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Contrasting N2O fluxes of source vs. sink in western Arctic Ocean during summer 2017

Posted by mmaheigan

· Wednesday, October 20th, 2021

During the western Arctic summer season both physical and biogeochemical features differ with latitude between the Bering Strait and Chukchi Borderland. The southern region (Bering Strait to the Chukchi Shelf) is relatively warm, saline, and eutrophic, due to the intrusion of Pacific waters that bring heat and nutrients in to the western Arctic Ocean (WAO). Because of the Pacific influence, the WAO is one of the most productive stretches of ocean in the world. In contrast, the northern region (Chukchi Borderland to the Canada Basin) is primarily influenced by freshwater originating from sea ice melt and rivers, and is relatively cold, fresh, and oligotrophic. A frontal zone exists between the southern region and northern region (~73°N) due to the distinct physicochemical contrast between mixing Pacific waters and freshwater. These regions support distinct bacterial communities also, making the environmental variations drivers extremely relevant to nitrous oxide (N₂O) dynamics.

A recent study published in Scientific Reports examined the role of the WAO as a source and a sink of atmospheric N₂O. There are obvious differences in N₂O fluxes between southern Chukchi Sea (SC) and northern Chukchi Sea (NC). In the SC (Pacific water characteristics dominate) N₂O emissions act as a net source to the atmosphere (Figure 1a). In the NC (freshwater dominant) absorption of atmospheric N₂O into the water column suggests that this region acts as a net sink (Figure 1a). The positive fluxes of SC occurred with relatively high sea surface temperature (SST), sea surface salinity (SSS), and biogeochemically-derived N₂O production, whereas the negative fluxes of NC were associated with relatively low SST, SSS, and little N₂O production. These linear relationships between N₂O fluxes and environmental variables suggest that summer WAO N₂O fluxes are remarkably sensitive to environmental changes.

Figure 1. (a) Map of the sampling stations using the Ice Breaking R/V Araon during August 2017. The sampling locations were coloured with N2O fluxes (blue to red gradient, see color bar; sink, air → sea (−), and source, sea → air (+). The southern Chukchi Sea (SC) extends from Bering Strait to Chukchi Shelf and the northern Chukchi Sea (NC) extends from Chukchi Borderland and Canada Basin. The frontal zone arises between SC and NC (black dotted line). (b) Illustration showing future changes in the distribution of the WAO N2O flux constrained by the positive feedback scenario of increasing inflow of Pacific waters and rapidly declining sea-ice extent under accelerating Arctic warming.

This study suggests a potential scenario for future WAO changes in terms of accelerating Arctic change. Increasing inflow of the Pacific waters and rapidly declining sea-ice extent are critical. The increasing inflow of warm nutrient-enriched Pacific waters will likely extend the SC N₂O source region northward, increasing productivity, and thereby intensifying nitrification. All of which would lead to a strengthening of the WAO’s role as an N₂O source. A rapid loss of the sea ice extent could ultimately lead to a sea-ice-free NC, and again, a northward shift, which would result in a diminished role of the NC as an N₂O sink (Figure 1b). While improving our understanding of WAO N₂O dynamics, this study suggests both a direction for future work and a clear need for a longer-term study to answer questions about both seasonal variations in these dynamics and possible interannual to climatological trends.

Authors:
Jang-Mu Heo (Department of Marine Science, Incheon National University)
Sang-Min Eom (Department of Marine Science, Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)
Hyo-Ryeon Kim (Department of Marine Science, Incheon National University)
Joo-Eun Yoon (Department of Marine Science, Incheon National University)
Il-Nam Kim (Department of Marine Science, Incheon National University)

