Cumulative effects of suspended sediments, organic nutrients and temperature stress on early life history stages of the coral Acropora tenuis

Coral reproduction is vulnerable to both declining water quality and warming temperatures, with simultaneous exposures likely compounding the negative impact of each stressor. We investigated how early life processes of the coral Acropora tenuis respond to increasing levels of suspended sediments in combination with temperature or organic nutrients. Fertilization success and embryo development were more sensitive to suspended sediments than to high temperatures or nutrient enrichment, while larval development (after acquisition of cilia) and settlement success were predominantly affected by thermal stress. Fertilization success was reduced 80% by suspended sediments, and up to 24% by temperature, while the addition of nutrients to suspended sediments had no further impact. Larval survivorship was unaffected by any of these treatments. However, settlement success of larvae developing from treatment-exposed embryos was negatively affected by all three stressors (e.g. up to 55% by suspended sediments), while exposure only during later larval stages predominantly responded to temperature stress. Environmentally relevant levels of suspended sediments and temperature had the greatest impacts, affecting more processes than the combined impacts of sediments and nutrients. These results suggest that management strategies to maintain suspended sediments at low concentrations during coral spawning events will benefit coral recruitment, especially with warming climate

Sediment characteristics influence the fertilisation success of the corals Acropora tenuis and Acropora millepora

Elevated suspended sediment concentrations (SSCs) often impact coral fertilisation success, but sediment composition can influence effect thresholds, which is problematic for accurately predicting risk. Here, we derived concentration–response thresholds and cause-effect pathways for SSCs comprising a range of realistic mineral and organic compositions on coral fertilisation success. Effect concentration thresholds (EC10: 10% fertilisation inhibition) varied markedly, with fertilisation highly sensitive to inshore organic-clay rich sediments and bentonite clay at 40 mg L−1). The effect thresholds for relevant sediment types were combined with in situ turbidity data from locations near dredging operations to assess the risks posed by dredging to coral fertilisation at these locations

Promoting inclusive metrics of success and impact to dismantle a discriminatory reward system in science

“The most dangerous phrase in the language is: We’ve always done it this way.” —Rear Admiral Grace HopperSuccess and impact metrics in science are based on a system that perpetuates sexist and racist “rewards” by prioritizing citations and impact factors. These metrics are flawed and biased against already marginalized groups and fail to accurately capture the breadth of individuals’ meaningful scientific impacts. We advocate shifting this outdated value system to advance science through principles of justice, equity, diversity, and inclusion. We outline pathways for a paradigm shift in scientific values based on multidimensional mentorship and promoting mentee well-being. These actions will require collective efforts supported by academic leaders and administrators to drive essential systemic change.Peer reviewe

Combined effects of water quality and temperature on the early life history stages of the broadcast spawning coral Acropora tenuis

