Profundal Sediments Research Articles

Nutrient Fluxes From Profundal Sediment of Ultra‐Oligotrophic Lake Tahoe, California/Nevada: Implications for Water Quality and Management in a Changing Climate

AbstractA warming climate is expected to lead to stronger thermal stratification, less frequent deep mixing, and greater potential for bottom water anoxia in deep, temperate oligotrophic lakes. As a result, there is growing interest in understanding nutrient cycling at the profundal sediment‐water interface of these rare ecosystems. This paper assessed nutrient content and nutrient flux rates from profundal sediment at Lake Tahoe, California/Nevada, USA. Sediment is a large reservoir of nutrients, with the upper 5 cm containing reduced nitrogen (∼6,300 metric tons) and redox‐sensitive phosphorus (∼710 metric tons) equivalent to ∼15 times the annual external load. Experimental results indicate that if deep water in Lake Tahoe goes anoxic, profundal sediment will release appreciable amounts of phosphate (0.13–0.29 mg P/m2·d), ammonia (0.49 mg N/m2·d), and iron to overlaying water. Assuming a 10 year duration of bottom water anoxia followed by a deep‐water mixing event, water column phosphate, and ammonia concentrations would increase by an estimated 1.6 µg P/L and 2.9 µg N/L, nearly doubling ambient concentrations. Based on historic nutrient enrichment assays this could lead to a ∼40% increase in algal growth. Iron release could have the dual effect of alleviating nitrate limitation on algal growth while promoting the formation of fine iron oxyhydroxide particles that degrade water clarity. If the depth and frequency of lake mixing decrease in the future as hydrodynamic models suggest, large‐scale in‐lake management strategies that impede internal nutrient loading in Lake Tahoe, such as bottom water oxygen addition or aluminum salt addition, may need to be considered.

Spatial variation in stable isotopic composition of organic matter of macrophytes and sediments from a small Arctic lake in west Greenland

ABSTRACT Stable isotope compositions of organic carbon (δ13Corg) and nitrogen (δ15N) in macrophytes and sediments are useful in assessing sources of lake productivity and diagenesis of organic matter from formation through sedimentation to decomposition. Despite the increasing importance of high-latitude landscapes to carbon cycling under amplified and accelerating warming in the Arctic, the high density of small closed-basin lakes in this landscape, and the utility of stable isotopes in the study of carbon dynamics, limited data are available on within-lake spatial variability of δ13Corg and δ15N in these systems. The goal of this study was to investigate the spatial variability in stable isotopic composition of three dominant macrophyte species (Hippuris vulgaris, Eriophorum angustifolium, Warnstorfia exannulata) and sediments from littoral and profundal areas of a single closed-basin system among the common small Arctic lakes that populate the ice-free margin of Greenland. The range in δ13Corg of macrophytes (−33.9‰ to −27.1‰) was within the typical range of plants utilizing the C3 pathway for carbon fixation. No notable differences were observed in δ13Corg between segments of the individual macrophytes (emergent, submergent, and root tissues), indicating that the isotopic fractionation of carbon was similar throughout the plant. Between-species variations in δ13Corg were small, but significant (p < 0.01), with the moss most depleted in 13C. The range of δ15N in littoral and profundal sediments (−0.52‰ to 1.33‰) was small, with littoral surface sediments 1‰ less enriched in 15N than surface sediments in the profundal zone. The C/N ratios of macrophytes (mean ± SD: 27.0 ± 12.6), littoral sediments (mean ± SD: 11.0 ± 1.0), and profundal sediments (mean ± SD: 9.1 ± 0.9) point to diagenetic alteration. Combined isotopic and elemental (C/N) compositions of littoral and profundal sediments suggest that organic matter accumulating in the study lake originate primarily from in-lake primary production of macrophytes. Terrestrial sources are likely minor because of the hydrologically closed basin and limited aeolian inputs, suggesting that the majority of organic matter produced by the dominant littoral macrophyte community was decomposed between production and sediment deposition.

