Photosynthetic O2 production can be an important source of oxygen in sub-surface ocean waters especially in permanently stratified oligotrophic regions of the ocean where O2 produced in deep ...chlorophyll maxima (DCM) is not likely to be outgassed. Today, permanently stratified regions extend across approximately 40% of the global ocean and their extent is expected to increase in a warmer ocean. Thus, predicting future ocean oxygen conditions requires a better understanding of the potential response of photosynthetic oxygen production to a warmer ocean. Based on our own and published observations of water column processes in oligotrophic regions, we develop a one-dimensional water column model describing photosynthetic oxygen production in the Sargasso Sea to quantify the importance of photosynthesis for the downward flux of O2 and examine how it may be influenced in a warmer ocean. Photosynthesis is driven in the model by vertical mixing of nutrients (including eddy-induced mixing) and diazotrophy and is found to substantially increase the downward O2 flux relative to physical-chemical processes alone. Warming (2°C) surface waters does not significantly change oxygen production at the DCM. Nor does a 15% increase in re-mineralization rate (assuming Q10 = 2; 2°C warming) have significant effect on net sub-surface oxygen accumulation. However, changes in the relative production of particulate (POM) and dissolved organic material (DOM) generate relatively large changes in net sub-surface oxygen production. As POM/DOM production is a function of plankton community composition, this implies plankton biodiversity and food web structure may be important factors influencing O2 production in a warmer ocean.
This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.
This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean ...acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.
Transgression of planetary boundaries by human activities have now brought humanity well beyond a “safe operating space.”
New production of organic matter from photosynthesis based on “new” nitrate transported into the illuminated surface layer fuels temperate ecosystems during periods of stratification when surface ...waters are nutrient limited. Published observations from the northeastern North Sea show a large spatial heterogeneity in vertical nitrate fluxes and suggest shelf edge mixing may be the major source for new production here during the stratified summer season. In the current study, we further investigate these empirical findings with a numerical model, where physical transports and mixing are evaluated against observations of temperature, salinity, nutrients and dissipation of turbulent kinetic energy. The relatively shallow central North Sea is separated from the deep Norwegian trench by a strong shelf edge current. This shelf edge frontal zone is characterized by a vertical separation of the surface and benthic boundary layers by an intermediate layer exhibiting low turbulence. A new nitrate assimilation model, driven by light and nitrate availability, is developed and applied for quantifying the potential for, and distribution of, new production in the area. New production in the frontal zone above the shelf edge is located in a narrow high productive (~100 mg C m−2 day−1) band. This is in qualitative accordance with observations. The model results also suggest, however, that new production of similar magnitude occurs above the deep Norwegian trench, where a shallow nutricline in combination with mesoscale eddy activity leads to increased transport of nitrate to the surface layer. Increased new production along the shelf edge could potentially impact ecosystem structure and may explain the relatively high species richness and fishing activity recorded in this part of the North Sea.
•A new “new production” model is developed for the northeastern North Sea.•Enhanced new production is simulated in a high productive belt above the shelf edge.•Enhanced new production is also associated with mesoscale eddies off the shelf edge.•Species richness and fishing activity coincide with high production at the shelf edge.
Global ocean primary production (PP) is a function of both light and nutrient availability. The vertical distribution of nutrients in the euphotic zone differs in both time and space. As a result, ...the vertical distribution of PP varies as well. Differences in the vertical distribution of PP have not, however, been systematically studied. Here, we focus on the open ocean and use nutricline depth, DNO3 (defined as the depth where NO₃⁻ = 1 μmol kg−1), as a proxy for nutrient availability in the euphotic zone. Using our own and archived (WOD, HOT, BATS, CARIACO) data, we show universal relationships between DNO3 and (1) depth of the deep chlorophyll maximum (DCM), (2) total water column PP and (3) vertical distribution of PP. When DNO3 is located between ~20 and 90 m, the DCM and DNO3 are juxtaposed. However, the DCM is located above nutriclines found at > ~90 m. The observed relationships between DCM and DNO3 depths can be explained with a simple model including light and nutrient limitation. The global PP estimates indicate that ~25% of ocean PP occurs in the upper 10 m. Estimating total global ocean PP from surface optical characteristics and the relationship between vertical PP distribution and DNO3 indicates that oligotrophic regions of the ocean may be more productive than usually assumed. The relationship shown here between water column PP and DNO3 suggests that considering stratification characteristics in a future ocean is critical for predicting climate change effects on global PP.
