Lake Tanganyika’s pelagic fish sustain the second largest inland fishery in Africa and are under pressure from heavy fishing and global warming related increases in stratification. The strength of water column stratification varies regionally, with a more stratified north and an upwelling-driven, biologically more productive south. Only little is known about whether such regional hydrodynamic regimes induce ecological or genetic differences among populations of highly mobile, pelagic fish inhabiting these different areas. Here, we examine whether the regional contrasts leave distinct isotopic imprints in the pelagic fish of Lake Tanganyika, which may reveal differences in diet or lipid content. We conducted two lake-wide campaigns during different seasons and collected physical, nutrient, chlorophyll, phytoplankton and zooplankton data. Additionally, we analyzed the pelagic fish–the clupeids Stolothrissa tanganicae, Limnothrissa miodon and four Lates species–for their isotopic and elemental carbon (C) and nitrogen (N) compositions. The δ¹³C values were significantly higher in the productive south after the upwelling/mixing period across all trophic levels, implying that the fish have regional foraging grounds, and thus record these latitudinal isotope gradients. By combining our isotope data with previous genetic results showing little geographic structure, we demonstrate that the fish reside in a region for a season or longer. Between specimens from the north and south we found no strong evidence for varying trophic levels or lipid contents, based on their bulk δ¹⁵N and C:N ratios. We suggest that the development of regional trophic or physiological differences may be inhibited by the lake-wide gene flow on the long term. Overall, our findings show that the pelagic fish species, despite not showing evidence for genetic structure at the basin scale, form regional stocks at the seasonal timescales. This implies that sustainable management strategies may consider adopting regional fishing quotas.

Lake Nyos, a deep crater lake, located in the north-west of Cameroon, was permanently stratified below 50 m depth due to subaquatic sources supplying warm, salty and CO₂-enriched water into the deepest reaches. The high CO₂ content in these source waters caused the 1986 limnic eruption. The deep inflowing water is denser than the hypolimnetic water and maintains the stability of the water column, which is double-diffusively stratified. During the dry season in Feb 2002, cooling triggered the formation of a double-diffusive (DD) staircase, a sequence of homogeneously mixed layers separated by distinct stable interfaces. The initiation of the staircase was slightly below the permanent chemocline at ~50 m depth, from where the staircase expanded vertically in a diffusion-type manner for ~750 days to a maximal vertical extension of ~37 m. The staircase pattern caused the upward heat fluxes to increase which depleted the driving temperature gradient. Subsequently, the density ratio increased and reduced the upward heat flux divergence until DD progressively weakened and finally the staircase structure eroded. Based on 39 CTD profiles, we describe the DD phenomenon, explain the three distinct phases of this unique DD event, which lasted for ~ 850 days, and discuss the vertical extension of the DD zone in relation to the rates of new layer formation and layer decay. To our knowledge, this is the only observation over the entire lifespan - "from birth to death" - of a DD event in a natural water body.

Understanding genetic connectivity plays a crucial role in species conservation decisions, and genetic connectivity is an important component of modern fisheries management. In this study, we investigated the population genetics of four endemic Lates species of Lake Tanganyika (Lates stappersii, L. microlepis, L. mariae, and L. angustifrons) using reduced-representation genomic sequencing methods. We find the four species to be strongly differentiated from one another (mean interspecific F_ST = 0.665), with no evidence for contemporary admixture. We also find evidence for strong genetic structure within L. mariae, with the majority of individuals from the most southern sampling site forming a genetic group that is distinct from the individuals at other sampling sites. We find evidence for much weaker structure within the other three species (L. stappersii, L. microlepis, and L. angustifrons). Our ability to detect this weak structure despite small and unbalanced sample sizes and imprecise geographic sampling locations suggests the possibility for further structure undetected in our study. We call for further research into the origins of the genetic differentiation in these four species—particularly that of L. mariae—which may be important for conservation and management of this culturally and economically important clade of fishes.

