Oceanographic Processes on Freshwater-Dominated Continental Shelves III Posters
Presiding: J H Muelbert, Fundacao Universidade Federal do Rio Grande; A Piola, Servicio de Hidrografia Naval (SHN)
OS23A-01 1330h
Enhanced Turbulent Mixing Over the Pinnacles on the Mississippi/Alabama Shelf
Topographic features, such as submarine canyons and banks, are often present on shelves and are known to enhance mixing at least one order of magnitude greater than mixing on shelves with gradually varying bathymetry. Such topographic features can easily alter ambient motion, intensify density gradients, accelerate mean currents, and modify vertical shear. Associated topographically induced turbulence may account for an appreciable fraction of the total shelf mixing and can exert a large bottom stress and dissipate considerable energy. The Pinnacles are an extensive system of drowned fossil reefs ranging from 5 to 15 m in height and 100 to 500 m in width covering an area approximately 8 by 20 km in 80 to 110 m of water on the shelf just west of the Desoto Canyon. This region is characterized by fairly vigorous currents, and it is thought that at times, these currents are energetic enough to induce hydraulically controlled flow over the Pinnacles. This flow, in turn, can produce strong turbulence in the bottom layer. A 24-hour time series of the currents and density measurements collected during NRL's first SEED cruise in the Pinnacles region suggest that such conditions may have occurred in May, 2004. The energetic flow was observed twice in the 20 to 40-m thick layer just above the bottom with speeds larger than 60 cm/s, while in the upper part of the water column, current speeds did not exceed 25 cm/s. Estimates of the composite internal Froude (>2.7) and Richardson (<0.35) numbers suggests that hydraulically control flow and enhanced turbulence may have been present during these two events.
OS23A-02 1330h
Granulometric composition of bottom sediments in the Southwest Atlantic shelf region under influence of the Plata River and the Patos Lagoon
Grain-size analysis was carried out with bottom sediments collected in the region of the southeastern South American continental shelf under influence of the La Plata River and the Patos-Mirim lagoon system outflows. The samples were collected with Box-Core during cruises the of Project LAPLATA, in the Austral winter of 2003 and summer of 2004. In both cases the study area extended from approximately 40°S to 28°S. In the analysis, we preliminarily separated the finest sediment fraction by laundering the samples with water and dried them in a greenhouse. Following, we submitted the samples to a X-ray diffraction analysis. The resulting data set was then processed by a computer program to calculate the relative composition of each considered mineral. A table with the percentages in mass of the components, as well as values of crystallization of illita and smectita, was made with the results. In the samples collected more to the South, we verified the predominance of largest-grain minerals. Amongst lesser minerals we found the illita the predominant, especially in the samples more to the north. The sediment brought by the La Plata River predominates in the region near the river mouth, decaying northward. More to the north, an increase in the amount of larger-grain sediments is verified and could be related to the influence of the Brazil Current.
