Lipids and pigments in deep-sea surface sediments and interfacial particles from the Western Crozet Basin

Deep-sea sediment samples were collected in the Western Crozet Basin (Indian sector of the Southern Ocean) through Permanently Open Ocean Zone (POOZ), Polar Frontal Zone (PFZ) and Sub-Antarctic Zone (SAZ). Lipid class and fatty acid compositions were investigated to determine the sources and fate of organic matter in the first centimeter of sediment and, above this layer, in the fluff (when present) and particles in the overlying water. The total lipid content varied from 74 to 1033 μg l⁻¹ in the overlying particles and fluffs, and from 24 to 97 μg g⁻¹ dry mass (DM) in surficial sediments. Lipid composition was always dominated by phospholipids in the first centimeter of sediment and often in the overlying particles. The amount of phospholipids (labile compounds representative of fresh material) was compared to the amount of chlorophyll a (Chl a), another compound that is susceptible to rapid degradation. A strong N–S gradient was observed in the distribution of these two compounds, which was attributed to the contrasting hydrodynamic of the study area. The high sedimentation rate in POOZ resulted in better preservation of Chl a in this zone than in other zones of the Crozet Basin (PFZ and SAZ). Phospholipid fatty acids suggested the presence of viable as well as morphologically intact organisms, and these organisms consisted essentially of bacteria with some diatom cysts in the fluff of POOZ. These spores were able to grow in the culture, indicating that they were still viable. Despite the strong hydrodynamic variability, phospholipid fatty acids analysed from the deep-sea surficial sediments were never representative of plankton. This pointed to the extremely labile nature of the phospholipids originally present in planktonic material compared with Chl a, which was always found in overlying particles and surficial sediments.     

Factors influencing fatty acid and hydrocarbon composition of sedimenting particles in the northeastern Adriatic Sea

Fatty acids and hydrocarbons of sedimenting particles were investigated in the northeastern Adriatic Sea from November 1988 to December 1989. Particles were collected at approximately monthly intervals, using sediment traps deployed at 30 m depth (2 m above bottom). Seasonal changes in sedimentation of particulate matter were very pronounced. Hydrocarbon fluxes and concentrations were found to vary significantly depending on the season. They averaged 2.69 ± 1.44 mg m⁻² day⁻¹ and 232.4 ± 90.93 μg g⁻¹ in winter, respectively. In late spring-early summer the corresponding values amounted to 0.045 ± 0.015 mg m⁻² day⁻¹ and 13.72 ± 5.56 μg g⁻¹, and they increased towards autumn, when mean values of 0.517 ± 0.228 mg m⁻² day⁻¹ and 98.86 ± 48.72 μg g⁻¹ were obtained. In contrast, fatty acid fluxes and concentrations were low during winter (0.26 ± 0.08 mg m⁻² day⁻¹ and 21.95 ± 3.35 μg g⁻¹), increased slightly towards the summer (0.48 ± 0.12 mg m⁻² day⁻¹ and 139.9 ± 44.6 μ g⁻¹) and reached maximum rate and concentration in autumn, when average values were 1.98 ± 1.30 mg m² day⁻¹ and 489.1 ± 186.7 μg g⁻¹, respectively. The differences in composition, concentrations and fluxes of the fatty acids and hydrocarbons were related to the sources of sedimenting material, reflecting the influence of resuspension of bottom sediments during winter and the appearance of mucus aggregates during summer and their subsequent deposition in autumn.     

The lipid geochemistry of antarctic marine sediments: Bransfield strait

Ten sections of a 10m long sediment core from Bransfield Strait, Antarctic region, have been examined for hydrocarbons, fatty acids, fatty alcohols, sterols, hopanoic acids, lignins and stable isotope ratios. The resolvable lipid compound classes generally occur in the following order of abundance: n-fatty acids > n-alkanes > n-alcohols > sterols > PAH with n-fatty acids comprising 50–80% of the lipids. The distribution of various lipid components indicate that they are principally from marine autochthonous sources, largely from diatoms and bacteria and to a lesser extent from dinoflagellates. There is no evidence from the lipid fraction of significant terrestrial plant input, which was also confirmed by the absence of methoxyphenols of the lignin degradation products. Aeolian transport from other continents appears to be minor or negligible. Minor amounts of epigenetic (mature) triterpenoids and polynuclear aromatic hydrocarbons found in all the samples are probably derived from submarine outcrops of weathered coaliferous shales. Stable carbon isotope ratios fall within a narrow range (−25 to −26‰), characteristic of Antarctic plankton. Coprostanol is the major stanol identified in the sediment core, most probably derived from the excreta of marine mammals, i.e. whales. Perylene, the dominant PAH, appears to be of marine origin in these Antarctic sediments. The presence of labile alkenes, unsaturated fatty acids and diploptene to a depth of 10m implies that little diagenetic alteration of the lipids has occurred. The sub-zero bottom water temperatures and the anoxic environmental conditions of this basin have preserved the total organic matter in the sediments, at least down to a depth of 10m.     

