Bottom Anoxia Research Articles

The Life and Death of Jamoytius kerwoodi White; A Silurian Jawless Nektonic Herbivore?

Jamoytius kerwoodi, is a primitive, eel-like jawless vertebrate found uniquely in an Early Silurian (Llandovery epoch; 444–433 Ma) horizon near Lesmahagow, Scotland. This species is a rare component of a low-diversity dominantly nektonic detritus-feeding and herbivorous fauna living over an anoxic bottom and is found at the transition from a marine-influenced, probably brackish-water, deep-water basin to a shallower-water, less saline and likely freshwater basin. In the absence of true teeth, Jamoytius was probably a detritivore or herbivore feeding on Dictyocaris. Jamoytius may have a common ancestor with living lampreys, especially as their ectoparasitic mode of life might have evolved from ancestral detritivores or herbivores.

Weathering Intensity, Paleoclimatic, and Progressive Expansion of Bottom-Water Anoxia in the Middle Jurassic Khatatba Formation, Southern Tethys: Geochemical Perspectives

The Jurassic Period was a significant phase of variable organic matter accumulation in paleo-shelf areas of the southern Tethys (Egypt). Reconstructing the paleoredox conditions, paleoclimate, and weathering intensity, along with the role of terrigenous sediment flux and mineralogical maturity, is important for understanding basin infill history and prevalent paleoenvironmental conditions. Here, inorganic geochemical data are presented from the Middle Jurassic Khatatba Formation and two samples from the underlying Ras Qattara and the overlying Masajid formations in the Jana-1x well, Shushan Basin, Western Desert. Twenty-four (24) whole-rock samples were analyzed for their major and trace element composition and carbonate content. The Khatatba Formation represents one of the major hydrocarbon source rocks in the North Western Desert, Egypt. Redox conditions were assessed based on enrichment factors of redox-sensitive elements Mo, V, U, and Co. Results revealed that the Khatatba Formation was deposited under predominant anoxic bottom and pore water conditions, in contrast to the oxic settings that were prevalent during the deposition of the Ras Qattara and Masajid formations. Continental weathering intensity and paleoclimate were reconstructed based on several proxies, such as the chemical index of alteration (CIA), K2O/Rb, Rb/Sr, Ln(Al2O3/Na2O), and Al/K ratios, indicating that the studied succession was deposited during alternating phases between weak and moderate weathering intensity under arid and warm-humid climates, respectively. Periods of enhanced continental weathering were associated with high values of clastic ratios such as Si/Al, Ti/Al, and Zr/Al, suggesting increased terrigenous sediment supply during intensified hydrological cycling. These ratios further provided inferences about the changes in sediment grain size, such as a change from shale to coarse silt- and sand-size fractions.

Paleoenvironmental reconstruction of the Berriasian organic-rich interval of the Vaca Muerta Formation (Neuquén Basin, Argentina): Insights for the characterization of unconventional hydrocarbon shale reservoirs

The 50 m-thick Berriasian organic-rich interval (BORI) of the Vaca Muerta Formation (Neuquén Basin, Argentina) represents one of the most attractive stratigraphic intervals for unconventional hydrocarbon exploration in the unit. Nevertheless, little is known about the environmental conditions prevailing during the accumulation of the sediments. To unravel the causes that favored the preservation of the organic matter (OM) in the BORI, a detailed multiproxy sedimentological approach was conducted. The study included the analyses of microfacies, palynological content, calpionellid-calcisphere dinocysts, mineralogy, spectral gamma-ray, inorganic geochemistry, Rock-Eval pyrolysis and stable carbon isotopes of whole rock carbonate (δ13Ccarb), and associated OM (δ13Corg). The BORI presents a late Early to early Late Berriasian age and is subdivided into a lower and upper interval. The lower interval (25.6 m-thick) is primarily formed by radiolaritic wackestones and presents higher total organic carbon (TOC) content (4.2 wt% on average), mainly in the form of amorphous OM (AOM), with variable contribution of phytoclasts. The geochemical analyses (Si, Ni, Cu, Mo, U, V) indicate high productivity of the water column and overall sea bottom anoxia. Oppositely, the upper interval (24.4 m-thick) is mostly constituted by peloidal packstones/grainstones and presents a lower TOC content (1.5 wt% on average) mainly constituted by phytoclasts, with variable contribution of AOM. A decrease of the productivity and an increase of sea bottom oxygenation is recorded based on the geochemical analyses. The high OM content of the BORI responds to the combination of high productivity of the water column and sea bottom anoxia in response to the Berriasian transgression and a worldwide paleoclimatic change towards more humid conditions. The results of our study position the BORI as an attractive interval for unconventional hydrocarbon production due to its high TOC, low bulk clay mineral and high biogenic quartz content, granting an adequate geomechanical behavior.

