Marine ecoregion and Deepwater Horizon oil spill affect recruitment and population structure of a salt marsh snail

Abstract

Abstract. Marine species with planktonic larvae often have high spatial and temporal variation in recruitment that leads to subsequent variation in the ecology of benthic adults. Using a combination of published and unpublished data, we compared the population structure of the salt marsh snail, Littoraria irrorata, between the South Atlantic Bight and the Gulf Coast of the United States to infer geographic differences in recruitment and to test the hypothesis that the Deepwater Horizon oil spill led to widespread recruitment failure of L. irrorata in Louisiana in 2010. Size-frequency distributions in both ecoregions were bimodal, with troughs in the distributions consistent with a transition from sub-adults to adults at~13 mm in shell length as reported in the literature; however, adult snails reached larger sizes in the Gulf Coast. The ratio of sub-adults to adults was 1.5-2 times greater in the South Atlantic Bight than the Gulf Coast, consistent with higher recruitment rates in the South Atlantic Bight. Higher recruitment rates in the South Atlantic Bight could contribute to higher snail densities and reduced adult growth in this region. The ratio of sub-adults to adults in Louisiana was lower in 2011 than in previous years, and began to recover in 2012-2014, consistent with widespread recruitment failure in 2010, when large expanses of spilled oil were present in coastal waters. Our results reveal an important difference in the ecology of a key salt marsh invertebrate between the two ecoregions, and also suggest that the Deepwater Horizon oil spill may have caused widespread recruitment failure in this species and perhaps others with similar planktonic larval stages.

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

“…The extent of shoreline oiling in northern Barataria Bay ( Michel et al, 2013 ; Nixon et al, 2016 ) and the extent and duration of oil in and on the water estimated by the Environmental Response Management Application (ERMA) ( https://response.restoration.noaa.gov/maps-and-spatial-data/environmental-response-management-application-erm ; MacDonald et al, 2015 ) indicates that populations of L. irrorata were likely affected in their ability to carry out their life cycle on the marsh and in the water column (as larvae) at all sites (including our reference sites) for some period following the spill ( Pennings et al, 2016 ). Even at 30 mo after the spill when this study began, the mean shell size of L. irrorata was smaller than would be predicted by regional averages at all oiling levels, including reference sites, and the recovery process was ongoing at all sites, as indicated by our analysis ( Fig.…”

“…We additionally classified snails >20 mm as “large adults”. Length-frequency distributions of L. irrorata are often bi- or tri-modal with the modes representing age class ( Hamilton, 1978 ; Pennings et al, 2016 ; Zengel et al, 2014 ; Zengel et al, 2015 ). Based on well-established growth estimates ( Stiven & Hunter, 1976 ), juveniles are considered to be <1 year of age, sub-adults ∼1 year, and adults 2 years and older ( Pennings et al, 2016 ).…”

“…Length-frequency distributions of L. irrorata are often bi- or tri-modal with the modes representing age class ( Hamilton, 1978 ; Pennings et al, 2016 ; Zengel et al, 2014 ; Zengel et al, 2015 ). Based on well-established growth estimates ( Stiven & Hunter, 1976 ), juveniles are considered to be <1 year of age, sub-adults ∼1 year, and adults 2 years and older ( Pennings et al, 2016 ). Evaluating changes in shell size over time by comparing size frequency histograms has previously been used as an aid to determine oiling impacts and to estimate recovery trajectories ( Pennings et al, 2016 ; Zengel et al, 2014 ; Zengel et al, 2015 ; Zengel et al, 2016c ).…”

“…Because fiddler crabs are mobile and direct observation is difficult, we used burrow density as a proxy for population density. A strong relationship exists between fiddler crab carapace width and burrow diameter ( Mouton & Felder, 1996 ), and burrow diameters were measured to the nearest millimeter with a transparent ruler (after Pennings et al, 2016 ). When present in the sampling site vicinity, fiddler crabs were captured, identified to species, and released.…”

“…Marsh periwinkles have also been shown to be sensitive to oil spills ( Bennett et al, 1999 ; Hershner & Moore, 1977 ; Hershner & Lake, 1980 ; Lee et al, 1981 ). McCall & Pennings (2012) , Pennings et al (2016) , Silliman et al (2012) , Zengel et al (2014) , Zengel et al (2015) , Zengel et al (2016a) and Zengel et al (2016c) investigated DWH impacts on the marsh periwinkle.…”