Species loss alters ecosystem function in plankton communities

Posted by mmaheigan

· Monday, February 8th, 2021

Climate change impacts on the ocean such as warming, altered nutrient supply, and acidification will lead to significant rearrangement of phytoplankton communities, with the potential for some phytoplankton species to become extinct, especially at the regional level. This leads to the question: What are phytoplankton species’ redundancy levels from ecological and biogeochemical standpoints—i.e. will other species be able to fill the functional ecological and/or biogeochemical roles of the extinct species? Authors of a paper published recently in Global Change Biology explored these ideas using a global three-dimensional computer model with diverse planktonic communities, in which single phytoplankton types were partially or fully eliminated. Complex trophic interactions such as decreased abundance of a predator’s predator led to unexpected “ripples” through the community structure and in particular, reductions in carbon transfer to higher trophic levels. The impacts of changes in resource utilization extended to regions beyond where the phytoplankton type went extinct. Redundancy appeared lowest for types on the edges of trait space (e.g., smallest) or those with unique competitive strategies. These are responses that laboratory or field studies may not adequately capture. These results suggest that species losses could compound many of the already anticipated outcomes of changing climate in terms of productivity, trophic transfer, and restructuring of planktonic communities. The authors also suggest that a combination of modeling, field, and laboratory studies will be the best path forward for studying functional redundancy in phytoplankton.

Figure caption: Examples of the modelled ecological and biogeochemical responses to the extinction of different phytoplankton species.Figure caption: Examples of the modelled ecological and biogeochemical responses to the extinction of different phytoplankton species.

Authors:
Stephanie Dutkiewicz (Massachusetts Institute of Technology)
Philip W. Boyd (Institute for Marine and Antarctic Studies, University of Tasmania)
Ulf Riebesell (GEOMAR Helmholtz Centre for Ocean Research Kiel)

How environmental drivers regulated the long-term evolution of the biological pump

Posted by mmaheigan

· Friday, January 22nd, 2021

The marine biological pump (BP) plays a crucial role in regulating earth’s atmospheric oxygen and carbon dioxide levels by transferring carbon fixed by primary producers into the ocean interior and marine sediments, thereby controlling the habitability of our planet. The rise of multicellular life and eukaryotic algae in the ocean about 700 million years ago would likely have influenced the physical characteristics of oceanic aggregates (e.g., sinking rate), yet the magnitude of the impact this biological innovation had on the efficiency of BP is unknown.

Figure. 1. The impact of biological innovations (left) and environmental factors (atmospheric oxygen level and seawater temperature; right) on the efficiency of marine biological pump (BP). Temperatures are ocean surface temperatures (SST), and atmospheric pO2 is shown relative to the present atmospheric level (PAL). The BP efficiency is calculated as the fraction of carbon exported from the surface ocean that is delivered to the sediment-water interface. The results indicate that evolution of larger sized algae and zooplanktons has little influence on the long-term evolution of biological pump (left panel). The change in the atmospheric oxygen level and seawater surface temperature as environmental factors, on the other hand, have a stronger leverage on the efficiency of biological pump (right panel).

The authors of a recent paper in Nature Geoscience constructed a particle-based stochastic model to explore the change in the efficiency of the BP in response to biological and physical changes in the ocean over geologic time. The model calculates the age of organic particles in each aggregate based on their sinking rates, and considers the impact of primary producer cell size, aggregation, temperature, dust flux, biomineralization, ballasting by mineral phases, oxygen, and the fractal geometry (porosity) of aggregates. The model results demonstrate that while the rise of larger-sized eukaryotes led to an increase in the average sinking rate of oceanic aggregates, its impact on BP efficiency was minor. The evolution of zooplankton (with daily vertical migration in the water column) had a larger impact on the carbon transfer into the ocean interior. But results suggest that environmental factors most strongly affected the marine carbon pump efficiency. Specifically, increased ocean temperatures and greater atmospheric oxygen abundance led to a significant decrease in the efficiency of the BP. Cumulatively, these results suggest that while major biological innovations influenced the efficiency of BP, the long-term evolution of the marine carbon pump was primarily controlled by environmental drivers such as climate cooling and warming. By enhancing the rate of heterotrophic microbial degradation, our results suggest that the anthropogenically-driven global warming can result in a less efficient BP with reduced power of marine ecosystem in sequestering carbon from the atmosphere.

