Variation in microbial biomass and community structure in sediments of peter the great bay (sea of japan/east sea), as estimated from fatty acid biomarkers

Variation in the microbial biomass and community structure found in sediment of heavily polluted bays and the adjacent unpolluted areas were examined using phospholipid fatty acid analysis. Total microbial biomass and microbial community structure were responding to environmental determinants, sediment grain size, depth of sediment, and pollution due to petroleum hydrocarbons. The marker fatty acids of microeukaryotes and prokaryotes - aerobic, anaerobic, and sulfate-reducing bacteria -were detected in sediments of the areas studied. Analysis of the fatty acid profiles revealed wide variations in the community structure in sediments, depending on the extent of pollution, sediment depth, and sediment grain size. The abundance of specific bacterial fatty acids points to the dominance of prokaryotic organisms, whose composition differed among the stations. Fatty acid distributions in sediments suggest the high contribution of aerobic bacteria. Sediments of polluted sites were significantly enriched with anaerobic bacteria in comparison with clean areas. The contribution of this bacterial group increased with the depth of sediments. Anaerobic bacteria were predominantly present in muddy sediments, as evidenced from the fatty acid profiles. Relatively high concentrations of marker fatty acids of sulfate-reducing bacteria were associated with organic pollution in this site. Specific fatty acids of microeukaryotes were more abundant in surface sediments than in deeper sediment layers. Among the microeukaryotes, diatoms were an important component. Significant amounts of bacterial biomass, the predominance of bacterial biomarker fatty acids with abundance of anaerobic and sulfate-reducing bacteria are indicative of a prokaryotic consortium responsive to organic pollution.     

Contrasting chemical and isotopic compositions of organic matter in Changjiang (Yangtze River) estuarine and East China Sea shelf sediments

To examine the source and preservation of organic matter in the shelf sediments of the East China Sea (ECS), we measured bulk C/N and isotopes, organic biomarkers (n-alkanes and fatty acids) and compound-specific (fatty acids) stable carbon isotope ratios in three sediment cores collected from two sites near the Changjiang Estuary and one in the ECS shelf. Contrasting chemical and isotopic compositions of organic matter were observed between the estuarine and shelf sediments. The concentrations of total n-alkanes and fatty acids in the shelf surface sediments (0–2 cm) were 5–10 times higher than those in estuarine surface sediments but they all decreased rapidly to comparable levels below the surface layer. The compositions of n-alkanes in the estuarine sediments were dominated by C₂₆-C₃₃ long-chain n-alkanes with a strong odd-to-even carbon number predominance. In contrast, the composition of n-alkanes in the shelf sediment was dominated by nC₁₅ to nC₂₂ compounds. Long-chain (>C₂₀) fatty acids (terrestrial biomarkers) accounted for a significantly higher fraction in the estuarine sediments compared to that in the shelf sediment, while short-chain (<C₂₀) saturated and unsaturated fatty acids were more abundant in the shelf surface sediments than in the estuarine sediments. Stable carbon isotopic ratios of individual fatty acids showed a general positive shift from estuarine to shelf sediments, consistent with the variations in bulk δ ¹³C_TOCTOC. These contrasts between the estuarine and shelf sediments indicate that terrestrial organic matter was mainly deposited within the Changjiang Estuary and inner shelf of ECS. Post-depositional diagenetic processes in the surface sediments rapidly altered the chemical compositions and control the preservation of organic matter in the region.     

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

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

Seasonal dynamics of elemental sulfur in two coastal sediments

A spectrophotometric method for elemental sulfur (S⁰) analysis without interference from other reduced sulfur compounds was adapted for the use in reducing sediments. The S⁰ distribution in two coastal sediments was studied regularly from summer to winter and compared to factors regulating the S⁰ accumulation, such as redox potentials, the rate of bacterial sulfide production and the general sulfur chemistry. Dense coatings of sulfur bacteria developed on the sediment surface of a sulfuretum which had an S⁰ concentration of up to 41 μmol S cm^?3. The 2·5-mm thick bacterial coating contained 40% of all S⁰ in the sediment. A more typical marine sediment with a few cm thick oxidized surface layer had an S⁰ maximum of 1–3 μmol S cm^?3 at 2–4 cm depth. The S⁰ maximum in both sediments increased from summer to winter as the sediments gradually became more oxidized. The deeper layers maintained a low S⁰ concentration. Most of the S⁰ in the upper few mm of a laboratory sulfuretum was present inside sulfur bacteria and actively migrated up and down with the bacteria depending upon the changing light and oxygen conditions.     

