Nan D. Walker

Genomic and physiological footprint of the Deepwater Horizon oil spill on resident marsh fishes

The biological consequences of the Deepwater Horizon oil spill are unknown, especially for resident organisms. Here, we report results from a field study tracking the effects of contaminating oil across space and time in resident killifish during the first 4 mo of the spill event. Remote sensing and analytical chemistry identified exposures, which were linked to effects in fish characterized by genome expression and associated gill immunohistochemistry, despite very low concentrations of hydrocarbons remaining in water and tissues. Divergence in genome expression coincides with contaminating oil and is consistent with genome responses that are predictive of exposure to hydrocarbon-like chemicals and indicative of physiological and reproductive impairment. Oil-contaminated waters are also associated with aberrant protein expression in gill tissues of larval and adult fish. These data suggest that heavily weathered crude oil from the spill imparts significant biological impacts in sensitive Louisiana marshes, some of which remain for over 2 mo following initial exposures.

Effects of River Discharge, Wind Stress, and Slope Eddies on Circulation and the Satellite-Observed Structure of the Mississippi River Plume

Abstract Satellite measurements of suspended sediment, temperature, and chlorophyll α are used in combination with surface current measurements to investigate surface circulation and structure of the Mississippi River plume. River discharge changes affect frontal locations, areal extent, and suspended sediment loads of the plume. During high river discharge (>20,000 m3 s−1) in spring, the sediment plume extends 23 km southwestward, covers 2700 km2, with maximum concentrations of 360 mg L−1. Plume temperatures vary seasonally from 10° to 28°C, with maximum surface fronts of 3.3°C km−1 in winter. East winds, prevalent in autumn, winter, and spring, drive a westward flow of river waters around the delta, linking two isolated shelf regions and increasing river discharge onto the Louisiana/Texas shelf. During peak river flow, this westward current exhibits velocities of 40–90 cm s−1, is 20 km wide, and transports 140,000–165,000 m3 s−1 of river and shelf water. It usually turns toward the coast between 89.5° W and 90° W, feeding a clockwise gyre in the Louisiana Bight and a westward coastal current. The prevalent east winds trap river water and associated nutrients on the shelf where hypoxia later develops in late spring/summer. During autumn and winter, short-term wind reversals from frontal passages rapidly reverse plume direction, initiate off-shelf transport, and reduce residence times for river waters and associated sediments, nutrients, phytoplankton, and carbon. During summer, persistent southwest and south winds force river water eastward, where cross-margin transport is likely due to the relatively narrow shelf. Slope eddies and the Loop Current control river water after leaving the shelf.

Satellite assessment of Mississippi River plume variability: Causes and predictability☆

Abstract The Mississippi River is the largest river in North America and 6th largest worldwide in terms of discharge. In this study, 5 years (1989–1993) of NOAA Advanced Very High Resolution Radiometer satellite data were used to investigate the variability of the Mississippi River sediment plume and the environmental forcing factors responsible for its variability. Plume variability was determined by extracting information on plume area and plume length from 112 cloud free satellite images. Correlation and multiple regression techniques were used to quantify these relationships for possible predictive applications. River discharge and wind forcing were identified as the main factors affecting plume variability. Seasonal and interannual variabilities in plume area were similar in magnitude and corresponded closely with large changes in river discharge. However, day-to-day variability in plume size and morphology was more closely associated with changes in the wind field. The plume parameters best predicted by the multiple regression models were plume area, east and west of the delta. Predictive models were improved by separating the data into summer and winter seasons. The best predictive model for the western area was obtained during summer when 64 % of plume variability was explained by river discharge, wind speed, and the east-west wind component. The best model for the eastern plume area was obtained during summer when river discharge, the north-south and east-west wind components explained 70 % of plume variability. The best model for the offshore extent of the sediment plume was obtained during summer when 53% of plume variability was explained by the east-west wind component, river discharge, and wind speed. All plume measurements were maximized by eastward winds from slightly different directions. During winter, the area and length of the western plume was additionally enhanced by offshore winds associated with winter storms. Anticyclonic curvature of the plume west of the delta was observed in 42% of the satellite images. This circulation pattern was observed primarily in association with westward winds.

