Louisiana ranks first among the coastal states accounting for 43% of the total open estuary surface and volume of the Gulf of Mexico. The ecosystems of coastal marshes in general and especially in Louisiana are threatened due to multiple human-induced stressors and climate change. Oil spills in particular have added to the degradation of marshes in the recent past, causing acute death of plants and animals as well as long-term effects, such as increased erosion, persistence of oil-derived components in the food web and overall decrease in ecosystem services. The catastrophic explosion of the Deep Water Horizon drilling platform caused the largest man-made marine oil spill to date, and it is estimated that roughly 75 linear km of East Louisiana's salt marshes experienced moderate to heavy oiling. Acute oiling in the Gulf of Mexico subsequent to the Deepwater Horizon platform collapse created visible impact on highly prominent vertebrates, particularly birds along the coast. Within the tidal marshes of Louisiana the PAH levels in subsurface water in the marsh locations remained high enough to have lethal effects on fish for up to two months, and it has been shown that high levels of oil were retained in the sediment of oiled marsh for much longer. Whether oil components remain in sediments of certain tidal marsh habitats and what effect these residues or degradation caused by oiling have on the invertebrate food web of vertebrates and invertebrates is now an important question. The aftermath of the oil spill provides unprecedented research opportunities for years to come. Tools must be developed not only to assess the impact of a particular oil spill but also to provide screening methods for time- and cost-efficient assessments of marsh health after future environmental insults to help guide remediation efforts. For future unpredicted insults on tidal marshes such as oil spills, techniques for rapid and intensive baseline sample collections with minimum impact on the fragile ecosystem will be needed. The measures of these techniques also could be used to monitor the health of the ecosystems during the process of land restoration and creation of marshland, e.g. by river diversions, etc. To address those needs, we propose to build on our previous studies and experiences to develop affordable and efficient measures of invertebrate food webs. Our previous studies began in May 2010, and we selected the saltmarsh greenhead (T. nigrovittatus) as an entomological model for coastal ecology for our studies. This species is native to and tightly bound to specific coastal marsh habitats. The larvae of T. nigrovittatus are apex predators that are dependent upon the invertebrate food web around them in the mud of the tidal marshes. Adult tabanids that are active in the spring and summer developed as larvae in the marshes over the past 9-12 months while adults active in the fall developed during the summer. During a two year period beginning immediately after the spill, we conducted studies on the abundance of greenhead horse fly populations at four locations (Grand Bayou and Grand Isle which were oiled and Cypremort Point and Cameron which remained pristine). Horse fly abundance estimates showed severe crashes of adult tabanid populations as well as reduced numbers of larvae recovered from the soil in oiled areas. We developed microsatellite primers to genotype 10 polymorphic microsatellite loci of T. nigrovittatus, and these genetic markers were used to compare population size and genetic structure of adult horse flies from pristine and oiled locations. Microsatellite genotyping of six pristine and seven oiled populations detected genetic bottlenecks in five of the oiled populations in association with fewer breeding parents, reduced effective population size, lower number of family clusters and fewer migrants among populations. We propose to follow up that study with a longitudinal population genetic study of horse flies as bioindicators of marsh health and recovery. In 2011, we conducted surveys of tabanid larvae and their surrounding sediment, and specimens were archived to initiate studies on establishing the food web within the sediments. We propose to use those and future collections for metagenetic analyses to compare the micro- and meiofauna community in larval guts to that in the immediate soil environment where tabanid larvae are either present or absent. Based on this knowledge we will develop a time- and cost-efficient PCR-based diagnostic method to differentiate between healthy and biologically depleted marsh soil for use in intensive sampling. The development of PCR diagnostic tools for food web detection fits our reductionist approach to develop and verify tools that can be used by coastal ecologists to evaluate tidal marsh health.
Project Research Overview (2016):
An overview of the proposed research activities from the GoMRI 2016 Meeting in Tampa.
Direct link to the Research Overview presentation.