Aluminum application to restore water quality in eutrophic lakes: maximizing binding efficiency between aluminum and phosphorus

ABSTRACTHuser BJ. 2017. Aluminum application to restore water quality in eutrophic lakes: maximizing binding efficiency between aluminum and phosphorus. Lake Reserv Manage. 33:143–151.Aluminum (Al), typically added to lakes to reduce internal cycling of legacy phosphorus (P) in sediment, was added to the littoral zone of Lake Harriet (Minnesota) to inhibit the uptake of sediment P by buoyancy regulating species of plankton. Analytical results from sediment collected over an 11-year period showed that the added Al (32 g/m2) started to move outside the treatment zone almost immediately after treatment, with <5% remaining within the treatment area after 6 months. Although the original treatment design failed, the application method was, unexpectedly, a success with respect to binding efficiency between Al and P in the sediment. As the Al drifted to deeper areas of the lake, internal P release in non-treated, deeper areas of the lake declined and the binding ratio between Al and Al-bound P decreased, reaching 2.1 (molar) in profundal sediments 10 years after treatment. The increased contact with available (mobile) sediment P increased binding efficiency, resulting in a 163–581% increase of P bound per unit Al compared to previous whole-lake aluminum treatments. The binding efficiency exceeded expectations showing that, in addition to the amount of Al added, treatment location and subsequent translocation of the Al floc can substantially affect binding efficiency and treatment effectiveness.

Using sulfur stable isotopes to assess mercury bioaccumulation and biomagnification in temperate lake food webs

Nitrogen and carbon stable isotopes (δ15 N, δ13 C) are commonly used to understand mercury (Hg) bioaccumulation and biomagnification in freshwater food webs. Though sulfur isotopes (δ34 S) can distinguish between energy sources from the water column (aqueous sulfate) and from sediments to freshwater organisms, little is known about whether δ34 S can help interpret variable Hg concentrations in aquatic species or food webs. Seven acidic lakes in Kejimkujik National Park (Nova Scotia, Canada) were sampled for biota, water, and sediments in 2009 and 2010. Fishes, zooplankton, and macroinvertebrates were analyzed for δ34 S, δ15 N, δ13 C, and Hg (methyl Hg in invertebrates, total Hg in fishes); aqueous sulfate and profundal sediments were analyzed for δ34 S. Within lakes, mean δ34 S values in sediments and sulfate differed between 0.53‰ and 1.98‰, limiting their use as tracers of energy sources to the food webs. However, log-Hg and δ34 S values were negatively related (slopes -0.14 to -0.35, R2 0.20-0.39, p < 0.001-0.01) through each food web, and slopes were significantly different among lakes (analysis of covariance, lake × δ34 S interaction term p = 0.04). Despite these relationships, multiple regression analyses within each taxon showed that biotic Hg concentrations were generally better predicted by δ15 N and/or δ13 C. The results indicate that δ34 S values are predictive of Hg concentrations in these food webs, although the mechanisms underlying these relationships warrant further study. Environ Toxicol Chem 2017;36:661-670. © 2016 SETAC.

Spatial variation in flux, δ13C and δ2H of methane in a small Arctic lake with fringing wetland in western Greenland

