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

Production and Fate of Transparent Exopolymer Particles in the Ocean

The production and fate of transparent exopolymer particles (TEP) have been investigated in various oceanic regions (tropical, temperate, and polar), from the sea surface microlayer (SML) to the deep ocean. Accumulation of TEP within the mixed layer was observed even in the absence of phytoplankton blooms, indicating abiotic processes are important in TEP production. The abiotic TEP aggregation rates measured in the tropical and temperate North Pacific and the Arctic Ocean averaged between 8 and 12 μmol C L-1 d-1. Depth profiles from under sea ice in the Arctic revealed the highest TEP concentrations, potentially released by sympagic algal activity at the bottom of the sea ice. The aggregation rates in the SML, the interfacial layer between the ocean and atmosphere, were generally enhanced over those in the bulk surface waters by factors of 2 to 30. This finding further strengthens a developing consensus on the gelatinous nature of the SML, which will also affect microbial life, light penetration, and surface wave properties. We present a conceptual model implying that abiotic aggregation is an important factor for TEP production in the ocean, in particular in sea surface microlayers, while consumption by zooplankton and protists recycle TEP, providing a new pool of dissolved precursor material. Overall, TEP is recycled within the water column through heterotrophic grazing and degradation, providing a new pool of TEP precursor materials, while enhanced aggregation rates of TEP in the SML indicates the importance of this thin surface film in the marine carbon cycle

Sensors and instruments for oceanic dissolved carbon measurements

Highly accurate and precise measurements of marine carbon components are required in the study of the marine carbon cycle, particularly when investigating the causes for its variability from seasonal to interannual timescales. This is especially true in the investigation of the consequences of anthropogenic influences. <br><br> The analysis of any marine carbon component requires elaborate instrumentation, most of which is currently used onboard ships, either in manual or automated mode. Technological developments result in more and more instruments that have sufficient long-term reliability so that they can be deployed on commercial ships, surface moorings, and buoys, whilst the great technological and operational challenges mean that only few sensors have been developed that can be used for sub-surface in situ measurements on floats, robots, or gliders. There is a special need for autonomous instruments and sensors that are able to measure a combination of different components, in order to increase the spatial and temporal coverage of marine carbon data. <br><br> This paper describes analytical techniques used for the measurement of the marine dissolved carbon components, both inorganic and organic: the fugacity of CO<sub>2</sub>, total dissolved inorganic carbon, pH, alkalinity, and dissolved organic carbon. By pointing out advantages, disadvantages, and/or challenges of the techniques employed in the analysis of each component, we aim to aid non-carbon marine scientists, sensor developers and technologists, in the decision of which challenges to address in further development

Bulk vs. aminoacid stable N isotope estimations of metabolic status and contributions of nitrogen fixation to size-fractionated zooplankton biomass in the subtropical N Atlantic.

A comparative analysis of natural abundance of stable N isotopes (δ15N) in individual amino acids and bulk organicmatter of size-fractionated plankton revealed the differential impact of nitrogen fixation through the food web in a transect across the subtropical North Atlantic. All δ15N measurements showed low values in the central region, followed by the western zone, while maximum δ15N values were found in the Eastern zone.These results were consistent with the prevalence of nitrogen fixation in the central and western zones, and the influence of the west Africa upwelling in the Eastern zone. Use of compound- specific amino acid isotope data (CSI-AA) revealed relatively low variability in the impact of diazotrophic nitrogen within the different plankton size fractions,while δ15N of bulk organic matter showed high variabilitywith size.Explicit CSI-AA trophic position estimates showed a small increase with mean plankton size class and varied in a relatively narrow range (1.8–2.5), with the lowest values in the central zone. High correlations between bulk plankton δ15N and individual amino acids (in particular Phe and Thr), as well as reconstructed total protein δ15N values,suggest a set of new relationships that may be important to tracing direct plankton contributions to nitrogen recycling in the ocean, including detrital organic nitrogen pools.Overall,these new results represent the most detailed investigation of CSI-AA data in plankton size clases to date, and indicated a greater importance of diazotrophic N than suggested by concurrent measurements of bulk δ15N, abundance of large nitrogen fixing organisms or nitrogen fixation rates.Malaspina-2010(CSD2008-00077), programa CONSOLIDER-INGENIO2010, NSF Grant OCE-1131816Versión del editor2,421

Mesozooplankton biomass and abundance in Cyprus coastal waters and comparison with the Aegean Sea (eastern Mediterranean)

