Experimental studies of changes produced by deposit feeders on pore water, sediment, and overlying water chemistry

Abstract

Two separate sets of laboratory experiments were performed to study the changes produced by deposit-feeding organisms in marine sediment and overlying water chemistry. The organisms used: Clymenella torquata, a sedentary tube dweller, and Yoldia limatula, a mobile subsurface deposit feeder, are representative of two important and distinctive deposit-feeding groups. Both species produce radical changes in pore-water profiles of Fe and Mn and increase the sediment-water flux of these metals. Clymenella did not feed during the experiment. Alterations of pore-water profiles compared to controls in this case are best explained by changes, brought about by burrow construction and irrigation, in the geometry of molecular diffusion in sediment. A non-steady-state radial-diffusion model is used to characterize transport geometry. On the other hand, Yoldia limatula is highly mobile, and pore-water transport in its presence can be characterized by a non-steady-state, composite-layer model in which an effective or biogenic diffusion coefficient acts in the zone of feeding, and molecular diffusion controls transport in underlying sediment. Pore water and flux data show that the effective biogenic diffusion coefficient for pore-water transport by Yoldia is -1 X 10-3 cm2/sec, Modeling of interstitial metabolites and solid phase properties also shows that Yoldia increases the rate of microbial metabolic activity and associated reactions in sediment, but that alteration of effective diffusion rates is most important in controlling pore water profiles. Control experiments indicate that HPO4- and HCO3- are consumed near the sediment-water interface by adsorption on Fe-oxides and oxidation of sulfides respectively. CaCO3 is also dissolved in the same region , at a rate of -9 mg/g sediment-yr, producing a Ca2+ maximum in pore water. These abiogenic reactions are largely masked in the presence of Yoldia by increased biogenic diffusion. Solid-phase Mn profiles typical of natural deposits are found to form on time scales of a month and to be influenced by Yoldia in accordance with the chemical behavior expected for Mn and the changes in transport caused by Yoldia. Production rates of dissolved Mn are 2 to 4 times higher in the presence of Yoldia than in its absence. The transport of particles and pore water during feeding, burrowing, and irrigation of sediment by both Yoldia and Clymenella influences different pore-water and solid-phase constituents to a variable degree consistent with their individual chemistry. In general, the effect of both organisms is to speed the rate of transfer between chemical reservoirs while at the same time masking the presence of specific reactions which would otherwise be expressed in standing concentrations of dissolved ions in interstitial or overlying water.