In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium-to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for a rapid vertical transport of nitrate for a thriving nitrate-assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

Identifying patterns in genetic structure and the genetic basis of ecological adaptation is a core goal of evolutionary biology and can inform the management and conservation of species that are vulnerable to population declines exacerbated by climate change. We used reduced-representation genomic sequencing methods to gain a better understanding of genetic structure among and within populations of Lake Tanganyika's two sardine species, Limnothrissa miodon and Stolothrissa tanganicae. Samples of these ecologically and economically important species were collected across the length of Lake Tanganyika, as well as from nearby Lake Kivu, where L. miodon was introduced in 1959. Our results reveal differentiation within both S. tanganicae and L. miodon that is not explained by geography. Instead, this genetic differentiation is due to the presence of large sex-specific regions in the genomes of both species, but involving different polymorphic sites in each species. Our results therefore indicate rapidly evolving XY sex determination in the two species. Additionally, we found evidence of a large chromosomal rearrangement in L. miodon, creating two homokaryotypes and one heterokaryotype. We found all karyotypes throughout Lake Tanganyika, but the frequencies vary along a north–south gradient and differ substantially in the introduced Lake Kivu population. We do not find evidence for significant isolation by distance, even over the hundreds of kilometres covered by our sampling, but we do find shallow population structure.

Biogeochemical processes are often confined to very narrow zones in aquatic systems. Therefore, highly resolved in situ measurements are required to study these processes. Potentiometric solid-contact ion selective electrodes (SC-ISEs) are promising tools for such measurements. SC-ISEs show good performance in analyses under controlled experimental conditions. Very few sensor designs, however, can sustain the challenges of natural water matrices and external environmental conditions during in situ applications. We fabricated ammonium and pH selective SC-ISEs with functionalized multiwalled carbon nanotubes (f-MWCNT) as a solid contact. Their functionality was tested in the laboratory and applied in situ for vertical profiling in a eutrophic lake. Sensors were insensitive to strong redox changes, high sulfide concentrations, and bright daylight conditions during the application in the lake. In addition, sensors are easily fabricated and exhibit short response times (<10 s). The proposed design of SC-ISEs based on f-MWCNTs is quite suitable for high-resolution in situ profiling of ionic species in fresh water lakes.

Velocity profile measurements at high spatial and temporal resolution are required for the detailed study of solute and momentum transfer close to the sediment–water interface. Still, not many devices allow such measurements in natural systems. Recently, a bistatic acoustic current profiler has become commercially available that allows the recording of profiles at down to 1-mm resolution with a maximum frequency of 100 Hz and a profile length of 3.5 cm. This study tested the ability to characterize the turbulent flow of this profiler in a laboratory flume and in a run of the river reservoir. The tests showed that average velocities were reliably measured in the upper 2.5 cm, while the flow statistics were affected by Doppler noise and signal decorrelation. The latter is caused by the decreasing overlap between the individual beam signals. Doppler noise can be estimated and accounted for by established correction procedures, but currently there is no method to quantify the influence of signal decorrelation. Both error sources mainly affect the measured variances of the velocities, while the Reynolds stresses are reliable as long as there is no interference with the solid bottom. In the field application, most problems arise because of the necessity of coordinate system rotation, since a perfect alignment of the profiler with the current is not possible. Also, because of the coordinate system rotation, the Reynolds stresses become contaminated by noise, which can be removed by low-pass filtering. Still, this filtering results in loss of the turbulent signal, which was estimated in this study to be between 2% and 10%.

A promising profiling setup for in situ measurements in lakes with potentiometric solid-contact ion-selective electrodes (SC-ISEs) and a data processing method for sensor calibration and drift correction are presented. The profiling setup consists of a logging system, which is equipped with a syringe sampler and sensors for the measurement of standard parameters including temperature, conductivity, oxygen and photosynthetically active radiation (PAR). The setup was expanded with SC-ISEs in galvanically separated amplifiers. The potential for high-resolution profiling is investigated by deploying the setup in the eutrophic Lake Rotsee (Lucerne, Switzerland), using two different designs of ammonium sensing SC-ISEs. Ammonium was chosen as a target analyte, since it is the most common reduced inorganic nitrogen species involved in various pathways of the nitrogen cycle and is therefore indicative of numerous biogeochemical processes that occur in lakes such as denitrification and primary production. One of the designs, which uses a composite carbon-nanotube–PVC-based membrane, suffered from sulfide poisoning in the deeper, sulfidic regions of the lake. In contrast, electrodes containing a plasticizer-free methacrylate copolymer-based sensing layer on top of a conducting polymer layer as a transducer did not show this poisoning effect. The syringe samples drawn during continuous profiling were utilized to calibrate the electrode response. Reaction hotspots and steep gradients of ammonium concentrations were identified on-site by monitoring the electrode potential online. Upon conversion to high-resolution concentration profiles, fine scale features between the calibration points were displayed, which would have been missed by conventional limnological sampling and subsequent laboratory analyses. Thus, the presented setup with SC-ISEs tuned to analytes of interest can facilitate the study of biogeochemical processes that occur at the centimeter scale.

