BUOYANCY AND GROWTH DYNAMICS OF FLOATING TYPHA MATS.

摘要

Floating mats formed by Typha (cattail) have become a problem in the management of dyked, freshwater impoundments in eastern Canada that were constructed to produce aquatic habitats for waterfowl. The relative importance of two mechanisms controlling mat buoyancy was investigated: (i) the living Typha and (ii) gas bubbles trapped in the dead organic material. The buoyancy contributed by Typha was 25 kg/m('2) in the 50 cm thick mats under study. This indicates that the contribution of Typha to mat buoyancy is important only on thin mats containing a low proportion of dead organic material.;The rate of organic matter deposition by Typha belowground components to floating mats was determined using a method designed to minimize errors caused by seasonal translocation of dry mass among the living organs. The estimated dry mass added (233 g m('-2) year('-1)) included approixmately equal quantities of material added by roots, rhizomes, shoots and shoot bases. The composition of these components in the organic material was consistent with the hypothesis that mats grow mainly by internal deposition of belowground organs rather than by accumulation of shoot material at the mat surface.;It is concluded that Typha mats are resilient systems that are unlikely to be altered substantially by disturbance events such as fire. The role of buoyancy as a factor controlling early peatland development and the observed trend toward ombrotrophic conditions on Typha floating mats is discussed.;Field and laboratory experiments were conducted to quantify seasonal variation in buoyancy and gas content of mats and to identify the major factors controlling this change. Experimentally isolated floating mats were most buoyant in late summer when water temperature in the mat was highest. Gas bubbles in the mat were composed mainly of nitrogen and methane. Mat samples incubated in the laboratory reached a gas content of 6-10% at 2(DEGREES)C and 4% at 22(DEGREES)C. Seasonal change in gas content, estimated at 2.5-4.7% of total mat volume, was caused by temperature-dependent change in the rate of anaerobic decomposition and gas solubility dynamics.