Decomposition of aquatic angiosperms. II. Particulate components

Abstract

The decomposition of particulate organic matter (POM) from five species of freshwater vascular macrophytes was analyzed over a 180-day period under controlled conditions of temperature and oxygen (10 and 25°C; aerobic and anaerobic). In addition, a series of seasonal decomposition experiments were conducted in situ under different lake conditions. Plant species studied included the emergent Scirpus acutus Bigelow, the floating-leaved lily Nuphar variegatum Engelm., and the submersed Myriophyllum heterophyllum Michx., Najas flexilis (Willd.) Rostk. & Schmidt, and the submersed bulrush Scirpus subterminalis Torrey. At 2, 4, 10, 24, 48, 90, and 180-day intervals, organic weight, ash, carbon, nitrogen, total non-structural carbohydrates (TNC), and fiber components (lignin, cellulose, and hemicellulose) were determined under each of the experimental conditions. A model was developed that fit the observed weight-loss data, which demonstrated that decay-rate coefficients decreased exponentially with time in response to increasing resistance to decomposition of residual organic constituents. The fastest decomposing species, Nuphar, exhibited losses from 60 (cold, anaerobic) to 90% (warm, aerobic) over a 180-day period; the slowest, Scirpus acutus, lost only 20 and 40% under comparable conditions. TNC content decreased more rapidly under warm aerobic than under cold anaerobic conditions. Reduction of TNC was greatest in Nuphar, less in Myriophyllum and Najas, and lowest in the two Scirpus species. Structural carbohydrates decreased slowly among all species with the lignin fraction being the most resistant to degradation. However, decay rates of the macrophytes were negatively correlated to total fiber content rather than to individual fiber components (hemicellulose, cellulose, and lignin). Floating-leaved plants contained the least amount of structural tissue and decayed most rapidly, followed by submersed and then emergent species. Scirpus subterminalis, although submersed, retains the structural characteristics of the emergent species of this genus and decayed in much the same way as S. acutus. The C:N ratios of tissue of Scirpus acutus were higher than for other species and continuously declined during decomposition under warm, aerobic conditions. In this species, the C:N ratios decreased very slowly under cold, anaerobic conditions. The C:N ratios of the other species were constant after initial rapid declines. Among all species, the proportion of nitrogen in the dissolved organic and inorganic matter of the media decreased rapidly to low levels during decomposition at both low and high temperatures under anaerobic conditions; reductions were less when oxygen was available and decreased least in cultures of the two Scirpus species. Thus, resistance of particulate tissue to microbial decomposition was strongly influenced by the composition of the plant species. The floating-leaved plant decomposed faster than the submersed plants, which decomposed more rapidly than the emergent species. Decay rates were related to initial nitrogen and fiber contents, with high-nitrogen, low-fiber plants decomposing most rapidly. Unlike the leached dissolved organic matter (DOM) of the plants whose rate of decomposition was found to be mostly influenced by oxygen availability, the rate of conversion of particulate matter to carbon dioxide and/or DOM was regulated primarily by temperature, tissue nitrogen, and fiber content. All species showed similar trends in decomposition under both laboratory and in situ conditions, but the species-specific rates differed under the various laboratory conditions of temperature and oxygen.