We examined methane (CH₄) oxidation pathways and associated methanotrophic communities in the water column of Lake Kivu using abundance and isotopic compositions of CH₄ and phospholipid fatty acids (PLFA), distributions of glycerol dialkyl glycerol tetraethers, and catalyzed reporter deposition fluorescence in situ hybridization (CARD–FISH) analysis. The carbon isotopic data of CH₄ indicate that aerobic CH₄ oxidation is the predominant pathway of microbial CH₄ consumption with an isotopic fractionation factor (α) of 1.022–1.038. A small amount of CH₄ is oxidized anaerobically, with an α of 1.002–1.006. Aerobic CH₄ oxidation is mediated by type II methane–oxidizing bacteria (type II MOB) based on the ¹³C depletion (δ¹³C of −40.5‰ to −43.7‰) of diagnostic C18:1ω7 fatty acids in the surface waters. CARD–FISH images and PLFA components C16:1ω7 and C16:1ω5 indicate the presence of type I MOB in the methane and nutrient–rich deep–water region. ¹³C depletion of C16:1ω7 and C16:1ω5 (δ¹³C, ∼ −40‰ to −50.6‰) in the lake water below 52 m suggests the involvement of type I MOB in methane oxidation in the anoxic deep–water regions of the lake. A novel cluster of anaerobic methane–oxidizing archaea (ANME) rather than the known ANME–1 and ANME–2 appear to be involved in anaerobic oxidation of methane (AOM). Sulfate reducing bacteria are associated with AOM in the lake based on the ¹³C depletion (δ¹³C, −38.2‰ to −45.0‰) of anteiso–methyl–C15:0 fatty acid. Methane constitutes an important carbon and energy source (up to 38%) for the heterotrophic and autotrophic communities in the lake.

Elements involved in biogeochemical cycles undergo rapid turnover at the oxic–anoxic interface of stratified lakes. Here, the presence or absence of oxygen governs abiotic and biotic processes and rates. However, achieving a detailed sampling resolution to precisely locate the oxic–anoxic interface is difficult due to a lack of fast, drift-free sensors in the working range of 10 to a few 1,000 nmol O₂ L⁻¹. Here, we demonstrate that conventional amperometric and optical microsensors can be used to resolve submicromolar oxygen concentrations in a continuous profiling mode. The amperometric drift was drastically reduced by anoxic preconditioning. In situ offset correction in the anoxic layer and a high amplification scheme allowed for an excellent detection limit of < 10 nmol L⁻¹. The optical microsensors also showed a similar performance with a detection limit of < 20 nmol L⁻¹. Their drift stability allowed for a laboratory calibration in combination with a minor in situ anoxic offset correction. The two different sensor systems showed virtually identical profiles during parallel use in stratified lakes. Both sensors were able to resolve the fine-scale structure at the oxic–anoxic interface and revealed hitherto unnoticed extended zones of submicromolar oxygen concentrations even below a steep oxycline. The zones extended up to several meters and showed substantial vertical variability. These results underline the need of a precise localization of the oxic–anoxic interface on a submicromolar scale in order to constrain the relevant aerobic and anaerobic redox processes.

