Organic matter from a sediment trap experiment in the equatorial north Atlantic: wax esters,steryl esters,triacylglycerols and alkyldiacylglycerols

The vertical flux and composition of wax esters, steryl esters, triacylglycerols, and alkyldiacylglycerols in particulate matter was determined in the equatorial Atlantic Ocean by deploying sediment traps at 389, 988, 3,755 and 5,068 m. Detailed compositional analyses of these lipids were carried out by high temperature glass capillary gas chromatography and gas chromatography/mass spectrometry.The distributions of these lipids are discussed in terms of potential biological sources. Zooplankton fecal matter and intact zooplankters may represent the most important input of these compounds to the shallower two traps, while the material in the deeper two traps appears to have been biogeochemically altered. The finding of these biochemically important compounds, often unsaturated, indicates that particle transit through the water column must be relatively fast.Wax esters were most abundant in the 389 m sediment trap and decreased with increasing trap depth. Compounds ranging from C₂₈–C₄₄ were present at all depths. The major homologs were C₃₂, C₃₄ and C₃₆, most often monounsaturated. The dominant alcohol/acid combinations in the 389 m trap were $\text{C}{}_{\mathrm{18:1}}\text{C}{}_{\mathrm{14:0}}$ and $\text{C}{}_{\mathrm{18:1}}\text{C}{}_{\mathrm{16:0}}$, but in the 988 m sample, $\text{C}{}_{\mathrm{16:0}}\text{C}{}_{\mathrm{18:1}}$ was the major wax ester. A flux maximum was observed for steryl esters at 988 m. Cholesteryl esters of C_14:0, C_16:1 and C_16:0, and $\text{C}{}_{\mathrm{18:1}}\text{C}{}_{\mathrm{18:0}}$ fatty acids were the dominant steryl esters. For triacylglycerols, fluxes in the 389 and 988 m traps were similar, while the deeper pair of traps contained much less triacylglycerol. C₄₆, C₄₈, C₅₀ and C₅₂ compounds were the major triacylglycerols. Constituent fatty acids in the 389 m and 988 m samples were mainly C_14:0, C_16:1, C_16:0, C_18:1 and C_18:0. In the 988 m material, C_20:5 and C_22:6 were also dominant. A homologous series of alkyldiacylglycerols was abundant in the 389 m trap material. The alkyldiacylglycerols consisted of C₄₆–C₅₆ compounds composed of C_16:0 alkyl moieties and C_14:0, C_16:0, C_18:1, and C_18:0 fatty acids.     

Sterols in interstitial waters of marine sediments

In order to attempt to elucidate the nature of biogeochemical processes occurring at the water-sediment interface, sterols have been analysed in near bottom sea and interstitial waters collected in the eastern and western intertropical Atlantic ocean. Free and esterified sterol concentrations range from 0.2 to 82 μg l^?1 and are much higher than those found in overlying sea water, which range from 0.2 to 1.7 μg l^?1 for the dissolved fraction and from 0.01 to 0.07 μg l^?1 for the particulate fraction. Cholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol are the dominant sterols in sea and interstitial waters. The variability encountered for the relative importance of minor sterols such as 24-methylcholesta-5,24(28)-dien-3β-ol and stanols, 5α-cholest-22(E)-en-3β-ol, 5α-cholestan-3β-ol and 24-ethyl-5α-cholestan-3β-ol in interstitial water and their variation with depth is discussed in terms of diversity of inputs and bacterial activity. For sediments cored off the Mauritanian coast, a productive area characterized by an intense upwelling, the chemical signatures observed in interstitial water through stanol/stenol ratios occur at levels of very high heterotrophic aerobic bacterial biomass estimations. The study of the sterol composition of interstitial water could constitute a valuable tool in appreciating the intensity of chemical and biological processes occurring in the first few metres of recent marine sediments.     

