Biochemical Composition Of Organic Matter Research Articles

Open-Ocean Minima in δ13C Values of Particulate Organic Carbon in the Lower Euphotic Zone

Extensive studies in the 1980s-1990s led to the characterization of latitudinal variations in sea surface δ13C values of particulate organic carbon (δ13CPOC), and relationships were found with CO2 concentrations, temperature, growth rates, and cell geometries. Surprisingly, no large-scale efforts have been made to describe variations in δ13CPOC values over depth in the water column. Here we compile published examples demonstrating a widespread isotopic pattern in particulate organic carbon of the upper water column. In 51 vertical profiles, δ13CPOC values in the lower euphotic zone on average are 1.4‰ lower than δ13CPOC values in the upper euphotic zone of open ocean settings. In a majority of locations this downward decrease in δ13CPOC values is > 2‰ and up to 5‰, larger than the commonly recognized vertical δ13C variation in dissolved inorganic carbon over the same depths. We briefly review hypotheses and supporting evidence offered by previous studies of individual water columns: The observed patterns could result from vertical differences in photosynthetic growth rates or community composition, biochemical composition of organic matter due to degradation, isotopic disequilibrium within the dissolved inorganic carbon pool, particle dynamics, or seasonal vertical mixing. Coordinated isotopic, biological, and seawater chemistry data are sparse, and consistent drivers of this widespread isotopic pattern are currently elusive. Further work is needed to adequately characterize the environmental conditions coinciding with this pattern, to test its origins, and to determine if the magnitude of upper water column δ13CPOC variations could be a useful marker of upper ocean carbon cycle dynamics.

Potentially toxic elements and biochemical components in surface sediments of NW Mexico: An assessment of contamination and trophic status

Abstract Concentrations of Potentially Toxic Elements (PTEs) were determined in sediments from Santa Maria La Reforma (SAMARE) coastal lagoon and Mocorito River, Mexico. Furthermore, biochemical composition of organic matter in sediments was analyzed to assess their trophic status. The PTEs concentrations followed the order Fe > Pb ∼ Zn > Cr ∼ Cu > Ni ∼ As ∼ Co > Cd. Except for As, PTEs showed correlation between them indicating a common source. As and Pb were above the threshold levels of toxic effect, but they did not exceed the levels of probable effect for biota. Metals were correlated with carbohydrates and As with lipids. Low protein/carbohydrate ratios suggest that decomposed detritus of nitrogen-rich algae as well as fresh mangrove litter can be present. These biochemical proxies suggest that meso-oligotrophic conditions prevail in SAMARE. Considering that fishing activities are practiced and that the northern portion of SAMARE was classified as a RAMSAR site, the information presented is useful to generate strategies for the management of this ecosystem.

Mediterranean Coastal Lagoons: The Importance of Monitoring in Sediments the Biochemical Composition of Organic Matter.

Transitional water ecosystems are targeted by the European Union (EU) Water Framework Directive (WFD, CE 2000/60) monitoring programs in coastal zones. Concerning sediments, activities performed for the WFD focus on a few variables concerning the biochemical composition of organic matter. Our research reports the effects of oxygen availability on the biochemical composition of organic matter in sediments to highlight levels of targeted variables in time and, according to the depth of sediment layer, both under oxygenated and anoxic conditions in a mesocosm study on sediment cores. Results provide evidence that tested factors of interest (i.e., disturbance type, oxygenic versus anoxic conditions; persistence time of disturbance, 0–14 days; penetration through sedimentary layers, 0–10 cm depth) are able to significantly affect the biochemical composition of organic matter in sediments. Large part of the variables considered in this study (total organic carbon (TOC), total phosphorous (TP), total sulphur (TS), Fe, carbohydrates (CHO), total proteins (PRT), biopolymeric carbon (BPC), chlorophyll-a (Chl-a) are significantly affected and correlated to the oxygenation levels and could be good early indicators of important changes of environmental conditions. Monitoring activities performed under WFD guidelines and management strategies of Mediterranean coastal lagoon ecosystems shall include the biochemical composition of organic matter in sediment to provide an exhaustive picture of such dynamic ecosystems.

