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Widespread nutrient co-limitation discovered in the South Atlantic

Posted by mmaheigan

· Thursday, March 15th, 2018

Unicellular photosynthetic microbes—phytoplankton—are responsible for virtually all oceanic primary production, which fuels marine food webs and plays a fundamental role in the global carbon cycle. Experiments to date have suggested that the growth of phytoplankton across much of the ocean is limited by either nitrogen or iron. But simultaneously low concentrations of these and other nutrients have been measured over large areas of the open ocean, raising the question: Are phytoplankton communities only limited by a single nutrient?

Authors of a study recently published in Nature tested this by conducting nutrient addition experiments on a GEOTRACES cruise in the nutrient-deficient South Atlantic gyre. Seawater samples were amended with nitrogen, iron, and cobalt both individually and in various combinations. Concurrent nitrogen and iron addition stimulated increased phytoplankton growth, yielding a ~40-fold increase in chlorophyll a. Supplementary addition of cobalt or cobalt-containing vitamin B₁₂ further enhanced phytoplankton growth in several experiments.

Experiments conducted throughout the southeast Atlantic GEOTRACES GA08 cruise transect (left panel) demonstrated that nitrogen and iron had to be added to significantly stimulate phytoplankton growth (right panel). Supplementary addition of cobalt (or cobalt-containing vitamin B12) stimulated significant additional growth.

In addition to co-limited sites, the study identified ‘singly’ and ‘serially’ limited sites. These limitation regimes could be predicted by the measured ambient seawater nutrient concentrations, demonstrating the potential for using nutrient datasets to make confident predictions about limitation at larger spatial scales, an approach that is being more widely used in programmes like GEOTRACES,.

Finally, a complex, state-of-the-art biogeochemical ocean model suggested a much smaller extent of nutrient co-limitation than the experiments indicated. Authors attributed this to relatively restricted microbial and nutrient diversity in the model. These findings have implications for how such models are constructed if they are to represent nutrient co-limitation in the ocean and accurately project changes in ocean productivity in the future.

Authors:
Thomas J. Browning (GEOMAR)
Eric P. Achterberg (GEOMAR)
Insa Rapp (GEOMAR)
Anja Engel (GEOMAR)
Erin M. Bertrand (Dalhousie University)
Alessandro Tagliabue (University of Liverpool)
Mark Moore (University of Southampton)

Increased temperatures suggest reduced capacity for carbon

Posted by mmaheigan

· Thursday, January 18th, 2018

The ocean’s biological pump works to draw down atmospheric carbon dioxide (CO₂) by exporting carbon from the surface ocean. This process is less efficient at higher temperatures, implying a possible climate feedback. Recent work by Cael et al. provides an explanation of why this feedback occurs and an estimate of its severity.

In a highly simplified view, carbon export depends on the balance between two temperature-dependent processes: 1) The autotrophic production and 2) the heterotrophic respiration of organic carbon. Cael and Follows (Geophysical Research Letters 2016) recently developed a mechanistic model based on established temperature dependencies for photosynthesis and respiration to explore feedbacks between export efficiency and climate. Heterotrophic growth rates increase more so than phototrophic rates with increasing temperature, which suggests that at higher temperatures, community respiration will increase relative to production, thereby decreasing export efficiency. Although simplistic, the model captures the temperature dependence of export efficiency observations.

Figure: Schematic of the mechanism on which the Cael and Follows (2016) model is based. (a) Photosynthesis (dark grey) and respiration (light grey) respond to temperature differently, yielding (b) a decline in export efficiency at higher temperatures.

More recently, Cael, Bisson, and Follows (Limnology and Oceanography 2017) applied this model to sea surface temperature records and estimated a ~1.5% decline in globally-averaged export efficiency over the past three decades of increasing ocean temperatures as a result of this metabolic mechanism. This ~1.5% decline is equivalent to a reduced ocean sequestration of approximately 100 million fewer tons of carbon annually, comparable to the annual carbon emissions of the United Kingdom. The model provides a framework in which to consider the relationship between climate and ocean carbon export that might also elucidate large-scale (e.g., glacial-interglacial) atmospheric CO₂ changes of the past.

