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Corresponding to their biology and ecology, the fish specifically modified base flow transport and surface of the sediments. We integrate these results into a conceptual framework that outlines the contribution of benthic animals to the patch dynamics of the gravel and sand transport in real streams. Over the past 2 decades, ecology progressed through the assessment of the patch dynamics of plant and animal populations or ecosystems such as forests and streams [e.
Thus both stream ecology and hydrology currently operate within a similar conceptual framework. This physical focus in stream hydrology neglects the potential bioconsolidation i. Meanwhile, other studies have shown that benthic stream invertebrates and fish also play a role as geomorphic agents or ecosystem engineers as they may affect the bed load transport of gravel or sand through either bioconsolidation or bioturbation of the bed sediments.
For example, silk production of an insect larva causes bioconsolidation of the bed sediments [ Statzner et al. Thus Statzner et al. These two benthic fish differ in their behavior and habitat use that, within each species, also varies among size classes or age groups and seasons [e. Therefore our description of their biology and ecology Table 1 focuses on details that are essential for the understanding of our experiments.
In addition, barbel is social and bioturbates both gravel and sand when feeding, whereas gudgeon is less social and feeds only from sand Table 1. In addition, the temporal pattern of the base flow sediment transport caused by the social barbel should be more variable i.
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This latter hypothesis is derived from the observation that disturbing the social environment of a barbel e. From experiments with crayfish we knew that their base flow activities affect bed form, physical particle consolidation, proportion of sand to gravel in the bed surface, cover of surface sand by gravel, and surface cover by filamentous algae [ Statzner et al.
Saltveit et al. Relating our results to those obtained for other benthic stream animals we assess the contribution of barbel and gudgeon to the biologically induced patch dynamics of the gravel and sand transport in real streams. Here we communicate only the essentials required for the understanding of the present experiments.
Statzner et al. Each outdoor flume was tilted, having its highest elevation at the beginning of the first straight line of the oval and its lowest elevation at the end of the second straight line of the oval Figure 1. We used the energy of water pumped into the flume to bring water uphill to the highest flume point, and excess water was discharged at the lowest flume point toward a reservoir that was common for all flumes and pumps. We equipped the flumes with sediment traps five units of smaller traps per flume straight, four larger traps in other parts of each flume to assess the base flow transport of gravel and sand during the experiments.
Thereby, the outdoor flumes were equipped as shown in Figure 1. Although gravel came from two different suppliers, its size D 16 , D 50 , and D 84 , in mm was similar in both experiments 7. We used calibration curves to transform sediment measures based on volume displacement to dry weight [cf. Particularly along the second straight line, flow decreased considerably from the beginning to the end e.
Overall, shear stress was too low to erode significant quantities of gravel at base flow in both the barbel and the gudgeon experiment Table 2.
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These sand accumulations produced a series of smaller bed forms similar to a series of dunes that were less frequent in the barbel than in the gudgeon experiment. Because of the differences in shear stress, we also removed more sand from the flumes prior to the barbel than to the gudgeon experiment see section 2. Thus, overall, the gudgeon flumes had sandier beds than the barbel flumes. We treated the fish with malachite green oxalate to avoid fungal infections and acclimated them to the experimental water keeping them in cages in a larger experimental channel barbel or in the reservoir of the outdoor flumes gudgeon.
Fish from these stocks served to replace animals of equal size that died in the experimental flumes note that mortality rates were low over the entire experimental periods, Table 2.
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To achieve the stabilization of the sand i. Because of the different shear stress conditions in the flumes see above , we removed more sand from the flumes in the period preceding the barbel than the gudgeon experiment in preliminary tests of the installations, the sand amount removed at conditions prevailing in the barbel experiment was about twice as much as that at conditions prevailing in the gudgeon experiment. At the start of each experiment, we added the appropriate number of randomly selected fish to randomly selected flumes. Each morning we measured the gravel and sand volume found in each sediment trap, applying the procedures described in detail by Statzner et al.
As the sediments found in the traps would have been displaced downstream by the fish in the absence of the traps, we distributed the captured and measured material uniformly across a transect immediately downstream from the trap where it was found. However, this was not possible in the barbel experiment as the tubes connecting pumps and flumes got an increasing number of fissures so that one tube finally broke in the night from 29 to 30 September, and the resulting jet of water almost hit the electric control panel of the entire installation.
Although rapidly repaired, we considered the installations as unsafe and stopped the experiment the following day and used stronger connection tubes in the gudgeon experiment. As a result of this unplanned event, our barbel experiment mimicked conditions occurring downstream of hydropeaking power plants. The discharge of the pump feeding the common reservoir of the 10 outdoor flumes with groundwater was too low to compensate for the water lost through the broken tube.
Therefore, in the morning of 30 September, the water level in the reservoir was very low and the pumps feeding the 10 flumes produced only minor flow. As a consequence, the water in the flumes was very shallow, and parts of the flume bottoms were even dry. Because of this accident, the discharge in the 10 flumes must have been rapidly decreased from normal base flow to that one observed in the morning of 30 September.
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After the repair of the damage, the discharge toward all flumes was instantaneously adjusted to the experimental one. Thus the fishless and the barbel flumes experienced conditions that, in sequence, resembled those prevailing downstream of a hydropeaking power plant at the end and the beginning of electricity production [cf. To assess the barbel behavior during rapidly decreasing discharge at nighttime, we exposed a group of eight fish to such conditions during early night after the experiment and observed them using a weak red light.
For this purpose, we could use data from recent experiments on species interactions from the summer of , which included three fishless and three barbel flumes with physical conditions as in the accidental barbel experiment and lasted 12 d without accidents.
We stopped the discharge into the flumes and collected data describing the bed sediments. Because of the precipitated end of the barbel experiment, we only took measures of the bed elevation on a median line along the flumes, assessed from the distance between the flume bottom and a ruler placed across the flume and sampled surface sediments mixing gravel and sand from various locations in the straight lines per sample for subsequent chlorophyll analyses see below.
After the gudgeon experiment, we also sampled surface sediments for subsequent chlorophyll analyses and assessed the sediment surface as described in Statzner et al. When not working on the outdoor flumes during the 3 d after the experiments, we adjusted their flow to conditions that were too weak to transport any of the sediment fractions. The hydrodynamic balance was integrated into a gravel bottom of the laboratory flume.
We visually determined incipient particle motion, which requires a good definition of that event [ Buffington , ]. If surface algal cover was low, we ignored the departure of single particles and waited until general transport mobilization of most surface sand or gravel particles occurred.
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If surface algal cover was high, large parts of the algal carpet suddenly departed together with sand or sand and gravel; we considered this moment as incipient sediment motion. Because of the precipitated end of the barbel experiment, the planned improvements of the hydrodynamic balance after its first routine use [ Statzner et al. To gain time during the measurements, we did not measure the force that acted on the round steel rods of the balance from which flow separated when general transport occurred.
Instead, we calculated this force from data given by Statzner et al. In the gudgeon experiment, our hydrodynamic balance had profiled rods so that flow did not separate from them. Therefore the forces acting on the profiled rods were negligible and corrections were not required [cf. The extract was then acidified by adding two drops of HCl and remeasured at nm, to correct chlorophyll a absorption for that by phaeopigments.