles. Laboratory experiments were conducted using individuals from one species (Gnathia aureamaculosa) from the Great Barrier Reef (GBR), and multiple unknown species from the central Philippines. Fed and unfed GBR gnathiids were held in temperature treatments of 29 °C to 32 °C and fed Philippines gnathiids were held at 28 °C to 36 °C. Gnathiids from both locations showed rapid mortality when held in temperatures 2 °C to 3 °C above average seasonal sea surface temperature (32 °C). This suggests environmental changes in temperature can influence gnathiid survival, which could have significant ecological consequences for host-parasite-cleaner fish interactions during increased temperature events

Impaired growth and survival of tropical macroalgae (Sargassum spp.) at elevated temperatures

Increasing ocean temperatures associated with ongoing climate change have resulted in regional reductions in the cover of live coral and increasing concerns that coral reefs will be overgrown by macroalgae. The likelihood of macroalgal overgrowth will, however, depend on the thermal sensitivities of the macroalgae themselves. We exposed recently settled propagules of the common canopy-forming macroalga Sargassum swartzii and adult thalli of three species of Sargassum (S. swatzii, S. cristaefolium, S. polycystum) to three experimental temperatures: ambient, + 2 degrees C, and + 3.5 degrees C, reflective of summer minimum, mean, and maximum temperatures for the region. Growth and survival of Sargassum swartzii propagules were assessed over 48 days, and the growth, physical toughness, elemental composition, and susceptibility to herbivory of adult thalli were assessed after short-term exposure (2-weeks) to experimental temperatures. Growth and survival of S. swartzii propagules were reduced by 43% and 84%, respectively, when cultured at the elevated (+ 3.5 degrees C) temperature compared to ambient temperature. Similarly, elevated temperature resulted in a 17-49% decline in the growth of adult Sargassum thalli relative to controls. Susceptability of S. swartzii and S. cristaefolium to herbivory (i.e. mass removed by herbivores) was 50% less for thalli cultured at elevated (+ 3.5 degrees C) compared to ambient temperature, but this pattern was not related to changes in the physical or chemical properties of the thalli as a result of elevated temperature. The negative effects of elevated temperatures on the growth and survival of both Sargassum propagules and adult thalli will likely restrict the capacity of Sargassum, and potentially other macroalgae, to establish in new areas, and may also threaten the persistence of existing macroalgal meadows under future ocean temperatures. The thermal sensitivities of tropical Sargassum, together with those of corals, suggest ongoing ocean warming may lead to novel reef ecosystems that are low in both coral cover and macroalgal cover