Microbial activity and decomposition of fine particulate organic matter in a Louisiana cypress swamp

Fine particulate organic matter (FPOM) is an important source of C and energy in aquatic systems, but the decomposition of FPOM has rarely been studied. Litterbags were used to study the decomposition of particles in 2 size fractions of FPOM in surface sediments of a cypress swamp in southeastern Louisiana for slightly over 1 y. Particles in both size fractions followed linear decay models, but fine particles (0.25-1 mm) derived from the remains of coarser material decomposed more slowly than very fine particles (0.063-0.25 mm) that probably were derived from a number of sources. The activities of 6 microbial extracellular enzymes involved in the degradation of lignocellulose also were analyzed, and enzymatic decomposition models were developed to relate the decomposition of particles in each size fraction of FPOM to microbial activity. Very fine particles decomposed more rapidly than fine particles, although enzyme activities were lower, suggesting that this material was more efficiently degraded than fine particles. Enzymatic activity also was measured on particles in 2 size fractions of unconfined FPOM in sediment, and the enzymatic decomposition models were used to predict instantaneous mass-loss rates. Unconfined fine particles decomposed 1.5x faster than fine particles confined in litterbags, whereas unconfined very fine particles decomposed 2 to 4x faster than very fine particles confined in litterbags. Confining FPOM within litterbags clearly led to underestimation of decomposition rates, which might be more effectively determined using enzymatic decomposition models. The differences in microbial activity and decomposition rates between particles in different size fractions show that the concept of FPOM as uniform particles <1 mm in diameter is simplistic, and that a more accurate concept would recognize the distinction between different sizes of particles that might be derived from different sources.