Incidence of piscine erythrocytic necrosis (PEN) andGlugea hertwigi (Weissenberg) in rainbow smelt (Osmerus mordax mitchill) in selected Massachusetts streams

Smelt (Osmerus mordax) from six rivers along the coast of Massachusetts were examined for the microsporidianGlugea hertwigi and piscine erythrocytic necrosis (PEN), a viral blood disease. Of the 485 smelt sampled, 13.4% were affected byGlugea. Incidence ranged from 18.0% in the Town River to 0% in the Parker River. PEN was observed in all river systems. Incidence ranged from 97% in the Jones to 61% in the Parker River. Individual intensity of infection was low, with the majority of fish having less than 1% of their erythrocytes affected. The highest intensity for an individual specimen was 4.68%.     

Effects of predation by sea-run juvenile alewise (Alosa pseudoharengus) on the zooplankton community at Hamilton Reservoir,Rhode Island

The zooplankton of Mill Pond and Hamilton Reservoir in the Annaquatucket River drainage system, North Kingstown, R.I., were compared. While the taxonomic composition of the heleoplankton of the two ponds was similar, the ponds differed in dominant species of Cladocera, distribution of species during the year, and abundance of each taxonomic group. These differences are attributed to the abundance of juvenile Alosa pseudoharengus in Hamilton Reservoir. Overexploitation of food resources is the probable cause of reduced growth of juvenile alewives from Hamilton Reservoir.     

Spatially explicit measures of production of young alewives in Lake Michigan: Linkage between essential fish habitat and recruitment

The identification and protection of essential habitats for early life stages of fishes are necessary to sustain fish stocks. Essential fish habitat for early life stages may be defined as areas where fish densities, growth, survival, or production rates are relatively high. To identify critical habitats for young-of-year (YOY) alewives (Alosa pseud oharengus) in Lake Michigan, we integrated bioenergetics models with GIS (Geographic Information Systems) to generate spatially explicit estimates of potential population production (an index of habitat quality). These estimates were based upon YOY alewife bioenergetic growth rate potential and their salmonine predators’ consumptive demand. We compared estimates of potential population production to YOY alewife yield (an index of habitat importance). Our analysis suggested that during 1994–1995, YOY alewife habitat quality and yield varied widely throughout Lake Michigan. Spatial patterns of alewife yield were not significantly correlated to habitat quality. Various mechanisms (e.g., predator migrations, lake circulation patterns, alternative strategies) may preclude YOY alewives from concentrating in areas of high habitat quality in Lake Michigan.     

Standing crops of marsh vegetation of two tributaries of Chesapeake Bay

A comparative study of the standing crop of marsh vegetation was made of the Patuxent River and Parker Creek, two tributaries of Chesapeake Bay. The biomass of marsh vegetation in the tidal freshwater and brackish regions of the Patuxent was relatively uniform with regard to salinity, seasonally high concentrations of dissolved nitrogen, and phosphorus and nutrient gradient. Maximum values of biomass occurred in the tidal freshwater and slightly brackish water region of Parker Creek, a system whose nutrient concentrations approximated 20% of those of Patuxent River. Biomass values for the Patuxent River and Parker Creek averaged about 1417 and 895 g m^?2 dry weight, respectively. Estimates of total annual marsh production based on the maximum standing crop was 27×10³ and 519 metric tons, respectively, for the Patuxent River and Parker Creek.     

Distribution of coliforms in the Seine River and estuary (France) studied by rapid enzymatic methods and plate counts

Fecal contamination in the Seine River and its estuary was studied for the first time by rapid enzymatic methods in parallel with traditional enumerations on selective culture media. The study consisted of four sampling campaigns focused on a 450-km stretch including the Parisian area and presenting highly variable levels of fecal pollution. Enzymatic assays (based on the activity of the β-D-galactosidase and β-D-glucuronidase enzymes in total [TC] and fecal [FC] coliforms, respectively) were in good agreement with classical plate counts of TC and FC. Both methods reflected the strong impact of the wastewater discharge from the Parisian area and of the presence of a maximum turbidity zone (at the mouth of the estuary) on the abundance of fecal bacteria in the river. Downstream from the Parisian outfalls, enzymatic measurements probably detected enzymatically active but nonculturable bacteria disregarded by plate counts. Enzymatic measurements in the Seine River downstream from the Parisian area were used to estimate net disappearance rates of coliforms in the river, which were close to total mortality rates measured with a method based on bacterial DNA labeling with tritiated thymidine. The part of total mortality due to grazing by protozoa was also investigated by the thymidine method. Grazing activity was responsible for 47% to 99% of the mortality of coliforms in the river. Attachment of coliforms to suspended matter (SM) was another factor which could be important in controlling the dynamics of coliforms in the Seine River and particularly in its estuary. Results of filtration and decantation experiments suggested that a significant part of the coliforms can be linked to SM depending on the nature and concentration of the SM.     