Meiofauna of the deep Eastern Mediterranean Sea: distribution and abundance in relation to bacterial biomass, organic matter composition and other environmental factors

Quantitative information on the abundance and biomass of metazoan meiofauna was obtained from samples collected at 15 deep-sea stations in the Eastern Mediterranean Sea (533–2400m). Meiofaunal abundance was compared to bacterial biomass and other environmental factors such as the total sedimentary organic matter content, the concentrations of the main biochemical classes of organic compounds (i.e. proteins, carbohydrates and lipids) and to ATP. To estimate the sedimentation potential of primary organic matter, sediment bound chloroplastic pigment equivalents (CPE) were assayed. Meiofaunal density was very low ranging from 4 ind.10cm⁻² (Station A4, 1658m depth) to 290 ind.10cm⁻² (Station A12, 636m depth). Nematodes were the numerically dominant taxon (68% of total meiofauna) and were usually confined to the top 6cm of the sediments. Total meiofaunal biomass ranged from 2.78μgC 10cm⁻² (Station A4) to 598.34μgC 10cm⁻² (Station 15A). There was a significant decrease in the density of metazoan meiofauna with water depth. Bacterial biomass largely dominated the total biomass (as the sum of bacterial and meiofaunal biomass) with an average of 73.2% and accounted for 35.8% of the living biomass (as ATP carbon) whereas meiofaunal biomass accounted only for 6.56%. Bacterial biomass was significantly related to the DNA concentrations of the sediment. A significant correlation between ATP concentration and CPE content was also found. No correlations were found between meiofauna, ATP and CPE, or between meiofauna and bacterial parameters. The significant relationship between meiofaunal density and the ratio of labile organic matter/total organic matter indicates that deep-sea meiofauna inhabiting an extremely oligotrophic environment (such as the Eastern Mediterranean) may be more nutritionally dependent upon the quality than on the quantity of sedimentary organic matter.     

Benthic microalgal biomass in sediments of Onslow Bay, North Carolina

Sediment samples were collected at stations along cross-shelf transects in Onslow Bay, North Carolina, during two cruises in 1984 and 1985. Station depths ranged from 11 to 285 m. Sediment chlorophyll a concentrations ranged from 0·06 to 1·87 μg g⁻¹ sediment (mean, 0·55), or 2·6–62·0 mg m². Areal sediment chlorophyll a exceeded water column chlorophyll a a at 16 of 17 stations, especially at inshore and mid-shelf stations. Sediment ATP concentrations ranged from 0 to 0·67 μg g⁻¹ sediment (mean, 0·28). Values for both biomass indicators were lowest in the depth range including the shelf break (50–99 m). Organic carbon contents of the sediments were uniformly low across the shelf, averaging 0·159% by weight. Photography of the sediments revealed extensive patches of microalgae on the sediment surface.Our data suggest that viable benthic microalgae occur across the North Carolina continental shelf. The distribution of benthic macroflora on the North Carolina shelf indicates that sufficient light and nutrients are available to support primary production out to the shelf break. Frequent storm-induced perturbations do not favour settling of phytoplankton, an alternative explanation for the presence of microalgal pigments in the sediments. Therefore, we propose that a distinct, productive benthic microflora exists across the North Carolina continental shelf.     

A laboratory and field comparison of sediment polycyclic aromatic hydrocarbon bioaccumulation by the cosmopolitan estuarine polychaete Streblospio benedicti (Webster)

Polycyclic aromatic hydrocarbon (PAH) bioaccumulation by the polychaete worm Streblospio benedicti (Webster) was measured under exposure to PAH-contaminated sediments in the field and for 28 d in the laboratory. Streblospio benedicti collected from field sediments contaminated at 2.94, 1.07, and l.52 μg g⁻¹ fluoranthene (FLU), benz[a]anthracene (BAA), and benzo[a]pyrene (BAP), bioaccumulated those PAHs at 1.53, 0.215, and 0.332 μg g⁻¹, while worms isolated from less contaminated field sediments (0.399, 0.228, 0.288 μg g⁻¹ FLU, BAA, and BAP) had FLU, BAA, and BAP body burdens of 0.543, 0.236, and 0.083 μg g⁻¹. Worms incubated for 28 d in PAH-spiked sediments (1.52, 0.991, 0.504 μg g⁻¹ FLU, BAA, and BAP) bioaccumulated those PAHs at 0.382, 0.966, and 0.602 μg g⁻¹, respectively. Data normalization to organism lipid and sediment organic carbon (biota-sediment accumulation factors [BSAFs] strongly suggest that Streblospio PAH bioaccumulation was directly related to percent sediment organic carbon, but BSAFs were substantially lower than predicted by equilibrium partitioning theory. BSAFs decreased with increasing PAH log K_ow, in worms collected from field sediments, but in spiked sediments BSAFs increased with increasing PAH hydrophobicity. This disparity may have been caused by insufficient spiked-sediment equilibration time (1.5h) in the case of the laboratory test sediments.     