Exploring the restricted Lontras Shale fossil record: Data biases or preservational casualties?

Throughout the Phanerozoic, numerous well-preserved fossiliferous sites have provided valuable insights into the evolution of ecosystems and organisms. Among these sites, the Lontras Shale Lagerstätte (LSL) within the Lontras Member in southern Brazil stands out as a rare glimpse into Carboniferous ecosystems of western Gondwana. The LSL is renowned for its diverse array of well-preserved fossils like insects, wood, fishes, larval cases, and poriferans, with different types of preservational pathways. Moreover, the Lontras Member is considered a maximum flooding surface that marks an interglacial warming peak of the final Late Paleozoic Ice Age, highlighting its paleoclimatological and paleogeographical relevance. However, the occurrence of fossils within the extensive Lontras Member is restricted to a few sites, raising questions about the factors influencing this constrained lateral and vertical distribution. Through meticulous literature review, this study aims not only to solidify the understanding of the Lontras Member's lithostratigraphy and paleontology but also to explore its broader implications and paleoenvironmental significance. We propose a comprehensive stratigraphic section for the Lontras Member, integrating local and regional sections from Santa Catarina and Paraná States, surveying paleontological sites and fossils exhaustively. By examining two additional fossil-rich sites in addition to the Lagerstätte outcrop, and analyzing vertical and horizontal distributions of the Member, we thoroughly explore the methodological, paleoenvironmental, and taphonomic factors that restricts this fossil record. This analysis extends beyond comparisons with paleontological insights from other units, locations, and Lagerstätten, encompassing modern analogous environments. The methodological biases are tied to stratigraphic focus, fieldwork scale, fossil size, and weathering degree of outcrops, constraining new fossil discoveries. Paleobiological and paleoclimatological factors, like transitional climatic conditions, limited organism diversity, inhospitable deep environments distant from the coast, and anoxic bottoms impact organism distribution and fossil recovery. The fossil transportability and the compositional resilience under widespread anoxia contributed as preservational constraints. While the LSL offers unique conditions for the proliferation and preservation of organisms, limiting the taphonomic window, facies of more energetic depositional conditions in other Lontras Member sites is more likely to harbor fossils, particularly transportable fragments and disarticulated pieces of chitinophosphatic or phosphatic composition. Addressing these biases unveils insights into flourishing ecosystems and organisms during this specific time period.

Thirty years of a bottom oxygen depletion-renewal cycle in the coastal yet deep environment of the West Saronikos Gulf (Greece): Its drivers and the impact on the benthic communities

In the period 1987–2017, a series of physical and chemical measurements related to oxygen variability at a trough area with a maximum depth of ~420 m in the West Saronikos Gulf, reveal the following: In the early 90s, deep winter mixing occurred resulting in an oxygenation down to ~420 m followed by an oxygen decline. This decline reached near-bottom hypoxic conditions (O2 < ~62 μM (μmol/L)) after 1998, while a denitrification phase occurred after 2000 and a complete bottom anoxia in 2005. In June 2012, an oxygenation down to ~350 m was detected that most likely occurred in winter 2012. The 2012 oxygenation raised the until-then anoxic bottom concentrations to hypoxic ones in the years towards 2017 via vertical diffusive oxygen transfer. Observations of the benthic communities during the hypoxia, severe hypoxia (O2 < ~15 μM) and oxygen recovery phases showed a peak of opportunists in the hypoxia and a long faunal depletion in the severe hypoxia phases. A reversal in the benthic community structure appeared after the oxygenation of 2012 with the (re)appearance of opportunists while, in 2017, the community showed signs of retreat to earlier stages. The main anthropogenic pressure that could tentatively affect the oxygen concentration in the study area is posed by the Athens treated-sewage outfall at ~40 km away from the trough, which inputs organic matter into the Saronikos Gulf through effluent water of reduced salinity that, in addition, may alter the stratification opposing the vertical mixing. We show that the treated sewage output had no influence on a) the stratification, b) the particulate and dissolved organic carbon and c) the sewage-derived organic matter. Instead, the long-term dissolved oxygen variability with the deep renewal events was mostly driven by the large-scale atmosphere-ocean conditions (heat exchange and evaporation-minus-precipitation budget) that determine the hydrographic characteristics and the winter mixing.