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

Catchments impacted by wildfire typically experience elevated rates of post-fire erosion and formation and deposition of pyrogenic carbon (PyC). To better understand the role of erosion in post-fire soil carbon dynamics, we determined distribution of soil organic carbon in different chemical fractions before and after the Gondola fire in South Lake Tahoe, CA. We analyzed soil samples from eroding and depositional landform positions in control and burned plots pre- and post-wildfire (in 2002, 2003, and 10-years post-fire in 2013). We determined elemental concentrations, stable isotope compositions, and biochemical composition of organic matter (OM) using mid-infrared (MIR) spectroscopy for all of the samples. A subset of samples was analyzed by 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy (CPMAS 13C-NMR). We combined the MIR and CPMAS 13C-NMR data in the Soil Carbon Research Program partial least squares regression model to predict distribution of soil carbon into three different fractions: 1) particulate, humic, and resistant organic matter fractions representing relatively fresh larger pieces of OM, 2) fine, decomposed OM, and 3) pyrogenic C, respectively. Samples from the post-fire eroding landform position showed no major difference in soil organic carbon (SOC) fractions one year post-fire. The depositional samples, however, had increased concentrations of all SOC fractions, particularly the fraction that resembles PyC, one year post-fire (2002), which had a mean of 160 g/kg compared with burned hillslope soils, which had 84 g/kg. The increase in all SOC fractions in the post-fire depositional landform position one year post-fire indicates significant lateral mobilization of the eroded PyC. In addition, our NMR analyses revealed a post-fire increase in both the aryl and O-aryl carbon compounds in the soils from the depositional landform position, indicating increases in soil PyC concentrations post-fire. After 10 years, the C concentration from all three fractions declined in the depositional landform position to below pre-fire levels likely due to further erosion or elevated rates of decomposition. Thus, we found, at this site, that both fire and erosion exert significant influence on the distribution of PyC throughout a landscape and its long-term fate in the soil system.

Quantity and biochemical composition of sedimentary organic matter around offshore gas extraction platforms of the Adriatic Sea

ABSTRACTGas platforms can exert relevant effects on various ecosystem properties of the hosting area, modifying patterns of productivity and particle sedimentation. We hypothesised that the presence of gas platforms is associated with higher organic matter (OM) contents and we tested the null hypothesis by which benthic trophic conditions do not vary significantly among gas structures with different architectural characteristics. To test these hypotheses, we investigated sedimentary OM contents and biochemical composition at increasing distance from two distinct structures: one subsea well-site and one four-leg platform (FLP). We found that sediments surrounding the structures were characterised by similar OM contents, but different biochemical compositions, and that sedimentary OM compositional change with increasing distance from the structure is confirmed only at the FLP. These results suggest that: (i) benthic eutrophication is not consistent among different structures; (ii) OM biochemical composition changes are more evident than those in OM quantity; and (iii) the effect, wherever present, disappears at a very narrow distance from the platform. As biochemical composition of OM reflects food availability for benthic organisms, our results suggest that the enrichment of macrozoobenthic communities around these structures could be the consequence of increased nutritional quality of resources.

Microbial metabolism mediates interactions between dissolved organic matter and clay minerals in streamwater.

Sorption of organic molecules to mineral surfaces is an important control upon the aquatic carbon (C) cycle. Organo-mineral interactions are known to regulate the transport and burial of C within inland waters, yet the mechanisms that underlie these processes are poorly constrained. Streamwater contains a complex and dynamic mix of dissolved organic compounds that coexists with a range of organic and inorganic particles and microorganisms. To test how microbial metabolism and organo-mineral complexation alter amino acid and organic carbon fluxes we experimented with 13C-labelled amino acids and two common clay minerals (kaolinite and montmorillonite). The addition of 13C-labelled amino acids stimulated increased microbial activity. Amino acids were preferentially mineralized by the microbial community, concomitant with the leaching of other (non-labelled) dissolved organic molecules that were removed from solution by clay-mediated processes. We propose that microbial processes mediate the formation of organo-mineral particles in streamwater, with potential implications for the biochemical composition of organic matter transported through and buried within fluvial environments.

