Breton Sound Research Articles

A Simplified Parametric Model for Fetch-Limited Peak Wave Frequency in Shallow Estuaries

ABSTRACT Karimpour, A. and Chen, Q., 2016. A simplified parametric model for fetch-limited peak wave frequency in shallow estuaries. A study was conducted on the wave energy and peak wave frequency in Breton Sound, Louisiana. A bottom-mounted Acoustic Doppler Velocimeter was used to measure waves and currents. The results of the study indicate that the asymptotic limits for the wave energy and peak wave frequency can be defined accurately, while the integral properties of the wave field estimated by the existing parametric wave models, particularly the peak wave frequency, suffer from a lack of accuracy. Based on the observed dependency of peak frequency on the ratio of wind fetch to water depth, a simplified parametric model is proposed in an effort to increase the accuracy of the estimated peak wave frequency. The physical boundaries between water depth and wind fetch for proper estimation of the peak frequency are introduced. The model was evaluated for the goodness of fit against measured data that ha...

Sediment Deposition at the Caernarvon Crevasse during the Great Mississippi Flood of 1927: Implications for Coastal Restoration

During the 1927 Mississippi flood, the levee was dynamited downstream of New Orleans creating a 2 km wide crevasse that inundated the Breton Sound estuary and deposited a crevasse splay of about 130 km2. We measured sediment deposition in the splay that consisted of a silty-clay layer bounded by aged peat below and living roots above. Based on coring, we developed a map of the crevasse splay. The clay layer ranged from 2 to 42 cm thick and occurred 24 to 55 cm below the surface. Bulk density of the clay layer decreased and soil organic matter increased with distance from the river. 210Pbexcess and 137Cs dating an age of ~1926–1929 for the top of the layer. During the flood event, deposition was at least 22 mm·month−1—10 times the annual post-1927 deposition. The crevasse splay captured from 55% to 75% of suspended sediments that flowed in from the river. The 1927 crevasse deposition shows how pulsed flooding can enhance sediment capture efficiency and deposition and serves as an example for large planned diversions for Mississippi delta restoration.

Increased Temperatures Combined with Lowered Salinities Differentially Impact Oyster Size Class Growth and Mortality

ABSTRACT Changes in the timing and interaction of seasonal high temperatures and low salinities as predicted by climate change models could dramatically alter oyster population dynamics. Little is known explicitly about how low salinity and high temperature combinations affect spat ( 75mm) oyster growth and mortality. Using field and laboratory studies, this project quantified the combined effects of extremely low salinities ( 30°C) on growth and survival of spat, seed, andmarket-sized oysters. In 2012 and 2013, hatchery-produced oysters were placed in open and closed cages at three sites in Breton Sound, LA, along a salinity gradient that typically ranged from 5 to 20. Growth and mortality were recorded monthly. Regardless of size class, oysters at the lowest salinity site (annualmean = 4.8) experienced significantly highermortality and lower growth than oysters located in higher salinity sites (annual means = 11.1 and 13.0, respectively); fur...

Analysis of Environmental Factors Influencing Salinity Patterns, Oyster Growth, and Mortality in Lower Breton Sound Estuary, Louisiana, Using 20 Years of Data

ABSTRACT La Peyre, M.K.; Geaghan, J.; Decossas, G., and La Peyre, J.F., 2016. Analysis of environmental factors influencing salinity patterns, oyster growth, and mortality in lower Breton Sound Estuary, Louisiana, using 20 years of data. Freshwater inflow characteristics define estuarine functioning by delivering nutrients, sediments, and freshwater, which affect biological resources and ultimately system production. Using 20 years of water quality, weather, and oyster growth and mortality data from Breton Sound Estuary (BSE), Louisiana, we examined the relationship of riverine, weather, and tidal influence on estuarine salinity, and the relationship of salinity to oyster growth and mortality. Mississippi River discharge was found to be the most important factor determining salinity patterns over oyster grounds within lower portions of BSE, with increased river flow associated with lowered salinities, while easterly winds associated with increased salinity were less influential. These patterns were consis...

Assessing shoreline exposure and oyster habitat suitability maximizes potential success for sustainable shoreline protection using restored oyster reefs.