Deoxygenation turns the coastal Red Sea lagoons into sources of nitrous oxide

Direct measurements of dissolved N2O concentrations, fluxes and saturation percentages undertaken for the first time in two coastal lagoons — Al-Shabab and Al-Arbaeen, along the east coast of the Red Sea, revealed the region as a significant source of N2O to the atmosphere. The exacerbated dissolved inorganic nitrogen (DIN) from various anthropogenic sources led to substantial oxygen depletion in both the lagoons, which turned to bottom anoxia at Al-Arbaeen lagoon during the spring season. We assume that the accumulation of N2O is caused by nitrifier-denitrification in the hypoxic/anoxic boundaries. In fact, the results indicated that oxygen-depleted bottom waters favoured denitrification when the oxygenated surface waters recorded nitrification signals. Overall, the N2O concentration ranged from 109.4 to 788.6 nM (40.6–325.6 nM) in spring and 58.7 to 209.8 nM (35.8–89.9 nM) in winter in the Al-Arbaeen (Al-Shabab) lagoon. The N2O flux ranged from 647.1 to 1763.2 μmol m−2 day−1 (85.9 to 160.2 μmol m−2 day−1) and 112.5 to 150.8 μmol m−2 day−1 (76.1 to 88.7 μmol m−2 day−1) in the spring and winter respectively, in the Al-Arbaeen (Al-Shabab) lagoons. The ongoing developmental activities may worsen the current situation of hypoxia and associated biogeochemical feedbacks; therefore, the present results underline the need for continuous monitoring of both lagoons to restrict more severe oxygen depletion in future.

Ecosystem Metabolism and Gradients of Temperature, Oxygen and Dissolved Inorganic Carbon in the Littoral Zone of a Macrophyte‐Dominated Lake

AbstractDense submerged macrophyte stands in lakes may promote alternating daytime stratification and nighttime convective mixing, driving extensive spatiotemporal variations in water temperature, oxygen, pH, and inorganic carbon (DIC). We set out to investigate environmental conditions and ecosystem metabolism in the macrophyte‐dominated littoral zone (0.6 m depth) of a shallow, mesotrophic lake and compared this with the pelagic zone (3.0 m deep). We found that, during summer, vertical water column gradients only occasionally occurred and were weak in the pelagic zone, while dense littoral macrophyte stands of charophytes exhibited strong diel changes and steep daytime temperature and oxygen depth gradients. Oxygen showed daytime surface supersaturation and bottom anoxia, alternating with nighttime mixing. In spring, before charophytes appeared, the vertical gradients were largely absent. Ecosystem metabolism was primarily positive in spring, but areal rates of daily gross primary production (GPP) and closely related respiration increased 6‐fold in summer. Ecosystem metabolism calculated based on oxygen or DIC was very similar, with a 1:1 M basis. Daytime DIC loss by CaCO3precipitation on charophyte surfaces averaged 6.3% of total DIC loss, and this loss was restored during nighttime. The contribution of shallow littoral macrophyte communities to whole‐lake summer GPP exceeded that estimated for phytoplankton. Environmental conditions in the macrophyte‐covered littoral zone differed markedly from pelagic waters with steep vertical gradients in temperature and chemistry during daytime including anoxic micro‐habitats in stark contrast to deep pelagic waters. The study highlights the significant influence of the littoral zone and macrophytes on whole‐lake ecosystem processes.

