Decomposition of aquatic angiosperms. III. Zostera marina L. and a conceptual model of decomposition

Abstract

The decomposition of particulate and dissolved organic matter of a marine angiosperm, Zostera marina L., was followed over 180 days in the laboratory under three conditions of oxygen (aerobic, anaerobic, aerobic-to-anaerobic) at each of two temperatures (10 and 25°C). Samples of detrital material were taken after 2, 4, 10, 24, 48, 90, and 180-day intervals for analysis of organic weight loss, ash content, particulate carbon and nitrogen, total non-structural carbohydrates, hemicellulose, cellulose, lignin, and for UV absorbance, fluorescence activity, and total dissolved organic carbon (DOC) in the synthetic seawater media. The DOC analyses were performed on four molecular weight fractions separated by membrane ultrafiltration and on the whole fraction. In addition, the microbial biomass and activity associated with the detrital material was determined each sampling day by ATP content and dehydrogenase activity assays. Zostera tissue was found to be much more resistant to decomposition than were freshwater angiosperms examined concurrently under similar conditions. Significant weight loss over the study period occurred only under warm, aerobic conditions. Concentrations of fiber components of Zostera before decomposition were within the range of those of freshwater species studied. The concentrations of the fiber components (hemicellulose, cellulose, lignin) of the particulate detritus changed little. Likewise, particulate nitrogen was initially similar in value to that of freshwater plants, and changed in time depending on the physical conditions of decomposition, usually first increasing then declining. ATP content of the particulate detritus exhibited similar trends, namely increasing then declining values, over various time scales depending on decomposition conditions of temperature and oxygen availability. Production and utilization of dissolved organic matter (DOM) was greatest at the higher temperature, but more resistant DOM, as determined by increased UV absorbance and fluorescence activity of the material, persisted under these conditions. The increased resistance of Zostera marina to decomposition in comparison with several freshwater species cannot be explained by initial contents of nitrogen or fiber components, as was found for the latter plants. It is hypothesized that such resistance results from the ultrastructure of the tissue, and is due to adaptation of this species to a physically harsh environment. Based on data of this and previous studies, a conceptual model is described in which all forms of decomposition of naturally occurring plant detritus in aquatic ecosystems are proposed to advance, with respect to rates of decomposition, through three phases. Rates of decay are controlled primarily by temperature, dissolved oxygen, nutrient availability, particle size, and tissue resistance, and these parameters interact directly with each other to determine the myriad of decay rates observed under natural and laboratory conditions.