Vertical profiles of bacterial abundance,productivity and growth rates in coastal sediments of the central Great Barrier Reef lagoon

Vertical patterns of bacterial densities, productivity and specific growth rates in coastal muds, quartz sands and muddy sands of the central Great Barrier Reef lagoon were examined in summer (February) and autumn (May) 1988. Variations in these parameters with station location, sediment depth and season were complex, exhibiting significant main and interaction effects in most instances. Some trends were apparent despite the large and complex variations. Bacterial densities did not vary seasonally, ranging from 2.9 to 38.1×10⁹ cells g^-1 dry wt, averaged over sediment depth (0 to 20 cm) and seasons. Trend analysis revealed that densities decreased with increasing sediment depth. Bacterial production (tritiated thymidine incorporation into DNA) was high, ranging from 0.4 to 5.7 gCm^-2 d^-1 (integrated over 10 cm depth), as were specific growth rates (grand mean, =0.25 d^-1; range=0.004 to 1.3 d^-1). Both were generally higher in summer than in autumn. Vertical profiles of productivity and specific growth rates revealed actively growing bacterial assemblages down to 20 cm depth. Factors which may account for these very abundant and productive communities are: (1) subsurface accumulations of detritus exported from adjacent mangrove forests, and (2) physical disturbance from tidal scouring and severe climate (e.g. cyclones, wet-season floods). Disturbance events occur frequently enough to inhibit the development of highly sulphidic conditions, but stimulate production of bacterial types (aerobes, fermenters) capable of incorporating labelled thymidine into their DNA.     

Sedimentation and sulfate reduction under a mussel culture

The sedimentation and dissimilatory sulfate reduction under a blue-mussel culture were quantified in order to gain information on the environmental impact of intense mussel farming. The sedimentation rate (3 g C·m^-2·d^-1) under a culture is nearly three times higher than at a nearby reference station. A build-up of sediment rich in organic material and sulfide takes place under the mussels. At 15°C the sulfate reduction rate was 30.5 mmol SO ₌ ⁴ ·m^-2·^-1 in the upper 10 cm of the mussel sediment. The increase in sedimentation under a mussel culture and the consequent effects should be considered when establishing mussel farms.     

Effects of mussel aquaculture on the nitrogen cycle and benthic communities in Kenepuru Sound,Marlborough Sounds,New Zealand

Nitrogen pools and transformations and benthic communities at a Perna canaliculus farm and a nearby reference site without direct influence of marine farming in Kenepuru Sound, New Zealand, were compared on four dates between September 1982 and May 1983. The organic nitrogen pool in the top 12 cm sediment was 7.4 to 10.8 mol m^-2 at the mussel farm and 6.1 to 8.9 mol m^-2 at the reference site. The nitrate and nitrite pools were similar in both sediments, but the ammonium pool in the mussel farm sediment was about twice as high as in the reference sediment. In January, the sediment ammonium concentrations ranged from 418 nmol cm^-3 (surface) to 149 nmol cm^-3 (12 cm depth) at the mussel farm and from 86 to 112 nmol cm^-3 at the reference site. The molar C:N ratio of the sediment organic matter was 6.2 to 7.2 at the mussel farm and 7.9 to 10.0 at the reference site. The molar N:P ratio of the sediment organic matter was 4.3 to 7.2 and 3.3 to 6.1 at mussel farm and reference site, respectively. The total nitrogen mineralisation rate in the top 12 cm sediment ranged from 21.7 to 37.1 mmol m^-2 d^-1 at the mussel farm and from 8.5 to 25.0 mmol m^-2 d^-1 at the reference site. Ammonium excretion by mussels was about 4.7% (January) and 7.4% (May) of the combined nitrogen mineralisation by mussels and sediment. The sediment-denitrification rate was 0.7 to 6.1 mmol m^-2 d^-1 at the mussel farm and 0.1 to 0.9 mmol m^-2 d^-1 at the reference site. In January, 76 and 93% of the nitrate reduced in the sediments were denitrified at the mussel farm and reference site, respectively. The denitrification rate on the mussel lines (determined on detritus-covered mussels) was twice the mussel farm sediment-denitrification rate and 10 times the reference sediment-denitrification rate. Total denitrification at the mussel farm was 21% higher than at the reference site. The loss of nitrogen through mussel harvest and denitrification was 68% higher at the mussel farm. The surface layers of both sediments contained about 75 mg m^-2 chlorophyll a. Sediment phaeophytin levels were 52 mg m^-2 at the reference site and 137 mg m^-2 at the mussel farm. While the benthic infauna of the mussel-farm sediment consisted only of polychaete worms, the reference sediment contained also bivalve molluscs, brittle stars and crustaceans.     

