Breakdown and nutrient dynamics of submerged Phragmites shoots in the littoral zone of a temperate hardwater lake

Breakdown and nutrient dynamics of submerged leaf blades, leaf sheaths and culms of the emergent macrophyte, Phragmites australis, were studied in a littoral reed stand of a temperate eutrophic hardwater lake. Mass loss of culms was negligible during the first 6 months, whereas leaves and leaf sheaths lost 48 and 57% of their initial mass during that time. Exponential breakdown rates of leaves and sheaths were similar, with k-values ranging from -0.0033 to -0.0051 per day depending on litter type and considered period. These rates were significantly higher than the breakdown rate of culms (k = -0.0015 per day), even when only the period after the initial 6-month lag is considered (k = -0.0026 per day). The slower breakdown of culms may have resulted from the substantially lower nutrient concentrations of culm litter together with a high fiber content and highly sclerenchymatous tissues in the outer portions. Leaf blades and sheaths also differ in regard to chemical composition and anatomical structure, but these differences apparently had no net effect on breakdown rates. Concentrations of N and P increased almost continuously over time in all litter types, with maximum values of 1.7, 3.0 and 3.7% N and 0.06, 0.18 and 0.21% P recorded in culms, sheaths and leaves, respectively. Sheaths and particularly culms immobilized substantial amounts of both N and P, whereas only a slight P and no net N immobilization was observed in decomposing leaves. Overall, the results of this study indicate that different fractions of dead Phragmites shoots differ considerably in quality and may exhibit different breakdown rates, patterns, and nutrient dynamics. Consequently, different plant parts (including leaf sheaths and culms) should be examined separately for a detailed description of litter turnover and nutrient dynamics in emergent macrophyte stands. (C)2000 Elsevier Science B.V. All rights reserved.