Porewater Sulfide Concentrations Research Articles

Anaerobic oxidation of methane at different temperature regimes in Guaymas Basin hydrothermal sediments

Anaerobic oxidation of methane (AOM) was investigated in hydrothermal sediments of Guaymas Basin based on δ(13)C signatures of CH(4), dissolved inorganic carbon and porewater concentration profiles of CH(4) and sulfate. Cool, warm and hot in-situ temperature regimes (15-20 °C, 30-35 °C and 70-95 °C) were selected from hydrothermal locations in Guaymas Basin to compare AOM geochemistry and 16S ribosomal RNA (rRNA), mcrA and dsrAB genes of the microbial communities. 16S rRNA gene clone libraries from the cool and hot AOM cores yielded similar archaeal types such as Miscellaneous Crenarchaeotal Group, Thermoproteales and anaerobic methane-oxidizing archaea (ANME)-1; some of the ANME-1 archaea formed a separate 16S rRNA lineage that at present seems to be limited to Guaymas Basin. Congruent results were obtained by mcrA gene analysis. The warm AOM core, chemically distinct by lower porewater sulfide concentrations, hosted a different archaeal community dominated by the two deep subsurface archaeal lineages Marine Benthic Group D and Marine Benthic Group B, and by members of the Methanosarcinales including ANME-2 archaea. This distinct composition of the methane-cycling archaeal community in the warm AOM core was confirmed by mcrA gene analysis. Functional genes of sulfate-reducing bacteria and archaea, dsrAB, showed more overlap between all cores, regardless of the core temperature. 16S rRNA gene clone libraries with Euryarchaeota-specific primers detected members of the Archaeoglobus clade in the cool and hot cores. A V6-tag high-throughput sequencing survey generally supported the clone library results while providing high-resolution detail on archaeal and bacterial community structure. These results indicate that AOM and the responsible archaeal communities persist over a wide temperature range.

Effects of upwelling on short-term variability in microbial and biogeochemical processes in estuarine sediments from Elkhorn Slough, California, USA

We examined short-term variability in sediment microbial processes and biogeochem- istry in response to decreased irradiance accompanying an upwelling event. Potential nitrification, extractable nutrients, solid phase iron and pore water sulfide concentrations were measured several times at 3 locations in Elkhorn Slough, California, USA. Benthic fluxes of nitrate+nitrite, ammonium, phosphate, sediment oxygen consumption and denitrification from N2/Ar flux were also measured. Fog that was associated with upwelling in Monterey Bay reduced photosynthetically active radiation in Elkhorn Slough, which led to increased duration of water column hypoxia. Solid phase Fe 3+ con- centrations and potential nitrification rates declined while pore water sulfide increased over this period. While potential nitrification rates from this study are lower than rates from estuaries that do not experience hypoxia or anoxia, nitrifiers in Elkhorn Slough appear to be capable of adjusting to changing environmental conditions, specifically of tolerating low levels of sulfide. The nitrifying assemblage was similar to the community of ammonia-oxidizing bacteria and ammonia-oxidizing archaea that was previously reported from this system. Eutrophic shallow estuaries like Elkhorn Slough are delicately poised such that their sediment biogeochemistry responds quickly to relatively small changes in the light environment.

Seasonal Variability of Diazotroph Assemblages Associated with the Rhizosphere of the Salt Marsh Cordgrass, Spartina alterniflora

Nitrogen fixation is the primary N source in the highly productive but N-limited North Inlet, SC, USA salt marsh system. The diverse assemblages of nitrogen-fixing (diazotrophic) bacteria associated with the rhizospheres of the short and tall growth forms of Spartina alterniflora were analyzed at two sites, Crab Haul Creek and Goat Island, which are in different tidal creek drainage systems in this marsh. The sites differed in proximity to the main channel for tidal intrusion and in several edaphic parameters. We hypothesized that either the differing abiotic environmental regimes of the two sites or the variation due to seasonal effects result in differences in the diazotroph assemblage. Rhizosphere samples were collected seasonally during 1999 and 2000. DNA was purified and nifH amplified for denaturing gradient gel electrophoresis (DGGE) analysis of diazotroph assemblage composition. Principal components analysis was used to analyze the binary DGGE band position data. Season strongly influenced assemblage composition and biplots were used to identify bands that significantly affected the seasonal and site-specific clustering. The types of organisms that were most responsive to seasonal or site variability were identified on the basis of DGGE band sequences. Seasonally responsive members of the anaerobic diazotrophs were detected during the winter and postsenescence conditions and may have been responsible for elevated pore water sulfide concentrations. Sequences from a diverse assemblage of Gammaproteobacteria were predominant during growth periods of S. alterniflora. Abiotic environmental parameters strongly influenced both the S. alterniflora and the diazotrophic bacterial assemblages associated with this keystone salt marsh plant species.