Tropical East African Lake Tanganyika hosts the Earth's largest anoxic freshwater body. The entire water column holds over 23 Tg of the potent greenhouse gas methane (CH₄). Methane is formed under sulphate-poor conditions via carbon dioxide reduction or fermentation from detritus and relict sediment organic matter. Permanent density stratification supports an accumulation of CH₄ below the permanent oxycline. Despite CH₄ significance for global climate, anaerobic microbial consumption of CH₄ in freshwater is poorly understood. Here we provide evidence for intense methanotrophic activity not only in the oxic but also in the anoxic part of the water column of Lake Tanganyika. We measured CH₄, ¹³C of dissolved CH₄, dissolved oxygen (O₂), sulphate (SO₄²⁻), sulphide (HS⁻) and the transient tracers chlorofluorocarbon-12 (CFC-12) and tritium (³H). A basic one-dimensional model, which considers vertical transport and biogeochemical fluxes and transformations, was used to interpret the vertical distribution of these substances. The results suggest that the anaerobic oxidation of CH₄ is an important mechanism limiting CH₄ to the anoxic zone of Lake Tanganyika. The important role of the anaerobic oxidation for CH₄ concentrations is further supported by high abundances (up to ∼33% of total DAPI-stained cells) of single living archaea, identified by fluorescence in situ hybridization.

The diffusive boundary layer (DBL) plays an important role in the transport of electron acceptors for mineralization and oxidation processes in highly reactive sediments. We used transient numerical modeling to characterize the effects of the DBL thickness on solute dynamics in the sediments of Lake Alpnach. Our model study shows that the DBL mainly influences short-term sedimentary denitrification by resisting transport. The DBL also governs the reoxidation of reduced compounds by controlling the oxygen penetration depth in the sediment. An increase of the DBL thickness from 0.25 to 1.5 mm diminished the oxygen flux into the sediment by more than 30% from 15 to 9.5 mmol m⁻² d⁻¹. At the same time, this change in DBL thickness had contrasting effects on the reoxidation of reduced solutes released in the anoxic sediment layers: While the rates of Fe(II) and Mn(II) oxidation decreased by up to 60%, the oxidation of methane changed by only 2%. Still, the contribution to the total oxygen uptake by these redox processes never exceeded 40%. Denitrification rates under steady state conditions were only 8% slower when the DBL was extended from 0.25 to 1.5 mm. The decreased nitrate supply was partially compensated by a stimulated denitrification process due to the lower oxygen penetration. However, fluxes of nitrogen species periodically deviated by more than 60% when an oscillating DBL thickness with periods of less than 6 h was modeled.

The intensity and pathways of mineralization of sedimentary organic matter were investigated in eutrophic Lake Zug, Switzerland. In a depth transect (25–180 m) from oxic to anoxic bottom water we recorded in situ sediment pore-water concentration profiles of O₂, NO₃^-, and NH₄⁺ with a benthic lander system equipped with both oxygen and ion-selective electrodes. Anaerobic sedimentary mineralization ranged from 13.1 to 34.9 mmol carbon (C) m^-2 d^-1 and increased linearly with water depth, as determined from the NH₄⁺ flux rates in the anoxic pore water and the molar C: nitrogen (N) ratio of the surface sediment. A parallel increase of the total organic carbon concentration of the sediment was attributed to lateral transfer of resuspended sedimentary matter. Denitrification was estimated from nitrate profiles and contributed only 1.5–3.2% to the total organic carbon mineralization at any water depth. Aerobic respiration and oxidation of reduced compounds were calculated from O₂ microprofiles and pore-water data of dissolved Mn(II), Fe(II), S(-II), and CH₄. When the O₂ concentration exceeded 0.15 mmol L^-1 in the sediment overlying water, 41–58%, or 12.4–18.1 mmol C m^-2 d^-1, was mineralized aerobically, whereas at lower concentrations (<0.04 mmol L^-1), >92% of organic carbon was mineralized anaerobically. Total benthic mineralization of organic carbon was 26.9–34.9 mmol C m^-2 d^-1. A budget including particulate as well as dissolved reduced compounds in the sediment indicated that >95% of the anaerobic mineralization was due to methanogenesis. Oxidation of CH₄ consumed 39–56% of the O₂ at the sediment–water interface. Oxygen exposure times for these sediments were estimated to be on the order of weeks to months. These time spans are too short to change the reactivity spectrum of sedimentary organic matter.

