Effects of organic particle deposition on porewater oxygenation and oxygen exchange in cohesive sediment

Simone M., Grant J.

Visually-based approaches using techniques such as sediment surface images as well as sediment profile imaging (SPI) have been applied previously in the assessment of benthic impacts of organic enrichment. New visually-based benthic health indices (VBH and Surface Index) that integrate various visual properties was developed for muddy and sandier sediments, respectively. Visual indices were compared to the existing sulfide-based thresholds often used in environmental monitoring programs (EMPs). We utilized both techniques as well as geochemical indicators of eutrophication to detect benthic impacts at shallow shellfish and finfish farms in Nova Scotia, Canada. The visual indices were able to capture a non-sulfidic anaerobic condition in the sediments missed by the current sulfide-based system. Our results indicate that as presently configured, the visual based index will provide a more stringent standard in the EMPs that currently rely on a sulfide-based classification system, and thus affect regulatory judgement.

Riekenberg P.M., Oakes J.M., Eyre B.D.

Estuarine sediments are important sites for the interception, processing, and retention of organic matter, prior to its export to the coastal oceans. Stimulated microbial co-metabolism (priming) po...

Hamoutene D., Salvo F., Donnet S., Dufour S.C.

Finfish aquaculture can be installed over hard and patchy substrates where grab sampling is challenging and use of video can be an appropriate tool to document benthic changes. Video monitoring can show visual indicators of enrichment, namely flocculent matter, Beggiatoa-like mats, and opportunistic polychaete complexes (OPC). We examined factors influencing presence of indicators using 52 video monitoring reports collected in Newfoundland, Canada. The main driving factor was distance to cage, with indicators showing a higher probability of occurrence within 10m from cages due to low current velocities. Indicators were less prevalent on sites dominated by hard substrates while OPC in particular were restricted to depths >35m. Beggiatoa-like bacteria covered a larger surface than the two other indicators; however, our results suggest the necessity of amalgamating information related to all the indicators (including bare stations that could indicate anoxia) to establish a more accurate evaluation of aquaculture impact.

Dyksma S., Bischof K., Fuchs B.M., Hoffmann K., Meier D., Meyerdierks A., Pjevac P., Probandt D., Richter M., Stepanauskas R., Mußmann M.

Marine sediments are the largest carbon sink on earth. Nearly half of dark carbon fixation in the oceans occurs in coastal sediments, but the microorganisms responsible are largely unknown. By integrating the 16S rRNA approach, single-cell genomics, metagenomics and transcriptomics with 14C-carbon assimilation experiments, we show that uncultured Gammaproteobacteria account for 70–86% of dark carbon fixation in coastal sediments. First, we surveyed the bacterial 16S rRNA gene diversity of 13 tidal and sublittoral sediments across Europe and Australia to identify ubiquitous core groups of Gammaproteobacteria mainly affiliating with sulfur-oxidizing bacteria. These also accounted for a substantial fraction of the microbial community in anoxic, 490-cm-deep subsurface sediments. We then quantified dark carbon fixation by scintillography of specific microbial populations extracted and flow-sorted from sediments that were short-term incubated with 14C-bicarbonate. We identified three distinct gammaproteobacterial clades covering diversity ranges on family to order level (the Acidiferrobacter, JTB255 and SSr clades) that made up >50% of dark carbon fixation in a tidal sediment. Consistent with these activity measurements, environmental transcripts of sulfur oxidation and carbon fixation genes mainly affiliated with those of sulfur-oxidizing Gammaproteobacteria. The co-localization of key genes of sulfur and hydrogen oxidation pathways and their expression in genomes of uncultured Gammaproteobacteria illustrates an unknown metabolic plasticity for sulfur oxidizers in marine sediments. Given their global distribution and high abundance, we propose that a stable assemblage of metabolically flexible Gammaproteobacteria drives important parts of marine carbon and sulfur cycles.

Valdemarsen T., Hansen P.K., Ervik A., Bannister R.J.

In this study the environmental impacts of two fish farms located over deep water (180-190 m) were compared. MC-Farm was located at a site with slightly higher water currents (mean current speed 3-5 cms(-1)) than LC-farm (

Keeley N., Cromey C., Goodwin E., Gibbs M., Macleod C.

Keeley N.B., Forrest B.M., Crawford C., Macleod C.K.

