Resolving Deepwater Horizon Impacts on Highly Variable Ichthyoplankton and Zooplankton Dynamics in the Northern Gulf of Mexico

Dr. Frank Hernandez at the University of Southern Mississippi (USM) was awarded an RFP-II grant at $455,812 to conduct the RFP-II project titled 'Resolving Deepwater Horizon Impacts on Highly Variable Ichthyoplankton and Zooplankton Dynamics in the Northern Gulf of Mexico'. The project consisted of 1 institution (USM), 1 principal investigator (Hernandez), 1 postdoc (Dr. Jesse Filbrun), 1 graduate student (John Ransom), and participation by several research technicians, interns and graduate students. The overall goal of this study was to examine how the documented shifts in zooplankton community structure during the Deepwater Horizon oil spill (Carassou et al. 2014, Environ. Res. Lett. 9 124003) affected larval fish feeding, growth and condition (critical factors related to survivorship and recruitment success). Relative to fisheries oceanography and recruitment theory, the overarching question was, 'Did the DWH event create a 'mismatch' scenario in the planktonic food web?'. At the core of this study was access to data collected as part of a unique plankton survey that is the only source of pre-DWH zooplankton and ichthyoplankton data for the northern Gulf of Mexico region collected at high temporal (monthly sampling throughout; twice monthly during the DWH event) and spatial resolutions (three cross-shelf stations, each with vertically-discrete sampling). This study took an 'ecosystem-based' approach by examining the early life stages of both ecologically- and economically-important fishes. The project was cost-effective in that most of the samples targeted for examination have already been collected, including five years of pre-spill, 'baseline' data, and many 'impact' samples collected during the DWH event. The objectives of this study are to examine larval fish feeding, growth and condition (using established methodologies) and compare these parameters among 'baseline', 'impact', and 'recovery' periods. Three target fishes were chosen for detailed ecological analyses based on (1) their importance in driving assemblage-level ichthyoplankton dynamics in historical collections, assessed using nonmetric multidimensional scaling, (2) their subjective ecological and commercial importance, (3) their total abundance and frequency of occurrence in historical samples, (4) the ease of removing and imaging intact otoliths, (5) the apparent ease of processing gut contents for diet analysis, and (6) the existence of peer-reviewed growth and diet studies. Based on these criteria, we selected Spanish mackerel (Scomberomorus maculatus, a piscivorous coastal pelagic species), red snapper (Lutjanus campechanus, a heavily managed commercial reef species), and Atlantic bumper (Chloroscombrus chrysurus, a zooplanktivorous forage species) for diet, growth, and condition analyses. The proposed work was highly responsive to Theme 3 of GoMRI RFP-II in that environmental impacts of the petroleum/dispersant system on critical water column food web dynamics (that have direct implications for fisheries recruitment) were assessed. While the ultimate goal of this study was to gain an understanding of the impacts of the DWH event, it proceeded with a deep understanding of the environmental factors that form the 'background noise' of fisheries recruitment. The project addressed recovery and resiliency of plankton food web dynamics by collecting additional samples in 2013, 2014, and 2015. The Principal Investigator was committed to actively integrating undergraduate, graduate, and post-doctoral researchers in the project, as well as outreach to K-12 and the general public through the Marine Education Center at the Gulf Coast Research Laboratory (University of Southern Mississippi). Also, an emphasis was placed on promoting diversity initiatives by leveraging other programs to support high school and undergraduate student interns from underrepresented demographics in marine sciences and fisheries. Three undergraduate summer interns supported by the NOAA Northern Gulf Institute's Diversity Internship program participated in our GoMRI project (Sandra Huynh, University of South Alabama; Jeffrey Fang, University of Portland; Katie Homa, Columbia University), as well as two high school summer interns supported by the American Fisheries Society's Hutton Program (Natasha Chawla, Ocean Springs High School; Adriona Horton, Ocean Springs High School). One of these interns (Fang) was also a junior author on a peer-reviewed paper. Dr. Hernandez also partnered with a non-profit group, ScienceJournalForKids.org, to publish an open-access version of one of their papers targeted for middle school and high school levels. Lastly, the Hernandez lab participated in the filming of the GoMRI-funded documentary, Dispatches from the Gulf (Screenscope). Research Highlights Dr. Hernandez's research, which included 18 outreach products and activities, resulted in 7 peer-reviewed publications and 7 datasets being submitting to the GoMRI Information and Data Cooperative (GRIIDC) , which are available to the public. Significant outcomes of their research (all related to GoMRI Research Theme 3) are highlighted below. Larval fish abundances and assemblage dynamics: Historical trends (2007–2011) in ichthyoplankton assemblages and target species abundances and distributions at two sampling stations (T20 and T35) were examined using the baseline survey data. Total larval fish abundance and family richness in 2010 were similar to years before and after the oil spill. All three target species were abundant in 2010, although presence varied substantially between stations. There were clear seasonal patterns in larval fish assemblages that followed expected patterns from adult spawning behavior. However, there were no differences in summer ichthyoplankton assemblages before, during, and after the oil spill. These results were presented at the 2014 Gulf of Mexico Oil Spill and Ecosystem Science meeting, and will be included in a pending synthesis manuscript. Larval Spanish mackerel diet, growth and condition: Comparisons of condition using morphometric analyses and length-weight relationships revealed that Spanish mackerel larvae were deeper-bodied and heavier during the 2010 (Deepwater Horizon period) relative to before and after spill periods. Additionally, larvae in June and July were in better condition than larvae in May across all years. Feeding incidence of subsampled Spanish mackerel larvae was low (18%), with diets comprised of unidentified larval fish and various crustacean zooplankters. The smallest piscivorous larva was only 3 mm long, and the guts of one 9-mm larva contained two larval fish. The most abundant prey items of larval Spanish mackerel were larval fishes, copepods, and ostracods. Diet composition did not differ significantly among the three time periods. Also, daily growth did not differ between larvae collected during and after the oil spill (no otoliths were available to estimate growth before 2010). Overall, our results suggest that larval Spanish mackerel were resilient to harmful effects of the Deepwater Horizon oil spill. These results were generated as part of graduate student and GoMRI Scholar John Ransom's MS thesis, and published as a peer-reviewed paper in Marine Ecology Progress Series (Ransom et al. 2016, Mar. Ecol. Prog. Ser. 558:143–152). Larval Atlantic bumper diet and condition: Larval Atlantic bumper diets were similar before, during, and after the oil spill. Feeding incidence was 30–40% across time periods, with copepods and cladocerans as the most common prey items. Larger fish generally had more prey items in their guts. Prey lengths were 0.1–1.2 mm. Larval C. chrysurus body condition did not differ before, during, and after the oil spill. However, body condition differed by month and station, with deeper-bodied larvae collected during the later months and at the more offshore stations. These results were presented at the 2014 joint meeting of the American Fisheries Society and Larval Fish Conference, and will be included in a pending synthesis manuscript. Larval red snapper condition: 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 2010 were in relatively poor condition even after accounting for variation in freshwater discharge and other environmental variables. By contrast, no differences in larval abundance were detected during these survey years. Taken together, larval supply did not change relative to the timing of the Deepwater Horizon oil spill, but larval condition was negatively impacted. These results were published as a peer-reviewed manuscript in Environmental Research Letters (Hernandez et al. 2016, Environ. Res. Lett. 11 094019).