Here we conduct the first comprehensive assessment of mesozooplankton abundance, biomass, and taxa composition in Cyprus coastal waters (Levantine Sea).  Mesozooplankton abundance and biomass sampled at several locations around the island ranged from 153 – 498 individuals m-3 and 0.7 – 5.2 mg dry weight m-3, respectively, with significantly larger biomass observed in winter-early spring (March) than in summer (September).  The community was dominated by calanoid and cyclopoid copepods throughout the year (80% of total numbers), with higher abundances of predatory taxa (chaetognaths and medusae) in winter and cladocerans in summer.  Overall, we find that coastal mesozooplankton communities around Cyprus appear to be more similar to communities in offshore waters or those around the island of Rhodes than to communities along the mainland Levantine coast.  We further highlight regional differences in the eastern Mediterranean by comparing our data with mesozooplankton in the western Aegean (Saronikos Gulf) and northeastern Aegean Sea (NEA).  Distinct spatial differences were observed, for example anthropogenic influences in the Saronikos Gulf and the outflow of Modified Black Sea Water in the NEA drove generally greater biomass and abundance in these regions.  Overall, our comparison supports the concept of a latitudinal gradient in oligotrophy in the eastern Mediterranean, with ultra-oligotrophic conditions found in the Levantine Sea

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

The oceans are an important global reservoir for mercury (Hg), and marine fish consumption is the dominant human exposure pathway for its toxic methylated form. A more thorough understanding of the global biogeochemical cycle of Hg requires additional information on the mechanisms that control Hg cycling in pelagic marine waters. In this study, Hg isotope ratios and total Hg concentrations are used to explore Hg biogeochemistry in oligotrophic marine environments north of Hawaii. We present the first measurements of the vertical water column distribution of Hg concentrations and the Hg isotopic composition in precipitation, marine particles, and zooplankton near Station ALOHA (22°45′N, 158°W). Our results reveal production and demethylation of methylmercury in both the euphotic (0–175 m) and mesopelagic zones (200–1,000 m). We document a strong relationship between Hg isotopic composition and depth in particles, zooplankton, and fish in the water column and diurnal variations in Δ199Hg values in zooplankton sampled near the surface (25 m). Based on these observations and stable Hg isotope relationships in the marine food web, we suggest that the Hg found in large pelagic fish at Station ALOHA was originally deposited largely by precipitation, transformed into methyl‐Hg, and bioaccumulated in situ in the water column. Our results highlight how Hg isotopic compositions reflect abiotic and biotic production and degradation of methyl‐Hg throughout the water column and the importance of particles and zooplankton in the vertical transport of Hg

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

The oceans are an important global reservoir for mercury (Hg), and marine fish consumption is the dominant human exposure pathway for its toxic methylated form. A more thorough understanding of the global biogeochemical cycle of Hg requires additional information on the mechanisms that control Hg cycling in pelagic marine waters. In this study, Hg isotope ratios and total Hg concentrations are used to explore Hg biogeochemistry in oligotrophic marine environments north of Hawaii. We present the first measurements of the vertical water column distribution of Hg concentrations and the Hg isotopic composition in precipitation, marine particles, and zooplankton near Station ALOHA (22°45â ²N, 158°W). Our results reveal production and demethylation of methylmercury in both the euphotic (0â 175 m) and mesopelagic zones (200â 1,000 m). We document a strong relationship between Hg isotopic composition and depth in particles, zooplankton, and fish in the water column and diurnal variations in Î 199Hg values in zooplankton sampled near the surface (25 m). Based on these observations and stable Hg isotope relationships in the marine food web, we suggest that the Hg found in large pelagic fish at Station ALOHA was originally deposited largely by precipitation, transformed into methylâ Hg, and bioaccumulated in situ in the water column. Our results highlight how Hg isotopic compositions reflect abiotic and biotic production and degradation of methylâ Hg throughout the water column and the importance of particles and zooplankton in the vertical transport of Hg.Key PointsMMHg bioaccumulated in fish is derived primarily from Hg (II) deposited in atmospheric precipitationMarine particles host the majority of Hg available for production of MMHg in the open oceanMethylation and demethylation of Hg occurs throughout the euphotic and mesopelagic zones in the North Pacific Subtropical GyrePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/150543/1/gbc20883.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150543/2/gbc20883_am.pd

Imaging Oxygen Distribution in Marine Sediments. The Importance of Bioturbation and Sediment Heterogeneity

The influence of sediment oxygen heterogeneity, due to bioturbation, on diffusive oxygen flux was investigated. Laboratory experiments were carried out with 3 macrobenthic species presenting different bioturbation behaviour patterns:the polychaetes Nereis diversicolor and Nereis virens, both constructing ventilated galleries in the sediment column, and the gastropod Cyclope neritea, a burrowing species which does not build any structure. Oxygen two-dimensional distribution in sediments was quantified by means of the optical planar optode technique. Diffusive oxygen fluxes (mean and integrated) and a variability index were calculated on the captured oxygen images. All species increased sediment oxygen heterogeneity compared to the controls without animals. This was particularly noticeable with the polychaetes because of the construction of more or less complex burrows. Integrated diffusive oxygen flux increased with oxygen heterogeneity due to the production of interface available for solute exchanges between overlying water and sediments. This work shows that sediment heterogeneity is an important feature of the control of oxygen exchanges at the sediment–water interface

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