Small lakes in northern latitudes represent a significant source of CH4 to the atmosphere that is predicted to increase with warming in the Arctic. Yet, whole-lake CH4 budgets are lacking as are measurements of δ13C-CH4 and δ2H-CH4. In this study, we quantify spatial variability of diffusive and ebullitive fluxes of CH4 and corresponding δ13C-CH4 and δ2H-CH4 in a small, Arctic lake system with fringing wetland in southwestern Greenland during summer. Net CH4 flux was highly variable, ranging from an average flux of 7 mg CH4 m−2 d−1 in the deep-water zone to 154 mg CH4 m−2 d−1 along the lake margin. Diffusive flux accounted for ~8.5 % of mean net CH4 flux, with plant-mediated and ebullitive flux accounting for the balance of the total net flux. Methane content of emitted ebullition was low (mean ± SD 10 ± 17 %) compared to previous studies from boreal lakes and wetlands. Isotopic composition of net CH4 emissions varied widely throughout the system, with δ13C-CH4 ranging from −66.2 to −55.5 ‰, and δ2H-CH4 ranging from −345 to −258 ‰. Carbon isotope composition of CH4 in ebullitive flux showed wider variation compared to net flux, ranging from −69.2 to −49.2 ‰. Dissolved CH4 concentrations were highest in the sediment and decreased up the water column. Higher concentrations of CH4 in the hypoxic deep water coincided with decreasing dissolved O2 concentrations, while methanotrophic oxidation dominated in the epilimnion based upon decreasing concentrations and increasing values of δ13C-CH4 and δ2H-CH4. The most depleted 13C- and 2H-isotopic values were observed in profundal bottom waters and in subsurface profundal sediments. Based upon paired δ13C and δ2H observations of CH4, acetate fermentation was likely the dominant production pathway throughout the system. However, isotopic ratios of CH4 in deeper sediments were consistent with mixing/transition between CH4 production pathways, indicating a higher contribution of the CO2 reduction pathway. The large spatial variability in fluxes of CH4 and in isotopic composition of CH4 throughout a single lake system indicates that the underlying mechanisms controlling CH4 cycling (production, consumption and transport) are spatially heterogeneous. Net flux along the lake margin dominated whole-lake flux, suggesting the nearshore littoral area dominates CH4 emissions in these systems. Future studies of whole-lake CH4 budgets should consider this significant spatial heterogeneity.

Microscale decoupling of sediment oxygen consumption and microbial biomass in an oligotrophic lake

Summary Sediments of aquatic ecosystems are hotspots for biological activity. Here, we address the question if, within surface sediments, oxygen consumption is linearly related to cell abundance. In addition, we identify habitat‐specific factors influencing underlying microbial processes. Sediment microcosms were established from three sites within oligotrophic Lake Ånnsjön, Sweden, to use microsensors for measuring oxygen profiles and estimate spatially resolved oxygen consumption rates at the water–sediment interfaces. To evaluate differences between habitats, we measured sediment carbon content and C : N : P as a proxy for diagenetic state and organic matter bioavailability. Epifluorescence microscopy was used to assess the microscale distribution and size of surface‐colonising microorganisms. There was no linear correlation between oxygen consumption rates and microbial cell abundances. Cell‐specific respiration rates were highest in the profundal compared to the littoral‐ and inflow‐sediment microcosms, whereas vertical variability in all these parameters was highest at the inflow, intermediate in the littoral and least variable in profundal sediments. Illumina sequencing of spatially resolved 16SrRNA genes was used to test for possible influence of bacterial diversity on spatially resolved oxygen consumption rates. Bacterial α‐diversity decreased over depth at each site, but was also lower in sediments from the most active profundal zones of the lake compared to the inflow. We suggest that bacteria in profundal sediments mainly use highly oxidised organic compounds, resulting in overall low growth yield despite high metabolic activity. In the lake inflow and the littoral, more reduced organic substrates of terrestrial origin are used at lower rates but with higher yield.

Large fractionations of C and H isotopes related to methane oxidation in Arctic lakes

