Coastal ecosystems can efficiently remove carbon dioxide (CO
) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH
) emissions from these ...ecosystems may counterbalance atmospheric CO
uptake. Still, knowledge of mechanisms sustaining such CH
emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH
in the range of 0.1 - 2.9 mg CH
m
d
to the atmosphere, revealing in situ CH
emissions from macroalgae that were sustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO
-equivalent CH
emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO
uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO
release of unvegetated sediments by 57%. Accounting for CH
alongside CO
sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.
The flux of nitrogen (N) to coastal marine ecosystems is strongly correlated with the "net anthropogenic nitrogen inputs" (NANI) to the landscape across 154 watersheds, ranging in size from 16 km
2
...to 279 000 km
2
, in the US and Europe. When NANI values are greater than 1070 kg N km
−2
yr
−1
, an average of 25%% of the NANI is exported from those watersheds in rivers. Our analysis suggests a possible threshold at lower NANI levels, with a smaller fraction exported when NANI values are below 1070 kg N km
−2
yr
−1
. Synthetic fertilizer is the largest component of NANI in many watersheds, but other inputs also contribute substantially to the N fluxes; in some regions, atmospheric deposition of N is the major component. The flux of N to coastal areas is controlled in part by climate, and a higher percentage of NANI is exported in rivers, from watersheds that have higher freshwater discharge.
Coastal waters have strong gradients in dissolved organic matter (DOM) quantity and characteristics, originating from terrestrial inputs and autochthonous production. Enclosed seas with high ...freshwater input therefore experience high DOM concentrations and gradients from freshwater sources to more saline waters. The brackish Baltic Sea experiences such salinity gradients from east to west and from river mouths to the open sea. Furthermore, the catchment areas of the Baltic Sea are very diverse and vary from sparsely populated northern areas to densely populated southern zones. Coastal systems vary from enclosed or open bays, estuaries, fjords, archipelagos and lagoons where the residence time of DOM at these sites varies and may control the extent to which organic matter is biologically, chemically or physically modified or simply diluted with transport off-shore. Data of DOM with simultaneous measurements of dissolved organic (DO) nitrogen (N), carbon (C) and phosphorus (P) across a range of contrasting coastal systems are scarce. Here we present data from the Roskilde Fjord, Vistula and Öre estuaries and Curonian Lagoon; four coastal systems with large differences in salinity, nutrient concentrations, freshwater inflow and catchment characteristics. The C:N:P ratios of DOM of our data, despite high variability, show site specific significant differences resulting largely from differences residence time. Microbial processes seemed to have minor effects, and only in spring did uptake of DON in the Vistula and Öre estuaries take place and not at the other sites or seasons. Resuspension from sediments impacts bottom waters and the entire shallow water column in the Curonian Lagoon. Finally, our data combined with published data show that land use in the catchments seems to impact the DOC:DON and DOC:DOP ratios of the tributaries most.
There is growing evidence that the release of phosphorus (P) from “legacy” stores can frustrate efforts to reduce P loading to surface water from sources such as agriculture and human sewage. Less is ...known, however, about the magnitude and residence times of these legacy pools. Here we constructed a budget of net anthropogenic P inputs to the Baltic Sea drainage basin and developed a three‐parameter, two‐box model to describe the movement of anthropogenic P though temporary (mobile) and long‐term (stable) storage pools. Phosphorus entered the sea as direct coastal effluent discharge and via rapid transport and slow, legacy pathways. The model reproduced past waterborne P loads and suggested an ~30‐year residence time in the mobile pool. Between 1900 and 2013, 17 and 27 Mt P has accumulated in the mobile and stable pools, respectively. Phosphorus inputs to the sea have halved since the 1980s due to improvements in coastal sewage treatment and reductions associated with the rapid transport pathway. After decades of accumulation, the system appears to have shifted to a depletion phase; absent further reductions in net anthropogenic P input, future waterborne loads could decrease. Presently, losses from the mobile pool contribute nearly half of P loads, suggesting that it will be difficult to achieve substantial near‐term reductions. However, there is still potential to make progress toward eutrophication management goals by addressing rapid transport pathways, such as overland flow, as well as mobile stores, such as cropland with large soil‐P reserves.