Phytoplankton community composition is important in establishing ecosystem structure and function. Intuitively, we recognize that water movements must be important for modifying spatial gradients and ...plankton diversity. However, identifying boundaries and exchange between habitats in the open ocean is not straightforward. Here, we use the abundance of nine phytoplankton species closely sampled in a mesoscale frontal system in the northeastern North Sea as a proxy for community composition and explore the relationship between phytoplankton biogeography and transport patterns. Subsurface community distributions could be related to modeled patterns in water movement. A methodology for analyzing pelagic diversity that includes a representation of plankton community composition and an Eulerian connectivity tracer was developed, and the relative importance of connectivity and geographical distance for phytoplankton species composition analyzed. The connectivity tracer identifies timescales and dispersal barriers in the open ocean. Connectivity was found to be superior in explaining pelagic plankton diversity and found to be a prerequisite for understanding the pelagic phytoplankton composition. This approach is a valuable tool for establishing the link between ocean transports, ecosystem structure and biodiversity and for informing the placement of marine protected areas.
Marine eutrophication and hypoxia caused by excess nutrient availability is a growing environmental problem. In this study, we explore marine nitrogen enrichment in the context of Absolute ...Environmental Sustainability Assessment (AESA), a method combining life cycle assessment (LCA) with environmental boundaries aiming to compare environmental impacts from an activity (product or system) with the safe operating space (SOS) for the activity. Specifically, we aim to increase the spatial resolution and improve life cycle impact assessment (LCIA) models for marine eutrophication for use in AESAs. By estimating a proxy of the areal extent of eutrophication and hypoxia in coastal large marine ecosystems (LME), we increased model resolution from 66 LMEs in the original LCIA method to 289 coastal LME subsegments and updated relevant LME parameters to the new scale (residence time, bottom water volume, reference O2 concentration, primary production rates and depths). The new method was tested and validated by comparing the global and spatially differentiated occupation of SOS by global nitrogen emissions with observations and it showed an improved ability to identify critical areas where the SOS is exceeded, in accordance with observations of hypoxic events. Despite limitations such as the estimation of benthic zone volume and low spatial differentiation of environmental boundaries, the method can be used by AESA and LCA practitioners wishing to assess the impact of nitrogen release on marine eutrophication with a higher and more relevant spatial resolution.
Display omitted
•Method assessing absolute marine eutrophication impacts due to nitrogen emissions•Spatially differentiated characterization of marine eutrophication impacts•Absolute environmental sustainability assessment of global nutrient emissions•Ability to identify critical areas where safe operating space is exceeded•Method shows coherence with observations of hypoxic events.
Primary production (PP) in the sub-polar region appears to be important for ocean carbon uptake but how the different water masses contribute to the PP occurring here has not yet been described. ...Using two models based on satellite observations of surface chlorophyll, light and temperature, seasonal patterns in the distribution of PP are shown here to differ in the sub-polar gyre south of the Greenland-Scotland Ridge (GSR) and surrounding water masses. Monthly averages of PP (2003–2013) were determined. Total and seasonal PP were similar in both models. Average PP in five of the domains (0.47–0.77 g C m
–2
d
–1
) was well above the global average (0.37 g C m
–2
d
–1
). Over the East Greenland shelf, however, total annual PP was estimated to be only 0.19 g C m
–2
d
–1
. The Norwegian shelf was the most productive of the regions studied. “Spring blooms” appear sporadically as spikes in the annual distribution of PP in some regions/years, but do not emerge as a dominant feature in the average annual development of PP in any of the domains. For all regions, ∼25% of the annual PP takes place in the period January-May. PP peaked over most of the study area at or around maximum insolation or temperature. PP in the study region as a whole appears to be more related to latitude or water masses than to bathymetry. In waters over the East Greenland shelf, the Norwegian shelf, and north of the GSR up to 50% of annual PP had taken place when ∼50% of the annual flux of light has reached the surface. In contrast, only about 35% of annual PP had taken place in the sub-polar gyre and waters over the southern open shelf by this time. Light-use efficiency differences may be explained by differences in mixed layer depth (MLD). Multi-model Earth System model studies have indicated that climate change may decrease the MLD in the sub-polar gyre and suggest this may lead to a decrease in the PP occurring here. The results presented here, however, suggest that a shallower MLD could lead to an increase in PP.
