The debate about drag and reconfiguration of freshwater macrophytes: comparing results obtained by three recently discussed approaches.

1. The paper by Sand-Jensen (2003, Freshwater Biology, 48, 271-283) on drag and reconfiguration of freshwater macrophytes stimulated comments and a reply about the use of variables in assessments of the drag coefficient (C-d) or the Reynolds number (Re) of such plants. Although the physical argument in this debate starts from the same equations, it diverges into approaches that address differently the dynamic behaviour of flexible plants in the boundary layer flow (the typical condition experienced by lotic macrophytes).

2. We compared three (potentially among many more) such different approaches using some preliminary experiments with Egeria densa by measuring drag (and other physical variables of interest) on (1) a single shoot exposed to varying flume flows; (2) a single shoot exposed to a constant flume flow, from which we subsequently pruned off pieces from the distal end; and (3) multiple shoots exposed to a constant flume flow after they had experienced replicated flow disturbances (causing shoot reconfiguration).

3. These experiments illustrated that the three approaches can produce opposite trends in the relationship between C-d and Re and that, for a given plant and flow, the C-d values obtained by these approaches can differ by about two orders of magnitude. Thus, conventions about the use of variables are required for experiments on drag and reconfiguration of freshwater macrophytes, otherwise the field will be plagued by a multitude of incomparable results.