Compositions and transport of lipid biomarkers through the water column and surficial sediments of the equatorial Pacific Ocean

A systematic investigation of fluxes and compositions of lipids through the water column and into sediments was conducted along the U.S. JGOFS EgPac transect from l2°N to l5°S at 140°W. Fluxes of lipids out of the euphotic zone varied spatially and temporally, ranging from ≈0.20 – 0.6 mmol lipid-C m⁻² day⁻¹. Lipid fluxes were greatly attenuated with increasing water column depth, dropping to 0.002-0.06 mmol lipid-C m⁻² day⁻¹ in deep-water sediment traps. Sediment accumulation rates for lipids were ≈ 0.0002 – 0.00003 mmol lipid-C m⁻² day⁻¹. Lipids comprised ≈ 11–23% of C_org in net-plankton, 10–30% in particles exiting the euphotic zone, 2–4% particles in the deep EgPac, and 0.1-1 % in sediments. Lipids were, in general, selectively lost due to their greater reactivity relative to bulk organic matter toward biogeochemical degradation in the water column and sediment. Qualitative changes in lipid compositions through the water column and into sediments are consistent with the reactive nature of lipids. Fatty acids were the most labile compounds, with polyunsaturated fatty acids (PUFAs) being quickly lost from particles. Branchedchain C₁₅ and C₁₇ fatty acids increased in relative abundance as particulate matter sank and was incorporated into the sediment, indicating inputs of organic matter from bacteria. Long-chain C₃₉ alkenones of marine origin and long-chain C₂₀-C₃₀ fatty acids, alcohols and hydrocarbons derived from land plants were selectively preserved in sediments. Compositional changes over time and space demonstrate the dynamic range of reactivities among individual biomarker compounds, and hence of organic matter as a whole. A thorough understanding of biogeochemical reprocessing of organic matter in the oceanic water column and sediments is, thus, essential for using the sediment record for reconstructing past oceanic environments.     

A comparison between the megafauna communities on the N.W. Iberian and Celtic continental margins—effects of coastal upwelling?

Megafauna biomass and feeding guilds were studied on the NW Iberian upwelling Continental Margin in order to determine the presence of enriched zones pointing to enhanced particle input. We compare these findings with similar data obtained from a transect across the Celtic Continental Margin that represents a regime without coastal upwelling. Additionally sediment concentrations of phytopigments (chlorophyll-a, phaeophorbides) representing recent inputs of algal production and of nucleic acids (DNA, RNA) are used as proxies for microbial biomass, to assess if there was a relation between these parameters and the megafauna distribution. The sediment on the upper slope (<1600 m) of the Iberian Margin was found to be inhabited by filter-feeding megafauna (26–73% of total invertebrate density, and 1–35% of biomass), and contained relatively low levels of phytopigments (3–6 ng/cm³ chlorophyll-a) and nucleic acids (12–16 μg⁻¹ DNA, 1.5–3.5 μg⁻¹ RNA). In contrast, on the upper slope of the Celtic Margin the dominant component of the megafauna were deposit-feeders (57–92% of total invertebrate density, and 23–90% of biomass) and the sediments contained higher concentrations of phytopigments and nucleic acid. These observations, supplemented by video records revealing the presence of current ripples on the Iberian upper slope, show that these upper slope regions are non-depositional, high energy environments. Conditions at the lower slope and the abyssal plain on the Iberian transect were more quiescent with large deposit-feeding holothurians dominating the megafauna (72–94% of invertebrate biomass), and with relatively high sediment concentrations of phytopigments (7–9 ng/cm³ chlorophyll-a, 157–170 ng/cm³ phaeophorbides) and nucleic acids (21–38 μg⁻¹ DNA, 2.4–5.5 μg⁻¹ RNA). On the basis of our data we argue that the benthic food for the deepest stations on the Iberian transect does not consist of shelf derived organic matter. More likely, fast sinking offshore blooms, possibly associated with filaments of upwelling water, form the major contribution to the annual food supply of the deep living megafauna.     