Microbial oxidation of methane (CH4) plays a central role in carbon cycling in Arctic lakes, reducing potential CH4 emissions associated with warming. Isotopic signatures of CH4 (δ13C and δ2H) are indicators of microbial oxidation, wherein the process strongly enriches 13C and 2H in residual CH4. We present δ13C and δ2H measurements obtained from sampling the water column and sediment for dissolved CH4 from three, small Arctic lakes in western Greenland under both open-water and ice-covered conditions from 2013 to 2014. Despite substantial variations in aquatic chemistry among the lakes, δ13C and δ2H of CH4 suggested that CH4 was produced predominantly by acetoclastic methanogenesis in the littoral sediments and hydrogenotrophic methanogenesis in the profundal sediments in all of the lakes. Surprisingly large variations for both δ13C and δ2H of CH4 were observed, with δ13C extending from −72‰ to +7.4‰ and δ2H from −390‰ to +250‰. The CH4 isotopic values reported here were significantly more enriched (p<0.0001) in both 13C and 2H than values reported from other Arctic freshwater environments. As is characteristic of methanotrophy, the enrichment in 13C and 2H was associated with low CH4 concentrations. We suggest that the CH4 most enriched in 13C and 2H may reflect unusually efficient methanotrophic communities in Arctic ice-margin lakes. This study provides the first measurement of δ2H for CH4 in an Arctic freshwater environment at concentrations <10μM. The extreme enrichment of 13C and 2H of CH4 from Arctic methanotrophy has significant implications for interpreting sources and sinks of CH4. Without knowledge of local geology, stable isotope values of CH4 higher than −30‰ for δ13C and −150‰ for δ2H could be misinterpreted as thermogenic, geothermal, or abiogenic origins. Given crystalline bedrock and the strong positive correlation between δ13C and δ2H throughout the water columns in three Arctic lakes confirms that CH4 heavily enriched in 13C and 2H is the result of methanotrophy.

Effects of chironomid density and dissolved oxygen on mercury efflux from profundal lake sediment

ABSTRACTBeutel MW, Cox SE, Gebremariam S. 2016. Effects of chironomid density and dissolved oxygen on mercury efflux from profundal lake sediment. Lake Reserv Manage. 32:158–167.Benthic macrofauna can dramatically affect the flux of dissolved compounds into and out of lake sediment. In this study, replicate experimental chambers containing profundal sediment from a relatively pristine lake were incubated under low (∼1000/m2) and high (∼3800/m2) chironomid densities and low (∼2.5 mg/L) and medium (∼5.0 mg/L) dissolved oxygen in chamber water. Dissolved methylmercury efflux rates were highest in high chironomid density/low oxygen chambers (0.35 ± 0.06 ng/m2/d, mean ± standard error, n = 4) and lowest in low density/medium oxygen chambers (0.06 ± 0.14 ng/m2/d). In contrast, dissolved total mercury efflux assessed at low dissolved oxygen (∼2.5 mg/L) was higher at low chironomid density (4.6 ± 0.7 ng/m2/d) relative to high chironomid density (2.5 ± 0.8 ng/m2/d, n = 4). Results indicate that oxygen addition, a common lake management practice, may enhance methylmercury efflux from profundal sediment as macrofauna recolonize previously anaerobic sediments. However, benthos-mediated methylmercury efflux rates are lower than hypolimnetic methylmercury accumulation rates under anaerobic conditions; therefore, relative to anaerobic conditions, oxygenation should lower net methylmercury accumulation rates in relatively pristine lakes.

Seven Millennia of human impact as reflected in a high resolution pollen profile from the profundal sediments of Litzelsee, Lake Constance region, Germany

A profundal core from Litzelsee, a small cirque lake in the western Lake Constance region, was investigated by pollen analysis and dated radiometrically. The upper part of the core, chronologically between 5000 cal bc and 1850 ad, was sampled continuously, resulting in a total of 449 samples, each with a sum of 1,000 arboreal pollen grains. Also in this huge data set, rare taxa, normally lacking or very scarce in pollen profiles, were registered. The ecological evaluation of these, with a focus on anthropogenic indicators, sheds light on environmental and human impact history from the Neolithic to Modern times. Further, the results are put in a regional context, together with seven other mostly unpublished pollen profiles studied in the same way.

The use of biopreparations in lake restoration − experimental research

Abstract The experiment was established in a set of containers. Every container held a 10 cm thick layer of profundal sediments and water from a badly polluted lake. The following five preparations were tested (brand names): AQUAKALGON+, BIOGEN REMEDIANT, BIODENN K, ECOTABS 1(TABLETS), ECOTABS 2(POWDER). The biological preparations were applied twice: phase I lasted 56 days and phase II was continued for 21 days. The research results demonstrated that considerably higher values, compared to the control, were initially (day 3) obtained for TOC after the application of ECOTABS 1, for total nitrogen − after BIODENN K and ECOTABS 1, and for total phosphorus − after BIODENN K, ECOTABS 1 and BIOGEN REMEDIANT. Multiplying bacteria might have been an additional source of organic matter in water, which was subsequently deposited in the bottom sediments as implicated by a higher percentage (by 1 to 7% of d.w.) of organic matter in the sediments at the termination of the experiment. Our preliminary experiment did not provide any justification for using the tested preparations in lake restoration efforts. Subsequent studies should focus on the question whether mineral phosphorus released to the lake water or a drastic change in the environmental conditions would not lead towards the dominance of invasive and highly toxic cyanobacteria.