Range-wide population structure of shortnose sturgeonAcipenser brevirostrum based on sequence analysis of the mitochondrial DNA control region

Riverine populations of shortnose sturgeon (Acipenser brevirostrum) once occurred in rivers and estuaries along the east coast of North America from the St. John River, New Brunswick, to the St. Johns River, Florida. Within this range, 19 population segments were identified by the U.S. Federal Shortnose Sturgeon Recovery Team; empirical data supporting this structure is limited. We obtained samples from 11 (12 including a small sample from the Cape Fear River, North Carolina) of these population segments and used PCR and direct sequence analysis of 440 base pairs of the mitochondrial DNA (mtDNA) control region to define the coast-wide genetic population structure of shortnose sturgeon. Collections from most population segments exhibited significant differences in haplotype frequencies with their nearest neighbors, including from the Ogeechee and Savannah Rivers, Georgia (despite the known movement of hatchery-reared offspring from the Savannah into the Ogeechee River). Collections from the Chesapeake Bay and Delaware River exhibited similar haplotype frequencies, suggesting that specimens collected in the Chesapeake Bay had dispersed from the Delaware River. Collections from the Kennebec River and Androscoggin River within a hypothesized single population segment did not exhibit significant differentiation of mtDNA haplotype frequencies. Haplotype frequencies were almost identical between collections from above and below the Holyoke Dam on the Connecticut River, indicating that these aggregations should be managed as a single unit. Our results support the population segment status afforded to shortnose sturgeon in at least the following 9 systems; St. John River, Kennebec-Androscoggin Rivers, upper-lower Connecticut River, Hudson River, Delaware River-Chesapeake Bay, Pee Dee River, Cooper River, Savannah River, and Ogeechee-Altamaha Rivers.     

Posthurricane Recovery of Riverine Fauna Reflected in the Diet of an Apex Predator

Hurricane Charley, a category 4 storm on the Safford–Simpson scale, passed over the Peace River, Florida, and its watershed on 13 August 2004 causing widespread hypoxia. An electrofishing study investigating the abundance of the euryhaline Centropomus undecimalis (common snook) began in the freshwater portions of the Peace River 3 months after the storm (November 2004) and included diet sampling. Samples were taken seasonally up to August 2006. Changes in the prey assemblages of C. undecimalis determined through multivariate analyses (e.g., nonmetric multidimensional scaling) were used as a proxy to estimate the recovery of riverine fauna. The initial posthurricane prey assemblages were dominated by species well adapted to low dissolved oxygen, namely Gambusia holbrooki and Hoplosternum littorale. A significant long-term serial change in assemblage structure occurred as the abundance of G. holbrooki and H. littorale decreased and other river fauna such as Procambarus spp. and ictalurid catfish increased. Assemblages were presumed as recovering 1 year after hurricane landfall, a posthurricane recovery period that is an order of magnitude longer than those reported for estuarine fish assemblages. The approach of using stomachs as sampling tools detected changes at the assemblage level that included an armored catfish species (e.g., H. littorale) and cryptic invertebrate species (e.g., Procambarus spp.) that went unnoticed in a study using standard electrofishing methods. Although sampling the diet of C. undecimalis was useful in detecting assemblage-level changes, other metrics—such as changes in health, sex ratio, relative abundance, and juvenile year-class strength of fishes and invertebrates collected by traditional fishing methods—are needed to fully evaluate the effects of the hurricane. Nonetheless, this study demonstrates that diet studies can supplement standard fishery gear to create a more comprehensive sampling strategy for aquatic fauna.     

