Habitat persistence for sedentary organisms in managed rivers: the case for the federally endangered dwarf wedgemussel (Alasmidonta heterodon) in the Delaware River

Abstract

Summary 1. To manage the environmental flow requirements of sedentary taxa, such as mussels and aquatic insects with fixed retreats, we need a measure of habitat availability over a variety of flows (i.e. a measure of persistent habitat). Habitat suitability measures in current environmental flow assessments are measured on a ‘flow by flow’ basis and thus are not appropriate for these taxa. Here, we present a novel measure of persistent habitat suitability for the dwarf wedgemussel (Alasmidonta heterodon), listed as federally endangered in the U.S.A., in three reaches of the Delaware River. 2. We used a two‐dimensional hydrodynamic model to quantify suitable habitat over a range of flows based on modelled depth, velocity, Froude number, shear velocity and shear stress at three scales (individual mussel, mussel bed and reach). Baseline potentially persistent habitat was quantified as the sum of pixels that met all thresholds identified for these variables for flows ≥40 m3 s−1, and we calculated the loss of persistently suitable habitat by sequentially summing suitable habitat estimates at lower flows. We estimated the proportion of mussel beds exposed at each flow and the amount of change in the size of the mussel bed for one reach. 3. For two reaches, mussel beds occupied areas with lower velocity, shear velocity, shear stress and Froude number than the reach average at all flows. In the third reach, this was true only at higher flows. Together, these results indicate that beds were possible refuge areas from the effects of these hydrological parameters. Two reaches showed an increase in the amount of exposed mussel beds with decreasing flow. 4. Baseline potentially persistent habitat was less than half the areal extent of potentially suitable habitat, and it decreased with decreasing flow. Actually identified beds and modelled persistent habitat showed good spatial overlap, but identified beds occupied only a portion of the total modelled persistent habitat, indicating either that additional suitable habitat is available or the need to improve habitat criteria. At one site, persistent beds (beds where mussels were routinely collected) were located at sites with stable substratum, whereas marginal beds (beds where mussels were infrequently collected or that were lost following a large flood event) were located in scoured areas. 5. Taken together, these model results support a multifaceted approach, which incorporates the effects of low and high flow stressors, to quantify habitat suitability for mussels and other sedentary taxa. Models of persistent habitat can provide a more holistic environmental flow assessment of rivers.

Influence of groundwater on distribution of dwarf wedgemussels (<i>Alasmidonta heterodon</i>) in the upper reaches of the Delaware River, northeastern USA

“…River slope and water depths were not measured at site 1. Maloney et al (2012) indicated that water depth, current velocity, and shear stress at site 1 are similar to site 2 during river discharge (less than about 100 m 3 s −1 ). Cole et al (2008) also indicated similar water depths between sites 1 and 2 in portions of the riverbed where DWM are known to be present.…”

“…Each site encompassed areas where DWM were found as well as similar adjacent or nearby areas where DWM had never been found. Previous studies at these same three sites investigated minimum flows and temperature stability (Cole et al, 2008) and modeled shear stress related to occurrence of DWM (Maloney et al, 2012). Site 1 extends along the right (descending) side of a mid-channel island (Fig.…”

“…Excessive shear stress also can reduce mussel populations (Hardison and Layzer, 2001;Layzer and Madison, 1995). Shear stress during high river stage in the three remaining DWMpopulated areas is smaller than the reach-averaged values (Maloney et al, 2012). Groundwater discharge may also be related to the location of mussel refugia (McRae et al, 2004), perhaps especially so in the upper Delaware River where groundwater discharge may locally diminish streambed dewatering (J.…”

Mollusc communities of lowland rivers and oxbow lakes in agricultural areas with anthropogenically elevated nutrient concentration

“…A higher number of mollusc species was recorded in the Wkra River, the width of which ranges from 0.5 to 60.0 m, than in its tributaries (width 0.8-15.0 m). According to Pérez-Quintero (2007) and MaLoney et al (2012, mussels, especially Unionidae, are sensitive to variation in hydrological conditions (lower-higher flows, velocity, river depth, bed stability) and prefer the permanent lowland watercourses of rivers. Thus, the higher number of unionid mussel species in the Wkra River may be explained by its less variable conditions compared to its tributaries.…”

“…In rivers, it may be the result of the interaction between alkalinity and the stream drainage area (DILLon & benfIeLd 1982), algal biomass, flow rate, substrate particle sizes, depth (crowL & schneLL 1990), hydrological conditions, or bed stability (MaLoney et al 2012). Distribution may also be a function of food supply (dILLon & benfIeLd 1982), conductivity, permanence, river width, turbidity (Pérez-Quintero 2011), environmental calcium concentration (BrIers 2003) or nutrient enrichment and periphyton abundance (HardIng et al 1999).…”