Title: Nitrogen processing by grazers in a headwater stream: riparian connections

Abstract

Summary Primary consumers play important roles in the cycling of nutrients in headwater streams, storing assimilated nutrients in growing tissue and recycling them through excretion. Although environmental conditions in most headwater streams and their surrounding terrestrial ecosystems vary considerably over the course of a year, relatively little is known about the effects of seasonality on consumer nutrient recycling these streams. In this study, we measured nitrogen accumulated through growth and excreted by the grazing snail Elimia clavaeformis (Pleuroceridae) over the course of 12 months in Walker Branch, identifying close connections between in‐stream nitrogen processing and seasonal changes in the surrounding forest. Nitrogen processing rates were positively correlated with ecosystem respiration, which was driven by leaf phenology on streamside trees. Snail nitrogen assimilation, growth and excretion were relatively high in spring before leaf emergence, low in summer when canopy shade was extensive and high again in autumn after leaf‐fall. During the time that snails grazed primarily on epilithon (winter, spring and summer), growth and excretion rates followed changes in light and epilithon biomass. In autumn, when snails primarily grazed fallen leaves, leaf‐associated microbes provided large subsidies of nitrogen for the snails. Nitrogen accumulation in snail biomass was greater in the 2 months followingmore » leaf‐fall than at any other time of the year. Snails were less important as nitrogen sinks than as sources of recycled nitrogen in Walker Branch. Over the course of the year, snails excreted approximately 12 times more nitrogen than they accumulated in biomass. Nitrogen accrued during growth in spring was subsequently lost in summer when primary production declined and snails underwent tissue loss. Catabolic losses represented >40% of the nitrogen excreted by the snails in summer. Net nitrogen growth efficiency (growth/assimilation), which varied with food availability, was only 8% for the entire year. Neither growth nor excretion was positively correlated with nitrogen concentrations in grazing substrata. Snails achieved high standing crops and were significant contributors to nitrogen spiralling in Walker Branch. On an areal basis, nitrogen in snail biomass (mgN m ⁻² ) was two to five times greater than that in epilithon biomass, depending on the season. Snails assimilated and excreted up to 50% of the nitrogen initially taken up by autotrophs and leaf microbes, and they were likely to have additional effects on nitrogen spiraling through egestion and the cropping of assimilative biomass. Primary consumers like Elimia are important catalysts of nutrient movement through headwater streams, decreasing residence times and facilitating fluxes to downstream waters.« less

Authors:

Hill, Walter R. ^[1]; Griffiths, Natalie A. ^[2]

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+ Show Author Affiliations

1. Univ. of Illinois, Champaign, IL (United States). Illinois Natural History Survey and Prairie Research Inst.
2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Climate Change Science Inst. and Environmental Sciences Division

Publication Date:

Wed Oct 19 00:00:00 EDT 2016

Research Org.:

Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Biological and Environmental Research (BER)

OSTI Identifier:

1354680

Alternate Identifier(s):

OSTI ID: 1401487

Grant/Contract Number:  

AC05-00OR22725; DE‐AC05‐00OR22275

Resource Type:

Journal Article: Accepted Manuscript

Journal Name:

Freshwater Biology

Additional Journal Information:

Journal Volume: 62; Journal Issue: 1; Journal ID: ISSN 0046-5070

Publisher:

Wiley

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; 59 BASIC BIOLOGICAL SCIENCES; excretion; grazers; growth; nutrient recycling; streams