The influence of deposit-feeding on chlorophyll-a degradation in coastal marine sediments

To determine how macrofaunal activity affects rates and mechanisms of Chlor ophyll-a (Chl-a) decomposition, we measured Chi-a concentrations during lab oratory incubations of surface sediment with varying abundances of a subsur face deposit-feeder, Yoldia limatula. Decomposition patterns of Chi-a in se diment cores with and without a layer of algal-enriched sediment added to t he surface were compared. Decomposition rate constants, k(d), were calculat ed from the loss of reactive Chi-a and further quantified using a nonsteady state, depth-dependent, reaction-diffusion model. Values of k(d) decreased approximately exponentially with depth and were directly proportional to t he number of Yoldia present. Yoldia increased the k(d) of both natural sedi mentary Chi-a and algal enriched Chi-a in the upper 2 cm by up to 5.7X. Sur face sediment porosity, penetration depths of a conservative tracer of diff usion (Br-), and oxidized metabolic substrates (e.g. Fe(III)) all increased significantly in the presence of Yoldia. Macrofaunal bioturbation increase d the importance of suboxic degradation pathways. These experiments demonst rated that organic compounds from a single source can have a continuum of d egradation rate constants as a function of biogenically determined environm ental conditions (Chl-a k(d) similar to 0.0043-0.20 d(-1)). In particular, Chi-a can have a continuum of k(d) values related to redox conditions, tran sport, and macrofauna abundance as a function of depth.