Authors:
B. B. Cael (MIT/WHOI)
Kelsey Bisson (UCSB)
Mick Follows (MIT)

WBC Series: Decadal variability of the Kuroshio Extension system and its impact on subtropical mode water formation

Posted by mmaheigan

· Friday, November 10th, 2017

Bo Qiu¹, Eitarou Oka², Stuart P. Bishop³, Shuiming Chen¹, Andrea J. Fassbender⁴

1. University of Hawaii at Manoa
2. The University of Tokyo
3. North Carolina State University
4. Monterey Bay Aquarium Research Institute

After separating from the Japanese coast at 36°N, 141°E, the Kuroshio enters the open basin of the North Pacific, where it is renamed the Kuroshio Extension (KE). Free from the constraint of coastal boundaries, the KE has been observed to be an eastward-flowing inertial jet accompanied by large-amplitude meanders and energetic pinched-off eddies (see Qiu 2002 and Kelly et al. 2010 for comprehensive reviews). Compared to its upstream counterpart south of Japan, the Kuroshio, the KE is accompanied by a stronger southern recirculation gyre that increases the KE’s eastward volume transport to more than twice the maximum Sverdrup transport (~ 60Sv) in the subtropical North Pacific Ocean (Wijffels et al. 1998). This has two important consequences. Dynamically, the increased transport enhances the nonlinearity of the KE jet, rendering the region surrounding the KE jet to have the highest mesoscale activity level in the Pacific basin. Thermodynamically, the enhanced KE jet brings a significant amount of tropical-origin warm water to the mid-latitude ocean to be in direct contact with cold, dry air blowing off the Eurasian continent. This results in significant wintertime heat loss from the ocean to atmosphere surrounding the Kuroshio/KE paths, contributing to the formation of North Pacific subtropical mode water (STMW; see Hanawa and Talley (2001) and Oka and Qiu (2012) for comprehensive reviews).

Figure 1. Yearly paths of the Kuroshio and KE plotted every 14 days using satellite SSH data (updated based on Qiu and Chen 2005). KE was in stable state in 1993–94, 2002–05, and 2010–15, and unstable state in 1995-2001, 2006–09, and 2016, respectively.

Although the ocean is known to be a turbulent medium, variations in both the level of mesoscale eddy activity and the formation rate of STMW in the KE region are by no means random on interannual and longer timescales. One important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits clearly defined decadal modulations between a stable and an unstable dynamical state (e.g., Qiu & Chen 2005, 2010; Taguchi et al. 2007; Qiu et al. 2007; Cebollas et al. 2009; Sugimoto and Hanawa 2009; Sasaki et al. 2013; Pierini 2014; Bishop et al. 2015). As shown in Figure 1, the KE paths were relatively stable in 1993–95, 2002–05, and 2010–15. In contrast, spatially convoluted paths prevailed during 1996–2001 and 2006–09. When the KE jet is in a stable dynamical state, satellite altimeter data further reveal that its eastward transport and latitudinal position tend to increase and migrate northward, its southern recirculation gyre tends to strengthen, and the regional eddy kinetic energy level tends to decrease. The reverse is true when the KE jet switches to an unstable dynamical state. In fact, the time-varying dynamical state of the KE system can be well represented by the KE index, defined by the average of the variance-normalized time series of the southern recirculation gyre intensity, the KE jet intensity, its latitudinal position, and the negative of its path length (Qiu et al. 2014). Figure 2a shows the KE index time series in the satellite altimetry period of 1993–present; here, a positive KE index indicates a stable dynamical state and a negative KE index, an unstable dynamical state. From Figure 2a, it is easy to discern the dominance of the decadal oscillations between the two dynamical states of the KE system.

Figure 2. (a) Time series of the KE index from 1993‑present; available at http://www.soest.hawaii.edu/oceanography/bo/KE_index.asc. (b) Year-mean SSH maps when the KE is in stable (2004 and 2011) versus unstable (1997 and 2008) states. (c) SSH anomalies along the zonal band of 32°-34°N from satellite altimetry measurements. (d) Time series of the PDO index from 1989-present; available at http://jisao.washington.edu/pdo/PDO.latest.

Transitions between the KE’s two dynamical states are caused by the basin-scale wind stress curl forcing in the eastern North Pacific related to the Pacific Decadal Oscillation (PDO). Specifically, when the central North Pacific wind stress curl anomalies are positive during the positive PDO phase (see Figure 2d), enhanced Ekman flux divergence generates negative local sea surface height (SSH) anomalies in 170°–150°W along the southern recirculation gyre latitude of 32°–34°N. As these wind-induced negative SSH anomalies propagate westward as baroclinic Rossby waves into the KE region after a delay of 3–4 years (Figure 2c), they weaken the zonal KE jet, leading to an unstable (i.e., negative index) state of the KE system with a reduced recirculation gyre and an active eddy kinetic energy field (Figure 2b). Negative anomalous wind stress curl forcing during the negative PDO phase, on the other hand, generates positive SSH anomalies through the Ekman flux convergence in the eastern North Pacific. After propagating into the KE region in the west, these anomalies stabilize the KE system by increasing the KE transport and by shifting its position northward, leading to a positive index state.