Total Lipid and Fatty Acid Classes in Decomposing Mangrove Leaves of <Emphasis Type="Italic">Bruguiera gymnorrhiza</Emphasis> and <Emphasis Type="Italic">Kandelia candel</Emphasis>: Significance with respect to Lipid Input

Changes in the concentration of total lipid and fatty acids (FAs) during the decomposition of mangrove leaves were investigated by field experiments using yellow leaves of Bruguiera gymnorrhiza (L.) Lamk. and Kandelia candel (L.) Druce, in order to quantify mangrove contribution to lipid and fatty acid inputs to marine sediments. Total lipid and total FA in the fresh (green and yellow) and decomposing leaves of both species were significantly higher during winter than summer. During decomposition, total lipid content and FA concentration, in particular branched chain fatty acids (BrFAs) and bacterial fatty acids (BFAs), increased to a maximum concentration in 45 days during winter and in 17 days during summer. Lipids were lost faster in K. candel leaf detritus than in B. gymnorrhiza leaf detritus in which >90% of the total lipid original weight was lost during the summer experiment and <60% during the winter experiment. The changes in the concentrations of total lipids and FAs in the decomposing leaves also indicate that mangrove leaves are significant sources of fatty acids and probably other lipid compounds to estuarine ecosystems and that tidal waters transport the lipids and FAs adsorbed to particulate matter from mangroves to adjacent estuarine sediments and the ocean.     

Suspended sediment transport of the Hangzhou Bay and its related hydrodynamic analyses

The 25-h measurements of current speed, flow direction, water depth, suspended sediment concentration and salinity were carried out at six anchored stations in the study area during spring and neap tides in winter of 1987 and summer of 1989. Caculations and analyses of the data obtained show that large amounts of suspended sediments are moved back and forth under the action of tidal current, and the net transport of sediment is small, with its predominance upstream in winter and downstream in summer. These calculations and analyses also suggest that the advective transport of sediment is dominant, while the vertical gravitational circulation of the suspended sediment comes next. Meantime, it is indicated that tidal currents play a major role in the suspended sediment transport, and residual flows have effect on the net transport of the suspended sediment, which is more remarkable during neap tide than during spring tide.     

Sulfate reduction and sulfur fixation in sediment of a historically meromictic lake,Lake Suigetsu,Japan

Vertical distributions of sulfate, hydrogen sulfide, and iron (II) concentrations in interstitial waters and of sulfur content in sediment have been studied in a sediment core (73 cm long) from a meromictic lake, Lake Suigetsu, which changed from fresh-water to brackish conditions in 1664 A.D. The diatom assemblage of the sediment has also been analyzed. A boundary between high (>1.5%) and low (<0.2%) sulfur contents is found at a depth of 52 cm in the core. In the high sulfur layer (above 52 cm), the maximum sulfur content is 6.8% at 35 to 37 cm. The diatom assemblage, however, indicates that the boundary between fresh-water and brackish sediments lies at 40 cm. The hydrogen sulfide and iron (II) profiles in the interstitial waters indicate a sink for these chemical species near a depth of 40 cm. The discrepancy between the chemically-defined boundary at 52 cm and the paleontologically-defined boundary at 40 cm seems to be due to the downward migration of hydrogen sulfide and deposition of iron sulfide after the lake became brackish.     