Cold-water stress in Florida Bay and northern Bahamas; a product of winter cold-air outbreaks

ABSTRACT During January 1977 three consecutive cold fronts crossed south Florida and the northern Bahamas which depressed shallow-water temperatures below the lethal limit for most reef corals. Digital thermal infrared data acquired by the NOAA-5 meteorological satellite, in situ water temperatures, and meteorological data were used to study the thermal evolution of Florida Bay and Bahama Bank waters. The third and most important frontal system depressed Florida Bay water below 16° C, a thermal stress threshold for most reef corals, for 8 days. The minimum water temperature recorded in situ was 12.6° C. Satellite data suggest that some Florida Bay coastal waters were at least 1° C cooler than water at this site. Cold-water plumes (detected on satellite imagery) suggest that offshelf or offbank movement of cold, dense water follows bathymetry-controlled routes. Absence of viable shelf reefs opposite tidal passes supports this contention. Coral mortality at Dry Tortugas was up to 91 percent during the 1977 event. Coral and fish kills were also reported from other parts of the Florida Reef Tract and northern Bahamas. Study results show that cold-water stress conditions can exist over vast shallow-water areas and have residence times of several days. These observations suggest that aperiodic chilling processes have a limiting influence on reef community development throughout the Florida Reef Tract and northern Bahamas.

An Organic Carbon Budget for the Mississippi River Turbidity Plume and Plume Contributions to Air-sea CO2 Fluxes and Bottom Water Hypoxia

We investigated seasonal variability in organic carbon (OC) budgets using a physical-biological model for the Mississippi River turbidity plume. Plume volume was calculated from mixed layer depth and area in each of four salinity subregions based on an extensive set of cruise data and satellite-derived suspended sediment distributions. These physical measurements were coupled with an existing food web model to determine seasonally dependent budgets for labile (reactive on time scales of days to weeks) OC in each salinity subregion. Autochthonous gross primary production (GPP) equaled 1.3×1012 g C yr−1 and dominated labile OC inputs (88% of the budget) because riverine OC was assumed mostly refractory (nonreactive). For perspective, riverine OC inputs amounted to 3.9×1012 g C yr−1, such that physical inputs were 3 times greater than biological inputs to the plume. Annually, microbial respiration (R) accounted for 65% of labile OC losses and net metabolism (GPP—R) for the entire plume was, autotrophic, equaling 5.1×1011 g C yr−1. Smaller losses of labile OC occurred via sedimentation (20%), advection (10%), and export to higher trophic levels (5%). In our present model, annual losses of labile OC are 10% higher than inputs, indicating future improvements are required. Application of our model to estimate air-sea carbon dioxide (CO2) fluxes indicated the plume was a net sink of 2.0×109 mol CO2 yr−1, of which 90% of the total drawdown was from biotic factors. In all seasons, low salinity waters were a source of CO2 (pCO2=560–890 μatm), and intermediate to high salinity waters were a sink of CO2 (pCO2=200–370 μatm). Our model was also used to calculate O2 demand for the development, of regional hypoxia, and our spring and early summer budgets indicated that sedimentation of autochthonous OC from the immediate plume contributed 23% of the O2 demand necessary for establishment of hypoxia in the region.

Recent land-cover/use change associated with land degradation in the Lake Baringo catchment, Kenya, East Africa: evidence from Landsat TM and ETM+

Many parts of East Africa are experiencing dramatic changes in land‐cover/use at a variety of spatial and temporal scales, due to both climatic variability and human activities. Information about such changes is often required for planning, management, and conservation of natural resources. Several methods for land cover/change detection using Landsat TM/ETM+ imagery were employed for Lake Baringo catchment in Kenya, East Africa. The Lake Baringo catchment presents a good example of environments experiencing remarkable land cover change due to multiple causes. Both the NDVI differencing and post‐classification comparison effectively depicted the hotspots of land degradation and land cover/use change in the Lake Baringo catchment. Change‐detection analysis showed that the forest cover was the most affected, in some sections recording reductions of over 40% in a 14‐year period. Deforestation and subsequent land degradation have increased the sediment yield in the lake resulting in reduction in lake surface area by over 10% and increased turbidity confirmed by the statistically significant increase (t = −84.699, p<0.001) in the albedo between 1986 and 2000. Although climatic variations may account for some of the changes in the lake catchment, most of the changes in land cover are inherently linked to mounting human and livestock population in the Lake Baringo catchment.

Relationships among satellite chlorophyll a, river inputs, and hypoxia on the Louisiana continental shelf, Gulf of Mexico