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OS23A-03 1330h
Variability of Chlorophyll-a From Ocean Color Images in the La Plata Continental Shelf Region
Ocean color data from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) have been used to investigate temporal and spatial variability of chlorophyll-a concentration over the La Plata continental shelf zone (25-40oS and 60-45oW). Our analysis is based on 276 weekly images (9km x 9km resolution) which span from October 1997 to September 2003. We separated the log-transformed chlorophyll concentration time signal into 12 harmonics and their amplitudes and phases were determined using Fourrier analysis. The time-series for each pixel has been reconstructed by using the known amplitudes and phases. Then it was calculated the percentage explained by one or a composition of several harmonics in the time series for each pixel. Results of the analysis show that the annual cycle is the most dominant signal, especially in the area over the Brazilian continental shelf (25-35oS). Regions close to the coastline and areas directly influenced by La Plata River show the weakest relation to annual cycle. An analysis of the spatial variability of amplitude and phase for each or a composition of several harmonics is also presented in this work. We conclude that SeaWiFS time-series satellite data can provide relevant information about biological cycles and their spatial extent over the La Plata region
OS23A-04 1330h
Evaluation of Bio-optical Algorithms for Chlorophyll Mapping in the Southwestern Atlantic
Efforts have been made over the past decade to study bio-optical properties of seawater in the Southwestern Atlantic for mapping chlorophyll concentration from space. Coastal regions deserve a greater attention due to the optical complexity from continental influence. Here we present an attempt to derive reliable bio-optical chlorophyll algorithms in the shelf region 25-40o S and 60-45o W. This area is subject to large optical interference by continental runoffs from La Plata River and Patos Lagoon. Spectral upwelling radiance and surface chlorophyll concentration data have been collected in the past years and have been used to generate a regional version of the NASA's OC2v4 model. The regional 2-band algorithm (termed OC2-LP), reduces chlorophyll positive bias to 11% as compared to the global SeaWiFS OC4v4 algorithm (bias = 27%). However, OC2-LP remains with an overall inaccuracy of over 40% in chlorophyll concentration, as calculated by the absolute percentage difference between in-situ and model-derived values. In-situ chlorophyll data from two cruises to the study region (La Plata I - winter of 2003 and La Plata II - summer of 2004) have been used to test the accuracy of the derived algorithm as well as the global version. A marked seasonal difference was found, where both OC4v4 and OC2-LP overestimate chlorophyll in summer at a higher magnitude than in the winter. These results indicate the need for other approaches rather than use of empirical band-ratio models in coastal waters of this region.
OS23A-05 1330h
Sea Surface Temperature Anomalies in the Western South Atlantic from 1982 to 2004
Twenty-six years of satellite-derived sea surface data were used to investigate spatial and temporal variabilities of SST anomalies in the Southwestern Atlantic (SWA). The analysis is focused on the large scale variations over the continental shelf and slope region with depths ≤~1000-m, between 42°S and 22°S. The purpose of this paper is two-fold. First, it revisits the work done by Lentini el al. (2001) motivated by the extension of the time series of SST from 1994 to September 2004. Second, it investigates the relationship between SST anomalies in the area of study and the occurrence of ENSO events in the Pacific for the same period. Northward-propagating cold and warm anomalies have average speeds of the order of 20 cm/s. The relationship between northward-propagating SST anomalies and anomalous alongshore winds is also investigated. Southward-advected cold and warm anomalies, on the other hand, have slower speeds. Significant differences were observed in the SST anomalies during El~Niño (EN), La~Niña (LN) and Neutral years (NT). On average, EN years are characterized by warm SST anomalies in regions directly influenced by the Brazil Current, whereas LN years are marked by cool SST anomalies in the Brazil Current vicinity. The first EOF mode for the three along-shelf transects show an in-phase oscillation, indicating that the SST variations take place almost simultaneously throughout the entire region. Investigation of the temporal structure for the first two PCA's modes account for more than 65% of the non-seasonal variabiliy over the shelf. These results seem to confirm the correlation between ENSO events and the northward-propagating (southward-advecting) SST anomalies along the SWA.