This study evaluates five benthic indicators (total abundance, number of taxa, redox potential, total free sulfides, total organic matter) and ten biotic indices (Margalef's d, Peilou's J’, Shannon H’, AMBI, M-AMBI, MEDOCC, BENTIX, BOPA, ITI, BQI), to identify those that best define organic enrichment gradients under different flow regimes. Performance was measured against Enrichment Stage (ES), a continuous variable characterising the full range of sediment conditions (natural to azoic). None of the 15 metrics were able to consistently discriminate over the full enrichment gradient for both flow environments. The most versatile indices were BQI > M-AMBI > AMBI > Log( N ) > BENTIX. Of these, M-AMBI best catered for different flow environments, while the BQI was the most effective under highly enriched conditions. Under strong enrichment, i.e. when macrofauna abundance is in decline, changes in redox, sulfides, number of taxa and abundance were reasonably clear. However, the more complex biotic indices were relatively insensitive at this level, highlighting a limited applicability beyond the ‘peak of opportunists’ (PO). Conversely, in high flow regimes, some of the biological indicators were relatively sensitive to low-to-moderate levels of enrichment that were not well discerned by the physico-chemical variables. A useful subset of variables for assessing enrichment status is recommended, comprising two of the best performing biotic indices that are based on alternative/independent classification schemes (i.e. EG's and ES50 0.05 ), total abundance, to aid in discerning PO, and a geochemical variable (redox or S 2− ). Inconsistencies between metrics were found to be more significant than the variability surrounding the predictive capacity of individual indicators, and as a result there is a risk of ES misclassification where only a single index is used. Whilst there is a recognised need to use combinations of indicators, this study also stresses the importance of focusing on a few regionally validated measures and down-weighting the importance placed on any that are not. Additionally, although using a combination of different indicators may produce a ‘safe’ average result, it may be inefficient, and the averaging effect has the potential to mask extreme conditions. Hence, there remains a need for expert judgement to select and appropriately weight indicator variables, to identify any erroneous results, and to reliably assess ecological quality status.

Valdemarsen T., Bannister R.J., Hansen P.K., Holmer M., Ervik A.

We investigated the environmental impact of a deep water fish farm (190 m). Despite deep water and low water currents, sediments underneath the farm were heavily enriched with organic matter, resulting in stimulated biogeochemical cycling. During the first 7 months of the production cycle benthic fluxes were stimulated >29 times for CO(2) and O(2) and >2000 times for NH(4)(+), when compared to the reference site. During the final 11 months, however, benthic fluxes decreased despite increasing sedimentation. Investigations of microbial mineralization revealed that the sediment metabolic capacity was exceeded, which resulted in inhibited microbial mineralization due to negative feed-backs from accumulation of various solutes in pore water. Conclusions are that (1) deep water sediments at 8 °C can metabolize fish farm waste corresponding to 407 and 29 mmol m(-2) d(-1) POC and TN, respectively, and (2) siting fish farms at deep water sites is not a universal solution for reducing benthic impacts.

Joyce S.

After the great Mississippi River flood of 1993, the hypoxic (or low-oxygen) "dead zone" in the Gulf of Mexico more than doubled its size, reaching an all-time high of over 7,700 square miles in July of 1999. Scientists attribute the Gulf of Mexico dead zone largely to nutrient runoff from agriculture in the Mississippi River basin. During the warm months, these nutrients fuel eutrophication, or high organic production, causing large algal blooms. When the algae decay, the result is hypoxia. Reports of such hypoxic events around the world have been increasing since the mid 1960s. Eutrophication and hypoxia have resulted in mortality of bottom-dwelling life in dozens of marine ecosystems and have stressed fisheries worldwide. Some algal blooms can alter the function of coastal ecosystems or, potentially, threaten human health. Anthropogenic nutrient loading from sources such as agriculture, fossil fuel emissions, and climate events is believed to be related to the global increase in frequency, size, and duration of certain algal blooms.

Brackley H.L., Blair N.E., Trustrum N.A., Carter L., Leithold E.L., Canuel E.A., Johnston J.H., Tate K.R.

The rivers that drain active, collisional margins of the southwest Pacific deliver up to 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y − 1 ) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ 13 C, 14 C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate that terrestrial OC is being transferred across the continental margin, with plant sterols, long-chain alcohols and long-chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1–10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

Folk R.L.

A system of grain-size nomenclature of terrigenous sediments and sedimentary rocks is introduced wherein fifteen major textural groups are defined on the ratios of gravel, sand, silt, and clay. Further subdivision of each class is based on the median diameter of each size fraction present. Next, the mineral composition of terrigenous sedimentary rocks is considered. A triangular diagram is used to define eight rock types (orthoquartzite, arkose, graywacke, and five transitional types) based on the mineralogy of the silt-sand-gravel fraction and ignoring clay content. The writer contends that the current practice of calling all clayey sandstones "graywackes" is not valid, inasmuch as it represents a confusion of texture with composition. It is suggested that sedimentary rocks may be best defined by the use of a tripartite name, based on the following pattern-(grain size): (textural maturity) (mineral composition).

Glud R.N.