Authors:
Mojtaba Fakhraee (Yale University, Georgia Tech, and NASA Astrobiology Institute)
Noah J. Planavsky (Yale University, and NASA Astrobiology Institute)
Christopher T. Reinhard (Georgia Tech, and NASA Astrobiology Institute)

Climate-driven pelagification of marine food webs: Implications for marine fish populations

Posted by mmaheigan

· Friday, January 22nd, 2021

Global warming changes the conditions for all ocean life, with wide-ranging consequences. It is particularly difficult to predict the impact of climate change on fish because fish production is conditioned on both temperature and food resource (zooplankton and benthic organisms) changes. Climate change projections from Earth system models show a negative amplification of changes in global ocean net primary production (NPP), with an approximate doubling of production decreases from net primary producers to mesozooplankton. This “trophic amplification” continues up the marine food web to fishes. A new study published in Frontiers in Marine Science illustrates this amplification clearly when fishes are defined by their maximum body size, which describes their position in the food web (Figure 1a). However, decreases in globally integrated biomass and production were not limited to differences in size alone. Importantly, reduced abundances also varied by fish functional type (Figure 1b).

Figure 1: a) Percent change in net primary production (NPP), mesozooplankton (MesoZ) production, all medium (M) fishes, and all large (L) fishes from Historic (1951-2000) to the RCP 8.5 Projection (2051-2100). b) Percent change in production of forage fish, large pelagic fish, demersal fish, and benthic invertebrates in Projection (2051-2100) from Historic (1951-2000). c) Absolute change in the ratio of zooplankton production to seafloor detrital flux as the difference of the Projection (2051-2100) from the Historic (1951-2000). d) Percent change in zooplankton production (dashed grey), percent change in seafloor detrital flux (solid grey), and absolute change in the ratio of their means during the Historic and Projection time periods relative to 1951.

Despite the “pelagification” of marine food webs caused by unequal decreases in secondary production (Figure 1d) and subsequent increases in pelagic zooplankton production relative to seafloor detritus production (Figure 1c,d), large pelagic fish (e.g., tunas and billfishes) suffered the greatest declines and the highest degree of projection uncertainty. The result was a shift from benthic-based ecosystems historically dominated by large demersal fish (e.g., cods and flounders) towards pelagic-based ones dominated by smaller forage fish (e.g., sardines and herring). Any positive impacts of the pelagification of food resources on large pelagic fish were overwhelmed by the negative impacts of the overall reduction in global productivity, compounded by warming-induced increases in metabolic demands. Both the degree of change in the productivity of large pelagic fish and the magnitude of trophic amplification were sensitive to the temperature dependence of metabolic rates. Thus, better constraints are needed on empirical estimates of the effect of temperature on physiological rates to project the impacts of climate change on fish biomass and marine ecosystem structure.

Ocean fish harvests currently supply ~15% of global protein demand. Reduced primary production will decrease the total amount of fish available to harvest for human food, while the pelagification of ecosystems could require large and expensive structural modifications to fisheries, including gear, location, regional and international management plans, consumer demands, and market values.

Authors:
Colleen M. Petrik (Texas A&M University)
Charles A. Stock (Geophysical Fluid Dynamics Laboratory)
Ken H. Andersen (Technical University of Denmark)
P. Daniël van Denderen (International Council for the Exploration of the Seas)
James R. Watson (Oregon State University)

Tiny phytoplankton seen from space

Posted by mmaheigan

· Thursday, November 19th, 2020

Picophytoplankton, the smallest phytoplankton on Earth, are dominant in over half of the global surface ocean, growing in low-nutrient “ocean deserts” where diatoms and other large phytoplankton have difficult to thrive. Despite their small size, picophytoplankton collectively account for well over 50% of primary production in oligotrophic waters, thus playing a major role in sustaining marine food webs.

In a recent paper published in Optics Express, the authors use satellite-detected ocean color (namely remote-sensing reflectance, R_rs(λ)) and sea surface temperature to estimate the abundance of the three picophytoplankton groups—the cyanobacteria Prochlorococcus and Synechococcus, and autotrophic picoeukaryotes. The authors analysed R_rs(λ) spectra using principal component analysis, and principal component scores and SST were used in the predictive models. Then, they trained and independently evaluated the models with in-situ data from the Atlantic Ocean (Atlantic Meridional Transect cruises). This approach allows for the satellite detection of the succession of species across ocean oligotrophic ecosystem boundaries, where these cells are most abundant (Figure 1).

