Organic Matter Sedimentation Rates Research Articles

Wastewater Treatment Using Active Microorganisms and Evaluation of Results

In this study, samples are taken from the facility where the wastewater is located, and tests and analyzes are made with an active microorganism (EM). EM Technology means technology using Active Microorganisms. EM is not composed of a single microorganism, but a mixture of various groups of microorganisms. It has a positive and healthy effect on people, animals, and nature. EM is effective in both aerobic and anaerobic conditions. For this reason, it would be beneficial to apply it at all stages of treatment in conventional wastewater treatment systems, except for the biological reactor. When used in EM-activated wastewater, various compounds are formed that may be beneficial for wastewater (enzymes with high hydrolytic activity and some antioxidant substances). These compounds together with EM microorganisms provide high-capacity purification of wastewater. As a result of analyses using EM, it is seen that the desired values are achieved. Heavy metal determination is made in Çankırı wastewater and water treated with EM using an ICP-OES device at a lower detection limit (LOD) value of the device at ppm level. As a result of treatment with EM, it has been determined that a high concentration of strontium fell to the lower detection limit. This result shows that EM has an effective role in removing heavy metals. It has been seen that the use of EM in wastewater is extremely beneficial in terms of eliminating bad odors, increasing the sedimentation rate of organic matter, and reducing chemical oxygen demand (COD), biochemical oxygen demand (BOD), and suspended solids amount (SSM). Besides, ionization is suppressed by the anti-oxidation effect. When EM is used, the operating cost will be lower as O2 consumption can be reduced. Since the water will still contain EM microorganisms after being treated, it is thought that wherever it is discharged from the wastewater plant to nature, a great improvement will be observed in terms of vegetation and the environment.

Palaeoenvironmental Evolution Based on Elemental Geochemistry of the Wufeng-Longmaxi Shales in Western Hubei, Middle Yangtze, China

The organic-rich shales found in the Wufeng–Longmaxi Formation are typically deposited in oxygen-deficient reducing environments. One of the primary sources of debate revolves around the question of whether the anoxic bottom water found in these shales is either euxinic or ferruginous, and this matter remains unresolved. Previous studies have mostly focused on the Wufeng–Longmaxi Formation as a whole in order to understand the key factors that control organic matter accumulation (OMA). However, research on OMA for each member, including the Wufeng Formation (WF), the lower Longmaxi Formation (LLM), and the upper Longmaxi Formation (ULM), has been insufficient. This paper aims to investigate the palaeoenvironmental conditions and OMA mechanisms of the Wufeng–Longmaxi shales in western Hubei by integrating data on total organic carbon (TOC) content, mineral compositions, major and trace elements, and iron speciation. The results indicate that the Wufeng–Longmaxi shales were deposited under highly restricted hydrographic conditions, except for relatively open and upwelling conditions in the upper WF. Silica in the upper WF was primarily biogenic origin and not hydrothermal. Ferruginous conditions were the primary redox conditions for organic-rich shales except for minor formations in the lower LLM that were deposited under euxinic conditions. Due to the tectonic uplift caused by the Kwangsian Orogeny in the upper LLM, the palaeoenvironment was characterized by a warmer and wetter climate, high terrigenous influx, oxic conditions, and low productivity as the result of the insufficient nutrients caused by the weak upwelling, leading to the turnover of graptolite biozones from LM5 to LM6. The factors influencing OMA changed vertically. TOC contents have a highly positive correlation with Al content, indicating that terrigenous influx was the main factor affecting OMA in the WF, which significantly differed from patterns found in other regions. This suggests that the sedimentation rate of organic matter was higher than the terrigenous dilution rate during the WF stage. The combination of redox conditions and productivity were the main factors affecting OMA in the LLM, while terrigenous influx was the key factor controlling OMA in the ULM, resulting in the dilution of organic matter. Regions in the eastern Yiling block, which are close to the Qinling Ocean, show better prospects for shale gas exploration. This research will further facilitate the development of shale gas in this area.