Impacts of mussel dredging on sediment phosphorus dynamics in a eutrophic Danish fjord

Effects of mussel dredging on sediment metabolism (oxygen uptake and sulfate reduction rates) and phosphorus dynamics (flux across sediment-water interface and sequential extraction) were examined in Limfjorden (Denmark) during spring (May) and summer (August). Sediment samples were taken during mussel dredging and in addition an experimental simulation of the dredging was performed to investigate short-term changes in phosphorus (P) dynamics. Iron-bound P was reduced by up to 2/3 in the surface layer in the dredging track (from 31 to 8?mmol?P?m^?2), whereas the dissolved P-pools and less reactive particulate pools were not affected by dredging. Sediment oxygen consumption was enhanced immediately after dredging, but returned to the initial level after 4 days (20–40?mmol?m^?2?d^?1). The enhanced consumption was attributed to reoxidation of reduced compounds released during dredging. Sulfate reduction rates were high in the area (13–15?mmol?m^?2?d^?1) and sulfides competed with P for oxidized iron resulting in low iron-bound pools in the area (<4% of total P pools). Sulfate reduction rates were stimulated by the resuspension of sediments, especially in August, where a subsurface maximum was found, possibly due to a mixing of labile organic matter into these layers. In contrast sulfate reduction rates were reduced in the dredging track due to removal of labile organic matter from the surface layers. The loss of P during dredging was to some extent counteracted by regeneration of iron-bound P pools in the surface layers. The release of P due to mussel dredging was estimated to be in the same order of magnitude as the annual loading from the catchments and point sources to Limfjorden.     

Rates of nitrification,distribution of nitrifying bacteria,and nitrate fluxes in different types of sediment from Danish waters

Nitrification rates, measured in different sediment types from Danish waters, are in the range of 0.3 to 1.4 mmol NO ₃ ^- m^-2 d^-1. There is no signification between sandy and muddy sediments, nor between shallow and deeper stations. The extent of nitrification is probably limited to the zone of oxygen penetration, 1.5 to 5.5 mm. There are, however, nitrifying bacteria located in the anoxic sediment layers. There relative numbers were found by measuring the nitrification potential of the sediment. These potential rates (22°C) can also be used to calculate actual rates of nitrification, by adjusting to in situ temperature and oxygen penetration. These calculated rates agree with the actual measured rates of nitrification for a wide range of sediment types and may be used for the estimation of actual nitrification rates. Nitrate flux out from the sediment/water interface is in the range of 0 to 0.7 mmol NO ₃ ^- m^-2 d^-1. There is no correlation between concentration gradients of nitrate across the sediment/water interface and the measured flux of nitrate. Approximately 50% of nitrate production is released to the water column. The remainder (0 to 0.35 mmol N m^-2 d^-1) may have been denitrified.     

Rates of seasonal sediment reworking in Axiothella rubrocincta (Polychaeta: Maldanidae)

Sediment reworking rates of Axiothella rubrocincta (Johnson, 1901) (Polychaeta: Maldanidae) were measured in situ in Tomales Bay, California (USA), from August, 1969 through July, 1970. On the average, each adult worm (approximate fresh weight=1 g) reworks about 5 g dry sediment d^-1 at a mean temperature of 13.4°C and a mean salinity of 31.8.Reworking rates are positively correlated with temperature and salinity, and negatively with sediment organic carbon, sedimentation rates and grain size. An inverse correlation exists between sediment reworking rates (g dry sediment g^-1 wet wt of worm d^-1) and g wet weight of worm. Sediment parameters deseribing unworked sediments are not significantly different from those for fecal sediments. Although these data suggest this species to be a non-selective deposit-feeder, it is more likely that it is faculatively selective.     