Sources of neutral lipids in a temperate intertidal sediment

The compositions of sterol, alcohol and ketone fractions from an oxic intertidal surface sediment from Corner Inlet. Victoria, Australia, have been examined by capillary gas chromatographymass spectrometry and related to the lipids of diatoms cultured from the sediment and to lipids of the sea-grass Zostera muelleri. Of the more than thirty sterols in the sediment most appear to derive from diatoms, including the major sterol 24-methylcholesta-5.22E-dien-3β-ol. Small amounts of 24-ethylcholesterol probably derive from Zostera, with a minor diatom contribution. 5α-stanols, both fully saturated and with a side-chain double bond, represented ca. 14% of the total sterols: a significant proportion of these are probably derived from marine invertebrates. These organisms also contribute C₂₆ sterols. most of the cholesterol and possibly small amounts of Δ-sterols. The data suggest that in situ biohydrogenation of stenols was not a major process in sterol transformation in the surface oxic sediment. Alcohols ranged from C₁₂ to C₃₀ and showed an unusual abundance of unsaturated components with Δ⁹-16:1 being the major constituent. The long-chain alcohols probably originate largely from Zostera but the shorter-chain. C₁₂-C₂₀. alcohols are at least partly derived from wax esters of unknown origin. Diatoms do not contribute significant amounts of alcohols. Ketones were not major constituents of the sediment and consisted mainly of 6.10.14-trimethylpentadecan-2-one and a series of n-alkan-2-ones ranging from C₁₆ to C₃₃. The latter showed a similar distribution to that of the n-alkanes but the correspondence was not sufficient to substantiate a product-precursor relationship. Very long-chain C₃- C₃₉ unsaturated methyl and ethyl ketones, which may originate from the marine unicellular alga Emiliania huxleyi. were present in low concentration.     

青海湖沉积物中的甾醇及其演化

Ｃ_２７，Ｃ_２８和Ｃ_２９甾醇（△５.２２，△２２，△５和△５.２４（２８）和５α－甾烷醇）在青海湖三孔近代沉积物（ＱＨ，ＱＥ和ＱＯ）中被检出。其中以Ｃ_２９甾醇占优势。在ＱＥ和ＱＧ孔沉积物中还检出了低浓度的三个Ｃ_３０甾醇（４α，２３，２４－三甲基－５α－胆甾－２２Ｅ－烯醇（甲藻甾醇），４α－甲基－２４－乙基－５α－胆甾－２２Ｅ－烯醇和４α－甲基－２４－乙基－５α－胆甾烷醇］。在ＱＨ和ＱＧ孔上部层段沉积物中，Ｃ_２７，Ｃ_２９.饱和的和不饱和的甾醇的碳数分布的平行性，甾烷醇/△５－甾醇及△２２－/△５.２２－甾醇的比值随沉积物埋深而增加提供了甾酸氢化作用的间接证据。青海湖沉积物中的甾醇主要来自湖区丰富的高等植物和湖中的藻类等。在ＱＧ孔８０～１００ｃｍ和１２０～１４０ｃｍ层段沉积物中，５ａ－胆甾烷醇浓度迅速增加可能反映特殊的生物（介形类）对该二层段沉积物中高浓度的胆甾烷醇的贡献。     

Aspects of the geochemistry of sedimentary sterols in the Chang Jiang estuary

Sediment samples have been collected during the Donghai 1 cruise in January 1986 in the Chang Jiang estuarine region for which solvent extractable sterols have been analysed by GC and GC/MS and the data examined using Correspondence Factorial Analysis (CFA). The main autochthonous sterols in these sediments were cholesterol, 24-methylcholesta-5,22-dien-3β-ol and 24-methylcholesta-5,24(28)-dien-3β-ol, whereas 24-ethylcholesterol, 24-ethylcholesta-5,22-dien-3β-ol and 24-methylcholesterol were principally allochthonous. Autochthonous sterols prevailed in regions of high primary production observed in summer, and allochthonous ones dominated the sterol distribution in the accumulation region of fine sediments just outside the river mouth and in the reaches of coastal currents. The cholesterol/cholesterol ratio values exhibited higher values close to the river mouth and in the region where the turbidity maximum extends. A comparison between sediment and suspended matter indicated a great difference in the sterol content in regions of high autochthonous production which suggested that sterols have been transformed in the course of sedimentation and at the sediment/water interface. In contrast, sedimentary and suspended sterol concentrations were comparable close to the mouth of the Chang Jiang River and in the region where the turbidity maximum extends, a feature that may be attributable to sediment resuspension episodes and lower production of autochthonous sterols. These results demonstrate the distinct impact of biogeochemical processes on the sediment sterol features in different zones of the estuary.     