Improving oxygen conditions in periodically stagnant basins using sea-based measures - Illustrated by hypothetical applications to the By Fjord, Sweden

To improve the water quality of a habitat or to stop eutrophication driven by internal loading of phosphorus from anoxic bottoms, the deepwater in inshore and offshore sea basins may be oxygenated using sea-based measures. The minimum oxygen concentration DOmin in periodically stagnant basins is controlled by three factors, namely (i) the rate of oxygen depletion dDO/dt. (ii) The residence time Te of the stagnant water. (iii) The oxygen concentration DOstart in the basin water at the beginning of a stagnation period. A theoretical framework is presented, suitable for evaluation of sea-based measures to improve the oxygen conditions in periodically stagnant basins. Five different sea-based measures to improve the oxygen conditions are recognized and discussed. The application of sea-based measures might be particularly attractive for habitat improvements in basins with poor oxygen conditions despite land-based measures are fully implemented. The sea-based measures are illustrated by hypothetical applications to the By Fjord on the west coast of Sweden, a salt-stratified fjord with periodically stagnant anoxic basin water confined behind the shallow Sunninge Strait in the mouth. An interesting result of one of the hypothetical applications is that dredging of the Sunninge Strait in 1958 and 1976 reduced tidal currents by a factor of 4. This reduced the tidal power supply to vertical mixing in the basin water, which led to prolonged residence time Te, from 2-3 years to 3–5 years, and deteriorated oxygen conditions in the basin water.

DYNAMICS OF DEPOSITION AND FOSSIL PRESERVATION AT THE EARLY EOCENE OKANAGAN HIGHLANDS OF BRITISH COLUMBIA, CANADA: INSIGHTS FROM ORGANIC GEOCHEMISTRY

ABSTRACT The early Eocene Okanagan Highland fossil sites of Washington (USA) and British Columbia (Canada) contain exquisitely preserved plant and insect fossils that showcase a critical time and place in the evolution of the Northern Hemisphere temperate deciduous biome. A comprehensive understanding of the dynamics of fossil deposition and preservation at these sites is not fully resolved but is critical for reliable reconstructions of these ancient forests. To expand on previous interpretations (e.g., deep, stratified, anoxic lake bottoms) and address uncertainties about the environment of deposition (e.g., distance to shore, influence of diatoms), we analyzed sediment samples from three Okanagan Highland fossil sites—McAbee, Falkland, and Driftwood Canyon—for organic biomarkers, their stable carbon isotopic compositions, and glycerol dialkyl glycerol tetraethers (GDGTs; at McAbee only). Terpenoids suggest relative trends in gymnosperm abundance between sites that agree with prior macrofossil evidence, though absolute values may overestimate local gymnosperm abundance. A combination of biomarker evidence indicates a predominantly autochthonous aquatic source (e.g., diatoms) for organic matter in shale and mudstone samples, even contributing to long chain n-alkanes and likely to branched GDGTs, which are often assumed to be terrestrially sourced. In combination with biomarker evidence for anoxia and stratification, fossiliferous shales are interpreted to have been deposited offshore in deep and mesotrophic lakes that were thermally stratified with an anoxic hypolimnion, away from in-flowing tributaries, while a coal horizon at Driftwood Canyon was deposited in a shallower, eutrophic, anoxic wetland. Anoxic conditions likely minimized some degradation-based biases and promoted high quality fossil preservation. Deposition of sediment and fossil remains offshore and away from inflowing tributaries suggest fossil plants were locally sourced but highlights the need for careful consideration of transport-induced biases.

Modeling of biogeochemical consequences of a CO2 leak in the water column with bottom anoxia

In this paper we investigate the spatial extent and biogeochemical properties of a known CO2 plume using the pelagic transport-biogeochemical model BROM (Bottom RedOx Model). The model consists of a biogeochemical module, a 2-dimensional vertical transport module and gas bubble fate module, parameterizing bubbles rising and dissolution according to existing approaches. A controlled CO2 release experiment was carried out in the Horten Inner Harbor, Norway, in September 2018. This isolated bay is characterized by limited water mixing and anoxia in the bottom layer. CO2 was released at a water depth of 18 m either in a gas phase or dissolved in seawater at leak rates ranging from 0.1 l/min to 15.8 l/min. The chemical response to the release events relative to background variations was measured using chemical sensors mounted on two seabed templates located 4 m and 15 m from the release point, respectively, and compared to the values predicted by the model. The measurements show elevated levels of pCO2 and simultaneously decreased values in pH corresponding to the controlled release experiments. The model's simulations were in good agreement with the baseline observations and the measured changes forced by the experimental leak. The model predicts that after a continuous leak of this magnitude in stagnant conditions of anoxic bottom water, a 2–3 weeks long restoration period occurs, after which the disturbances disappear. This work confirms that the footprint of a potential CO2 leak is localized in the vicinity of the source (tens of meters) where it can be detectable with available chemical sensors.