Coastal developments and increased agricultural activities are causing long-term changes to marine ecosystems at local scales through exposure to elevated levels of sediments and nutrients. At global scales, marine ecosystems are also experiencing warming seawater temperatures due to climate change. Coral reproduction is vulnerable to both declining water quality and warming temperatures, with simultaneous exposures likely to compound their negative impacts and further inhibit successful replenishment of coral populations. A quantitative evaluation of the effects of nutrient enrichment, suspended sediments and warming temperatures, and of interactions among these stressors, on the early life history stages of corals is crucial to understand how local (i.e. water quality) and global stressors (i.e., temperature) directly affect replenishment of coral populations. This research was conducted experimentally under controlled conditions, using Acropora tenuis, a common species in the Indo-Pacific and the Red Sea, as a model species. Results provide important evidence that processes during the early life of hard corals are directly and negatively affected by water quality declines, warming temperatures and their interactions, and that thresholds for stress are dependent on specific combinations of these stressors. As single stressors, nutrient enrichment, suspended sediments and elevated temperatures are each known to reduce coral cover and biodiversity; but their combined effects on early life history processes are unknown. In Chapter 2, a series of experiments were conducted to test the individual and combined effects of nutrient enrichment (three levels: 0, 0.3 and 0.6 mg organic carbon l⁻¹) and elevated seawater temperature (up to five levels: 27, 29, 30, 31 and 32°C) on the early life history stages of Acropora tenuis. Gamete fertilization, larval survivorship and larval settlement were all significantly reduced as temperature increased, but only fertilization was further affected by simultaneous nutrient enrichment. Combinations of high temperatures and nutrient enrichment affected fertilization in an additive manner, whereas embryo abnormalities increased synergistically. Higher than normal temperatures (32°C) increased coral juvenile growth rates 1.6-fold, but mortality also increased by 50%. However, when the high temperatures co-occurred with nutrient enrichment, juvenile mortality declined from 50% to 36%, ameliorating temperature stress (antagonistic interaction). Overall, the types of effect (additive vs synergistic or antagonistic) and their magnitude varied among life stages. Gamete and embryo stages were more affected by temperature stress and, in some cases, also by nutrient enrichment than the juvenile corals. Results suggest that coastal runoff events are likely to exacerbate the impacts of warming temperatures on fertilization if these events co-occur during corals spawning. The cumulative impacts of simultaneous exposure to nutrient enrichment and elevated temperatures over all early life history stages increases the likelihood for failure of larval supply and recruitment for this coral species. Chapter 3 complemented Chapter 2, and contains data on how early life history processes (gamete fertilization, larval survival and larval settlement) of the coral A. tenuis, responded to environmentally-relevant levels of suspended sediments (up to five levels: 0, 5, 10, 30 and 100 mg l⁻¹), when tested both individually and in combination with either elevated nutrients (three levels: 0, 0.3 and 0.6 mg organic carbon l⁻¹) or temperatures (three levels: 27, 30 and 32°C). Results reveal that key early life processes differ greatly in their sensitivities to these pressures. Fertilization success was reduced by as much as 80% by sediments, and up to 24% by temperature, but the addition of nutrients had no further impact. Larval survivorship was unaffected by any of these treatments. However, the settlement success of larvae developing from treatment-exposed embryos was negatively affected by all three stressors: by 33% for nutrient enrichment, 14% for increased temperature and up to 55% for suspended sediments. When exposed to treatments only during later larval stages, larval settlement success was affected only by temperature, decreasing by 23% at 32°C. In combination, sediments and temperature had the greatest impacts, affecting more processes than the combined impacts of suspended sediments and nutrients. The combined effects of suspended sediments and nutrients, and of suspended sediments and temperature, on early life stages were additive, indicating that cumulative pressures aggravate negative impacts on coral recruitment in nearshore environments. Results suggest that management strategies to maintain suspended sediments at low concentrations in inshore areas during coral spawning events could ameliorate the impacts of thermal stress likely to be experienced with climate change. Chapter 4 contains data on the effects of co-exposing juveniles of three coral species (Acropora tenuis, Acropora millepora and Pocillopora acuta), common on inshore Indo-Pacific and Red Sea reefs, to suspended sediments (four levels: 0, 10, 30 and 100 mg l⁻¹) and nutrient enrichment (two levels: 0 and 0.6 mg organic carbon l⁻¹) for 40 days. Effects of the treatments on survival, growth and physiology varied among species, indicating species-specific strategies to cope with stress. Suspended sediments reduced survival in A. millepora to 64% at 100 mg l⁻¹ but did not affect survival in A. tenuis or P. acuta, instead reducing their growth rates to 73 and 60%, respectively, compared with rates under control conditions. Suspended sediments did not affect maximum quantum yields in any of the three species, but increased the effective quantum yield in A. millepora and A. tenuis by 30 and 40%, respectively; no effects were obtained for P. acuta. High levels of suspended sediments (100 mg l⁻¹) also enhanced respiration rates by 13 and 64%, evidence of an energy cost associated with exposure to suspended sediments. Nutrient enrichment up to a concentration of 0.6 mg organic carbon l⁻¹ did not have any significant effect on any of the variables measured, suggesting resilience of juveniles to this stressor. To improve understanding of the effects of early life history stages on coral population dynamics, a size-based model considering all life history stages (i.e. from gametes to adult colonies) was built for Acropora tenuis in Chapter 5. The model was constructed based on empirical published data of the demography of juveniles and adult coral colonies from inshore reefs of the Great Barrier Reef, together with the experimental information of early life history stages (gamete fertilization, larval survivorship and settlement success; Chapter 2 and 3) were used to construct the model. Impacts of contrasting water quality (i.e., nutrient enrichment and suspended sediments) and stressinducing high temperatures were modelled during the period of coral spawning, to examine their potential effects on the annual population growth rates. This theoretical exercise provides new insight into the effects of local (i.e. water quality) and global (i.e. temperature) stressors during the development of early life history stages on coral population dynamics. Results highlight the importance of considering early life history stages in demographic analyses aimed at understanding how coral cover is likely to change when spawning events are affected by local and global stressors. Collectively, this research provides important evidence that early life history stages and processes of hard corals are directly affected by water quality declines, warming temperatures and their interactions, and that clear threshold values exist for the effects of these combined stressors. The additive nature of simultaneous exposure to these stressors during gamete fertilization, larval development, larval settlement juvenile physiology and fitness underscore the need to improve water quality associated with river discharges and dredging activities, given that ocean warming can only be managed at global scales. Changes in these fundamental processes that regulate coral assemblages may have long-term repercussions on coral health, population size distributions and consequently, on coral cover on inshore reefs