Plain Language Summary
All life depends on phosphorus (P), which is why it is an important crop fertilizer. Humans generally consume more P than needed and the excess ends up in sewage systems. Past management of P in fertilizer and human sewage has led to the accumulation of P in soils and sediments of lakes and streams. This accumulation is called “legacy” P because it can leak for decades to downstream lakes and coastal areas where it contributes to environmental problems. We developed a model to understand P dynamics for the entire drainage basin of the Baltic Sea since 1900. This model included a rapid transport pathway that represented sources such as runoff from cropland and a slow pathway that represented leakage from mobile legacy sources. The model suggests that loss from the mobile pool contributes about half of current waterborne inputs to the sea; as a result, it could be difficult to make substantial near‐term reductions. However, there are opportunities to meet environmental goals by slowing the accumulation of P in the landscape and by implementing measures that address the rapid transport pathway, such runoff from cropland, and the mobile stores, such as cropland with large soil‐P reserves.
Key Points
A three‐parameter, two‐box model describes phosphorus dynamics in the Baltic Sea drainage basin since 1900
Transfers from “legacy” phosphorus pools that accumulated in previous years contribute about half of current waterborne loads to the sea
After decades of accumulation, the mobile legacy pool appears to be depleting, suggesting that future loads to the sea could decrease
The Laptev and East Siberian Seas have been proposed as a substantial source of methane (CH4) to the atmosphere. During summer 2014, we made unique high‐resolution simultaneous measurements of CH4 in ...the atmosphere above, and surface waters of, the Laptev and East Siberian Seas. Turbulence‐driven sea‐air fluxes along the ship's track were derived from these observations; an average diffusive flux of 2.99 mg m−2 d−1 was calculated for the Laptev Sea and for the ice‐free portions of the western East Siberian Sea, 3.80 mg m−2 d−1. Although seafloor bubble plumes were observed at two locations in the study area, our calculations suggest that regionally, turbulence‐driven diffusive flux alone accounts for the observed atmospheric CH4 enhancements, with only a local, limited role for bubble fluxes, in contrast to earlier reports. CH4 in subice seawater in certain areas suggests that a short‐lived flux also occurs annually at ice‐out.
Key Points
Methane sea‐air flux in the East Siberian Arctic shelf region appears larger than other shelf seas
An under‐ice accumulation of methane during ice‐covered seasons is rapidly released at ice melt
Sea‐air methane flux is regionally dominated by turbulence‐driven diffusive fluxes, not bubble fluxes
Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, ...however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955–2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.
In 2009, following approval of the European Marine Strategy Framework Directive (MSFD, 2008/56/EC), the European Commission (EC) created task groups to develop guidance for eleven quality descriptors ...that form the basis for evaluating ecosystem function. The objective was to provide European countries with practical guidelines for implementing the MSFD, and to produce a Commission Decision that encapsulated key points of the work in a legal framework. This paper presents a review of work carried out by the eutrophication task group, and reports our main findings to the scientific community. On the basis of an operational, management-oriented definition, we discuss the main methodologies that could be used for coastal and marine eutrophication assessment. Emphasis is placed on integrated approaches that account for physico–chemical and biological components, and combine both pelagic and benthic symptoms of eutrophication, in keeping with the holistic nature of the MSFD. We highlight general features that any marine eutrophication model should possess, rather than making specific recommendations. European seas range from highly eutrophic systems such as the Baltic to nutrient-poor environments such as the Aegean Sea. From a physical perspective, marine waters range from high energy environments of the north east Atlantic to the permanent vertical stratification of the Black Sea. This review aimed to encapsulate that variability, recognizing that meaningful guidance should be flexible enough to accommodate the widely differing characteristics of European seas, and that this information is potentially relevant in marine ecosystems worldwide. Given the spatial extent of the MSFD, innovative approaches are required to allow meaningful monitoring and assessment. Consequently, substantial logistic and financial challenges will drive research in areas such as remote sensing of harmful algal blooms,
in situ sensor development, and mathematical models. Our review takes into account related legislation, and in particular the EU Water Framework
Directive (WFD – 2000/60/EC), which deals with river basins, including estuaries and a narrow coastal strip, in order to examine these issues within the framework of integrated coastal zone management.
