Acute catastrophic events can cause significant damage to marine environments in a short time period and may have devastating long-term impacts. In April 2010 the BP-operated Deepwater Horizon (DWH) offshore oil rig exploded, releasing an estimated 760 million liters of crude oil into the Gulf of Mexico. This study examines the potential effects of oil spill exposure on coral larvae of the Florida Keys. Larvae of the brooding coral, Porites astreoides, and the broadcast spawning coral, Montastraea faveolata, were exposed to multiple concentrations of BP Horizon source oil (crude, weathered and WAF), oil in combination with the dispersant Corexit® 9500 (CEWAF), and dispersant alone, and analyzed for behavior, settlement, and survival. Settlement and survival of P. astreoides and M. faveolata larvae decreased with increasing concentrations of WAF, CEWAF and Corexit® 9500, however the degree of the response varied by species and solution. P. astreoides larvae experienced decreased settlement and survival following exposure to 0.62 ppm source oil, while M. faveolata larvae were negatively impacted by 0.65, 1.34 and 1.5 ppm, suggesting that P. astreoides larvae may be more tolerant to WAF exposure than M. faveolata larvae. Exposure to medium and high concentrations of CEWAF (4.28/18.56 and 30.99/35.76 ppm) and dispersant Corexit® 9500 (50 and 100 ppm), significantly decreased larval settlement and survival for both species. Furthermore, exposure to Corexit® 9500 resulted in settlement failure and complete larval mortality after exposure to 50 and 100 ppm for M. faveolata and 100 ppm for P. astreoides. These results indicate that exposure of coral larvae to oil spill related contaminants, particularly the dispersant Corexit® 9500, has the potential to negatively impact coral settlement and survival, thereby affecting the resilience and recovery of coral reefs following exposure to oil and dispersants.

Funding: Funding for this project was provided via Protect Our Reefs 2010 Emergency funding to KB Ritchie, NSF (National Science Foundation) REU-1004181 funding to DG, NSF OCE-0926822 funding to Mary Alice Coffroth, and the Dart Foundation to KBR. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Copyright: © 2013 Goodbody-Gringley et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

The Gulf of Mexico serves as a major source of crude oil for much of the Western hemisphere. It is estimated that over 1.5 million barrels of oil are extracted each day from offshore oil platforms in the Gulf [1], many of which are located within close proximity to the coastline. This intensive extraction and traffic of crude oil has, as seen with the Exxon Valdez and Deepwater Horizon (DWH) spills, the potential to result in large-scale environmental catastrophes with significant environmental impacts.

Remediation of oil spills often involves the application of dispersant chemicals, which can be mixtures of solvents and surface-active agents, along with other compounds. By reducing the interfacial tension between oil and water, dispersants enhance the breakup of an oil slick into small oil droplets that stabilize in the water column. While dispersants do not reduce the amount of oil entering the environment, they affect the fate, transport, and potential effects of an oil spill by altering the oil's physical properties [2].

The decision to use dispersant chemicals poses trade-offs for oil spill responders. While a dispersed surface oil slick is rendered less likely to reach the shore [3], treatment of major oil spills with dispersant chemicals has been shown to result in significant environmental degradation as a result of increased hydrocarbon dissolution and surfactant toxicity [4]. The effects of dispersed oil on the marine environment are also dependent on the degree of the dispersant's application as well as corresponding weather conditions [5]. While technological advancements have reduced the number of extraction- and transport-related incidents in recent years, the magnitude and potential impact of oil spills, such as the recent DWH spill in the Gulf of Mexico, affirms the fact that oil pollution constitutes a major threat to the marine environment [6], [7].

A major percentage of global offshore oil traffic occurs in close proximity to coral reef ecosystems [8]. The sensitivity of many coral species to environmental perturbation, as well as the current decline in reef cover worldwide has prompted a considerable amount of research into the effects of oil pollution on coral reef communities [7], [8], [9], [11]. Studies have shown that exposure of adult coral colonies to crude oil can result in a range of effects including inhibited growth rate, reduced reproductive activity, and tissue loss [10], [11]. Evidence also suggests that dispersed oil is significantly more toxic to corals than crude oil alone. Shafir et al. [7] found that in a survivorship assay of Stylophora pistillata and Pocillopora damicornis nubbins, concentrations of oil-dispersant mixtures above 25% caused 100% mortality to nubbins of both species, while none of the crude oil water-soluble fractions (WSF's) had any significant effect on coral survivorship.

While many field and laboratory experiments have noted significant damage to coral reef communities by oil contamination, the effects of oil exposure on coral reproduction and larval fitness and recruitment have received less attention. Loya and Rinkevich [11] noticed that exposure of Stylophora pistillata colonies to crude oil induced immediate mouth opening, followed by the premature release of underdeveloped larvae. Epstein et al. [4] found that exposure of S. pistillata planulae to increasing concentrations of dispersed oil resulted in reduced settlement and survivorship over the course of 96 hours. Similarly, Harrison [12] reported a reduction in metamorphosis of Acropora tenuis larvae when exposed to dispersed oil.

Coral larvae play a significant role in reef ecology. In scleractinian corals, planula larvae are the result of sexual reproduction, and have the ability to recruit to new substrate and contribute to genetic diversity [13]. Successful settlement of coral larvae involves the sampling of available substrate, followed by adherence and metamorphosis into a competent juvenile polyp [14]. Evidence suggests that larval settlement is strongly influenced by chemical cues, which are believed to stem from naturally occurring biofilms on marine substratum [8], [15], [16]. Consequently, surfactants alone and in the presence of oil could alter the physical and chemical properties of the ideal biofilm required for settlement or interrupt these sensitive chemical cues. Given the importance of successful larval recruitment in maintaining the reef environment, it is imperative to gain a fundamental understanding of toxicological effects on larval ecology.

Methods for conducting toxicity tests of chemical pollutants such as oil and dispersants have been developed and applied to teleost larvae and other organisms [17], [18]. The purpose of the present study was to evaluate effects of exposure to the water accommodated fractions (WAFs) of DWH oil (fresh and weathered), chemically enhanced water accommodated fractions (CEWAFs) of the oil, and Corexit® 9500 dispersant on planula larvae of the scleractinian corals Porites astreoides and Montastraea faveolata from the Florida Keys. Both species are common on reefs throughout the Caribbean, where P. astreoides is an early succession species contributing to reef recovery, and M. faveolata is an important reef building species. These species also differ in mode of reproduction; P. astreoides is a brooding coral that undergoes internal fertilization and releases semi-mature planula larvae from January to September [19], while M. faveolata is a broadcasting coral that spawns gametes in synchrony 1–2 times a year (Aug./Sept.) and requires external fertilization [20]. The longevity and extent of an oil spill in the water column can vary based on weather conditions [21] and microbial interactions [5], and may persist for several days, weeks or years [22]. This study examined the effects of exposure to both fresh and weathered DWH oil and dispersants using short-term assays (≤96-hr) to monitor larval settlement rates, survivorship, and behavioral responses. While the Deepwater Horizon spill did not occur in close proximity to the Florida Keys, it is plausible that oil pollution from similar events could eventually reach these coral reefs via offshore current movement [23], thereby affecting reef health.