Amazon River discharge estimated from TOPEX/Poseidon altimetry

This paper presents an application of the TOPEX/Poseidon (T/P) satellite altimetry data to estimate river discharge at three sites along the Amazon River. We discuss the methodology to establish empirical relationships between satellite-derived water levels and daily estimations of river discharges based on rating curves and in situ level measurements at gauging stations. Three sites are chosen: Manacapuru (River Solimões), Jatuarana (nearby the confluence of the Solimões and Rio Negro rivers) and Óbidos (Amazon River). We then reconstruct the satellite-based river discharge over a 10-year time span (1992–2002). Comparison between satellite-derived and river discharge at the gauging stations shows that the T/P data can successfully be used for hydrological studies of large rivers, in providing in particular discharge estimates when in situ data are not available. To cite this article: E.A. Zakharova et al., C. R. Geoscience 338 (2006).     

The application of HVSR method in detecting sediment thickness in karst collapse area of Pearl River Delta,China

The microtremor horizontal-to-vertical spectral ratio (HVSR, or H/V spectral ratio) method is an effective tool for detecting sediment thickness. Firstly, single-station microtremor measurements were taken beside 52 boreholes located in Pearl River Delta, China. Sediment thicknesses revealed by those boreholes range from 7.9 to 39.6 m. Then, those microtremor data were analyzed using HVSR method, and peak frequency of each site was extracted. According to those peak frequencies of HVSR and corresponding sediment thicknesses, the frequency-to-thickness fitting (f–Z) equation which is suitable for Pearl River Delta area was established. And this equation was compared with several fitting equations derived from different regions by other researchers. Finally, this fitting equation was applied in a collapse area in Guangzhou city to estimate site sediment thickness. Compared with data from five boreholes in this collapse area, the consistency between estimated sediment thickness and real value was relatively good. Karst sinkholes are generally located in regions where thickness of sediments is shallow. Therefore, the method described in this paper can provide support for delineating potential collapse areas.     

Parasites and cyclopoid predators of age-0 fish in the Roanoke River,North Carolina

Altered river flow has been suggested as a cause for the low recruitment of striped bass,Morone saxatilis, in the Roanoke River (North Carolina) because of its effect on the proximity of zooplankton and larval striped bass. This results in unsuccessful feeding and subsequent starvation, which was considered to be a major mortality factor. Other mortality factors, such as parasitism and copepod predation on age-0 fish, may also be regulated to some extent by changes in river flow. The relationship of cestode plerocercoids, trematode metacercaria, mussel glochidia, and cyclopoid copepod predators with age-0 fish was evaluated in the lower Roanoke River and western Albemarle Sound from plankton net collections made in 1984 to 1986 and 1988. Plerocercoid prevalence was higher under low river flow conditions than under high flow conditions in darters (Percidae; 16.7% vs. 9.2%), minnows (Cyprinidae; 28.8% vs. 4.7%), andMorone (1.9% vs. 0%). Gut analysis of the age-0 fish revealed that copepods (source of the plerocercoids) were a major diet component ofMorone and darters but not of minnows or herring (Clupeidae). Decreases in river flow were associated with increases in copepod density (Pearson r=?0.62; p=0.0001) and plerocercoid prevalence inMorone (Pearson r=?0.29; p=0.03). The low correlation value forMorone may be quite strong considering the complexity of the variables associated with prevalence. Metacercaria were found only inMorone and minnows, and prevalence and mean intensity were less than that found for plerocercoids. Mussel glochidia prevalence was less than 0.5% for all affected taxa, an order of magnitude less that that found in other studies. The low value may indicate that the mussel population in the Roanoke River is declining. Prevalence of attacks by the predatory copepodMesocyclops edax on age-0 fish was similar to that in Chesapeake Bay, and striped bass was the primary prey. Spatial and temporal proximity of copepods and fish prey may be the key factors in regulating copepod attacks. The low prevalence of parasites and copepod predators seen in this tudy would suggest that mortality from these sources may not be a major factor in age-0 recruitment in this system. Confirmation of these conclusions would require a more controlled experimental approach.     