Influence of Particle Mixing on Vertical Profiles of Chlorophyll a and Bacterial Biomass in Sediments of the German Bight, Oyster Ground and Dogger Bank (North Sea)

In May and September 1999 11 stations were sampled in the southern and central North Sea, located in the German Bight, eastern Oyster Ground and Dogger Bank. The study focused on the influence of particle mixing on transport of chlorophyll a to deeper sediment layers and vertical bacterial distribution (max. DEPTH=10 cm). The sampling stations were chosen to reflect a gradient in environmental conditions in the North Sea. The sampling stations differed in respect to redox potential (eH up to −243 mV in the German Bight and up to 274 mV in the offshore regions), silt content (up to 54% in the German Bight and 0·34% at the northern Dogger Bank) and different proportion of fresh organic material on total organic matter content (C/N ratios ranging from 9·27 in the German Bight up to 1·72 in the offshore sediments). Although bacterial densities (8·55×10⁹ g⁻¹in the German Bight up to 0·35×10⁹ g⁻¹in offshore sediments) were significantly correlated to chlorophyll a content in the sediment (P<0·01), inconsistencies in the temporal pattern of both variables in the surficial sediment layer suggested, that the dynamics of bacterial densities is generally controlled by food supply but also by other variables. The chlorophyll a content in the surficial sediments of the German Bight (up to 1·84 μg g⁻¹) was significantly higher than in the Oyster Ground (up to 0·58 μg g⁻¹) and the Dogger Bank area (up to 0·68 μg g⁻¹). With increasing chlorophyll a input to the benthic realm a subsequent enhanced burial of this compound into deeper sediment layers was expected either by biological (bioturbation) or by physical sediment mixing. However, the vertical profile of chlorophyll a decreased steeply in the sediments of the German Bight. Contrary, subsurface peaks were measured in the offshore areas. It was concluded from these results, that the vertical distribution of organic matter in sediments is less limited by the quantitative input from the water column but concomitant with particle mixing itself. The extent and possible mechanisms of particle mixing in the different study areas in relation to specific environmental factors is discussed.     

A depocenter of organic matter at 7800 m depth in the SE Pacific Ocean

The Atacama trench, the deepest ecosystem of the southern Pacific Ocean (ca. 8000 m depth) was investigated during the Atacama Trench International Expedition. Sediments, collected at three bathyal stations (1040–1355 m depth) and at a hadal site (7800 m) were analyzed for organic matter quantity and biochemical composition (in terms of phytopigments, proteins, carbohydrates and lipids), bacterial abundance, biomass and carbon production and extracellular enzymatic activities. Functional chlorophyll-a (18.0±0.10 mg m⁻²), phytodetritus (322.2 mg m⁻²) and labile organic carbon (16.9±4.3 g C m⁻²) deposited on surface sediments at hadal depth (7800 m) reached concentrations similar to those encountered in highly productive shallow coastal areas. High values of bacterial C production and aminopeptidase activity were also measured (at in situ temperature and 1 atm). The chemical analyses of the Atacama hadal sediments indicate that this trench behaves as a deep oceanic trap for organic material. We hypothesize that, despite the extreme physical conditions, benthic microbial processes might be accelerated as a result of the organic enrichment.     

Aquatic surface microlayer contamination in chesapeake bay

The aquatic surface microlayer (SMIC), 50 μm thick, serves as a concentration point for metal and organic contaminants that have low water solubility or are associated with floatable particles. Also, the eggs and larvae of many fish and shellfish species float on, or come in contact with, the water surface throughout their early development. The objectives of this study were (1) to determine the present degree of aquatic surface microlayer pollution at selected sites in Chesapeake Bay, and (2) to provide a preliminary evaluation of sources contributing to any observed contamination.Twelve stations located in urban bays, major rivers, and the north central bay were sampled three times, each at 5-day intervals during May 1986. Samples of 1.4–4.1 each were collected from the upper 30–60-μm water surface (surface microlayer, SMIC) using a Teflon-coated rotating drum microlayer sampler. One sample of subsurface water was collected in the central bay.At all stations, concentrations of metals, alkanes, and aromatic hydrocarbons in the SMIC were high compared with one bulk-water sample and with typical concentrations in water of Chesapeake Bay and elsewhere. SMIC contamination varied greatly among the three sampling times, but high mean contaminant levels (total polycyclic aromatic hydrocarbons, 1.9–6.2 μg 1⁻¹; Pb, 4.9–24 μg 1⁻¹; Cu, 4–16 μg 1⁻¹; and Zn, 34–59 μg 1⁻¹) were found at the upper Potomac and northern bay sites. Three separate areas were identified on the basis of relative concentrations of different aromatic hydrocarbons in SMIC samples - the northern bay, the Potomac River, and the cleaner southern and eastern portions of the sampling area.Suspected sources of surface contamination include gasoline and diesel fuel combustion, coal combustion, and petroleum product releases. Concentrations of metals and hydrocarbons, at approximately half the stations sampled, are sufficient to pose a threat to the reproductive stages of some fish and shellfish. Sampling and analysis of the surface microlayer provides a sensitive tool for source identification and monitoring of potentially harmful aquatic pollution.     