Organic matter of the Kiziltashskie Limans

Data on biochemical ecosystem monitoring of Kiziltashskie Limans are given. The limans are the only natural water bodies in the Russian Black Sea where pascual breeding of very valuable commercial fish is implemented. The presented concentrations of dissolved and particulate organic matter are 2–5 times greater than the values typical of the coastal zone of the Black Sea (from 7.9 to 27.7 mg/L and from 0.3 to 1.5 mg/L, respectively). The main biochemical component of the dissolved organic matter is carbohydrates, and that of particulate matter is protein. The share of particulate organic matter in the Kiziltashskie Limans never exceeds 7%. The elemental (C, N, P) and biochemical composition of organic matter is studied (carbohydrates, proteins, lipids). The rates of enzymatic reactions vary from season to season. Thus, processes of hydrolytic splitting of phosphor-organic compounds during primary production are very intensive in the spring, while processes of organic matter oxidation, catalyzed by enzymes of electron-transport system, are intensive in the summer.

Organic matter composition and macrofaunal diversity in sediments of the Condor Seamount (Azores, NE Atlantic)

In recent years increasing knowledge has been accumulated on seamounts ecology; however their sedimentary environments and associated biological communities remain largely understudied. In this study we investigated quantity and biochemical composition of organic matter and macrofaunal diversity in sediments of the Condor Seamount (NE Atlantic, Azores). In order to test the effect of the seamount on organic matter distribution, sediment samples were collected in 6 areas: the summit, the northern and southern flanks and bases, and in an external far field site. Macrofauna abundance and diversity were investigated on the summit, the southern flank and in the far field site. The organic matter distribution reflected the complex hydrodynamic conditions occurring on the Condor. Concentrations of organic matter compounds were generally lower on the whole seamount than in the far field site and on the seamount summit compared to flanks and bases. A clear difference was also evident between the northern and southern slopes of the Condor, suggesting a role of the seamount in conditioning sedimentation processes and distribution of food resources for benthic consumers. Macrofauna assemblages changed significantly among the three sampling sites. High abundance and dominance, accompanied by low biodiversity, characterized the macrofauna community on the Condor summit, while low dominance and high biodiversity were observed at the flank. Our results, although limited to five samples on the seamount and two off the seamount, do not necessarily support the paradigm that seamounts are more biodiverse than the surrounding seafloor. However, the abundance (and biomass), functional diversity and taxonomical distinctiveness of the macrofaunal assemblages from the Condor Seamount suggest that seamounts habitats may play a relevant role in adding to the regional biodiversity.

Meiofauna assemblages of the Condor Seamount (North-East Atlantic Ocean) and adjacent deep-sea sediments

Seamounts are currently considered hotspots of biodiversity and biomass for macro- and megabenthic taxa, but knowledge of meiofauna is still limited. Studies have revealed the existence of highly diverse meiofauna assemblages; however most data are mainly qualitative or focused only on specific groups, thus preventing comparisons among seamounts and with other deep-sea areas. This study, conducted on Condor Seamount (Azores, North-East Atlantic Ocean), describes variation in abundance, biomass, community structure and biodiversity of benthic meiofauna from five sites located on the Condor Seamount: and one site away from the seamount. While the summit of the seamount hosted the highest alpha biodiversity, the flanks and the bases showed a rich meiofauna assemblage in terms of abundance and biomass. The observed marked differences in grain size composition of sediments reflected the oceanographic conditions impacting different sectors of the Condor seamount, and could play an important role in the spatial distribution of different meiofaunal taxa. Trophic conditions (biochemical composition of organic matter) explained 78% of the variability in the meiofauna biomass pattern while sediment grain influenced the vertical distribution of meiofauna and only partially explained meiofaunal taxa composition. This study provides a further advancement in the knowledge of meiofaunal communities of seamounts. Only a deeper understanding of the whole benthic communities (including meiofauna) will allow to elaborate effective management and conservation tools for seamount ecosystems.

