Diffusion of organic and inorganic solutes through macrofaunal mucus secretions and tube linings in marine sediments

Transport models in sediments commonly assume that diffusion occurs through water saturated pore space and that diffusive properties are largely homogeneous and isotropic. The bioturbated zone of marine sediments is characterized by sediment pores filled with mucus gel and criss-crossed by organic membranes that line macrofaunal tubes and burrows. Diffusion experiments utilizing pedal mucus from the naticid snails, Neverita (=Polinices) duplicata and Euspira (=Lunatia) heros, and organic tube linings from the polychaetes Onuphis jenneri, Diopatra cupria, and Chaetopterus variopedatus, demonstrated that the diffusion of both organic and inorganic solutes is inhibited by these common biogenic components. Diffusion of porewater DOC and Br− tracer through mucus is reduced by factors typically 3–8X relative to free solution. Diffusion rates of DOC and Br− through mucus and tube linings demonstrate that both charge and size inhibition commonly occur, however, charge discrimination was not observed for a range of inorganic solutes within mucus cements formed by the polychaete Melinna cristata. Diffusion of polystyrene sulphonates having varied molecular weights shows that inhibition of diffusion by mucus gel increases regularly with molecular size. No size exclusion or cutoff was observed up to molecular weights of at least 100 kDa. Although increases of solution viscosity by mucus (up to ∼ 170 mpoise), could explain solute diffusion inhibition to some extent, size and charge inhibition patterns imply that both mucus and tube linings behave as polyelectrolyte, fibrous meshworks with species specific properties (e.g. open channel patterns) rather than as polyelectrolyte solutions per se. The measured diffusion rates of bulk porewater DOC (0.387 cm2 d−1, 5°C) and of specific polystyrene sulphonates in sea water are substantially higher than predicted by extrapolation from measurements in distilled water, presumably as a result of ionic strength effects on molecular conformations. The transport of solutes, particularly DOC, in the bioturbated zone is greatly complicated by the presence of semipermeable mucus secretions and tube linings. Differential inhibition by biogenic secretions of the transport of specific classes of organic molecules such as exoenzymes, may be especially important to understanding faunal adaptations, processes governing the remineralization of organic matter, and linkages between macrofauna and microbial activities

Miniature thermistor chain for determining surficial sediment porewater advection

A miniature thermistor chain (mTc) was developed to measure the subdiurnal variability of temperature in the upper layers of subtidal coastal permeable (sandy) sediments and across the sediment-water interface (SWI). The mTc has 15 precision thermistors (0.002°C accuracy) attached by narrow tines to a stainless steel backbone that connects to an electronics module, all of which is buried in the top 20 cm of the sediment. Instrument performance was tested by deploying the mTc in nearshore permeable sediment at the Kilo Nalu Observatory, Oahu, Hawaii over an 80-d period. The mTc reached thermal equilibrium with the adjoining sediment within a few days after deployment and then recorded the advective propagation of the sub-daily water-column temperature variation into the sediment. The data produced are consistent with predicted effects of surface waves on advective porewater transport: transport rate increased with wave height and decreased with depth below the SWI, and temperature time lag increased with depth below the SWI. Data from an independent, more deeply buried thermistor are in good agreement with the mTc time-series data, showing attenuated temperature variability and similar (but longer, as expected) thermal time lags. Because thermal variations in surficial sediments is dominated by advection in wavy environments, mTc subdiurnal temperature propagation data can be used to calculate advective transport across the SWI and as deep as 20 cm into the sediment (i.e., over depths where advection dominates over thermal diffusion).This is the publisher’s final pdf. The published article is copyrighted by the American Society of Limnology and Oceanography, Inc. and can be found at: http://aslo.org/lomethods/index.html

Organic matter cycling and nutrient dynamics in marine sediments

Thesis (Ph.D.)--University of Hawaii at Manoa, 2008.This dissertation explores two recently recognized pathways of organic matter supply to marine sediments and their impact on sedimentary biogeochemistry: The entrapment and decomposition of particulate organic matter in permeable sediments and the resulting nutrient dynamics were investigated with a specially designed experimental laboratory microcosm that allows permeable sediment incubation under controlled physical forcing. Microcosm generated enhanced solute transport rates were roughly proportional to sediment column permeability. Comparison with field observations revealed that the enhanced transport rates induced by the experimental conditions were lower than those observed in the field, and this was also reflected in the enhanced buildup of pore water nutrients relative to concentrations in field sediments. Particulate organic matter and nutrient enrichment experiments, conducted with the microcosm, demonstrated the rapid uptake of both particles and solutes by the permeable sediment column under physical forcing, the rapid decomposition of the removed particles, and the lack of regenerated nutrient build-up. Nutrient enrichment experiments with sediment plug chambers demonstrated the rapid uptake and retention of nutrients by surface permeable sediments, as well as the loss of nitrogen though benthic denitrification in sediment grain microzones. The processing of large organic matter packages on the deepsea floor by deep-sea communities was investigated using natural and experimental whale, kelp and wood falls in the California Borderland Basins region, focusing on their quality and its impact on sedimentary organic enrichment and redox shifts. Labile organic material generated by the processing of whale falls and kelp falls generated substantial pore water sulfide levels in impacted sediment, despite a small organic enrichment signal. In contrast, wood derived material input did not result in sulfidic conditions, despite generating very high organic enrichment in impacted sediments, due to its refractory nature. A constructed model incorporating metazoan processing and dispersal of fragmented material revealed that whale fall processing is characterized by higher rates of tissue removal, organic carbon release, and carbon sedimentation rates than wood fall processing. However, organic carbon release rates, when normalized to fall weight, are similar between the two types of falls, due to differences in their carbon content and in absorption rates during metazoan ingestion.Includes bibliographical references.Also available by subscription via World Wide Web468 leaves, bound 29 c

The Role of a Permeable Sand Column in Modifying Tidal Creek Nutrient Inputs into the Coastal Ocean

Estuarine tidal creeks are an important conduit for freshwater run-off into the coastal ocean. In Long Bay, South Carolina, tidal creeks terminate in swashes—broad sandy fields constantly reworked by discharged creek water. We examined the role of a highly permeable sandy column in altering the nutrient loading of the passing water at Singleton Swash, Myrtle Beach, South Carolina. Seasonal transects along the swash’s primary channel documented gradients in physical and biogeochemical parameters. The nutrient and chlorophyll a concentrations were higher in the sediment than in the overlying water, consistent with coastal sediments as a major site of organic matter degradation, nutrient regeneration, and benthic primary productivity. Oxygen, nutrient, and chlorophyll concentrations exhibited a strong seasonal component, explained by a photosynthesis–respiration balance shift between summer and winter. The conservative mixing model approach to elucidate the sink–source patterns was moderately informative due to the lack of a gradual salinity gradient from land to ocean, due to substantial tidal flushing and observable nutrient-rich surface freshwater discharges along the channel that fueled substantial submerged aquatic macroalgal growth. Future studies should focus on the role of benthic photosynthesizers, both microbial and macroalgal, in retaining land-derived nutrients in irrigation freshwater inputs prior to them reaching the coastal ocean