351
Specifically, the innermost zone covering 28 km
2
exhibited injuries including
smothering and direct toxicity, resulting in a reduction by half in the diversity of
sediment-dwelling infauna. The second and third concentric zones of 195 and
793 km
2
, respectively, experienced mortality of hardground corals and reductions in
sediment infauna diversity. The fourth concentric zone of 1275 km
2
was fouled by
PAH concentrations in excess of toxicity values (i.e., site-specific LC20 and LC50
values for Leptocheirus) and experienced decreased abundance of benthic foraminifera (Trustees 2016).
This pattern of impacts demonstrated the following:
• Pathways of injury were multiple and varied. Based on experience from other
spills, certain pathways were expected, including direct impacts from oil exposure in the nearfield environment and indirect injuries caused as a result of
response efforts. However, the Trustees did not anticipate marine snow to be a
significant pathway; yet studies showed this to be a significant transport mechanism for moving petroleum constituents from the surface and water column to
the seafloor.
• Exposure was patchy. Likely due in part to currents present in the deep sea, exposure was patchy. This patchiness was apparent across multiple biological
resources, including between and within hardground coral sites, as well as across
the seafloor sediment, and even within various areas of the mesophotic reefs.
• A broad range of organisms experienced harm, though some organisms benefitted. For the most part, at least within the vicinity of the wellhead, adverse impacts
were readily identifiable. While this could very well be related to the scope of the
spill, it was somewhat surprising to see that in an environment that experiences
natural oil seepage, the Trustees were able to identify overt injuries across a
spectrum of organisms. Such findings hint at the potential for perturbations to
propagate throughout the ecosystem. However, we also observed resiliency in
certain organisms, and in some cases, certain species tended to benefit; for example, some of the more pollution-tolerant nematodes and polychaete worms
increased in abundance in oil-impacted sediments.
• The severity of impacts tended to decrease with increasing distance. Not surprisingly, the Gulf of Mexico exhibited a certain capacity to dilute the impacts of the
oil, particularly with increasing distance from the release point at the wellhead.
As alluded to above, with distance, exposure became more patchy, and injuries
became more difficult to identify, let alone directly link back to the spill. These
difficulties were in part due to the scale of the area over which the Trustees were
working to identify impacts.
• Subtle and/or longer term impacts were difficult to identify. Due primarily to a
general dearth of scientific knowledge of many resources of the deep sea, the
Trustees were unable to identify or quantify injuries that were not overt. That is,
the Trustees focused on injuries that were obvious and easily tied to the release.
However, some chronic impacts from the spill that were possible, or even likely,
were insufficiently supported by data for the Trustees to claim damages for had
the NRDA case gone to trial. Some spill-related injuries predicted to ecological
21 Overview of Ecological Impacts of Deep Spills: Deepwater Horizon
Specifically, the innermost zone covering 28 km
2
exhibited injuries including
smothering and direct toxicity, resulting in a reduction by half in the diversity of
sediment-dwelling infauna. The second and third concentric zones of 195 and
793 km
2
, respectively, experienced mortality of hardground corals and reductions in
sediment infauna diversity. The fourth concentric zone of 1275 km
2
was fouled by
PAH concentrations in excess of toxicity values (i.e., site-specific LC20 and LC50
values for Leptocheirus) and experienced decreased abundance of benthic foraminifera (Trustees 2016).
This pattern of impacts demonstrated the following:
• Pathways of injury were multiple and varied. Based on experience from other
spills, certain pathways were expected, including direct impacts from oil exposure in the nearfield environment and indirect injuries caused as a result of
response efforts. However, the Trustees did not anticipate marine snow to be a
significant pathway; yet studies showed this to be a significant transport mechanism for moving petroleum constituents from the surface and water column to
the seafloor.
• Exposure was patchy. Likely due in part to currents present in the deep sea, exposure was patchy. This patchiness was apparent across multiple biological
resources, including between and within hardground coral sites, as well as across
the seafloor sediment, and even within various areas of the mesophotic reefs.
• A broad range of organisms experienced harm, though some organisms benefitted. For the most part, at least within the vicinity of the wellhead, adverse impacts
were readily identifiable. While this could very well be related to the scope of the
spill, it was somewhat surprising to see that in an environment that experiences
natural oil seepage, the Trustees were able to identify overt injuries across a
spectrum of organisms. Such findings hint at the potential for perturbations to
propagate throughout the ecosystem. However, we also observed resiliency in
certain organisms, and in some cases, certain species tended to benefit; for example, some of the more pollution-tolerant nematodes and polychaete worms
increased in abundance in oil-impacted sediments.
• The severity of impacts tended to decrease with increasing distance. Not surprisingly, the Gulf of Mexico exhibited a certain capacity to dilute the impacts of the
oil, particularly with increasing distance from the release point at the wellhead.
As alluded to above, with distance, exposure became more patchy, and injuries
became more difficult to identify, let alone directly link back to the spill. These
difficulties were in part due to the scale of the area over which the Trustees were
working to identify impacts.
• Subtle and/or longer term impacts were difficult to identify. Due primarily to a
general dearth of scientific knowledge of many resources of the deep sea, the
Trustees were unable to identify or quantify injuries that were not overt. That is,
the Trustees focused on injuries that were obvious and easily tied to the release.
However, some chronic impacts from the spill that were possible, or even likely,
were insufficiently supported by data for the Trustees to claim damages for had
the NRDA case gone to trial. Some spill-related injuries predicted to ecological
21 Overview of Ecological Impacts of Deep Spills: Deepwater Horizon