SeaWiFS ocean color measurements were used to investigate interannual, monthly, and weekly variations in chlorophylla (chla) on the Louisiana shelf and to assess relationships with river discharge, nitrate load, and hypoxia. During the study period (2000–2003), interannual changes in shelf-wide chla concentrations averaged over January–July ranged from +57% to −33% of the 4-yr average, in accord with freshwater discharge changes of +20% to −29% and nitrate load changes of +20% to −35% from the Mississippi and Atchafalaya Rivers. Chla variations were largest on the shelf between the Mississippi and Atchafalaya Deltas. Within this region, which corresponds spatially to the area of most frequent hypoxia, lowest January–July mean chla concentrations (5.5 mg m−3 over 7,000 km2) occurred during 2000, the year of lowest freshwater discharge (16,136 m3 s−1) and nitrate load (55,738 MT N d−1) onto the shelf. Highest January–July mean chla concentrations (13 mg m−3 over 7,000 km2) were measured in 2002, when freshwater discharge (27,440 m3s−1) and nitrate load (101,761 MT N d−1) were highest and second highest, respectively. Positive correlations (R2=0.4–0.5) were found between chla and both fresh water and nitrate loads with 0 to 1 month lags, with the strongest relationships just west of the Mississippi Delta. In 2001, unusually clear skies allowed the identification of distinct spring and summer chla blooms west of the Mississippi Delta 4–5 wk after peaks in river discharge. East of the delta, the chla concentrations peaked in June and July, following the seasonal reversal in the coastal current. A clear linkage was not detected between satellite-measured chla and hypoxia during the 4-yr period, based on a time series of bottom oxygen concentrations at one station within the area of most frequent hypoxia. Clear relationships are confounded by the interaction of physical processes (wind stress effects) with the seasonal cycle of nutrient-enhanced productivity and are influenced by the prior years nitrate load and carbon accumulation at the seabed.

The Great Flood of summer 1993: Mississippi River discharge studied

In the summer of 1993, the Mississippi River basin in the midwestern United States experienced anomalously high rainfall. Record flooding resulted from an abnormally persistent atmospheric weather pattern consisting of a quasi-stationary jet stream positioned over the central part of the nation, where moist, unstable air flowing north from the Gulf of Mexico converged with unseasonably cool, dry air moving south from Canada. In concert with the persistent weather pattern over the United States, highly anomalous circulation patterns were observed over much of the Northern Hemisphere [Richards, 1994]. The rainfall anomalies over the central United States produced abnormally high river discharges along the Louisiana coastline from the Mississippi and Atchafalaya Rivers during July and August, traditionally months of low river discharge. Some of the river water discharged into the northern Gulf of Mexico reached the Straits of Florida by September 1993 [Lee et al., 1994].

A major perturbation in the Agulhas retroflection area in 1986

Abstract During late 1985 and in 1986 a major perturbation in the retroflection of the Agulhas Current occurred. A combination of climatological data and satellite-derived sea surface temperature observations are used to describe the evolution of the disturbance which resulted in 1986 being the warmest year on record in the southeast Atlantic. Possible causes of the changes in the retroflection and the associated input of Agulhas water into the southern Benguela Current region are discussed. Results show that, following intermittent leakage of elements of the Agulhas Current around the Cape of Good Hope, during the second half of 1985 and the first part of 1986, an offshoot of the Current developed in June as a flow into the Atlantic around the edge of the Agulhas Bank. Subsequently a major change in the retroflection occurred, and in August 1986 a large body of warm water joined to the Current moved northwards and was situated within 20 km of the Cape Peninsula and Cape Columbine. Boundary thermal gradients associated with this warm intrusion were 3–4°C in 25 km. This intrusion moved progressively northwards and westwards at 4.5–9.5 cm s −1 during the winter and spring of 1986 and had a typical zonal width of 240 km. The intrusion was evidently terminated when filaments of subtarctic water moved northwards, flooding much of the area with cold water during December 1986 and January 1987. Although intermittent leakage of Agulhas water into the southeast Atlantic continued during 1987, it was not substantial. By late 1987 the situation in the retroflection area had returned to normal. The observations of the Agulhas intrusion, when viewed in the light of changes in wind stress in the southwest Indian Ocean and south of Africa, are consistent with published numerical modelling results which predict a substantial flow of Agulhas water around the Cape of Good Hope under certain conditions.

Property fields in an effluent plume of the Mississippi River

Surface property distributions were mapped in the Mississippi River plume during May and August 1993 while following surface drifters. Prevailing winds were the primary factor controlling the orientation of the plume. In May, under typical southeasterly winds, the plume turned anticyclonically towards the coast, while in August, under anomalous westerly winds, the plume turned east. Remote imagery of sea surface temperature and suspended sediments confirmed the direction of the plume. Optimally interpolated maps of surface salinity, temperature, chlorophyll a fluorescence, and transmissivity from underway sampling, and periodic nutrient samples, reveal the plume structure. In May concentrations of nitrate, silicate, and phosphate decreased linearly with increasing salinity. Chlorophyll a increased to peak concentrations of 10 μg 1−1 in the plume, although higher pigment biomass was observed near the coast. In August nitrate and silicate concentrations decreased conservatively near the mouth of SW Pass, except where pigment biomass was enhanced in a convergent surface front. Surface nutrient concentrations in the plume also decreased with increasing salinity. The observations provide the first Lagrangian view of surface property distributions in the Mississippi River plume, and indicate that significant temporal variability exists in physical and biological properties within a day after waters are discharged from the river delta.