Use of microbial enrichments for the study of the anaerobic degradation of cholesterol

To demonstrate the potential of model microbial assemblages in studies of the biogeochemistry of complex organic molecules, anaerobic microbial populations capable of degrading cholesterol (cholest-5-en-3β-ol) have been enriched from marine sediment sources. The bacterial enrichments actively mineralized the C-4 nucleus of the cholesterol ring system to carbon dioxide when nitrate was present as the terminal electron acceptor. Nitrite was found as an intermediate in the reduction of nitrate, indicating the presence of denitrifying bacteria in the enrichments. When sulfate was supplied as the sole electron acceptor, active dissimilation of cholesterol was not observed. In enrichments containing 5 mM nitrate, 95–98% of the added cholesterol was recovered as carbon dioxide (2–5%), transformation products (20–30%), or as the unmodified sterol (70–80%). Cholesterol transformation products thus far identified include 5α- and 5β-cholestan-3β-ol, cholest-4-en-3-one, 5α-androstan-3, 17-dione and androst-4-en-3, 17 dione.     

Amino acid composition of suspended particles,sediment-trap material,and benthic sediment in the Potomac Estuary

Sediment trap deployments in estuaries provide a method for estimating the amount of organic material transported to the sediments from the euphotic zone. The amino acid composition of suspended particles, benthic sediment, and sediment-trap material collected at 2.4 m, 5.8 m, and 7.9 m depths in the Potomac Estuary was determined in stratified summer waters, and in well-mixed oxygenated waters (DO) in late fall. The total vertical flow, or flux, of material into the top traps ranged from 3 g m^?2 d^?1 in August to 4.9 g m^?2 d^?1 in October. The carbon and nitrogen fluxes increased in the deepest traps relative to the surface traps during both sampling periods, along with that of the total material flux (up to 47.3 g m^?2 d^?1 in the deepest trap), although the actual weight percent of organic carbon and organic nitrogen decreased with depth. Amino acid concentrations ranged from 129 mg g^?1 in surface water particulate material to 22 mg g^?1 in particulate material in 9-m-deep waters and in the benthic sediment. Amino acid concentrations from 2.4-mg-depth sediment traps averaged 104±29 mg g^?1 in stratified waters and 164±81 mg g^?1 in well-mixed waters. The deep trap samples averaed, 77.3±4.8 mg g^?1 amino acids in summer waters and 37±16 mg g^?1 in oxygenated fall waters. Amino acids comprised 13% to 39% of the organic carbon and 12% to 89% of the orgnaic nitrogen in these samples. Analysis of the flux results suggest that resuspension combined with lateral advection from adjacent slopes can account for up to 27% of the material in the deep traps when the estuary was well-mixed and unstratified. When the estuary was stratified in late summer, the amino acid carbon produced by primary productivity in the euphotic zone decreased by 85% (86% for total organic carbon) at the pycnocline at 6 m depth, leaving up to 15% of the vertical organic flux available for benthic sediment deposition.     

