Pierre Sagnes

A submersible device for measuring drag forces on aquatic plants and other organisms

Abstract This paper describes a device, the submersible drag gauge (SDG), that can be used for drag measurements in studies of flow‐biota interactions at low (setup A) and high (setup B) load applications. The device is designed so that solely drag forces acting on a test object in a boundary layer now are measured. The SDG can measure drag on organism(s), such as macrophtyes, crayfish or mussels, as well as a combination of such organisms and sediments assembled in a variety of arrangements in both laboratory and field conditions. Laboratory calibrations over the range of 0 to 4 N showed that setup A registered forces as low as 0.02 N, whereas setup B often did not register forces up to c. 0.15 N. The relative standard error of drag measurements was <3% for forces >0.2 N (setup A) or <4% for forces >1 N (setup B). Using a rigid cylinder, and natural and artificial (plastic) Egeria densa as examples, showed how the SDG can serve in studies of flow‐biota interactions such as drag‐velocity relations, assessments of drag coefficients, or the frequency spectrum of macrophyte drag.

Field evidence of reproduction impairment through sperm DNA damage in the fish nase (Chondrostoma nasus) in anthropized hydrosystems

This work aims to explore in the field the relationship between the integrity of sperm DNA and the quality of offspring as a possible cause of the decline of a feral fish population through reproduction impairment. Mature nase (Chondrostoma nasus) were caught during the breeding season in three locations (A-C) of the Rhône River basin and gametes collected by stripping. Sampling locations were chosen according to the following gradient of contamination due to human activities on the watershed: A≤B<C. Samples of a pool of collected oocytes were fertilized with the sperm of individual males and then incubated individually back in the lab to study embryo-larval development as well as using sperm samples to assess DNA integrity. Genetic analysis clearly showed the absence of a difference in genetic structure between the three studied C. nasus populations from the Rhône basin. Sperm DNA integrity was significantly lower in males from station C compared to other ones while sperm biochemical characteristics and fertilization rate remained almost unchanged whatever the station. Mortality and abnormality rates measured at both hatching and at the end of yolk sac resorption stages followed the same trend as the sperm DNA damage, demonstrating an impact of river water quality on nase fitness through a loss of sperm DNA integrity. Since the level of both abnormalities and mortality measured in offspring of fish caught in the most contaminated area reached high values up to 15% and 80%, respectively, the hypothesis that the observed nase decline in Rhône River stemming through selection forces can be put forward.

Effects of flume characteristics on the assessment of drag on flexible macrophytes and a rigid cylinder

Abstract Previous research, using rigid objects, showed that the magnitude of mean drag on submerged bodies can be influenced by the blockage ratio (ratio of the frontal projection area of an object to the wetted flume cross‐sectional area). In addition, drag measurements, under similar flow conditions, are likely to be affected by other flume characteristics. these factors lead to some uncertainty in drag measurement comparisons between identical objects for a given reference flow velocity. We measured drag on three pairs of replicate objects (two types of flexible artificial macrophyte and a rigid cylinder) in two flumes differing in width (0.25 and 0.75 m). Owing to different flume widths, the blockage ratios differed between the flumes for the macrophytes (c. 7% and c. 22% at intermediate flow rates) and the cylinder (2.5% and 7.5%). For theoretically identical mean cross‐sectional velocities, the between‐flume difference in mean centreline channel velocities across the range of flows used in this study was 6.4%, indicating similar bulk flow conditions for the two flumes. We found no between‐flume differences in drag‐velocity relationships for the cylinder, whereas one macrophyte had a distinctly higher drag in the narrower flume at low approach velocities (c. 0.2 m s–1 ). This difference of drag on the macrophyte likely indicates blockage effects in the narrower flume at low velocities, when plant bending and streamlining was low. At higher approach velocities, between‐flume differences in drag on the macrophytes were minor, except for one type of macrophyte that had a higher drag at the highest velocity in the wider flume (possibly related to plant reconfiguration and fluttering).