Lake Pontchartrain Estuary Research Articles

Variability in estuarine habitat use of a threatened species in the northern Gulf of Mexico: implications for coastal restoration

Coastal restoration projects often require sediment resources borrowed from estuarine or marine ecosystems. A generalized additive modeling (GAM) framework was used to characterize fish-habitat relationships and predict spatiotemporal and ontogenetic shifts of Gulf sturgeon distribution within an impacted estuary in the northern Gulf of Mexico, Lake Pontchartrain. Habitat use and movement of Gulf sturgeon (n = 103) was monitored using an array of acoustic receivers (n = 81) from 2016 to 2019. Telemetry data was paired with environmental conditions hindcast from a high-resolution hydrodynamic model to develop GAMs for three life stages of Gulf sturgeon: juvenile, sub-adult, and adult. Model results demonstrated clear spatial, temporal, and ontogenetic shifts in habitat use for Gulf sturgeon in the Lake Pontchartrain estuary. The importance of spatial and temporal variables in the final models of all life stages suggests that distribution of Gulf sturgeon within the estuary is more likely driven by biology (prey distribution) rather than environment. Interestingly, dependence of Gulf sturgeon on the estuary for overwintering habitat declined with ontogeny. The northeastern shoreline of Lake Pontchartrain was identified as potential nursery habitat for overwintering juvenile sturgeon from the Pearl River, while eastern portions of the estuary and associated channels provide critical staging habitat and corridors of connectivity for migrating sub-adult and adult sturgeon. Given the clear need to prioritize conservation of Gulf sturgeon while restoring coastal marshes, findings from this study provide needed information to shape future coastal restoration and inform the siting and timing of future dredging operations in Lake Pontchartrain.

The Future of Cyanobacteria Toxicity in Estuaries Undergoing Pulsed Nutrient Inputs: A Case Study from Coastal Louisiana

Harmful cyanobacteria blooms (cyanoHABs) are a global phenomenon, especially in calm, warm, and nutrient-rich freshwater and estuarine systems. These blooms can produce various potent toxins responsible for animal poisoning and human health problems. Nutrient-rich freshwater pulsed into estuaries affects turbidity, water temperature, salinity, and nutrient concentrations and ratios at irregular intervals, creating a highly dynamic habitat. However, the underlying processes that lead to the selective development of cyanoHABs for certain species and the fate of their toxins are still uncertain. This paper draws upon the rich body of research available for one such system, the Lake Pontchartrain Estuary, Louisiana, to generate insights about future research directions in pulsed-nutrient-delivery estuaries. Toxin-producing cyanobacteria blooms in river-dominated Louisiana coastal ecosystems have already been documented at high concentrations, presenting a potential risk to human health as $2.4 billion worth of Louisiana’s fish and shellfish are consumed by millions of people throughout the US every year. Recent studies have shown that the Lake Pontchartrain Estuary, just north of New Orleans, Louisiana has been experiencing cyanoHABs, likely connected to combinations of (a) high interannual variability in nutrient loading associated with seasonal and episodic rainfall, (b) the timing, duration, and magnitude of the flood-stage Mississippi River water diverted into the Lake Pontchartrain Estuary, and (c) saltwater inputs from tropical storms. It is expected that cyanoHABs will become more frequent in Louisiana with a warming climate and changes to the timing and magnitude of river water diverted into the Lake Pontchartrain Estuary, which will play a dominant role in the development of blooms in this region. More studies are needed to focus on the environmental conditions that control the succession or/and co-existence of different cyanobacteria species and their toxins, optimally culminating in a near-term forecasting tool since this information is critical for health agencies to mitigate or to provide early warnings. Toxin forecasts for pulsed-nutrient estuaries, including Lake Pontchartrain, could directly inform state and municipal health agencies on human exposure risks to upcoming cyanobacteria toxicity events by predicting cyanobacteria species shifts, potency, and toxin modality along the freshwater-to-marine continuum while also informing a longer-term projection on how the changing climate will impact the frequency and potency of such blooms.