Free,esterified and residual bound sterols in Black Sea Unit I sediments

Detailed compositional data for the sterols isolated from a surface, Unit I, sediment from the Black Sea are reported. A procedure based on digitonin precipitation has been used to separate the more abundant free sterols from those occurring in esterified forms. Saponification of the solvent extracted sediment residue liberated only a small quantity of residual bound sterols in contrast to studies of other sediments. 4-Methylsterols are much more abundant than 4-desmethylsterols in both the free and esterified sterol fractions which we attribute to a major dinoflagellate input, as in deeper Unit II sediment. The desmethylsterol fraction appears to derive from a variety of sources including dinoflagellates, coccolithophores, diatoms, terrigenous detritus and perhaps invertebrates. 5α(H)-Stanols are particularly abundant in the free sterol fraction. An analysis of the stanol/stenol ratios suggests that the 4-desmethyl-5α(H)-stanols are the result of specific microbial reductions of Δ⁵-sterols and/or the reflection of a contribution of stanol containing source organisms.     

Vertical sediment fluxes and wave-induced sediment resuspension in a shallow-water coastal lagoon

The present study describes variations in the vertical fluxes measured concurrently with sediment traps at both a shallow water (4 m) and a deeper water (7.5 m) position in a coastal lagoon in April 1995. A tripod equipped with five sediment traps (trap openings at 0.35 m, 0.75 m, 1.05 m, 1.40 m, and 1.80 m above the seabed) was placed at the shallow water position. This tripod was deployed three times during the study period and deployment periods varied between 2 d and 5 d. The second sediment trap, placed at the deep water position in the central part of the lagoon, measured vertical flux for intervals of 12 h at 1.4 m above the seabed. The horizontal distance between the sediment traps was 8 km. The average maximum vertical flux at the shallow water position reached 27.9 g m⁻² d⁻¹ during a period of high, westerly wind speeds, and a maximum vertical flux of 16.9 g m⁻² d⁻¹ was reached at the deep water position during a period of high, easterly wind speeds. Both strong resuspension events were closely related to increased wave shear stress derived from surface waves. Maximum wave-induced resuspension rate was 10 times higher at the shallow water position and 3.8 times higher at the deep water position compared with the net sedimentation rate in the lagoon. Small resuspension events occurred at the shallow water position during periods of increased current shear stress, Estimations of conditions for transport of sediment between shallow water and deep water showed that particles must be resuspended to a height between 3 m and 4 m and that current speeds must be higher than about 0.1 m s⁻¹. An average sedimentation rate of 3.8 g m⁻² d⁻¹ was obtained at the shallow water position during a period without wave shear stress and low current shear stress. This rate measured by sediment traps is similar to a net sedimentation rate in the lagoon of 4.4 g m⁻² d⁻¹, which was determined by radiocarbon dating of a sediment core (Kristensen et al. 1995).     

Organic geochemical studies of a recent Inner Great Barrier Reef sediment—I. Assessment of input sources

The concentration of solvent-extractable (SE) and bound hydrocarbons, ketones, alcohols, sterols, monocarboxylic acids, hydroxyacids and α,ω-dicarboxylic acids of a surface sample (0–2.5 cm) from a recent carbonate tropical sediment taken at 60ft (18m) depth north-west of the Low Isles, North Queensland, are reported in detail. n-Alkanes are a minor constituent in the SE and bound hydrocarbon fractions with the major component being unresolved complex hydrocarbon material which is not an anthropogenic input. 6,10,14-Trimethylpentadecan-2-one derived from chlorophyll, as well as phytol, were identified as major components of the ketone and alcohol constituents. No chlorophyll pigments or pigmented degradation products were present, implying degradation before incorporation into the sediment. This conclusion is consistent with evidence for higher-plant input in which all sensitive lipids have been degraded before incorporation. SE and bound fractions have been studied in detail and show considerable differences probably arising from the importance of viable biomass in the SE fraction. Inputs to the sediment are determined as higher plants, algae, bacteria, fungi and meiofauna in descending order of importance.     