Effect of algal bloom deposition on sediment respiration and fluxes

Using sediment cores collected in November 1989 from Aarhus Bight, Denmark, the fluxes of O₂, CO₂ (total CO₂), NH ₄ ⁺ , NO ₃ ^– +NO ₂ ^– and DON (dissolved organic nitrogen) across the sediment-water interface were followed for 20 d in an experimental continous flow system. On day 7, phytoplankton was added to the sediment surface, to see the result of simulated algal bloom sedimentation. Benthic O₂ consumption and CO₂ efflux, 38 to 41 mmol O₂ m^-2 d^-1 and 25 to 30 mmol CO₂ m^-2 d^-1, respectively, immediately increased by 39% and 100% after the algal addition, but gradually declined to the orginal level. Fluxes of NH ₄ ⁺ (1.0 to 1.2 mmol m^-2 d^-1) and DON (0.3 to 0.9 mmol m^-2 d^-1) increased due to the organic substrate addition. NH ₄ ⁺ and NO ₃ ^– flux changed direction, becoming an efflux and influx, respectively, for a few days and a large amount of DON (max. rate 4.0 mmol m^-2 d^-1) was immediately produced either by bacterial hydrolytic activity or from autolysis of the algae. DON was the most significant nitrogen component in pore water and in terms of N-flux from sediment. A temporary stimulation of anaerobic respiration processes (sulfate reduction and denitrification) and a decrease in nitrification were indicated. After the effect of the organic addition had declined, the fluxes gradually reverted to the original rates. The halflife of the added algal material, of which 20 to 30% was very labile, was estimated to be 2 to 3 wk.     

Nitrogenous nutrition in the euphotic zone of the Central North Pacific Gyre

The uptake rates of the three nitrogen compounds ammonium, nitrate, and urea were measured in the oligotrophic North Central Pacific Gyre in August–September 1985. The measurements were performed by using ¹⁵N-labelled substrates and incubating for short-time periods (3 to 4 h) under simulated in situ conditions. Ambient concentrations of the nitrogenous nutrients were generally below 0.10 mol l^-1. The average total daily nitrogen uptake rate, integrated over the euphotic zone, was 12.5 mmol N m^-2 d^-1. Diel studies in the upper water mass resulted in a calculated phytoplankton growth rate of 1.3 d^-1. Ammonium was the dominating nutrient, accounting for on the average 54% of the total nitrogen uptake, while urea uptake represented 32% and nitrate 14%. Ammonium uptake rates at a coastal station off the Hawaiian Islands were very close to the rates found at the oceanic station. Organisms <3 m dominated the nitrogen assimilation, being responsible for about 75% of the ammonium uptake. The nitrogen uptake rates in this study seem to be higher than those found by earlier investigations in the area, but correlated well with other productivity measurements performed during the same cruise.     

Impact of the polychaete Capitella sp. I on microbial activity in an organic-rich marine sediment contaminated with the polycyclic aromatic hydrocarbon fluoranthene

Polychaetes belonging to the genus Capitella are often present in high numbers in organic-rich sediments polluted with, e.g., oil components, and Capitella spp. may have a great impact on the biogeochemistry of these sediments. We examined the influence of Capitella sp. I on microbial activity in an organic-rich marine sediment contaminated with the polycyclic aromatic hydrocarbon, fluoranthene. Capitella sp. I were added to microcosms (10 000 ind m⁻²) and the impact of a pulse-sedimentation of fluoranthene-contaminated sediment (3 mm layer) was studied for a period of 12 d after sedimentation. The sediment oxygen uptake and total sediment metabolism (TCO₂ production) increased in cores with worms (71 to 131%), whereas the anaerobic activity, measured as sulfate reduction rate 12 d after sedimentation, was lower compared to cores without worms. The effect of fluoranthene on sulfate reduction was most pronounced in the presence of worms, with a 34% reduction versus 16% in cores without worms. The reduced sulfur pools in cores with worms were smaller than in cores without worms, suggesting that the reduced anaerobic activity was caused by increased oxidation of the sediment, which may favor O₂ and other electron-acceptors (e.g. NO₃ ⁻, Fe³⁺, Mn⁴⁺) in organic matter decomposition. The sediment oxygen uptake and TCO₂ production did not show significant changes due to fluoranthene treatment, indicating that these parameters were either less sensitive to fluoranthene stress or recovered more rapidly (i.e. within 48 h) than sulfate reduction rates. Bioturbation by Capitella sp. I altered the depth profile of fluoranthene such that fluoranthene was found in deeper sediment layers (down to 2 cm) where diffusional loss and microbial breakdown probably are reduced relative to surface layers. In cores without worms, fluoranthene was found down to 1 cm, with 75% remaining in the upper 5 mm. Received: 5 December 1996 / Accepted: 11 February 1997     