► Eutrophication guidance for the EU Marine Strategy Framework Directive (MSFD). ► Operational, management-oriented definition of eutrophication. ► Integrated assessment of physico-chemical and biological components. ► Assessment models combine both pelagic and benthic symptoms of eutrophication. ► Innovative approaches required for meaningful monitoring and assessment.
The Arctic Ocean is an important sink for atmospheric CO2. The impact of decreasing sea ice extent and expanding marginal ice zones on Arctic air‐sea CO2 exchange depends on the rate of gas transfer ...in the presence of sea ice. Sea ice acts to limit air‐sea gas exchange by reducing contact between air and water but is also hypothesized to enhance gas transfer rates across surrounding open‐water surfaces through physical processes such as increased surface‐ocean turbulence from ice‐water shear and ice‐edge form drag. Here we present the first direct determination of the CO2 air‐sea gas transfer velocity in a wide range of Arctic sea ice conditions. We show that the gas transfer velocity increases near linearly with decreasing sea ice concentration. We also show that previous modeling approaches overestimate gas transfer rates in sea ice regions.
This study provides the first estimate of silicate and carbonate weathering rates and derived CO2 consumption rates for the Baltic Sea catchment using river chemistry data of 78 rivers draining into ...the Baltic Sea. The silicate weathering rates (denoted as total dissolved solids) of individual river basin range from 0.014Tg/year to 4.55Tg/year and the carbonate weathering rates range from 0.079Tg/year to 6.49Tg/year. The total CO2 consumption across the Baltic catchment is approximately 3.9TgC/year and is almost equally shared by silicates and carbonates. Uncertainty associated with the weathering estimate is around 32%, which is mainly caused by incomplete pollution correction for a few major rivers in the south. The calculations for the boreal river basins have higher certainties because of less human impacts. The CO2 consumption rate of individual river basin vary between 0.53 and 5.66gC/m2/year with an average of 2.97gC/m2/year, in which carbonates consume CO2, 1.4gC/m2/year and silicates take 1.5gC/m2/year. This is in the same range as has been reported for the Mackenzie River and Siberian river basins, but at the lower range of tropical rivers, suggesting the Baltic Sea catchment holds solid weathering signals for high-latitude systems, especially in the pristine boreal silicate-dominated areas. The amount of CO2 consumed by weathering in the Baltic Sea catchment accounts for approximately 3–30% of the net ecosystem carbon exchange (10–100gC/m2/year), implying that weathering contributes as a significant sink of atmospheric CO2. Although many studies have shown the positive relation between temperature and weathering rates in various river catchments, multiple regression analysis using the 40-year continuous records of river chemistry in the boreal area of the Baltic Sea catchment reveals a strong correlation between weathering flux and precipitation, but no statistically significant correlation between weathering and temperature. This suggests not only that temperature is not necessarily to be primary controlling factor for weathering rates, but also besides precipitation, other factors, like increased soil organic matter contents and water path changes may have high impact on weathering rates. The 40-year data analysis also shows generally increasing weathering fluxes by 10–20% in the pristine boreal area over the past decades. This indicates that increased CO2 consumption by weathering and the resulting elevated dissolved inorganic carbon delivery to the ocean act as a negative feedback for ocean acidification, such as the Arctic Ocean that has become more acidic due to high terrestrial organic carbon delivery together with increased river water input.
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.
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