The dynamical state of the KE system exerts a tremendous influence upon the STMW that forms largely along the paths of the Kuroshio/KE jet and inside of its southern recirculation gyre (e.g., Suga et al. 2004; Qiu et al. 2006; Oka 2009). Figure 3a shows the monthly time series of temperature profile, constructed by averaging available Argo and XBT/CTD/XCTD data inside the KE southern recirculation gyre (see Qiu and Chen 2006 for details on the constructing method). The black line in the plot denotes the base of the mixed layer, defined as where the water temperature drops by 0.5°C from the sea surface temperature. Based on the temperature profiles, Figure 3b shows the monthly time series of potential vorticity. STMW in Figure 3b is characterized by water columns with potential vorticity of less than 2.0 x 10^-10 m^-1s^-1 beneath the mixed layer. From Figure 3, it is clear that both the late winter mixed layer depth and the low-potential vorticity STMW layer underwent significant decadal changes over the past 25 years. Specifically, deep mixed layer and pronounced low-potential vorticity STMW were detected in 1993–95, 2001–05, and 2010–15, and these years corresponded roughly to the periods when the KE index was in the positive phase (cf. Figure 2a).

Figure 3. Monthly time series of (a) temperature (°C) and (b) potential vorticity (10-10 m-1 s-1) averaged in the KE’s southern recirculation gyre. The thick black and white lines in (a) and (b) denote the base of the mixed layer, defined as where the temperature drops by 0.5°C from the surface value. Red pluses (at the top of each panel) indicate the individual temperature profiles used in constructing the monthly T(z, t) profiles. The potential vorticity, Q(z,t) = fα∂T(z,t)/∂z, where f is the Coriolis parameter and α the thermal expansion coefficient.

The close connection between the dynamical state of the KE system and the STMW formation has been detected by many recent studies based on different observational data sources and analysis approaches (Qiu and Chen 2006; Sugimoto and Hanawa 2010; Rainville et al. 2014; Bishop and Watts 2014; Oka et al. 2012; 2015; Cerovecki and Giglio 2016). Physically, this connection can be understood as follows. When the KE is in an unstable state (or a negative KE index phase), high-regional eddy variability infuses high-potential vorticity KE and subarctic-gyre water into the southern recirculation gyre, increasing the upper-ocean stratification and hindering the development of deep winter mixed layer and formation of STMW. A stable KE path with suppressed eddy variability (in the positive KE index phase), on the other hand, favors the maintenance of a weak stratification in the recirculation gyre, leading to the formation of a deep winter mixed layer and thick STMW.

Since the STMW is renewed each winter, due to combined net surface heat flux and wind stress forcing that modulate on interannual timescales, a question arising naturally is the timescale on which the dynamical state change of the KE system is able to alter the upper ocean stratification and potential vorticity inside the recirculation gyre. If the influence of the KE dynamical state acts on interannual timescales, one may expect a stronger control on the STMW variability by the wintertime atmospheric condition (e.g., Suga and Hanawa 1995; Davis et al. 2011). Intensive observations from the Kuroshio Extension System Study (KESS) program, spanning the period from April 2004 to July 2006, captured the 2004–05 transition of the KE system from a stable to an unstable state. The combined measurements by profiling Argo floats, moored current meter, current and pressure inverted echo sounder (CPIES), and the Kuroshio Extension Observatory (KEO) surface mooring revealed that the KE dynamical state change was able to change the STMW properties both significantly in amplitude and effectively in time (Qiu et al. 2007; Bishop 2013; Cronin et al. 2013; Bishop and Watts 2014). Relative to 2004, the low-potential vorticity signal in the core of STMW was diminished by one-half in 2005, and this weakening of STMW’s intensity occurred within a period of less than seven months. These significant and rapid responses of STMW to the KE dynamical state change suggests that the variability in STMW formation is more sensitive to the dynamical state of the KE than to interannual variations in overlying atmospheric conditions over the past 25 years.

