The Atchafalaya River Basin Floodway (ARBF), a regulated river/floodplain distributary of the Mississippi River, experiences an annual flood pulse that strongly influences floodplain physicochemistry. We developed several metrics to investigate the relationship between the timing and magnitude of the flood pulse and floodplain hypoxia, which in most years is a spatially extensive and temporally prolonged problem in the lower ARBF. Principal components analysis of flood metrics from 2001 to 2009 revealed contrasting flood types (early cool and late warm), but component‐based general linear models were unable to predict the magnitude of hypoxia in ARBF water management areas (WMAs). Further analyses based on temperature and geographic information system‐determined WMA inundation with generalized additive models (GAMs) revealed WMA‐specific patterns of hypoxia, but the likelihood of hypoxia consistently increased when temperatures approached 20°C and inundation rose above 20–30%. Validation with held‐out data based on logistic regression indicated that the models constructed with the 2001–2009 temperature and inundation data were able to accurately predict the probabilities of hypoxia in two WMAs based on data collected from 2010 to 2013. The GAMs were an effective tool for visualizing and predicting the probability of hypoxia based on two easily generated parameters. Our analyses indicate that modification of the Atchafalaya River flood pulse could reduce the magnitude of hypoxia within the lower ARBF, subject to engineering (control structure operation) and economic (commercial fisheries production) constraints, by minimizing floodplain inundation after water temperatures reach 20°C. Copyright © 2015 John Wiley & Sons, Ltd.

We used conservative isotope tracers (deuterium and oxygen-18) and biologically relevant water quality measurements to assess connectivity of the Atchafalaya River to other waterways in its floodplain during the rising limb, peak, and falling limb of the 2011 flood pulse. We compared isotope tracers and water quality (dissolved oxygen and specific conductance) in biweekly samples at 83 sites in two areas that differed in their connectivity. We also compared tracers to an 8-year dataset of water quality measurements from the same sites. Although tracers clearly described differences in connectivity between the two floodplain areas and were correlated with concurrent measures of water quality, relationships were mediated by a strong temporal component and site-level variation in aquatic vegetation. Our results suggest a delay in floodplain water quality response to water inputs, and a strong influence of aquatic vegetation that locally overwhelms connectivity as a primary driver of local water quality.

We evaluated the relative effectiveness of continuous versus point electrofishing methods for describing fish assemblage metrics in the lower Atchafalaya River Floodway system, Louisiana. Continuous data were collected by boat electrofishing opposing shores of a 100‐m reach for a total of 480 s. Point data were collected in an adjacent 100‐m reach by boat electrofishing for 60 s at four points (240 s total) at 25‐m intervals on alternating shorelines. We tested for significant differences between the methods with a linear model for species richness and CPUE (fish/min) and a generalized linear model for species diversity. We also compared the length frequencies of Largemouth Bass Micropterus salmoides between methods. We found no difference in diversity between methods but significantly higher species richness (mean = 1.46 species) and significantly lower CPUE (mean = −24.63 fish/min) for continuous electrofishing. We found no difference in Largemouth Bass length frequency between methods, but point electrofishing captured more individuals between 300 and 400 mm. Our results indicate that point electrofishing may be an efficient, effective technique for capturing target species and larger individuals that may otherwise avoid capture. When coupled with continuous electrofishing, point electrofishing may bolster assemblage and population size structure data, particularly in shallow, turbid environments. Received July 9, 2014; accepted December 8, 2015 Published online March 31, 2016

Although rivers support significant unionid mussel (Unionida: Unionidae) diversity, Gulf of Mexico tributary rivers have been subject to changes in water quality and habitat due to altered watershed land use. We quantified mussel species richness and relative abundance and environmental factors in small tributary streams of the Pearl River, Mississippi-Louisiana. Freshwater mussel and habitat surveys were conducted at 27 stream sampling sites over two summers (9 sites revisited), and coverage of seven land use categories and seven geological categories above each reach were calculated. Mussels were patchily distributed (53% of sites sampled yielded mussels) and typically not abundant (only 26% of sites yielded >10 mussels). Surveys revealed nine species, with total abundance ranging from 0–66 mussels and richness ranging from 0–5 species per site. Assemblages were driven by an upper to lower watershed gradient of decreasing CPUE and richness, with microhabitat and water quality, land cover, and geology locally modifying this gradient. Environmental variables did not seem of sufficient magnitude to account for the patchy distributions and low abundances of mussels at most study sites, and we hypothesize that high discharge events related to tropical storm passage may have exerted an overriding influence on mussel assemblages in these streams through direct mortality and/or altered availability of suitable glochidial hosts.

Little is known about water movement, volume, or residence time (RT), and how those characteristics affect sediment trapping efficiency (TE) and dissolved oxygen concentrations (DO) in the United States' largest remaining bottomland hardwood swamp, the Atchafalaya River Basin. To better understand these dynamics, this study used bathymetry, lidar, and stage records to determine volumes in the Basin's hydrologically distinct water management units (WMUs). Discharge measurements determined flow distribution and RT. Residence time was compared with DO to identify conditions that coincided with DO increases or decreases. Suspended sediment concentrations (SSC) were used to determine TE relative to calculated and measured discharge and RT. Discharge through units (85–2,200 m3/s) and RT (0.37–231 d) depended on connectivity and river stage. At high stages, with water temperatures >20°C, DO in the largest WMU declined by −0.21 mg/l/day. DO trends indicated less well‐connected areas of the WMU contributed hypoxic waters as the flood wave lengthened and stages fell. In the two WMUs examined for TE, TE (−266% to 99% and up to 38 Gg/day) correlated with hydrologic connectivity, SSC, RT, water volume, and, in one WMU, discharge losses. Long RT and high TE indicated a high potential to process nutrients. These relationships varied among WMUs. Large volumes of sediment‐laden water moving over the floodplain combined with long RT, high TE, and hypoxia indicate that this ecosystem has continental‐scale importance in reducing nutrient loads to the northern Gulf of Mexico. Reports from other systems suggest similar processes may be operating on other large river floodplains globally.