Floodplains include some of the most productive habitats in the world and play a key role in nutrient retention and organic matter dynamics of riverine landscapes. The complex and dynamic array of aquatic, amphibious and terrestrial landscape elements; reates a considerable temporal and spatial heterogeneity, enables high biodiversity and increases hydrological retention.Lateral exchange processes between the main channel and slack water areas are crucial for the biogeochemical cycling in riverine landscapes. In functionally intact systems, the pulsing of river discharge determines the degree of connectivity and the exchange processes of matter and organisms across river floodplains. The net effect of floodplains on downstream water quality depends upon their hydrologic interaction with river water, as well as the nutrient flux and transformation processes that occur within them. More specifically, frequently connected side-arms that carry considerable flow could affect water quality in the main channel. Hydrological retention in side-arms is associated with lower flows and increased transparency of the water column and therefore, combined with nutrient inputs from the main channel, provides optimum conditions for enhanced algal primary productivity.
The biota in the river corridor depends upon two major sources of organic carbon: allochthonous organic matter from the catchment and autochthonous material derived from primary production within the aquatic environment. Most of the downstream transport of carbon involves refractory dissolved organic carbon (DOC) and small particles with low biological activity; carbon from autochthonous production, on the other hand, is commonly more labile and easier to assimilate and supports the food web. showed for the Orinoco River that the original C source for both invertebrates and fish is algae (phytoplankton and periphyton), even though macrophytes and litter-fall from the floodplain forest composed 98% of potentially available C. Thus, enhanced algal production is expected to be crucial for riverine food webs, especially in regulated systems.
The historically braided River Danube at Vienna, Austria, is now significantly altered and listed among a huge number of endangered braided river stretches. In Austria, for example, only 1% of the formerly braided river sections remains intact. As in all large rivers in Europe and North America, regulation and damming have led to habitat destruction and fragmentation and disrupted the structure and function of lotic ecosystems. Carbon transport and transformation processes in the river course are impaired. Particularly in braided rivers, like the Danube downstream of Vienna, restoration needs to focus on the availability of side-arms, specifically those with an active hydrologic exchange at a wide range of discharge conditions. This is a key approach to improving the ecological integrity of these riverine systems. As a major initiative in Austria, the ‘Danube Restoration Project’ (DRP) was developed in 1996 to enhance the hydrological connectivity between the main channel and former side-arms by increased the duration of lotic conditions and hydrologic exchange. The first step of the DRP was to rehabilitate the side-arm system of Regelsbrunn southeast of Vienna.
Before rehabilitation, identified three phases of river-floodplain connectivity in the Regelsbrunn side-arm system and linked them with ecological processes. Periods of disconnection were designated as ‘biological interaction phase’, where internal processes dominate and the ecosystem is mainly biologically controlled. Periods of inflow through seepage, with massive nutrient inputs, relatively high retention and therefore high algal biomass, were classified as the ‘primary production phase’. Finally, periods of surface connection, restricted to floods (where most particular matter transport takes place), were defined as the ‘transport phase’. The increased connectivity through restoration created a more gradual change between these phases. For example, the duration of surface connection between side-arm and main channel, and thereby the potential exchange of matter, was enhanced from 4% to 46% of the time. A promising way to investigate functional river-floodplain interactions is to measure the balance between transport and biological transformation processes.
The main objective of the present study was to describe the relative importance of autochthonous and allochthonous carbon within a restored side-arm and the impact of this side-arm system (Regelsbrunn) on the carbon dynamics of the Danube downstream of Vienna (Austria). The aims were (i) to explore how the hydrological stage affects biological activity and the contributions of the pelagic and benthic systems to total primary production and (ii) to assess to what extent the increase in hydrological connectivity due to the restoration alters the export of algal carbon to the main channel. Therefore, to quantify basic carbon transformation processes within the side-arm system, we measured primary production and respiration of pelagic algae and the whole side-arm community, and compared them with the total carbon transported in the main channel and the side-arm under contrasting hydrological situations.