The decadal variability of STMW in the KE’s southern recirculation gyre is able to affect the water property distributions in the entire western part of the North Pacific subtropical gyre (Oka et al. 2015). Measurements by Argo profiling floats during 2005–14 revealed that the volume and spatial extent of STMW decreased (increased) in 2006–09 (after 2010) during the unstable (stable) KE period in its formation region north of ~28°N, as well as in the southern, downstream regions with a time lag of 1-2 years. Such decadal subduction variability affects not only physical but also biogeochemical structures in the downstream, interior subtropical gyre. Shipboard observations at 25°N and along the 137°E repeat hydrographic section of the Japan Meteorological Agency exhibited that, after 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon. Changes in dissolved inorganic carbon, pH, and aragonite saturation state were opposite their long-term trends.

KE State and the Ocean Carbon Cycle

Western boundary current (WBC) regions display the largest magnitude air-to-sea carbon dioxide (CO₂) fluxes of anywhere in the global ocean. STMW formation processes are thought to account for a majority of the anthropogenic CO₂ sequestration that occurs outside of the polar, deep water formation regions (Sabine et al. 2004; Khatiwala et al. 2009). Once subducted and advected away from the formation region, mode waters often remain out of contact with the atmosphere on timescales of decades to hundreds of years, making them short-term carbon silos relative to the abyssal carbon storage reservoirs. One of the physical impacts on carbon uptake via air-sea CO₂flux is due to the temperature dependence of the solubility of pCO₂ in the surface waters. Cooler surface waters during the wintertime months reduce the oceanic pCO₂and subsequently enhance the CO₂flux into the ocean. This carbon uptake corresponds with the timing of peak STMW formation.

As mentioned above, the formation of STMW is modulated by the dynamic states of the KE, with less STMW forming during unstable states and more during stable states. To complicate matters, more enhanced levels of surface chlorophyll (Chla) have also been observed from satellite ocean color during unstable states (Lin et al. 2014), which points to the potential importance of biophysical interactions on carbon uptake. Elevated levels of Chla can further modify the pCO₂ of surface waters and enhance carbon export at depth from sinking of particulate organic matter following an individual bloom. Given that submesoscale processes result from deep wintertime mixed layers and from the presence of the larger mesoscale lateral shear and strain fields (McWilliams 2016), it is expected that submesoscale processes are also important in STMW formation during unstable states of the KE. An open question in the research community is to what extent do elevated levels of mesoscale and submesoscale eddy activity modulate STMW formation and carbon uptake during unstable states of the KE? With large variations in STMW formation occurring in concert with decadal variability in the mesoscale eddy field, it is possible that submesoscale processes may impact STMW formation through restratification of the mixed layer within density classes encompassing STMW and timing of the spring bloom. These mesoscale and submesoscale processes may then also impact the uptake of CO₂ in the North Pacific on interannual to decadal timescales.

References

Bishop, S. P., 2013: Divergent eddy heat fluxes in the Kuroshio Extension at 143°-149°E. Part II: Spatiotemporal variability. J. Phys. Oceanogr., 43, 2416-2431, doi: 10.1175/JPO-D-13-061.1.

Bishop, S. P., and D. R. Watts, 2014: Rapid eddy-induced modification of subtropical mode water during the Kuroshio Extension System Study. J. Phys. Oceanogr., 44, 1941-1953, doi:10.1175/JPO-D-13-0191.1.

Bishop, S. P., F. O. Bryan, and R. J. Small, 2015: Bjerknes-like compensation in the wintertime north Pacific. J. Phys. Oceanogr., 45, 1339-1355, doi:10.1175/JPO-D-14-0157.1.

Ceballos, L., E. Di Lorenzo, C. D. Hoyos, N. Schneider, and B. Taguchi, 2009: North Pacific Gyre oscillation synchronizes climate variability in the eastern and western boundary current systems. J. Climate, 22, 5163-5174, doi:10.1175/2009JCLI2848.1.

Cerovecki, I., and D. Giglio, 2016: North Pacific subtropical mode water volume decrease in 2006–09 estimated from Argo observations: Influence of surface formation and basin-scale oceanic variability. J. Climate, 29, 2177-2199, doi:10.1175/JCLI-D-15-0179.1.

Cronin, M. F., N. A. Bond, J. T. Farrar, H. Ichikawa, S. R. Jayne, Y. Kawai, M. Konda, B. Qiu, L. Rainville, and H. Tomita, 2013: Formation and erosion of the seasonal thermocline in the Kuroshio Extension Recirculation Gyre. Deep-Sea Res. II, 85, 62-74, doi:10.1016/j.dsr2.2012.07.018.