Experimental manipulation of sediment organic content and water column aeration reduces Zostera marina (eelgrass) growth and survival

The effect of synergy between sediment organic enrichment and lack of night oxygen renewal in the water column on growth and survival of Zostera marina, and how it is reflected in the sulfur parameters in the plants ( δ 34 S, TS and S 0) was studied experimentally. An experiment consisting of Z. marina mesocosms with different levels of organic enrichment and water column aeration was established, and the effects on sediment conditions, sulfide invasion and growth and survival of Z. marina were examined over a 4 week period. Shoots growing in Ambient Organic matter–sediments showed signs of sulfide invasion, as TS increased in all plant compartments and δ 34 S of the plant tissues decreased during the experiment, but neither growth rate nor survival were significantly affected. The lack of night oxygen renewal had no evident effects in non-enriched sediments as porewater sulfide concentrations, AVS- and CRS-pools were not different from the corresponding 24 h aeration treatment. Plant growth rate and survival were neither different from the corresponding 24 h aeration treatment. On the contrary, shoots growing on High Organic matter-sediments suffered a massive sulfide invasion and it was directly correlated to the observed decrease in growth rates. Even though the lack of night oxygen renewal had no evident effects on sediment variables there were, however, strong indications that the different aeration levels affected plant performance, suggesting a lower sulfide oxidation capacity and confirming that low water column oxygen concentrations reduces the defense capacity of the shoots against sulfide invasion. Although δ 34 S, TS and S 0 concentrations together provided a powerful set of indicators to detect the invasion of sulfide in Z. marina shoots, this study enlightens the need for a deeper investigation of sulfide intrusion in seagrasses and the relation between plant sulfur parameters and sediment conditions.

Sediment nitrate manipulation using porewater equilibrators reveals potential for N and S coupling in freshwaters

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 54:233-241 (2009) - DOI: https://doi.org/10.3354/ame01272 Sediment nitrate manipulation using porewater equilibrators reveals potential for N and S coupling in freshwaters E. K. Payne1,2, A. J. Burgin2,3,*, S. K. Hamilton2 1Life Sciences and Society Program, University of Michigan School of Public Health, 109 South Observatory Road, Ann Arbor, Michigan 48105, USA 2W.K. Kellogg Biological Station and Department of Zoology, Michigan State University, 3700 Gull Lake Drive, Hickory Corners, Michigan 49060, USA 3Cary Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545, USA *Corresponding author. Email: burginam@gmail.com ABSTRACT: Anthropogenic nitrogen (N) loading to agricultural and populated landscapes has resulted in elevated nitrate (NO3–) concentrations in ground water, streams and rivers, ultimately causing problems in coastal marine environments such as eutrophication, hypoxia and harmful algal blooms. Nitrate removal along hydrologic flow paths through landscapes intercepts much of the N before it reaches coastal zones. We used traditional porewater equilibrators in a novel way to add nitrate to the sediment porewater of 8 wetlands in southwestern Michigan. Nitrate losses and changes in porewater chemistry were examined to elucidate N removal processes, with particular focus on the potential coupling of bacterial sulfur (S) oxidation to (1) dissimilatory nitrate reduction to ammonium (DNRA) and (2) denitrification. We hypothesized that, if S oxidizers utilized the added nitrate, porewater sulfide concentrations should decrease and sulfate concentrations should increase. Additionally, if the nitrate is used in DNRA, ammonium concentrations should increase as well. Nitrate additions caused decreases in dissolved hydrogen sulfide and increases in sulfate relative to controls at all sites. Ammonium also tended to increase, though the response was less consistent due to a high background ammonium pool. These results provide evidence that microbial S transformations may play an important role in nitrate removal in these freshwater wetland sediments. KEY WORDS: Denitrification · Nitrate removal · Dissimilatory nitrate reduction to ammonium · DNRA · Sulfur oxidation · Porewater equilibrators · Freshwater sediments · Sulfate Full text in pdf format PreviousNextCite this article as: Payne EK, Burgin AJ, Hamilton SK (2009) Sediment nitrate manipulation using porewater equilibrators reveals potential for N and S coupling in freshwaters. Aquat Microb Ecol 54:233-241. https://doi.org/10.3354/ame01272 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 54, No. 3. Online publication date: February 24, 2009 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2009 Inter-Research.