Assessment of bottom sediment quality in Niterói harbor (Brazil, South America) through ecological indexes concerning nutrients and trace metals.

Bottom sediment quality from the Niterói harbor at Guanabara Bay (Rio de Janeiro, Brazil) was evaluated based on concentrations of organic phosphorus (OP) and inorganic phosphorus (IP) and bioavailability of trace metals through BCR fractionation analysis (Zn, Cu, Pb, Cr, Ni, Cd). The study area revealed elevated concentrations of fine sediments and organic matter (TOC: 2.26-7.31%). OP presented extremely elevated concentrations between 0.57 and 47.04 μmol/g, whereas IP reached a maximum concentration of 4.99 μmol/g. The anoxic bottom at the study area was confirmed by high TOC/OP values in most stations. Phosphorus enrichment index (PEI) varied between 0.07 and 2.57, pointing to ecological risk at some stations. Trace metals were mostly bonded to the bioavailable fractions (exchangeable, reducible, oxidizable), and decreasing order of mobility was Zn > Cu > Pb > Cr > Ni > Cd. The Risk Assessment Code (RAC) suggested a high risk of bioavailability for Zinc and a medium risk for the other metals. Overall, the Niterói harbor revealed poor sediment quality suggesting a strong anthropogenic pressure in the area.

Evaluating sedimentary DNA for tracing changes in cyanobacteria dynamics from sediments spanning the last 350\xa0years of Lake Tiefer See, NE Germany

Since the beginning of the Anthropocene, lacustrine biodiversity has been influenced by climate change and human activities. These factors advance the spread of harmful cyanobacteria in lakes around the world, which affects water quality and impairs the aquatic food chain. In this study, we assessed changes in cyanobacterial community dynamics via sedimentary DNA (sedaDNA) from well-dated lake sediments of Lake Tiefer See, which is part of the Klocksin Lake Chain spanning the last 350 years. Our diversity and community analysis revealed that cyanobacterial communities form clusters according to the presence or absence of varves. Based on distance-based redundancy and variation partitioning analyses (dbRDA and VPA) we identified that intensified lake circulation inferred from vegetation openness reconstructions, δ13C data (a proxy for varve preservation) and total nitrogen content were abiotic factors that significantly explained the variation in the reconstructed cyanobacterial community from Lake Tiefer See sediments. Operational taxonomic units (OTUs) assigned to Microcystis sp. and Aphanizomenon sp. were identified as potential eutrophication-driven taxa of growing importance since circa common era (ca. CE) 1920 till present. This result is corroborated by a cyanobacteria lipid biomarker analysis. Furthermore, we suggest that stronger lake circulation as indicated by non-varved sediments favoured the deposition of the non-photosynthetic cyanobacteria sister clade Sericytochromatia, whereas lake bottom anoxia as indicated by subrecent- and recent varves favoured the Melainabacteria in sediments. Our findings highlight the potential of high-resolution amplicon sequencing in investigating the dynamics of past cyanobacterial communities in lake sediments and show that lake circulation, anoxic conditions, and human-induced eutrophication are main factors explaining variations in the cyanobacteria community in Lake Tiefer See during the last 350 years.