Davis, X. J., L. M. Rothstein, W. K. Dewar, and D. Menemenlis, 2011: Numerical investigations of seasonal and interannual variability of North Pacific subtropical mode water and its implications for Pacific climate variability. J. Climate, 24, 2648-2665, doi:10.1175/2010JCLI3435.1.

Hanawa, K., and L. D. Talley, 2001: Mode waters. Ocean Circulation and Climate: Observing and Modelling the Global Ocean, G. Siedler, J. Church, and J. Gould, Eds., Academic Press, 373-386.

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Kelly, K. A., R. J. Small, R. M. Samelson, B. Qiu, T. M. Joyce, Y.-O. Kwon, and M. F. Cronin, 2010: Western boundary currents and frontal air-sea interaction: Gulf Stream and Kuroshio Extension. J. Climate, 23, 5644-5667, doi:10.1175/2010JCLI3346.1.

Lin, P., F. Chai, H. Xue, and P. Xiu, 2014: Modulation of decadal oscillation on surface chlorophyll in the Kuroshio Extension. J. Geophys. Res., 119, 187–199, doi:10.1002/2013JC009359.

McWilliams, J. C., 2016: Submesoscale currents in the ocean. Proc. Roy. Soc. A, 472, doi:10.1098/rspa.2016.0117..

Oka, E., 2009: Seasonal and interannual variation of North Pacific subtropical mode water in 2003–2006. J. Oceanogr., 65, 151-164, doi:10.1007/s10872-009-0015-y.

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Oka, E., B. Qiu, S. Kouketsu, K. Uehara, and T. Suga, 2012: Decadal seesaw of the central and subtropical mode water formation associated with the Kuroshio Extension variability. J. Oceanogr., 68, 355-360, doi: 10.1007/s10872-015-0300-x.

Oka, E., B. Qiu, Y. Takatani, K. Enyo, D. Sasano, N. Kosugi, M. Ishii, T. Nakano, and T. Suga, 2015: Decadal variability of subtropical mode water subduction and its impact on biogeochemistry. J. Oceanogr., 71, 389-400, doi: 10.1007/s10872-015-0300-x.

Pierini, S., 2014: Kuroshio Extension bimodality and the North Pacific Oscillation: A case of intrinsic variability paced by external forcing. J. Climate, 27, 448-454, doi:10.1175/JCLI-D-13-00306.1.

Qiu, B., 2002: The Kuroshio Extension system: Its large-scale variability and role in the midlatitude ocean-atmosphere interaction. J. Oceanogr., 58, 57-75, doi:10.1023/A:1015824717293.

Qiu, B., and S. Chen, 2005: Variability of the Kuroshio Extension jet, recirculation gyre and mesoscale eddies on decadal timescales. J. Phys. Oceanogr., 35, 2090-2103, doi: 10.1175/JPO2807.1.

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Qiu, B., P. Hacker, S. Chen, K. A. Donohue, D. R. Watts, H. Mitsudera, N. G. Hogg and S. R. Jayne, 2006: Observations of the subtropical mode water evolution from the Kuroshio Extension System Study. J. Phys. Oceanogr., 36, 457-473, doi:10.1175/JPO2849.1.

Rainville, L., S. R. Jayne, and M. F. Cronin, 2014: Variations of the North Pacific subtropical mode water from direct observations. J. Climate, 27, 2842-2860, doi:10.1175/JCLI-D-13-00227.1.

Sabine, C. L., Feely, R. A., Gruber, N., Key, R. M., Lee, K., Bullister, J. L., Wanninkhof, R., Wong, C., Wallace, D. W. R., Rilbrook, B., Millero, F. J., Peng, T.-H., Kozyr, A., Ono, T., and Rios, A. F., 2004. The oceanic sink for anthropogenic CO₂. Science, 305, 367–371.

Sasaki, Y. N, S. Minobe, and N. Schneider, 2013: Decadal response of the Kuroshio Extension jet to Rossby waves: Observation and thin-jet theory. J. Phys. Oceanogr., 43, 442-456, doi:10.1175/JPO-D-12-096.1.

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Phytoplankton increase projected for the Ross Sea in response to climate change

Posted by mmaheigan

· Thursday, October 26th, 2017

How will phytoplankton respond to climate changes over the next century in the Ross Sea, the most productive coastal waters of Antarctica? Model projections of physical conditions suggest substantial environmental changes in this region, but associated impacts on Ross Sea biology, specifically phytoplankton, remain unclear.

