Periodic Anoxia Research Articles

Carbonate-hosted manganese deposits and ocean anoxia

Late Devonian (ca. 360 Ma), Early Carboniferous (ca. 330 Ma), and Early Triassic (ca. 250 Ma) manganese deposits in the South China Block support an emerging view that some Mn carbonates form through direct synsedimentary (authigenic) precipitation. These Mn carbonates accumulated on distal shelves and are interbedded with lime mudstone and heterozoan carbonates that accumulated in coastal upwelling environments. Lithofacies, Ce anomalies combined with vanadium, uranium, and molybdenum enrichments indicate that the Mn carbonates were primarily precipitated under anoxic conditions. We provide a robust dataset to clarify the precipitation pathway of these Mn carbonates and explore the implications of using them as proxies of regional shallow water and deep-ocean anoxia. In the studied deposits, Mn calcite encases dolomite rhombs (<10 μm) or aggregates, indicating heterogenous nucleation, which is supported by similar lattice parameters of both minerals with a small lattice mismatch (<2.1%). The presence of micro-sized euhedral rhombs and subhedral aggregates of rhombs, the non-stoichiometric compositions, and characteristic organic matter Raman spectroscopy peaks near the D and G bands in the dolomite nuclei suggest an organogenic origin. Authigenic organogenic dolomite precipitation is interpreted to have preconditioned the sediment with nucleation sites for the subsequent precipitation of Mn carbonates in the shallow seafloor. This is consistent with δ13C values (−2.61‰) of Mn carbonate bulk samples that are slightly lower than coeval seawater (0.31‰, from host rocks), suggesting incorporation of 12C during microbial respiration of sedimentary organic matter during the precipitation of organogenic dolomite and hydrothermal sourced inorganic carbon. The absence of Mn in shallow facies suggests terrestrial input of Mn was minimal. Geochemical and sedimentologic evidence suggests that coastal upwelling delivered manganous anoxic deep waters to the shelves. Eu/Eu⁎ ratios (<1.5) and decoupling of Fe and Mn imply long-range transport of hydrothermal Mn, during which hydrothermal fingerprints were muted through mixing with ambient seawater. The transport of hydrothermal Mn to shelves indicates that the regional deep oceans at the time must have been anoxic, as Mn2+ would be easily oxidized and become insoluble under oxygen-rich conditions, preventing its long-distance transport. This precipitation pathway and depositional model implies that Mn carbonates are a largely unrecognized proxy for tracking ancient oceanic redox fluctuations, particularly during times of well-oxygenated global oceans. Hence, these Mn deposits record at least transient regional deep-ocean oxygen depletion in the generally well-oxygenated Late Paleozoic and Early Triassic oceans. The recognition that this periodic anoxia was broadly coeval with the anoxia-related Hangenberg, Serpukhovian, and Permian-Triassic biotic crises suggests a potential relationship between Mn carbonate accumulation and extinction events.

Paleoenvironmental changes of the last 16,000 years based on diatom and geochemical stratigraphies from the varved sediment of Holzmaar (West-Eifel Volcanic Field, Germany)

Lacustrine environmental archives with annually laminated (varved) sediments allow a calendar-year chronology and the calculation of precise sediment accumulation rates. We present multiproxy analyses of a varved sediment record from Holzmaar (West-Eifel Volcanic Field, Germany) for the last 16,000 years providing a continuous diatom stratigraphy supported by physical and chemical variables with a centennial resolution. Patterns of diatom assemblages infer the trophic history of Holzmaar. There is a distinct variation at the Pleistocene/Lateglacial transition characterized by the replacement of Staurosira construens with Stephanodiscus minutulus, increases of Ca/Ti, total organic carbon/total nitrogen (TOC/TN) ratios and biogenic silica, together suggesting an increase in lacustrine productivity and a shift of the lake's trophic status from oligotrophic to mesotrophic. These conditions remain during the Bölling/Alleröd interstadial. During the Younger Dryas stadial, there is a decrease in organic productivity as reflected by TOC, a subtle increase in benthic and epiphytic diatoms indicating colder and dry conditions, and S. minutulus remains dominant suggesting increased winds. The Postglacial is dominated by Lindavia radiosa, S. minutulus and Nitzschia paleacea. The latter occurs between 9000 and 6000 cal yr BP together with an increase of TOC/TN and total sulphur (TS) suggesting eutrophication with periodic anoxia during the Holocene Thermal Maximum. After 2200 cal yr BP, L. radiosa and Pantocsekiella comensis are dominant suggesting warmer conditions (especially during the Medieval Climate Anomaly) and thermal stability. Increased surface runoff is reflected by higher values of Ti and magnetic susceptibility, suggesting more humidity and enhanced anthropogenic disturbance. Moreover, the appearance of Aulacoseira subarctica is related to a cold and wet period coinciding with the Little Ice Age. In addition to environmental changes, our multiproxy analyses track human impact since the Middle Neolithic.

