Cumulative Effects of Nutrient Enrichment and Elevated Temperature Compromise the Early Life History Stages of the Coral Acropora tenuis.

Adriana Humanes,Katharina E Fabricius,Andrew P Negri,Bette L Willis,Sam H C Noonan,Covadonga Orejas

doi:10.1371/journal.pone.0161616

Abstract

Inshore coral reefs are experiencing the combined pressures of excess nutrient availability associated with coastal activities and warming seawater temperatures. Both pressures are known to have detrimental effects on the early life history stages of hard corals, but studies of their combined effects on early demographic stages are lacking. We conducted a series of experiments to test the combined effects of nutrient enrichment (three levels) and elevated seawater temperature (up to five levels) on early life history stages of the inshore coral Acropora tenuis, a common species in the Indo-Pacific and Red Sea. Gamete fertilization, larval survivorship and larval settlement were all significantly reduced as temperature increased, but only fertilization was further affected by simultaneous nutrient enrichment. Combined 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%. The co-occurrence of nutrient enrichment with high temperatures reduced juvenile mortality 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 juveniles. The data suggest that coastal runoff events might 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. Our results suggest that improving the water quality of river discharges into coastal areas might help to enhance the thermal tolerances of early life history stages in this common coral species.

Highlights

Coral reefs around the world are facing increasing pressures from coastal human activities and climate change [1], with warming sea surface temperatures (SST) and nutrient enrichment among their most harmful stressors [2]
The reduction in fertilization success compared to the control treatment was 5 ± 6% at nutrient enrichment = ‘high’, and 8 ± 7% at temperature = 32°C, while temperature = 32°C and nutrient enrichment = ‘high’ in combination resulted in a 14 ± 10% decline in fertilization
When the highest levels of both factors co-occurred, the proportion of abnormal embryos increased (27 ± 16%) to values higher than expected for the addition of the individual effects of temperature and nutrient enrichment (Fig 2), indicating a multiplicative synergistic interaction of both stressors according to the Generalized linear models (GLM) model with the log-link function

Summary

Introduction

Coral reefs around the world are facing increasing pressures from coastal human activities and climate change [1], with warming sea surface temperatures (SST) and nutrient enrichment among their most harmful stressors [2]. The simultaneous and cumulative effects of elevated SST and nutrient enrichment on demographic processes may lead to significant declines in coral cover [3] This highlights the importance of studying their joint effects, on the sensitive early life history stages that maintain and replenish coral populations, including gamete fertilization, larval supply, settlement and juvenile survivorship [4]. Since the beginning of the 20th century SST has risen by a global average of ~1°C [5] and is projected to increase by a further 2 to 3°C by the end of the century under a moderate Representative Concentration Scenario of the Intergovernmental Panel on Climate Change (IPCC RCP 4.5 scenario) [6]. River runoff, resulting in the eutrophication of nearshore tropical marine habitats has been reported to cause: reductions in coral biodiversity [11], increases in macroalgae cover [12], proliferation of macro-bioeroding organisms (i.e., sponges, molluscs, polychaetes and sipunculans) that weaken the structural integrity of coral reefs [13], increases in the frequency and severity of coral diseases [14], and changes in the composition of biofilms that provide conditioned surfaces for larval settlement and metamorphosis of many sessile organisms [15,16,17]

Methods

Results

Conclusion