In a recent study, Kaufman et al (2017) generated and analyzed model scenarios for the mid- and late-21^st century using a combination of a biogeochemical model, hydrodynamic simulations forced by a global climate projection, and new data from autonomous gliders. These scenarios indicate increases in the production of phytoplankton in the Ross Sea and increases in the downward flux of carbon in response to environmental changes over the next century. Modeled responses of the different phytoplankton groups to shoaling mixed layer depths shift the biomass composition more towards diatoms by the mid 21^st century. While diatom biomass remains relatively constant in the second half of the 21^st century, the haptophyte Phaeocystis antarctica biomass increases as a result of earlier seasonal sea ice melt, allowing earlier availability of low light, in which P. antarctica thrive.

Modeled climate scenarios for the 21st century project phytoplankton composition changes and increases in primary production and carbon export flux, primarily as a result of shoaling mixed layer depths and earlier available low light.

The projected responses of phytoplankton composition, production, and carbon export to climate-related changes can have broad impacts on food web functioning and nutrient cycling, with wide-ranging potential effects as local deep waters are transported out from the Ross Sea continental shelf. Future changes to this ecosystem have taken on a new relevance as the Ross Sea became home this year to the world’s largest Marine Protected Area, designed to protect critical habitat for highly valued species that rely on the Ross Sea food web. Continued coordination between modeling and autonomous observing efforts is needed to provide vital data for global ocean assessments and to improve our understanding of ecosystem dynamics and climate change impacts in this sensitive and important region.

For other relevant work on observing phytoplankton characteristics in the Ross Sea using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

And for assimilation of bio-optical glider data in the Ross Sea please see: https://doi.org/10.5194/bg-2017-258.

Authors:
Daniel E. Kaufman (VIMS, College of William and Mary)
Marjorie A. M. Friedrichs (VIMS, College of William and Mary)
Walker O. Smith Jr. (VIMS, College of William and Mary)
Eileen E. Hofmann (CCPO, Old Dominion University)
Michael S. Dinniman (CCPO, Old Dominion University)
John C. P. Hemmings (Wessex Environmental Associates; now at the UK Met Office)

Sinking particles as biogeochemical hubs for trace metal cycling and release

Posted by mmaheigan

· Thursday, September 14th, 2017

The extent to which the return of major and minor elements to the dissolved phase in the deep ocean (termed remineralization) is decoupled plays a major role in setting patterns of nutrient limitation in the global ocean. It is well established that major elements such as phosphorus, silicon, and carbon are released at different rates from sinking particles, with major implications for nutrient recycling. Is this also the case for trace metals?

A recent publication by Boyd et al. in Nature Geoscience provides new insights into the biotic and abiotic processes that drive remineralization of metals in the ocean. Particle composition changes rapidly with depth with both physical (disaggregation) and biogeochemical (grazing; desorption) processes leading to a marked decrease in the total surface area of the particle population. The proportion of lithogenic metals in sinking particles also appears to increase with depth, as the biogenic metals may be more labile and hence more readily removed.

Findings from GEOTRACES process studies revealed that release rates for trace elements such as iron, nickel, and zinc vary from each other. Microbes play a key role in determining the turnover rates for nutrients and trace elements. Decoupling of trace metal recycling in the surface ocean and below may result from their preferential removal by microbes to satisfy their nutritional requirements. In addition, the chemistry operating on particle surfaces plays a pivotal role in determining the specific fates of each trace metal. Teasing apart these factors will take time, as there is a complex interplay between chemical and biological processes. Improving our understanding is crucial, as these processes are not currently well represented by state-of-the-art ocean biogeochemical models.

Figure caption: Rapid changes in the characteristics of sinking particles over the upper 200 m as evidenced by: a) differential release of trace metals from sinking diatoms; b) changes in proportion of lithogenic versus biogenic materials; and c) ten-fold decrease in total particle surface area.

Authors:
Philip Boyd (IMAS, Australia)
Michael Ellwood (ANU, Australia)
Alessandro Tagliabue (Liverpool, UK)
Ben Twining (Bigelow, USA)

Relevant links:
GEOTRACES Digest: Iron Superstar

Joint workshop with GEOTRACES in August 2016: Biogeochemical Cycling of Trace Elements within the Ocean

Tiny marine animals strongly influence the carbon cycle

Posted by mmaheigan

· Thursday, August 31st, 2017

What controls the amount of organic carbon entering the deep ocean? In the sunlit layer of the ocean, phytoplankton transform inorganic carbon to organic carbon via a process called photosynthesis. As these particulate forms of organic carbon stick together, they become dense enough to sink out of the sunlit layer, transferring large quantities of organic carbon to the deep ocean and out of contact with the atmosphere.