Oyster reefs provide valuable ecosystem services that contribute to coastal resilience. Unfortunately, many reefs have been degraded or removed completely, and there are increased efforts to restore oysters in many coastal areas. In particular, much attention has recently been given to the restoration of shellfish reefs along eroding shorelines to reduce erosion. Such fringing reef approaches, however, often lack empirical data to identify locations where reefs are most effective in reducing marsh erosion, or fully take into account habitat suitability. Using monitoring data from 5 separate fringing reef projects across coastal Louisiana, we quantify shoreline exposure (fetch + wind direction + wind speed) and reef impacts on shoreline retreat. Our results indicate that fringing oyster reefs have a higher impact on shoreline retreat at higher exposure shorelines. At higher exposures, fringing reefs reduced marsh edge erosion an average of 1.0 m y−1. Using these data, we identify ranges of shoreline exposure values where oyster reefs are most effective at reducing marsh edge erosion and apply this knowledge to a case study within one Louisiana estuary. In Breton Sound estuary, we calculate shoreline exposure at 500 random points and then overlay a habitat suitability index for oysters. This method and the resulting visualization show areas most likely to support sustainable oyster populations as well as significantly reduce shoreline erosion. Our results demonstrate how site selection criteria, which include shoreline exposure and habitat suitability, are critical to ensuring greater positive impacts and longevity of oyster reef restoration projects.

Effects of salinity and particle concentration on sediment hydrodynamics and critical bed-shear-stress for erosion of fine grained sediments used in wetland restoration projects

Abstract. Sea-level rise, the increasing number and intensity of storms, oil and groundwater extraction, and coastal land subsidence are putting people and property at risk along Louisiana’s coast, with major implications for human safety and economic health of coastal areas. A major goal towards re-establishing a healthy and sustainable coastal ecosystem has been to rebuild Louisiana’s disappearing wetlands with fine grained sediments that are dredged or diverted from nearby rivers, channels and lakes to build land in open water areas. A thorough geo-hydrodynamic characterization of the deposited sediments is important in the correct design and a more realistic outcome assessment of the long-term performance measures for ongoing coastal restoration projects. This paper evaluates the effects of salinity and solid particle concentration on the re-suspension characteristics of fine-grained dredged sediments obtained from multiple geographic locations along the Gulf coast. The critical bed-shear-stress for erosion has been evaluated as a function of sedimentation time. The sediment hydrodynamic properties obtained from the laboratory testing were used in a numerical coastal sediment distribution model to aid in evaluating sediment diversions from the Mississippi River into Breton Sound and Barataria Bay.

Phytoplankton Community Shifts and Harmful Algae Presence in a Diversion Influenced Estuary

River water entering estuaries affects the physical and chemical environment at irregular intervals creating a highly dynamic aquatic habitat. Phytoplankton are important primary producers in estuaries that respond quickly to their changing environment. Over a 12-month period, the phytoplankton response was examined in terms of biomass, abundance, community composition, and potential phycotoxin production to seasonal changes in river input entering the Breton Sound Estuary in Louisiana. Chlorophyll a (chl a) measurements estimated phytoplankton biomass and light microscopy identified phytoplankton abundance and community composition. Phycotoxins were measured using ultra-sensitive enzyme-linked immunosorbent assay (ELISA). During the study period, chl a and cell abundances exhibited an inverse relationship with the seasons, which were determined by temperature and diversion input rates. The community was dominated by cyanobacteria for most of the year, mainly corresponding to the warm-low flow season. For the rest of the year, cyanobacteria decreased while chlorophytes and centric diatoms increased to approximately equal contributions. Overall, the phytoplankton community composition was most commonly moderated by temperature, salinity, and dissolved inorganic nitrogen (DIN) availability using BIO-ENV. Microcystins (MCs) were detected throughout the year ranging from below detection to 2.92 μg MCs l−1. MC levels were highest during the warm-low flow season and toward the lower estuary. The detection of MCs for the first time in the Breton Sound Estuary illustrates a potential risk to human health and economically important estuarine food webs.

A numerical study of vegetation impact on reducing storm surge by wetlands in a semi-enclosed estuary

Coastal wetlands play a unique role in extreme hurricane events. The impact of wetlands on storm surge depends on multiple factors including vegetation, landscape, and storm characteristics. The Delft3D model, in which vegetation effects on flow and turbulence are explicitly incorporated, was applied to the semi-enclosed Breton Sound (BS) estuary in coastal Louisiana to investigate the wetland impact. Guided by extensive field observations, a series of numerical experiments were conducted based on variations of actual vegetation properties and storm parameters from Hurricane Isaac in 2012. Both the vegetation-induced maximum surge reduction (MSR) and maximum surge reduction rate (MSRR) increased with stem height and stem density, and were more sensitive to stem height. The MSR and MSRR decreased significantly with increasing wind intensity. The MSRR was the highest with a fast-moving weak storm. It was also found that the MSRR varied proportionally to the expression involving the maximum bulk velocity and surge over the area of interest, and was more dependent on the maximum bulk surge. Both MSR and MSRR appeared to increase when the area of interest decreased from the whole BS estuary to the upper estuary. Within the range of the numerical experiments, the maximum simulated MSR and MSRR over the upper estuary were 0.7m and 37%, respectively.