Nitrogen removal processes in lakes of different trophic states from on-site measurements and historic data

Freshwater lakes are essential hotspots for the removal of excessive anthropogenic nitrogen (N) loads transported from the land to coastal oceans. The biogeochemical processes responsible for N removal, the corresponding transformation rates and overall removal efficiencies differ between lakes, however, it is unclear what the main controlling factors are. Here, we investigated the factors that moderate the rates of N removal under contrasting trophic states in two lakes located in central Switzerland. In the eutrophic Lake Baldegg and the oligotrophic Lake Sarnen, we specifically examined seasonal sediment porewater chemistry, organic matter sedimentation rates, as well as 33-year of historic water column data. We find that the eutrophic Lake Baldegg, which contributed to the removal of 20 ± 6.6 gN m−2 year−1, effectively removed two-thirds of the total areal N load. In stark contrast, the more oligotrophic Lake Sarnen contributed to 3.2 ± 4.2 gN m−2 year−1, and had removed only one-third of the areal N load. The historic dataset of the eutrophic lake revealed a close linkage between annual loads of dissolved N (DN) and removal rates (NRR = 0.63 × DN load) and a significant correlation of the concentration of bottom water nitrate and removal rates. We further show that the seasonal increase in N removal rates of the eutrophic lake correlated significantly with seasonal oxygen fluxes measured across the water–sediment interface (R2 = 0.75). We suggest that increasing oxygen enhances sediment mineralization and stimulates nitrification, indirectly enhancing denitrification activity.

Response of Microphytobenthos and Benthic Bacteria Viability to Eutrophication in a Benthic–Pelagic Coupling Mesocosm Experiment

Excessive primary productivity due to nutrient inputs is a potential problem in coastal areas when resulting in high organic matter sedimentation rates. Microphytobenthos and heterotrophic bacteria are two components of the benthic ecosystem that contribute to nutrient cycling and decomposition of organic matter. In this context, the effects of nutrient addition and the associated in situ produced organic matter on microphytobenthos community composition and benthic bacterial viability were assessed in a mesocosm experiment for 58 days. The experimental setup included triplicate mesocosms filled with sediment and water under three levels of nutrient addition (‘control’, ‘low’ and ‘high’). Benthic algal community composition was assessed using chemotaxonomy and bacterial viability was estimated using flow cytometry and a double-staining protocol. Multivariate analysis detected a significant effect of treatment and time on microphytobenthic community composition indicating a difference between control and low mesocosms and also between low and high treatments at Day 12 and Day 24 of the experiment. Nonetheless, microphytobenthos implied high resistance and redundancy of benthic algae to disturbance as all three treatments showed no significant difference in community structure between Day 0 and Day 58. Bacterial viability responded quickly to the high nutrient addition and was significantly lower than in the ‘control’ and ‘low’ treatments at Days 6 and 12. Both pelagic and benthic environmental variables were correlated to these changes in benthic community.

Temporal change in the spatial distribution of visual organic enrichment indicators at aquaculture sites in Newfoundland, Canada

Growing salmon in sea cages results in the deposition of uneaten feed and wastes which can smother organisms, increase biological oxygen demand, and alter seafloor communities. At some sites with mainly hard-bottom substrates, opportunistic annelids and mat-forming bacteria are used as visual indicators of aquaculture waste and are monitored via drop camera imaging. Additionally, high organic matter sedimentation rates can result in the proliferation of microbes that release gas; this offgassing could also serve as a useful visual enrichment indicator. Little is known of how the distribution of opportunistic annelids, microbial mats, and offgassing might change over time and space, although coverage has important repercussions in some regulatory schemes. Here, we explore spatiotemporal distributions of offgassing, bacterial mats, annelids, and epibenthic taxon richness based on existing video-monitoring data collected along transects at four Newfoundland aquaculture sites in 2011–2012. A high degree of spatial and temporal patchiness was observed at both fallow and production sites, and combinations of time, transect, and distance to cage explained the distribution of bacterial mats, annelids, and taxon richness. Expected successional patterns resulting from organic enrichment or remediation were evident in some patches. Offgassing was observed only at fallow sites and may signal a distinct and persistent phase in organic matter degradation. Results highlight the spatial heterogeneity of benthic organic enrichment indicators across transects at Newfoundland aquaculture sites, with indicators appearing to form a mosaic of patches undergoing different stages of enrichment or recovery.