However, all is not still in the dark ocean. Microbial organisms such as bacteria, and zooplankton consume the sinking, carbon-rich particles and convert the organic carbon back to its original inorganic form. Depending on how deep this occurs, the carbon can be physically mixed back up into the surface layers for exchange with the atmosphere or repeat consumption by phytoplankton. In a recent study published in Biogeosciences, researchers used field data and an ecosystem model in three very different oceanic regions to show that zooplankton are extremely important in determining how much carbon reaches the deep ocean.

Figure 1. Particle export and transfer efficiency to the deep ocean in the Southern Ocean (SO, blue circles), North Atlantic Porcupine Abyssal Plain site (PAP, red squares) and the Equatorial Tropical North Pacific (ETNP, orange triangles) oxygen minimum zone. a) particle export efficiency of fast sinking particles (Fast PEeff) against primary production on a Log10 scale. b) transfer efficiency of particles to the deep ocean expressed as Martin’s b (high b = low efficiency). Error bars in b) are standard error of the mean for observed particles, error too small in model to be seen on this plot.

In the Southern Ocean (SO), zooplankton graze on phytoplankton and produce rapidly sinking fecal pellets, resulting in an inverse relationship between particle export and primary production (Fig. 1a). In the North Atlantic (NA), the efficiency with which particles are transferred to the deep ocean is comparable to that of the Southern Ocean, suggesting similar processes apply; but in both regions, there is a large discrepancy between the field data and the ecosystem model (Fig. 1b), which poorly represents particle processing by zooplankton. Conversely, much better data-model matches are observed in the equatorial Pacific, where lower oxygen concentrations mean fewer zooplankton; this reduces the potential for zooplankton-particle interactions that reduce particle size and density, resulting in a lower transfer efficiency.

This result suggests that mismatches between the data and model in the SO and NA may be due to the lack of zooplankton-particle parameterizations in the model, highlighting the potential importance of zooplankton in regulating carbon export and storage in the deep ocean. Zooplankton parameterizations in ecosystem models must be enhanced by including zooplankton fragmentation of particles as well as consumption. Large field programs such as EXPORTS could help constrain these parameterisation by collecting data on zooplankton-particle interaction rates. This will improve our model estimates of carbon export and our ability to predict future changes in the biological carbon pump. This is especially important in the face of climate-driven changes in zooplankton populations (e.g. oxygen minimum zone (OMZ) expansion) and associated implications for ocean carbon storage and atmospheric carbon dioxide levels.

Authors:
Emma L. Cavan (University of Tasmania)
Stephanie A. Henson (National Oceanography Centre, Southampton)
Anna Belcher (University of Southampton)
Richard Sanders (National Oceanography Centre, Southampton)

Do rivers supply nutrients to the open ocean?

Posted by mmaheigan

· Wednesday, May 24th, 2017

Rivers carry large amounts of nutrients (e.g., nitrogen and phosphorus) to the sea, but we do not know how much of that riverine nutrient supply escapes biological and chemical processing in shallow coastal waters to reach the open ocean. Most global ocean biogeochemical models, which are typically unable to resolve coastal processes, assume that either all or none of the riverine nutrients entering coastal waters actually contribute to open ocean processes.

While we know a good deal about the dynamics of individual rivers entering the coastal ocean, studies to date have been limited to a few major river systems, mainly in in developed countries. Globally, there are over 6,000 rivers entering the coastal ocean. In a recent study, Sharples et al (2017) devised a simple approach to obtain a global-scale estimate of riverine nutrient inputs based on the knowledge that low-salinity waters entering the coastal ocean tend to form buoyant plumes that turn under the influence of Earth’s daily rotation to flow along the coastline. Using published data on such flows and incorporating the effect of Earth’s rotation, they obtained estimates of typical cross-shore plume width and compared them to the local width of the continental shelf. This was used to calculate the residence time of riverine nutrients on the shelf, which is the key to estimating how much of a given nutrient is consumed in shelf waters vs. how much is exported to the open ocean.

Global distribution of the amount of riverine dissolved inorganic nitrogen that escapes the continental shelf to reach the open ocean.