Lake Kivu is one of the large African Rift lakes situated between the Democratic Republic of the Congo and Rwanda. In its permanently stratified hypolimnion, unusually high methane concentrations have increased further in recent decades. Because methanogenesis is, in part, dependent on supply of organic material from the photic zone, it is necessary to quantify upward nutrient fluxes from the saline, nutrient-rich deep waters. These upward fluxes are mainly driven by advection caused by subaquatic springs. Biogenic calcite precipitation drives surface-water depletion and deep-water enrichment of Ca²⁺, Sr²⁺, and Ba²⁺. Methane is mainly oxidized aerobically at the redox interface at 60 m, with a small contribution of anaerobic methane oxidation. A subaquatic spring that sustains the major chemocline at 250 m depth was depleted of N, P, and CH₄, and concentrations of major ions were slightly lower than in the lake water of the same depth. Enrichment of the deep waters with nutrients and CH₄ are driven by mineralization of settling organic material, whereas SiO₂ is influenced by uptake and mineralization of diatoms and inputs through subaquatic springs. Dissolved inorganic phosphorus and Si fluxes supplied by internal loading through upwelling were found to be lower than the estimations for Lakes Malawi and Tanganyika. In contrast, N flux was within the lower range for Lake Malawi, whereas it was assumed to be totally lost by denitrification in Lake Tanganyika. In Lake Kivu, nutrient uptake by primary production is three times higher than nutrient upward fluxes.

The currents in the lowest few millimeters of the bottom boundary layer of lakes are highly important for the dissipation of kinetic energy and for chemical processes such as oxygen transfer into the sediment. So far, no highresolution flow velocity profiles close to the sediment–water interface have been reported for such systems because a suitable flow meter was lacking. This article introduces a novel sensor for the measurement of extremely lowflow velocities. The sensor is based on a gas transducer surrounded by a gas reservoir. It measures the change in the partial pressure of a tracer gas outside the reservoir tip due to advective transport. The sensor is suitable for measurements of velocities smaller than 1 mm s^-1, with a spatial resolution of 100 to 250 μm. The flow measurements prove to be insensitive to temperature changes between 5 and 15 °C. The sensor is robust against relative pressure changes, and angular differences in the sensitivity can be calibrated. We present high-resolution in situ measurements at the bottom of a pre-alpine lake with shear velocities as low as 0.13 ± 0.02 cm s^–1. The velocity profile nicely resolves the transition zone between the viscous and the logarithmic boundary layer.

In sediments, diffusive transport of ions and molecules is basically influenced by two sediment characteristics: tortuosity and porosity. For the first time, the formation factor F, which combines the effect of tortuosity and porosity on diffusion, was quantified in freshwater sediments at submillimeter resolution. Sediment cores were treated with KCl and F was determined using a resistivity sensor and K⁺ selective electrodes.
F was determined in sediments from different water depths of the eutrophic Lake Zug (Switzerland): In sandy sediments from a shallow site (12 m depth), F increased by approximately 50% within a few millimeters below the sediment surface. In clayey and silty sediments from the oxic (80 m depth) and seasonally anoxic (80–120 m depth) zones of the lake, the initial increase in F was only 20%. In the permanent anoxic zone (>160 m depth), F increased by only 10% just below the sediment surface. Values of F were correlated with the porosity at each depth. We found close correlations of F = 1.02 · φ^-1.81 for clay-silt sediments, and F = 1.04 · φ^-1.21 for sandy sediments. The exponents are considerably smaller in Lake Zug than found for marine sediments, thus, diffusive transport seems to be less affected by tortuosity in this freshwater system.

