The response of stream fish to local and reach‐scale variation in the occurrence of a benthic aquatic macrophyte

Abstract

Summary 1. The aquatic macrophyte Podostemum ceratophyllum has been shown to increase stream productivity, abundance and biomass of benthic invertebrates, and local occurrences of some stream fishes. However, experimental evidence that fishes preferentially associate with Podostemum is lacking, and the value of Podostemum as a predictor of stream fish assemblage composition has not been studied. 2. We conducted two short‐term (2 week), small‐scale (36 m2) experimental manipulations of Podostemum cover in the Conasauga River (Georgia and Tennessee, U.S.), and found higher abundances of benthic insectivorous fishes in patches with augmented (>80%) compared to reduced (7%) Podostemum cover. In an observational study, we quantified associations among percent cover of Podostemum, fish species richness, land cover, shoal length and base‐flow turbidity at 20 randomly selected shoals from a 39‐km reach that spanned a gradient of decreasing forest land cover. 3. Richness of all fish species and of lotic fishes peaked in the centre of the study reach, and richness was weakly correlated with predictor variables. Occupancy models for individual species also indicated that longitudinal position was a strong covariate for 13 of 19 species examined, with little support that Podostemum cover influenced occupancy. 4. Local associations may reflect choices by benthic fishes to utilise Podostemum, whereas downstream decline in fish species richness and Podostemum cover may reflect altered capacity of the system to support native species.

Use of recent and historical records to estimate status and trends of a rare and imperiled stream fish, Percina jenkinsi (Percidae)

“…cf. M. aestivalis), and tricolor shiner (Cyprinella trichroistia)), as well was a decline in the submerged aquatic macrophyte riverweed (Podostemum ceratophyllum) and an apparent increase in algal production in the Conasauga River, including extensive benthic algal blooms during two summers Argentina et al 2010;B.J. Freeman, personal observations).…”

“…We included a covariate on probability of detection to distinguish seine from snorkel survey methods. To evaluate evidence for the hypothesis that species were less likely to occupy shoals in the lower portion of the study reach (where declines in riverweed and other fish species appear most likely; Argentina et al 2010), we included a covariate on the probability of occupancy that indicated whether the site was located in the downstream-most portion of the study reach (Fig. 1).…”

Long-Term Monitoring Data Provide Evidence of Declining Species Richness in a River Valued for Biodiversity Conservation

“…However, elevated nutrient loading may be responsible for changes in water clarity and benthic habitats that we have observed in the Conasauga River. In particular, recent synoptic surveys along the mainstem have documented increases in water turbidity from ,2 nephelometric turbidity units in national forest, to .10 nephelometric turbidity units within our study reach (Argentina et al 2010b), even during periods of limited runoff. This contrasts sharply with conditions during the late 1980s, when at least two of the monitored shoals in our study reach had sufficiently low turbidity (e.g., ,5 nephelometric turbidity units) to permit snorkeling observations of Amber Darter habitat use (Freeman and Freeman 1994).…”

“…Forty-one of the 202 samples were repeated within-year surveys at particular study shoals, including 29 early summer (31 May-30 June) samples collected at most sites during the first 3 y of the study (1995)(1996)(1997) in addition to October samples in those same years. The remaining within-year replicates were 10 summer (June-September) and 2 spring (March) surveys made in conjunction with other studies (e.g., Argentina et al 2010b) using the same methods as described below.…”

“…Shifts in benthic habitat in the Conasauga study reach include loss of previously lush growths of the benthic macrophyte Podostemum ceratophyllum (Freeman and Freeman 1994). Podostemum provides habitat for shoal-dwelling fishes and their macroinvertebrate prey (Hutchens et al 2004;Argentina et al 2010b), and currently is sparse at most sites in the study reach in contrast to thick growths at sites farther upstream (Argentina et al 2010a;Baker 2012) and also in the majority of the Etowah River study sites. Finally, since about 2000, we have observed extensive mats of benthic algae in the study shoals during low-flow conditions of dry summers (Freeman et al 2007).…”