Temporally-displaced Mississippi River spring flood pulse shows muted aquatic ecosystem response in estuarine waters: A climate change warning for coastal foodwebs

Mississippi River water levels typically rise in the early spring after snow melt in the extensive watershed. However, in 2016, warm air temperatures coupled with high precipitation led to a historically early river flood pulse, resulting in the opening of a flood release valve (Bonnet Carré Spillway) in early January to protect the city of New Orleans, Louisiana. The goal of this research was to determine the ecosystem response of this wintertime nutrient flood pulse on the receiving estuarine system and compare it to historical opening responses, which are generally several months later. Nutrients, TSS, and Chl a were measured along a 30 km transect in the Lake Pontchartrain estuary, before, during, and after the river diversion event. In the past, NOx concentrations were quickly reduced to below detection in the estuary in <4 weeks post-event accompanied by a moderate phytoplankton bloom. However, due to seasonal limitations (cold water temperatures and light limitation) during the 2016 event, NOx remained elevated for >2 months post-closure and Chl a values were low, indicating limited assimilation of nutrients into phytoplankton biomass. Consequently, much of the bioavailable nitrogen was denitrified by sediments and dispersed to the coastal ocean over time, limiting the transfer of nutrients into the food web by means of a spring phytoplankton bloom. An increasing warming trend in temperate and polar river watersheds is leading to earlier spring flood pulses, altering the timing of coastal nutrient transport, decoupled from conditions supporting primary production, which could significantly affect coastal food webs.

Impacts of a Major Mississippi River Freshwater Diversion on Suspended Sediment Plume Kinematics in Lake Pontchartrain, a Semi-enclosed Gulf of Mexico Estuary

MODIS satellite imagery and in situ water measurements were used to quantify inorganic sediment concentration (ISC) and, subsequently, sediment plume motion and evolution during the 2016 diversion of the Mississippi River through the Bonnet Carre Spillway (BCS) within Lake Pontchartrain (LP) estuary in Louisiana, USA. Satellite images revealed that sediment plume areal distribution was closely linked to BCS discharge rate. The plume flowed eastward along the southern LP coastline at an average speed of ~ 0.3 m s−1 during the first week of the event and attained its maximum areal extent (~ 1117 km2; 68% of the total LP area) on 23 January. In an attempt to examine the plume kinematics and potential sediment resuspension impacts, wind information was also assessed. Following BCS closure, the prevailing wind direction, but not wind speed, corresponded to significant differences in LP ISC. LP was further subdivided into four separate quadrants to assess relationships between ISC within each quadrant to BCS discharge rate and winds. While the BCS was open, the plume’s eastward motion was associated with a high correlation (r2 = 0.93) between BCS discharge and ISC in the southeast quadrant. However, the weak relationship (r2 = 0.42) between BCS discharge and ISC in the northwest quadrant was attributed to influences from other tributaries. These results will benefit future research regarding impacts of freshwater diversions on coastal and estuarine processes that are sensitive to sediment influx by providing a satellite-based sediment map time series that can be used to help validate numerical circulation and sediment transport models.

Numerical Experiments on Variation of Freshwater Plume and Leakage Effect From Mississippi River Diversion in the Lake Pontchartrain Estuary

AbstractA Finite Volume Community Ocean Model is used to investigate how wind impacts the circulation and evolution of a freshwater plume from Mississippi River diversion in the Lake Pontchartrain Estuary. Results show that northerly and southerly winds tend to stretch the plume in the east‐west directions, while easterly and westerly winds constrain the plume in the north‐south directions. Increasing wind magnitude tends to increase the total salt content of the estuary except under weak westerly wind (&lt;6 m/s) during which salt content decreases. A no‐motion middepth interface is found (by the model and verified by the data), separating the top layer downwind flow and bottom layer upwind flow. Increasing wind magnitude can enhance the two‐layered flows and lower the no‐motion plane between the two opposite flows. Apparent small leakage of the river water through the diversion structure prior to its opening is found to impact the vertical structure of flows and salinity: Mixing is facilitated by the large amount of freshwater leaked into the lake prior to the opening of the diversion; wind‐driven gyres are diminished; the average potential energy demand, a quantity used to measure the vertical stratification, is reduced to very low values; more deviation from the quasi‐steady state balance tends to occur; and a total of 3.7 × 108 kg of salt is reduced during the opening period of the Bonnet Carré Spillway. The Lake Pontchartrain Estuary is completely dominated by the river water within about 25 days, when salinity drops from an average value of 4 g/kg to essentially zero.