Spatial and Temporal Variations in Nutrients and Water-Quality Parameters in the Mississippi River-Influenced Breton Sound Estuary

ABSTRACT Lundberg, C.J.; Lane, R.R., and Day, J.W., Jr., 2014. Spatial and temporal variations in nutrients and water-quality parameters in the Mississippi River-influenced Breton Sound estuary. The purpose of this study is to investigate the long-term temporal and spatial nutrient patterns in the Breton Sound estuary, an estuarine wetland complex in coastal Louisiana that is highly influenced by the Caernarvon river diversion. A water-quality data set spanning 8 years of monthly sampling was analyzed. Analysis of salinity mixing diagrams indicates the estuary to be a source of ammonium and chlorophyll a, and a sink for nitrate, total nitrogen, and total suspended sediments. The estuary served as either a source or sink for phosphate, total phosphorus, and silicate depending on season. The NOx loading rate ranged from 1.1 g N m−2 y−1 during fall to 4.9 g N m−2 y−1 during spring, with an overall mean of 3.5 g N m−2 y−1. Nitrate removal efficiency varied seasonally, with highest efficiency during the fall (...

Simulating Fish Movement Responses to and Potential Salinity Stress from Large‐Scale River Diversions

AbstractRiver diversions are a major but controversial management approach to restoring coastal wetlands and mitigating offshore oil spills in the northern Gulf of Mexico. One of the controversies concerns the potential displacement of and salinity stress on commercially and recreationally important fish species in response to the widespread and prolonged freshening of habitat. We developed a coupled hydrodynamics–fish movement model and applied it to the Caernarvon diversion located in the Breton Sound estuary, Louisiana. Hydrodynamics model output was used as input to the individual‐based fish movement model. The period of model simulation was from April 1 to July 1, 2010. We simulated three diversion scenarios: baseline, pulse, and oil spill mitigation. We first used field data from Bay Anchovy Anchoa mitchilli and showed that the model predicted downestuary shifts similar to those observed in field studies under large diversions. We then defined generic low‐ and intermediate‐salinity fish species and simulated each under the three diversion scenarios. Compared with the baseline diversion scenario, more than 50% of the intermediate‐salinity individuals moved about 15 km farther downestuary under the pulse diversion scenario and moved more than 35 km under the oil spill mitigation diversion scenario. The effects of the diversions on the low‐salinity species were evidenced by individuals becoming more dispersed (i.e., spreading out downestuary) and more exposed to bursts of too high salinity. Our conclusions agreed with those from earlier field and modeling analyses that focused on average (rather than transitory) fish responses.Received November 4, 2013; accepted November 7, 2013

Differences in extreme low salinity timing and duration differentially affect eastern oyster (Crassostrea virginica) size class growth and mortality in Breton Sound, LA

Understanding how different life history stages are impacted by extreme or stochastic environmental variation is critical for predicting and modeling organism population dynamics. This project examined recruitment, growth, and mortality of seed (25–75 mm) and market (>75 mm) sized oysters along a salinity gradient over two years in Breton Sound, LA. In April 2010, management responses to the Deepwater Horizon oil spill resulted in extreme low salinity (<5) at all sites through August 2010; in 2011, a 100-year Mississippi River flood event resulted in low salinity in late spring. Extended low salinity (<5) during hot summer months (>25 °C) significantly and negatively impacted oyster recruitment, survival and growth in 2010, while low salinity (<5) for a shorter period that did not extend into July (<25 °C) in 2011 had minimal impacts on oyster growth and mortality. In 2011, recruitment was limited, which may be due to a combination of low spring time salinities, high 2010 oyster mortality, minimal 2010 recruitment, cumulative effects from 10 years of declining oyster stock in the area, and poor cultch quality. In both 2010 and 2011, Perkinsus marinus infection prevalence remained low throughout the year at all sites and almost all infection intensities were light. Oyster plasma osmolality failed to match surrounding low salinity waters in 2010, while oysters appeared to osmoconform throughout 2011 indicating that the high mortality in 2010 may be due to extended valve closing and resulting starvation or asphyxiation in response to the combination of low salinity during high temperatures (>25 °C). With increasing management of our freshwater inputs to estuaries combined with predicted climate changes, how extreme events affect different life history stages is key to understanding variation in population demographics of commercially important species and predicting future populations.