Since the catastrophic CO₂ eruption in 1986, Lake Nyos has been investigated in detail by several research groups. However, no signs of double-diffusive convection were observed before December 2002, when a set of 26 well-mixed layers with thicknesses of 0.2-2.1 m and sharp interfaces in between were discovered at 53-74 m depth. Such pronounced steps are a characteristic feature of double-diffusive convection of the diffusive regime. A temperature time series measured at 62 m depth indicates that the double-diffusive convection started in the second half of March 2002. The trigger was most probably the cooling at the top of this layer caused by relatively strong seasonal convective mixing down to 52.5 m depth during the dry season in February 2002. The heat fluxes calculated by the heat budget method and the thicknesses of the layers agree within the uncertainties with the values expected from the double-diffusive flux laws. The heat fluxes increased by an order of magnitude since the establishment of the double-diffusive convection and reached values comparable to the heat input by a source of warm and CO₂-enriched water to the deepest zone of the water column. In contrast, the CO₂ fluxes caused by double diffusion are negligible compared to the input by this source. Because the double-diffusive heat fluxes were higher in the upper layers of the staircase compared to the lower ones, the temperature gradient between 60 and 75 m depth approximately doubled from March 2002 to December 2002, whereas the total dissolved solids gradient remained almost constant during this period. Consequently, this process is reducing the stability of the staircase and could potentially lead to a complete homogenization of this zone within a few years. It cannot be excluded that a similar double-diffusive event could have been the trigger of the CO₂ eruption in 1986.

We investigated the nutrient uptake capacity of three lakes (Uzlina, Matita and Rosu) within the Danube Delta during high water level in June and low water level in September 1999. Special emphasis was placed on nutrient cycling at the sediment-water interface and on the self-purification capacity of the lakes in the Danube Delta. In order to estimate the nutrient uptake of selected lakes we present in this paper the results of water analyses, benthic flux chamber experiments and deck incubation experiments of ¹⁵N-labeled sediment cores at the inflow and the outlet of the lakes. The external input of dissolved inorganic nitrogen and silica into the lakes decreases with increasing distance to the main Danube branches whereas the total dissolved phosphorus input is independent of the hydrological distance to the main branches. The nutrient loading is highest in the inflow channels, and decreases towards the outflow of the lakes. In June, the uptake of NO₃^-, TDP and Si(OH)₄ in the lakes was higher than in September. In contrast, NH₄⁺ uptake was more intense in September, when benthic release was more intense as well. On average, about 76% of the external plus internal nitrogen and phosphorus input into the lakes was taken up by macrophytes and phytoplankton during the growing season, whereas the uptake of external nutrient input amounted to about 43%. The benthic release of ammonia and silica increases from June to September and indicates, that part of the nutrients taken up during the growing season might be released during winter. We estimate the net impact of the Delta on the nutrient reduction of the Danube during the growing season is about 4.3%, assuming 10% of the Danube water is flowing through the Delta.

Benthic fluxes of nutrients and metals were measured in the coastal zone of the north-western Black Sea, which is influenced by the Danube and Dniestr rivers. The results from the benthic flux chambers deployed during two EROS 21 cruises in summer 1995 and in spring 1997 yield information on benthic nutrient cycling and diagenetic pathways at the sediment-water interface. This information is discussed in the light of benthic activity as well as pore-water data. The benthic recycling of nutrients varied seasonally due to the availability of oxygen and organic material, and spatially due to river influence. Areas of high benthic fluxes near-shore and of low benthic fluxes offshore on the shelf were distinguished. Nutrients and suspended particulate matter discharged by the Danube is kept nearshore by the coastal current. The oxygen concentrations in the bottom water varied strongly between the two seasons. In summer, the bottom water at the near-shore stations turned anoxic. In spring, the higher oxygen concentrations are related to more intense mixing of the water column due to stormy periods and high river discharge. Highest oxygen concentrations were found on the offshore continental shelf. However, the benthic oxygen consumption rates in spring (13–23 mmol m⁻² day⁻¹) were as high as in summer. Areas with highest nutrient concentrations in the overlaying bottom water were found at the Danube delta front and Danube prodelta. On the Danube delta front and the Dniestr mouth, ammonia fluxes were lower in spring (1·1–1·7 mmol m⁻² day⁻¹) than in the summer (2·6–4·4 mmol m⁻² day⁻¹) due to higher nitrification rates and the lower influx of organic matter. In spring, the concentrations of dissolved iron and manganese in the bottom water were one order of magnitude lower than during the summer. This decrease in flux rates is related to the higher oxygen concentrations in the bottom water in spring. A rough comparison of the river’s nutrient load and the benthic recycling over a certain area showed that the near-shore benthic phosphate and silica recycling account for 50% and 35% of the Danube input in summer, respectively. The fluxes of ammonia from benthic recycling and from the Danube discharge were at the same order of magnitude in both seasons. Nitrogen is introduced by the rivers mostly as nitrate. Benthic recycling is the dominant source of ammonia. The Danube input is phosphate deficient. In brief, benthic nutrient recycling is an important factor in sustaining high productivity of the system.