Mineralogy and geochemistry of deeply-buried marine sediments of the Vaca Muerta-Quintuco system in the Neuquén Basin (Chacay Melehue section), Argentina: Paleoclimatic and paleoenvironmental implications for the global Tithonian-Valanginian reconstructions

The Vaca Muerta-Quintuco (VM-Q) system of Tithonian-early Valanginian age was studied in the Chacay Melehue section of the Neuquén Basin (western Argentina) by means of sedimentological, mineralogical and geochemical analyses in order to determine the main driving factors that triggered the paleoenvironmental change from a carbonate ramp (Vaca Muerta Formation, VMFm) to a mixed siliciclastic/carbonatic marine environment (Quintuco Formation, QFm). The VMFm was divided into two stratigraphic intervals: Lower VMFm (LVMFm) and Upper VMFm (UVMFm), whereas the QFm is subdivided into the Puesto Barros Member (PBMb) and the Cerro La Visera Member (CVMb), which can be correlated to other sections in the basin (e.g., Puerta Curaco). Isolated, turbiditic sandstone beds, correlated to the Huncal Member, are included in the QFm.The LVMFm (Tithonian) and the UVMFm (Berriasian) are constituted by dark, well-laminated marls, mudstones, calcite concretions and tuffs. The PBMb (earlier early Valanginian) is constituted by marls and sandstones, whereas the CVMb (later early Valanginian) is constituted by marls, mudstones, siltstones, sandstones and coquinas. The LVMFm (Total organic carbon, TOC ~ 1–4 wt%) is characterized by the enrichment of redox sensitive trace elements (RSTE), where the enrichment of Ni and Cu suggest high productivity in the water column, and the enrichment of Mo, U, V points to sea bottom anoxia, with periods of increased oxygenation as deduced from higher P concentrations in marls and mudstones. The clay mineral association is constituted by mixed-layer illite/smectite formed by the transformation of smectitic layers. The predominance of smectite in coeval sucessions, less overprinted by burial diagenesis, suggests a temperate and semi-arid climate in the adjacent continent. The UVMFm (TOC ~ >1 wt%) is characterized by a gradual decrease of the RSTE pointing to a decrease in productivity and a slight increase in the oxygenation of the sea bottom. In addition, a change towards more humid conditions in the continent is inferred by both the increase in the Chemical Index of Alteration and the Al2O3/TiO2 ratio. The PBMb (TOC ~1 wt%) has even lower content of RSTE indicating diminished sea water productivity and a gradual rise of the oxygenation of the sea bottom. In this interval, increased illite contents suggest periods of enhanced physical weathering, probably related to the tectonic uplift of the Huincul Ridge. The RSTE in the CVMb (TOC <1 wt%) documents a fully oxygenated sea bottom, where the productivity of the water column was negligible. The presence of kaolinite and the increment of the detrital sedimentation in the CVMb indicate a change towards more humid conditions in the hinterlands. The enhanced runoff caused by this paleoclimatic change towards more humid conditions that started in the early Berriasian and increased during the later early Valanginian triggered the change from carbonate ramp to mixed siliciclastic/carbonatic to siliciclastic marine paleoenvironments. The organic carbon isotope composition (δ13Corg vs. VPDB) of the VMFm ranges between −30.0 and −23.4‰, whereas in the QFm values range between −29.0 and −23.9‰. Two positive carbon isotope excursions (PCIE) are recorded in the system: PCIE-A in the lower part of the VMFm (early Tithonian) with δ13Corg values ~ −25‰, and PCIE-B in the upper part of the QFm (later early Valanginian) with δ13Corg values ~ −24.5‰. The shift in δ13Corg at PCIE-B is up to +4.2‰ and marks the onset of the Weissert Event. This is the first time that the onset of this event is recorded in the Neuquén Basin, within the Lissonia riveroi ammonite zone. The results of our study confirm that clay mineralogy, trace elements and stable isotopes are valuable proxies of past ocean-climate variability even in sediment deposits that underwent near-surface and deep burial diagenesis.

The Eutrophication of the Baltic Sea has been Boosted and Perpetuated by a Major Internal Phosphorus Source

The phosphorus (P) concentration c1 in the surface layer of the Baltic proper in winter depends on the land-based P source LPS, and the ocean P source OPS, which are known. It also depends on the internal P source IPS from anoxic bottoms and the sum of internal and external P sinks TPsink, which are estimated in this paper. IPS is parameterized as fs·Aanox, where fs is the specific annual mass flux of P from anoxic sediments and Aanox is the area of anoxic bottoms, and TPsink is parameterized as c1·TRVF, where TRVF is the total removal volume flux. We use a time-dependent P budget model, and 47 years of observational data, and the method of least squares to determine the best estimates of the unknown parameters fs and TRVF. The result is TRVF = 3,000 km3 year−1 and fs = 1.22 tons P km−2 year−1. With these parameter values, the model gives a quite good description of the observed evolution of c1. The observed runaway evolution of c1, with increasing c1 since the 1980s although the land-based supply LPS has been halved, is well-described by the model. It is concluded that the internal P source IPS provides a positive feedback mechanism that has boosted and perpetuated the eutrophication of the Baltic proper and that IPS is the major driver of the Baltic Sea eutrophication since the late 1990s. It is suggested that measures to eliminate IPS should be included in the management strategy to reduce the eutrophication of the Baltic proper.