OS23A-06 1330h
Space-Time Variability of the Plata Plume Inferred From Sea Spectral Reflectance Data
The Plata River, discharges on average 23,000 cubic meters per second of freshwater into the western South Atlantic. The Plata diluted waters are laden with sediments and organic matter, which strongly affect the physical and bio-geo-chemical properties and the circulation over the neighboring ocean. Historical hydrographic data and numerical models show large seasonal variations in the northward and offshore extension of the river plume. Though the Plata variations induce a large impact on the coastal ecosystem, little is known about the interannual and intra-seasonal variability. The Plata plume induces a strong radiance signal in the visible band associated to the combination of productivity and suspended particles. To characterize the space-time variability of the Plata plume we use five years (1998-2002) of surface chlorophyll-a concentrations (chl-a) derived from NASA's Sea-viewing Wide-field of View Sensor. Though the data is too noisy to allow computation of surface salinity from satellite observations, comparison of in-situ surface salinities obtained during the Plata expeditions with 1 km resolution chl-a data, shows that the later can be used to clearly distinguish the Plata derived waters from open ocean waters. Salinities < 30 (> 35) are associated with log chl-a > 0.8 (< -0.4) mg/m3. Our analysis reveals northeastward plume penetrations beyond 28S during the austral winter and retractions in summer to 32S, qualitatively similar to those inferred from historical hydrographic data and numerical simulations. High chl-a (low salinity) waters advance and retract along-shore at a mean rate of about 800km/3months, similar to results from numerical simulations. Maximum along-shore plume penetrations (>1100km) are observed in winter 2002, preceded by a period of normal outflow and relatively strong southeasterly wind. In contrast the plume presented the least along-shore penetration (400 km) in winter 1999, also preceded by near average outflow but northeasterly winds north of 31S. These results suggest that sea spectral reflectance (SSR) imagery can be used to trace the variations in the extension of the Plata plume over the continental shelf.
OS23A-07 1330h
Modeling the Transport Success and Retention of Anchovy (Engraulis anchoita) Early Stages in the Southern Brazilian Shelf
The Southern Brazilian Shelf (SBS) is one of the most productive fishing areas in the Southwestern Atlantic. Eggs and larvae of about 88 fish species distributed among 48 families occur in the plankton community in this area. The processes involving the circulation in this area and its influence on eggs/larvae transport are poorly understood. Recent studies described the spawning activity of anchovy relating it to oceanographic processes. These studies suggest that the predominant onshore Ekman transport, which occurs mainly in the winter and spring periods, keeps the eggs and larvae in coastal areas. It is also known that the continental runoff is a primary factor to the formation of a retention zone for planktonic organisms over the SBS. The formation of a stagnant zone, where the residual currents are weak, favors the patchiness character of plankton communities and nutrients. In our ongoing study we investigate with data and model the dispersion pattern of eggs/larvae of Engraulis anchoita in this region, considering both biological and physical processes. In the modelling approach, surface velocity fields from runs with the Princeton Ocean Model (POM) are used as input data for an Individual Based Model (IBM). This IBM treats every single particle as an individual with its own features, tracking particles (eggs/larvae) trajectories using a Runge-Kutta scheme for spatial interpolation. The analyses of our preliminary results seems to confirm the expected pattern in which the majority of eggs and larvae are transported to their nursery grounds, remaining in these areas.
OS23A-08 1330h
Hydro-Meteorological Conditions over the Southeastern South America during ENSO years and its Impact on the Adjacent Continental Shelf
An investigation of the hydro-meteorological conditions during ENSO years and its impact over the adjacent continental shelf has been carried out using monthly mean time series of the la Plata river discharge, the latitude of separation of the Brazil and Malvinas current, and NCEP winds. From the ENSO composites two major results can be highlighted. First, a considerable difference in the amplitude response from one event to another for the same ENSO phase is clearly observed. Second, there is a tendency for larger amplitude anomalies during El Niño (EN) years than during La Niña (LN) years. Although the river's response to ENSO events shows the same tendency for positive (negative) anomalies along the EN (LN) composites, the largest peak of the la Plata river associated with EN occurs in January(+1) instead of December(0). The Brazil and Malvinas latitude of separation composites show positive (negative) monthly anomalies during EN (LN) events from July(0) at least until January(+1). Analysis of the NCEP alongshelf anomaly wind suggests a relationship between river runoff and the latitude of separation of the Brazil Current.