Gulf of Mexico hydrocarbon seep communities—IX. Sterol biosynthesis of seep mussels and its implications for host-symbiont association

The sterol analysis of six hydrocarbon seep mussels (mytilid II and mytilid Ib) from the Alaminos Canyon in the Gulf of Mexico is reported. The sterol composition of the mussel-symbiotic bacteria complex reflects a preponderant synthesis of 4α-methyl sterols (seep mytilid II), and a predominant biosynthesis of 4-desmethyl sterols with some amounts of 4α-methyl sterols (seep mytilid Ib). This suggests a methane-based symbiotic relationship between the mussels and methanotrophic bacteria. It also suggests that the biosynthesis of sterols in the mussel-bacteria complex is completed to the level of cholest-5-en-3ß-ol (mytilid II) or 5α-cholestan-3ß-ol (mytilid Ib).     

Sterols as source indicators of organic materials in sediments

The sterol contents of plankton and sediment samples have been determined. Cholesterol, 22-dehydrocholesterol, brassicasterol, campesterol, 24-methylenecholesterol, stigmasterol, and β-sitosterol are the major sterols in these samples. Systematic variations have been observed in the compositions of the sterols from bay and Gulf of Mexico plankton and from sediments collected in the mouth of the Aransas River, Capano Bay, Aransas Bay, Corpus Christi Bay and the Gulf near Port Aransas. The concentration of cholesterol increases as the concentration of β-sitosterol decreases in the sample sequence of river inlet sediments-bay sediments-Gulf sediments-bay and Gulf plankton. Calculations based on either the cholesterol or the β-sitosterol contents of the sterol fractions of the fresh water sediments and the Gulf plankton and sediments suggest that 34% of the sterols in the Gulf sediments are derived from terrigenous sources.     

Free and bound lipids in recent (1835–1987) sediments from Santa Barbara Basin

In order to study the first steps of incorporation of lipids in recent organic-rich sediments into free and bound fractions, we have selected the Santa Barbara Basin, off California. This basin with a maximum of 590 m water depth is characterized by high phytoplankton production in surface waters and a low oxygen content in bottom waters. Sediments show the following features: high sedimentation rate ≈ 4 mm/yr, no bioturbation, and development of a bacterial mat community at the surface with predominance of sulphur-oxidizing bacteria trapping particles, thus preventing re-distribution of sediment, which permit a unique observations of organic sedimentation on a new few years basis.A sediment core has been divided into 2 cm thick slices corresponding to a time resolution of ≈5 years, from 1835 up to 1987. Samples have been analyzed using a multi-parameter approach, such as for plankton species identification, carbon stable isotope ratios and, as reported here, for lipid organic tracers. Organic tracers have been analyzed in the sterol and fatty series for both free and bound compounds by gas chromatography and gas chromatography/mass spectrometry.The incorporation processes of sterols with depth appear different for free and bound compounds. Total free sterol concentrations show high values in surface sediments (≈ 100 μg/g dry sediment), rapidly decrease up to ≈ 10 cm depth and remain at a constant value of 30–40 μg/g. Total bound sterol concentrations show low values in surface sediment (13.6 μg/g), and vary irregularly with depth up to a value of 55 μg/g at 7.3 m, and then remain constant at 25–26 μg/g.Profiles of evolution with depth of free C₂₇, C₂₈ sterols show a regular decrease, whereas C₂₉ sterols show an irregular decrease with anomalies at 7.3 cm (approximately age: 1977–1978) and at 14.1 cm (1962–1964). Profiles of bound compounds are rather different, very regular for C₂₈ sterols, irregular with oscillations for C₂₇ and C₂₉ sterols at 7.3, 12.2 and 18.0 cm (1954–1956), suggesting a different mode of incorporation and probably different inputs for C₂₇, C₂₉ and C₂₈ sterols.The study of the kinetics of degradation of sterols between surface (1987) and 10.5 cm (1968–1972) shows that C₂₇ compounds are degraded at a slight higher rate (0.53 μg/μg of initial C₂₇ concentration/ year) than are C₂₈ and C₂₉ compounds (0.047 μg/μg of initial concentration/year). An intermediate value is found for brassicasterol: 0.049 μg/μg/year.     