Rapid unpredicted changes in the stratification of marine lake Mogilnoe (Kildin Island, the Barents Sea) through the early 21st century

Lake Mogilnoe is a rare example of an anchialine lake (with subterranean connection to the ocean) in the Arctic, a refuge for landlocked populations of marine organisms. The lake has been the subject of intensive studies since the end of the 19th century. Here we demonstrate that between the 2003–07 and 2015–18 observation periods this permanently stratified lake experienced significant changes. The surface salinity increased and exceeded the tolerance limits of many freshwater organisms. The bottom anoxia expanded from one-fifth to one-third of the lake volume. Such a turn in stratification affected both composition and distribution of biota: freshwater zooplanktonic species virtually disappeared, while benthic communities shifted to shallower depths. Although recent changes in the lake stratification are consistent with the long-term trend, their scale is much larger than has been observed during the past 120 years. It was earlier considered that the lake dynamics were mainly affected by human activity in the vicinity of the lake. However, lack of human activity around Mogilnoe during last decades persuades us to search for the natural causes of the recorded changes.

Seasonal interactive effects of pCO2 and irradiance on the ecophysiology of brown macroalga Fucus vesiculosus L.

ABSTRACT Stochastic upwelling of seawater in the Baltic Sea from the deep, anoxic bottoms may bring low-pH water rich in CO2 close to the surface. Such events may become more frequent with climate change and ongoing ocean acidification (OA). Photoautotrophs, such as macroalgae, which are important foundation species, have been proposed to benefit from increased carbon availability due to reduced energetic cost in carbon acquisition. However, the exact effects of CO2 fertilization may depend on the ambient light environment, as photosynthesis rates depend on available irradiance. In this experimental study, interacting effects of CO2 addition and irradiance on the habitat-forming macroalga Fucus vesiculosus were investigated during two seasons – winter and summer – in the northern Baltic Sea. Growth rates remained unaffected by CO2 or irradiance during both seasons, suggesting that direct effects of elevated CO2 on mature F. vesiculosus are small. Increases in CO2 affected algal elemental ratios by increasing carbon and decreasing nitrogen content, with resulting changes in the C:N ratio, but only in winter. In summer, chlorophyll a content increased under low irradiance. Increases in CO2 caused a decline in light-harvesting efficiency (decrease in Fv/Fm and α) under high irradiance in summer, and conversely increased α under low irradiance. High irradiance caused increases in the maximum relative electron transport rate (rETRmax) in summer, but not in winter. Differences between winter and summer indicate that F. vesiculosus responses to CO2 and irradiance are season-specific. Increases in carbon content during winter could indicate slightly positive effects of CO2 addition in the long run if the extra carbon gained may be capitalized in growth. The results of this study suggest that increases in CO2, either through upwelling or OA, may have positive effects on F. vesiculosus, but these effects are probably small.

Late-Holocene palaeoenvironmental changes at Lake Esmeralda (Vega Island, Antarctic Peninsula) based on a multi-proxy analysis of laminated lake sediment

We have studied laminated sediments from Lake Esmeralda, Vega Island, in order to reconstruct its history. We describe both inorganic and organic components of the sediment using a combination of the following analytical methods: x-ray fluorescence (XRF), x-ray diffraction (XRD), magnetic susceptibility measurement, chemical analysis for determination of cation exchange capacity, grain size determination, geochemical analyses (total inorganic carbon (TIC), total organic carbon (TOC), total sulphur (TS)), spectrophotometry, high-pressure liquid chromatography, and diatom assemblage and faunal remains characterization. The geochronology of the core was based on modelling optically stimulated luminescence ages and supported by laminae counting. The dating results imply a maximum age of ~400 years for the 177-cm long core and a period covered of ~200 years, suggesting (quasi-)annual laminae formation. Such a young age contradicts previous findings based on radiocarbon dating. Geomorphological evidence indicates that river capture isolated the lake catchment from upslope sediment delivery, effectively terminating accumulation ~230 years ago. Conversely, our short-term palaeoenvironmental record yields a subdecadal temporal resolution, which is unparalleled in comparison with other Antarctic palaeolimnological studies. Our interpretations of the geochemical and mineralogical proxy data give us insight into the past lake catchment and waterbody evolution, and lead us to recognize periods of enhanced weathering, bottom anoxia and to distinguish major lake level changes.