Diagenetic pathways linked to labile Mg-clays in lacustrine carbonate reservoirs: a model for the origin of secondary porosity in the Cretaceous pre-salt Barra Velha Formation, offshore Brazil

Abstract The lacustrine carbonate reservoirs of the South Atlantic host significant accumulations of chemically reactive and Al-free Mg-silicate minerals (e.g. stevensite, kerolite and talc). Petrographic data from units such as the Cretaceous Barra Velha Formation in the Santos Basin suggest that Mg-silicate minerals strongly influenced, and perhaps created, much of the observed secondary porosity. The diagenetic interactions between reactive Mg-silicate minerals and carbonate sediments are, however, poorly known. Here we develop a conceptual model for the origin of secondary porosity in the Barra Velha Formation guided by considerations of the chemistry that triggers Mg-silicate crystallization, as well as the geochemical and mineralogical factors that act as prerequisites for rapid Mg-silicate dissolution during early and late diagenesis. We conclude that sub-littoral zones of volcanically influenced rift lakes would have acted as the locus for widespread Mg-silicate accumulation and preservation. Organic-rich profundal sediments, however, would be especially prone to Mg-silicate dissolution and secondary porosity development. Here, organic matter diagenesis (especially methanogenesis) plays a major role in modifying the dissolved inorganic carbon budget and the pH of sediment porewaters, which preferentially destabilizes and then dissolves Mg-silicates. Together, the sedimentological, stratigraphic and geochemical predictions of the model explain many enigmatic features of the Barra Velha Formation, providing a novel framework for understanding how Mg-silicate–carbonate interactions might generate secondary porosity more broadly in other lacustrine carbonate reservoirs across the South Atlantic.

Managing internal phosphorus loading and vertical entrainment in a weakly stratified eutrophic lake

Unsuccessful control of internal phosphorus (P) loading in Cedar Lake, Wisconsin, via artificially induced destratification led to reexamination of management strategies and linkages between profundal sediment diffusive P flux, P entrainment, and cyanobacterial bloom development. The destratification system, operated since 1991, was suspended in 2009–2010 and turned on in 2011 to evaluate P and phytoplankton dynamics under stratified and destratified conditions. In 2009–2010, the lake was susceptible to wind-induced mixing and P exchange into the epilimnion due to its shallow morphometry, long fetch, and low Osgood Index (2.7). Anaerobic diffusive P flux from bottom sediment was high at 12 mg/m2/d. While hypolimnetic soluble reactive P (SRP) exceeded 0.8 mg/L, dissolved iron (DFe) was low, resulting in a low DFe:SRP mass ratio of 0.9:1 despite high sediment Fe concentrations. Incomplete Fe binding control of P during entrainment events may have been due to Fe reaction to an inert form (chelation with organic carbon and reaction with sulfur) in sediment and removal from recycling. High anaerobic diffusive P flux coupled with entrainment resulted in potentially toxin-producing cyanobacteria blooms in excess of 50–100 μg/L chlorophyll that were sustained from late August through October. Operation of the destratification system in 2011 did not prevent anoxia and actually exacerbated diffusive P flux, resulting in >0.1 mg/L SRP in the epilimnion and nitrogen-limited phytoplankton growth. Fe–P linkages and Fe:P ratio stoichiometry need to be considered in management and control of internal P loading. For Cedar Lake, internal P loading management should consider application of materials that can irreversibly bind sediment P instead of destratification.

Mineralization pathways of organic matter deposited in a river-lake transition of the Rhone River Delta, Lake Geneva.