Sterol diagenesis in Recent sediments from Buzzards Bay,Massachusetts

Sections of a 42 cm core from one area and surface sediments from two other areas of Buzzards Bay, Massachusetts were analyzed for sterols. The distribution of sterols in surface sediments appears to be influenced by benthic faunal composition or factors controlling this composition. A comparison of sterols extracted by Soxhlet-extraction with sterols extracted after subsequent saponification of the sediment showed that the former sterols decrease in concentration while the latter saponified sterols increase in concentration with depth. In both fractions, the individual sterol compositions are similar. A transformation between Soxhlet-extractable and non-Soxhlet-extractable saponified sterols appears to be occurring over the 130 yrs sampled by the core. This transformation is probably due to chemical/physical processes rather than biological processes. Comparison with fatty acid data shows that early diagenesis of sterols is slower than early diagenesis of fatty acids.     

Decadal Change in Sediment Community Oxygen Consumption in the Abyssal Northeast Pacific

Long time-series studies are critical to assessing impacts of climate change on the marine carbon cycle. A 27-year time-series study in the abyssal northeast Pacific (Sta. M, 4000 m depth) has provided the first concurrent measurements of sinking particulate organic carbon supply (POC flux) and remineralization by the benthic community. Sediment community oxygen consumption (SCOC), an estimate of organic carbon remineralization, was measured in situ over daily to interannual periods with four different instruments. Daily averages of SCOC ranged from a low of 5.0 mg C m^?2 day^?1 in February 1991 to a high of 31.0 mg C m^?2 day^?1 in June 2012. POC flux estimated from sediment trap collections at 600 and 50 m above bottom ranged from 0.3 mg C m^?2 day^?1 in October 2013 to 32.0 mg C m^?2 day^?1 in June 2011. Monthly averages of SCOC and POC flux correlated significantly with no time lag. Over the long time series, yearly average POC flux accounted for 63 % of the estimated carbon demand of the benthic community. Long time-series studies of sediment community processes, particularly SCOC, have shown similar fluctuations with the flux of POC reaching the abyssal seafloor. SCOC quickly responds to changes in food supply and tracks POC flux. Yet, SCOC consistently exceeds POC flux as measured by sediment traps alone. The shortfall of ~37 % could be explained by sediment trap sampling artifacts over decadal scales including undersampling of large sinking particles. High-resolution measurements of SCOC are critical to developing a realistic carbon cycle model for the open ocean. Such input is essential to evaluate the impact of climate change on the oceanic carbon cycle, and the long-term influences on the sedimentation record.     

Calcite dissolution in two deep eutrophic lakes

The calcium cycle, in particular carbonate dissolution, was analyzed in two deep eutrophic lakes, Lago di Lugano (288 m maximum depth) and Sempachersee (87 m) located in Switzerland. A box model approach was used to calculate calcite dissolution in the water column and at the sediment-water interface based on various lake monitoring data such as sediment traps, sediment cores, water and pore-water analysis. A model for stationary conditions allowing the calculation of calcite dissolution in the water column for a given particle size distribution was developed. The relative values of the simulated flux were consistent with sediment trap observations. The best fit of the dissolution rate constant of sinking calcite in Lago di Lugano was on the same order of magnitude (3 · 10⁻¹⁰ kg^1/3 s⁻¹) as published laboratory values for this surface controlled process.Both lakes show a similar specific calcite precipitation rate of 170 g Ca m⁻² a⁻¹. The diffusive flux across the sediment-water interface amounts to about 15 and 10% of total calcite precipitation in Sempachersee and Lago di Lugano, respectively. However, 61% of the precipitated calcite is dissolved in the water column of Lago di Lugano compared to only 13% in Sempachersee. These results point towards the importance of grain size distributions and settling times in stratified deep waters as the two most important factors determining calcite retention in sediments of hard water lakes.     