The role of Mississippi River discharge in offshore phytoplankton blooming in the northeastern Gulf of Mexico during August 2010

Abstract A phytoplankton bloom covering an area > 11,000 km 2 has been reported in the northeastern Gulf of Mexico east of the Mississippi River Delta in August 2010 (30°–28° N, 90°–86° W) based on NASA Moderate Resolution Imaging Spectrometer (MODIS) chlorophyll fluorescence line height (FLH) images. The bloom appeared to be anomalous relative to a historical monthly chlorophyll FLH mean (August 2002 to 2010), and was attributed by others to the Deepwater Horizon ( DWH ) oil spill accident. Here we tested an alternative hypothesis that the eastward dispersal of the Mississippi River plume entrained in wind-driven currents contributed to the development of this phytoplankton bloom. We examined Mississippi River discharge and nutrient records, ship-based surface salinity data, and the time-series of MODIS ocean color and FLH images. Wind measurements from buoys showed winds > 6 m s − 1 first in a southerly (northward) direction August 8–10, and then in a northerly (southward) direction until August 26. Results from the American Seas-Navy Coastal Ocean Model (AmSeas-NCOM) show how coastal water was advected in wind-driven currents into the area of the northeastern Gulf of Mexico where the bloom was documented. Between July 9 and September 8, 2010, the discharge of the Mississippi River exceeded the 20-year mean. During this time, the northeastern Gulf received an estimated excess volume of approximately 8.3 × 10 9 m 3 relative to the 20-year mean. Together with findings from a particle trajectory model, we conclude that nutrients and other materials (e.g., sediments) from the excessive discharge of the Mississippi River and adjacent marshes and wind-driven currents likely contributed to the anomalous bloom observed during August 2010 and to the anomalously high organic matter sedimentation rates observed in the northeastern Gulf of Mexico in the aftermath of the Deepwater Horizon oil spill.

Ecological effects of co-culturing sea cucumber Apostichopus japonicus (Selenka) with scallop Chlamys farreri in earthen ponds

Monthly changes in sedimentation and sediment properties were studied for three different culture treatments: sea cucumber monoculture (Mc), sea cucumber and scallop polyculture (Ps-c) and scallop monoculture (Ms). Results indicated that the survival rate of sea cucumber was significantly higher in Ps-c cultures than in Mc cultures. Sea cucumber yield was 69.6% higher in Ps-c culture than in Mc culture. No significant differences in body weight and scallop shell length were found between Ps-c and Ms cultures. The mean sedimentation rate of total particulate matter (TPM) was 72.2 g/(m2·d) in Ps-c cultures, with a maximum of 119.7 g/(m2·d), which was markedly higher than that of Mc (mean value). Sedimentation rates of organic matter (OM), total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) in Ps-c cultures were also significantly higher than those in Mc cultures. TOC and TN contents of sediment increased rapidly in the first 5 months in Ms cultures and remained at a high level. TOC and TN contents in Mc and Ps-c cultures decreased during sea cucumber feeding seasons and increased during sea cucumber dormancy periods (summer and winter). The study demonstrates that co-culture of sea cucumber and scallop in earthen ponds is an alternative way to alleviate nutrient loads and improve water quality in coastal aquaculture systems. Moreover, it provides the additional benefit of an increased sea cucumber yield.