Bacterial Community Dynamics in a Seagrass (Posidonia oceanica) Meadow Sediment

Some traits of the bacterial community dynamics associated to the rhizosphere of the Mediterranean seagrass Posidonia oceanica, growing in carbonate sediments, were analyzed during a 2-year period in an enclosed bay of the Balearic Islands. The diversity of the bacterial community was studied by the construction of 16S rDNA clone library. For testing temporal and vertical differences in the abundance of total cells and active Bacteria, we used 4′,6-diamidino-2-phenylindole (DAPI) staining and fluorescence in situ hybridization (FISH). Moreover, some relevant groups of sulfate-reducing bacteria (SRB) were occasionally assessed by FISH. Despite the observed decrease in the total DAPI-stained cells, bacterial counts, and sulfate reduction rates throughout the sampling time, we found an increase in both the pore-water sulfide concentration and the proportion of SRB. Overall, the results revealed a very high bacterial diversity and indicated shifts in bacterial dynamics that could not be related to temperature-dependent factors, suggesting a link between the documented regression of the seagrass meadow and the decline of the microbial community, likely due to large organic matter inputs to the bay.

Effect of increased sediment sulfide concentrations on the composition of stable sulfur isotopes (δ 34S) and sulfur accumulation in the seagrasses Zostera marina and Posidonia oceanica

The effect of increased sediment sulfide concentrations on the sulfur isotopic composition (δ 34S), total sulfur (TS) and elemental sulfur (S 0) concentrations in plant tissues was studied for the two seagrasses Zostera marina (3 weeks in laboratory) and Posidonia oceanica (4 months in situ). Porewater sulfide concentrations were experimentally regulated and plants exposed to high sediment sulfide concentrations had δ 34S signals closer to the δ 34S of sulfide, whereas plants exposed to no / low sulfide concentrations had δ 34S signals closer to the δ 34S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to high sulfide concentrations. The δ 34S varied between the plant tissues in both species with the leaves having more positive δ 34S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS and S 0 concentrations were higher in plants exposed to sulfide in both experiments suggesting that sulfur derived from sediment sulfide accumulated in the plants. The δ 34S signal in S 0 was similar to sediment sulfide verifying that S 0 found in the seagrasses originated from sediment sulfide. Direct comparisons of δ 34S in the two different seagrasses and across the treatments were not possible due to large differences in δ 34S of the sulfur sources. F sulfide adjusted for these differences and may be a useful alternative, when δ 34S of the sulfur sources varies between study sites. There were no significant effects of sulfide exposure on plant growth and mortality in Z. marina and P. oceanica after 3 weeks and 8 weeks exposure, respectively, but P. oceanica showed indications of reduced growth and higher mortality after 16 weeks of sulfide exposure probably due to sulfide invasion/toxicity.

Lucinid clam influence on the biogeochemistry of the seagrassThalassia testudinum sediments

Lucinid bivalves dominate the infauna of tropical seagrass sediments. While the effect of seagrass on lucinids has been studied, the reverse effect has largely been ignored. Lucinids can alter porewater chemistry (i.e., increase porewater nutrients by suspension feeding and decrease porewater sulfides by oxygen introduction and bacterial oxidation), which can potentially change seagrass productivity and growth morphology. To observe correlations between porewater chemistry and lucinid presence, a field survey and laboratory microcosm experiment were conducted. Survey sampling sites with lucinids had significantly lower sulfide and higher ammonium concentrations than sampling sites without lucinids. There was no difference in phosphate concentration among sampling sites. Both lucinid species used in the microcosm experiment (Ctena orbiculata andLucinesca nassula) significantly lowered sulfide concentrations in the sediment porewater. Microcosm and field survey results were incorporated into a sulfide budget. In seagrass sediments, lucinids remove 2–16% of the total sulfide produced. Sulfide is a major stressor to both plants and animals in Florida Bay sediments; this removal may be important to maintaining seagrass productivity and health. Oxygen introduction into sediments byC. orbiculata was estimated in a dye experiment.C. orbiculata were added to small tubes containing sieved mud and incubated in a bath of seawater with a Rhodamine WT. Rhodamine WT accumulation in the sediment was measured. A first order estimate showed that oxygen introduction can account for less than 5% ofC. orbiculata sulfide removal.