Spatial variation of cold front wind-driven circulation and quasi-steady state balance in Lake Pontchartrain Estuary

Circulation in a low-salinity estuary with restricted openings to the ocean is mainly driven by wind and to a lesser extent by water level fluctuations at the open boundaries. Numerical model experiments are conducted using a baroclinic Finite Volume Community Ocean Model (FVCOM) for wind-driven circulations during a period encompassing a total of 16 cold fronts in the Lake Pontchartrain Estuary. The work is focused on the examination of the spatial structure of circulation in response to local and remote winds. A quasi-steady state balance between the surface slope and wind stress is further analyzed. The accuracy of the balance is evaluated by comparing the FVCOM calculated surface slope with that from the steady state balance equation. Results show that this balance is more accurate in the cross-estuary direction than that in the along-estuary direction (R2 ∼ 0.94 vs. 0.60). This difference in the accuracy of the quasi-steady state balance between the cross- and along-estuary directions is caused by the open boundary - a tidally-induced mean slope exists. In addition, even if the tidal effect is removed, the accuracy still decreases toward the open end for slopes in both directions. Remote wind effect and residual flow through the eastern open boundary tend to introduce a departure from the quasi-steady state balance in both along- and cross-estuary directions. Remote wind effect decreases into the interior due to bottom friction. Local wind effect tends to produce downwind flows in coastal, shallow water regions and on the surface, but upwind flows near the bottom, a result consistent with barotropic wind-driven circulations; while the remote wind effect is important mostly near the open boundary. Furthermore, quasi-steady state balance is more accurate in the along-estuary direction before cold front passages then after, because of the relatively higher occurrence of the wind in that direction before the cold fronts than after. In contrast, this quasi-steady state balance is less accurate in the cross estuary direction before cold front passages than after, because of the relatively lower occurrence of the wind in that direction before the cold fronts than after.

Flow Regimes and Adjustment to Wind‐Driven Motions in Lake Pontchartrain Estuary: A Modeling Experiment Using FVCOM

AbstractLake Pontchartrain is a brackish estuary and restricted lagoon in the microtidal northern Gulf of Mexico. Using a validated Finite Volume Community Ocean Model (FVCOM), we study the flow regimes and wind‐driven adjustment of circulation in this system. It is found that tidal currents are only significant in the eastern end near the open boundary. Local wind inside the estuary is important in driving the circulation including coastal currents. Numerical experiments confirm that the subtidal wind‐driven flow is in a quasi‐steady balance between wind stress and surface slope. Our results show that the adjustment of surface level inside the system to a sudden change in wind is a transient seiche, which dissipates within two to three cycles, each lasting ~3 hr. After the oscillations, the system reaches a new equilibrium. We show in theory that this is a damped oscillation and as wind changes, an adjustment to a new equilibrium goes much faster than a complete tidal period. This leads to the quasi steady state balance of subtidal wind‐driven flows with essentially no phase lag with wind even though wind changes with time continuously. Wind‐induced circulation behaves differently from the Csanady model with Earth rotation but similar to the prismatic lake model of Engelund without Coriolis: The coastal and shallow regions tend to have flows in the direction of wind, while the interior and deeper waters have return flows against the wind especially in bottom layer. The surface flow may not be in the direction of wind, while the bottom usually has broad and uniform counter wind flows.

Investigation of an early season river flood pulse: Carbon cycling in a subtropical estuary

The January 2016 Bonnet Carré Spillway (BCS) opening resulted in a large-scale Mississippi River (MR) flood discharge that qualitatively and quantitatively impacted the dissolved organic matter (DOM) cycling in the Lake Pontchartrain Estuary (LPE) located in Louisiana, USA. This early season flood event was a result of the delay of snow formation caused by warmer than normal watershed temperatures. During the diversion period and the subsequent weeks, the LPE water temperature remained lower than pre-flood water temperatures, suppressing carbon cycling. Following that period, the water temperature increased, leading to an increase in the rate of abiotic and biological carbon processing (i.e., mineralization, degradation, and consumption). There were multiple and abnormally high discharges into LPE from the northern tributaries, totaling 43% of the MR flood discharge. As a secondary DOM source, the northern tributaries discharge was qualitatively and quantitatively different from the discharge originating from the river or estuarine sources. The dominant DOM source was determined using satellite images in conjunction with UV–Vis, fluorescence EEMs, and PARAFAC indicators. Overall, the three sources (MR, northern tributaries, and LPE) characteristics were identified by UV–Vis, fluorescence EEMs, and PARAFAC parameters, namely: i) spectral slope (S275), serving as an indicator of lignin-like compounds' molecular weights, with a trend of MR > northern tributaries > LPE; ii) biological index (BIX), indicating freshness of DOM, with a trend of LPE > MR > northern tributaries; and iii) Fluorophore T intensity, serving as an indicator of the amount of terrestrial-like sourced DOM, with a trend of northern tributaries > LPE > MR. It was possible to identify DOM sources and monitor DOM transformation in the water column, increasing our understanding of DOM, carbon, and nitrogen ecological processing.

