Effects of detritus accumulation on the growth of Scirpus maritimus under greenhouse conditions

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Abstract
The effects of accumulation of dead belowground plant parts (detritus) of the emergent macrophyte Scirpus maritimus on the growth of this species were studied under greenhouse conditions. In the first experiment, the growth of S. maritimus seedlings decreased when the thickness of the detritus mat, covered by a layer of sediment, was increased. The position of a 5 cm thick detritus mat in a 15-cm column filled up with sediment had only a minor effect on the growth of this species. In the second experiment, fragmentation of detritus turned out to increase the growth-reducing effects found in the first experiment. Unfertilized seedlings with the lowest dry mass proportionally allocated most dry matter to their roots. The supply of nutrients either did not or only partly compensated for differences in plant growth among substrate types. In all instances the supply of nutrients strongly reduced the proportional dry matter allocation to roots, whereas differences in nutrient concentrations in plants diminished. In most instances the soil redox potential indicated a reduction of iron, but iron concentrations in the shoots never reached toxic levels. The most likely explanations for the poor growth of S. maritimus in its own detritus are the release of toxic compounds into the substrate and deficiency of oxygen to the roots due to reduced substrate conditions. Key words: freshwater lakes, emergent macrophytes, organic matter, degeneration.