Characteristics of vesicomyid clams and their environment at the Blake Ridge cold seep, South Carolina, USA

Spatial distributions and patchiness of dominant megafaunal invertebrates in deep-sea seep environments may indicate heterogeneities in the flux of reduced chemical compounds. At the Blake Ridge seep off South Carolina, USA, the invertebrate assemblage includes dense populations of live vesicomyid clams (an undescribed species) as well as extensive clam shell beds (i.e. dead clams). In the present study, we characterized clam parameters (density, size-frequency distribution, reproductive condition) in relation to sulfur chemistry (sulfide and sulfate concentrations and isotopic compositions, pyrite and elemental sulfur concentrations) and other sedimentary metrics (grain size, organic content). For clams >5 mm, clam density was highest where the total dissolved sulfide concentration at 10 cm depth (ΣH2S10cm) was 0.4 to 1.1 mmol l -1 ; juvenile clams (<5 mm) were most dense where ΣH2S10cm was lowest. Clams were reproductively capable across a broad range of ΣH2S10cm (0.1 to 6.4 mmol l -1 ), and fe- males in the sampled populations displayed asynchronous gametogenesis. Sulfide concentrations in porewaters at the shell-sediment interface of cores from shell beds were high, 3.3 to 12.1 mmol l -1 , com- pared to <1 mmol l -1 sulfide concentrations at the clam-sediment interface in live clam beds. Concen- tration profiles for sulfide and sulfate in shell beds were typical of those expected where there is active microbial sulfate reduction. In clam beds, profiles of sulfide and sulfate concentrations were also consis- tent with rapid uptake of sulfide by the clams. Sulfate in shell beds was systematically enriched in 34 S rel- ative to that in clam beds due to microbial fractionation during sulfate reduction, but in clam beds, sul- fate δ 34 S matched that of seawater (~20‰). Residual sulfide values in clam and shell beds were correspondingly depleted in 34 S. Based on porewater sulfide concentrations in shell beds at the time of sampling, we suggest that clam mortality may have been due to an abrupt increase in sulfide concen- tration and sulfide toxicity, but other alternatives cannot be eliminated.

Mangrove zonation in the dry life zone of the Gulf of Fonseca, Honduras

The structural attributes and zonation patterns of mangrove forests in a dry environmental setting were evaluated in relation to gradients of soil resources, soil regulators, and hydroperiod between October 2000 and August 2001 in the Gulf of Fonseca, Honduras. Transects perpendicular to the edge of tidal channels were established at four mangrove sites, each sectioned into three mangrove zones (fringe, transition, and scrub) based upon distinct tree-height gradients. Porewater sulfide concentrations among zones were normally below levels of detection (<0.03 mM) and mean redox values ranged from 163.4±9.9 to −42.4±15.8 mv, indicating slightly reducing conditions in all zones. Mean porewater NOx − and PO4 3− concentrations were not significantly different among zones and ranged from 3.3±0.5 to 4.5±0.4 μM and from 0.05 ±0.02 to 0.18±0.04 μM, respectively. Soil nitrogen concentrations were 1–3 mg g−1 in all zones, and mean nitrogen:phosphorus atomic ratios were transition) and were coupled with differences in height within species and zones.Avicennia germinans occupied areas with higher elevations associated with higher salinities ranging from 80 to 140 g kg−1 in the dry season.Rhizophora mangle dominated lower elevations where salinities ranged from 38 to 57 g kg−1 in both the dry and wet seasons. Spatial analysis on mangrove seedlings along transects confirmed that seedling distribution along the intertidal profile was statistically correlated with conspecific adults, indicating that survival and growth of seedlings in different intertidal locations are closely matched to canopy membership. The sharp transition (<100 m) in forest structure and distribution of mangrove species along the intertidal zone of subhumid coastal environments demonstrate common zonation patterns where species distribution and species-specific habitat suitability are mediated closely with tolerance to soil regulators (high soil salinity) rather than resource availability.