High carbon losses from oxygen-limited soils challenge biogeochemical theory and model assumptions.

Oxygen (O2 ) limitation contributes to persistence of large carbon (C) stocks in saturated soils. However, many soils experience spatiotemporal O2 fluctuations impacted by climate and land-use change, and O2 -mediated climate feedbacks from soil greenhouse gas emissions remain poorly constrained. Current theory and models posit that anoxia uniformly suppresses carbon (C) decomposition. Here we show that periodic anoxia may sustain or even stimulate decomposition over weeks to months in two disparate soils by increasing turnover and/or size of fast-cycling C pools relative to static oxic conditions, and by sustaining decomposition of reduced organic molecules. Cumulative C losses did not decrease consistently as cumulative O2 exposure decreased. After >1year, soils anoxic for 75% of the time had similar C losses as the oxic control but nearly threefold greater climate impact on a CO2 -equivalent basis (20-year timescale) due to high methane (CH4 ) emission. A mechanistic model incorporating current theory closely reproduced oxic control results but systematically underestimated C losses under O2 fluctuations. Using a model-experiment integration (ModEx) approach, we found that models were improved by varying microbial maintenance respiration and the fraction of CH4 production in total C mineralization as a function of O2 availability. Consistent with thermodynamic expectations, the calibrated models predicted lower microbial C-use efficiency with increasing anoxic duration in one soil; in the other soil, dynamic organo-mineral interactions implied by our empirical data but not represented in the model may have obscured this relationship. In both soils, the updated model was better able to capture transient spikes in C mineralization that occurred following anoxic-oxic transitions, where decomposition from the fluctuating-O2 treatments greatly exceeded the control. Overall, our data-model comparison indicates that incorporating emergent biogeochemical properties of soil O2 variability will be critical for effectively modeling C-climate feedbacks in humid ecosystems.

Habitat characteristics mediated partitioning of economically important bivalves in a tropical monsoon-influenced estuary.

Bivalves are benthic organisms inhabiting coastal marine habitats, especially estuarine tidal and mudflats. Due to their high stocking density and rich protein content, they form a major part of the artisanal fishery resource around the world. A clear partitioning in the population of bivalves, Paphia malabarica (Chemnitz, 1782), and Meretrix casta (Gmelin, 1791) in southern (Chicalim) and northern (Siridao) bank of a tropical Zuari estuary influenced by the monsoon along the Indian west coast, is evidenced. This study unravels the reasons for their partitioning in this estuary. Paphia malabarica is an exclusive inhabitant of Chicalim which has silty-sandy sediment, whereas M. casta is exclusive to Siridao, a sandy habitat. Observations showed that this segregation is facilitated by the semi-enclosed nature of habitat at Chicalim with the high amount of degraded and aged sediment organic carbon, high chlorophyll a, elemental, and biochemical components, whereas Siridao experiences the high impact of tidal currents, low sediment organic carbon, and high water column chlorophyll a. The habitat in Siridao gets exposed to UV radiation during low tide, reducing the photosynthetic oxygen production, turning the habitat to periodic anoxia indicated by differences in the TOC:TS ratio. However, such conditions may not influence M. casta, which can derive oxygen from the water column. The fatty acids specific to diatoms, dinoflagellates, higher plants, and partially degraded organic matter in the tissues of P. malabarica indicate their ability to source the food from the sediment and water column, whereas in tissues of M. casta, higher dinoflagellate-specific fatty acids followed by diatom and bacteria indicate water column-derived food. Chicalim can be considered an actively coupled benthic-pelagic habitat, and Siridao as an uncoupled habitat. Thus, the diverse flux of food particles, species-specific feeding ecology, and local hydrodynamics operating at these habitats could be the determining factors in the partitioning of the bivalves. Graphical abstract .