Coupling early-diagenesis and pelagic biogeochemical models for estimating the seasonal variability of N and P fluxes at the sediment–water interface: Application to the northwestern Adriatic coastal zone

An integrated numerical model for the simulation of both pelagic and benthic biogeochemical processes was obtained by fully coupling a transient early diagenesis model with a reaction-transport pelagic biogeochemical model. The integrated model was applied to a shallow coastal area of the Northern Adriatic Sea, approximately 1400 km 2 wide, with the aim of investigating: 1) the seasonal dynamics of the fluxes of macronutrients, namely N and P, at the sediment–water interface; 2) the spatial variability in both sediment concentration profiles and benthic–pelagic fluxes of NH 4 +, NO 3 −, DIP and O 2. Model results concerning the pelagic state variables compare well with field data. The outputs of the benthic biogeochemical sub-model were found to be consistent with a set of literature data on sediment biogeochemistry collected within the model domain. Even though these comparisons should not be regarded as a formal validation, they provide evidence that the model correctly simulates the main patterns of both primary production and organic matter mineralisation. Model results indicate a strong coupling between benthic and pelagic systems, since the oxygen penetration depth is small (2–4 mm) and the sediment oxygen demand, up to 20 mmol m − 2 day − 1 , is high. This is also confirmed by the importance of fluxes of macronutrients from sediments, 17.5 10 6 kg N year − 1 and 1.7 10 6 kg P year − 1 on the whole model domain, which accounted for approximately 14% and 50% of the total N and P reaching the water column on a yearly basis. Fluxes of dissolved inorganic compounds at the SWI are higher in the proximity of the coastline, where the sedimentation rate of organic matter is also more elevated. The integrated model could be further applied for research investigations and management purposes. To this regard, its use for the assessment of aquaculture impacts on sediment geochemistry and nutrient recycling looks appropriate.

Influência de um cultivo de camarão sobre o metabolismo bêntico e a qualidade da água

Atividades de cultivo de camarão tendem a aumentar a taxa de sedimentação de matéria orgânica nos corpos hídricos receptores de seus efluentes, devido à excessiva produção de produtos de excreção e sobras de ração. Dependendo das condições ambientais, o sistema pode não apresentar condições de autodepuração, gerando um grande acúmulo de nutrientes ao longo da camada sedimentar e intensificação do metabolismo bêntico levando, muitas vezes, o ambiente a um processo de eutrofização e possível situação de anoxia. Utilizaram-se, no presente estudo, incubações in situ com câmaras bentônicas transparentes e opacas, para avaliar o metabolismo bêntico em uma área de cultivo de camarão no estuário da Lagoa dos Patos, RS. Ficou evidente a existência de uma clara interferência do cultivo nos processos de regeneração bêntica de nutrientes, incrementando os fluxos de nitrogênio (na forma de amônio) e fósforo (fosfato). Embora a interferência desta atividade se tenha mostrado de abrangência local e temporária (apenas 4 a 5 meses do ano), é conveniente salientar que ela ganha maior importância em locais rasos e de circulação restrita, como é o caso de baías e enseadas, locais em que o cultivo de camarão é desenvolvido no estuário da Lagoa dos Patos.

Modelling nutrient exchange at the sediment–water interface of river systems

Summary In-stream benthic processes can play a significant role on the water quality of overlying waters flowing through a river network. In order to better understand and quantify the fate of nutrients (nitrogen, phosphorus and silica) during their travel through the river continuum, a deterministic benthic sub-model was developed with the purpose of being connected to a drainage network model. This benthic sub-model resolves the differential equations representing early diagenesis in the sediment, linking the sedimentation rate of organic matter onto the sediment to the resulting flux of nutrients across the sediment–water interface. The model has been developed for conditions where sedimentation prevails as well as for situations where net erosion prevents the built-up of a significant sediment layer and where only a biofilm can develop, attached to solid substrates. The benthic model was tested independently of the main water column biological–hydrological model to which it is intended to be coupled. For this, three case studies were chosen from the literature representing various sedimentation/erosion conditions: the 8th order river Seine (France), the water storage basin of Mery s /Oise (France), and the headwater stream Orneau (Belgium). The general benthic model has been validated for ammonium, nitrate, oxygen and phosphorus fluxes across the sediment–water interface. The capability of the model to correctly predict the observed nutrients profiles within the sediment was also validated for organic carbon, ammonium and phosphorus. An uncertainty analysis showed that using two modelling objectives (observed fluxes and concentration profiles in the sediment) strongly reduces the uncertainty in parameters calibration. A sensitivity analysis illustrated the complexity of the interacting reactions driving each variable, and justifies the usefulness of the model as a tool for understanding and predicting the behaviour of the benthic compartment of river systems.