MECHANISMS GENERATING MODIFICATION OF BENTHOS FOLLOWING TIDAL FLAT INVASION BY A SPARTINA HYBRID

Many coastal habitats are being substantially altered by introduced plants. In San Francisco Bay, California, USA, a hybrid form of the eastern cordgrass Spartina alterniflora is rapidly invading open mudflats in southern and central sections of the Bay, altering habitat, reducing macrofaunal densities, and shifting species composition. The invasion has resulted in significant losses of surface-feeding amphipods, bivalves, and cirratulid polychaetes, while subsurface feeding groups such as tubificid oligochaetes and capitellid polychaetes have been unaffected. In the present paper, we document the causes and mechanisms underlying the changes observed. Through a series of in situ manipulative experiments we examined the influence of hybrid Spartina canopy on a range of physical, chemical, and biological properties. The hybrid Spartina canopy exerted a strong influence on the hydrodynamic regime, triggering a series of physical, chemical, and biological changes in the benthic system. Relative to tidal flats, water velocity was reduced in hybrid patches, promoting deposition of fine-grained, organic-rich particles. The resulting changes in the sediment environment included increased porewater sulfide concentrations and anoxia, which led to poor survivorship of surface feeders such as bivalves, amphipods, and polychaetes. These are key taxa that support higher trophic levels including migratory shorebirds that feed on tidal flats. Altered flow in the Spartina canopy further contributed to changes in barnacle recruitment and resuspension of adult benthic invertebrates. Increased crab-induced predation pressure associated with Spartina invasion also contributed to changes in benthic invertebrate communities. Our results suggest that multiple physical, chemical, biotic, and trophic impacts of the Spartina invasion have resulted in substantial changes in benthic communities that are likely to have important effects on the entire ecosystem.

Effects of drift macroalgae and light attenuation on chlorophyll fluorescence and sediment sulfides in the seagrass Thalassia testudinum

We examined fluorescence parameters, chlorophyll indices, and sediment sulfides in the leaves of the subtropical seagrass Thalassia testudinum in relation to shading by drift macroalgae and artificial light reduction in a shallow meadow near Port Aransas, Texas, USA. Experiments conducted during a 5-week period in autumn 2003 included three cages in 0.7 m water depths covered with neutral density screening to achieve four light treatments: a control at full insolation ( I o) and at 10%, 33%, and 50% I o. Seagrasses in six remaining cages were covered with a 50% cover of drift macroalgae similar to that observed in situ. Effective quantum yield ( Y), non-photochemical quenching (NPQ), and minimal fluorescence ( F o) at the apex and base portions of six leaves were measured under the four different light regimes and under the drift algal treatment once a week for four weeks. Values of fluorescence quantum yield in older distal segments were lower than in proximal younger segments in all treatments, likely reflecting differences in tissue age. Y decreased at the leaf apex in both the light reduction and drift macroalgae treatments following the first week of the study. At the end of two weeks Y had also decreased at the leaf base and continued to decrease in both leaf areas over the course of the experiment in all treatments relative to the control. Shading by drift algae and artificial light reduction also induced increases in NPQ. The increase was highest at the 10% I o treatment. F o values were stable beneath the drift macroalgae treatment but increased in plants subjected to artificial light reduction. Leaf chlorophyll (chl) a content, measured in each treatment at the end of the study, ranged from 4 to 5.2 mg chl g dry weight − 1 . Chl a increased at 50% and 33% I o, showed no change in the drift macroalgal treatment, but decreased significantly at 10% I o. Chl a : b ratios at 10% I o (1.3) were also significantly lower than all other treatments (range 1.9 to 2.2), including the control. The changes in pigment concentrations under various regimes of light reduction suggest various degrees of photoacclimation relative to controls and plants shaded by macroalgae. Coincident with the changes in Y and NPQ, sediment porewater sulfide concentrations increased in drift macroalgal and artificial light reduction treatments, and were inversely correlated to light. Highest sulfide values were measured in the 10% I o treatment but were significantly different at each experimental light level. These preliminary results suggest that chlorophyll fluorescence is a useful and non-destructive indicator of seagrass condition that has potential as a monitoring tool for management of an important but threatened coastal resource.

