365
organisms are known to the species level, for example, only 40% (207 of 517) of the
polychaete species and 25% (31 of 124) of the amphipod species found in the
DGoMB study could be identified to the species level. The isopods contained more
than 60 new to science (Wilson 2008). Species richness was greatest at depths
around 1200–1500 m. The lowest richness was in the Sigsbee Abyssal Plain. There
were no isopod diversity trends with longitude.
22.3.2 Soft-Bottom Habitats During DWH
The DWH oil spill occurred in the lease block named Macondo 252 and continued
until 15 July 2010 when it was closed by a cap. Field missions to examine the deep
seafloor for oil were conducted on the R/V Gyre and R/V Ocean Veritas in September
through October 2010. Oil did end up on the bottom, and these contaminants pose
risks to benthic fauna, particularly the infauna that lives within or in close association with bottom substrates and unable to avoid exposure due to their relatively
sedentary existence. The study below describes the effects of the DWH oil spill on
the macrofauna and meiofauna 2–3 months after the well was capped.
The study collected samples at 58 stations, and assessments of chemical, physical, and biological parameters were carried out using multivariate analyses to reduce
the dataset to one new variable representing the oil spill footprint (Montagna et al.
2013). The new variable contained scores for each sample where high total petroleum hydrocarbon (TPH), polycyclic aromatic hydrocarbons (PAH), and barium
(Ba) concentrations were inversely correlated to meiofauna and macrofauna diversity and the nematode/copepod (NC) ratio. The NC ratio is a sensitive bioindicator
used to classify impacts of pollution and organic enrichment in the field (Shiells and
Anderson 1985), mesocosm studies (Gee et al. 1985), and impacts of offshore drilling activities in the GoM (Peterson et al. 1996). Thus, the new variable is easy to
interpret because as pollution goes up, bioindicator metrics go down. The most
severe relative reduction of faunal abundance and diversity extended to 3 km from
the wellhead in all directions covering an area about 24 km
2
(about 9 square miles;
Fig. 22.4). Moderate impacts were observed up to 17 km toward the southwest and
8.5 km toward the northeast of the wellhead, covering an area of 148 km
2
(about 57
square miles). Benthic effects were correlated to TPH, PAHs, and Ba concentrations
and distance to the wellhead, but not distance to hydrocarbon seeps. Thus, benthic
effects are more likely due to the oil spill, and not natural hydrocarbon seepage.
A later analysis of this data focusing on the meiofauna alone at 66 stations confirmed that the NC ratio is a good indicator (Baguley et al. 2015). Nematode dominance increased significantly with increasing impacts, and NC spiked near the
wellhead. Copepod abundance decreased where impacts were most severe. The
increasing abundance and dominance by nematodes with increasing pollution could
represent a balance between organic enrichment and toxicity. Using just meiofauna
diversity and NC yielded a larger spatial footprint because stations that had
22 Deep-Sea Benthic Faunal Impacts and Community Evolution Before, During…
organisms are known to the species level, for example, only 40% (207 of 517) of the
polychaete species and 25% (31 of 124) of the amphipod species found in the
DGoMB study could be identified to the species level. The isopods contained more
than 60 new to science (Wilson 2008). Species richness was greatest at depths
around 1200–1500 m. The lowest richness was in the Sigsbee Abyssal Plain. There
were no isopod diversity trends with longitude.
22.3.2 Soft-Bottom Habitats During DWH
The DWH oil spill occurred in the lease block named Macondo 252 and continued
until 15 July 2010 when it was closed by a cap. Field missions to examine the deep
seafloor for oil were conducted on the R/V Gyre and R/V Ocean Veritas in September
through October 2010. Oil did end up on the bottom, and these contaminants pose
risks to benthic fauna, particularly the infauna that lives within or in close association with bottom substrates and unable to avoid exposure due to their relatively
sedentary existence. The study below describes the effects of the DWH oil spill on
the macrofauna and meiofauna 2–3 months after the well was capped.
The study collected samples at 58 stations, and assessments of chemical, physical, and biological parameters were carried out using multivariate analyses to reduce
the dataset to one new variable representing the oil spill footprint (Montagna et al.
2013). The new variable contained scores for each sample where high total petroleum hydrocarbon (TPH), polycyclic aromatic hydrocarbons (PAH), and barium
(Ba) concentrations were inversely correlated to meiofauna and macrofauna diversity and the nematode/copepod (NC) ratio. The NC ratio is a sensitive bioindicator
used to classify impacts of pollution and organic enrichment in the field (Shiells and
Anderson 1985), mesocosm studies (Gee et al. 1985), and impacts of offshore drilling activities in the GoM (Peterson et al. 1996). Thus, the new variable is easy to
interpret because as pollution goes up, bioindicator metrics go down. The most
severe relative reduction of faunal abundance and diversity extended to 3 km from
the wellhead in all directions covering an area about 24 km
2
(about 9 square miles;
Fig. 22.4). Moderate impacts were observed up to 17 km toward the southwest and
8.5 km toward the northeast of the wellhead, covering an area of 148 km
2
(about 57
square miles). Benthic effects were correlated to TPH, PAHs, and Ba concentrations
and distance to the wellhead, but not distance to hydrocarbon seeps. Thus, benthic
effects are more likely due to the oil spill, and not natural hydrocarbon seepage.
A later analysis of this data focusing on the meiofauna alone at 66 stations confirmed that the NC ratio is a good indicator (Baguley et al. 2015). Nematode dominance increased significantly with increasing impacts, and NC spiked near the
wellhead. Copepod abundance decreased where impacts were most severe. The
increasing abundance and dominance by nematodes with increasing pollution could
represent a balance between organic enrichment and toxicity. Using just meiofauna
diversity and NC yielded a larger spatial footprint because stations that had
22 Deep-Sea Benthic Faunal Impacts and Community Evolution Before, During…