Geology topics

Megan K. La Peyre

Publications and source records attributed to Megan K. La Peyre.

27 records · Page 2Linked to original sources

USGS2021

Ecological engineering with oysters enhances coastal resilience efforts

Coastal areas are especially vulnerable to habitat loss, sea-level rise, and other climate change effects. Oyster-dominated eco-engineered reefs have been promoted as integral components of engineered habitats enhancing coastal resilience through provision of numerous ecological, morphological, and socio-economic services. However, the assessed ‘success’ of these eco-engineered oyster reefs remains variable across projects and locations, with their general efficacy in promoting coastal resilience, along with related services, often mixed at best. Understanding factors influencing the success of these eco-engineered habitats as valuable coastal management tools could greatly inform related future efforts. Here, we review past studies incorporating reef-building oysters for coastal resilience and enhanced ecosystem services. Our aims are to better understand their utility and limitations, along with critical knowledge gaps to better advance future applicability. Success depends largely on site selection, informed by physical, chemical and biological factors, and adjacent habitats and bottom types. Better understanding of oyster metapopulation dynamics, tolerance and adaptation to changing conditions, and interactions with adjacent habitats will help to better identify suitable locations, and design more effective eco-engineered reefs. These eco-engineered reefs provide a useful tool to assist in developing coastal resilience in the face of climate change and sea level rise.

Ecological Engineering↗

USGS2021

Effects of sample gear on estuarine nekton assemblage assessments and food web model simulations

Long-term fisheries-independent sampling data inform population status and trends of species-specific biomass and are often used to drive biomass-based food web models such as the Comprehensive Aquatic Systems Model (CASM). Indicators such as total biomass and mean trophic level derived from these data and from CASM outputs inform management and facilitate assessments of on-going and predicted coastal change and restoration activities on fisheries, but rely on consistent sampling to enable comparisons across space and time. Changes in coastal estuarine gradients, combined with the availability of new sampling technologies, highlight a need to assess the potential consequences of changing sampling technologies on fisheries data and the cascading impact on model outputs. In Louisiana, USA, CASM models are used to inform coastal restoration projects, relying on 40 years of fisheries-independent data derived from 50′ seine sampling. However, alternative use of electrofishers as a sampling method has been proposed to replace the seine sampling. In this study, we examine data from concurrent seine and electrofisher sampling in Barataria Basin, Louisiana, and compare biomass, assemblage data and CASM outputs related to species biomass, food web structure and energy cycling. In a paired comparison of data in 2018–2019, the electrofisher captured higher total catch and diversity compared to the seine. The electrofisher samples were dominated by shrimp (grass, white, brown) and larger bodied fish, while seine samples were dominated by smaller-bodied fish (i.e., anchovy, menhaden). Ecosystem indicators derived from running the CASM using biomass data from seine and electrofisher sampling separately in two different simulation exercises provide contrasting results. In Simulation Exercise 1, the use of different datasets (long-term CASM calibration, 2018–2019 seine, 2018–2019 electrofisher) to initialize the CASM biomasses did not result in large or long-running changes in the simulated biomasses over time. In contrast, in Simulation Exercise 2, CASM model outputs using adjusted gear ratios indicated changes in biomass structure when using electrofisher data, with a doubling of total food web biomass due to the increased shrimp count, and a 13% increase in total energy flow through the food web. Conversions based on area and gear efficiency for overall catch may be useful in maintaining the continuity of historical data. However, differences in species-specific catch due to gear selectivity could have large consequences for constructing and calibrating fish and ecosystem models and remain difficult to reconcile. These differences in assemblages, and estimated biomasses for key food web species, suggest careful consideration in changing gears.

Louisiana↗

USGS2021

Tolerance of northern Gulf of Mexico eastern oysters to chronic warming at extreme salinities

