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

Experimental studies of the deposit-feeders Heteromastus flliformis, 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 conservative tracer, one-dimensional transport models show that the effective diffusion coefficient, De, in the presence of these macrofauna is ∼2–5× the molecular diffusion value in the upper 8–12 cm of sediment. In general, the exact value of De is time dependent as shown 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 or 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 HC03− consumption at a rate \u3e24 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. Although no measurable effects of infauna on Si(OH)4 reaction constants could be demonstrated from pore water profiles, lowered concentrations result in higher net production rates, and sediment-water fluxes of Si(OH)4 increased in the presence of macrofauna by ∼1.4–1.6×, in agreement with theoretical models. Despite their limitations, the transport-reaction models and the two-dimensional or nonlocal parameterization models in particular, provide a consistent basis for description of the effects of macroinfauna on bulk sediment properties, and allow for comparison of different species at similar population abundances

Biogeochemistry of tube-dwellings: A study of the sedentary polychaete Amphitrite ornata (Leidy)

Most studies of near interface sediments assume that chemically and biologicalJy important properties are stratified vertically in a deposit. Sampling patterns reflect this assumption and little attention has been paid to three-dimensional heterogeneity. In this study the effects of burrow structures formed by Amphitrite ornata on the distribution of physical, chemical, and biological characteristics of a deposit are investigated...

Comparative biogeochemistry of water in intertidal Onuphis (polychaeta) and Upogebia (crustacea) burrows: temporal patterns and causes

The burrows of macrobenthos represent an important class of sedimentary microenvironments which significantly influence chemical, biological, and physical characteristics of a deposit. In intertidal regions, the time-dependent changes in composition during ebb-tide of water contained in burrows is a sensitive indicator of biogenic and abiogenic chemical reactions in burrow walls and adjacent sediment. Comparison of time series water samples taken from Onuphis jenneri (polychaete) and Upogebia affinis (crustacean) burrows in the same tidal flat demonstrate substantial differences in biogeochemical microenvironments despite the spatial proximity of the two species. Both types of burrows are influenced by the same general kinds of biogeochemical reactions but the relative intensity of these reactions differs in each case. Evidence for both heterotrophic and chemoautotrophic metabolic activity in each burrow type comes from the build-up or consumption patterns in burrow water of solutes such as NH4+, NO3−, Mn++, l−, HPO4−, and HCO3−. Burrow irrigation models and the stoichiometry of solute build-up imply that Upogebia burrows are sites of more intense nitrification-denitrification and microbial activity generally than are Onuphis tubes. Upogebia burrow water is also distinctly undersaturated with respect to carbonate minerals and has high numbers of bacteria relative to Onuphis. In addition to reaction rates, burrow geometry and the adsorption-diffusive permeability properties of the burrow wall also affect transient behavior of solutes. The organic burrow lining of Onuphis shows linear adsorption isotherms for positive, negative, and neutrally charged solutes represented by NH4−, HP04−, and Si(OH)4. Diffusion-reaction modeling demonstrates that adsorption, in particular, can significantly lower the transient state concentrations of burrow water trace solutes even for tube wall thicknesses of only 200 μm. The observed differences between burrow microenvironments of the two species living in close proximity suggest distinct biogeochemical associations between microbes and species specific biogenic structures