Effects of spatial scale on assessment of dissolved oxygen dynamics in the Atchafalaya River Basin, Louisiana

Abstract

Low levels of dissolved oxygen (DO) are potential stressors of fishes and invertebrates in freshwater and marine systems. Because of the incredibly nutrient-rich and warm nature of the Atchafalaya River Basin (ARB), microbial decomposition in backwater areas with limited flow often results in potentially stressful, if not lethal, DO levels during and after the annual flood pulse. As water temperatures increase and flood levels fall, DO levels often stratify, restricting fish habitat. In order to better guide development of water management projects, we investigated the relationship between DO and DO stratification (DOD; surface DO -bottom DO) and a suite of environmental variables potentially linked to DO and DOD. Based on continuous monitoring data collected from 2000 to 2007, we used a variable selection procedure to construct statistical generalized linear models to describe the relationship between DO levels and DOD and several potentially important predictor variables at three spatial scales: the entire sampled area of the ARB; individual water management units (WMUs) or subunits (WMSs); and individual sampling sites. We conducted these analyses using models that assume the normal distribution and compared them to models that incorporate alternate error terms based on other error term distributions. Goodness of fit statistics suggested that our analyses best described the relationship between DO and structuring variables at the WMU/WMS scale. At this scale, water temperature, river stage height, current velocity, and to a lesser extent, water depth appeared to be the most important measured variables that influence DO levels. We suggest that management decisions at the WMU and WMS scales will offer the greatest potential benefits for improvement of DO conditions in the ARB.

Aquatic vegetation mediates the relationship between hydrologic connectivity and water quality in a managed floodplain

“…To test for correlations between isotopes and the concurrently measured water quality variables, data were paired by sample site and date for the 2011 flood period, with ''paired'' samples taken within a maximum of 7 days of each other, a time period that has been demonstrated to provide comparable water quality (Kaller et al, 2011). We followed the general procedure for constructing and selecting generalized linear models outlined by Bolker et al (2008).…”

“…9.3) with isotope concentrations as a response and explanatory variables consisting of contemporaneous DO, DOD, and specific conductance or of regime-based variables, based on results of the ordination. Quadratic water quality variables were included in the models because DO relationships with flooding are not linear (Kaller et al, 2011). Both sets of models (contemporaneous and regime-based) also included random variables: autoregressive sampling date, subregion nested within sampling date, sampling site nested within subregion, and aquatic vegetation coverage.…”

“…Ecosystem processes on the mainly forested floodplain are strongly influenced by the annual flood pulse, which significantly alters water quality (Sabo et al, 1999a, b;Kaller et al, 2011) and biotic production throughout the ARB (Davidson et al, 1998;Fontenot et al, 2001;Rutherford et al, 2001;Alford & Walker, 2013). The Atchafalaya River is a regulated distributary of the Mississippi River that carries 30 % of the combined flow of the Mississippi and Red rivers and receives comparatively little water from local rainfall.…”

“…The U.S. Army Corps of Engineers (USACE) manages the ARB floodplain as part of the Atchafalaya River Floodway System through water management units (WMUs) that vary in connectivity to the Atchafalaya River, depth and duration of flooding, and water quality (Sabo et al, 1999a, b;Kaller et al, 2011). Our comparison focused on two WMUs in the western portion of the ARB that vary in their connectivity to the mainstem Atchafalaya River .…”

“…Mitch. ), has a really extensive but temporally variable impacts on local, site-specific water quality in this system (Kaller et al, 2011). Expansive and dense beds of aquatic vegetation promote hypoxic conditions in the water column (Carpenter & Lodge, 1986;Colon-Gaud et al, 2004;Troutman et al, 2007;Turner et al, 2010a) because complete surface coverage, lack of wind-driven turbulence, and consumption of dissolved oxygen (DO) related to decomposition and respiration in dense beds overwhelm production by aquatic plants.…”