Influence of growth faults on coastal fluvial systems: Examples from the late Miocene to Recent Mississippi River Delta

The details of how fluvial systems respond to spatial changes in land-surface subsidence produced by active faulting remain incompletely understood. Here, we examine the degree to which the positioning of individual channels and channel-belts is affected by local maxima in subsidence associated with the hanging walls of growth faults. The channel forms and faults are imaged using a seismic volume covering 1400km2 of Breton Sound and Barataria Bay in southern Louisiana, USA. We look at the consequences of interactions between channels, channel-belts, and faults in late Miocene to Recent strata. More than fifty individual channels that crossed the traces of active growth faults were examined. Of these channels, only three appear to have been redirected by the faults. There also appeared to be no systematic change in the cross-sectional geometries of channels or channel-belts associated with crossing a fault, though the orientation of the channel-belts appears to be more influenced by faulting than the orientation of individual channels. Seven out of ten mapped channel-belts appear to have been steered by growth faults. We propose that channel belts are more likely to be influenced by faults than individual channels because channel-belts are longer lived features, unlikely to shift their overall position before experiencing a discrete faulting event. In addition, the style of influence in the few cases where an individual channel is affected by a fault is different from that of larger systems. While downstream of a fault channel-belts generally become oriented perpendicular to fault strike, the individual channels are directed along the hanging wall of the fault, running parallel to the fault trace. We relate this to the ratio of the length-scale of fault rollover relative to the channel or channel-belt width. Fluvial-fault interactions with higher values for this ratio are more likely to be carried parallel to the fault trace than systems with lower ratio values.

Ammonification and denitrification rates in coastal Louisiana bayou sediment and marsh soil: Implications for Mississippi river diversion management

The Caernarvon Diversion directs Mississippi River water into coastal marshes in the Breton Sound Estuary. Elevated nitrogen levels in the Mississippi River water result in nutrient loading to these coastal marsh systems and consequently to the coastal ocean. The goal of this study is to determine the potential nitrate removal rates for two different substrates. Bayou sediments represent the low flow conditions, when the water is constrained within the canals with high potential for transport to the coastal ocean. The marsh soil represents the high flow diversion events when flood water inundates up into the marshes. We sought to remove the plant effect by using cores containing bayou sediment and marsh soil, removing all roots and flooding with a water column containing 2mg NO3-NL−1. Water column nitrate and ammonium concentration were monitored over 9d. Net nitrate loss in bayou sediments was 9.5±1.5mg Nm−2d−1 while the nitrate loss was significantly less at 7.2±0.9mg Nm−2d−1 in the marsh soil. A comparison of nitrate reduction rates in vegetated and non-vegetated marsh soils indicated that the rate of denitrification increased tenfold in vegetated soils. This increase could be attributed to the “plant effect”. Our results suggest that operating diversions on the high flow end of the spectrum would promote nitrate delivery over the vegetated marsh rather than flowing only through canals. Flooding of the vegetated marsh maximizes the potential for removal of riverine nitrate and limits delivery of nitrate to the coastal ocean, thereby mitigating expressions of eutrophication including algal blooms and hypoxia.

Vegetation and Soil Dynamics of a Louisiana Estuary Receiving Pulsed Mississippi River Water Following Hurricane Katrina