Organic matter transformations in the upper mesopelagic zone of the North Pacific: Chemical composition and linkages to microbial community structure

Transformation processes in the euphotic and mesopelagic zones are of crucial importance to the biological pump and global elemental cycles. In this study, elemental stoichiometries and chemical compositions of particulate and dissolved organic matter (DOM) were investigated in the euphotic and upper mesopelagic zones of the North Pacific Subtropical Gyre. The distributions of bacterial biomarkers (D‐amino acids, muramic acid) and major biochemicals (amino acids, neutral sugars, amino sugars) indicated a direct link between microbial community structure and the biochemical composition of organic matter. Bacteria were major sources of organic C, N, and P in the upper mesopelagic zone. Heterotrophic bacterial transformations were important in the formation of biorefractory organic matter that is retained in the ocean on timescales of decades to millennia. Net removal rates for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and major biochemicals were calculated for the upper mesopelagic zone (110–300 m). Dissolved hydrolyzable amino acids, neutral sugars, and amino sugars comprised 5–18% of DOC and 4–5% of DON removed in the upper mesopelagic zone, indicating these biochemicals were important components of semilabile DOM. Net removal rates of neutral sugars were 3–10 times higher than net removal rates of amino acids and amino sugars. This suggested that neutral sugars were the most reactive component among the three classes of biochemicals. Depth‐integrated net DOC removal rates indicated that DOC comprised 19–31% of total carbon export flux in the North Pacific gyre and supplied 27–93% of bacterial carbon demand in the upper mesopelagic zone.

Benthic trophic status of sediments in a metropolitan area (Rio de la Plata estuary): Linkages with natural and human pressures

We evaluated the benthic trophic status of the Montevideo coastal zone-Rio de la Plata estuary using the quantity and the biochemical composition of sedimentary organic matter as synthetic descriptors. The spatio-temporal patterns in the biochemical composition of sedimentary organic matter were related to the presence of some natural and human pressures. Biochemical features were analyzed correlatively with the type and proximity of certain impacts to investigate the usefulness of this biochemical approach for assessing the levels of natural and anthropogenic perturbations. Chlorophyll-a, phaeopigment and the biopolymeric carbon concentrations were similar to those reported in very productive, eutrophic and anthropised estuarine areas. Total proteins (PRT) and lipids (LIP) showed the highest concentrations in the inner portion of Montevideo Bay, decreasing towards the nearby coastal areas of Punta Carretas and Punta Yeguas. Total carbohydrates (CHO) presented the lowest values in the outer stations of Montevideo Bay, but similar and higher concentrations were recorded in the inner stations of the bay and the adjacent coastal zones. PRT:CHO ratios >1 were always observed for the inner stations of Montevideo Bay thus suggesting intense detritus mineralization and an increment in their protein content due to bacterial activity. The biopolymeric carbon showed the same spatial trend observed for PRT, LPD and the PRT: CHO ratios with highest concentrations in the inner bay than in Pta. Carretas and Pta. Yeguas. Elevated contributions of PRT and LIP in the inner Montevideo Bay may be associated with anthropogenic inputs of organic matter such as sewage, food industry and petroleum hydrocarbons. Conversely, high CHO contributions in the nearby coastal areas of Pta. Carretas and Pta. Yeguas seemed to be related to autochthonous primary production and CHO temporal variability with natural oscillations in the productivity of the system. Biochemical composition of organic matter indicates hypertrophic conditions and poor environmental quality in the sediments of the inner stations of Montevideo Bay due to strongest human impact. However, biochemical descriptors showed a relative improvement of the benthic trophic status in the external coastal areas subjected to moderate levels of anthropogenic pressure. This study confirmed the utility of this biochemical approach to establish zones with distinct benthic trophic status associated to different degrees of natural and human pressures. Nevertheless, our results suggest that threshold levels for PRT, BPC and the algal contribution to the BPC that properly reflect hypertrophic conditions should be further investigated.