Effects of organic effluents from a salmon farm on a fjord system. I. Vertical export and dispersal processes

Vertical flux of organic waste from a Norwegian salmon farm (which produced 2910 tonnes of fish in 19 months) located in 230-m-deep water was measured nine times in the course of 2 years by sediment traps along a transect stretching from the farm and 3 km out towards the sea. The chemical composition of the trapped material and the sediment below the traps were analyzed. The results showed that when the fish farm was empty, sedimentation rates of organic matter were similar all along the transect. Throughout the production cycle high and very variable sedimentation rates of particulate organic matter, particulate organic carbon and particulate organic nitrogen were measured within 250 m from the farm, while between 550 and 3000 m away from the farm sedimentation rates were lower and constant throughout the cycle. During the second year of production, the annual vertical flux of particulate organic carbon to the bottom adjacent to the farm was 365 g m − 2 , nine times as high as what was found 3 km away from the farm. While the sedimentation rates showed that most of the waste matter settled within 250 m of the farm, the fatty acid composition and δ 13C isotope ratio of the material in the bottom traps indicate that some components of the organic waste were transported as far as 550 to 900 m, probably due to resuspension of surface sediment. Despite the high sedimentation rates the content of sedimentary organic matter, total organic carbon and total organic nitrogen was not elevated in the sediment around the farm. However, phosphorus was found in higher concentrations in the sediments close to the farm, indicating that organic matter had settled on the sediment and been decomposed. The unchanged content of organic matter in the sediment during the production cycle showed that at this site the local resuspension and dispersal conditions and the decomposition capacity of the benthos were sufficient to prevent overloading of the locality.

Impact of suspended and off-bottom Eastern oyster culture on the benthic environment in eastern Canada

The impact of Eastern oyster culture ( Crassostrea virginica) on the benthic environment at a shellfish farm in New Brunswick, Canada, was assessed using recommended methods for routine environmental monitoring, specifically measurements of sediment redox and sulfide levels. Maximum culture density was equivalent to 4000 oyster bags per hectare, or a final oyster biomass of 8 kg m − 2 . Two culture sites, one with floating bags and one with oyster tables, as well as two reference sites were monitored over 17 months (June 2002–October 2003). Seasonal variations in sediment redox and sulfide levels were observed, but no significant differences were detected between the culture and the reference sites. Biodeposition associated with the oyster biomass contributed to increased sedimentation rates of organic matter at the oyster table site, but there was no indication of organic enrichment in the sediment. Macrofauna biomass, abundance and number of species were higher at the oyster table site than at the other sites in September 2002, but values were similar for all sites in September 2003. In this region of eastern Canada, the bays are typically shallow and the upper layers of the sediment are frequently subjected to re-suspension by wave activity and physical erosion by winter ice. Given these highly dynamic conditions and the relatively low stocking densities per hectare, we would argue that the potential impact of oyster culture on the environment should be assessed on the basis of parameters other than sediment redox and sulfide levels.