The melting of tidewater outlet glaciers from the Greenland Ice Sheet contributes significantly to global sea level rise. Accelerated mass loss is related to melt processes in front of calving ...glaciers, yet the role of ocean heat transports is poorly understood. Here we present the first direct measurements from a subglacial plume in front of a calving tidewater outlet glacier. Surface salinity in the plume corresponded to a meltwater content of 7%, which is indicative of significant entrainment of warm bottom water and, according to plume model calculations, significant ice melt. Energy balance of the area near the glacier showed that ice melt was mainly due to ocean heat transport and that direct plume‐associated melt was only important in periods with high meltwater discharge rates of ~100 m3 s−1. Ocean mixing outside of the plume area was thus the primary heat source for melting glacier ice.
Key Points
XCTD measurements in a subglacial plume in front of a calving glacier
Plume entrainment amplifies subglacial discharge by a mixing ratio of 1:13
Energy balance in front of the tidewater outlet glacier implies large ocean mixing
New production, i.e. that driven by allochthonous nutrient inputs, is the
only form of primary production that can lead to net increases in organic
material and is, therefore, important for ...understanding energy flow in marine
ecosystems. The spatial distribution of new production is generally, however,
not well known. Using data collected in July 2016, we analyse the potential
for vertical mixing to support new production in the upper layers of the
northeastern portion of the North Sea. Relatively large (up to >0.5 mmol N m−2 d−1) nitrate fluxes due to turbulent vertical
mixing into the euphotic zone were found at some stations over the shelf
edge, while low values (< 0.1 mmol N m−2 d−1) were found in
the deeper open area north of the shelf edge. The low vertical mixing rates
(dissipation rates of turbulent kinetic energy below 10−8 W kg−1,
corresponding to vertical turbulent diffusion coefficients of
10−6–10−5 m2 s−1) implied f
ratios of <0.02 in the open waters
north of the shelf edge. In the shallow (<50 m) southern and central part
of the study area, inorganic nutrients were low and nitrate undetectable,
suggesting negligible new production here, despite relatively high
concentrations of chlorophyll a being found in the bottom layer. Thus, high
rates of new production seem to be concentrated around the shelf-edge zone
and in association with localized features exhibiting enhanced vertical
mixing. We find that the nutricline depth is significantly deeper at the
shelf edge and interference with increased mixing in this deeper depth range
can explain the increased diapycnal nitrate fluxes. Overall, this suggests
that the shelf-edge zone may be the major nutrient supplier to the euphotic
zone in this area during the period of summer stratification.
1. Phytoplankton assemblages in the open ocean are usually assumed to be mixed on local scales unless large semi-permanent density discontinuities separating water masses are present. Recent ...modelling studies have, however, suggested that ephemeral submesoscale oceanographic features leading to only subtle density discontinuities may be important for controlling phytoplankton alpha- and betadiversity patterns. Until now, no empirical evidence has been presented to support this hypothesis. 2. Using hydrographic and taxonomic composition data collected near Iceland during the period of the 2008 spring bloom, we show that the distribution of phytoplankton alpha- and beta-diversity was related to submesoscale heterogeneity in oceanographic conditions. Distinct phytoplankton communities as well as differences in richness were identified on either side of a front delimiting surface waters of slightly different (∼0.03) salinities. 3. Alpha-diversity was significantly higher on the high salinity side of the front compared to the low salinity side. The difference was primarily driven by the presence of several large diatom species in the high salinity region, especially of the genus Chaetoceros which dominated the biomass here. By investigating beta-diversity in relation to environmental and spatiotemporal variables, we show that the regional distribution of phytoplankton taxa was influenced by both different environmental conditions on either side of the front and dispersal limitation across the front. Changes in beta-diversity were primarily driven by turnover rather than nestedness and were apparently controlled by different processes in each region. 4. Synthesis. This study shows that small-scale and ephemeral density discontinuities created by submesoscale frontal dynamics can play a major role in structuring patterns of phytoplankton diversity. Evidence is presented that they can generate changes in environmental conditions (leading to environmental filtering) and act as physical (dispersal) barriers for phytoplankton transport. The study suggests that dispersal barriers are potentially of much greater importance for phytoplankton diversity at local scales than currently recognized and indicates that drivers of marine phytoplankton diversity are similar to those structuring diversity of land plants.
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