Patterns of micro- and meiofaunal abundance in marine sediments,measured with the adenosine triphosphate assay

Adenosine triphosphate (ATP) measurements are used to determine vertical and seasonal distributions of microorganisms and meiofauna in sediments from a 14 m-deep mud bottom in central Long Island Sound on 12 sampling dates from April 1975 to October 1976. Below the topmost 1 cm of sediment, ATP measurements can be useful in estimating and comparing standing stocks of microorganisms and meiofauna. In the top 1 cm, however, large quantities of newly settled bivalves (Yoldia limatula, Nucula annulata, and Mulinia lateralis) and juvenile polychaetes (Owenia fusiformis) in summer and fall months account for total ATP concentrations. The ATP content of individual meiofauna ranges from 1.97 ng individual copepod nauplius^-1 to 190.7±60 ng individual M. lateralis ^-1. In general, the total ATP content of individual polychaetes and bivalves is much higher than that of individuals of other groups. However, on a mg ATP per g wet or dry tissue basis, the ATP content of micro- and meiofaunal taxa are not significantly different. In addition to providing a means for comparing micro- and meiofaunal standing stocks, ATP measurements permit examination of the relative contribution of different meiofauna to the total living biomass of meiofauna in sediments. Total sediment ATP concentrations are greatest in the top 1 cm at all seasons, and decrease with increasing depth in the sediment. Annual concentrations in the topmost centimeter average 4.22 g g dry sediment^-1 and range seasonally from 1.09 to 7.64 g g dry sediment^-1. At a depth of 10 cm, values average 0.16 and range from 0.019 to 0.35 g g dry sediment^-1. High ATP concentrations in surface sediment reflect high concentrations of microorganisms and meiofauna at the sediment-water interface. The top 2 cm of sediment contain 71% of all meiofauna, with 41% occurring in the topmost cm. In general, densities are lowest in the winter and highest during the spring and summer, averaging 490 individuals 10 cm^-2, and varying from 87 to 1366 individuals 10 cm^-2. Because of wide variation in recruitment patterns of the benthos in Long Island Sound, the extremes of the range in meiofaunal densities can be observed in the same month in two different years. In order to monitor and compare standing stocks of organisms less than 1 mm in size in sediments, the ATP assay can save hours of processing time compared with alternate methods such as direct counts.     

Interrelationships between sulfate reducing and methane producing bacteria in coastal sediments with intense sulfide production

Coastal sediments receiving different amounts of organic carbon through sedimentation were investigated with respect to sulfate reduction and methanogenic activity. Sampling was carried out at sediment temperatures of 7° and 15°C. Sulfate-reducing and methanogenic bacteria were found at all depths. Sulfate reduction decreased with depth and the highest sulfide concentrations were found a few centimeters below the sediment surface, up to 15 mM at 15°C and pH 7.1. In the same segments a maximum in the methane concentration was also found, 0.91 mM. The high sulfide concentration inhibited the methane formation from acetate but not from carbon dioxide. In the organic rich sediment sulfate reduction was limited by the diffusion of SO ₄ ⁺ into the sediment and methane production from acetate by sulfide diffusion out of the sediment. When electron acceptor concentration limits sulfate reduction, thermodynamic calculations show that the utilization of electron donors more reduced than acetate is favored. In the sediment with the high carbon-input, acetate predominated at 15°C whereas in the low carbon-input sediment hardly any short chain organic acids were detected. The possibility of a shift in sulfate reduction from acetate oxidation to acetate production is discussed.     

Microbial RNA and DNA synthesis in marine sediments

A technique for measuring rates of RNA and DNA synthesis in sedimentary microbial communities has been adapted from methods developed for marine and freshwater microplankton research. The procedure measures the uptake, incorporation and turnover of exogenous [2, ³H]-adenine by benthic microbial populations. With minor modification, it is applicable to a wide range of sediment types. Measurement of nucleic acid synthesis rates are reported from selected benthic marine environments, including coral reef sediments (Kaneohe Bay, Oahu, Hawaii), intertidal beach sands (Oahu and southern California) and California borderland basin sediment (San Pedro Basin), and comparisons are made to selected water-column microbial communities. Biomass-specific rates of nucleic acid synthesis in sediment microbial communities were comparable to those observed in water-column assemblages (i.e., 0.02 to 2.0 pmol deoxyadenine incorporated into DNA [ng ATP]^-1 h^-1 and 0.2 to 8.9 pmol adenine incorporated into RNA [ng ATP]^-1 h^-1). DNA synthesis rates were used to calculate carbon production estimates ranging from 2 g C cm^-3 h^-1 in San Pedro Basin sediment (880 m water depth) to 807 g C cm^-3 h^-1 in coral reef sediment from the Kaneohe Bay. Microbial community specific growth rate, (d^-1), estimated from DNA synthesis rates in surface sediments ranged from 0.1 in San Pedro Basin to 4.2 in Scripps Beach (La Jolla, California) intertidal sand.     