Effect of iron enrichment on the dynamics of transparent exopolymer particles in the western subarctic Pacific

Dynamics of transparent exopolymer particles (TEP) was studied during the first in situ iron-enrichment experiment conducted in the western subarctic Pacific in July–August 2001, with the goal of evaluating the contribution of TEP to vertical flux as a result of increased primary production following iron enrichment in open ocean ecosystems. Subsequent to the enhancement of phytoplankton production, we observed increase in TEP concentration in the surface layer and sedimentation of organic matter beneath it. Vertical profiles of TEP, chlorophyll a (Chl a) and particulate organic carbon (POC) were obtained from six depths between 5 and 70 m, from a station each located inside and outside the enriched patch. TEP and total mass flux were estimated from the floating sediment traps deployed at 200 m depth. Chl a and TEP concentrations outside the patch varied from 0.2 to 1.9 μg L⁻¹ and 40–60 μg XG equiv. L⁻¹, respectively. Inside the patch, Chl a increased drastically from day 7 reaching the peak of 19.2 μg L⁻¹ on day 13, which coincided with the TEP peak of 189 μg XG equiv. L⁻¹. TEP flux in the sediment trap increased from 41 to 88 mg XG equiv. m⁻² d⁻¹, with 8–14% contribution of TEP to total mass flux. This forms the basic data set on ambient concentrations of TEP in the western subarctic Pacific, and evaluation of the effect of iron enrichment on TEP.     

Spatial and temporal variability of particulate matter in the benthic boundary layer at the N.W. European Continental Margin (Goban Spur)

Near bottom water samples and sediments were taken during five cruises to 6 stations forming a transect across the N.W. European Continental Margin at Goban Spur. Flow velocity spot measurements in the benthic boundary layer (BBL) always increased from the shelf to the upper slope (1470 m) from 5 to 9 cm s⁻¹ in spring/summer and from 15 to 37 cm s⁻¹ in autumn/winter. Decreasing values were detected at the lower slope (2000 m) and the lowest values of ca. 2 cm s⁻¹ at the continental rise at 4500 m water depth. Long term measurements with a benthic lander at 1470 m show that currents have a tidal component and reach maximum velocities up to 20 cm s⁻¹, sufficiently high periodically to resuspend and transport phytodetritus. During these long-term observations, currents were always weaker in spring/summer than in autumn/winter. Critical shear velocities of shelf/slope sediments increased with depth from 0.5 to 1.7 cm s⁻¹ and major resuspension events and Intermediate Nepheloid Layers (INLs) should occur around 1000 m. Chloroplastic Pigment Equivalents (CPE) ranged from 0.0 to 0.21 μg dm⁻³, Particulate Organic Carbon (POC) from 12 to 141 μg dm⁻³ and Total Particulate Matter (TPM) from 0.2 to 10.0 mg dm⁻³. Aggregates in the BBL occurred with a median diameter of 152 to 468 μm. Data on suspended particulate matter in the near-bottom waters showed that hydrodynamic sorting within the particulate organic fraction occurred. Phytodetritus was packaged in relatively large aggregates and contributed little to the total organic carbon pool in nearbottom waters (CPE/POC ca.0.2%). The main organic fraction has low settling velocities and high residence times within the benthic boundary layer. As POC was not concentrated in the near bed region the degree to which carbon is accessible to the benthic community depends on aggregate formation, subsequent settling and/or biodeposition of the POC. Close to the sea bed downslope transport may dominate. Under flow conditions high enough to resuspend fresh phythodetritus from sediments at the productive shelf edge, this could be transported to 1500 m (Goban Spur) or abyssal depth (Canyon site between Meriadzek and Goban Spur) within 21 days.     

