Effects of the marine deposit-feeders Heteromastus filiformis (Polychaeta), Macoma balthica (Bivalvia), and Tellina texana (Bivalvia) on averaged sedimentary solute transport, reaction rates, and microbial distributions

Abstract

Experimental studies of the deposit-feeders Heteromastus filiformis, Tellina texana, and Macoma balthica demonstrate that, at natural population abundances, each species has major effects on sediment overlying water solute transport, bulk sediment reaction rates, and microbial distributions. Using Cl- as a conservation tracer, one-dimensional transport models show that the effective diffusion coefficient, De, in the presence of these macrofauna is .apprx. 2-5.times. the molecular diffusion value in the upper 8-12 cm of sediment. In general, the extact value of De is time dependent as show by time-course experiments with Heteromastus and by comparison of one-dimensional model predictions with a transient-state, two-dimensional cylindrical coordinate model. This latter model takes into account changes in diffusion geometry caused by irrigated burrow structures. The magnitude of apparent time variation in De depends on burrow abundance, size, and depth of burrowing; larger values of De are measured at longer times of tracer transport. In contrast, the simple nonlocal parameter required to mimic the two-dimensional model distributions is essentially constant with time and can be related to different solutes by the ratio of their diffusion coefficients. Models of pore water distributions demonstrate that the production rates of NH4+ in sediments are increased by at least 20-30% in the presence of macrofauna compared to controls of anoxic incubations, regardless of the model used. This is presumably due to the overall lowering of inhibitory metabolite concentrations as well as stimulation of bacteria during grazing. Total bacterial numbers increase at depth in the presence of Heteromastus and Tellina relative to controls but are depleted in fecal material. A substantial increase in ATP/bacteria ratios occurs in fecal and surface sediment, presumably indicative of active growth and conversion from anaerobic to aerobic metabolism. Apparent elevated numbers and stimulation of metabolic activity is consistent with a "microbial gardening" effect by macrofauna. Most of the NH4+ produced in experimental sediment was apparently oxidized at the sediment-water interface and, along with the oxidation of sulfide and metals, results in a major surficial zone of low pH and HCO3- consumption at a rate > 24 meq/m2/d. This should cause substantial dissolution of CaCO3 as shown in previous studies. A zone of elevated Si(OH)4 production is also associated with the redoxcline, but Si(OH)4 is otherwise produced at a sufficiently slow rate that detectable decreases in concentration occur in irrigated sediments.