Assessment of ecological flow rate by flow duration and environmental management class in the Geum River, Korea

This study attempted to analyze flow duration in a basin using a method to estimate environmental flow developed by the International Water Management Institute, and simulate the effects of runoff characteristics unique to a river and flow variability due to basin developments on aquatic ecosystems. To do so, KModSim, a simulation model for basin-wide water distribution, was used to assess flow duration in the Geum River basin, one of the four major river basins in Korea, by environmental management class (EMC). Flow duration curves by EMC at Sutong and Gongju sites were derived on the basis of natural flow in the Geum River basin. As a result, they were found to be consistent with the results of previous studies. Time series of mean monthly flow data by EMC were plotted together with those of simulated flow data by reservoir operation scenario; Sutong and Gongju points both showed flow behaviors corresponding almost to “A” in EMC. In addition, the characteristics of habitats by species of fish were identified through monitoring fish habitat at the Sutong site, so that optimal ecological flow rate was estimated. For this purpose, relations between flow discharge and weighted usable area for Coreoleuciscus splendidus and Pseudopungtungia nigra were projected using physical habitat simulation system, and EMCs consistent with flow duration curves (estimated taking in-stream flow) were assessed. The results or findings reported in this study are expected to serve as basic data for making a plan to efficiently monitor and manage aquatic ecosystems in the Geum River basin.     

Variability in fault size scaling due to rock strength heterogeneity: a finite element investigation

Practical application of fault size scaling relationships usually involves extrapolation of a limited data set over a scale range beyond that observed or into adjacent unstudied areas. Here we investigate the validity of such extrapolations by considering the variability in the size–frequency distributions of fault populations that develop under identical tectonic conditions using a numerical model to generate conjugate, normal fault populations in cross-section. The deforming material is modelled using a strain-softening, Von Mises rheology with Gaussian heterogeneity in yield strength distributed randomly throughout the mesh. We present eight deformation experiments that differ only in the random spatial pattern of yield strengths. We observe power law size–frequency scaling, i.e. N=ax^−c (where N is the cumulative number of faults and x is a measure of fault size) but with a range of values of c. The ensemble average value of c decreases with increasing percentage extension. However, for individual model runs the dependence of c on total strain shows significant variability that we can relate to small but important differences in fault growth and strain localisation. At any particular strain, the range of values of c is ∼10 times greater than the error estimate derived from least squares regression of the cumulative frequency data. Our results suggest therefore that large uncertainties should be associated with extrapolating fault population data from one scale or region to another even if the lithology and tectonic history are similar.     

The structure of the global solar magnetic field excited by the turbulent dynamo mechanism

A mixing-length approximation is used to calculate Kλ for a Parker dynamo wave excited by the dynamo mechanism near the base of the solar convection zone (K is the wave number of the dynamo wave and λ the extent of the dynamo region). In a turbulent-dynamo model, this number characterizes the modes of the global magnetic field generated by a mechanism based on the joint action of the mean helical turbulence and solar differential rotation. Estimates are obtained for the helicity and radial angular-velocity gradient using the most recent helioseismological measurements at the growth phase of solar cycle 23. These estimates indicate that the dynamo mechanism most efficiently excites the fundamental antisymmetric (odd), dipole, mode of the poloidal field (Kλ≈?7) at low latitudes, while the conditions at latitudes above 50° are more favorable for the excitation of the lowest symmetric (even), quadrupole, mode (Kλ≈+8). The resulting north-south asymmetry of the poloidal field can explain the magnetic anomaly (“monopole” structure) of the polar fields observed near solar-cycle maxima. The effect of α quenching increases the calculated period of the dynamo-wave propagation from middle latitudes to the equator to about seven years, in rough agreement with the observed duration of the solar cycle.     

Catchment scale geostatistical simulation and uncertainty of soil erodibility using sequential Gaussian simulation

Soil erodibility (K) affects sediment delivery to streams and needs to be appropriately quantified and interpolated as a fundamental geographic variable for implementing suitable catchment management and conservation practices. The spatial distribution of K for erosion modelling at non-sampling grid locations has traditionally been estimated using interpolation algorithms such as kriging which do not adequately represent the uncertainty of estimates. These methods cause smoothing effects through overestimating the low values and underestimating the large values. In this study observed values were used to implement a sequential Gaussian simulation (SGS) procedure to evaluate the certainty of modelled data. Soil erodibility values were computed using 41 soil samples taken from the top 10 cm soil layer regularly distributed across four catchments, 367–770 ha in area, within Kangaroo River State forest, New South Wales (NSW). One hundred realisations were applied in the simulation process to provide spatial uncertainty and error estimates of soil erodibility. The results indicated that values simulated by the SGS algorithm produced similar K values for the neighbouring cells. At the pixel level, the SGS approach generated a reliable estimation of soil erodibility in most areas. Spatial variation of the K factor in this study was strongly related to soil landscape differences across the catchments; within catchments slope gradient did not have a substantial impact on the numerical values of the K factor using pixel-by-pixel comparisons of raster grid maps.     