Massive hard coral loss after a severe bleaching event in 2010 at Los Roques, Venezuela

Thermal anomalies have become more severe, frequent and well-documented across the Caribbean for the past 30 years. This increase in temperature has caused coral bleaching resulting in reef decline. At Los Roques National Park, Venezuela, temperature has been monitored at four reef sites. In mid-September 2010, seawater temperature reached 30.85°C at 5 m depth in Los Roques, an archipelago only slightly affected by previous bleaching events. For example, bleaching in Los Roques in 2005 was mild compared to the rest of the Caribbean and to the results in this study. In 2010, seawater temperatures remained above 29.0°C from mid- August until the first week of November, resulting in +16 Degree Heating Weeks by that time. Our annual survey of four reef sites indicated that 72% of 563 scleractinian colonies were partial or totally bleached (white) or pale (discolored) in October 2010. In February 2011, there were still 46% of coral colonies affected; but most of them were pale and only 2% were bleached. By February, coral cover had declined 4 to 30% per transect, with a mean of 14.3%. Thus, mean coral cover dropped significantly from 45 to 31% cover (a 34% reduction). In addition to bleaching, corals showed a high prevalence (up to 16%) of black band disease in October 2010 and of white plague (11%) in February 2011. As a consequence, coral mortality is expected to be larger than reported here. Reef surveys since 2002 and personal observations for more than 20 years indicated that this bleaching event and its consequences in Los Roques have no precedent. Our results suggest that reef sites with no previous record of significant deterioration are more likely to become affected by thermal anomalies. However, this archipelago is relatively unaffected by local anthropogenic disturbance and has a high coral recruitment, which may contribute to its recovery.Durante las últimas décadas las anomalías térmicas han sido más frecuentes y severas en el Caribe, quedando pocos arrecifes exentos de eventos masivos de blanqueamiento (EMB). En el Parque Nacional Los Roques, Venezuela, un archipiélago poco afectado previamente por EMB, la temperatura del agua a 5m de profundidad alcanzó 30,85°C en septiembre 2010, y fue >29,0°C entre mediados de agosto y la primera semana de noviembre en cuatro arrecifes. El 72% de 563 colonias de escleractinios estaban blanqueadas o pálidas para octubre de 2010, mientras que para febrero 2011, el 46% de las colonias aún estaban afectadas. Para febrero 2011, la cobertura béntica coralina promedio disminuyó de 45 a 31%. Además, los arrecifes mostraron una alta prevalencia (de hasta 16%) de enfermedad de banda negra en Octubre 2010, y de plaga blanca (11%) en Febrero 2011. Como consecuencia, es probable que la mortalidad coralina resulte mayor a la reportada acá. Sin embargo, Los Roques es poco afectado por perturbaciones antropogénicas y cuenta con un alto reclutamiento de corales, lo cual podría contribuir a su recuperación