We monitored wetland biomass, decomposition, hydrology, and soil porewater chemistry at the Breton Sound estuary, which receives Mississippi River water from the Caernarvon river diversion structure. The estuary was in the direct path of hurricane Katrina in 2005, which caused a dramatic loss of wetlands in the upper basin. From March 2006 to October 2007, we made duplicate measurements at three distance classes from the diversion structure along the estuarine gradient as well as at a reference area, designated Near (N1&2), Mid (M1&2), Far (F1&2), and Ref (R1&2). Above- and belowground live biomass, porewater nutrients (NOx, NH4, and PO4), salinity, sulfide, and soil Eh were measured every 2 months. Water level was monitored with gauges. Above- and belowground decomposition was measured using the litterbag (both) and cotton strip (belowground only) methods. Analysis of porewater parameters showed that stress factors affecting biomass production (porewater salinity, sulfide, flooding, and redox potential) were generally low to moderate, while measurable porewater nutrient concentrations occurred at all sites. Aboveground end of season live (EOSL) standing crop in October ranged from 423 g/m2 at site M2 to 1,515 at site F1, and was significantly greater at site N1 than at sites N2, M1, or M2. Aboveground EOSL biomass during this study was significantly lower than previously measured in 1999, 2000, and 2001. Peak belowground biomass ranged from 8,315 g/m2 at site R2 to 17,890 g/m2 at site N1, which is among the highest reported in the literature, and there were significant increases throughout the study, suggesting recovery from hurricane Katrina. The decomposition bag data did not indicate any significant differences; however, the cotton strip decomposition rate was significantly lower at the lowest depth. Wetland surface vertical accretion ranged from 0.49 cm/year at N2 to 1.24 cm/year at N1, with site N1 significantly greater than N2, M1, F2, and R1, and site N2 significantly less than all other sites except site R1. These findings show that marsh productivity and stability is related to a number of factors and no one factor can explain the impacts of the hurricanes.

Predicting the Effects of Proposed Mississippi River Diversions on Oyster Habitat Quality; Application of an Oyster Habitat Suitability Index Model

ABSTRACT In an attempt to decelerate the rate of coastal erosion and wetland loss, and protect human communities, the state of Louisiana developed its Comprehensive Master Plan for a Sustainable Coast. The master plan proposes a combination of restoration efforts including shoreline protection, marsh creation, sediment diversions, and ridge, barrier island, and hydrological restoration. Coastal restoration projects, particularly the large-scale diversions of fresh water from the Mississippi River, needed to supply sediment to an eroding coast potentially impact oyster populations and oyster habitat. An oyster habitat suitability index model is presented that evaluates the effects of a proposed sediment and freshwater diversion into Lower Breton Sound. Voluminous freshwater, needed to suspend and broadly distribute river sediment, will push optimal salinities for oysters seaward and beyond many of the existing reefs. Implementation and operation of the Lower Breton Sound diversion structure as proposed wou...

Landscape-Scale Analysis of Wetland Sediment Deposition from Four Tropical Cyclone Events

Hurricanes Katrina, Rita, Gustav, and Ike deposited large quantities of sediment on coastal wetlands after making landfall in the northern Gulf of Mexico. We sampled sediments deposited on the wetland surface throughout the entire Louisiana and Texas depositional surfaces of Hurricanes Katrina, Rita, Gustav, and the Louisiana portion of Hurricane Ike. We used spatial interpolation to model the total amount and spatial distribution of inorganic sediment deposition from each storm. The sediment deposition on coastal wetlands was an estimated 68, 48, and 21 million metric tons from Hurricanes Katrina, Rita, and Gustav, respectively. The spatial distribution decreased in a similar manner with distance from the coast for all hurricanes, but the relationship with distance from the storm track was more variable between events. The southeast-facing Breton Sound estuary had significant storm-derived sediment deposition west of the storm track, whereas sediment deposition along the south-facing coastline occurred primarily east of the storm track. Sediment organic content, bulk density, and grain size also decreased significantly with distance from the coast, but were also more variable with respect to distance from the track. On average, eighty percent of the mineral deposition occurred within 20 km from the coast, and 58% was within 50 km of the track. These results highlight an important link between tropical cyclone events and coastal wetland sedimentation, and are useful in identifying a more complete sediment budget for coastal wetland soils.

Simulating Hurricane Storm Surge in the Lower Mississippi River under Varying Flow Conditions

AbstractHurricanes in southeastern Louisiana develop significant surges within the lower Mississippi River. Storms with strong sustained easterly winds push water into shallow Breton Sound, overtop the river’s east bank south of Pointe a la Hache, Louisiana, penetrate into the river, and are confined by levees on the west bank. The main channel’s width and depth allow surge to propagate rapidly and efficiently up river. This work refines the high-resolution, unstructured mesh, wave current Simulating Waves Nearshore + Advanced Circulation (SWAN+ADCIRC) SL16 model to simulate river flow and hurricane-driven surge within the Mississippi River. A river velocity regime–based variation in bottom friction and a temporally variable riverine flow-driven radiation boundary condition are essential to accurately model these processes for high and/or time-varying flows. The coupled modeling system is validated for riverine flow stage relationships, flow distributions within the distributary systems, tides, and Hurric...

