Spatial and seasonal variation in the ecological significance of nutrient recycling by larval salamanders in Appalachian headwater streams Author(s): S. Conor Keitzer and Reuben R. Goforth Source: Freshwater Science, Vol. 32, No. 4 (December 2013), pp. 1136-1147 Published by: The University of Chicago Press on behalf of Society for Freshwater Science Stable URL: http://www.jstor.org/stable/10.1899/13-002.1 Accessed: 08-02-2017 15:48 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms Society for Freshwater Science, The University of Chicago Press are collaborating with JSTOR to digitize, preserve and extend access to Freshwater Science This content downloaded from 128.192.114.19 on Wed, 08 Feb 2017 15:48:23 UTC All use subject to http://about.jstor.org/terms Freshwater Science, 2013, 32(4):1136–1147 ’ 2013 by The Society for Freshwater Science DOI: 10.1899/13-002.1 Published online: 17 September 2013 Spatial and seasonal variation in the ecological significance of nutrient recycling by larval salamanders in Appalachian headwater streams 1 2 S. Conor Keitzer AND Reuben R. Goforth Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana 47907 USA Abstract. Salamanders are abundant consumers in many temperate streams and may be important recyclers of biologically essential nutrients, but their ecological role is poorly understood. The ecological significance of nutrient recycling by salamanders may vary spatially and seasonally because of their potentially patchy distribution in streams and the dynamic nature of stream hydrology and other nutrient fluxes. We examined the spatial and seasonal variation of salamander-driven nutrient recycling in 3 headwater streams in the southern Appalachian Mountains. We quantified the aggregate areal excretion + rates of N (NH4 -N) for the larvae of the 2 most abundant salamander species in these steams before and after leaf fall to examine spatial and seasonal variation in the supply of nutrients from salamanders. We used short-term nutrient additions in each stream to examine temporal heterogeneity in the ecosystem + demand for NH4 -N. Before leaf fall, salamanders were capable of meeting ,10% of the ecosystem + demand for NH4 -N and could turn over the ambient nutrient pool in ,3 km. The significance of this contribution declined to ,3% after leaf fall and the turnover length increased 73. The ecological significance of salamander nutrient excretion varied by as much as 173 within streams and was as high as 30% of the nutrient demand in some stream sections, a result suggesting that salamanders may create biogeochemical hotspots in these nutrient-limited ecosystems. Thus, salamanders appear to be capable of contributing substantially to stream nutrient cycles through the excretion of limiting nutrients and may be underappreciated members of headwater stream ecosystems, particularly at small spatial scales. However, this contribution varied substantially seasonally and spatially. Key words: consumer-driven nutrient recycling, amphibian ecological roles, headwater stream, salamander, Desmognathus quadramaculatus, Eurycea wilderae. Amphibian populations are declining globally streams, where their biomass can be .10 g/m2 (Davic (Stuart et al. 2004, Wake and Vredenburg 2008), and and Welsh 2004). They are environmentally sensitive this decline could have substantial consequences for species, whose abundance and diversity are expected terrestrial and aquatic ecosystems (Whiles et al. 2006). to change in response to multiple interacting factors However, the ecological roles of many amphibians are (Milanovich et al. 2010, Blaustein et al. 2011). poorly understood, so the ecosystem-level conse- However, the ecological implications of their potential quences of these population declines are difficult to decline or loss from stream ecosystems are largely estimate. For example, the catastrophic decline of unknown (Davic and Welsh 2004). anurans in tropical streams of Central America has Larval salamanders of stream-breeding species are had substantial effects on these ecosystems (Ranvestel generalist consumers of aquatic insects and reach et al. 2004, Connelly et al. 2008, Colon-Gaud et al. their highest abundance and diversity in small, 2009, 2010, Rugenski et al. 2012), but the importance fishless streams (Petranka 1998). Salamanders may of these species was largely unknown until many affect stream ecosystems by recycling biologically were in the process of local extirpation by disease important nutrients, such as N and P, a key ecosystem (Lips et al. 1996). Larval