Recent sediments, sediment accumulation and carbon burial at Goban Spur, N.W. European Continental Margin (47–50°N)

To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur results in along-slope and periodic off-slope directed transport of particles, possibly by entrainment of particles in a detached bottom or intermediate nepheloid layer. From the shelf to the abyssal plain the surface sediments on the Goban Spur change from terrigenous sandy shelf sediments into clayey silts. ²¹⁰Pb activity decreases exponentially down core, reaching a stable background value at 10 cm (shallower stations) to 5 cm (deeper stations) sediment depth. ²¹⁰Pb profiles of repeatedly sampled stations indicate negligible annual variability of mixing and flux. The ²¹⁰Pb_xs flux to the sediment shows a decreasing trend with increasing water depth. Below about 2000 m the average ²¹⁰Pb_xs flux is about 0.3 dpm cm⁻² y⁻¹, a third of the fluxes measured on the shelf and upper slope stations. Sediment mixing rates (D_b) correlate with macro- and meiofaunal density changes and are within the normal oceanic ranges. Lower mixing rates on the lower slope likely reflect lower organic carbon fluxes there. Mass accumulation rates on Meriadzek Terrace are at maximum 80 g m⁻² y⁻¹, almost twice as high as at Goban Spur stations of comparable depth. A minimum accumulation rate of 16.6 g m⁻² y⁻¹ is found at the Goban Spur upper slope break. Organic carbon burial rates are low compared to other margins and range from a lowest value of 0.05 g m⁻² y⁻¹ at the upper slope break to 0.11 g m⁻² y⁻¹ downslope. A maximum organic carbon burial rate of 0.41 g m⁻² y⁻¹ is found on Meriadzek Terrace. Carbonate burial rates increase along the northern transect from the shelf (13 g m⁻² y⁻¹) via a low (9.3 g m⁻² y⁻¹) on the upper slope break to the deep sea (30.7 g m⁻² y⁻¹). Carbonate burial is highest on Meriadzek Terrace (44.5 g m⁻² y⁻¹). The N.W. European Margin at Goban Spur and Meriadzek Terrace cannot be considered a major carbon depocenter.     

Seasonal changes and biochemical composition of the labile organic matter flux in the Cretan Sea

Downward fluxes of labile organic matter (lipids, proteins and carbohydrates) at 200 (trap A) and 1515 m depth (trap B), measured during a 12 months sediment trap experiment, are presented, together with estimates of the bacterial and cyanobacterial biomasses associated to the particles. The biochemical composition of the settling particles was determined in order to provide qualitative and quantitative information on the flux of readily available organic carbon supplying the deep-sea benthic communities of the Cretan Sea. Total mass flux and labile carbon fluxes were characterised by a clear seasonality. Higher labile organic fluxes were reported in trap B, indicating the presence of resuspended particles coming from lateral inputs. Particulate carbohydrates were the major component of the flux of labile compounds (on annual average about 66% of the total labile organic flux) followed by lipids (20%) and proteins (13%). The biopolymeric carbon flux was very low (on annual average 0.9 and 1.2 gC m⁻² y⁻¹, at trap A and B). Labile carbon accounted for most of the OC flux (on annual average 84% and 74% in trap A and B respectively). In trap A, highest carbohydrate and protein fluxes in April and September, corresponded to high faecal pellet fluxes. The qualitative composition of the organic fluxes indicated a strong protein depletion in trap B and a decrease of the bioavailability of the settling particles as a result of a higher degree of dilution with inorganic material. Quantity and quality of the food supply to the benthos displayed different temporal patterns. Bacterial biomass in the sediment traps (on average 122 and 229 μgC m⁻² d⁻¹ in trap A and B, respectively) was significantly correlated to the flux of labile organic carbon, and particularly to the protein and carbohydrate fluxes. Cyanobacterial flux (on average, 1.1 and 0.4 μgC m⁻² d⁻¹, in trap A and B, respectively) was significantly correlated with total mass and protein fluxes only in trap A. Bacterial carbon flux, equivalent to 84.2 and 156 mgC m⁻² y⁻¹, accounted for 5–6.5% of the labile carbon flux (in trap A and B respectively) and for 22–41% protein pool of the settling particles. These results suggest that in the Cretan Sea, bacteria attached to the settling particles represent a potential food source of primary importance for deep-sea benthic communities.     

A piscine glutathione S-transferase which efficiently conjugates the end-products of lipid peroxidation

A cDNA clone for glutathione S-transferaseA (GSTA) from plaice (Pleuronectes platessa) was expressed in Eschericia coli (E. coli) and purified to homogeneity by S-hexylglutathione affinity chromatography. When compared to literature values for a variety of purified mammalian GSTs, the heterologously expressed purified plaice enzyme had moderate activity towards the model substrate 1,2-chloro-2,4-dinitrobenzene (CDNB) and exhibited a Km of 2.5 ± 2 mM and Vmax of 30.9 ± 2.3 μmol min⁻¹ mg⁻¹. It had little or no activity towards several other model GST substrates including 1,2-dinitrochloro-4-benzene (DCNB), ethacrynic acid (EA), and p-nitrobenzylchloride (NBC). However plaice GSTA was a relatively efficient catalyst for the conjugation of a series of alk-2-enals and alk-2,4-dienals and also 4-hydroxynonenal. The highest activity observed with this series of substrates was with trans-non-2-enal with a Km of 17.9 ± 2.2 μM and a Vmax of 3.01 ± 0.57 μmol min⁻¹ mg⁻¹. These unsaturated alkenals have been identified in cells and cell extracts as highly toxic products arising from peroxidation of unsaturated fatty acids particularly during periods of oxidative stress. Fish are relatively rich in polyunsaturated fatty acids and thus GSTA mediated conjugation may be an important mechanism for detoxifying peroxidised lipid breakdown products.     