Cold Front Driven Flows Through Multiple Inlets of Lake Pontchartrain Estuary

AbstractWith in situ observations using acoustic Doppler current profilers (ADCPs) and numerical experiments using the Finite Volume Coastal Ocean Model (FVCOM), this study investigates atmospheric cold front induced exchange of water between Lake Pontchartrain Estuary and coastal ocean through multiple inlets. Results show that the subtidal hydrodynamic response is highly correlated with meteorological parameters. Northerly and westerly winds tend to push water out of Lake Pontchartrain, while south and east winds tend to produce currents flowing into it. For most cases, the subtidal water level is inversely correlated with the east wind, with the correlation coefficient being ∼0.8. The most important finding of this work is that, contrary to intuition, the cold front induced remote wind effect has the greatest contribution to the overall water level variation, while the local wind stress determines the surface slope inside the estuary. It is found that wind driven flow is roughly quasi steady state: the surface slope in the north‐south direction is determined by the north‐south wind stress, explaining ∼83% of the variability but less so in the east‐west direction (∼43%). In other words, the north‐south local wind stress determines the water level gradient in that direction in the estuary while the overall water level change is pretty much controlled by the open boundary which is the “remote wind effect,” a regional response that can be illustrated only by a numerical model for a much larger area encompassing the estuary.

Changes in estuarine sediment phosphorus fractions during a large-scale Mississippi River diversion

Ongoing deterioration and loss of wetlands in the Mississippi River delta threatens the survival of Louisiana's coastal ecosystems and human settlements. In response, the State of Louisiana has proposed a $50 billion, 50-year restoration program. A central piece of this program is the reintroduction of Mississippi River water into the deltaic plain using managed diversions that mimic natural flood pulses. These diversions would transport critically needed sediment, but also deliver large nutrient loads. Coastal eutrophication is therefore a concern, particularly blooms of toxin-producing cyanobacteria. The Bonnet Carré Spillway (BCS) is an existing large flood diversion that protects New Orleans and provides an opportunity to investigate diversion nutrient transport. Here, we quantify sediment phosphorus (P) deposited by the BCS for the first time, and use a sequential P fractionation scheme to evaluate the likelihood of future sediment P release to the water column of the Lake Pontchartrain Estuary. In 2011, we collected sediment cores in the estuary for determination of P fractions before and after the discharge of 21.9km3 of river water through the BCS in just under 6weeks. We observed the greatest net increases in sediment total P, inorganic P forms, and more labile organic P in the region near the inflow. We estimate that the diversion deposited ≥5000 metric tons of P in the sediments of the Lake Pontchartrain Estuary. The sum of readily available inorganic P, Fe/Al-bound inorganic P, and more labile organic P equaled approximately 20–30% of post-diversion sediment total P. These fractions are more likely to be released to the water column than the other sediment P forms we quantified. Diversion designs that encourage sedimentation in coastal marshes versus open bays can likely reduce the chances that deposited particulate P creates eutrophication risk.

Will Mississippi River diversions designed for coastal restoration cause harmful algal blooms?

Large-scale river diversions have been proposed as a way to transport sediment to the deteriorating Mississippi River deltaic plain. These diversions would also transport immense nutrient loads to shallow receiving estuaries, including open water areas where there may be potential for blooms of toxin-producing cyanobacteria. We examined environmental variables and cyanobacteria dynamics in the Lake Pontchartrain Estuary during the spring-summer period of two years: (a) 2011 when the system received a massive inflow of diverted Mississippi River water from the Bonnet Carré Spillway, and (b) 2012 when no diversion occurred. Cluster analysis of 180 samples collected over the course of both years identified five distinct water types grouped according to salinity, temperature, clarity, bioavailable nutrients, and chlorophyll a. Three of these water types were unique to the 2011 diversion event. We found no evidence of immediate, direct cyanobacteria bloom stimulation by diverted nutrients in the “high phytoplankton biomass” cluster associated with the 2011 diversion. Substantial nitrate assimilation by non-harmful phytoplankton occurred as the plume of diverted freshwater traveled through the system that year. The “summer” cluster characterized by nitrogen-limited conditions and including data from both years was characterized by significantly greater cyanobacteria abundance than all other clusters. There was no difference in cyanobacteria abundance between years within the summer cluster. Microcystin levels detected suggested low probability of acute health risks for recreational waters and were not correlated to cyanobacteria abundance. Site-specific understanding of the following is needed to guide diversion management: (i) interannual phytoplankton community variability in non-diversion years, (ii) cyanobacteria inputs from local tributaries, (iii) interactions between plume physics, chemistry, and phytoplankton dynamics, (iv) internal phosphorus feedbacks that may indirectly link diversions to summer blooms, and (v) controls on cyanotoxin production.