OS23A-09 1330h
Influence of ENSO-Related Wind Stress Variability on the Coastal Upwelling off Peru
The Eastern Tropical and Subtropical Pacific, particularly the coastal region off the western South America, is negatively affected by the El Niño-Southern Oscillation (ENSO) events. The rise of coastal sea temperatures due to the weakening of coastal upwelling causes the reduction of important fish stocks and other coastal resources of economical significance, especially the sardine and anchovy populations. Here, variability in the intensity of upwelling along the Peruvian coast is studied using monthly means of the ERS-1 and ERS-2 scatterometer wind stress. In the study area, the Trade Winds blow predominantly parallel to the coast, making the upwelling system highly sensitive to variability in the intensity of the wind stress. On its turn, the wind stress intensity is expected to suffer strong variation during ENSO periods. In our ongoing work we try to understand the relationship between coastal upwelling and different ENSO events in the 1991-2000 decade. In this period, two warm events where observed, the relatively weaker 1991-1994 and the stronger 97-98 El Niños. In both cases we found that in the region from 12°S to 3°S, the intensity of alongshore wind stress is reduced considerably, with the coastal upwelled waters being replaced by warmer waters. On the other hand, in the region between 12°S and 19°S the coastal upwelling maintains its intensity because only the cross-shore component of the wind stress was reduced. With respect to the two cold events, the 1996-1997 and the 1998-2000 La Niñas, the intensity of the alongshore windstress was higher than the climatological values. This resulted in more intense upwelling in the entire region.
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OS23A-10 1330h
Nutrient Distribution and Benthic Foraminiferal Assemblages on the South Brazilian Shelf: Relationship With Water Masses and Freshwater Influx
Using distributions of nutrients and benthic foraminifera, we tracked water masses on the South Brazilian Shelf and assessed the seasonality of continental runoff and the dilution of coastal waters. The data were obtained from water and sediment samples collected in winter 2003 and summer 2004. Silicate concentrations were 0.02-83.52 YM in winter and 0.05-41.16 YM in summer. The high silicate content in the surface water was associated with low salinity (minima 21.22 in winter and 21.96 in summer), confirming a significant input of freshwater. Phosphate values, a secondary tracer of the terrestrial input, were 0.00-3.30 YM in winter and 0.03-2.26 YM in summer, with the shallow-water phosphate content indicating a freshwater influence. The nitrate evidence showed the presence of new water in the region, derived from South Atlantic Central Water by upwelling processes. Nitrate, nitrite, and oxygen correlations indicated a denitrification process in bottom water, associated with the oxygen minimum layer, and with the suspended matter content. Dissolved oxygen values were 3.41-7.06 mL.L-1 in winter and 2.65 mL-6.85 mL.L-1 in summer. The lowest salinities were frequently associated with >20>XC temperatures. In winter, an inner-shelf salinity front was present, and the strong influence of freshwater led to a silicate maximum (27.79 YM), increased concentrations of phosphate (2.70 YM), Si(OH)4 (10.88 YM), NO3 (1.01 YM), and Total Dissolved Nitrogen (22.98 YM), together with a high value of suspended matter (44.80 mg/L). The terrestrial input was significantly reduced with increasing distance from the coast, but high values of nutrients were maintained in subsurface waters, because of SACW upwelling. At shallow sampling stations, the influence of freshwater runoff was also demonstrated by (1) the dominance of benthic foraminifera such as lagoon-related Elphidium spp. and several agglutinated species, including Gaudryina exillis and Trochammina spp; and (2) an abundance of diatoms. The assemblage at offshore stations, in contrast, was characterized by smaller forms and higher species diversity. The foraminiferal data correlate well with nutrient distributions. Our findings suggest that during winter, when freshwater influence is stronger, the front along its plume has a strong effect on physical-chemical-biological interactions. In summer, the foraminiferal species Bulimina marginata, Cancris sacra, Cassidulina subglobosa, and Uvigerina peregrina found in the area suggest the influence of SACW nutrients brought up by upwelling processes.