This chapter illustrates the capabilities of in situ applications of potentiometric chemical sensors at the sediment-water interface of deep lakes. A lander system is described which allows positioning of the sensors with a stepping motor and a video-endoscope. A procedure has been developed to determine the position of the sediment-water interface directly from the standard deviation of acquired data. Preliminary studies illustrate the ability of the device to detect spatial heterogeneity and the seasonality in pore-water profiles. The different ion-selective sensors available for freshwater systems allow us to gain new insights in the nitrogen cycle and calcite dissolution.

The exchange of O₂, NH₄⁺, Si(OH)(4), ortho-phosphate, Fe²⁺, Mn²⁺ and H₂S between the sediment and the overlying water (benthic flux) was determined at 6 locations on the northwestern shelf of the Black Sea using an in situ benthic lander. Oxygen consumption by the sediments ranged from 0 to 33 mmol m^-2 d^-1 depending mostly on the initial oxygen concentration in the bottom water. During the time period of deployment of 22 h, no reduction of sulfate was observed within the lander and no release of methane. H2S was released from the highly eutrophic sediments close to the Dniester river with a flux of 0.3 mmol m^-2 d^-1. Fluxes of phosphate, ammonia and silica close to the mouths of the Danube and Dniester rivers were high (0.5, 2.6-4.4 and 6.0 mmol m^-2 d^-1, respectively) and comparable with values measured in other estuaries such as Naragansett and Chesapeake Bay, but higher than fluxes measured in shelf-slope seas like the Skagerrak. The benthic fluxes of phosphorus and silica are in the same order of magnitude as the annual nutrient input by the Danube river, whereas ammonia fluxes from the sediment amounted to ~ 10% of the Danube input. Hence, benthic fluxes from the sediments are responsible for a significant flux of nutrients to the waters of the northwestern shelf of the Black Sea.

Porewater profiles often are used to identify and quantify important biogeochemical processes occurring in lake sediments. In this study, multiple porewater profiles were obtained from two eutrophic Swiss lakes using porewater equilibrators (peepers) in order to examine spatial and seasonal trends in biogeochemical processes. Variability in profile shapes and concentrations was small on spatial scales of a few meters, but the uncertainty in calculated diffusive fluxes across the sediment surface was, on average, 35%. Focusing of Fe and Mn oxides toward the lake center resulted in systematic increases in porewater concentrations and diffusive fluxes of Fe²⁺ and Mn²⁺ with increasing water depth; these fluxes are postulated to be regulated by the pH-dependent dissolution of reduced- metal phases. Despite higher concentrations of inorganic carbon, NH₄⁺, Si and P in pelagic compared to littoral sites, diffusive fluxes of these substances across the sediment surface increased only slightly or not at all with increasing water depth. Porewater profiles did reveal temporal changes in Fe²⁺, Mn²⁺, Ca²⁺ and Mg²⁺ that were an indirect result of the large, seasonal changes in seston deposition, but no clear seasonal variations were found in diffusive fluxes of nutrients across the sediment surface. The intense mineralization occurring at the sediment surface was not reflected in the porewater profiles nor in the calculated diffusive fluxes. Calculated diffusive fluxes across the sediment surface resulted from decomposition occurring primarily in the top 5-7 cm of sediment. Diffusive fluxes from this subsurface mineralization were equal to the solute release from mineralization occurring at the sediment-water interface. Buried organic matter acts as a memory of previous lake conditons; it will require at least a decade before reductions in nutrient inputs to lakes fully reduce the diffusive fluxes into the lake from the buried reservoir of organic matter.