Figure 1. Cell abundances of the three major picophytoplankton groups (the cyanobacteria Prochlorococcus and Synechococcus, and a collective group of autotrophic picoeukaryotes) in surface waters of the Atlantic Ocean. Abundances are shown for the dominant group in terms of total biovolume (converted from cell abundance).

Since these organisms can be used as proxies for marine ecosystem boundaries, this method can be used in studies of climate and ecosystem change, as it allows a synoptic observation of changes in picophytoplankton distributions over time and space. For exploring spectral features in hyperspectral R_rs(λ) data, the implementation of this model using data from future hyperspectral satellite instruments such as NASA PACE’s Ocean Color Instrument (OCI) will extend our knowledge about the distribution of these ecologically relevant phytoplankton taxa. These observations are crucial for broad comprehension of the effects of climate change in the expansion or shifts in ocean ecosystems.

Authors:
Priscila K. Lange (NASA Goddard Space Flight Center / Universities Space Research Association / Blue Marble Space Institute of Science)
Jeremy Werdell (NASA Goddard Space Flight Center)
Zachary K. Erickson (NASA Goddard Space Flight Center)
Giorgio Dall’Olmo (Plymouth Marine Laboratory)
Robert J. W. Brewin (University of Exeter)
Mikhail V. Zubkov (Scottish Association for Marine Science)
Glen A. Tarran (Plymouth Marine Laboratory)
Heather A. Bouman (University of Oxford)
Wayne H. Slade (Sequoia Scientific, Inc)
Susanne E. Craig (NASA Goddard Space Flight Center / Universities Space Research Association)
Nicole J. Poulton (Bigelow Laboratory for Ocean Sciences)
Astrid Bracher (Alfred-Wegener-Institute Helmholtz Center for Polar and Marine Research / University of Bremen)
Michael W. Lomas (Bigelow Laboratory for Ocean Sciences)
Ivona Cetinić (NASA Goddard Space Flight Center / Universities Space Research Association)

Warming counteracts acidification in temperate crustose coralline algae communities

Posted by mmaheigan

· Friday, November 6th, 2020

Seawater carbonate chemistry has been altered by dramatic increases in anthropogenic CO₂ release and global temperatures, leading to significant changes in rocky shore habitats and the metabolism of most marine organisms. There has been recent interest in how these anthropogenic stresses affect crustose coralline algae (CCA) communities because CCA photosynthesis and calcification are directly influenced by seawater carbonate chemistry. CCA is a foundation species in temperate macroalgal communities, where species succession and rocky shore community structure are particularly susceptible to anthropogenic disturbance. In particular, the disappearance of turf and foliose macroalgae caused by climate change and herbivore pressure results in the dominance of CCA (Figure 1a).

Figure 1: (a) Examples of crustose coralline algae (CCA)-dominated seaweed bed in the East Sea of Korea showing barren ground dominated by CCA (bright white and pink color on the rock; see arrows) on a rocky subtidal zone grazed by sea urchins. (b) Specific growth rate of marginal encrusting area under future climate conditions.

In a recent study published in Marine Pollution Bulletin, the authors conducted a mesocosm experiment to investigate the sensitivity of temperate CCA Chamberlainium sp. to future climate stressors, as simulated by three experimental treatments: 1) Acidification: doubled CO₂; 2) Warming: +5ºC; and 3) Greenhouse: doubled CO₂ and +5ºC. After a 47-day acclimation period, when compared with present-day (control: 490 μatm and 20ºC) conditions, the Acidification treatment showed decreased photosynthesis rates of Chamberlainium sp, whereas the Warming treatment showed increased photosynthesis. The Acidification treatment also showed reduced encrusting growth rates relative to the Control, but when acidification was combined with warming in the Greenhouse treatment, encrusting growth rates increased substantially (Figure 1b). Taken together, these results suggest that the negative ecophysiological responses of Chamberlainium sp to acidification are ameliorated by elevated temperatures in a greenhouse world. In other words, if the foliose macroalgal community responses negatively in the greenhouse environment, the dominance of CCA will increase further, and the biodiversity of the algae community will be reduced.