OS23A-11 1330h
On the Color of the Orinoco River Plume
In situ measurements were used to study the bio-optical properties of marine waters within the Gulf of Paria (GOP, Venezuela) and in the Southeastern Caribbean Sea (SEC) as they are affected by the seasonal discharge of the Orinoco River plume. The main purpose of this study was to determine the impact of colored dissolved organic matter (CDOM) (also known as Gelbstoff), phytoplankton, and total suspended matter (TSM) in the color of the Orinoco River plume. This information is essential for regional ocean color algorithms development. Salinity and silica values indicate that the GOP and SEC waters were under the influence of the Orinoco River plume during both seasons. This riverine influence resulted in high values of Gelbstoff absorption, ag(λ), which contributed to up to 90% of the total absorption at 440 nm in both the GOP and SEC regardless of the season. Phytoplankton absorption contributions were normally around 5%, but during the dry season these values reached 20% in the SEC. Ratios of ag(440) to ph(440) were extremely large, with most of the values ranging from 10 to 50. Due to the strong absorption by Gelbstoff, light at the blue wavelengths (412 nm, 440 nm and 490 nm) was attenuated to 1% of the subsurface irradiance in the first 5 m of the water column within the GOP, and in the first 10 m of the water column in the SEC. Furthermore, the absorption by Gelbstoff significantly decreased the water leaving radiance (Lw(λ)) in the blue wavelengths along the Orinoco River plume. As ag(λ) relatively decreased from the GOP to the SEC (mean ~1.6 m-1 and mean ~0.9 m-1, respectively), a shift in the maximum peak of Rrs(λ) spectra (Rrsmax(λ)), towards shorter wavelengths (from ~ 580 nm to ~500 nm) was observed. Similar to Gelbstoff, concentrations of TSM normally decreased from the stations near the Delta to the stations in the SEC. The impact of TSM on the color of the Orinoco plume was represented by a reduction in the magnitude of Rrsmax(λ) of ~50% going from the waters near the Orinoco delta to the SEC, indistinctively of the season.
OS23A-12 1330h
Freshwater Influence Over the South Brazilian Continental Shelf: Indications From Foraminiferal Species Dominance
The 250-km-long Patos Lagoon covers an area of 10,360 km2 along the coastal plain of Rio Grande do Sul in southern Brazil. To the north, the 40-km-long Laguna estuarine system along the Santa Catarina coastline is essentially a series of choked coastal lagoons, with elliptical cells connected to the ocean via a single long and narrow channel. We examined the distribution of benthic foraminifera in samples collected from two continental-shelf transects, labeled the Albardäo transect (32° 58'S) and Santa Marta Cape transect (28° 34'S), in order to track the influence of the Patos Lagoon and the Laguna estuarine system in the dilution of shelf waters. In the Albardäo transect, we notice the dominance of Bulimininella elegantissima and Bolivina striatula live individuals in the shallower stations (15-42 m). These low-oxygen tolerant species are also found in muddy sediments of the Patos lagoon and near the mouth of the Plata River; their presence in shelf sediments indicates continental runoff in the region. Very large individuals of the agglutinated species Arenoparella mexicana and Gaudryina exilis (present in brackish waters Patos Lagoon) are found in water depths 19-54 m, also suggesting a strong and deep freshwater influence. At the 90-m isobath, (1) the species diversity shows a rise, and (2) larger agglutinated species and Buccela peruviana, a temperate/cold-water hyaline species, are occasionally present. B. peruviana is characteristic of assemblages influenced by the Malvinas Current, and indicates the presence of Sub-Antarctic Shelf Water (SASW). In contrast, the dominant species at 53 m on the Santa Marta Cape transect are Bulimina marginata and Pseudononion atlanticum. These species, together with small individuals of Arenoparella mexicana and Gaudryina exilis, show that the Santa Marta Cape area is under a major influence of Subtropical Shelf Waters (STSW), and that fresh-water influence is minor. The assemblage from the deepest stations on this transect (75, 109m) is dominated by Uvigerina peregrina and Cassidulina subglobosa, which indicates the presence of the cold, nutrient-rich South Atlantic Central Water (SACW).