Early Middle Triassic trace fossils from the Luoping Biota, southwestern China: Evidence of recovery from mass extinction

Trace fossils have proven useful for studying the timing and process of biotic recovery after the Permian–Triassic Mass Extinction (PTME). Recovery stages are defined by comparing successive ichnoassemblages from the latest Permian to the early Middle Triassic. Lower Triassic trace fossils have been explored in some detail, but those of the lower Middle Triassic are less well known. Here, well-preserved fossil materials from the Luoping Biota from Yunnan Province, South China suggest that a fully recovered shallow-marine ecosystem was re-established by the early Middle Triassic. Trace fossil assemblages of the Luoping Biota are characterized by high ichnodiversity, with 14 ichnogenera in the shallow-marine environment of an intra-carbonate platform basin, and nine ichnogenera in the subtidal environment. Such moderate to high ichnodiversity, together with a marked increase in burrow sizes and the common occurrence of key ichnotaxa (e.g. Rhizocorallium and Thalassinoides), suggests that the ichnofauna had reached recovery stage four. In contrast, non-turbiditic strata of the offshore setting record only three ichnogenera, with bioturbation indices never exceeding one. Periodic anoxia in bottom waters was presumably the main control for such a protracted trace fossil recovery in an offshore setting, which otherwise aided the fine preservation of body fossils of the Luoping Biota. Furthermore, event sedimentation (turbidite deposits) in the offshore setting incorporated moderate ichnodiversity and moderate to high bioturbation indices, both interpreted as a result of short-term colonization by transported infaunal animals from proximal settings. The occurrence of variable crustacean traces (e.g. Sinusichnus, Spongeliomorpha, and Thalassinoides) at Luoping and the locomotion traces of marine reptiles, together with abundant fishes and fossil decapods, highlights the value of trace fossils in ecosystem reconstruction after the PTME.

Effects of seasonal anoxia on the distribution of phosphorus fractions in the surface sediments of southeastern Arabian Sea shelf

The seasonal upwelling along the southeastern Arabian Sea (SEAS) brings cold, nutrient-rich low oxygen subsurface water to the continental shelf. The subsurface oxygen deficiency due to upwelling is severe in some years, the intensity of which could profoundly influence the nutrient cycling along the SEAS. Herein, we studied the effect of seasonal anoxia on fractionation of phosphorus during the peak upwelling period of August 2013. Abundance of five fractions of phosphorus (P), namely exchangeable or loosely sorbed P (Pads), iron-bound P (PFe), authigenic P (Paut), detrital apatite plus other inorganic P (Pdet) and organic P (Porg), in surface sediments of SEAS shelf has been studied using a sequential extraction procedure (SEDEX) to examine their distributions and sources. Total P (TP) concentrations ranged from 209 to 1081µgg−1 with an average of 508 ± 256µgg−1. Among the five P fractions, the authigenic P was the dominant species, representing about 60% of TP. The relative abundance of P fractions was in the order: Paut>Pdet>Porg>PFe> Pads. Multivariate analyses revealed that the P fractions were primarily associated with the organic constituents and fine sediments. The hypoxic bottom condition associated with summer monsoon upwelling significantly reduced the concentrations of PFe, Pads and Porg in the surface sediments. The enhanced release of organic bound and iron oxides bound P under low oxygen condition was evident from the enhanced C/P and N/P ratios and lower C/N ratio in the sediments. The intense biomineralization of organic matter and reduction of Fe-Mn oxides due to the periodic anoxia resulted in the transformations of their associated P fractions and enhanced accumulation as authigenic calcium phosphate mineral.