Organic Material: The Primary Control on Mercury Methylation and Ambient Methyl Mercury Concentrations in Estuarine Sediments

Estuarine environments that have no direct sources of mercury (Hg) pollution may have sediment concentrations of methylmercury (MeHg) as high as those of polluted marine environments. In this study we examined the biogeochemical factors affecting net methylation and sediment MeHg concentrations in an unpolluted estuarine environment, the Ore River estuary, which discharges into the Bothnian Bay (20-120 ng total Hg g(-1) dry sediment, salinity 3-5% per hundred). We analyzed the spatial and temporal differences in surface sediment profiles of MeHg concentration, Hg methylation, MeHg demethylation, and concentrations of sulfide and oxygen between accumulation and erosion type bottoms. The main difference between the bottoms studied was in the proportion of organic material (OM) in the sediment, ranging between 0.8% and 10.8%. The pore water sulfide concentration profiles also differed considerably between sites and seasons, from 0 to 20 microM, with 100 microM as the extreme maximum. The sediment MeHg concentration profiles (0-10 cm) mostly varied between 0.1 and 7 ng g(-1) dry weight (dw, as Hg). The MeHg demethylation rates were relatively low and the depth profiles of the rates were relatively constant over season, site, and depth. In contrast, both rates and depths of maximum Hg methylation differed between the bottoms. The results indicate that the amount of OM accumulated at the bottoms was the main factor affecting net MeHg production, while the total amount of Hg had little or no influence on the amount of MeHg in the sediment.

Environmental controls on the speciation and distribution of mercury in coastal sediments

Methylmercury production by sulfate reducing bacteria in coastal sediments leads to bioaccumulation of mercury in fish, shellfish, and ultimately humans. Sulfur, organic carbon, and sediment structure and composition can all affect methylmercury production by changing the amount of bioavailable inorganic mercury and by stimulating the activity of methylating microbes. This study investigates total and methylmercury in solids and porewaters relative to total sulfide concentration, redox potential, sediment grain size, and total organic carbon in a range of sediment types from the Bay of Fundy region of Canada. Using these data, we construct a conceptual model of the biogeochemical environment surrounding methylating microbes in high sulfide, organically enriched sediments. Whereas other studies of methylmercury dynamics measured porewater sulfide concentrations in relatively low-sulfide systems (∼20–300 μM), we measured total sulfide levels using a method developed to indicate organic enrichment across a much wider range of sulfidic sediments (10–4000 μM). We observed that higher sulfide concentrations correspond to an elevated fraction of mercury in methylated form suggesting higher net methylation rates in these sediments. This relationship is strongest in sediments that are moderately impacted by organic enrichment, but weak in less impacted, aerobic sediments. Higher sulfide concentrations in porewaters containing dissolved organic matter appear to yield a geochemical environment that is conducive to uptake of Hg(II) by methylating bacteria. Data collected in this study imply that moderate levels of organic enrichment through fish farming may enhance methylmercury production in the Bay of Fundy.

Living benthic foraminifera in sediments influenced by gas hydrates at the Cascadia convergent margin, NE Pacific

Living (Rose Bengal stained) foraminifera in gas-hydrate-influenced sediments at the Cascadia convergent margin were investigated. Foraminiferal assemblages from the southern Hydrate Ridge and neighboring basins were compared in terms of abundances, vertical distribution, diversity, and species composition. At Hydrate Ridge, the presence of shallow gas hydrates and increased porewater sulfide concentrations was indicated by extensive bacterial mats of Beggiatoa sp. and clam beds of the bivalve mollusk Calyptogena sp., generating different biological zones. Living foraminifera were found in all biological zones, in sediment layers down to 5 cm. They showed highly variable densities within all zones. The average abundance of benthic foraminifera at Hydrate Ridge differs from neighboring basins. Average species diversities are comparable between biological zones, while the average number of species increases from bacterial mats to clam fields and surrounding sediments. Foraminifera can be characterized by 5 principal component communities which explain 97.3% of the variance of the live assemblages at the southern Hydrate Ridge and neighboring basins. At Hydrate Ridge, 2 foraminiferal zones can be distinguished: (1) an Uvigerina peregrina community which characterizes sediments covered with bacterial mats and clam fields; (2) a ?Spiroplectammina biformis community in the surrounding non-seep sediments. Foraminiferal assemblages in the neighboring Western and Eastern Basin differ from the Hydrate Ridge stations.