OS23A-13 1330h
Quantifying Sediment Transport Determined From Grain-Size Distributions
A technique derived from McLaren & Bowles (1985) has been applied to investigate sediment dynamics in the Patos Lagoon estuary (Brazil). Qualitative sediment transport in the access channel of the estuary was inferred from changes in statistical properties describing grain-size distributions. Assuming the influence of a single transport function, the spatial gradient of particle mean size, sorting and skewness was used to determine the transport direction along the channel. A long-average net sediment deposition rate in the area was estimated using digitalized historical nautical charts. This deposition rate was used to quantify the sediment transport inside the estuary, through a simple application of Green's Theorem. Results show a net seaward transport in the deep channel of approximately 50 m3 day-1, accompanied by a net inward transport in the shallower channel margin of similar intensity. The estimated net sediment transport was validated against a numerical model output, with good agreement in terms of direction and intensity.
OS23A-14 1330h
Sources of Organic Carbon to a River-Dominated Coastal Shelf: the use of Chemical Biomarkers to Track Inputs to Sediments
The oceans act as a global sink for excess carbon dioxide not only via oceanic primary production, but also through sinking organic matter and burial. Coastal sediments, particularly along river-dominated ocean margins, can be important repositories for both allocthonous and autochthonous organic carbon. Using molecular biomarker analyses, this study examines the sources of organic carbon to the Louisiana shelf within the dispersive path of the Mississippi River. Two cruises were conducted near the mouth of the river on the Louisiana shelf, during periods of high and low river discharge. Sediment samples were collected on a grid of 50 sampling sites that spanned the area of the plume and the surrounding waters, and analyzed for bulk carbon and specific molecular biomarkers. Total organic carbon (TOC) in surface sediment ranged from 0.28 to 1.8%OC. The Δ 13C values generally ranged from -20.6 to -22.9 ‰, with a few stations showing more enriched values. It is not possible, therefore, to precisely distinguish between marine and terrestrial inputs based on the bulk carbon and Δ13C values alone. Chlorophyll in the sediment (0.1 to 8.9 Μ g/g) was used as an indicator of marine organic carbon inputs, while lignin (Λ8, 0.3 to 4.7 mg/100 mg OC) was used as a biomarker for terrestrial organic carbon. An isotope mixing model incorporating the bulk isotope and the biomarker data suggests ~ 32 to 41% of the TOC in surface sediments is composed of terrestrial material during high discharge, while only 14 to 29% of the TOC is of terrestrial origin during the low discharge period. Compound specific isotope analyses (CSIA) show differences up to 7 ‰ in syringyl compounds and up to 10 ‰ in cinnamyl compounds, indicating both C3 and C4 terrestrial inputs. The relative proportion of marine and terrestrial carbon deposition on the Louisiana shelf varies with river flow and the amount of in situ production. However, even with the high productivity during spring months there is still a substantial terrestrial contribution to the surface sediments, likely due to the recalcitrant character, and consequently longer residence time of this material.