Environmental Conditions in a Carpathian Deep Sea Basin During the Period Preceding Oceanic Anoxic Event 2 - A Case Study from the Skole Nappe

Abstract Hemipelagic green clayey shales and thin muddy turbidites accumulated in a deep sea environment below the CCD in the Skole Basin, a part of the Outer Carpathian realm, during the Middle Cenomanian. The hemipelagites contain numerous radiolarians, associated with deep-water agglutinated foraminifera. These sediments accumulated under mesotrophic conditions with limited oxygen concentration. Short-term periodic anoxia also occurred during that time. Muddy turbidity currents caused deposition of siliciclastic and biogenic material, including calcareous foramini-fers and numerous sponge spicules. The preservation and diversity of the spicules suggests that they originate from disarticulation of moderately diversified sponge assemblages, which lived predominantly in the neritic-bathyal zone. Analyses of radiolarian ecological groups and pellets reflect the water column properties during the sedimentation of green shales. At that time, surface and also intermediate waters were oxygenated enough and sufficiently rich in nutri-ents to enable plankton production. Numerous, uncompacted pellets with nearly pristine radiolarian skeletons inside show that pelletization was the main factor of radiolarian flux into the deep basin floor. Partly dissolved skeletons indicate that waters in the Skole Basin were undersaturated in relation to silica content. Oxygen content might have been depleted in the deeper part of the water column causing periodic anoxic conditions which prevent rapid bacterial degra-dation of the pellets during their fall to the sea floor.

Modeling the dissolved CO2 system in the redox environment of the Baltic Sea

Generation and depletion of total alkalinity (AT) were added to a Baltic Sea numerical model. The vertical distribution of generation and depletion of total alkalinity were described and attributed to different processes in the Eastern Gotland basin at the Gotland deep station (BY15) during the 1995‐2004 period. At this site, the mean annual generation (28.2 µmol kg−1 yr−1) and depletion (−25.8 µmol kg−1 yr−1) were almost balanced, though the transient rates were much faster (+125/−340 µmol kg−1 yr−1). The mean volume‐integrated AT content increased up to 50 µmol kg−1 when generation and depletion were added to the model. The AT changes were coupled to oxidation‐reduction (redox) reactions and the model budget indicates that internal generation and depletions is as important as lateral transports, including riverine input, at this site. Model predictive capability in marine environments with strong biogeochemical gradients was improved by coupling within the dissolved CO2, and the biogeochemical, systems. This enables evaluation of eutrophication, acidification, and climate change simultaneously, and is important specifically in regions with permanent or periodic anoxia.

Origin of the extreme polymetallic enrichment (Cd, Cr, Mo, Ni, U, V, Zn) of the Late Cretaceous–Early Tertiary Belqa Group, central Jordan

The sedimentary formations of the Late Cretaceous to Early Tertiary Belqa Group correspond to pure to slightly clayey limestones and minor black shales characterized by a remarkable enrichment in phosphorous and various redox sensitive elements (Cd, Cr, Mo, Ni, U, V, Zn). Phosphorous enrichment is related to the great phosphorite deposition event that has occurred at the southern margin of the Tethys Ocean during this period. The very low organic matter contents in the limestones, despite their strong enrichment in redox sensitive elements, is attributed to a deposition in anoxic water but under shallow water conditions permitting the bacterial degradation of a large part of the organic matter. Most of the elements constituting the Belqa Group sediments have been deposited from the sea water either directly by chemical precipitation (most Ca and redox sensitive elements) or indirectly through biogenic activity (P, part of Ca and U), except for the minor detritial contribution presents in some limestones and more significantly in the shales. Such an origin is also supported by the REE patterns of the limestones which are similar to that of the present sea water. The anomalously high concentrations of some redox sensitive trace metals (Cr, Ni and probably Cd) compared to worldwide black shales is explained by an exogenic metal flux, corresponding to the leaching of the huge amounts of ophiolites obducted during the collision between the African-Arabian and Eurasian plates at the same time as the deposition of the Belqa Group sediments. Uranium is substituted for calcium in biogenic apatite in the limestones, but was also directly precipitated from sea water in the black shales in relation with the suboxic environment with the periodic anoxia developed during their deposition in restricted deeper basins.The pyrometamorphism resulting from the burning of organic-rich levels in clayey limestones has created a further enrichment of redox sensitive elements in the resulting marbles by a volume loss resulting from decarbonatation reactions and the combustion of the organic matter.

