Stream Ecosystem Functioning Under Reduced Flow Conditions

Abstract

Assessments of flow reduction in streams often focus on changes to biological communities and in-stream physical characteristics, with little consideration for changes in ecosystem functioning. It is unclear whether functional indicators of ecosystem condition may be useful for assessing the impacts of reduced discharge on small streams. Using weirs and diversions to reduce stream discharge during summer baseflow conditions, we tested the response of leaf breakdown, coarse particulate organic matter (CPOM) retention, and primary production to one month of water abstraction in before-after, control-impact (BACI) designed experiments. Discharge at impact (downstream) reaches decreased by over 85% in each of three small New Zealand streams compared to controls (upstream). There also were decreases in velocity, depth, and wetted width. Sediment cover increased at impact reaches, but there were only small changes to conductivity, pH, and surface water temperature. We installed mesh bags filled with willow leaves in-stream for one month to measure leaf breakdown. Reduced discharge had little influence on leaf breakdown rate in these streams. Travel distances and retention structures for CPOM were evaluated using releases of paper strips and wooden dowelling over a range of discharges. The distance traveled by released CPOM increased with increasing discharge, and the importance of riffles as retention structures increased at lower discharges. We measured the accumulation of chlorophyll a after one month on artificial substrates as an estimate of the relative primary production of control and impact reaches. The differences in chlorophyll a concentrations between control and impact reaches were inconsistent among streams. These ecosystem functions have responded inconsistently to water removal in these streams. However, the strong response of CPOM retention to reduced discharge could complement measures of biological community structure when the influence of reduced discharge is assessed. We recommend further investigation in a wide range of streams to assess the utility of these processes as functional indicators of reduced discharge.

Drought impact on stream detritivores: experimental effects on leaf litter breakdown and life cycles

“…To our knowledge, few other studies have examined the effects of reduced flow or drought on ecosystem functions. Dewson et al (2007) reported little influence on leaf breakdown rate by reduced discharge. However, their study was performed in New Zealand streams where specialized leaf-shredding taxa are rare.…”

Mesocosm Experiments as a Tool for Ecological Climate-Change Research

“…5A). Correlational data on the effects of flow in running waters, however, are considerable, as it is a well-known central controlling variable in stream ecology (Beniston et al, 2007;Dahm et al, 2003;Daufresne et al, 2007;Dewson et al, 2007;Schindler and Donahue, 2006;Walters and Post, 2011) and a rich literature on the impacts of floods in lotic ecosystems has accrued since the 1980s, when there was a strong focus on understanding the role of flow refugia in determining community structure and dynamics (James et al, 2008;Statzner et al, 1988). Many of these studies are based on correlational field data, although some manipulative experiments and field bioassays were also conducted on the effects of (usually high) flow on the biota and their interactions (Gjerlov et al, 2003;Lancaster, 1996).…”

Extreme Climatic Events Alter Aquatic Food Webs

“…Experiments have been advocated as useful tools to explore the impact of unpredictable extreme events in natural systems (Jentsch et al, 2007), but research on drought and low flows in streams has tended to be phenomenological (but see, e.g. Dewson et al, 2007b;Walters and Post, 2011) and based on field surveys that are often confounded by environmental gradients or which lack the pre-impact data necessary to demonstrate causation (James et al, 2008). In the present study, we simulated hydrologic drought in a series of stream mesocosms over 2 years to capture intra-and intergenerational responses to habitat loss caused by periodic dewatering of benthic habitat (see Ledger et al, 2008Ledger et al, , 2011Ledger et al, , 2012Ledger et al, , 2013.…”