广东典型湿地环境沉积物及鱼体中多环芳烃的污染特征及风险评估

为了解广东典型湿地环境表层沉积物及鱼体中多环芳烃(PAHs)的污染特征,分别于2014年10月和2015年4月采集沉积物及鱼类样品,分析其中16种US EPA优控多环芳烃的主要来源和风险。结果表明,广东典型湿地环境表层沉积物中多环芳烃的含量范围为139.4～1 134.3 ng·g~(-1)干重,鱼类肌肉中多环芳烃含量范围为11.1～33.9 ng·g~(-1)湿重。表层沉积物中有机碳与不同环数的多环芳烃含量均呈现显著的正相关关系。来源分析的结果表明,研究区域表层沉积物中多环芳烃的主要来源为石油排放及燃烧来源的混合。风险评估的结果表明,该区域表层沉积物中多环芳烃存在一定的生态风险,需引起重视;通过食用鱼类造成的致癌风险为2.25×10~(-6)～4.23×10~(-6),略高于美国环保局(US EPA)推荐的可接受风险(10~(-6)),存在一定的潜在致癌风险。对于成年人来说,研究区域鱼类肌肉中多环芳烃产生致癌风险允许的最大日食用量(CR_(lim))范围为124.5～234.6 g·d~(-1),尽管食用这几种鱼的致癌风险不大,居民摄入时仍应加以控制。     

The sulfur cycle of a marine sediment model system

In a laboratory sediment model consisting of sand and chopped Zostera marina leaves, the sulfur cycle and the succession and zonation patterns of sulfur bacteria were studied for a period of 7 months. The pool size of different forms of sulfur was quantified, and the rate of sulfate reduction was measured with an in situ radio-tracer technique at regular intervals. From these data, transfer rates of the sulfur cycle in the system could be calculated for different periods. The rate of sulfate reduction was initially 80 nMS·cm³·day^-1; this decreased to 25 nMS·cm³·day^-1 towards the end of the experiment as the system started to become oxidized. There was a transient accumulation of elemental sulfur in the bacterial plate at the sediment surface. More than 50% of the total mineralization of the organic material was due to sulfate reduction. Comparison with the net oxygen uptake showed that, during the first 2 to 3 months (when the sediment closely resembled a natural, reduced shallow water sediment), SO₄ ^- was equally as important as O₂ for transporting oxidacion equivalents from the water to the sediment.     

Detrital pathways in a coral reef lagoon

Coral reef lagoons have generally been regarded as sinks for organic matter exported from more productive reef front and reef flat zones. The object of this study was to examine the importance of detritus as a carbon source for benthic communities in the lagoon at Davies Reef, central Great Barrier Reef. We report the results of seasonal measurements, taken in 1986, of bacterial numbers and production, protozoan numbers, community primary production and respiration in the sediments of Davies Reef lagoon. Deposition rates of organic matter in the lagoon were also measured. Deposition rates (±1 SE) of carbon ranged from 9.2 (±1.5) to 140.7 (±10.3) mg Cm^-2d^-1. Deposition rates were highest in winter and spring, lowest in summer. Rates of bacterial production ranged from 4.7 (±0.2) pmol thymidine incorporated g^-1 dry wt (DW) h^-1 in winter to 23.5 (±1.0) pmol thymidine incorporated g^-1 DW h^-1 in spring. The number of ciliates ranged from 65 (±10) to 356 (±50) cm^-3 through the year and the number of large (20 m) flagellates from 38 (±7) to 108 (±16) cm^-3. There were no clear relationships between the sediment organic content, detrital input or temperature and the rates of bacterial processes, community metabolism or the standing stocks of microbes in the lagoon. The relative significance of detritus and in situ primary production as sources of carbon in the lagoon varied with season. In summer and autumn, detritus was less important than primary production as a source of carbon (4 to 27% of total carbon input). In winter and spring, detritus input became more significant in supply of carbon to the sediments (32 to 67% of the total carbon input). The lagoon does not simply act as a sink for carbon exported from the reef flat. We calculate that only 5% of the net reef flat primary production reached lagoon sediments in summer, but nearly 40% in winter.     