salamanders are the most process performed by many freshwater consumers abundant amphibians in many North American (Vanni 2002). By consuming biologically unavailable organic nutrients and releasing them in dissolved 1 E-mail addresses: [email protected] inorganic forms, freshwater consumers can supply 2 [email protected] nutrients to primary producers and heterotrophic 1136 This content downloaded from 128.192.114.19 on Wed, 08 Feb 2017 15:48:23 UTC All use subject to http://about.jstor.org/terms 2013] HETEROGENEITY IN SALAMANDER NUTRIENT RECYCLING 1137 microbes that form the base of aquatic food webs 2003, Cross et al. 2006, Greenwood et al. 2007, (Vanni 2002). Despite their high biomass in temperate Rosemond et al. 2008). Thus, salamanders may forests (Burton and Likens 1975b), salamanders may contribute substantially to headwater stream nutrient play a minor role as nutrient sinks in forest nutrient cycling, but the significance of this contribution may cycles (Burton and Likens 1975a, but see Milanovich vary both seasonally and spatially. 2010, J. R. Milanovich and M. E. Hopton, US We investigated nutrient recycling by larval sala- Environmental Protection Agency, unpublished data), manders in southern Appalachian headwater streams but their role as nutrient recyclers in streams is poorly to examine: 1) the ecosystem-level significance of understood. nutrient recycling by larval salamanders and 2) Larval salamanders are patchily distributed within spatial and temporal variation in the significance of streams as the result of abiotic factors, such as flow this contribution. We predicted that larval salaman- rate and stream substratum composition (Davic and ders would contribute substantially to nutrient cycles Orr 1987, Tumlison et al. 1990, Baumgartner et al. because their high densities would result in a large 1999, Barr and Babbitt 2002, Bruce 2003, Smith and supply of nutrients to heterotrophic bacteria and Grossman 2003), and biotic factors that include fungi that form the base of detritus-based food webs. competition and predation (Resetarits 1991, 1995, We expected that strong spatial variation in this Wiltenmuth 1997, Barr and Babbitt 2002, 2007, Lowe contribution would result from the patchy distribu- and Bolger 2002). Temporal patterns in larval sala- tion of salamanders and that seasonal changes in mander communities also can be influenced by other nutrient fluxes would mediate the ecosystem- species-specific life-history attributes because species level significance of this contribution. vary in the timing of hatching, length of larval period, size at metamorphosis, and subsequent timing of Methods stream emergence (Petranka 1998). This heterogeneity in salamander distribution would increase variability Study site in the rate they supply nutrients to stream ecosystems We conducted this study in 3 unnamed headwater both temporally and spatially with potentially impor- streams (hereafter, Streams A, B, and C) in the tant implications for stream nutrient cycling. For Nantahala National Forest, a large forested tract of example, the spatial distribution of consumers can land in the Appalachian Mountains of southwestern create biogeochemical hotspots and moments, which North Carolina, USA (lat 35u19N, long 83uW). In each are areas and periods of high nutrient cycling and stream, we established a 50-m study reach .20 m biological activity (McClain et al. 2003). Some evi- upstream from the nearest road crossing. Stream dence exists for spatial variation in stream consumer reaches were predominantly riffle habitat with an nutrient recycling (e.g., McIntyre et al. 2008, Benstead occasional pool and had mixed gravel, cobble, and et al. 2010, Moslemi et al. 2012). Temporal variation sand substrata. Streams were similar in size (mean 6 has been less well documented (Benstead et al. 2010), SE, wetted width = 1.66 6 0.09 m, depth = 8.02 6 but may also be important in determining the 0.67 cm, discharge = 3.89 6 1.02 L/s) and had low ecosystem-level significance of nutrient recycling by + ambient nutrient concentrations (NH4 -N = 6 6 salamanders. 0.92 mg/L, soluble reactive P [SRP] = 6 6 1.46 mg/ The ecological significance of nutrient recycling by L). Streams were heavily shaded by a dense canopy salamander communities will be mediated by spatial (,85%) of predominantly red maple (Acer rubrum), and temporal heterogeneity in other components of poplar (Liriodendron spp.), and oak (Quercus spp.) stream nutrient cycles, which can be considerable with a thick understory of rhododendron