Microbial photosynthesis in coral reef sediments (Heron Reef, Australia)

We investigated microphytobenthic photosynthesis at four stations in the coral reef sediments at Heron Reef, Australia. The microphytobenthos was dominated by diatoms, dinoflagellates and cyanobacteria, as indicated by biomarker pigment analysis. Conspicuous algae firmly attached to the sand grains (ca. 100 μm in diameter, surrounded by a hard transparent wall) were rich in peridinin, a marker pigment for dinoflagellates, but also showed a high diversity based on cyanobacterial 16S rDNA gene sequence analysis. Specimens of these algae that were buried below the photic zone exhibited an unexpected stimulation of respiration by light, resulting in an increase of local oxygen concentrations upon darkening. Net photosynthesis of the sediments varied between 1.9 and 8.5 mmol O₂ m⁻² h⁻¹ and was strongly correlated with Chl a content, which lay between 31 and 84 mg m⁻². An estimate based on our spatially limited dataset indicates that the microphytobenthic production for the entire reef is in the order of magnitude of the production estimated for corals. Photosynthesis stimulated calcification at all investigated sites (0.2–1.0 mmol Ca²⁺ m⁻² h⁻¹). The sediments of at least three stations were net calcifying. Sedimentary N₂-fixation rates (measured by acetylene reduction assays at two sites) ranged between 0.9 to 3.9 mmol N₂ m⁻² h⁻¹ and were highest in the light, indicating the importance of heterocystous cyanobacteria. In coral fingers no N₂-fixation was measurable, which stresses the importance of the sediment compartment for reef nitrogen cycling.     

Benthic denitrification and nitrogen cycling at the slope and rise of the N.W. European Continental Margin (Goban Spur)

The rate of benthic denitrification in slope and rise sediments of a transect across the N.W. European Continental Margin (Goban Spur) was evaluated from 31 pore water nitrate profiles obtained during six cruises between May and October. All profiles had well separated zones of nitrification and denitrification. High near-surface nitrate concentrations prevented the influx of nitrate from the bottom water. The denitrification rates obtained from steady-state-modelling ranged from 0.13 to 2.56 μmol N cm⁻² y⁻¹ and showed an exponential increase both with decreasing water depth and with increasing rate of organic carbon degradation. Denitrification rates in a nearby canyon, which did not follow these relationships, were estimated to be much higher as a result of erosion and redistribution of organic matter. Denitrification at the Goban Spur slope and rise is much lower than previously reported for similar environments in the Pacific resulting predominantly from the different oxygen and nitrate concentrations in the bottom water. A weighted average for the whole slope and rise sediment system shows that 17% of the particulate organic nitrogen input (8.93 μmol N cm⁻² y⁻¹) is denitrified and only 1% is buried, the rest being released as nitrate. Although being ten times higher compared with basin sediments, denitrification on the slope and rise is several times lower than on the adjacent shelf.     

On the oxidation and burial of organic carbon in sediments of the Iberian margin and Nazaré Canyon (NE Atlantic)