The sulphur isotope systematics of the Taolin lead-zinc ore deposit,China

Sulphur isotope data from coexisting sulphides and sulphates from the Taolin Pb-Zn ore deposit have been used to estimate the temperatures of sulphur mineral precipitation. The data indicate that sulphide was the dominant sulphur species in solution at high temperatures and that sulphate was dominant at low temperatures. Also the data show that the δ³⁴S value of total sulphur in solution was close to zero at high temperatures (~325°C) but had high positive values (+15%.) at low temperatures (~250°C). We interpret this phenomenon in terms of the effects of mineral precipitation on the isotopic composition of the solution. The increase in the δ³⁴S value of total sulphur with decreasing temperature was brought about by the removal from the system, by precipitation, of isotopically light sulphides.     

Stock Origins of Subadult and Adult Atlantic Sturgeon, Acipenser oxyrinchus, in a Non-natal Estuary, Long Island Sound

The anadromous acipenserid Atlantic sturgeon Acipenser oxyrinchus was listed in 2012 under the U.S. Endangered Species Act as having four endangered and one threatened distinct population segment (DPS) in American waters. Anthropogenic activities outside of natal estuaries, particularly bycatch, may hinder the abilities of some populations to rebuild. Most Atlantic sturgeon are residential for their first 2–6 years within their natal estuaries, whereas older subadults and adults may migrate to non-natal estuaries and coastal locations. Previous studies demonstrated that subadults and adults aggregate during summer at locations in Long Island Sound (LIS) and its tributary, the Connecticut River; however, the population origin of these fish is unknown. Because of its geographic proximity and relatively robust population, we hypothesized that the LIS and Connecticut River aggregations were almost solely derived from the Hudson River. We used microsatellite nuclear DNA analysis at 11 loci and mitochondrial DNA control region sequence analyses to estimate the relative contributions of nine Atlantic sturgeon populations and the five DPS to these aggregations using individual-based assignment tests and mixed-stock analysis. From 64 to 73 % of specimens from LIS were estimated to be of Hudson origin. Similarly, 66–76 % of specimens from the Connecticut River were of Hudson origin. However, moderate numbers of specimens were detected from distant spawning populations in the southeastern DPS and from two populations once thought to be extirpated or nearly so, the James River (6–7.3 %), and the Delaware River (7.6–12 %). Additionally, specimens were detected from all five DPS in both the LIS and Connecticut River collections. These results highlight the difficulty of evaluating the status of individual Atlantic sturgeon populations because of the propensity of subadults and adults to migrate for extended duration to distant sites where they may be vulnerable to anthropogenic disturbances.     

Detecting trends in seagrass abundance using aerial photograph interpretation: Problems arising with the evolution of mapping methods

In the late 1970s and early 1980s an inventory of seagrass distribution in southeast Australia was made from multiscale aerial photographs using a simple analog mapping method, referred to here as theCamera lucida technique. Four priority estuaries were remapped in 2003, also using multi-scale aerial photographs, but using a geographic information system (GIS)-based analog mapping technique. We quantified the degree of error that arose when estimates of seagrass distribution were compared for the two techniques be ressessing. with GIS, the same aerial photographs used earlier for four estuaries: Port Hacking, Bermagui River, Merimbula Lake, and St Georges Basin.Camera lucida produced a larger estimate of total seagrass area than GIS in all four locations: 8% of total seagrass area or 12.9 ha in Port Hacking, 15% or 5 ha in Bermagui River, 20% or 38.0 ha in Merimbula Lake, and 243% or 502.2 ha in St Georges Basin. The consistent and moderate discrepancy in Port Hacking, Bermagui River, and Merimbula Lake was attributed mainly to differences betweenCamera lucida and GIS, such as the tendency ofCamera lucida to overestimate seagrass area by amalgamating disjunct seagrass patches into continuous meadows. The large discrepancy at St Georges Basin was attributed to interoperator error; i.e., the operators who mapped photographs taken in 1979 had widely differing interpretations of the aerial photographs. A 20-yr trend was calculated for each estuary using either theCamera lucida or revised GIS value as the start point. Perceived management status of the seagrass depended greatly on the initial data used. In St Georges Basin, a catastrophic decline (65%, 553.9 ha) occurred based on theCamera lucida-GIS comparison but only a small decline (14.7%, 51.8 ha) when all photos were GIS assessed, In view of the technical developments in habitat mapping, evaluation of change in seagrass abundance using remote sensing needs an assessment of intra-operator error, inter-operator error, and systemic error before maps produced by superseded technology are considered useful baselines.     