Authors:
Ju-Hyoung Kim (Faculty of Marine Applied Biosciences, Kunsan National University)
Il-Nam Kim (Department of Marine Science, Incheon National University)

Austral summer vertical migration patterns in Antarctic zooplankton

Posted by mmaheigan

· Thursday, October 15th, 2020

Sunrise and sunset are the main cues driving zooplankton diel vertical migration (DVM) throughout the world’s oceans. These marine animals balance the trade-off between feeding in surface waters at night and avoiding predation during the day at depth. Near-constant daylight during polar summer was assumed to dampen these daily migrations. In a recent paper published in Deep-Sea Research I, authors assessed austral summer DVM patterns for 15 taxa over a 9-year period. Despite up to 22 hours of sunlight, a diverse array of zooplankton – including copepods, krill, pteropods, and salps – continued DVM.

Figure caption: Mean day (orange) and night (blue) abundance of (A) the salp Salpa thompsoni, (B) the krill species Thysanoessa macrura, (C) the pteropod Limacina helicina, and (D) chaetognaths sampled at discrete depth intervals from 0-500m. Horizontal dashed lines indicate weighted mean depth (WMD). N:D is the night to day abundance ratio for 0-150 m. Error bars indicate one standard error. Sample size n = 12 to 22. Photos by Larry Madin, Miram Gleiber, and Kharis Schrage.

The Palmer Antarctica Long-Term Ecological Research (LTER) Program conducted this study using a MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) to collect depth-stratified samples west of the Antarctic Peninsula. The depth range of migrations during austral summer varied across taxa and with daylength and phytoplankton biomass and distribution. While most taxa continued some form of DVM, others (e.g., carnivores and detritivores) remained most abundant in the mesopelagic zone, regardless of photoperiod, which likely impacted the attenuation of vertical carbon flux. Given the observed differences in vertical distribution and migration behavior across taxa, ongoing changes in Antarctic zooplankton assemblages will likely impact carbon export pathways. More regional, taxon-specific studies such as this are needed to inform efforts to model zooplankton contributions to the biological carbon pump.

Authors:
John Conroy (VIMS, William & Mary)
Deborah Steinberg (VIMS, William & Mary)
Patricia Thibodeau (VIMS, William & Mary; currently University of Rhode Island)
Oscar Schofield (Rutgers University)

Marine heatwave implications for future phytoplankton blooms

Posted by mmaheigan

· Thursday, October 15th, 2020

Ocean temperature extreme events such as marine heatwaves are expected to intensify in coming decades due to anthropogenic warming. Although the effects of marine heatwaves on large plants and animals are becoming well documented, little is known about how these warming events will impact microbes that regulate key biogeochemical processes such as ocean carbon uptake and export, which represent important feedbacks on the global carbon cycle and climate.

Figure caption: Relationship between phytoplankton bloom response to marine heatwaves and background nitrate concentration in the 23 study regions. X-axis denotes the annual-mean sea-surface nitrate concentration based on the model simulation (1992-2014; OFAM3, blue) and the in situ climatology (WOA13, orange). Y-axis denotes the mean standardised anomalies (see Equation 1 of the paper) of simulated sea-surface phytoplankton nitrogen biomass (1992-2014; OFAM3, blue) and observed sea-surface chlorophyll a concentration (2002-2018; MODIS, orange) during the co-occurrence of phytoplankton blooms and marine heatwaves.

In a recent study published in Global Change Biology, authors combined model simulations and satellite observations in tropical and temperate oceanographic regions over recent decades to characterize marine heatwave impacts on phytoplankton blooms. The results reveal regionally‐coherent anomalies depicted by shallower surface mixed layers and lower surface nitrate concentrations during marine heatwaves, which counteract known light and nutrient limitation effects on phytoplankton growth, respectively (Figure 1). Consequently, phytoplankton bloom responses are mixed, but derive from the background nutrient conditions of a study region such that blooms are weaker (stronger) during marine heatwaves in nutrient-poor (nutrient-rich) waters.

Given the projected expansion of nutrient-poor waters in the 21st century ocean, the coming decades are likely to see an increased occurrence of weaker blooms during marine heatwaves, with implications for higher trophic levels and biogeochemical cycling of key elements.