As a contribution to the EC-OMEX-II program, sediment carbon and nitrogen budgets are presented for the Iberian Margin (northeastern Atlantic). The budgets for degradable organic carbon and associated nitrogen were calculated from sediment and pore water properties, using a steady-state version of a numerical coupled diagenetic model, OMEXDIA. Data were collected throughout the major upwelling period along five transects, four of which were located on the open margin and one positioned in a major submarine canyon, the Nazaré Canyon.A comparison of in situ oxygen profiles measured with monocathodic microelectrodes and with Clark type microelectrodes showed that monocathodic electrodes overestimate the oxygen concentration gradient near the sediment–water interface. This artifact probably results from the loss in sensitivity of the monocathodic microelectrode during profiling. Shipboard time course measurements with Clark type electrodes demonstrated transient conditions upon sediment retrieval on deck and indicated enhanced rates of oxygen consumption in the surface sediment, presumably as a result of lysis or exudation of oxidisable substrates by infauna. As a result, oxygen fluxes calculated from shipboard oxygen profiles overestimated in situ fluxes by up to a factor of 5 for water depths >1000 m.The sediments from the canyon and from a depositional area on the shelf were enriched in organic carbon (3–4.5 wt%) relative to the open margin stations (0.5–2 wt%) and showed C/N ratios exceeding Redfield stoichiometry for marine organic matter, indicating there was deposition of organic carbon of terrestrial origin in these areas. The oxidation of organic carbon on the open margin declined from ˜11 gCm⁻²y⁻¹ on the shelf to 2 gCm⁻²y⁻¹ at 5000 m water depth, and was dominated by aerobic oxidation. The reactivity of the degradable organic carbon at the time of deposition was <2.5 y⁻¹ on the shelf, and declined to <0.5 y⁻¹ offshore. The burial of refractory organic carbon at the stations along the open margin transects also declined with increasing water depth from ˜5 gCm⁻²y⁻¹ on the shelf to <1 gCm⁻²y⁻¹ at 2000 m depth, whereas the burial of particulate inorganic carbon declined from ˜20 gCm⁻²y⁻¹ to <5 gCm⁻²y⁻¹. A comparison of the estimated total organic carbon deposition and predicted delivery for the shelf suggest that 58 to 165 gCm⁻²y⁻¹ is oxidized in the water column, laterally advected, or focused into one of the canyons.Anaerobic oxidation, denitrification and, therefore, total oxidation of organic carbon was enhanced within the canyon relative to the open margin. Total organic carbon oxidation decreased with water depth from 22 gCm⁻²y⁻¹ at the head of the canyon to 3 gCm⁻²y⁻¹ over its fan. The reactivity of the organic carbon deposited in the canyon was lower than those of the shelf stations, suggesting that the canyon is being enriched in older, laterally advected organic matter. The burial of refractory organic carbon in sediments from the Nazaré Canyon was considerably higher than in the sediments from the open margin; it also decreased with depth from 20 gCm⁻²y⁻¹ at 343 m to ˜2.5 gCm⁻²y⁻¹ at 4298 m water depth. The burial of particulate inorganic carbon was slightly lower than that of refractory organic carbon.The burial of refractory organic carbon and the deposition of degradable organic carbon were both positively correlated with the sedimentation rates for the Iberian Margin, and indicated burial efficiencies were 0.6 to 48%. A single trend for burial efficiency versus sedimentation rate for both the canyon and the open margin indicates that the sedimentation rate was the master variable for the geographical distribution of organic carbon oxidation and carbon preservation on the NW Iberian Margin.     

Nondetrital and total metal distribution in core sediments from the U-Tapao Canal, Songkhla, Thailand

The U-Tapao Canal is the main source of freshwater draining into the outer part of Songkhla Lake, which is the most important estuarine lagoon in Thailand. Songkhla Lake is located in southern Thailand between latitudes 7°08' and 7°50' N and longitudes 100°07' and 100°37' E. Acetic acid (HOAc)-soluble Cu, Fe, Mn, Pb, and Zn and the total concentration of these metals along with Al concentration, organic carbon, carbonate, sand, silt, and clay contents were determined in 4 sediment cores obtained at selected intervals from the mouth of the canal to 12 km upstream. Readily oxidizable organic matter in the cores varies from 1.52% to 7.30% and is generally found to decrease seaward. Total concentrations of Al (61.7–99.0 g kg⁻¹; 2.29–3.67 mol kg⁻¹), Cu (12.4–28.2 mg kg⁻¹; 195–444 μmol kg⁻¹), Fe (25.2–42.0 g kg⁻¹; 451–752 mmol kg⁻¹), Mn (0.22–0.49 g kg⁻¹; 4.0–8.9 mmol kg⁻¹), Pb (16.7–43.1 mg kg⁻¹; 80.6–208 μmol kg⁻¹), and Zn (48.6–122.7 mg kg⁻¹; 0.74–1.88 mmol kg⁻¹) vary to a certain extent vertically and seaward in the U-Tapao Canal core sediments. These concentrations are at or near natural levels and show no indication of anthropogenic contamination.Overall, the data show that total metal concentrations in the surface and near surface core sediments are enriched in varying degrees relative to Al in the order of Zn>Mn>Pb>Fe>Cu. Chemical partitioning shows that the enrichment in the surface and near surface sediments is related to the relatively high proportion of the total metal concentrations (Mn>Zn>Fe>Cu>Pb) that occur in the acetic acid-soluble (nondetrital) fraction, and they generally decrease with depth. Nondetrital Cu, Pb, and Zn likely derive from those metals held in ion exchange positions, certain carbonates, and from easily soluble amorphous compounds of Mn and perhaps those of Fe. Diagenetic processes involving Mn and to a lesser extent, Fe compounds, as well as the vertical changes in the oxidizing/reducing boundaries, appear to be the most important factors controlling the behavior of the metals in these cores. Organic matter and the aluminosilicate minerals, however, appear to be less important carriers of the metals studied.