The eastern oyster, Crassostrea virginica , provides critical ecosystem services and supports valuable fishery and aquaculture industries in northern Gulf of Mexico (nGoM) subtropical estuaries where it is grown subtidally. Its upper critical thermal limit is not well defined, especially when combined with extreme salinities. The cumulative mortalities of the progenies of wild C. virginica from four nGoM estuaries differing in mean annual salinity, acclimated to low (4.0), moderate (20.0), and high (36.0) salinities at 28.9 °C (84 °F) and exposed to increasing target temperatures of 33.3 °C (92 °F), 35.6 °C (96 °F) or 37.8 °C (100 °F), were measured over a three-week period. Oysters of all stocks were the most sensitive to increasing temperatures at low salinity, dying quicker (i.e., lower median lethal time, LT 50 ) than at the moderate and high salinities and resulting in high cumulative mortalities at all target temperatures. Oysters of all stocks at moderate salinity died the slowest with high cumulative mortalities only at the two highest temperatures. The F1 oysters from the more southern and hypersaline Upper Laguna Madre estuary were generally more tolerant to prolonged higher temperatures (higher LT 50 ) than stocks originating from lower salinity estuaries, most notably at the highest salinity. Using the measured temperatures oysters were exposed to, 3-day median lethal Celsius degrees (LD 50 ) were estimated for each stock at each salinity. The lowest 3-day LD 50 (35.1–36.0 °C) for all stocks was calculated at a salinity of 4.0, while the highest 3-day LD 50 (40.1–44.0 °C) was calculated at a salinity of 20.0.

Louisiana, Texas↗

USGS2020

Living on the edge: Multi-scale analyses of bird habitat use in coastal marshes of Barataria Basin, Louisiana, USA

Coastal marsh loss, combined with expected sea-level rise, will cause inundation and extensive shifts to vegetation and salinity regimes that may affect the bird species dependent on coastal ecosystems worldwide. Within coastal marsh habitats, birds provide key targets for coastal management goals. However, limited information on bird-habitat relationships within coastal marshes inhibits the development of restoration projects targeted to bird species. We surveyed birds bi-monthly within Barataria Basin, LA from July 2014 to December 2015 to compare their use between fresh and saline coastal marshes. Additionally, we examined habitat use at finer spatial scales to assess preference for marsh edge microhabitats. Edge habitat supported 1.8 times more bird species (guild) richness than emergent and open water habitat. We concluded that future modelling efforts would be improved if models incorporate edge effects for birds in coastal marshes that extend 20 m from emergent vegetation into open water, with a reduced effect if marsh types convert from fresh to saline. Our data will be useful to simulate the effects of changes in marsh type, area, and edge on habitat quality for birds in coastal Louisiana and will inform habitat restoration and management decisions aimed at optimizing bird use.

Louisiana↗

USGS2020

Estuarine submerged aquatic vegetation habitat provides organic carbon storage across a shifting landscape

Submerged aquatic vegetation (SAV) thrives across the estuarine salinity gradient providing valuable ecosystem services. Within the saline portion of estuaries, seagrass areas are frequently cited as hotspots for their role in capturing and retaining organic carbon (C org ). Non-seagrass SAV, located in the fresh to brackish estuarine areas, may also retain significant soil C org , yet their role remains unquantified. Given rapidly occurring landscape and salinity changes due to human and natural disturbances, landscape level carbon pool estimates from estuarine SAV habitat blue carbon estimates are needed. We assessed C org stocks in SAV habitat soils from estuarine freshwater to saline habitats (interior deltaic) to saline barrier islands (Chandeleur Island) within the Mississippi River Delta Plain (MRDP), Louisiana, USA. SAV habitats contain C org stocks equivalent to those reported for other estuarine vegetation types (seagrass, salt marsh, mangrove). Interior deltaic SAV C org stocks (231.6 ± 19.5 Mg C org ha −1 ) were similar across the salinity gradient, and significantly higher than at barrier island sites (56.6 ± 10.4 Mg C org ha −1 ). Within the MRDP, shallow water SAV habitat covers up to an estimated 28,000 ha, indicating that soil C org storage is potentially 6.4 ± 0.1 Tg representing an unaccounted C org pool. Extrapolated across Louisiana, and the Gulf of Mexico, this represents a major unaccounted pool of soil C org . As marshes continue to erode, the ability of coastal SAV habitat to offset some of the lost carbon sequestration may be valuable. Our estimates of C org sequestration rates indicated that conversion of eroding marsh to potential SAV habitat may help to offset the reduction of C org sequestration rates. Across Louisiana, we estimated SAV to offset this loss by as much as 79,000 Mg C yr −1 between the 1960s and 2000s.