Heavy metals and metalloid contamination in Louisiana Lake Pontchartrain Estuary along I-10 Bridge

In this study, HMs concentration in sediments and water from the Lake Pontchartrain estuary along the I-10 Bridge were investigated in two seasons to evaluate the level of contamination and to assess the effect of vehicular traffic. Vehicular traffic has led to HMs such as As, Cd, Cr, Pb, Zn, Ni, and Cu accumulation in sediments along the bridge. The HMs content in water especially in summer increased due to high temperature which led to release of HMs from the sediments. The level of pollution attributed to anthropogenic activities was evaluated using several pollution indicators. The overall HMs bioavailability and EFs was mostly high in summer and the concentration of the HMs in both sediments and water was highly influenced by the season and the distance from the highways. The content of As and Cd in lake sediments were higher than the background values (BGV) for both seasons (spring and summer), and Cr and Pb in sediments were higher than the BGV in most sampling points in the spring. According to Sediments Quality Guidelines (SQGs), heavy metals except for As have low biotoxicity, and As could potentially result in harmful effects on the aquatic organisms. The relationship between As and vehicular traffic is less studied so our finding stress the need for further study on source of As in Lake Pontchartrain sediments.

Spectroscopic measurements of estuarine dissolved organic matter dynamics during a large-scale Mississippi River flood diversion

The Mississippi River Flood of 2011 prompted the opening of the Bonnet Carré Spillway (BCS) in southeastern Louisiana to protect the City of New Orleans. The BCS diverted approximately 21.9km3 of river water into the oligohaline Lake Pontchartrain Estuary over the course of 43days. We characterized estuarine dissolved organic matter (DOM) dynamics before, during, and after the diversion in order to better understand the biogeochemical dynamics associated with these immense freshwater inflows. Dissolved organic carbon (DOC) exhibited a large degree of variability during and after the period of elevated primary productivity that occurred following the diversion. Furthermore, DOC analysis provides limited insight into carbon cycling during these dynamic periods. In order to overcome the limitations of DOC, spectroscopic methods were used to gain insights into chemical composition dynamics. Both ultraviolet visible (A254, A350, SUVA254, spectral slope, and normalized UV/Vis) and fluorescence spectroscopy (excitation emission matrices and fluorescence and biological indices) were used to study the compositional changes of DOM over time. Collectively, our results document a perturbation in DOM chemistry in Lake Pontchartrain due to the diversion and a subsequent return toward pre-diversion conditions. Immediate increases in A350 indicate that BCS freshwater contained elevated concentrations of lignin of terrestrial origin. Ensuing declines in A350, along with changes in the fluorescence and biological indices, indicate that DOM rapidly became more microbial in composition. Our results provide insights into estuarine DOM dynamics relevant to systems receiving flood pulses of freshwater due to either hydrologic manipulation or precipitation events.

Internal loading of phosphorus from sediments of Lake Pontchartrain (Louisiana, USA) with implications for eutrophication