Elemental and biochemical composition of organic matter in the waters of the Russian sector of the Arctic Basin under the present conditions

The considered area of the Russian sector of the Arctic Basin was characterized in August–September of 2008 by the wide horizontal and vertical variability of the concentrations of dissolved and particulate organic matter (OM), as well as of its elemental and biochemical composition. The concentration ranges amounted to 51.6–434 and 2.2–18.6 μM, respectively, for the dissolved and particulate Corg; up to 1.9–30.2 μM for Norg; and up to 0.08–1.53 μM for Porg. The maximum values were characteristic for the Russian Arctic shelf. The analysis of the authors’ and published data showed that a pronounced accumulation of OM, mainly in the dissolved form, took place in the Arctic Basin within the past 12 years. The concentrations of dissolved OM were higher in the western sector of the Russian Arctic than those in the eastern sector. The main biochemical components in all the waters constituting the Arctic Ocean are carbohydrates and lipids for dissolved OM and proteins and carbohydrates for particulate matter.

Effects of artificial disturbance on quantity and biochemical composition of organic matter in sediments of a coastal lagoon

The eutrophication of the coastal lagoon of Burano (Tuscany, Italy) produces periodic toxic-anoxic events. The possibility of mitigating eutrophication of a lagoon by resuspension of sediment was tested in a three-year field experiment conducted in 2008–2009. An unreplicated before-after control-impact (BACI) study design was used to ascertain variations in the quantity and biochemical composition of organic matter in sediment artificially disturbed by a specially equipped boat. In October 2008, before beginning disturbance, January 2009, half way through the disturbance period, and May 2009, at the end of disturbance, sediment was sampled in a disturbed area and an undisturbed control area to determine chlorophyll-a (Chl-a), phaeopigments (PHAE), proteins (PRT), carbohydrates (CHO), lipids (LIP), labile organic matter (LOM), refractory organic matter (ROM), total organic carbon (TOC) and total nitrogen (TN). The disturbed area, measuring 44 ha, was divided into nine subareas, eight of which were subjected to four different frequencies of disturbance (from 2 to 5) in the period October 2008 – April 2009. Sediment was sampled in each sub-area in the three months mentioned above to determine labile and total organic matter. The results were processed by univariate and multivariate analysis using Primer 6.0, Permanova + and Prism 5.0 software. The findings were as follows: (1) a large proportion of the biopolymeric carbon consisted of labile matter throughout the lagoon; (2) higher abatement of labile organic matter was recorded in the disturbed area; (3) a lower protein:carbohydrate ratio was found in the disturbed than in the undisturbed area; (4) the C:N ratio of sediment was much lower in the undisturbed area than in the disturbed area at the end of the study period; (5) sediment and macroalgal C:N ratios did not significantly change in response to disturbance; (6) initial accumulation of organic matter from phytoplankton was greater in the disturbed area than the undisturbed area; (7) chlorophyll-a, phaeopigments and the pigment diversity index did not reflect any decline in photosynthesis in the disturbed area; (8) treatment effects increased with increasing frequency of disturbance. These findings indicate less availability of labile organic matter and therefore lower risk of extended anoxic events in the disturbed area and support our hypothesis that artificial disturbance of sediment can be used to manage lagoon environments and prevent the consequences of eutrophication.

Ecosystem health of Montevideo coastal zone: A multi approach using some different benthic indicators to improve a ten-year-ago assessment

A study on the benthic ecosystem health was performed to assess the environmental quality of Montevideo coastal zone, in view of the construction of a new sanitation system. Data were compared to previous research undertaken 10 years ago, and biochemical composition of organic matter, heavy metals, organic matter, phytopigments, benthic diatoms, macrofauna community structure and a biotic index (AMBI) were used as proxies. Results indicate an environmental quality-gradient, with the worst conditions within the inner stations of Montevideo Bay and an improvement towards the adjacent coastal zone. Higher levels of chromium, lead, phaeopigments, organic biopolymers and poor benthic macrofauna and diatom communities, characterised the hypertrophic innermost portion of Montevideo Bay. Data indicated a clear deterioration of the adjacent coastal zone comparatively to that observed 10 years ago. The complementary use of approaches not applied before (benthic diatoms and organic biopolymers) with those formerly applied improve our assessment of the trophic status and the environmental health of the area.

