Frank J. Hernandez

Novel Pathways for Injury from Offshore Oil Spills: Direct, Sublethal and Indirect Effects of the Deepwater Horizon Oil Spill on Pelagic Sargassum Communities

The pelagic brown alga Sargassum forms an oasis of biodiversity and productivity in an otherwise featureless ocean surface. The vast pool of oil resulting from the Deepwater Horizon oil spill came into contact with a large portion of the Gulf of Mexico’s floating Sargassum mats. Aerial surveys performed during and after the oil spill show compelling evidence of loss and subsequent recovery of Sargassum. Expanding on the trends observed in the aerial surveys, we conducted a series of mesocosm experiments to test the effect of oil and dispersants on the vertical position and weight of the Sargassum complex (Sargassum natans and S. fluitans), as well as on the dissolved oxygen concentrations surrounding the algae. Dispersant and dispersed-oil had significant effects on the vertical position of both species of Sargassum over a period of 72 hours. Similarly, dissolved oxygen concentrations were lowest in dispersant and dispersed-oil treatments, respectively. Cumulatively, our findings suggest three pathways for oil-spill related injury: (1) Sargassum accumulated oil on the surface exposing animals to high concentrations of contaminants; (2) application of dispersant sank Sargassum, thus removing the habitat and potentially transporting oil and dispersant vertically; and (3) low oxygen surrounded the habitat potentially stressing animals that reside in the alga. These pathways represent direct, sublethal, and indirect effects of oil and dispersant release that minimize the ecosystem services provided by floating Sargassum – the latter two effects are rarely considered in assessing impacts of oil spills or response procedures.

Change and recovery of coastal mesozooplankton community structure during the Deepwater Horizon oil spill

The response of mesozooplankton community structure to the Deepwater Horizon oil spill in the northern Gulf of Mexico was investigated using data from a long-term plankton survey off the coast of Alabama (USA). Environmental conditions observed in the study area during the oil spill (2010) were compared to historical observations (2005–2009), to support the contention that variations observed in zooplankton assemblage structure may be attributed to the oil spill, as opposed to natural climatic or environmental variations. Zooplankton assemblage structure observed during the oil spill period (May–August) in 2010 was then compared to historical observations from the same period (2005–2009). Significant variations were detected in assemblage structure in May and June 2010, but these changes were no longer significant by July 2010. The density of ostracods, cladocerans and echinoderm larvae were responsible for most of the differences observed, but patterns differed depending on taxa and months. Many taxa had higher densities during the oil spill year, including calanoid and cyclopoid copepods, ostracods, bivalve larvae and cladocerans, among others. Although this result is somewhat surprising, it is possible that increased microbial activity related to the infusion of oil carbon may have stimulated secondary production through microbial-zooplankton trophic linkages. Overall, results suggest that, although changes in zooplankton community composition were observed during the oil spill, variations were weak and recovery was rapid. S Online supplementary data available from stacks.iop.org/ERL/9/124003/mmedia

Cross-Shore, Seasonal, and Depth-Related Structure of Ichthyoplankton Assemblages in Coastal Alabama

Abstract Investigations of the spatial and temporal structure of larval fish assemblages are essential for a better understanding of the dynamics of fish populations and their resilience to environmental change. This study provides an original typology of the spatial, seasonal, and depth-related structure of ichthyoplankton assemblages collected along a 77-km cross-shore gradient in Alabama coastal waters. This typology is based on a depth-discrete ichthyoplankton survey conducted across multiple years at a high spatial and temporal resolution in the northern Gulf of Mexico. A total of 350,766 larvae were collected, among which >95% could be identified to families. The total density of fish larvae was significantly higher inshore, whereas the number of families increased offshore. Multivariate regression trees and Dufrene–Legendre indices were used to identify seven assemblages dominated by different families of larvae. These assemblages were primarily structured by distance from the shore, followed by se...

Environmental Influences on Juvenile Fish Abundances in a River-Dominated Coastal System