The demise and recovery of seagrass in the northern Indian River Lagoon, Florida

Seagrass both disappeared and recovered within 4 yr in one region of northern Indian River Lagoon (IRL). For the specific area referred to as Turnbull Bay, a relatively pristine area of the IRL, over 100 ha of seagrass completely disappeared from 1996 to 1997 and then recovered by 2000. Based on lagoon-wide mapping from aerial photographs taken every 2–3 years since 1986, coverage of seagrass in Turnbull Bay declined from 124 ha in 1989 to 34 ha by 1999 and increased to 58 ha in 2003. Bi-annual monitoring of fixed seagrass transects tells a more detailed story. Species composition along the Turnbull transect shifted fromHalodule wrightii toRuppia maritima beginning in 1995, and macroalgal abundance increased. By the summer of 1997, seagrass completely disappeared along the transect, as well as in most of the surrounding areas in Turnbull Bay; macroalgae covered much of the sediment surface. No significant water quality changes were detected. Light attenuation and suspended solid values did increase after the seagrass disappeared. Porewater sulfide concentrations, taken after all the grass was gone in 1997, were high (2,000 μM), but did improve by 1998 (1,200 μM). Seagrass recovery was rapid and occurred in the reverse sequence of species loss. Seedlings ofR. maritima were the first colonizers, then patches ofH. wrightii appeared. In 2000,Halophila engelmannii returned in the deeper water (>0.6m). By the summer of 2000, the beds had completely recovered. We conclude that this demise was a natural event caused by a long-term buildup of seagrass biomass and a thick (10–15 cm) layer of organic detritus and ooze. We surmise that such a crash and subsequent recovery may be a natural cycle of decline and recovery within this semirestricted, poorly-flushed area. The frequency of this cycle remains uncertain.

Nutrient, sulfur and carbon dynamics in a hypersaline lagoon

We measured benthic and water column fluxes in a hypersaline coastal system (Baffin Bay, Texas) in 1996–1997, a period of decreasing salinity (increased freshwater input) and turbidity. Salinity decreased from a mean of 60 to 32 practical salinity units (psu) and turbidity decreased from a mean of 78 to 25 NTU over the study period. Associated with hydrological changes, there were important changes in nutrient fluxes and metabolism. There was a shift of total respiration from the water column to the sediments and an increased amount of the benthic metabolism (2–67%) was attributed to sulfate reduction in this system when salinity was lowest, perhaps a consequence of increased benthic light levels and photosynthetic production of labile carbon in the sediments. The sediments were a large sink for both N and P. Sediment particulate C:N (9.8) and C:P (119) ratios were lower than those in the water column. However, ammonium:phosphate fluxes increased coincident with increased sulfate reduction rates and porewater sulfide concentrations. Efficient N-retention mediated through dissimilative nitrate reduction to ammonium, and high rates of N-fixation in shallow, hypersaline systems may facilitate transitions from N-limitation to P-limitation. During the most hypersaline period, seston exhibited some of the most extreme nutrient ratios ever reported for a marine ecosystem (C:N 10–37 and C:P 200–1200) and suggest that plankton are likely to be P-limited or are very well adapted to low P availability. When salinity and N:P and C:P ratios were highest, the plankton was dominated by a brown tide alga ( Aureoumbra lagunensis), supporting evidence that this organism is adapted to low P, long residence time systems.