During the éLEMO endeavour (a research project in which the Russian MIR submersibles were used for studying Lake Geneva) four sediment cores were retrieved on a transect from the delta of the Rhone River towards the profundal part of the lake. The degradation pathways of organic material (OM) were investigated considering different electron acceptors. Essentially, OM at the delta sites had a higher fraction of terrestrial material than the lake sites indicated by higher C/N ratios, and higher long-chain n-alkane and alcohol concentrations. The concentrations of chlorins were higher at the distant sites indicating more easily degradable OM in the sediments. However, the chlorin index that was used to determine the degradation state of the OM material indicated that pigment derived OM of deltaic sediments was less degraded than that of the profundal sediments. The fluxes of reduced species from the sediments decreased from the delta to the profundal for CH4 (from 2.3 to 0.5 mmol m(-2) d(-1)) and NH4(+) (from 0.31 to 0.13 mmol m(-2) d(-1)). Fluxes of Fe(ii) and Mn(ii), however, increased although they were generally very low (between 9 × 10(-5) and 7.6 × 10(-3) mmol m(-2) d(-1)). Oxygen concentration profiles in the pore waters revealed lower fluxes close to the river inflow with 4.3 and 4.1 mmol m(-2) d(-1) compared to two times higher fluxes at the profundal sites (8.8 and 8.2 mmol m(-2) d(-1)). The rates for totally mineralized OM (Rtotal) at the shallower sites (4.7 mmol C m(-2) d(-1)) were only half of those of the deeper sites (9.7 mmol C m(-2) d(-1)). Accordingly, not only the rates but also the mineralization pathways differed between the shallow and profundal sites. Whereas only 0-6% of the OM was mineralized aerobically at the shallow sites (since almost all O2 was used to oxidize the large flux of CH4 from below) the situation was reversed at the deeper sites and the fraction of aerobically degraded OM was 72-78%. We found a better efficiency in CH4 production per carbon equivalent deposited at the deeper sites as a result of the higher degradability of the mainly autochthonous OM in spite of the lower deposition rate and the higher degradation state of the OM compared to the delta sites.

Response of methylmercury, total mercury, iron and manganese to oxygenation of an anoxic hypolimnion in North Twin Lake, Washington

This study evaluated the impact of oxygenation on the accumulation of methylmercury (MeHg), total mercury (Hg), iron (Fe), and manganese (Mn) in the anoxic hypolimnion of North Twin Lake, Washington (mean depth = 9.7 m). Within 8 hours of the start of the oxygenation test, mean hypolimnetic total Fe dropped from 833 to 243 μg/L (58% dissolved), and Mn decreased from 119 to 32 μg/L (100% dissolved). After 17 days of oxygenation, mean hypolimnetic total Hg decreased from 0.77 to 0.58 ng/L, and MeHg decreased from 0.20 to 0.10 ng/L. A month after the end of the oxygenation test, MeHg, total Hg, Fe, and Mn concentrations rebounded to pre-oxygenation levels. Metal dynamics were explained by the differing redox characteristics of Fe (fast oxidation kinetics; reduction at lower redox potential) and Mn (slow oxidation kinetics; reduction at higher redox potential) in natural waters, and the tendency for Fe oxides to co-precipitate with reduced Mn, ionic Hg, and MeHg. From a lake management perspective, the study highlights the role that oxidized metals in surfacial sediments play in retaining Hg in profundal sediments and suggests that sorption characteristics of metal oxides could be exploited to manage Hg bioaccumulation in aquatic biota. The study also suggests that for oxygenation to effectively repress the accumulation of redox-sensitive compounds in the hypolimnion, oxygenation systems should (1) start before anoxic conditions are established; (2) operate continuously through the entire stratified season; and (3) maintain oxygen levels at the sediment–water interface to ensure oxygen penetration into surfacial sediments.

