Coastal systems can act as filters for anthropogenic
nutrient input into marine environments. Here, we assess the processes
controlling the removal of phosphorus (P) and nitrogen (N) for four sites ...in
the eutrophic Stockholm archipelago. Bottom water concentrations of oxygen
(O2) and P are inversely correlated. This is attributed to the seasonal
release of P from iron-oxide-bound (Fe-oxide-bound) P in surface sediments and from
degrading organic matter. The abundant presence of sulfide in the pore water
and its high upward flux towards the sediment surface (∼4 to
8 mmol m−2 d−1), linked to prior deposition of organic-rich
sediments in a low-O2 setting (“legacy of hypoxia”), hinder the
formation of a larger Fe-oxide-bound P pool in winter. This is most
pronounced at sites where water column mixing is naturally relatively low
and where low bottom water O2 concentrations prevail in summer. Burial rates
of P are high at all sites (0.03–0.3 mol m−2 yr−1), a combined
result of high sedimentation rates (0.5 to 3.5 cm yr−1) and high
sedimentary P at depth (∼30 to 50 µmol g−1).
Sedimentary P is dominated by Fe-bound P and organic P at the sediment
surface and by organic P, authigenic Ca-P and detrital P at depth. Apart
from one site in the inner archipelago, where a vivianite-type Fe(II)-P
mineral is likely present at depth, there is little evidence for
sink switching of organic or Fe-oxide-bound P to authigenic P minerals.
Denitrification is the major benthic nitrate-reducing process at all sites
(0.09 to 1.7 mmol m−2 d−1) with rates decreasing seaward from the
inner to outer archipelago. Our results explain how sediments in this
eutrophic coastal system can remove P through burial at a relatively high
rate, regardless of whether the bottom waters are oxic or (frequently)
hypoxic. Our results suggest that benthic N processes undergo annual cycles
of removal and recycling in response to hypoxic conditions. Further nutrient
load reductions are expected to contribute to the recovery of the eutrophic
Stockholm archipelago from hypoxia. Based on the dominant pathways of P and
N removal identified in this study, it is expected that the sediments will
continue to remove part of the P and N loads.
Estuarine sediments are key sites for removal of phosphorus
(P) from rivers and the open sea. Vivianite, an Fe(II)-P mineral, can act as
a major sink for P in Fe-rich coastal sediments. In this ...study, we
investigate the burial of P in the Öre Estuary in the northern Baltic
Sea. We find much higher rates of P burial at our five study sites (up to
∼0.145 molm-2yr-1) when compared to more southern
coastal areas in the Baltic Sea with similar rates of sedimentation. Detailed
study of the sediment P forms at our site with the highest rate of
sedimentation reveals a major role for P associated with Fe and the presence
of vivianite crystals below the sulfate methane transition zone. By applying
a reactive transport model to sediment and porewater profiles for this site,
we show that vivianite may account for up to ∼40 % of total P
burial. With the model, we demonstrate that vivianite formation is promoted
in sediments with a low bottom water salinity and high rates of sedimentation
and Fe oxide input. While high rates of organic matter input are also
required, there is an optimum rate above which vivianite formation declines.
Distinct enrichments in sediment Fe and sulfur at depth in the sediment are
attributed to short periods of enhanced input of riverine Fe and organic
matter. These periods of enhanced input are linked to variations in rainfall
on land and follow dry periods. Most of the P associated with the Fe in the
sediment is likely imported from the adjacent eutrophic Baltic Proper. Our
work demonstrates that variations in land-to-sea transfer of Fe may act as a
key control on burial of P in coastal sediments. Ongoing climate change is
expected to lead to a decrease in bottom water salinity and contribute to
continued high inputs of Fe oxides from land, further promoting P burial as
vivianite in the coastal zone of the northern Baltic Sea. This may enhance
the role of this oligotrophic area as a sink for P imported from eutrophic
parts of the Baltic Sea.