Evaluation of Genetic Diversity in Zostera japonica (Aschers. et Graebn.) for Seagrass Conservation in Brackish Lower Reaches of the Hii River System, Japan

This study examined the genetic diversity and composition of 15 populations of Zostera japonica in the Hii River system, Japan. Genetic and genotypic diversity were consistently higher in populations in the Ohashi River than those in Lake Nakaumi, and the local tidal movements may explain the observed patterns of genetic diversity. Pairwise fixation index values among the populations in Lake Nakaumi were higher than among those in the Ohashi River, even though the ranges of the distances among the populations in the two locations were almost the same. The gene flow from the Ohashi River populations was important for some populations in Lake Nakaumi. The low genotypic diversity and gene flow in Lake Nakaumi seemed similar to those of marginal populations. If this low genotypic diversity and genetic differentiation were partly caused by human activities, the promotion of gene flow should be taken into account in conserving the populations in Lake Nakaumi.     

The distribution of heavy metals in the hard clam,Mercenaria mercenaria,in the lower Chesapeake Bay region

Populations of the hard clam,Mercenaria mercenaria, were sampled at thirty sites in the lower Chesapeake Bay region in 1972 and 1973. Subsamples were taken for analysis for the trace metals cadmium, copper and zinc. Emphasis was placed on samples from the York and James rivers, two very productive and commercially utilized clam grounds. The levels of metals determined were comparable to those reported from other regions except Southampton Water in the United Kingdom. Differences in levels between the York and James rivers were statistically significant (P<0.001) which indicates that the James River probably suffers from contamination by these metals. One metal, copper, varied significantly with the age of the organism whereas the other two, cadmium and zinc, varied with salinity. InMercenaria the salinity of the sampling site must be considered before cadmium and zinc data can be interpreted. A standard action-level to denote pollution from copper would be adequate in this species.     

Seasonal and spatial water quality changes in the outflow plume of the Atchafalaya River,Louisiana, USA

The objective of this study was to examine the interaction between the Atchafalaya River and the Atchafalaya Delta estuarine complex. Measurements of suspended sediments, inorganic nutrients (NO₃ ^?, NH₄ ⁺, PO₄ ^3?), chlorophylla (chla), and-salinity were taken monthly from December 1996 to January 1998. These data were compiled by season, and the Atchafalaya River plume data were also analyzed using the Generalized Additive Model technique. There were significant decreases in NO₃ ^? concentrations during summer, fall, and winter as river water passed through the estuary, that were attributable to chemical and biological processes rather than dilution with ambient water. In some regions there were higher chla concentrations during summer and fall compared to winter and spring, when river discharge and the introduction of inorganic nutrients were highest, suggesting biological processes were active during this study. The presence of NH₄ ⁺, as a percentage of available dissolved inorganic nitrogen, increased with distance from the Atchafalaya River, indicative of remineralization processes and NO₃ ^? reduction. Mean PO₄ ^3? concentrations were often higher in the estuarine regions compared to the Atchafalaya River. During summer total suspended solid (TSS) concentrations increased with distance from the river mouth, suggesting a turbidity maximum. Highest chla concentrations were found in the bayous and shallow water bodies of the Terrebonne marshes, as were the lowest TSS concentrations. The low chla concentrations found in other areas of this study, despite high inorganic nutrient concentrations, suggest light limitation as the major control of phytoplankton growth. Salinity reached near seawater concentrations at the outer edge of the Atchafalaya River plume, but much lower salinities (<10 psu) were observed at all other regions. The Atchafalaya Delta estuarine complex buffers the impact of the Atchafalaya River on the Louisiana coastal shelf zone, with a 41% of 47% decrease in Atchafalaya River NO₃ ^? concentrations before reaching Gulf waters.