Characterization and phylogeny of a novel methanotroph, Methyloglobulus morosus gen. nov., spec. nov

A novel methanotrophic gammaproteobacterium, strain KoM1, was isolated from the profundal sediment of Lake Constance after initial enrichment in opposing gradients of methane and oxygen. Strain KoM1 grows on methane or methanol as its sole source of carbon and energy. It is a Gram-negative methanotroph, often expressing red pigmentation. Cells are short rods and occur sometimes in pairs or short chains. Strain KoM1 grows preferably at reduced oxygen concentrations (pO2=0.05–0.1bar). It can fix nitrogen, and grows at neutral pH and at temperatures between 4 and 30°C. Phylogenetically, the closest relatives are Methylovulum miyakonense and Methylosoma difficile showing 91% 16S rRNA gene sequence identity. The only respiratory quinone is ubiquinone Q8; the main polar lipids are phosphatidyl ethanolamine and phosphatidyl glycerol. The major cellular fatty acids are summed feature 3 (presumably C16:1ω7c) and C16:1ω5c, and the G+C content of the DNA is 47.7mol%. Strain KoM1 is described as the type strain of a novel species within a new genus, Methyloglobulus morosus gen. nov., sp. nov.

Extreme water level decline effects sediment distribution and composition in Lake Alexandrina, South Australia

Water level decline affects the biophysical environment of shallow lakes. Unprecedented drought in Australia’s Murray–Darling Basin resulted in extreme water level drawdown in the large, shallow Lake Alexandrina at the end of the River Murray. Surface sediment was collected from 22 sites in the lake before and after water levels declined to assess the integrated limnological changes over the period of drawdown. Results indicate an increase in the proportion of organic particles in profundal sediments, as well as an increase of fine particles (<19.9 μm) in peripheral sediments. These changes to sediment composition corresponded to higher concentrations of suspended particles at low water levels. Increased autochthony and a shift in primary production from macrophytes to phytoplankton in Lake Alexandrina support these findings. Inorganic carbon and other nutrients were lost from sandy sediments most likely through carbonate dissolution driven by a localized decrease in pore water pH from increased mineralisation of organic matter.

A new dimension to sediment magnetism: Charting the spatial variability of magnetic properties across lake basins

We have investigated the variability of the magnetic properties of surface sediments across eight Minnesota lake basins. The measured magnetic properties are controlled by the competing fluxes of allochthonous and autochthonous magnetic particles, and differ according to location in the basin. Shoreline sediments are dominated by detrital magnetic particles, whereas littoral and profundal sediments are characterized by a combination of bacterial magnetosomes and detrital particles. The position of the oxic–anoxic interface, which may occur in the water or within the sediment column, controls the depth at which living magnetotactic bacteria occur, and determines the degree of preservation of their magnetosome chains in the surface sediment. The preservation potential of undisturbed chains is higher for bacterial magnetite formed at the top of the sediment column in the littoral area than for magnetosomes originating in the water column in the profundal area. Bacterial magnetite in the profundal facies will contain a higher proportion of chains collapsed during settlement through the water column to the lake bottom. This process increases the fraction of interacting magnetosomes, which in turn artificially lowers the ARM ratio (χARM/IRM), which ceases to be a reliable grain size indicator in the profundal environment. Our results indicate that a holistic approach to interpreting limnologically-derived paleoecological data should be employed. Specifically, a thorough understanding of evolving and interrelated factors such as basin morphology and limnologic conditions is crucial for a more confident interpretation of the sedimentary record in terms of environmental conditions at the time of sediment deposition.

Internal nutrient loading may increase microcystin concentrations in freshwater lakes by promoting growth ofMicrocystispopulations