Oxygen depletion in coastal waters may lead to release of toxic sulfide from sediments. Cable bacteria can limit sulfide release by promoting iron oxide formation in sediments. Currently, it is ...unknown how widespread this phenomenon is. Here, we assess the abundance, activity, and biogeochemical impact of cable bacteria at 12 Baltic Sea sites. Cable bacteria were mostly absent in sediments overlain by anoxic and sulfidic bottom waters, emphasizing their dependence on oxygen or nitrate as electron acceptors. At sites that were temporarily reoxygenated, cable bacterial densities were low. At seasonally hypoxic sites, cable bacterial densities correlated linearly with the supply of sulfide. The highest densities were observed at Gulf of Finland sites with high rates of sulfate reduction. Microelectrode profiles of sulfide, oxygen, and pH indicated low or no in situ cable bacteria activity at all sites. Reactivation occurred within 5 days upon incubation of an intact sediment core from the Gulf of Finland with aerated overlying water. We found no relationship between cable bacterial densities and macrofaunal abundances, salinity, or sediment organic carbon. Our geochemical data suggest that cable bacteria promote conversion of iron monosulfides to iron oxides in the Gulf of Finland in spring, possibly explaining why bottom waters in this highly eutrophic region rarely contain sulfide in summer.
Abstract
In coastal waters, methane-oxidizing bacteria (MOB) can form a methane biofilter and mitigate methane emissions. The metabolism of these MOBs is versatile, and the resilience to changing ...oxygen concentrations is potentially high. It is still unclear how seasonal changes in oxygen availability and water column chemistry affect the functioning of the methane biofilter and MOB community composition. Here, we determined water column methane and oxygen depth profiles, the methanotrophic community structure, methane oxidation potential, and water–air methane fluxes of a eutrophic marine basin during summer stratification and in the mixed water in spring and autumn. In spring, the MOB diversity and relative abundance were low. Yet, MOB formed a methane biofilter with up to 9% relative abundance and vertical niche partitioning during summer stratification. The vertical distribution and potential methane oxidation of MOB did not follow the upward shift of the oxycline during summer, and water–air fluxes remained below 0.6 mmol m−2 d−1. Together, this suggests active methane removal by MOB in the anoxic water. Surprisingly, with a weaker stratification, and therefore potentially increased oxygen supply, methane oxidation rates decreased, and water–air methane fluxes increased. Thus, despite the potential resilience of the MOB community, seasonal water column dynamics significantly influence methane removal.
Methane-oxidizing bacteria in coastal waters can mitigate diffusive methane emissions. Their metabolic versatility and resilience are potentially high. Yet, methane removal is insufficient and highly variable throughout the year.
Oceanic Anoxic Event 2 (OAE2), a ∼ 600 kyr episode close to the Cenomanian–Turonian boundary (ca. 94 Ma), is characterized by relatively widespread marine anoxia and ranks amongst the warmest ...intervals of the Phanerozoic. The early stages of OAE2 are, however, marked by an episode of widespread transient cooling and bottom water oxygenation: the Plenus Cold Event. This cold spell has been linked to a decline in atmospheric pCO2, resulting from enhanced global organic carbon burial. To investigate the response of phytoplankton to this marked and rapid climate shift we examined the biogeographical response of dinoflagellates to the Plenus Cold Event. Our study is based on a newly generated geochemical and palynological data set from a high-latitude Northern Hemisphere site, Pratts Landing (western Alberta, Canada). We combine these data with a semi-quantitative global compilation of the stratigraphic distribution of dinoflagellate cyst taxa. The data show that dinoflagellate cysts grouped in the Cyclonephelium compactum–membraniphorum morphological plexus migrated from high to mid-latitudes during the Plenus Cold Event, making it the sole widely found (micro)fossil to mark this cold spell. In addition to earlier reports from regional metazoan migrations during the Plenus Cold Event, our findings illustrate the effect of rapid climate change on the global biogeographical dispersion of phytoplankton.
Anthropogenic activities are influencing aquatic environments through increased chemical pollution and thus are greatly affecting the biogeochemical cycling of elements. This has increased greenhouse ...gas emissions, particularly methane, from lakes, wetlands, and canals. Most of the methane produced in anoxic sediments is converted into carbon dioxide by methanotrophs before it reaches the atmosphere. Anaerobic oxidation of methane requires an electron acceptor such as sulphate, nitrate, or metal oxides. Here, we explore the anaerobic methanotrophy in sediments of three urban canals in Amsterdam, covering a gradient from freshwater to brackish conditions. Biogeochemical analysis showed the presence of a shallow sulphate–methane transition zone in sediments of the most brackish canal, suggesting that sulphate could be a relevant electron acceptor for anaerobic methanotrophy in this setting. However, sediment incubations amended with sulphate or iron oxides (ferrihydrite) did not lead to detectable rates of methanotrophy. Despite the presence of known nitrate‐dependent anaerobic methanotrophs (Methanoperedenaceae), no nitrate‐driven methanotrophy was observed in any of the investigated sediments either. Interestingly, graphene oxide stimulated anaerobic methanotrophy in incubations of brackish canal sediment, possibly catalysed by anaerobic methanotrophs of the ANME‐2a/b clade. We propose that natural organic matter serving as electron acceptor drives anaerobic methanotrophy in brackish sediments.