Horizontal and vertical variations in sedimentation and resuspension rates in a stratifying lake – effects of internal seiches

The temporal and spatial variations in the rate of sedimentation and sediment resuspension in Lake Rehtijärvi (southern Finland) were studied by sediment traps deployed in the stratifying and non‐stratifying regions of the lake. Both the gross sedimentation rate and the resuspension rate recorded by near‐bottom traps were significantly higher in the stratifying region (>20 m depth) than in the shallow part of the lake (2 m depth). These rate changes were caused by temporarily elevated hypolimnetic resuspension rates exceeding 40 g dry weight m^?2 day^?1 in July 2005, whereas in the shallow station no such peak was observed and the resuspension rate remained below 30 g dry weight m^?2 day^?1 throughout the study. The elevated resuspension rate in the hypolimnion could not be attributed to surface wave action or sediment disturbance by fish, but was due to the occurrence of internal seiches. The seiche amplitude was on average the highest in July, although the wind velocities were low; this was because winds temporarily were blowing along the longitudinal axis of the lake. Due to the steep slopes, the sediments of Lake Rehtijärvi are prone to redistribution and sediment focusing towards the accumulation areas also affected trap catches in the deep stations.     

A study of oxic/anoxic effects on degradation of sterols at the simulated sediment–water interface of coastal sediments

Laboratory incubation experiments were conducted to determine the behavior of sterols at the sediment–water interface in oxic and anoxic sediments. Both plankton and 4- -cholesterol were used as tracers. Cholesterol was rapidly degraded at the sediment–water interface: 55% of -cholesterol was lost from sediments under anoxic conditions and 78% under oxic conditions over three and one-half months. About 3% of initially-added free radiolabel was incorporated into a bound pool that was released only by saponification of solvent-extracted sediment. Less than 1% of initially-added radiolabel remained in pore waters after three and one-half months. Rate constants for degradation of cholesterol in oxic and anoxic surficial sediments were estimated by tracking variations in radioactivity and sterol concentration as a function of time. We discuss our results in terms of factors affecting sterol degradation in coastal marine sediments, including molecular structure, sediment matrix effect, and redox conditions.     

Environmental impact of salmon net-pen culture on marine benthic communities in Maine: A case study

The environmental impacts of salmon net-pen aquaculture on the benthic environment were investigated at a commercial fish farm located in coastal Maine waters. This site has a sandy mud bottom and low current velocities, is subjected to episodic sediment resuspension, and way in production for 3 yr prior to this study: We examined both the increase in carbon flux to the benthos caused by the net-pen and the effects of the elevated flux on sediment biogeochemistry and the microbenthic communities. The experimental design involved the establishment of two study sites, an ambient site ca. 100 m from the net-pen and a treatment site around the pen. Sediment traps deployed 1 m above the sediment-water interface indicated that carbon flux to the benthos was increased 1-fold to 6-fold (to a maximum of 5 g m^?2d^?1) at the edge of the net-pen with little or no increase in carbon flux 10 m from the pen. Unlike carbon flux rates, sediment organic matter inventories showed a complex pattern of change over time. Mineral surface area, organic carbon and nitrogen, digestible protein, and sterol content were initially (April 1991) lower beneath the pen than in ambient sediments. During 1991 ambient sediment accumulated organic matter until July after which it decreased, to a low during November. In contrast, organic matter inventories of sediment beneath the pen remained low until July and then increased to a high during November. These latter gains were associated with the development of bacterial mats at the sediment-water interface. Beneath the pen, microbial and macrofaunal communities were shifted toward those commonly associated with organic enrichment but seasonal trends and storm-related resuspension events also significantly affected these sediment communities. When abundant, most epibenthic organisms were more numerous near the pen than in adjacent ambient areas. These results suggest that net-pen aquaculture can alter the benthic ecosystem in Maine Coastal waters but indicate that the effects are spatially limited.