Diffusive flux of bioavailable soluble reactive phosphorus (SRP) across the sediment–water interface is one mechanism by which sediments can be a source of phosphorus to the water column in aquatic systems and contribute to primary productivity. This process is dependent on sediment biogeochemistry and SRP concentration gradients at the sediment–water interface. In systems subjected to episodic external pulses of nutrient-rich water, SRP concentration gradients can have potential implications for diffusive flux. In this study, we sought to investigate two hypotheses: (1) diffusive flux of SRP from sediments is a significant source of SRP in the annual budget for the oligohaline Lake Pontchartrain estuary and (2) under SRP-depleted water column conditions following large episodic, external pulses of nitrogen-rich Mississippi River water to the estuary, internal SRP loading by diffusive flux can regenerate SRP in the water column to previously observed levels rapidly. Our specific objectives were to: (i) determine sediment, water column, and phytoplankton characteristics at multiple locations in the estuary, (ii) measure rates of SRP diffusive flux from sediments using intact cores under aerobic and anaerobic incubations, (iii) estimate the potential for water column SRP regeneration by diffusive flux under SRP-depleted conditions using a simple model, and (iv) estimate the annual load of SRP from the sediments by diffusive flux. Results indicate that diffusive flux of SRP from Lake Pontchartrain sediments likely contributes ~30–44% of the annual SRP load to the estuary. Further, internal SRP loading by diffusion has the potential to regenerate SRP in SRP-depleted waters to previously observed concentrations in <60 days. Our findings suggest that a sequence of events is feasible where external pulses of nitrogen-rich water produce phosphorus-limited conditions, followed by an internal pulse of SRP from sediments to restore nitrogen-limited conditions. This internal SRP load may be an important contributor in promoting blooms of nitrogen-fixing harmful algae under summertime low-nutrient conditions.

Molecular and morphological divergence in the inland silverside (Menidia beryllina) along a freshwater-estuarine interface

Populations that vary across ecological gradients or that have invaded novel habitats are important to elucidate the association between adaptive divergence and gene flow, factors that may play an important role in speciation of silverside fishes. The inland silverside, Menidia beryllina, is an ideal organism for this kind of research, displaying a great diversity in morphology among freshwater and coastal brackish habitats. Using a combination of geometric morphometrics and mitochondrial (mt) DNA, we investigated patterns of variation within and among the nominal freshwater Menidia audens and coastal M. beryllina, spanning the transition from freshwater to tidally influenced semi-brackish waters of the lower Mississippi River to brackish waters of the Lake Pontchartrain estuary. Although we found no evidence for a phylogenetic split between M. audens and M. beryllina, our results indicate that significant genetic divergence corresponds with body shape differences among the two, with a clear distinction at the interface of freshwater and brackish water. Patterns in mtDNA suggest that freshwater populations referred to as M. audens are of recent origin with evidence for habitat-based divergence compared to coastal populations referred to as M. beryllina. Our findings add to a growing body of evidence that ecological shifts, following colonization of novel habitats, may promote rapid adaptive divergence and reduced gene flow among silverside populations in adjacent environmental regimes.

Storm surge induced flux through multiple tidal passes of Lake Pontchartrain estuary during Hurricanes Gustav and Ike

In September 2008, Hurricanes Gustav and Ike generated major storm surges which impacted the Lake Pontchartrain estuary in Louisiana. This paper presents analyses of in situ measurements acquired during these storm events. The main data used in the analyses were from three bottom mounted moorings equipped with conductivity, temperature, and depth sensors, acoustic Doppler current profilers (ADCPs), and a semi-permanent laterally mounted horizontal acoustic Doppler profiler (ADP). These moorings were deployed in the three major tidal channels that connect Lake Pontchartrain with the coastal ocean. A process similar to tidal straining was observed: the vertical shear of the horizontal velocity was negligible during the inundation stage, but a shear of 0.8 m/s over a less than 5 m water column was recorded during the receding stage, 2–3 times the normal tidal oscillations. The surge reached its peak in the Industrial Canal 1.4–2.1 h before those in the other two channels. The inward flux of water lasted for a shorter time period than that of the outward flux. The inward flux was also observed to have much smaller magnitude than the outward flux (∼960–1200 vs. 2100–3100 million m 3). The imbalance was believed to have been caused by the additional water into Lake Pontchartrain through some small rivers and inundation over the land plus rainfall from the hurricanes. The flux through the Industrial Canal was 8–12%, while the flux through the other two tidal passes ranged between 17% and 70% of the total, but mostly split roughly half-half of the remaining (∼88–92% of the total).

