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

To determine how macrofaunal activity affects rates and mechanisms of Chlorophyll-a (Chl-a) decomposition, we measured Chl-a concentrations during laboratory incubations of surface sediment with varying abundances of a subsurface deposit-feeder, Yoldia limatula. Decomposition patterns of Chl-a in sediment cores with and without a layer of algal-enriched sediment added to the surface were compared. Decomposition rate constants, k(d), were calculated from the loss of reactive Chl-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 the number of Yoldia present. Yoldia increased the k(d) of both natural sedimentary Chl-a and algal enriched Chl-a in the upper 2 cm by up to 5.7X. Surface sediment porosity, penetration depths of a conservative tracer of diffusion (Br^-), and oxidized metabolic substrates (e.g. Fe(III)) all increased significantly in the presence of Yoldia. Macrofaunal bioturbation increased the importance of suboxic degradation pathways. These experiments demonstrated that organic compounds from a single source can have a continuum of degradation rate constants as a function of biogenically determined environmental conditions (Chl-a k(d) ~ 0.0043-0.20 d^-1). In particular, Chl-a can have a continuum of k(d) values related to redox conditions, transport, and macrofauna abundance as a function of depth.