Louisiana↗

USGS2020

Eastern oyster clearance and respiration rates in response to acute and chronic exposure to suspended sediment loads

Coastal Louisiana supports some of the most productive areas for the eastern oyster, Crassostrea virginica . Changing conditions from restoration and climate change alter freshwater and sediment inflows into critical estuarine areas affecting water quality, including salinity and concentrations of suspended sediment. This study examined the effects of acute (1 h) and chronic (8 weeks) exposure of suspended sediment concentrations on the eastern oyster's respiration and clearance rates. Acute exposure at six sediment concentrations (0, 10, 50, 200, 500, 1000 mg L −1 ) and one salinity (15) indicated that sediment concentration significantly affected oyster clearance rates, with increasing clearance rates as suspended sediment concentrations increased up to 500 mg L −1 . Respiration rates were not affected by sediment concentration ( p = .12). Chronic exposure at two salinities (6 and 15) and three sediment concentrations (0, 50, 400 mg L −1 ) found no significant effect of sediment, salinity or their interaction on clearance rates. Respiration rate was reduced at higher sediment concentrations (50 and 400 mg L −1 versus 0 mg L −1 ) and lower salinity. As clearance and oxygen consumption rates critically inform oyster energetic models, these data provide valuable insight to more accurately predict eastern oyster population dynamics and inform harvest models in the face of changing estuarine conditions. Changes in rates of growth through altered energetic demands ultimately can impact not just the economic viability of the industry, but also the ability for the populations to maintain sustainable reefs.

Louisiana↗

USGS2020

Gear comparison study for sampling nekton in Barataria Basin marshes

This project was funded by the Louisiana Trustee Implementation Group (LA TIG) to support decisions related to investments in long-term monitoring. The LA TIG seeks to ensure long-term monitoring informs coastal restoration activities with the goal of sustaining and improving fisheries impacted by the Deepwater Horizon (DWH) Oil Spill. The project objective was to compare nekton catch across an estuarine gradient using different sampling gear with the goal of identifying trade-offs among nekton sampling approaches. To accomplish this objective, Louisiana Department of Wildlife and Fisheries (LDWF), The Water Institute of the Gulf (the Institute), Dynamic Solutions, LLC, Louisiana State University Agricultural Center (LSU AgCenter), and the U.S. Geological Survey (USGS) completed a field gear comparison study from 2018 to 2019. This work compared electrofisher and seine sampling at 12 fixed stations in Barataria Basin using data collected by LDWF. In addition, and in conjunction with LDWF monthly sampling, the same 12 fixed stations were sampled in May 2019 using a throw trap to compare nekton catch and assemblages collected with the throw trap, seine and electrofisher. LDWF has been conducting seine sampling since 1986, and seine data are used by the State of Louisiana to assess juvenile shrimp, crab and fish abundances, sizes and overall assemblages. In 2018, LDWF began conducting electrofisher sampling at 12 Barataria Basin seine stations in order to determine if the two gear types sample similar species and assemblages for potential future replacement of long-term seine sampling with electrofishing. Throw traps were included as they provide density estimates, which are ultimately the desired statistic used in modeling trophic webs, and are used in assessing habitat restoration outcomes. The project compared the nekton catch and assemblages collected using seine, electrofisher, and throw trap data from marsh edge habitats located across the estuarine gradient in Barataria Basin. Specifically, catch per unit effort (CPUE), species richness, species-specific total length (mm) distribution and nekton assemblages were compared between gear types. The first dataset was collected in May 2019 with throw trap (Appendix A), seine (LDWF data), and electrofisher (LDWF data) gear, and the second dataset (collected by LDWF) spanned 14 months of seine and electrofisher monthly sampling occurring from May 2018 through June 2019 at 12 stations in Barataria Basin. Key findings include that gear bias was not evident across the range of water quality conditions (salinity, temperature, o C, dissolved oxygen, mg L-1 , turbidity, NTU; Appendix B: scatter plots) captured during this pilot study, but differences in nekton catch per unit effort (CPUE) and assemblages were evident between gear types. However, those differences largely depended on the parameter examined. For example, the overall CPUE was highest for electrofishing, followed by seine, and then throw trap. When grass shrimp (the most abundant taxon collected) were removed from CPUE, the electrofisher and seine results were similar in CPUE. When CPUE was corrected for gear efficiency and total area sampled, the throw trap had the highest reported density of nekton sampled, followed by electrofisher and seine results. Electrofishing captured the highest number of species, which included more unique species compared to seine or throw trap catches, though all gear types captured at least one unique species. These highlight a need for caution in interpreting assemblage and density data when comparing datasets derived from different sampling methodologies. These key findings can help inform implementation and interpretation of long-term monitoring data in Louisiana as management decisions are made about coastal restoration projects to sustain and improve fisheries. There are trade-offs in selecting gear types for estuarine nekton monitoring of density, abundance, species richness, and assemblages. The table below (Table 1) summarizes some considerations when selecting gear types for long-term monitoring of estuarine nekton. In addition to biological and ecological considerations, other important considerations include cost, the labor required to conduct sampling, logistics, and potential uncertainties related to how effective each gear type is for sampling the wide variety of conditions found across Louisiana’s coastal habitats. For example, although electrofishing may capture higher CPUE, the equipment is more expensive to obtain and maintain compared to the other gear types. Most importantly, this table highlights differences in the nekton assemblages sampled by each gear type; this consideration is critical when designing the goals of a long-term monitoring program as it will inform how the data can be used and interpreted in the future. This report provides caveats, assumptions, and recommendations that can help support the Louisiana Coastal Protection and Restoration Authority (CPRA), LDWF and the LA TIG in comparing data from different gear types, and in making decisions for future monitoring. Findings from this study are limited to the range of water quality conditions occurring during these data collection events; these data and analyses could benefit from sampling across a wider range of water quality conditions, and collection of habitat structure and bottom type data which are not routinely collected but critically influence nekton. Further investigation examining how relative differences detected in key species abundances between gear types might impact ecosystem indicators and energetics in a modeled food web would provide valuable input to understand outputs of the Comprehensive Aquatic System Model for Barataria Basin, including the potential impacts of nekton monitoring decisions on food web models.