Isotope geochemistry tracks the maturation of submarine massive sulfide mounds (Iberian Pyrite Belt)

The massive sulfide deposits of the southern Iberian Pyrite Belt (IPB) occur as shale-hosted exhalative mounds or shallow sub-seafloor replacive bodies that formed at the Devonian-Carboniferous boundary in anoxic bottoms. The geochemistry of the mineralization and the host shale from some of the major deposits, including a detailed 50-m-thick section through the Migollas orebody, shows a marked textural and vertical zonation that is also reflected in the chemical, textural, and isotopic variations of pyrite and carbonates. Pyrite evolves from primary framboids and colloform aggregates to late euhedral crystals, which accompanies a redistribution of the contained base metals and grain coarsening. SIMS analyses depict an extreme variability of δ34S values from − 42.3 to + 4.4‰ with a gradual increase that is related to the textural maturation. There is also a systematic rise in the 87Sr/86Sr ratios (0.70846 to 0.71354), δ13C (− 12.2 to − 5.2‰) and δ18O values (+ 14.1 to + 27.8‰) of the associated carbonates, which evolve from early dolomite to late siderite. This evolution evidences the maturation of the exhalative mineralization during mixing between two fluids: modified seawater enriched in biogenically derived reduced sulfur and upflowing hydrothermal fluids. Early sulfides precipitated in the seawater-mound interface. Towards the core of the mounds, there was an increasing influence of deep fluids that modified the early mineral textures and the isotopic signatures. This evolution is similar to that observed in modern submarine hydrothermal systems and confirms the importance of mound refining in the evolution of fossil exhalative mounds and the formation of economic ores.

The Chaoborus pump: Migrating phantom midge larvae sustain hypolimnetic oxygen deficiency and nutrient internal loading in lakes

Hypolimnetic oxygen demand in lakes is often assumed to be driven mainly by sediment microbial processes, while the role of Chaoborus larvae, which are prevalent in eutrophic lakes with hypoxic to anoxic bottoms, has been overlooked. We experimentally measured the respiration rates of C. flavicans at different temperatures yielding a Q10 of 1.44–1.71 and a respiratory quotient of 0.84–0.98. Applying the experimental data in a system analytical approach, we showed that migrating Chaoborus larvae can significantly add to the water column and sediment oxygen demand, and contribute to the observed linear relationship between water column respiration and depth. The estimated phosphorus excretion by Chaoborus in sediment is comparable in magnitude to the required phosphorus loading for eutrophication. Migrating Chaoborus larvae thereby essentially trap nutrients between the water column and the sediment, and this continuous internal loading of nutrients would delay lake remediation even when external inputs are stopped.

Any payoff to ecological engineering? Cost-benefit analysis of pumping oxygen-rich water to control benthic release of phosphorus in the Baltic Sea

Abstract This paper examines the economic aspects of pumping of oxygen-rich water to anoxic bottoms of the Baltic Sea from two different angles. Drawing on the recent pumping experiments in the Swedish archipelago (Lannerstasundet) and in the Finnish archipelago (Sandofjarden), we generalize the findings to the anoxic coastal areas of the Gulf of Finland. For nutrient policies we find that under the current abatement levels and abatement costs in WWTPs, reduction of external loads produces higher annual net benefits and higher present value than oxygenating anoxic bottoms by pumping in the coastal areas of the Gulf of Finland. For pumping a means of speeding up the recovery after a hypothetical heavy reduction in nutrient loads we find that it might provide positive net benefits in the coastal anoxic bottoms in the Gulf of Finland but not in the open Gulf of Finland.