Longitudinal patterns of metabolism in a southern Appalachian river

Abstract

Abstract. We investigated longitudinal patterns of ecosystem metabolism (primary production and respiration) at 4 sites along a 37-km segment of the Little Tennessee River (LTR), North Carolina. These sites corresponded to 4th-to 6th-order reaches in the LTR in an attempt to identify the transition from heterotrophic to autotrophic conditions in this river ecosystem. In addition, we compared autochthonous C production to supply of coarse organic material from direct litter fall and entrainment from the floodplain during floods to determine the contributions of each to river energetics on an annual basis. Metabolism was measured at several times of year at each site using the singlestation diel oxygen change method and reaeration estimated by the energy dissipation method. Gross primary production (GPP) ranged from 0.07 to 1.92 g C m Ϫ2 d Ϫ1 and increased ϳ3-fold from upstream to downstream. Respiration (R) ranged from 0.27 to 2.32 g C m Ϫ2 d Ϫ1 but did not change along the river continuum. Net ecosystem production (NEP) and P/R consistently showed that metabolism was heterotrophic in upstream sites and became autotrophic in the site furthest downstream. Calculated transitional P/R (i.e., where heterotrophic respiration is supported equally by autochthonous and allochthonous C sources) suggested that this heterotrophy-autotrophy shift occurred further upstream than where P/R ϭ 1. Annual rates of GPP were 3 times higher than litter fall and floodplain inputs of C, but R was higher than total C input suggesting that unmeasured C sources must be important for C dynamics in the LTR. The difference between measured C inputs and R decreased along the river continuum because of a 3-fold increase in GPP with little change in allochthonous input and R. Our results suggest that the LTR changes from heterotrophic to autotrophic along this stretch of river and that autochthonous C sources become more important for respiration and secondary production at downstream sites.

Organic matter breakdown and ecosystem metabolism: functional indicators for assessing river ecosystem health

“…For example, a key prediction of the river continuum concept is that GPP/ER should change in a predictable manner from the headwaters to the lower reaches of natural river systems (Vannote et al 1980). This prediction has been tested in many locations and generally has been supported by data (Table 2; Naiman 1983, Bott et al 1985, Chessman 1985, Naiman et al 1987, Minshall et al 1992, McTammany et al 2003, although some fundamental differences have been seen in grassland/prairie systems (Wiley et al 1990, Young andHuryn 1996) and in rivers with strong floodplain connections (Junk et al 1989, Meyer andEdwards 1990). Thus, metabolic rates at potentially impacted sites should be compared with rates measured at (more) pristine sites that are characterized by similar stream order and size.…”

“…The 2 nd approach compares values at test sites with Comments References GPP light limited, but not in grassland (P/R high) Junk et al 1989, Meyer and Edwards 1990, Wiley et al 1990, Young and Huryn 1996 P/R decreases downstream in grassland Young and Huryn 1996 Where strong floodplain connection, high organic input and ER (P/R,,1) Naiman 1983, Bott et al 1985, Chessman 1985, Naiman et al 1987, Junk et al 1989, Meyer and Edwards 1990, Minshall et al 1992, McTammany et al 2003 Algal biomass higher, GPP sometimes higher Flecker and Townsend 1994, Huryn 1998, Biggs et al 2000 Mainly based on the amount of light reaching the riverbed; affected by season, cloud cover, canopy cover, and turbidity Naiman 1983, Bott et al 1985, 2006, Webster et al 1995, Hill et al 2001, Acuñ a et al 2004, Ortiz-Zayas et al 2005, McTammany et al 2007 Only weak evidence Phinney and McIntire 1965, Bott et al 1985, Hill and Gardner 1987, Hedin 1990, Howarth et al 1992, Webster et al 1995, DeNicola 1996, Sinsabaugh 1997, Hill et al 1998, 2002 More organic matter Hedin 1990, Hill et al 1998 Algal production higher on large stable particles Rosenfeld and Roff 1991, Rier and King 1996, Biggs et al 2001…”

Abiotic controls and temporal variability of river metabolism: multiyear analyses of Mississippi and Chattahoochee River data

“…The importance of downstream heterotrophy has been confirmed for whole-river metabolism measured over shorter periods of time (Minshall et Our results also provide some support for the idea that autochthonous production could be an important food source in these large rivers, at least at some times of the year, given that we did regularly detect positive rates of GPP. Food webs of some large rivers are more reliant upon autochthonous C sources than would be expected (Thorp and Delong 2002), and GPP can increase with river size (McTammany et al 2003). We did not analyze relationships between metabolism and secondary consumer production, but during winter and early spring significant GPP occurred in both rivers.…”

The relative influence of nutrients and habitat on stream metabolism in agricultural streams

“…Numerous studies utilizing changes in DO and/or CO 2 to estimate primary production and stream metabolism have been conducted in a variety of environmental settings (Odum 1956;Hall 1972;Bott et al 1978Bott et al , 1985Bott et al , 2006Wiley et al 1990;Marzolf et al 1994;Young and Huryn 1996;Mulholland et al 1997;Biggs et al 1999;Young and Huryn 1999;Hall and Tank 2003;McTammany et al 2003;Houser and Mulholland 2005). Similarly, a small number of studies have analyzed the effect of increasing nitrate concentrations on the efficiency of biotic uptake and denitrification of nitrate, as related to ecosystem photosynthesis and respiration (Peterson et al 2001;Duff et al 2008;Mulholland et al 2008).…”