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

Abstract

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.

Figure 1

Percentage fertilization success for eggs of Acropora tenuis exposed to five levels of suspended sediments (0, 5, 10, 30 and 100 mg l⁻¹) and either (a) nutrient enrichment [3 levels: low (open circles), medium (grey circles), high (black circles)] at 27 °C, or (b) elevated temperature (3 levels: 27, 30, and 32 °C) without organic nutrient enrichment (Experiment 1). Solid lines indicate generalised linear model fits, while dashed lines are 95% confidence intervals. Points are jittered horizontally for clarity.

Figure 2

Percent larval settlement success for Acropora tenuis after 8-h old embryos were reared for 28 h (until ciliated) in a suspended sediment treatment (0, 5, 10, 30 or 100 mg l⁻¹) combined with either (a) nutrient enrichment (low, medium, high) at 27 °C; or (b) elevated temperatures (27, 30, or 32 °C) without organic nutrient enrichment (Experiment 2). After the 28 h exposure larvae were maintained and settled under control conditions (0 mg l⁻¹ suspended sediements, low nutrient enrichment and 27 °C). Solid lines indicate generalised linear model fits, while dashed lines are 95% confidence intervals. Points are jittered horizontally for clarity.

Figure 3

(a) Settlement of 3-d old larvae previously exposed for 48 h to combined suspended sediments (0, 5, 10, 30 or 100 mg l⁻¹) and contrasting temperature (27, 30, or 32 °C) treatments (Experiment 3). After the exposure, larvae were settled under control conditions (0 mg l⁻¹ suspended sediements, low nutrient enrichment and 27 °C). (b) Settlement of 5-d old larvae exposed only during settlement to suspended sediments (0, 5, 10, 30 or 100 mg l⁻¹) and elevated temperatures (27, 30, or 32 °C) (Experiment 4). Diamonds indicate means at each temperature level (error bars are standard deviation), and circles indicate means for each level of suspended sediments at the corresponding temperature.

Figure 4

Comparative impacts of these three stressors on processes occurring during the early life history processes of Acropora tenuis exposed to: (a) suspended sediments (0, 5, 10, 30 or 100 mg l⁻¹), and either high nutrients or high temperatures, (b) nutrient enrichment only (low, medium, high), and (c) a temperature treatment only (27, 30, or 32 °C). Bars represent the percent change in the response of each treatment level (main effects) compared to control conditions (0 mg l⁻¹ suspended sediments, low nutrient enrichment and 27 °C). Asterisks indicate factors that were statistically significant (p < 0.05).

Figure 5

Experiments performed with early life history stages of Acropora tenuis exposed to suspended sediments (SS: 0, 5, 10, 30 or 100 mg l⁻¹) in combination with either (a) a nutrient enrichment treatment (low, medium, high), or (b) a temperature treatment (27, 30 or 32 °C) treatments. Black bars indicate the processes or stages involved in each experiment (modified from Jones et al.23).