Spatial characteristics of sediment trace metals in an eastern boundary upwelling retention area (St. Helena Bay, South Africa): A hydrodynamic–biological pump hypothesis

St. Helena Bay, a retention zone located in the southern Benguela upwelling system, is an important fish nursery. However, it suffers from seasonal bottom water hypoxia causing major economic losses. Anoxic conditions are linked to sulfide fluxes from bottom sediments defined by a high sedimentation rate of organic matter. It is proposed that trace metals may play an important role in alleviating part of the ecological stress by forming sulfide complexes in such systems. A spatially intensive data set of sediment biogeochemical characteristics showed that POC and trace metals (Cr, Cu, Zn, Ni, etc.) accumulated in the central zone of the Bay. Furthermore, trace metal concentrations were strongly correlated with both POC and Al. To explain the observed biogeochemical relationships in St. Helena Bay, we propose a hypothesis that links the upwelling retention hydrodynamics, primary productivity and sediment trace metal distribution. Trace metals are incorporated into phytoplankton cells in the euphotic zone but rapidly sediment along with particulate organics, on their senescence. Both, the biological pump and the dispersion of particulates are primarily controlled by the hydrodynamics prevalent within St. Helena Bay, which also govern the retention zone in the shadow of one of the major upwelling cells. The dynamics of entrainment–stratification drives the productivity, while a residual cyclonic gyre concentrates the surface productivity within the bay. Bed-shear stresses spatially constrain the accumulation of biogenic organic matter, which governs the trace metal biogeochemistry of the sediments, along a narrow terrigenous mud belt.

The role of biogeochemical cycling for the formation and preservation of varved sediments in Soppensee (Switzerland)

We analyzed seasonally aggregated observations of temperature, conductivity, dissolved oxygen and dissolved inorganic carbon from Soppensee (District of Lucerne, Switzerland) for the yrs 1980 to 1993. Holomictic Soppensee is characterized by a strong summer stratification with a thin epilimnion separated from an anoxic hypolimnion by a strong pycnocline formed by thermal and chemical gradients. A vertical one-dimensional model was developed to simulate the observed seasonal cycles of carbon and oxygen. The processes of net community production, mineralization of organic matter, precipitation and dissolution of calcite, gas exchange, in- and outflow, sedimentation and vertical eddy diffusion are included. According to the model, the annual net community production is estimated to about 110 g C m-2 yr-1 and the annual net primary production to about 330 to 440 g C m-2 yr-1, which is a typical value for eutrophic lakes. A mass balance of the carbon cycle indicates that most of the inflow comes from groundwater which is super-saturated with respect to atmospheric CO2. Therefore the surface waters exhibit a large capacity for calcite precipitation. The results of the model are used to constrain the conditions that favor the formation of varved sediments in Soppensee during thousands of yrs. Model calculations show that the deep waters would still turn anoxic even if the sedimentation rate of organic matter were decreased to 25%. Several physical factors such as biogenic stabilization of the deep waters due to calcite dissolution and low input of wind energy are responsible for the long term anoxia in Soppensee.

Responses of deep-sea benthos to sedimentation patterns in the North-East Atlantic in 1992

In an extended deep-sea study the response of the benthic community to seasonally varying sedimentation rates of organic matter were investigated at a fixed abyssal site in the NE Atlantic (BIOTRANS station or JGOFS station L2 at 47°N–20°W, water depth >4500 m) on four legs of METEOR expedition 21 between March and August 1992. The vertical flux at 3500 m depth and temporal variations in the chloroplastic pigment concentration, a measure of phytodetritus deposition, and of total adenylates and total phospholipids, measures of benthic biomass, and of activity of hydrolytic enzymes were observed. The flux patterns in moored sediment traps of total chlorophyll, POC and total flux showed an early sedimentation peak in March/April 1992, followed by low fluxes in May and intermediate ones from June to August. Thus 1992 differed from other years, in which one large flux peak after the spring phytoplankton bloom was observed. Unusually high concentrations of chloroplastic pigments were consistently observed in March 1992, reflecting the early sedimentation input. At the same time biomass of small benthic organisms (bacteria to meiobenthos) and activity of hydrolytic enzymes were higher compared to values from March 1985 and from the following months in 1992. In May and August 1992 pigment concentrations and biomass and activity parameters in the sediment were lower than during previously observed depositions of phytodetrital matter in summer. The data imply that the deep ocean benthic community reacts to small sedimentation events with transient increases in metabolic activity and only small biomass production. The coupling between pelagic and benthic processes is so close that interannual variability in surface water production is “mirrored” by deep-sea benthic processes.

