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.