Living benthic foraminiferal assemblages along a latitudinal transect at 1000m depth off the Portuguese margin

The relationships between the quantity and biochemical composition of organic matter and the species diversity and community structure of living benthic foraminiferal assemblages have been investigated at four sites on the Portuguese margin (NE Atlantic). All of the sites are located at approximately 1000m depth. Two out of the four sites are located in the Nazaré and Cascais submarine canyons, while the other two are positioned on the adjacent open slope. The composition and vertical distribution in the sediment of the foraminiferal assemblages have been investigated in the topmost 10cm for the >150µm fraction and in the top cm for the 63-150µm size fraction. Foraminiferal abundance and species richness are related to the quantity and biochemical composition of the sedimentary organic matter, as well as to the stability of the sea floor. The open slope stations are characterised by a relatively low quantity and nutritional quality of organic matter. The faunas of the two open slope stations are much poorer than those found in Nazaré canyon. At both stations, there is a succession of shallow, intermediate and deep infaunal species, suggesting a fairly deep oxygen penetration into the sediment. At the northern open slope station, the faunal density is about two times lower than at the southern station, where the fauna is very largely concentrated in the uppermost half cm. This difference coincides with a lower nutritional quality of organic matter at the northern station. The faunas of the two canyons, where the quantity and nutritional quality of bio-available organic matter are higher, are very different. The rich fauna of Nazaré canyon is characterised by a strong dominance of intermediate and deep infaunal species (e.g. Melonis barleeanus and Chilostomella oolina) in superficial sediment layers, suggesting a low bottom-water oxygen concentration and a minimal oxygen penetration into the sediment. InCascais canyon themuch poorer faunas of the superficial sediment layers are characterised by the co-occurrence of shallow, intermediate and deep infaunal taxa, again suggesting a rather limited oxygen penetration into the sediment.We suggest that the relatively low densities in the Cascais canyon could reflect an early stage of ecosystem colonisation after a recent turbidite deposition.

Spatial-temporal variations in organic matter of the Sea of Azov

The spatial-temporal variations in the amount and biochemical composition of organic matter and the rates of its transformations in the ecosystems of the Russian part of the Sea of Azov are analyzed. Maximum OM concentrations are typical for Taganrog Bay. A characteristic feature of the Sea of Azov is a large proportion of particulate organic matter, which in summer in Taganrog Bay exceeded 35%. It is shown that not only the concentration of organic matter changes from season to season, but also its elementary (Corg, Norg, and Porg) and biochemical composition (proteins, carbohydrates, and lipids). The major biochemical compound of dissolved organic matter is shown to be carbohydrates (13–28%), and that of particulate matter is protein (44–51%). The hydrolytic (phosphatase and protease) and oxidation-reduction enzymes of electron-transport system demonstrate a high activity in summer. The estimated short turnover times of phosphates and protein suggest the rapid and complete utilization of organic matter in the Sea of Azov.

Viruses, prokaryotes and biochemical composition of organic matter in different types of mucilage aggregates

It has been hypothesised that several microbial processes contribute to mucilage formation and transformation. However, to date, none of these mechanisms have been consistently tested using different types of mucilage. We investigated the biochemical composition and the microbial activities occurring in 9 different types of mucilage aggregates (macroflocs, stringers, cobwebs, ribbons, stringers/cobwebs clouds, big creamy surface layer, false bottoms and anoxic false bottoms) collected during summer 2000, 2002 and 2003 in the Adriatic Sea. Larger and aged aggregates, characterized by the accumulation of carbohydrate and a low lipid fraction, displayed significant biochemical differences when compared with younger aggregates. Prokaryote abundance, C production and extracellular enzymatic activities increased from small to medium-sized aggregates (corresponding approximately to mid life span) while they decreased in larger (and aged) aggregates. The highest prokaryotic C production and enzymatic activities were coupled with highest viral abundance. In mucilage displaying the highest viral abundance a reduced incorporation of enzymatically degraded C into prokaryote biomass was observed. This result suggests a potential involvement of viruses in the impairment of the microbial loop functioning in marine aggregates. Applying a multivariate analysis to the microbial variables, 3 stages of mucilage life span can be identified: (1) an early stage characterized by a large prokaryote colonization of the aggregate and DOM accumulation; (2) a mature stage characterized by a decreased capability of incorporating degraded C into prokaryote biomass and high viral abundance and (3) an aged stage characterized by the decrease in both viral and prokaryote abundance and prokaryote activity. These results provide new insights into the microbial ecology of marine aggregates and the processes influencing their life span.

