Shoreline oiling effects and recovery of salt marsh macroinvertebrates from the Deepwater Horizon Oil Spill

Abstract

Salt marshes in northern Barataria Bay, Louisiana, USA were oiled, sometimes heavily, in the aftermath of the Deepwater Horizon oil spill. Previous studies indicate that fiddler crabs (in the genus Uca) and the salt marsh periwinkle (Littoraria irrorata) were negatively impacted in the short term by the spill. Here, we detail longer-term effects and recovery from moderate and heavy oiling over a 3-year span, beginning 30 months after the spill. Although neither fiddler crab burrow density nor diameter differed between oiled and reference sites when combined across all sampling events, these traits differed among some individual sampling periods consistent with a pattern of lingering oiling impacts. Periwinkle density, however, increased in all oiling categories and shell-length groups during our sampling period, and periwinkle densities were consistently highest at moderately oiled sites where Spartina alterniflora aboveground biomass was highest. Periwinkle shell length linearly increased from a mean of 16.5 to 19.2 mm over the study period at reference sites. In contrast, shell lengths at moderately oiled and heavily oiled sites increased through month 48 after the spill, but then decreased. This decrease was associated with a decline in the relative abundance of large adults (shell length 21-26 mm) at oiled sites which was likely caused by chronic hydrocarbon toxicity or oil-induced effects on habitat quality or food resources. Overall, the recovery of S. alterniflora facilitated the recovery of fiddler crabs and periwinkles. However, our long-term record not only indicates that variation in periwinkle mean shell length and length-frequency distributions are sensitive indicators of the health and recovery of the marsh, but agrees with synoptic studies of vegetation and infaunal communities that full recovery of heavily oiled sites will take longer than 66 months.

Keywords: Deepwater Horizon oil spill; Fiddler crab; Littoraria irrorata; Marsh periwinkle; Uca.

Figure 1. Sampling site locations within Barataria Bay, Louisiana.

Sampling locations represented by squares, circles, and triangles for the reference (RF), Moderate (MD) and Heavily (HV) oiled stations, respectively (background shape file Digital Ortho Quarter Quad from Atlas: Louisiana GIS: https://atlas.ga.lsu.edu/).

Figure 2. Live total aboveground biomass from 30 to 66 mo after the DWH oil spill.

Values are means (±SE, n = 7) for reference (RF), moderately oiled (MD), and heavily oiled (HV) shoreline marsh sites in northern Barataria Bay, LA.

Figure 3. Density (m⁻²) of Uca burrows from 30 to 66 mo after the DWH oil spill.

Values are means (+SE, n = 7) for reference (RF), moderately oiled (MD), and heavily oiled (HV) shoreline marsh sites in northern Barataria Bay, LA.

Figure 4. Diameter of Uca burrows (mm) from 30 to 66 mo after the DWH oil spill.

Values are means (+SE, n = 7) for reference (RF), moderately oiled (MD), and heavily oiled (HV) shoreline marsh sites in northern Barataria Bay, LA.

Figure 5. Total Littoraria irrorata density (m⁻²) from 30 to 66 mo after the DWH oil spill.

Values are means (+SE, n = 7) for reference (RF), moderately oiled (MD), and heavily oiled (HV) shoreline marsh sites in northern Barataria Bay, LA.

Figure 6. Mean Littoraria irrorata shell length (mm) from 30 to 66 mo after the DWH oil spill.

Values are means of sites (+SE, n = 7) for reference (RF), moderately oiled (MD), and heavily oiled (HV) shoreline marsh sites in northern Barataria Bay, LA.

Figure 7. The trend for the average shell length (mm) of Littoraria irrorata over time from the beginning of the study (30 mo after the spill) to the last sampling period (66 mo after the spill), provided by the slopes and intercepts of the ANCOVA regression means model in Table 3, at the reference (RF), moderately oiled (MD), and heavily oiled (HV) shoreline marsh sites in northern Barataria Bay, LA.

Figure 8. The relative frequency of juvenile (<6 mm shell length), subadults (6–13 mm), small adult (13–20 mm), and large adult (>20 mm) shell length categories of Littoraria irrorata yearly from the beginning of the study at 30 months after the spill to 66 months after the spill.