In situ measurements of saltwater flux through tidal passes of Lake Pontchartrain estuary by Hurricanes Gustav and Ike in September 2008

Storm surge induced saltwater flux through multiple inlets has never been documented. This article reports in situ measurements of saltwater flux through major tidal passes by two consecutive storm surges impacting Lake Pontchartrain estuary in Louisiana during the first two weeks in September 2008. The main peak of inward saltwater flux lasted for 1.9 days during the flood, followed by more than 8 days of outward salt flux. The salt flux rate reached close to 200 tons/sec through the northern channel and 16 tons/sec through the southern channel. Outward flux of salt during Gustav amounted to more than 16 million tons during the receding stage, more than twice of that measured for the flux into the lake. Apparently, overland inundation, which was not measured, caused this biased estimate of influx of salt. Hurricane Ike, however, did not cause such a dramatic difference in influx and outflux of salt.

Circular plumes in Lake Pontchartrain estuary under wind straining

Circular shaped density plumes of low turbidity, low fecal indicator ( Escherichia coli and enterococci) concentrations, and high salinity have been observed near the Industrial Canal in Lake Pontchartrain, north of the City of New Orleans. A conceptual model in polar coordinates and a numerical model are developed, together with data analysis, to illustrate the dense plume. It is demonstrated that the northward expansion of the plume occurs under northerly winds. The northward expansion of the plume occurs under northerly winds that drive downwind flow at the surface and upwind radial flow at the bottom. Northerly wind-induced straining, similar to tidal straining, promotes vertical stratification. As a result, the water becomes stratified near a thin bottom layer (<1 m), within which density currents are facilitated. The stability of the stratified plume suppresses wind-induced turbulent mixing inside the plume. The bottom water outside of the plume is more effectively stirred by the wind, the result being that the suspended sediment concentration outside of the plume area is much higher than inside. This contrast in mixing makes the plume visible from the surface by satellites even though the stratification is at the bottom. Laterally, wind stress produces a torque (vorticity) in areas of non-uniform depth such that upwind flow is developed in deep water and downwind flow in shallow water. The continuity requirement produces an upwind flow along the axis of the Industrial Canal (IC). The upwind flow is balanced by the downwind flow over the shallower peripheral areas along the coast.

Enumeration and characterization of culturable arsenate resistant bacteria in a large estuary

Arsenic is a toxic element that exists in two major inorganic forms, arsenate and arsenite. A number of bacteria have been shown to resist arsenic exposure, and even more bacteria appear to possess the genes for arsenic resistance. In this study, the numbers of culturable arsenate-resistant bacteria present in water at three coastal sites in the Lake Pontchartrain estuary, Louisiana, was determined. Despite insignificant (less than 1.33 μM) levels of arsenic in this system, 20–50% of the viable count of bacteria showed appreciable arsenate resistance, suggesting that arsenic-resistant bacteria are common and widespread. A diverse array of arsenate-resistant isolates was obtained, with 16S rRNA sequence analysis indicating 37 different bacterial strains, representing six major bacterial groups. Many of these isolates were affiliated with groups of bacteria that have been poorly characterized in terms of arsenic resistance, such as the Betaproteobacteria or Flavobacteria. Some isolates were capable of tolerating very high (>100 mM) levels of arsenate, although arsenite resistance was generally much lower. The results suggest that arsenic-resistant bacteria are common, even in environments with insignificant arsenic contamination, and that many different groups of aquatic bacteria show appreciable arsenic resistance.

Impact of urban stormwater runoff on estuarine environmental quality

Urban stormwater runoff could contribute to the deterioration of water quality of a receiving water body. In this research, field studies and laboratory experiments were conducted to assess the microbial contamination resulting from urban stormwater runoff into the Lake Pontchartrain estuary. Fecal coliform, Escherichia coli and enterococci were used as indicator organisms. The specific objectives of the research were to examine the distribution of the indicator organisms in different environmental elements (water column, suspended particles and sediment) and to further investigate the mechanisms related to their fate. Results of the research indicated satisfactory water quality at the study sites during dry weather periods. However, a significant increase was observed in the concentrations of the indicator organisms in the water columns and sediment at specific study sites following a given stormwater event. Three to seven days were needed for the elevated indicator organisms to return back to their background levels in the water column and sediment, respectively. The mechanism of sedimentation contributed to a reduction in the microbial concentration in the water column, as the indicator organisms were found to attach to the suspended particles in the stormwater. The percentage of fecal coliform, E. coli and enterococci attached to the suspended particles was found to be within the range of 9.8–27.5%, 21.8–30.4%, and 8.4–11.5% of the total indicator organisms in the stormwater loaded into the estuary, respectively. About 75–80% of the total indicator organisms remained free-floating for some distance in the water column before dying off.