Louisiana↗

USGS2019

Salinity and water clarity dictate seasonal variability in coastal submerged aquatic vegetation in subtropical estuarine environments

Spatial and temporal variability characterize submerged aquatic vegetation (SAV) assemblages, but understanding the complex interactions of environmental drivers of SAV assemblages remains elusive. We documented SAV composition and biomass across a salinity gradient in a coastal estuary over 12 mo. Ten macrophyte species were identified. The dominant species, Ceratophyllum demersum and Myriophyllum spicatum, accounted for over 40% of total biomass. Only Ruppia maritima occurred across the salinity gradient. Salinity, water depth and clarity delineated 3 assemblages: a saline assemblage, and 2 groups of fresher-water species, one associated with deeper water and lower water clarity and the other associated with shallow water and higher water clarity. These assemblages exhibited intra-annual variation, with at least 5 times more biomass in late spring/mid-summer compared to early winter. This pattern was consistent across the estuary, although the difference between peak and low biomass varied by habitat type; brackish exhibited the greatest magnitude. This variation is likely due to higher variation in salinity and the species composition of this habitat. As climate change and coastal restoration impact timing and range of salinity, water depth and clarity in this region, these data can be used to help inform predictive models and management decisions.

Louisiana↗

USGS2018

Measuring carbon and nitrogen bioassimilation, burial, and denitrification contributions of oyster reefs in Gulf coast estuaries

The eastern oyster ( Crassostrea virginica ) and the reefs they create provide significant ecosystem services. This study measured their possible role in nutrient mitigation through bioassimilation, burial, and oyster-mediated sediment denitrification in near-shore shallow water (< 1 m water depth) and deep-water (> 1 m water depth) oyster reefs in Louisiana. Nitrogen (N) and carbon (C) in shell and tissue differed by oyster reproductive status, size, and habitat type. Changes in tissue percent N and C post-spawning combined with significant reductions in tissue dry weight from the release of gametes, resulted in 20 and 46% reductions in tissue N and C load (mg), respectively, for a 100-mm oyster. Oyster reefs did not enhance burial rates, with burial range rates estimated at 1.4–2.6 g N m −2 year −1 , and 26.9–43.8 g C m −2 year −1 . Closed-system ex situ incubations indicated net denitrification in all habitat types studied, with the highest rates exceeding 600 µmol N m −2 h −1 during the summer, but no enhancement attributable to oyster reefs specifically. Within the highly productive, organic-rich wetland complex systems of coastal Louisiana, oyster reefs were not associated with enhanced denitrification, likely due to the organic-rich setting, and redundant supplies of organic nitrogen and carbon from adjacent marshes. Context remains critical in determining ecosystem provision of habitats, and efforts to extrapolate and predict nitrogen removal across locations necessitates consideration of local conditions. Considering the large extent of reefs and oyster production across coastal Louisiana, oyster habitats may still contribute to N and C mitigation, but their unique contribution likely comes from bioassimilation, and removal of the oysters from the system.

Louisiana↗