Summary
in situ cylindrical porewater sampler, with interval stackers and a nondegradable membrane, is described. Both horizontal and vertical gradients of dissolved constituents in porewater can be assessed. Sampling at 4-cm intervals within Myriophyllum spicatum root zones showed that steep depletion contours of dissolved phosphate occur, delimiting the zone of root influence.
Methodology
We report here a dialysis technique for the in situ measurement of concentration gradients of dissolved nutrients in macrophyte root zones. Using this technique, we have measured depletion of porewater phosphate both horizontally and vertically within Myriophyllum root zones. We thank D. Omond for technical and field assistance. S. Painter provided the idea for this study. G. Bayly drafted the figures. We sampled vertical porewater profiles using an in situ dialysis sampler of cylindrical design. By placing samplers at close intervals along a short transect passing out- ward from the Myriophyllum root crown, we could measure both horizontal and vertical gradients of dissolved nutrients within the root zone. The dialysis probe (Fig. 1) was a modified version of Hesslein’s (1976) design in which dissolved constituents diffused from the sediment porewater across a membrane and into deionized water in the sampler compartments initially free of the dissolved species. Externally, the probe was similar to that described by Mayer (1976), who used a perforated Lucite tube to house the dialysis materials. Our sampler consisted of a perforated Lexan sleeve (40 x 3 cm) with a solid tapered base and a threaded top portion which was capped. Additional sections of sleeve, threaded at both ends, could be joined to the main sampler body when deeper sampling was desired. The perforated sleeve helped protect the membrane surface inside.