Causes of sulfur isotope variability in the seagrass, Zostera capricorni

Sulfur has been proposed as a useful element to employ in addition to carbon and nitrogen in stable isotope studies of marine food webs, but variability in δ 34S of primary producers may prevent food web resolution. δ 34S values in green leaves of the seagrass, Zostera capricorni, showed considerable variability (12.7–17.6‰) in a survey in Moreton Bay, Australia. We demonstrated that δ 34S values were correlated with sediment organic matter (OM) content and height of seagrass on the tidal gradient, but these relationships were opposite to those expected from work elsewhere. In our survey, δ 34S values were relatively depleted at sites higher on the shore and with lower OM content. We did find the expected relationship of depleted δ 34S values where sediment porewater sulfide concentrations were higher. Any influence of OM content on δ 34S values would have been confounded in the survey by the relationship between height on shore and OM content itself. We separated the effects of height and OM content by creating the following treatments at one height on the shore: (1) OM added, (2) procedural control, and (3) untouched control. δ 34S values of seagrass in OM added plots were significantly depleted (5.6‰) relative to procedural (10.1‰) and untouched (11.0‰) controls 8 weeks after the manipulation. This demonstrated that OM content on its own does have the expected effect on δ 34S values of seagrass, so in the initial survey another factor, probably related to height on shore, must have overridden the influence of OM content. Seagrass roots are able to exude excess oxygen produced during photosynthesis, reoxidising sulfides in surrounding porewater. We demonstrated that the above and below-ground biomass of seagrass was higher low on the shore, and contend that higher seagrass productivity low on the shore results in greater reoxidation of sulfides and leads to more enriched δ 34S values of seagrass.

Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone

The coastal upwelling system off central Namibia is one of the most productive regions of the oceans and is characterized by frequently occurring shelf anoxia with severe effects for the benthic life and fisheries. We present data on water column dissolved oxygen, sulfide, nitrate and nitrite, pore water profiles for dissolved sulfide and sulfate, 35S-sulfate reduction rates, as well as bacterial counts of large sulfur bacteria from 20 stations across the continental shelf and slope. The stations covered two transects and included the inner shelf with its anoxic and extremely oxygen-depleted bottom waters, the oxygen minimum zone on the continental slope, and the lower continental slope below the oxygen minimum zone. High concentrations of dissolved sulfide, up to 22 mM, in the near-surface sediments of the inner shelf result from extremely high rates of bacterial sulfate reduction and the low capacity to oxidize and trap sulfide. The inner shelf break marks the seaward border of sulfidic bottom waters, and separates two different regimes of bacterial sulfate reduction. In the sulfidic bottom waters on the shelf, up to 55% of sulfide oxidation is mediated by the large nitrate-storing sulfur bacteria, Thiomargarita spp. The filamentous relatives Beggiatoa spp. occupy low-O 2 bottom waters on the outer shelf. Sulfide oxidation on the slope is apparently not mediated by the large sulfur bacteria. The data demonstrate the importance of large sulfur bacteria, which live close to the sediment-water interface and reduce the hydrogen sulfide flux to the water column. Modeling of pore water sulfide concentration profiles indicates that sulfide produced by bacterial sulfate reduction in the uppermost 16 cm of sediment is sufficient to account for the total flux of hydrogen sulfide to the water column. However, the total pool of hydrogen sulfide in the water column is too large to be explained by steady state diffusion across the sediment-water interface. Episodic advection of hydrogen sulfide, possibly triggered by methane eruptions, may contribute to hydrogen sulfide in the water column.

Spatial heterogeneity of macrofauna at northern California methane seeps: influence of sulfide concentration and fluid flow

Relationships among fluid flow, sulfide concentration, sulfur bacteria and macrofaunal assemblages were examined at methane seeps on the northern California margin, near the mouth of the Eel River (512 to 525 m). Over a 6 mo period, sediments covered with microbial mats exhibited significant but variable outflow of altered fluids, with no flow reversals. This fluid flow was associated with high porewater sulfide concentrations (up to 20 mM) and almost no oxygen penetration of sedi- ments ( 300 µm), but biomass and diversity (no. species per core, E(S100), H ') were lower and composition varied in the sulfidic microbial mat sediments relative to clam-bed and non-seep sediments. The community in microbial mat-covered sediments consisted largely (82%) of 6 species in the polychaete family Dorvilleidae, whereas the clam-bed and non-seep microhabitats supported a mixture of annelids, peracarid crustaceans, nemerteans, and mollusks. Vertical microprofiling of sulfide in animal cores indicated that most taxa avoid H2S concentrations >1 mM. However, sulfide-oxidizing filamentous bacteria, dorvilleid polychaetes and bivalves (mainly V. pacifica) exhibited highest densities at sul- fide concentrations of 1 to 5 mM sulfide. Horizontal and vertical patterns of sulfide availability have a strong influence on the fine-scale distribution, structure and composition of macrofaunal assem- blages inhabiting methane seeps and must be accounted for when characterizing the microbiology and ecology of seep habitats.