Nutrient release from lake sediments may increase concentrations of harmful algal toxins - such as microcystins - by stimulating blooms of toxigenic cyanobacteria. This hypothesis is supported by a series of experiments in which intact cores of sediment were incubated under different environmental conditions, after which the water overlying the sediments was harvested as a culture medium for growing a toxic strain of the common cyanobacterium Microcystis. Both littoral and profundal sediments from Lake Kinneret, the largest freshwater lake in Israel, released substantial amounts of dissolved phosphorus (1.0 and 4.0 mg.m x2 .d x1 , respectively) and nitrogen (44.2 and 24.3 mg.m x2 .d x1 , respectively) under simulated summer conditions in the laboratory. In comparison, nutrient fluxes from sediments under simulated winter conditions were considerably smaller or negative. The addition of nutrient-rich overlying water harvested from profundal sediments, and to a lesser extent from littoral sediments, increased both chlorophyll a and microcystin concentrations in Microcystis cultures. In contrast, when Microcystis cells were inoculated in natural surface waters only, the cultures did not grow or produce microcystins, and soon collapsed. This study provides experimental evidence of a link between internal nutrient loading from sediments and microcystin concentrations in freshwaters, and demonstrates how environmental factors may indirectly exert control over toxin concentrations in freshwater lakes.

Allochthonous contribution to seasonal and spatial variability of organic matter sedimentation in a deep oligotrophic lake (Lake Constance)

Previous studies analysing the chemical composition of profundal sediments of Lake Constance suggest a overall large contribution of allochthonous material to total sedimentation but also a large spatial variability in the relative importance of allochthonous versus autochthonous sedimentation. Here we analyse sedimenting particulate organic matter (POMsed) during an annual cycle at two sites differing in their position relative to the major inflow and thus in their proposed contribution of allochthonous matter to the sedimentary flux, i.e., site [AL] characterised by a more allochthonous contribution und the site [AU] characterised by a more autochthonous contribution. Chemical and mineralogical composition (chlorite contents) of sedimenting matter were used for discrimination of autochthonous and allochthonous sources in addition to stable isotope signatures (δ13C, δ15N) of POMsed. Generally, δ13C values were “heavier” at site AL (−26.5 to −30.9‰) than at site AU (−33.10 to −29.30‰), especially during the main sedimentation period from April to September. In contrast differences between sites AL und AU in respect to δ15N-values were small (averages: +5.10‰ and +5.80‰, respectively). δ13C values were shown to be inversely correlated to contents of chlorophyll a of POMsed (i.e. lighter values with higher concentration), whereas they were positively correlated to chlorite contents and the C:N ratio of sedimenting material. The contribution of autochthonous and allochthonous POMsed changed also seasonally at both sampling sites. These differences resulted on the one hand from the seasonal succession of plankton, and on the other hand from the discharge characteristics of the inflowing rivers with regular (snow melting) and irregular maxima (flood events). A mixing model suggests that on average the allochthonous POMsed from river loads contributes at site AL 73.40% of total organic matter sedimentation and at the site AU 33.80%.

Incorporating spatial variation of nitrification and denitrification rates into whole‐lake nitrogen dynamics

Despite dramatic increases in nitrogen (N) loading to fresh waters and growing scientific attention on the changing N cycle, measurements of nitrification and denitrification rates in lakes are lacking. In particular, we know little about how these processes vary spatially within a lake, and how this potential spatial variation contributes to a lake's N dynamics. We measured sediment nitrification and denitrification rates at 40 sites in Gull Lake, Michigan (USA), and found that the shallow edge sediments (&lt;2 m deep) of the lake were hot spots of N transformation. Nitrification rates were comparable in sediments at all depths, while sediment denitrification rates were highest in the shallow edge habitat, and lowest in the profundal sediments (&lt;2 m and &gt;10 m deep, respectively). We scaled‐up our sediment transformation rates across the lake to illustrate spatial variability in nitrification and denitrification. For whole‐lake nitrification, the contribution of shallow edge, littoral, and profundal sediments followed in proportion to lake surface area of each habitat. In contrast, the contribution of each of these areas to whole‐lake denitrification was not proportional to their respective surface areas, and instead was equal across the 3 habitat types, with each area contributing roughly 30% of the total N loss via denitrification. Spatially representative characterization of nitrification and denitrification in lentic ecosystems requires incorporation of the spatial variation in these transformations with a particular focus on littoral sediments, and this is often overlooked in studies of lentic N cycling. Furthermore, anthropogenic changes to lake shorelines that influence N cycling in littoral sediments may have a disproportionate effect on whole‐lake ecosystem function.