黄东海沉积物中的还原无机硫

Cold diffusion methods are used to separate and quantify the three reduced inorganic sulfur species into acid volatile sulfide(AVS), pyrite–S and element sulfur(ES) in the sediments of the Yellow and East China Seas. The results show that up to 25.02 μmol/g of AVS, 113.1 μmol/g of pyrite–S and 44.4 μmol/g of ES are observed in the sediments of the Yellow Sea and East China Sea. Pyrite–S is the predominant sulfide mineral in the sediments, while the concentration of AVS is quite low at most stations in the study area. The amounts and reactivity of organic matter are the primary limited factor for the sulfide formation, while an iron limitation and a sulfate limitation are not observed in the sediments of the Yellow Sea and East China Sea. The irregular profiles of the three reduced inorganic sulfur species also reflected the comprehensive influence of sediment composition and sedimentation rates.     

Functional structure of laminated microbial sediments from a supratidal sandy beach of the German Wadden Sea (St. Peter-Ording)

Hidden for the untrained eye through a thin layer of sand, laminated microbial sediments occur in supratidal beaches along the North Sea coast. The inhabiting microbial communities organize themselves in response to vertical gradients of light, oxygen or sulfur compounds. We performed a fine-scale investigation on the vertical zonation of the microbial communities using a lipid biomarker approach, and assessed the biogeochemical processes using a combination of microsensor measurements and a ¹³C-labeling experiment. Lipid biomarker fingerprinting showed the overarching importance of cyanobacteria and diatoms in these systems, and heterocyst glycolipids revealed the presence of diazotrophic cyanobacteria even in 9 to 20 mm depth. High abundance of ornithine lipids (OL) throughout the system may derive from sulfate reducing bacteria, while a characteristic OL profile between 5 and 8 mm may indicate presence of purple non-sulfur bacteria. The fate of ¹³C-labeled bicarbonate was followed by experimentally investigating the uptake into microbial lipids, revealing an overarching importance of cyanobacteria for carbon fixation. However, in deeper layers, uptake into purple sulfur bacteria was evident, and a close microbial coupling could be shown by uptake of label into lipids of sulfate reducing bacteria in the deepest layer. Microsensor measurements in sediment cores collected at a later time point revealed the same general pattern as the biomarker analysis and the labeling experiments. Oxygen and pH-microsensor profiles showed active photosynthesis in the top layer. The sulfide that diffuses from deeper down and decreases just below the layer of active oxygenic photosynthesis indicates the presence of sulfur bacteria, like anoxygenic phototrophs that use sulfide instead of water for photosynthesis.     

The key features of the bottom sediment composition in the anoxic Lake Mogil’noe

In the summer 2006, integrated geological, geochemical, hydrological, and hydrochemical studies were performed on the relict anoxic Lake Mogil’noe (down to 16 m depths) located on Kil’din Island in the Barents Sea. The chemical and grain-size composition of the bottom sediments were compared for the lake (a permanently anoxic basin) and the Baltic Sea Deeps (periodically anoxic basins). The vertical location of the hydrogen sulfide layer boundary in the lake (9–11 m depths) was practically the same from 1974 up to now. The concentrations of suspended particulate matter in the lake in June and July 2006 appeared to be close to its summer concentrations in the seawaters of the open part of the Baltic Sea. The mud from Lake Mogil’noe compared to those of the Baltic Sea Deeps are characterized by fluid and flake consistency and by pronounced admixtures of sandy and silty fractions probably of eolic origin. The lacustrine mud contain much plant remains; iron sulfides and vivanite were also found in ooze. The concentrations of 22 elements determined in the lacustrine bottom sediments were of the same levels as those found here 33 years ago. The concentrations also appeared to be close to those in the corresponding grain-size types of the bottom sediments in the Baltic Sea. The low C_org/N value (5% on average) in the mud of Mogil’noe Lake compared to the values for the mud of the Baltic Sea Deeps (10% on average) points to the considerable planktogenic component in the organic matter composition of the lacustrine mud. No indications were reveled for anthropogenic contaminations of the lacustrine bottom sediments with toxic metals.     