Precession-driven monsoon variability at the Permian–Triassic boundary — Implications for anoxia and the mass extinction

By the end of the Late Permian, most continents had collided to form the supercontinent of Pangea. The associated climatic changes at the Permian–Triassic boundary coincided with the most severe mass extinction in the Phanerozoic. One extinction hypothesis favors a climatic response to an increase in large-scale volcanism resulting in ocean stagnation and widespread anoxia with fatal consequences for marine and land organisms. Recent interpretations of geochemical data suggest that orbitally-driven periodic upwelling of toxic hydrogen-sulfide rich water masses contributed to the extinction of species.In this paper, we use the Community Climate System Model (CCSM3) in order to explore the effect of eccentricity-modulated changes of the precession on the strength of Pangean megamonsoons and their impact on productivity and oxygen distribution. The climate model simulates high variability in monsoonal precipitation, trade winds and equatorial upwelling in response to precessional extremes, leading to remarkable fluctuations in the export of carbon from the euphotic zone and hence reduction in dissolved oxygen concentrations in subsurface layers. These findings are in general agreement with increased primary productivity, intensified euxinia within the oxygen-minimum zone, and decimation of the radiolarian zooplankton community as inferred from Japanese marine sections.Strong changes in river run-off linked to precipitation oscillations possibly led to a high variability in the nutrient supply to the Tethys Ocean, thus affecting regional productivity and oxygen distribution. The model results suggest that orbital variability in the sedimentary record and the associated extinction of species are related rather to periodic anoxia in near surface-to-intermediate depth than to widespread anoxic events in the Panthalassic deep-sea.

Effects of temperature and cadmium exposure on the mitochondria of oysters (Crassostrea virginica) exposed to hypoxia and subsequent reoxygenation

Intertidal bivalves are commonly exposed to multiple stressors including periodic hypoxia, temperature fluctuations and pollution, which can strongly affect energy metabolism. We used top-down control and elasticity analyses to determine the interactive effects of intermittent hypoxia, cadmium (Cd) exposure and acute temperature stress on mitochondria of the eastern oyster Crassostrea virginica. Oysters were acclimated at 20°C for 30 days in the absence or presence of 50 μg l(-1) Cd and then subjected to a long-term hypoxia (6 days at <0.5% O(2) in seawater) followed by normoxic recovery. Mitochondrial function was assessed at the acclimation temperature (20°C), or at elevated temperature (30°C) mimicking acute temperature stress in the intertidal zone. In the absence of Cd or temperature stress, mitochondria of oysters showed high resilience to transient hypoxia. In control oysters at 20°C, hypoxia/reoxygenation induced elevated flux capacity of all three studied mitochondrial subsystems (substrate oxidation, phosphorylation and proton leak) and resulted in a mild depolarization of resting mitochondria. Elevated proton conductance and enhanced capacity of phosphorylation and substrate oxidation subsystems may confer resistance to hypoxia/reoxygenation stress in oyster mitochondria by alleviating production of reactive oxygen species and maintaining high aerobic capacity and ATP synthesis rates during recovery. Exposure to environmental stressors such as Cd and elevated temperatures abolished the putative adaptive responses of the substrate oxidation and phosphorylation subsystems, and strongly enhanced proton leak in mitochondria of oysters subjected to hypoxia/reoxygenation stress. Our findings suggest that Cd exposure and acute temperature stress may lead to the loss of mitochondrial resistance to hypoxia and reoxygenation and thus potentially affect the ability of oysters to survive periodic oxygen deprivation in coastal and estuarine habitats.