Ten Years of Artificial Mixing and Oxygenation: No Effect on the Internal Phosphorus Loading of Two Eutrophic Lakes

Hypolimnetic aeration was used in conjunction with reductions in phosphorus (P) inputs in order to lower the P concentrations in two eutrophic lakes. Based on more than 10 years of experience with artificial mixing and hypolimnetic oxygenation we conclude that the lakes' internal P cycling was not affected by increased hypolimnetic dissolved oxygen (D.O.) concentrations. We show that irrespective of oxic conditions in the hypolimnion the sediment/water interface remained anoxic due to unchanged high sedimentation rates of organic matter. This may explain why oxygenation did not increase the P retention capacity of the sediment. However, the presented information indicates that, contrary to general assumption, anoxic sediment surface and high P release rates from lake sediments may not be cause-effect related but simply two parallel symptoms of one common cause: excessive organic matter and P sedimentation exhausting the stock of hypolimnetic D.O. and exceeding the P retention capacity of the sediment after diagenesis. Therefore, it is not surprising that fighting one symptom (an anoxic hypolimnion and sediment surface) does not solve the other one (a high P release rate). These findings and considerations based on more than 10 years of experimental experience warrant a reevaluation of the well accepted theoretical management strategy of limiting lake internal P cycling by maintaining an aerobic hypolimnion and sediment surface.

Estimation of export production in the coastal Baltic Sea: effect of resuspension and microbial decomposition on sedimentation measurements

Hydrography, nutrient concentrations, primary production and sedimentation of particulate matter were studied during spring, late summer and autumn in the coastal area of the northern Baltic Sea, SW Finland. Vernal phytoplankton productivity peak and biomass maximum in early May were followed by high sedimentation rates of organic matter at the end of May. In summer, sedimentation rates of organic material were generally low. The decay rates of organic carbon in the sediment traps, estimated by measuring oxygen uptake of settled organic matter, varied between 0.005 to 0.08 d−1 and were on average 0.02 d−1. Decomposition of organic matter inside the sediment traps was mainly controlled by temperature, while also organic contents of settled material were significant. Microbial decomposition decreased sedimentation rates of organic carbon and nitrogen on average by 11% and 15%, respectively, during the whole study period of ca. 6 months. Resuspension of organic matter from sediment surface was estimated to contribute ca. 17 and 24% of the total sedimentation of organic carbon during spring and summer, respectively. Export production (i.e. primary sedimentation of organic carbon corrected by decomposition) was estimated to be 32% of the net primary production during the whole productive season and 42% in spring when the flux of primary settling material was greatest. Sedimentation of the spring bloom was the major annual supply of organic matter to the benthos (>80% of the total primary sedimentation).

Profundal Sediment Organic Content and Physical Character Do Not Reflect Lake Trophic Status, but Rather Reflect Inorganic Sedimentation and Exposure

An analysis of profundal sediment data from 83 north-temperate lakes shows that increasing inorganic sedimentation and exposure (or lake surface area) results in lower organic content and water content, and greater bulk density. Because sedimentation rates are unavailable for most lakes, we estimate sedimentation rates from readily available catchment sediment loads using a mass-balance model. The mass-balance estimate of sediment retention (per square metre of depositional zone) is an excellent predictor of measured inorganic sedimentation rates for a data set covering 19 lakes (R2 = 0.92). Multiple regressions are used to predict organic content, water content, and bulk density of profundal sediment from inorganic sedimentation rates and either exposure or lake surface area, which are surrogates for the energy of the depositional environment. These analyses explain 76, 74, and 66% of the between-lake variation in the three sediment parameters, respectively. Sediment organic content is not related to lake trophic status (chlorophyll a) and is negatively correlated with net organic matter sedimentation rates. The common occurrence of organic-rich sediments in oligotrophic shield lakes is, therefore, not a reflection of high organic matter inputs, but rather the extremely low inputs of mineral sediments to these lakes.