Effects of suspended sediments and nutrient enrichment on juvenile corals

Three to six-month-old juveniles of Acropora tennis, A. millepora and Pocillopora acute were experimentally co-exposed to nutrient enrichment and suspended sediments (without light attenuation or sediment deposition) for 40 days. Suspended sediments reduced survivorship of A. millepora strongly, proportional to the sediment concentration, but not in A. tennis or P. acuta juveniles. However, juvenile growth of the latter two species was reduced to less than half or to zero, respectively. Additionally, suspended sediments increased effective quantum yields of symbionts associated with A. millepora and A. tennis, but not those associated with P. acuta. Nutrient enrichment did not significantly affect juvenile survivorship, growth or photophysiology for any of the three species, either as a sole stressor or in combination with suspended sediments. Our results indicate that exposure to suspended sediments can be energetically costly for juveniles of some coral species, implying detrimental longer-term but species-specific repercussions for populations and coral cover

Massive hard coral loss after a severe bleaching event in 2010 at Los Roques, Venezuela

Thermal anomalies have become more severe, frequent and well-documented across the Caribbean for the past 30 years. This increase in temperature has caused coral bleaching resulting in reef decline. At Los Roques National Park, Venezuela, temperature has been monitored at four reef sites. In mid-September 2010, seawater temperature reached 30.85°C at 5 m depth in Los Roques, an archipelago only slightly affected by previous bleaching events. For example, bleaching in Los Roques in 2005 was mild compared to the rest of the Caribbean and to the results in this study. In 2010, seawater temperatures remained above 29.0°C from mid-August until the first week of November, resulting in +16 Degree Heating Weeks by that time. Our annual survey of four reef sites indicated that 72% of 563 scleractinian colonies were partial or totally bleached (white) or pale (discolored) in October 2010. In February 2011, there were still 46% of coral colonies affected; but most of them were pale and only 2% were bleached. By February, coral cover had declined 4 to 30% per transect, with a mean of 14.3%. Thus, mean coral cover dropped significantly from 45 to 31% cover (a 34% reduction). In addition to bleaching, corals showed a high prevalence (up to 16%) of black band disease in October 2010 and of white plague (11%) in February 2011. As a consequence, coral mortality is expected to be larger than reported here. Reef surveys since 2002 and personal observations for more than 20 years indicated that this bleaching event and its consequences in Los Roques have no precedent. Our results suggest that reef sites with no previous record of significant deterioration are more likely to become affected by thermal anomalies. However, this archipelago is relatively unaffected by local anthropogenic disturbance and has a high coral recruitment, which may contribute to its recover

Emergent increase in coral thermal tolerance reduces mass bleaching under climate change

Abstract Recurrent mass bleaching events threaten the future of coral reefs. To persist under climate change, corals will need to endure progressively more intense and frequent marine heatwaves, yet it remains unknown whether their thermal tolerance can keep pace with warming. Here, we reveal an emergent increase in the thermal tolerance of coral assemblages at a rate of 0.1 °C/decade for a remote Pacific coral reef system. This led to less severe bleaching impacts than would have been predicted otherwise, indicating adaptation, acclimatisation or shifts in community structure. Using future climate projections, we show that if thermal tolerance continues to rise over the coming century at the most-likely historic rate, substantial reductions in bleaching trajectories are possible. High-frequency bleaching can be fully mitigated at some reefs under low-to-middle emissions scenarios, yet can only be delayed under high emissions scenarios. Collectively, our results indicate a potential ecological resilience to climate change, but still highlight the need for reducing carbon emissions in line with Paris Agreement commitments to preserve coral reefs