Long-term changes in North Adriatic soft-bottom polychaete assemblages following a dystrophic crisis

In the Northern Adriatic Sea, the combination of natural and anthropogenic processes causes periodic hypoxia and anoxia, which can cause mass mortality of benthic organisms. Here, we report the results of a 20-year monitoring programme carried out after an anoxic event in 1989 that caused benthic mass mortality over an area of about 1200 km2. The recovery dynamics of the polychaete fauna was followed at three stations (including one sampled a year before the anoxic event). Samples (0.1 m2 Van Veen grab, 2-mm sieve) were collected from 1989 to 1994 and from 2003 to 2008. A total of 6519 specimens belonging to 186 species were identified. The analyses of univariate biodiversity indexes highlighted higher temporal variation of assemblage diversity in the period following the dystrophic crisis, followed by higher stability in the next decade. PERMANOVA suggested that the highest component of assemblage variation belonged to the factor year. However, differences across years varied in magnitude, depending on stations and were portrayed by non-metric multidimensional scaling plots of each year's centroids for each of the three stations. SIMPER analysis identified the species most characterizing the assemblages in each year for each of the three stations. Our results highlight the importance of analysing long-term data sets in order to understand assemblage dynamics following strong disturbance events. Assuming the 1989 anoxia as the event determining the biodiversity change, and considering our outcomes, which suggest that the assemblages still show a pattern of non-random changes through years, our data indicate that the polychaete fauna is still recovering without having reached a pattern of among-years stability. Focusing on the biology of the species characterizing the different periods might help in understanding the ecological processes that have determined the observed pattern.

Heat Shock Protein 70 Expression in Juvenile Eastern Oysters,Crassostrea virginica(Gmelin, 1791), Exposed to Anoxic Conditions

ABSTRACT Anoxic water events in conjunction with summer high temperatures are thought to be one of the causes of declines in natural oyster reefs on the eastern shore of Mobile Bay. Work is underway to determine whether tolerance to low oxygen can be selected for in hatchery-produced oysters. As a component of this work, the expression of heat shock protein 70 (HSP 70) was examined in control (normoxia) and anoxia-challenged juvenile oysters. Parental Eastern oysters, Crassostrea virginica were collected from 2 sites, Cedar Point Reef (CP), an area considered to have normoxic conditions, and White House Reef (WH), an area suspected to experience periodic anoxia. F1 generation oysters were produced from CP and WH parents that survived an anoxic exposure of 96 h. Control F1 generation oysters from both parental stocks not exposed to anoxia were also produced. The F1 generation oysters were subsequently exposed to anoxia or control normoxic conditions, and differences in expression of HSP 70 were examined. N...

Natural oligotrophy vs. pollution-induced eutrophy on the SE Mediterranean shallow shelf (Israel): Environmental parameters and benthic foraminifera

The eastern Mediterranean is naturally highly oligotrophic, but urbanization along the Levant coast has led to raised organic and nutrient loads. This study tracks living foraminiferal assemblages at two sites near an activated sewage sludge outfall from 11/2003 to 5/2004. Oligotrophic site PL29 shows seasonal variations in O2, chlorophyll a, and organic carbon, and has an abundant, diverse benthic foraminiferal assemblage living at various in-sediment depths. At eutrophic site PL3, ∼16years of sludge injection favor a depleted assemblage primarily of opportunist foraminifera. This site shows less seasonality, is subjected to organic matter overload, O2-stress, and periodic anoxia, foraminifera are less abundant and diverse, and live at shallower depths. The assemblages at both sites represent a common pool of species, with Ammonia tepida highly dominant. Benthic foraminifera were therefore found to be sensitive to trophic trajectories, respond on sub-seasonal time-scales, and track injection and dispersal of organic loads on the shelf.

