Biological relations to DO minima
A problem inherent to threshold analysis is that it is rare in nature that one independent variable will influence biological assemblages to the degree that there are obvious changes in assemblage data that can be attributed to that variable. However, if viewed with an understanding of the direct and indirect influences that DO can have on species (abundance) with different tolerances (Hynes, 1960), the analysis of tolerance information for both assemblages can be used for indicating a general threshold.
Distributions for the three invertebrate and fish taxa of varying tolerance indicated that a DO threshold could be near 2.5 mg/L (Figure 5). Although the highest RA percentages for the three pair of taxa representing the three tolerance classifications were observed at different locations along the gradient of DO minima, RA percentage seemed to fluctuate dramatically at sites when DO minima declined to approximately 2.5 mg/L. More specifically, conditions >2.5 mg/L were much more favourable than conditions <2.5 mg/L for all but the extremely tolerant species.
DO concentrations <2 mg/L seemed to be inhibiting the two taxa selected to represent the moderately tolerant classification and also, but to a slightly lesser degree, the two taxa representing the tolerant classification. There was a fairly dramatic fluctuation in the RA percentage for both moderately tolerant taxa when DO minima were <2 mg/L. The pattern associated with the RA percentage of the two tolerant taxa, side swimmers and smallmouth buffalo, is typical of biological distributions that are subjected to intermediate disturbances (Connell, 1978). In theory, diversity and other measures of biological condition can be highest in response to moderately disturbed conditions (i.e. when DO minima were between 2.0 and 3.0 mg/L in this situation) as tolerant taxa begin to compete with other less tolerant taxa (Ward et al., 1983; Petraitis et al., 1989).
The pattern typical of the two extremely tolerant taxa, bloodworms and spotted gar, was most different of the three tolerance groups in that highest RA percentages for both taxa occurred at sites when DO minima were <2 mg/L (Figure 5). Extremely tolerant taxa that use atmospheric oxygen [e.g. mosquito larvae, (culicids)], store atmospheric oxygen [e.g. gar, bowfin (Amia calva), diving beetles (dytiscids) and back swimmers (notonectids)] or are capable of anaerobic respiration (e.g. tubificid worms and bloodworms) are often found in high abundances in hypoxic waters when other less tolerant invertebrate and fish taxa that compete with or predate on them are absent (Hynes, 1960; Gaufin, 1974; Justus and Harp, 1992). Related to the latter adaptation in particular, dominance by what is usually a small number of extremely tolerant taxa can result in exceedingly high numbers of individuals, especially in the case of invertebrates (Del Rosario et al., 2002; Ortiz and Puig, 2007).
Similar to plots for the three pair of taxa selected to represent the tolerance classifications, LOESS lines within scatterplots comparing taxa richness, diversity and total abundance metrics to DO minima also indicate that changes occurred in biological conditions when DO minima declined to approximately 2.5 mg/L (Figure 6). Detailed analysis of the scatterplot data associated to taxa richness, diversity and abundance with piecewise regression indicates that in most cases, statistically significance thresholds were found when DO minima were approximately 2.6 and 2.3 for invertebrates and fish, respectively.
For both assemblages, relations between DO minima and total abundance were weaker than relations between DO minima and taxa richness and diversity. Although, a few fish taxa have behavioural (e.g. positioning near the surface) or physiological (e.g. swim bladders that can function as lungs) adaptations that enable them to withstand low DO conditions, compared with invertebrates, there are relatively few fish taxa with adaptations that enable them to withstand hypoxic conditions. Our data indicate that fish abundance can be low or high when DO concentrations are near the estimated biological threshold.
The three metrics that were evaluated with piecewise regression—taxa richness, diversity and total abundance—were some of the first metrics used to describe biological assemblages (Gaufin and Tarzwell, 1956) and are precursors to many of the metrics that have been used for indices of biological integrity (Karr, 1981; Davis and Simon, 1995). It should be noted, however, that some overlap probably exists in the analyses with particular regard to diversity and taxa richness. Diversity is calculated with taxa richness and total abundance data, and it is certain that the three metrics will sometimes be correlated (e.g. for both assemblages, the Spearman rho correlation between taxa richness and diversity was approximately 0.80, and for diversity and total abundance was < 0.26). That being stated, the ability of the metrics for demonstrating the response of the two assemblages to DO minima was considered to exceed the negative aspect associated with a small part of the analyses being redundant.
DO thresholds would be expected to be below DO criteria commonly established for the protection of aquatic life but well above the minimum DO concentration that is lethal to species native to lowland streams. The average DO thresholds determined for the invertebrate and fish assemblage (2.6 and 2.3 mg/L) slightly exceed DO criteria that are being applied to some coastal streams in Louisiana and Texas. Louisiana has recently applied a minimum DO criterion of 2 mg/L for some coastal streams (Louisiana Department of Environmental Quality, 2009), and the Texas criterion for the daily minimum DO concentrations for some coastal streams is 2 mg/L (Texas Commission on Environmental Quality, 2007).
There are numerous references indicating that a large number of invertebrate and fish species are capable of tolerating DO concentrations of 1 mg/L (Moore, 1942; Doudoroff and Shumway, 1970; Davis, 1975; Kilgore and Hoover, 2001), which is slightly less than half of the average thresholds for the two assemblages. The time that fish can withstand low DO concentrations may depend on several factors (e.g. fish size, water temperature and behaviour). Doudoroff and Shumway (1970) reported that some species (e.g. bluegill, Lepomis macrochirus; orangespotted sunfish, Lepomis humilus; warmouth, Lepomis gulosus; and plains minnow, Hybognathus placitus) could tolerate DO concentrations around 1 mg/L for 18 h or longer when provided access to the surface, but survival was much lower when they could not access the surface.
DO criteria considerations for lowland regions and implications to land use
The impetus for investigating DO thresholds stems not only from the need to establish DO criteria but also because DO can be related to nutrients and other water-quality variables (USEPA, 2000; Robertson et al., 2001). Relatedly, there are efforts in many regions to establish links between DO concentrations and anthropogenic sources of nutrient enrichment (i.e. associated processes related to photosynthesis and decomposition). Much of the guidance for nutrient criteria assumes there is a strong, positive connection between nutrient water quality and the amount of vegetated buffer (USEPA, 2000); however, this association may not always apply to DO in lowland regions. Although DO minima generally had an inverse relation to the amount of agriculture in the buffer area, DO concentrations at three least-disturbed sites with low amounts of agriculture also declined to less than 2.5 mg/L. Ice and Sugden (2003) found that in the summer, almost 60% of the least-impaired or reference streams in forested streams of northern Louisiana had DO concentrations less than 3 mg/L. Thus, indications are that in some lowland settings, the link between DO and degree of aeration and organic decomposition (i.e. flushing, Mallin et al., 2006) will sometimes be stronger than the link between DO and stream–nutrient concentrations. Further, although DO may fall below a concentration known to impair biological assemblages, sources of this impairment will sometimes be related to natural settings.