Effects of bottom trawling on the quantity and biochemical composition of organic matter in coastal marine sediments (Thermaikos Gulf, northwestern Aegean Sea)

The effects of bottom trawling on quantity and the biochemical composition of sediment organic matter were assessed, along a gradient of trophic state in a coastal area of the Gulf of Thermaikos (Aegean Sea). Total organic carbon concentrations and other organic variables (such as biopolymeric C concentrations) displayed a significant increase immediately after the initiation of trawling activities, reaching unexpectedly high values when compared to a control (i.e. samples collected before trawling events). The lack of significant changes in phytopigment content of the sediments, observed after trawling events, indicates that primary production processes did not change markedly. Since also sediment properties did not vary significantly during the period of investigation, the observed changes in sediment organic content can be related with organic matter uplift from deeper sediment layers, caused by reworking (bottom up transfer) and/or reduced C and N consumption by benthic biota. Trawling determined also significant changes in the biochemical composition of sediment organic matter. Indeed, sediment resuspension increased the hydrolysable fractions of protein and carbohydrate pools, likely to be the result of increased degradation rates under oxic conditions. Such an effect indicates that trawling may increase the quality and bioavailability of organic C to consumers; this, in turn, could modify the energetic and trophic state of the benthic systems. We conclude that bottom trawling might have important trophodynamic consequences, for benthic microbial and meiofaunal assemblages.

Sediment organic matter and meiofauna community response to long-term fish-farm impact in the Ligurian Sea (Western Mediterranean)

Quantitative and qualitative changes in meiofauna community structure were investigated to assess the impact of a fish farm, which was operating continuously for 15 years (La Spezia Gulf, W Mediterranean). Sediment samples were collected in June, July, September, October 2000 and February 2001 for the analysis of phytopigments (chlorophyll-a and phaeopigments), the biochemical composition of organic matter (proteins, carbohydrates and lipids) and related to meiofaunal parameters. Sediment organic matter reached extremely high concentrations beneath the fish cages when compared to the control. Particularly lipids, carbohydrates and chlorophyll-a were significantly higher in fish-farm sediments. On a long-term basis meiofauna displayed adaptations in sediments beneath the cages resulting in an increase of density. Organic impact on meiofaunal community structure was evident in terms of an increase of the nematodes to copepods (Ne/Co) and nauplius to copepods (Na/Co) ratios in fish farm sediments. Cumaceans and kinorhynchs were encountered in control sediments, but disappeared in fish-farm samples. These data suggest that meiofauna is a sensitive tool for evaluating the effects of organic enrichment in fish farm impacted areas.

Organic matter composition in sediments of the Oman Margin

The quantity and biochemical composition of organic matter (OM) in the sediments underlying and below oxygen minimum zone (OMZ) in the Arabian Sea were studied to provide information on the diagenetic processes of organic carbon under different environmental conditions. Concentrations of total organic carbon (TOC), total nitrogen (TN) and total hydrolysable amino acids (THAA) were significantly higher in sediments within rather than below OMZ, while those of total carbohydrates (TCHO) were slightly lower in the latter, suggesting the presence of a larger supply of labile compounds into the sea bottom at the shallower site (i.e. within OMZ). Hydrolysable amino acid and carbohydrate contribution to TOC were even lower (about 10% in surficial sediments) than the values obtained from the abyssal oligotrophic North Pacific, suggesting that OM food availability in the Oman Margin sediments within OMZ was lower than that observed at abyssal depths. The presence of the highest THAA and TCHO concentrations in the top 40 mm of the sediment core at both sites reflected the presence of bioturbation processes. In contrast with the general view of the deep sea as a stable and constant system, below OMZ in the Arabian Sea sediments, some differences were observed in the two investigated cores indicating the presence of a certain spatial variability in OM content and diagenesis.