Study site
At Vienna the Danube is a ninth order river and drains an area of 104 000 km2. The annual flow is characterized by an alpine regime with highly variable and stochastic patterns; the mean discharge is 1950 m3 s-1. The side-arm system between Haslau and Regelsbrunn is part of the semi-natural reach of the Danube between Vienna and Bratislava (48°7′N; 16°43′-16°47′E), which represents one of the last remnants of river-floodplain systems in Europe. Although strongly impacted by regulation measures, the key functional attributes of floodplains - a dynamic hydrology with flood and flow pulses and bed load transport - are partially operative. The area was designated as a National Park (‘Alluvial Zone National Park’) in 1996.
The side-arm at Regelsbrunn is dominated by a former river channel with a total length of 10 km, an average width of 100 m and an average depth of 1.5 m at mean river discharge. It was cut off artificially from the main river channel at the end of the 19th century, reducing the hydrological connection of side-arm and main channel to floods and subsurface water exchange. Several weirs divided the water body into distinct basins. These weirs contain culverts which allow flows between the basins when the water level in the main channel is high enough.
The hydrological connectivity and dynamics of the side-arm was restored in 1996. Lowering of the riverside embankments, additional artificial openings and lowered culverts within the system increased the hydrological connectivity of the side-arm with the main channel of the Danube from <10 to >200 days year-1 and provided a more continuous water flow within the side-arm system.
Hydrological conditions
The hydrology of the side-arm system is determined mainly by its upstream surface connection with the main channel. The frequency and duration of the connection, and the amount of inflowing water, depends on the water level of the river, the height of the inflow and the morphology of the river embankments.
At low water, the mean discharge of the Danube is about 1340 m3 s-1. Water inflow to the side-arm system is reduced to seepage of river water through the ground; this is equivalent to about 0.1% of the river discharge (c. 1.3 m3 s-1). Conditions in the side-arm system are essentially lentic at such times.
Surface connection with the main channel is established at 0.5 m below mean water level (MW), where the river discharge is about 1.700 m3 s-1. River water enters the system additionally via the restored inflow areas resulting in flowing conditions. About 0.8% (c. 13.6 m3 s-1) of the main channel discharge flows through the side-arm at MW level (Austrian River Authority, unpublished report). Approximately 120 days year-1 (mean 1997-2003), water level is in the range of MW ±0.5 m.
During floods, mainstream water overflows the river embankment and the whole floodplain becomes inundated. At a river discharge of 5000 m3 s-1, approximately 12% (c. 600 m3 s-1) of the main channel flow enters the side-arm. The rehabilitation changed the situation in the side-arm at medium water levels, from MW to 0.5 m to annual flooding height. Below and above, the conditions remained unchanged.
The software program ‘regels’ was used to calculate water inflow and various hydrological and morphological parameters, such as basin volume, water surface area, mean flow velocity and discharge within the side-arm. The program also computes an implicit groundwater infiltration rate, depending on water levels in the side-arm and the main channel and the distance to the main channel. The main output of the software is ‘water age’, which is used as a measure of residence time of the water within a multi-input side-arm system and is defined as how long on average the water has been in the side-arm; more details are provided.
Low age implies conditions in the side-arm like these in the main channel. The longer water spends in the side-arm, the more that biological processes in the open-water and benthic systems can change water quality. Hence, water age is an inverse measure of the connectivity to the main channel, with low age indicating high connectivity.
Sampling and chemical analysis
To model transport, we used data from several sampling stations in the main channel and the side-arm system from 1997 (the year the rehabilitation measures were finished) to 2003. In this study, water was sampled every 2 weeks from March to September 2003 in the main channel and the side-arm system. In addition, data from previous studies from 1997 to 1999, 70 samples per site in total were included.
Water samples were taken from 30 cm below the surface with 10-L polyethylene bottles. Within 4 h after sampling, 500-2000 mL were filtered through pre-combusted filters (Whatman GF/F; 2.5 h, 490 °C), dried at 60 °C for 24 h, and then re-weighed to determine total suspended solids (TSS). The filters were combusted (490 °C for 2.5 h) and weighed again to obtain the particulate organic matter (POM) and particulate inorganic matter. For analysis of DOC, 20 mL of filtrate were pipetted into acid-rinsed, pre-combusted glass vials. The DOC concentrations were determined with a Shimadzu TOC-5000 (Shimadzu Corporation, Kyoto, Japan)
No Comments, Comment or Ping