苏北废黄河三角洲沉积物的时空变化特征

基于苏北废黄河三角洲近岸冬夏两季的表层沉积物粒度参数分析结果,研究了沉积物的时空变化特征。结果表明,沉积物分布具有显著的时空变化特征:空间上由岸向海、由北侧至南侧,沉积物粒度呈变粗的趋势;时间上,沉积物粒径冬季粗于夏季。沉积物分选普遍较差,偏态为正偏,峰态尖锐。受季节性风的影响,冬季沉积动力环境比夏季复杂,沉积物粒度参数变化幅度较夏季大,冬季样品粒度参数等值线梯度明显大于夏季。在粒级-标准偏差曲线上,高峰值在52.55～88.39μm,为敏感粒级。研究区在潮流和季风驱动的风浪作用下,沉积环境具有波-流联合作用复杂的特点。     

Methane in the water and bottom sediments of the mouth area of the Severnaya Dvina River during the winter time

The spatial distribution of methane in the water and bottom sediments of the mouth area of the Severnaya Dvina River shows quite a similar character during the ice-cover and summer periods. It is characterized by the increase of the gas content from the head of the mouth area towards the delta with maximum values at the sites subjected to permanent anthropogenous impact. The difference consists in the higher level of the methane content in the bottom sediments during the winter period. At the same time, the excess of the winter to summer concentrations in the water was registered only at the stations within the impact area of the sources of anthropogenous contamination. As in the summer, the bottom sediments have a prevailing role in the formation of the level of the methane content in the water of the mouth area. The linear dependence between the concentrations of the total hydrogen sulfide and methane in the surface layer of the riverine sediments is probably caused by the parallel and nonconcurrent proceeding of the generation of these gases, which are controlled by the bacterial communities being not inhibitory to each other.     

Sediments of lakes Rotoroa and Rotoiti,South Island,New Zealand

The chemistry and mineralogy of the sediments of Lakes Rotoroa and Rotoiti, South Island, New Zealand (41° 51 S, 172° 38 E and 41° 50 S, 172° 50 E respectively) , were studied. In general, the cation exchange properties, element concentrations, and mineral compositions of the sediments show little variation between samples from each lake. However, some nutrient concentrations, notably exchangeable calcium, magnesium, and potassium, and total nitrogen and sulphur, are higher in sediments from weed beds than in sediments underlying deeper water; iron and manganese concentrations tend to show the opposite trend. Concentrations of exchangeable calcium and total nitrogen, sulphur, manganese, and iron generally decrease down the sediment profiles from the sediment/water interface. Differences between lakes, notably higher concentrations of illite and of the elements potassium, aluminium, titanium, and zirconium, and lower concentrations of calcium in the sediments of Lake Rotoiti than in those from Lake Rotoroa, are probably due mainly to the different geological settings rather than to differences in the diagenetic processes operating within the lakes. Mineralogical analyses indicate that the lake sediments are derived mainly from the valleys of the major inflow rivers rather than from the slopes at the sides of each lake.     

Review of recent advances in the interpretation of eastern Mediterranean sapropel S1 from geochemical evidence

The sediments of the eastern Mediterranean basin contain C_org-enriched layers (sapropels) interbedded with the C_org-poor sediments which form by far the greater part of the record. While it is generally appreciated that different surface ocean productivity and bottom water conditions are necessary for the formation and preservation of these two sediment types, less attention has been paid to diagenetic effects which are an expected consequence of transitions between dramatically different bottom water oxygenation levels. A geochemical interpretation has emerged of post-depositional oxidation of the most recent sapropel (S1), initially based on the relationship of the Mn, Fe, C_org and S concentration/depth profiles observed around S1, and the characteristic shapes of these elemental profiles known from other situations. This indicates that post-depositional oxidation has removed approximately half of the visual evidence of the sapropel (∼6 cm from a total of ∼12 cm in the deep basin). The oxidation interpretation from redox-sensitive element redistribution profiles has subsequently been consolidated with evidence from pore water (O₂, NO⁻₃, Mn²⁺ and Fe²⁺) studies, from characteristic solid phase Ba profiles which yield palaeoproductivity records, and from oxidation-sensitive indicator trace elements (I and Se). So far, these geochemical observations have been concentrated in the deeper central parts of the basin, where sediment accumulation rates are lower than on the basin margins, and radiocarbon dating indicates that S1 formation occurred between 5.3 and 9.0 ky (uncorrected conventional radiocarbon time). It remains to be demonstrated whether or not these times are applicable to the entire E. Mediterranean basin. The implications of these findings to guide sampling in future work on the S1 productivity episode and on older sapropels for palaeoenvironmental investigations are discussed.     