OS23A-15 1330h
Sources and Transformations of Organic Matter in Surface Sediments Across Different Dispersal Pathways on the Louisiana Continental Shelf/Slope: The Application of Plant Pigments and Lignin-Phenols as Biomarkers
Bulk surface sediments from the Mississippi Margin were analyzed for lignin phenols (a biomarker of vascular plant material) and plant pigment biomarkers. Sediment samples were collected at five stations along a transect extending from the mouth of the Mississippi River (SW pass) to the Mississippi River Canyon (MRC)and a second transect from outside SW Pass extending to Barataria Bay, along-shelf at the 50 meter isobath. An additional smaller transect comprised of three stations from just inside Barataria Bay to the inner shelf was also sampled. The average lambda-8 (e.g., lignin-phenols normalized to 100 mg carbon) and (Ad/Al)v (e.g., decayed lignin) values for all shelf stations was 1.12 ug/100g OC and 0.49, respectively. On the cross-margin transect, lamda-8 and Ad/Al)v values ranged from 1.45 to 0.34 and 0.35-0.78, respectively, and from 1.05 to 0.42 and 0.45-0.68 along the 50 m isobath transect, respectively. The Barataria Bay transect showed higher lambda-8 values compared to mid and outer-shelf sites and ranged from 0.93 to 4.85. Terrestrially-derived organic matter (TOM) (based on lignin-phenol concentrations) showed similar trends of decreasing concentrations and increasing degradation in surface sediments along both major transects: 1) across shelf to the MRC and 2) westward along the 50 m isobath. Similar decreases in lignin abundance and increased degradative state of TOM along either major transport pathways indicate that decomposition of TOM in mobile muds in shallow waters and/or across deeper waters to the MRC are effective at oxidizing terrestrial material, yet over different time periods. Lignin-phenols also indicated fresh loading of local marsh material from Barataria Bay to the inner shelf, emphasizing that local marshes are sources of TOM the the coast. Pigment data indicated much of the marine-derived organic matter (MDOM) is dominated by diatoms. Increasing pigment concentrations along the 50 m isobath transect co-occurred with decreasing lignin concentrations suggesting local sources of phytoplankton inputs outside the Mississippi River plume. The occurrence of fresh diatom pigments in the MRC may suggest transport of MDOM from the shelf to the MRC. Lignin data will also be contrasted between pre- and -post hurricane Ivan concentrations as a major disturbance event.
OS23A-16 1330h
Abundance and Bulk Composition of DOM in the Lower Mississippi and Pearl Rivers (USA)
Here we report on temporal changes in the composition of dissolved organic carbon (DOC) and nitrogen (DON) collected in the tidal freshwater region of the lower Mississippi and Pearl Rivers (MR and PR) (USA). Bulk stable carbon isotopes and 13C nuclear magnetic resonance (NMR) spectrometry were used to examine the composition of high molecular weight (< 0.2 æm > 1 kDa) dissolved organic matter (HMW DOM). Monthly water samples were collected at one station in each river from August 2001 to July 2003. Surveys of spatial variability (225 km downstream in the MR and from Jackson to Stennis Space Center in the PR) in total DOC and DON were also conducted in both rivers in June 2003. Higher total DOC (336 to 1156 uM), DON (9.3 to 59.5 uM), % HMW DOM (25 to 47 %), ultraviolet (UV) absorption (0.13 to 0.70 /m), and more depleted delta-15N (0.76 to 2.16 per mil) delta-13C (-25.1 to -28.0 permil) were observed in the PR than in the lower MR (223 to 380 uM, 6.1 to 13.4 uM, 16 to 38 %, 0.08 to 0.17 /m, 0.76 to 2.16 permil, -25.7 to -27.1 permil, respectively). 13C-NMR spectra revealed that alkyl and carbohydrate carbons were dominant in HMW DOC in both rivers. However, a significantly lower percentage of aromatic C (13.2 to 16.6 %) and higher carboxyl C (17.1 to 25.8 %) were observed in the lower MR than in the PR (16.9 to 21.3 % and 12.3 to 20.9 %). Total DOC, DON, HMW DOM, and percent aromaticity of HMW DOM were higher in the PR during local flooding events, and lower during low discharge, indicating a coupling between local carbon inputs (soil and wetlands) and regional precipitation events in the PR. Conversely, seasonal variability of total DOC, DON, and HMW DOM in the lower MR was controlled by spatial variability of an integrative signal from watershed inputs and in-situ production from upriver sources, resulting in a more phytoplankton-derived 13C-NMR signature of HMW DOM. Spatially, very little change occurred in total DOC (259 to 282 uM) and DON (8.85 to 13.3 uM) in the downstream survey of the lower MR, compared to decreases of 24 % and 50 % in DOC and DON, respectively, in the PR. Once again local inputs are more important in the PR compared to the MR and likely account for higher variability. Recent lab incubation experiments also suggest that photochemical oxidation, coupled with bacterial degradation of DOM, accounts for significant alteration of DOM in these lower floodplain rivers.