Abstract Benthic O2 availability regulates many important biogeochemical processes and has crucial implications for the biology and ecology of benthic communities. Further, the benthic O2 exchange rate represents the most widely used proxy for quantifying mineralization and primary production of marine sediments. Consequently, numerous researchers have investigated the benthic O2 dynamics in a wide range of environments. On the basis of case studies – from abyssal sediments to microbial phototrophic communities – I hereby try to review the current status on what we know about controls that interrelate with the O2 dynamics of marine sediments. This includes factors like: sedimentation rates, bottom water O2 concentrations, diffusive boundary layers, fauna activity, light, temperature, and sediment permeability. The investigation of benthic O2 dynamics represents a challenge in resolving variations on temporal and spatial scales covering several orders of magnitude. Such an effort requires the use of several complementary measuring techniques and modeling approaches. Recent technical developments (improved chamber approaches, O2 optodes, eddy-correlation, benthic observatories) and advances in diagenetic modeling have facilitated our abilities to resolve and interpret benthic O2 dynamics. However, all approaches have limitations and caveats that must be carefully evaluated during data interpretation. Much has been learned during the last decades but there are still many unanswered questions that need to be addressed in order to fully understand benthic O2 dynamics and the role of sediments for marine carbon cycling.

Preisler A., de Beer D., Lichtschlag A., Lavik G., Boetius A., Jørgensen B.B.

The ecological niche of nitrate-storing Beggiatoa, and their contribution to the removal of sulfide were investigated in coastal sediment. With microsensors a clear suboxic zone of 2–10 cm thick was identified, where neither oxygen nor free sulfide was detectable. In this zone most of the Beggiatoa were found, where they oxidize sulfide with internally stored nitrate. The sulfide input into the suboxic zone was dominated by an upward sulfide flux from deeper sediment, whereas the local production in the suboxic zone was much smaller. Despite their abundance, the calculated sulfide-oxidizing capacity of the Beggiatoa could account for only a small fraction of the total sulfide removal in the sediment. Consequently, most of the sulfide flux into the suboxic layer must have been removed by chemical processes, mainly by precipitation with Fe2+ and oxidation by Fe(III), which was coupled with a pH increase. The free Fe2+ diffusing upwards was oxidized by Mn(IV), resulting in a strong pH decrease. The nitrate storage capacity allows Beggiatoa to migrate randomly up and down in anoxic sediments with an accumulated gliding distance of 4 m before running out of nitrate. We propose that the steep sulfide gradient and corresponding high sulfide flux, a typical characteristic of Beggiatoa habitats, is not needed for their metabolic performance, but rather used as a chemotactic cue by the highly motile filaments to avoid getting lost at depth in the sediment. Indeed sulfide is a repellant for Beggiatoa.

Sayama M., Risgaard-Petersen N., Nielsen L.P., Fossing H., Christensen P.B.

ABSTRACT Experiments demonstrated that Beggiatoa could induce a H 2 S-depleted suboxic zone of more than 10 mm in marine sediments and cause a divergence in sediment NO 3 − reduction from denitrification to dissimilatory NO 3 − reduction to ammonium. pH, O 2 , and H 2 S profiles indicated that the bacteria oxidized H 2 S with NO 3 − and transported S 0 to the sediment surface for aerobic oxidation.

Kennedy P., Kennedy H., Papadimitriou S.

We investigated the effects of sample acidification on the stable carbon and nitrogen isotopic composition (delta13C and delta15N), as well as the organic carbon (OC) and total nitrogen (TN) composition, of an algal culture and a marine sediment. Replicate measurements of untreated and acid-treated samples were made using 1 M, 2 M and 6 M HCl, 6% H2SO3 and 1 M H3PO4. For all treatments the precision of the analysis for the acid-treated sample was equal to or less than that in the non-acidified sample. For the algae, analysis of variance (ANOVA) indicated no significant differences in the mean OC and TN concentration, or delta13C and delta15N composition, between any acid treatment and non-acidified samples. For the sediment sample a comparison could only be made between the different acid treatments because the untreated contained significant amounts ( approximately 30%) of carbonate carbon. ANOVA indicated that the mean OC determined in sediment samples after the 1 M HCl treatment and the mean delta13C values after the 6% H2SO3 and 1 M H3PO4 treatments were significantly different (p < 0.013 and < .05, respectively) from all other treatments. Mass balance calculations indicate that in some instances delta13C values were biased due to a contribution from unreacted carbonate carbon. There were no significant differences in the mean TN between any acid-treated and non-acidified samples. The mean delta15N values after 6 M HCl, 6% H2SO3 and 1 M H3PO4 treatments were significantly different from the untreated sediment sample (p < 0.044). Based on the significant bias observed for the delta15N and delta13C values, a weak (1-2 M) HCl solution is confirmed as the most appropriate acid for the removal of inorganic carbon from natural materials requiring elemental and isotopic analysis.