冬、夏季渤黄海表层沉积物粒度特征差异及其成因分析

对渤海、黄海海域冬、夏两季表层沉积物取样,通过激光粒度仪得出粒度参数,进而分析讨论冬季强的沿岸流的作用、黄海暖流、夏季冷水团的影响以及地形、海底地貌特征、物源特征等对表层沉积物分布造成的影响。结果表明,冬、夏两季渤黄海表层沉积物粒度特征总体上相差不大,但部分海域如渤海中北部、渤海中南部、北黄海西北部近渤海海峡北部海域、山东半岛东北部海域、南黄海中部沉积物粒度特征存在明显季节性差异。表层沉积物粒度特征季节性差异与地形地貌、沿岸流、黄海暖流、黄海冷水团及物源密切相关。本研究对于探讨渤黄海不同季节表层沉积物沉积特征的影响机制、了解渤黄海区海洋动力过程的季节差异有积极意义。     

Vertical transport of organic materials in the northern North Pacific as determined by sediment trap experiment

Sediment traps were deployed at 5 depths of 100 through 5,250 m to collect suspended sediments in the northern North Pacific (47°51.1'N; 176°20.6'E, 5,300 m deep) in the summer of 1978. Fatty acid composition was determined in the samples of phytoplankton, particulate matter, trap sediment and bottom sediment.Fatty acid composition of the trap sediments revealed no significant vertical trend throughout the water column from depths of 100 to 5,250 m, and were also similar to those of the phytoplankton and the particulate matter from the euphotic layer. However, a marked difference in the fatty acid composition was observed between the trap sediments and the particulate matter from deep waters. Therefore, it can be concluded that the source of fatty acids in the trap sediments is the particulate matter from the euphotic layer but not from deep waters.Unsaturated fatty acids highly susceptible to biological agents were rather abundant in the trap sediments as well as in the phytoplankton and particulate matter from the euphotic layer, however no unsaturated fatty acid was found in the particulate matter from deep waters. From these findings, it is clear that the particulate matter of the euphotic layer is transported to deep waters very rapidly. As the sinking rate of fecal pellets produced by zooplankton is in the range of ten to hundreds of meters a day, fecal pellets are assumed to be the most likely carrier of rapid-transport of organic matter including fatty acids from the euphotic layer to deep waters.     

Trace element and molecular markers of organic carbon dynamics along a shelf–basin continuum in sediments of the western Arctic Ocean

Molecular organic biomarkers together with trace element composition were investigated in sediments east of Barrow Canyon in the western Arctic Ocean to determine sources and recycling of organic carbon in a continuum from the shelf to the basin. Algal biomarkers (polyunsaturated and short-chain saturated fatty acids, 24-methylcholesta-5,24(28)-dien-3β-ol, dinosterol) highlight the substantial contribution of organic matter from water column and sea-ice primary productivity in shelf environments, while redox markers such as acid volatile sulfide (AVS), Mn, and Re indicate intense metabolism of this material leading to sediment anoxia. Shelf sediments also receive considerable inputs from terrestrial organic carbon, with biomarker composition suggesting the presence of multiple pools of terrestrial organic matter segregated by age/lability or hydrodynamic sorting. Sedimentary metabolism was not as intense in slope sediments as on the shelf; however, sufficient labile organic matter is present to create suboxic and anoxic conditions, at least intermittently, as organic matter is focused towards the slope. Basin sediments also showed evidence for episodic delivery of labile organic carbon inputs despite the strong physical controls of water depth and sea-ice cover. Principal components analysis of the lipid biomarker data was used to estimate fractions of preserved recalcitrant (of terrestrial origin) and labile (of marine origin) organic matter in the sediments, with ranges of 12–79%, 14–45%, and 37–66% found for the shelf, slope, and basin cores, respectively. On average, the relative preserved terrestrial organic matter in basin sediments was 56%, suggesting exchange of organic carbon between nearshore and basin environments in the western Arctic.     