Surge Generation Mechanisms in the Lower Mississippi River and Discharge Dependency

The Lower Mississippi River protrudes into the Gulf of Mexico, and manmade levees line only the west bank for 55 km of the Lower Plaquemines section. Historically, sustained easterly winds from hurricanes have directed surge across Breton Sound, into the Mississippi River and against its west bank levee, allowing for surge to build and then propagate efficiently upriver and thus increase water levels past New Orleans. This case study applies a new and extensively validated basin- to channel-scale, high-resolution, unstructured-mesh ADvanced CIRCulation model to simulate a suite of historical and hypothetical storms under low to high river discharges. The results show that during hurricanes, (1) total water levels in the lower river south of Pointe à La Hache are only weakly dependent on river flow, and easterly wind-driven storm surge is generated on top of existing ambient strongly flow-dependent river stages, so the surge that propagates upriver reduces with increasing river flow; (2) natural levees and adjacent wetlands on the east and west banks in the Lower Plaquemines capture storm surge in the river, although not as effectively as the manmade levees on the west bank; and (3) the lowering of manmade levees along this Lower Plaquemines river section to their natural state, to allow storm surge to partially pass across the Mississippi River, will decrease storm surge upriver by 1 to 2 m between Pointe à La Hache and New Orleans, independent of river flow.

Measuring Changes in Consumer Resource Availability to Riverine Pulsing in Breton Sound, Louisiana, USA

Resource pulses are thought to structure communities and food webs through the assembly of consumers. Aggregated consumers represent a high quality resource subsidy that becomes available for trophic transfer during and after the pulse. In estuarine systems, riverine flood pulses deliver large quantities of basal resources and make high quality habitat available for exploitation by consumers. These consumers represent a change in resources that may be available for trophic transfer. We quantified this increased consumer resource availability (nekton density, biomass, energy density) provided by riverine flood pulsing in Breton Sound, Louisiana, USA. We used water level differences between an area subject to two experimental riverine flood pulses (inflow) and a reference area not receiving inflow to identify the percentage of nekton standing stock and energy density that may be attributable solely to riverine pulsing and may represent a consumer resource subsidy. Riverine pulsing accounted for more than 60% of resident nekton density (ind m−2), biomass (g m−2), and energy density (cal m−2) on the flooded marsh surface during two experimental pulse events in 2005. Our results document the potential subsidy of resident nekton standing stock from a riverine flood pulse available for export to subtidal habitats. Given predicted large scale changes in river discharge globally, this approach could provide a useful tool for quantifying the effects of changes in riverine discharge on consumer resource availability.

Mercury bioaccumulation in estuarine food webs

We tested for unintended mercury contamination problems associated with estuarine floodplain restoration projects of the Louisiana coastal zone, USA. Barataria Bay and Breton Sound are two neighboring deltaic estuaries that were isolated by levees from the Mississippi River about 100 years ago. These estuaries recently have been reconnected to the nutrient-rich Mississippi River, starting major river diversion (input) flows in 1991 for Breton Sound and in 2004 for Barataria Bay. We collected > 2100 fish over five years from 20 stations in these estuaries to test two hypotheses about Hg bioaccumulation: (H1) Background Hg bioaccumulation in fish would be highest in low-salinity upper reaches of estuaries, and (H2) recent river inputs to these upper estuarine areas would increase Hg bioaccumulation in fish food webs. For H1, we surveyed fish Hg concentrations at several stations along a salinity gradient in Barataria Bay in 2003-2004, a time when this estuary lacked strong river inputs. Results showed that average Hg concentrations in fish communities were lowest (150 ng/g dry mass) in higher salinity areas and -2.4x higher (350 ng/g) in low-salinity oligohaline and freshwater upper reaches of the estuary. For H2, we tested for enhanced Hg bioaccumulation following diversion onset in both estuaries. Fish communities from Breton Sound that had long-term (> 10 years) diversion inputs had -1.7x higher average Hg contents of 610 ng/g Hg vs. 350 ng/g background values. Shorter-term diversion inputs over 2-3 years in upper Barataria Bay did not result in strong Hg enrichments or stable C isotope increases seen in Breton Sound, even though N and S stable-isotope values indicated strong river inputs in both estuaries. It may be that epiphyte communities on abundant submerged aquatic vegetation (SAV) are important hotspots for Hg cycling in these estuaries, and observed lesser development of these epiphyte communities in upper Barataria Bay during the first years of diversion inputs may account for the lessened Hg bioaccumulation in fish. A management consideration from this study is that river restoration projects may unintentionally fertilize SAV and epiphyte-based food webs, leading to higher Hg bioaccumulation in river-impacted floodplains and their food webs.