Authors:
Hakase Hayashida (University of Tasmania)
Richard Matear (CSIRO)
Pete Strutton (University of Tasmania)

Will global change “stress out” ocean DOC cycling?

Posted by mmaheigan

· Tuesday, September 29th, 2020

The dissolved organic carbon (DOC) pool is vital for the functioning of marine ecosystems. DOC fuels marine food webs and is a cornerstone of the earth’s carbon cycle. As one of the largest pools of organic matter on the planet, disruptions to marine DOC cycling driven by climate and environmental global changes can impact air-sea CO2 exchange, with the added potential for feedbacks on Earth’s climate system.

Figure 1. Simplified view of major dissolved organic carbon (DOC) sources (black text) and sinks (yellow text) in the ocean.

Since DOC cycling involves multiple processes acting concurrently over a range of time and space scales, it is especially challenging to characterize and quantify the influence of global change. In a recent review paper published in Frontiers in Marine Science, the authors synthesize impacts of global change-related stressors on DOC cycling such as ocean warming, stratification, acidification, deoxygenation, glacial and sea ice melting, inflow from rivers, ocean circulation and upwelling, and atmospheric deposition. While ocean warming and acidification are projected to stimulate DOC production and degradation, in most regions, the outcomes for other key climate stressors are less clear, with much more regional variation. This synthesis helps advance our understanding of how global change will affect the DOC pool in the future ocean, but also highlights important research gaps that need to be explored. These gaps include for example a need for studies that allow to understand the adaptation of degradation/production pathways to global change stressors, and their cumulative impacts (e.g. temperature with acidification).

Authors:
C. Lønborg (Aarhus University)
C. Carreira (CESAM, Universidade de Aveiro)
Tim Jickells (University of East Anglia)
X.A. Álvarez-Salgado (CSIC, Instituto de Investigacións Mariñas)

Little big exporters

Posted by mmaheigan

· Wednesday, April 8th, 2020

In the Southern Ocean, coccolithophores are thought to account for a major fraction of marine carbonate production and export to the deep sea. Despite their importance in the ocean carbon cycle, we lack fundamental information about Southern Ocean coccolithophore abundance, species composition, and contribution to carbonate export.

Figure caption: Heliscosphaera carteri (left), Coccolithus pelagicus (right) and Emiliania huxleyi (bottom right, partially behind C. pelagicus) coccospheres retrieved from the subantarctic waters south of Tasmania. Image Ruth Eriksen, courtesy AAD EMU.

A recent study in Biogeosciences has generated annual observations of coccolithophore species composition and contribution to calcium carbonate fluxes at two sites that are representative of a large portion of the Subantarctic zone. Coccolithophores account for roughly half of the annual calcium carbonate exported to the deep sea. Notably, it is not the most abundant species (Emiliania huxleyi), but rather the less abundant and larger species (e.g. Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus) that make the greatest contribution to carbonate export to the deep sea. Since these larger species exhibit substantially different ecological traits from the opportunistic E. huxleyi, predictions of future response of Southern Ocean coccolithophore communities should not be based on the physiological results from experiments with E. huxleyi. Rather, new physiological response experiments of those less abundant, larger coccolithophore species are urgently needed to constrain responses of these important carbonate exporters to environmental change in the Southern Ocean. This study underscores the importance of phytoplankton ecological traits on the regulation of the marine carbon cycle and emphasizes the need for more species-specific studies to improve predictions of marine ecosystem response to ongoing climate change.

Authors
Andrés S. Rigual Hernández (Universidad de Salamanca)
Thomas W. Trull (CSIRO and ACE CRC)
Scott D. Nodder (NIWA)
José A. Flores (Universidad de Salamanca)
Helen Bostock (University of Queensland,)
Fátima Abrantes (Portuguese Institute for Sea and Atmosphere and CCMAR)
Ruth S. Eriksen (CSIRO and IMAS)
Francisco J. Sierro (Universidad de Salamanca)
Diana M. Davies (CSIRO and ACE CRC)
Anne-Marie Ballegeer (Universidad de Salamanca)
Miguel A. Fuertes (Universidad de Salamanca)
Lisa C. Northcote (NIWA)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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