Sediment and nutrient accumulation rates in sediments of twelve New Zealand lakes: influence of lake morphology, catchment characteristics and trophic state

Intact sediment cores were collected from the deepest basins of 12 lakes in the Rotorua District, New Zealand, to test for effects of morphological features, catchment characteristics and lake trophic state on net sedimentation rates and sediment nutrient concentrations. Multiple linear regression was used to show that 68% of the variation in net sedimentation rates across the lakes could be explained by lake trophic state and catchment area. Comparison of 2006 data with results from a survey in 1995 showed that surficial sediment (0–2 cm) total phosphorus concentrations (TP) have increased in three of the 12 lakes, at rates ranging from 27.5 to 114.4 mg P kg–1 dry wt y–1. Total nitrogen (TN) concentrations in surficial sediments have increased in nine of the 12 lakes at rates ranging from 51.8 to 869.2 mg N kg–1 dry wt y–1. Temporal changes in sediment TP and TN concentrations were not significantly linearly related (P = 0.12–0.88) to catchment area or different water column indices considered to reflect lake trophic state, including annual mean water column concentrations of TP, TN or chlorophyll a. It is concluded that between-lake variations in sediment TP and TN concentrations are influenced by a range of complex interacting factors, such as sediment redox conditions (and periodic anoxia in the hypolimnion of some lakes) as well as variations in sediment mineral composition (which influences retention and release of various sediment phosphorus and nitrogen species). Subsequently, these factors cause sediment TP and TN concentrations across the 12 lakes to respond differently to temporal changes in water column TP and TN concentrations.

Internal Phosphorus Loading in Shallow Lakes: Importance and Control

Decreasing the algal biomass and increasing transparency in shallow, unstratified lakes is usually more difficult than for deep, stratified lakes. Eutrophic unstratified lakes (or shallow, stratified lakes susceptible to metalimnion erosion) have typically responded slowly to reduced external nutrient loading, usually because of longevity of internal loading. That is because sediment-released nutrients (especially phosphorus) readily enter the trophogenic zone of shallow lakes during the growing season and result in high lake concentrations. In stratified lakes, metalimnia may serve as barriers to phosphorus transport into the trophogenic zone. Although the whole water column in shallow lakes is usually aerobic, several mechanisms can combine to produce relatively high sediment phosphorus release rates in these lakes. These include: 1) wind resuspension and bioturbation, combined with high pH or low Fe/P ratio that maintains high P solubility, 2) periodic anoxia and reducing conditions promoted by calm, warm weather, and 3) macro-phyte senescence. Attempts to reduce algal biomass by controlling internal phosphorus loading have often been effective.

The impact of climate and shore-level displacement on the late-Holocene environmental development of Havstens Fjord and Koljö Fjord, Swedish west coast

Koljö Fjord and Havstens Fjord on the Swedish west coast are, like many silled fjords in Scandinavia, characterized by strong stratification and stagnant bottom water with periodically occurring low-oxygen and anoxic conditions. High organic production together with a stable water-column, very low tidal activity and existence of the sill create an ideal foundation for low-oxygen conditions to develop. The aim of this study was to find out how the fjord environments developed during the later part of the Holocene and, especially, how and when the low-oxygen conditions evolved. To achieve these goals, sediment cores were dated (210Pb and 14C) and x-rayed, the distribution of benthic foraminifera was analysed and their content of the stable isotopes d18O and d13C was investigated. In both fjords, the climate proved to be of importance for the environment and it seems that increased freshwater runoff increased the primary production between 500 bc and ad 500. Increasing freshwater runoff not only increased primary production but, at least since ad 1880, also made the water stratification stronger and the deep water more stagnant. Stagnant conditions are the main cause of the development of periodic anoxia and formation of laminated sediments in Koljö Fjord from ad 1930 to 1980 and in Havstens Fjord from ad 1950. In Koljö Fjord, the isostatic land rise and the shallow sill are the most important reasons for a general change from an almost normal saline deep-water environment to a brackish environment in ad 500. At that time the sill depth passed the pycnocline mean depth of 15 m. For Koljö Fjord, this is a threshold value of the depth for the quality of the deep water, marine or brackish.

