Physico-chemical characterization of channel types in a glacial floodplain ecosystem (Val Roseg, Switzerland)

Abstract

With 10 figures and 5 tables in the text Abstract: Val Roseg in the Swiss Alps is a complex alluvial valley formed in glacial outwash. The braided flood plain, 2.6 km long and 130-510 m wide, begins 1.2 km downstream of the glacier terminus and extends to a "knickpoint" at 1990 m a.s.!. where water upwells before entering a constrained reach. A long-term study has been initiated to investigate habitat heterogeneity and how such heterogeneity (I) contribu tes to the biodiversity of benthos, groundwater fauna, and periphyton in a harsh envi ronment and (2) influences ecosystem processes such as productivity and decomposi tion dynamics. As a first step we have distinguished different channel types based on the correspondence between hydrological connectivity and physico-chemical attrib utes. This functional characterization will serve as a habitat template to structure future ecological research in the Val Roseg flood plain. Six distinct channel types have been identified within the fl oodplain ecosystem: (i) Main channel. (ii) Side channels, (iii) Intermittently-connected channels. (iv) Mixed channels, (v) Ground water channels, and (vi) Tributaries. Distinct seasonal and daily runoff patterns, caused by ice melt, change the hydrological connectivity between individual channel types. Results clearly demonstrate that the whole flood plain shifts from dominance by surface water at high summer discharge to a groundwater-controlled system in winter. Temporal variability, rather than the means of environmental values, has been used to differentiate between individual floodplain channel types. Groundwater chan nels exhibit the highest spatial but the lowest temporal variability. In contrast, inter mittently-connected channels are characterized by a low spatial but an extraordinary temporal variability. High spatio-temporal heterogeneity resulting from a diversity of channel types is believed to play a major role in maintaining what appears to be re markably high biodiversity in this glacial flood plain.

Nutrients and organic matter in a glacial river—floodplain system (Val Roseg, Switzerland)

“…In summer, concentrations can be as low as 0.1 mg L Ϫ1 . Exceptionally high DOC concentrations in surface water of the Roseg floodplain (i.e., 3.4 mg L Ϫ1 ) were measured once during a rainfall event that caused leaching of soils (Tockner et al 1997). Increasing DOC concentrations during groundwater-dominated base flow may also reflect in-stream production by algae, although this process was not evidenced by our mixing model.…”

“…Similarly, in many floodplains the combination of varying inputs from rivers and local basins result in large seasonal fluctuations in the nutrient composition. However, in contrast to the Val Roseg floodplain, where lentic water bodies are almost lacking, significant biological uptake of DIN and SRP occur in most temperate and tropical floodplains, such as the Amazon or the Danube (Forsberg et al 1988;Lewis et al 1990;Tockner et al 1999).…”

“…Three different types of water bodies (three sites of each type) were separated within the floodplain: (a) hillslope groundwater channels (disconnected waters fed by the hillslope aquifer; H1, H2, H3); (b) sites fed by either alluvial groundwater or subsurface seepage water from the main channel (subsurfaceconnected channels; A1, A2, A3), and (c) surface-connected channels, including one main channel site and two sites permanently connected to the main channel at their upstream ends (sites 7, U1, and U2). For comparison with published work, sites numbered 1 to 10 correspond to codes M-15, M-10, M-1, M5, M10, M12, M15, M20, M30, and M40 used in previous papers (Tockner et al 1997;Malard et al 1999Malard et al , 2000. Sites H1, H2, H3 correspond to G1, G5, G8; sites A1, A2, A3 correspond to G2, G3, and G4, and, sites U1 and U2 correspond to X3 and S3 .…”

“…Several distinct channel types can be distinguished within the floodplain, including the main channel and upstream connected side channels, subsurface-connected (seepage-fed) and disconnected (hillslope groundwater fed) channels. Tockner et al (1997) and Malard et al (1999) described the catchment and the floodplain in detail.…”

“…Glacial melt water carries high amounts of suspended matter, and specific conductance is generally very low. Proglacial floodplains, characteristic geomorphic features of many glacial rivers, have attracted only minor interest in alpine stream ecology until recently (Tockner et al 1997;Malard et al 1999;Ward et al 1999). There, the interaction between processes acting at the catchment scale (snow melting and glacial runoff) and the floodplain scale (e.g., upwelling of groundwater) increases the complexity of nutrient and organic matter dynamics.…”