Nutrient regeneration in shallow-water sediments of the estuarine plume region of the nearshore Georgia Bight,USA

Benthic community respiration and the cycling of N and P were seasonally investigated in the unprotected, sandy sediments (Z5m) of the nearshore zone of the Georgia Bight, USA in 1981 and 1982. Nutrient exchange across the sediment-water interface was calculated from a diffusive model, measured by in-situ enclosure experiments and estimated from whole core incubations. Seasonally changing pore water profiles indicated that the sediments were not in steady-state with respect to N and P and showed the characteristics of enhanced interstitial water movement by benthic animals. Over an annual period the total flux of nitrogen measured in situ averaged 1812 mol m^-2 d^-1 from the sediments. NH ₄ ⁺ flux accounted for the vast majority of the total directly measured N flux (77%), followed by nitrate + nitrite (14%), and dissolved organic nitrogen (9%). Phosphorus flux averaged 537 mol m^-2 d^-1. A large ratio of in-situ fluxes to calculated diffusive fluxes (5.2:1) indicated flux enhancement due to benthic animal activity. ammonium fluxes measured in situ did not agree well with the rate of NH ₄ ⁺ produced in incubated whole cores (11.7 mmol m^-2 d^-1). Relative rates of C, N and P release throughout the year fluctuated considerably. Generally, nutrient fluxes were not simply related to respiration or temperature. As respiration was highly correlated with temperature, however, this suggested that respiration-regeneration was temporarily decoupled from exchange across the sediment-water interface. The annual C-N-P flux stoichiometry was 130:3.1:1. Using the rate at which NH ₄ ⁺ was produced in incubated cores the stoichiometry was 120:21:1. The anomalously low N flux measured in situ was attributed to a combination of denitrification and wave-and current-induced sediment nutrient flushing. The potential for sediment flushing is high as experiments showed that sediments were fluidized or resuspended down to 25 cm during large storms. Benthic nutrient flux contributed 40% to the annual P but only 11% to the annual N requirements of the pelagic primary producers.This is Contribution No. 558 from the University of Georgia Marine Institute. This work was supported by the Georgia Sea Grant College Program maintained by the National Oceanic and Atmospheric Administration, US Department of Commerce, under Grant No. NA80AA-D-00091     

Relationships between microbial distributions and the anaerobic decomposition of organic matter in surface sediments of long Island Sound,USA

Relative rates of the anaerobic decomposition of organic matter in the upper 10 cm of sediment from two stations in central Long Island Sound, USA, were compared. Sediment samples from discrete depth intervas were incubated anoxically and changes in SO ₄ ⁼ , NH ₄ ⁺ , bacterial numbers, extractable adenosine triphosphate (ATP), organic matter, and organic carbon were measured as a function of time and temperature. At both stations (15 and 34 m water-depths, respectively), the calculated rates of SO ₄ ⁼ reduction and NH ₄ ⁺ production decreased exponentially (approximately) with depth below the sediment-water interface. Over the same depth interval, ATP concentrations dropped by a factor of 6 to 7 and bacterial numbers were lower by a factor of 2 to 3. These decreases in SO ₄ ⁼ reduction, NH ₄ ⁺ production, bacterial numbers, and ATP, reflect a change in the physiological state of microbial populations with depth in the sediment and are consistent with the conclusions that the quantity of easily utilizable organic matter changes rapidly below the sediment surface and that food limitation controls the basic depth distribution of microbial activity. The average rates of SO ₄ ⁼ reduction, 29 to 39 mM year^-1 (22°C), in the top 10 cm are similar at both stations studied here, as well as at an additional station from a previous study. In contrast, average NH ₄ ⁺ production differs by a factor of 2 at the two stations, reflecting differences in the C:N ratio of the organic matter supplied to the sediment surface and differences in particle reworking by macrofauna at each site. The apparent activation energy of SO ₄ ⁼ reduction was 19±1 kcal mole^-1 and that of NH ₄ ⁺ production, 18±3 kcal mole^-1. The overall quantity of carbon required to support the calculated average SO ₄ ⁼ reduction rate in the top 10 cm is 23 g C m^-2 year^-1 and represents 36% of all the carbon available to the benthos annually and 11% of the net primary production in the water column. Directly measured fluxes of NH ₄ ⁺ from sediments to overlying water at both stations agree well with those predicted from production rates obtained by the incubation techniques.     