Periodic anoxic shelf in the Early-Middle Ordovician transition: ichnosedimentologic evidence from west-central Utah, USA

Ichnosedimentologic evidence of periodic anoxic shelf in the Early-Middle Ordovician transition includes lower ichnodiversity, shallower bioturbation and burrowing depth (＜4 cm ) , rare domichnia, tiny Chondrites occupying shallower or shallowest tiering, widely distributed nodules of limonite pseudomorphs after pyrite, occurrence of trace fossils being closely associated with the storm event layers, and stratigraphic successions with orbital cyclostratigraphic architecture. It is suggested that lower atmospheric oxygen level during the Early Paleozoic, the Ordovician radiation, dramatic transgression and warmer temperatures would result in the periodic anoxia in the Early-Middle Ordovician transition. This episode began at the later Early Ordovician and lasted about 3.4 Ma on the basis of orbital cyclostratigraphy.

Comparative Sublethal Actions of 3-Trifluoromethyl-4-nitrophenol in Marine Molluscs as Measured by in Vivo31P NMR

Abstract The sublethal actions of the piscicide, 3-trifluoromethyl-4-nitrophenol, were characterized in two marine molluscs by in vivo31P nuclear magnetic resonance spectroscopy. This noninvasive technique is ideal for examination of cellular respiratory toxicity since inorganic phosphate, phosphoarginine, and adenosine 5′-triphosphate levels, and the arginine kinase rate constant, can be measured. These parameters were characterized in both red abalone (Haliotis rufescens) and owl limpets (Lottia gigantea) during 5-h exposures to 3 mg L−1 (14.5 μM) 3-trifluoromethyl-4-nitrophenol, followed by 5-h recovery periods. In exposed abalone, phosphoarginine decreased by 50%, inorganic phosphate increased by 900%, and the arginine kinase rate constant quadrupled—all compared to controls and consistent with an uncoupling of oxidative phosphorylation. Limpets were less severely impacted, showing no change in phosphoarginine, an increase of only 200% in inorganic phosphate, and an approximate doubling of the arginine kinase rate constant. Adenosine 5′-triphosphate levels remained constant in all control and exposed molluscs. In addition, at the cessation of dosing, all biochemical parameters in abalone returned to preexposure levels within 3–4 h, consistent with rapid depuration, whereas the limpet recovery data were inconclusive. We suggest that the owl limpet's greater tolerance toward the pesticide could result from an increased capability for anaerobic glycolysis, which is required to survive the periodic anoxia associated with residing higher in the intertidal zone. Thus, the natural histories of nontarget organisms should be considered when the effects of pesticide exposure are determined.

Recent Eutrophication Histories in Lac Ste. Anne and Lake Isle, Alberta, Canada, Inferred Using Paleolimnological Methods

ABSTRACT A paleolimnological analysis of Lac Ste. Anne and Lake Isle, two eutrophic lakes in central Alberta, Canada, confirmed that they were naturally eutrophic, but have become increasingly eutrophic after the 1960s as 27% and 24% of their watersheds, respectively, were developed for urban and agricultural purposes. This was shown by a predominance of Chironomidae head capsules indicating periodic anoxia in bottom waters, high abundances of eutrophic diatoms (e.g., Aulacoseira granulata, Fragilaria crotonensis, Stephanodiscus niagarae and S. parvus), and, in Lac Ste. Anne, an enrichment of pigments from filamentous cyanobacteria (e.g., myxoxanthophyll) in post-1960 sediments. Evidence of periodic anoxia in the profundal zone was also provided by a scarcity of reducible phosphorus in the sediments. Lac Ste. Anne showed the largest changes during the 1960s and 1970s, when increases were observed in the abundance of hyper-eutrophic diatoms (e.g., S. parvus) as well as sedimentary phosphorus fractions and algal pigments. The changes in Lake Isle were similar, but more subtle. Results indicate that reduced anthropogenic activities would still result in eutrophic systems, but some improvements in water quality would be achieved, particularly in Lac Ste. Anne.