Degradation of algal lipids by deep-sea benthic foraminifera: An in situ tracer experiment

We conducted an in situ feeding experiment using ¹³C-labeled unicellular algae in Sagami Bay, Japan (water depth, 1450 m), in order to understand the fate of lipid compounds in phytodetritus at the deep-sea floor. We examined the incorporation of excess ¹³C into lipid compounds extracted from bulk sediments and benthic foraminiferal cells. ¹³C-enriched fatty acids derived from ¹³C-labeled algae were exponentially degraded during 6 days of incubation in the sediment. Subsequent enrichments in ¹³C in sedimentary n-C_15, anteiso-C₁₇, and C₁₇ fatty acids indicated the microbial degradation of algal material and production of bacterial biomass in the sediment. We observed the incorporation of ¹³C-labeled algal phytol and fatty acids into foraminiferal cells. The compositions of ¹³C-labeled algal lipids in foraminiferal cells were different from those in the bulk sediments, indicating that foraminiferal feeding and digestion influenced the lipid distribution in the sediments. Furthermore, some sterols in Globobulimina affinis (e.g., 24-ethylcholesta-5,22-dien-3β-ol, 24-ethylcholest-5-en-3β-ol, and 23,24-dimethylcholesta-5,22E-dien-3β-ol) were newly produced via the modification of dietary algal sterols within 4–6 days. In addition to the effects of bacteria, feeding by benthic foraminifera can result in a significant reorganization of the composition of organic matter and influence benthic food webs and carbon cycling at the deep-sea floor.     

Peculiarities in the distribution of hydrocarbons in the southeastern part of the Baltic Sea

Data on the contents and compositions of the hydrocarbons (HCs)—aliphatic (AHCs) and polycyclic aromatic (PAHs)—are provided in comparison with the contents of the total organic carbon (C_org), the lipids in the particulates, and the C_org in bottom sediments. Particular attention has been paid to the distribution of the HCs in the water area of the Kravtsov oil field. It has been established that the concentrations of AHCs in the water are governed by the content of particulates, and the elevated AHC concentrations are confined to the coastal areas. In the water area of platform D-6, the sandy bottom sediments were notable for the great variability of the HC concentrations, both laterally and from year to year. In the summer of 2010, the content of AHCs averaged 40 μg/g (19% in the C_org), and that of PAHs, 23 ng/g. Natural seepage from the sediment mass is considered to be a source of HCs along with oil contamination.     

Microbial biomass response to different quantities and sources of organic matter in Brazilian coastal sediments

This study investigates the benthic microbial responses to organic matter (OM) variations in quantity and sources in two shallow water bays (Fortaleza and Ubatuba Bays) on the SE coast of Brazil on six occasions during the year. The pelagic and benthic compartments of the bays were evaluated by: (i) nutrients and chlorophyll a (Chl a) in the water column; (ii) quantity and sources of OM in the sediment (Chl a, total organic carbon and total nitrogen and lipid biomarker composition); and (iii) microbial biomass in sediments as an indicator of active benthic response. Although there were changes in water‐column nutrients during the year, Chl a was fairly constant, suggesting a regular supply of microalgae‐derived OM to the sea bottom. Based on the composition of lipid biomarkers in sediments, OM sources were classified as mostly marine and with high contributions of labile (microalgae‐derived) OM. Labile OM composition varied from diatoms in the summer to phytoflagellates in the winter and tended to accumulate in areas protected by physical disturbances in one of the bays. Microbial biomass followed this trend and was 160% higher in protected than in exposed areas. This study suggests that the coupling between labile OM and benthic microbial biomass occurs primarily in protected areas, irrespective of the time of the year. Since meio‐ and macrofaunal assemblages depend upon secondary microbial production within the sediments, this coupling may have an important role for the benthic food‐web.