Nutrient cycling between the water column and a marine sediment. I. Organic carbon

The carbon flow through the sediments at a station located in 18.3 m of water off the Scripps Institution of Oceanography, San Diego, California (USA) was determined. The parameters studied [and their mean rates of input (+) or output (-) to the benthos] were macro-detritus (+0.028 gC m^-2day^-1), fallout of particulate debris (+3.3 gC m^-2day^-1), benthic net photosynthesis during the day (-0.06 gC m^-2 daylight period^-1), burial (0 gC m^-2day^-1), benthic respiration at night (-0.28 gC m^-2 night period^-1), and resuspension (-3.0 gC m^-2day^-1). Resuspension of sediment at this station was found to have a controlling effect on the sediment organic carbon content. Benthic photosynthesis was able to provide 79% of the organic carbon required by the benthos for respiration during the daylight hours. A carbon-flow diagram linking together all of the above measurements is presented.     

Nitrogen cycling in microbial mats: rates and patterns of denitrification and nitrogen fixation

Spatial and temporal variations in nitrogen fixation and denitrification rates were examined between July 1991 and September 1992 in the intertidal regions of Tomales Bay (California, USA). Microbial mat communities inhabited exposed mudflat and vegetated marsh surface sediments. Mudflat and marsh sediments exhibited comparable rates of nitrogen fixation. Denitrification rates were higher in marsh sediments. Nitrogen fixation rates were lowest during January at both sites, whereas highest rates occurred during summer and fall. Denitrification rates were highest during fall and winter months in marsh sediments, while rates in mudflat sediments were highest during summer and fall. In mudflat sediments, nitrogen fixation and denitrification rates, integrated over 24 h, ranged from 6 to 79 mg N m^-1 d^-1 and 1 to 10 mg N m^-2 d^-1, respectively. Rates of denitrification represented between 6 and 20% of nitrogen fixation rates during the day, but exceeded or were equivalent to nitrogen fixation rates at night. The highest integrated rates of both nitrogen fixation and denitrification occurred during July, whereas, the highest percent loss occurred during spring when denitrification rates amounted to 20% of nitrogen fixation rates during the day. Over an annual cycle, inputs of fixed N to mudflat communities occurred exclusively during daylight. These results underscore the importance of determining integrated diel rates of both nitrogen fixation and denitrification when constructing N budgets. Using this approach, it was shown that microbial denitrification can represent a significant loss of combined nitrogen from mats on daily as well as monthly time scales.     

Growth of juvenile Mercenaria mercenaria and the effect of resuspended bottom sediments

The influence of silt on growth of juvenile hard clams Mercenaria mercenaria (L.) (9 mm in mean shell length) was investigated in the laboratory using mixed suspensions of algae (50x10⁶ Pseudoisochrysis paradoxa cells l^-1) and fine-grained bottom sediments (0 to 44 mg l^-1). Growth rates, expressed as percent increase in ash-free dry tissue weight, were not significantly affected by sediment concentrations up to 25 mg l^-1. Significant reduction in growth (by 16% relative to controls fed only algae), and condition of clams, occurred at 44 mg silt l^-1. The results of the 3-week growth experiment agree well with predictions made in an earlier study by integrating results of shortterm physiological measurements. Growth rates obtained with experimental algal-silt diets at 21°C (2.6 to 3.3% increase in dry tissue weight d^-1) were comparable to those determined at ambient concentrations of Great South Bay particulates at 20°C (0.9 to 4.0% d^-1). Levels of particulate inorganic matter in seawater from Great South Bay, New York, exhibited pronounced daily changes, and ranged from 6 to 126 mg dry weight l^-1. Growth enhancement by the addition of silt to an algal diet, reported in mussels, surf clams and oysters, was not found in M. mercenaria. It is suggested that these three species are better suited than hard clams for culturing efforts in inshore turbid waters above uncompacted, muddy bottoms.Contribution No. 452 from the Marine Sciences Research Center, State University of New York at Stony Brook, USA