Abstract We investigated the influence of climatic and environmental factors on interannual variations in juvenile abundances of marine fishes in a river-dominated coastal system of the north-central Gulf of Mexico, where an elevated primary productivity sustains fisheries of high economic importance. Fish were collected monthly with an otter trawl at three stations near Mobile Bay from 1982 to 2007. Fish sizes were used to isolate juvenile stages within the data set, and monthly patterns in juvenile fish abundance and size were then used to identify seasonal peaks for each species. The average numbers of juvenile fish collected during these seasonal peaks in each year were used as indices of annual juvenile abundances and were related to corresponding seasonal averages of selected environmental factors via a combination of principal components analysis and co-inertia analysis. Factors contributing the most to explain interannual variations in juvenile fish abundances were river discharge and water temperature during early spring—early summer, wind speed and North Atlantic Oscillation index during late fall—winter, and atmospheric pressure and wind speed during summer—fall. For example, juvenile abundances of southern kingfish Menticirrhus americanus during summer—fall were positively associated with atmospheric pressure and negatively associated with wind speed during this period. Southern kingfish juvenile abundances during late fall—winter were also negatively associated with wind speed during the same period and were positively associated with river discharge during early spring—early summer. Juvenile abundances of the Atlantic croaker Micropogonias undulatus during early spring—early summer were negatively associated with river discharge and North Atlantic Oscillation during late fall—winter. Overall, the importance of river discharge for many of the species examined emphasizes the major role of watershed processes for marine fisheries production in coastal waters of the north-central Gulf of Mexico.

Sargassum coverage in the northeastern Gulf of Mexico during 2010 from Landsat and airborne observations: Implications for the Deepwater Horizon oil spill impact assessment.

Using high-resolution airborne measurements and more synoptic coverage of Landsat measurements, we estimated the total Sargassum coverage in the northeastern Gulf of Mexico (NE GOM) during 2010, with the ultimate purpose to infer how much Sargassum might have been in contact with oil from the Deepwater Horizon oil spill. Mean Sargassum coverage during the four quarters of 2010 for the study region was estimated to range from ~3148±2355km(2) during January-March to ~7584±2532km(2) during July-September (95% confidence intervals) while estimated Sargassum coverage within the integrated oil footprint ranged from 1296±453km(2) (for areas with >5% thick oil) to 736±257km(2) (for areas with >10% thick oil). Similar to previous studies on estimating Sargassum coverage, a direct validation of such estimates is impossible given the heterogeneity and scarcity of Sargassum occurrence. Nonetheless, these estimates provide preliminary information to understand relative Sargassum abundance in the NE GOM.

Condition of larval red snapper (Lutjanus campechanus) relative to environmental variability and the Deepwater Horizon oil spill

TheDeepwaterHorizon oil spill (DWHOS) spatially and temporally overlappedwith the spawning of many fish species, including Red Snapper, one of themost economically important reef fish in the Gulf ofMexico. To investigate potential impacts of theDWHOSon larval Red Snapper, data from a long-term ichthyoplankton survey off the coast of Alabamawere used to examine: (1) larval abundances among pre-impact (2007–2009), impact (2010), and post-impact (2011, 2013) periods; (2) proxies for larval condition (size-adjustedmorphometric relationships and dryweight) among the same periods; and (3) the effects of background environmental variation on larval condition.We found that larval Red Snapper were in poorer body condition during 2010, 2011, and 2013 as compared to the 2007–2009 period, a trend that was strongly (and negatively) related to variation in Mobile Bay freshwater discharge. However, larvae collected during and after 2010were in relatively poor condition even after accounting for variation in freshwater discharge and other environmental variables. By contrast, no differences in larval abundancewere detected during these survey years. Taken together, larval supply did not change relative to the timing of theDWHOS, but larval condition was negatively impacted. Even small changes in condition can affect larval survival, so these trends may have consequences for recruitment of larvae to juvenile and adult life stages.

Environmentally driven fluctuations in condition factor of adult Gulf menhaden (Brevoortia patronus) in the northern Gulf of Mexico

We evaluated the effects of a suite of environmental predictors on the relative condition (the relationship between individual weight and length) of adult Gulf Menhaden Brevoortia patronus sampled from the commercial fishery, from 1964 to 2011. A hierarchical linear model (HLM) was used to examine the association between relative condition and Mississippi River discharge. Generalized Additive Models (GAMs) were constructed to determine the impact on relative condition by El Niño Southern Oscillation (ENSO) and the spatially-variant influence of sea surface temperature (SST°C), chlorophyll a concentration (mg m) and wind vector components. The HLM revealed a positive correlation between Mississippi River discharge and relative condition that was consistent throughout the fishing season (April to October). Comparisons of spatially–variant and –invariant GAMs indicated that the effects of SST was consistent in the northern Gulf of Mexico (NGOM) and that the greatest relative condition was at temperatures associated with the Mississippi River plume front. The effect of wind vector components was spatially-variable, likely due to geographic differences in wind-related transport of productive plume waters across the NGOM. Relative condition was greatest during positive ENSO anomalies. Relative condition exhibited intra-annual variability with a small peak during April and May and increasing condition from August until November, likely caused by increased food availability and provisioning by individuals for spawning. We show that multiple bottom-up processes impact the individual dynamics of Gulf Menhaden in the NGOM and these results can be used to predict their impacts on the fisheries and ecology of the NGOM.