Urban canals are ubiquitous in cities yet their methane cycling microbial community is relatively unexplored. Here, we investigated the anaerobic oxidation of methane in the canals of Amsterdam and determined that the most likely electron acceptor for anaerobic methane oxidation is natural organic matter, even when sulphate is present at a concentration similar to the Baltic Sea.
The potential and drivers of microbial methane removal in the water column of seasonally stratified coastal ecosystems and the importance of the methanotrophic community composition for ecosystem ...functioning are not well explored. Here, we combined depth profiles of oxygen and methane with 16S rRNA gene amplicon sequencing, metagenomics and methane oxidation rates at discrete depths in a stratified coastal marine system (Lake Grevelingen, The Netherlands). Three amplicon sequence variants (ASVs) belonging to different genera of aerobic Methylomonadaceae and the corresponding three methanotrophic metagenome‐assembled genomes (MOB‐MAGs) were retrieved by 16S rRNA sequencing and metagenomic analysis, respectively. The abundances of the different methanotrophic ASVs and MOB‐MAGs peaked at different depths along the methane oxygen counter‐gradient and the MOB‐MAGs show a quite diverse genomic potential regarding oxygen metabolism, partial denitrification and sulphur metabolism. Moreover, potential aerobic methane oxidation rates indicated high methanotrophic activity throughout the methane oxygen counter‐gradient, even at depths with low in situ methane or oxygen concentration. This suggests that niche‐partitioning with high genomic versatility of the present Methylomonadaceae might contribute to the functional resilience of the methanotrophic community and ultimately the efficiency of methane removal in the stratified water column of a marine basin.
Hypoxia has occurred intermittently in the Baltic Sea since the establishment of brackish‐water conditions at ∼8,000 years B.P., principally as recurrent hypoxic events during the Holocene Thermal ...Maximum (HTM) and the Medieval Climate Anomaly (MCA). Sedimentary phosphorus release has been implicated as a key driver of these events, but previous paleoenvironmental reconstructions have lacked the sampling resolution to investigate feedbacks in past iron‐phosphorus cycling on short timescales. Here we employ Laser Ablation (LA)‐ICP‐MS scanning of sediment cores to generate ultra‐high resolution geochemical records of past hypoxic events. We show that in‐phase multidecadal oscillations in hypoxia intensity and iron‐phosphorus cycling occurred throughout these events. Using a box model, we demonstrate that such oscillations were likely driven by instabilities in the dynamics of iron‐phosphorus cycling under preindustrial phosphorus loads, and modulated by external climate forcing. Oscillatory behavior could complicate the recovery from hypoxia during future trajectories of external loading reductions.
Plain Language Summary
Hypoxia in the coastal ocean is expanding worldwide, and inputs of nutrients from waste water and agriculture are mainly to blame. Nutrients feed plankton blooms, which then consume oxygen as they decay. Because much of this decay takes place at the seafloor, sediments play an important role in deoxygenation, and in the recycling of nutrients in coastal regions. It is known that the amount of iron oxides in sediments has a strong effect on nutrient recycling during algal decay. Iron oxide‐rich sediments in healthy oxygen‐rich areas can trap phosphorus, a key nutrient element from decaying algae. In contrast, iron oxide‐poor sediments in deoxygenated areas release phosphorus back to the water to fuel more algal growth. Our study shows that changes in the distribution of iron oxides between deep and shallow areas of the Baltic Sea led to self‐sustaining variability (oscillations) in oxygen stress on decadal timescales during past intervals in the Sea's 8000‐year history. We use a model to demonstrate that under certain conditions of climate and nutrient pressure, such variability may occur naturally, and therefore may influence the future recovery of the Baltic Sea from its present nutrient‐rich state.
Key Points
Past hypoxic intervals in the Baltic Sea were characterized by multidecadal oscillations in oxygen stress
Regularly paced internal oscillations caused by feedbacks in coupled iron‐phosphorus dynamics
External loading of phosphorus and climate forcing dictate the amplitude of internal oscillatory behavior
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