The results indicate that, on a global scale, 75% (80%) of the nitrogen (phosphorus) supplied by rivers reaches the open ocean, whereas 25% (20%) of the nitrogen (phosphorus) is consumed on the shelf (e.g., fueling coastal productivity). Limited knowledge of nutrient cycling and consumption in shelf waters represents the primary source of uncertainty in this study. However, well-defined global patterns related to human land use (e.g., agricultural fertilizer use in developed nations) emerged from this analysis, underscoring the need to understand how land-use changes and other human activities will alter nutrient delivery to the coastal ocean in the future.

Authors:
Jonathan Sharples (School of Environmental Sciences, University of Liverpool, UK)
Jack Middelburg (Department of Earth Sciences, Utrecht University, Netherlands)
Katja Fennel (Department of Oceanography, Dalhousie University, Canada)
Tim Jickells (School of Environmental Sciences, University of East Anglia, UK)

Reconciling fisheries catch and ocean productivity in a changing climate

Posted by mmaheigan

· Thursday, March 16th, 2017

Phytoplankton provide the energy that fuels marine food webs, yet differences in fisheries catch across global ecosystems far exceed accompanying differences in phytoplankton production. Nearly 50 years ago, John Ryther hypothesized that this contrast must arise from synergistic interactions between phytoplankton production and food webs. New perspectives on global fish catch, fishing effort, and a prototype high-resolution global earth system model allowed us to revisit Ryther’s supposition and explore its implications under climate change. After accounting for a small number of lightly fished ecosystems, we find that stark differences in regional catch can be explained with an energetically constrained model that a) resolves large inter-regional differences in the benthic and pelagic energy pathways connecting phytoplankton and fish; b) reduces trophic transfer efficiencies in warm, tropical ecosystems; and, less critically, c) associates elevated trophic transfer efficiencies with benthic systems. The same food web processes that accentuate spatial differences in phytoplankton production in the contemporary ocean also accentuated temporal trends under climate change, with projected fish catch changes in some areas exceeding 50% (Figure 1). Our results, recently published in PNAS, demonstrate the importance of marine resource management strategies that are robust to potentially significant changes in fisheries productivity baselines. These results also provide impetus for efforts to improve constraints on regional ocean productivity projections that often disagree in present earth system models.

Figure 1: Projected percent changes in net phytoplankton production (left) and fisheries catch (right) between 2050-2100 and 1950-2000 under a high greenhouse gas emission scenario (RCP8.5) in GFDL’s ESM2M-COBALT Earth System Model. Contours are shown for +/- 50%.

Authors: Charles A. Stock^a, Jasmin G. John^a, Ryan R. Rykaczewski^b,c, Rebecca G. Asch^d, William W.L. Cheung^e, John P. Dunne^a, Kevin D. Friedland^f, Vicky W.Y. Lam^e, Jorge L. Sarmiento^d, and Reg A. Watson^g

^aGeophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration
^bSchool of the Earth, Ocean, and Environment, University of South Carolina
^cDepartment of Biological Sciences, University of South Carolina
^dAtmospheric and Oceanic Sciences Program, Princeton University
^eNippon Foundation-Nereus Program, Institute of Oceans and Fisheries, The University of British Columbia
^fNational Marine Fisheries Service, Narragansett, RI
^gInstitute for Marine and Antarctic Studies, University of Tasmania, Australia

Oceanic fronts enhance carbon transport to the ocean’s interior through both subduction and amplified sinking

Posted by mmaheigan

· Wednesday, March 1st, 2017

Mesoscale fronts are regions with potentially enhanced nutrient fluxes, phytoplankton production and biomass, and aggregation of mesozooplankton and higher trophic levels. However, the role of these features in transporting organic carbon to depth and hence sequestering CO₂ from the atmosphere has not previously been determined. Working with the California Current Ecosystem Long Term Ecological Research (CCE LTER) program, we determined that the flux of sinking particles at a stable front off the coast of California was twice as high as similar fluxes on either side of the front, or in typical non-frontal waters of the CCE in a recent study by Stukel et al. (2017) published in Proceedings of the National Academy of Sciences.

This increased export flux was tied to enhanced silica-ballasting by Fe-stressed diatoms and to an abundance of mesozooplankton grazers. Furthermore, downward transport of particulate organic carbon by subduction at the front led to additional carbon export that was similar in magnitude to sinking flux, suggesting that these fronts (which are a common feature in productive eastern boundary upwelling systems) are an important conduit for carbon sequestration. These enhanced carbon export mechanisms at episodic and mesoscale features need to be included in future biogeochemical forecast models to understand how a changing climate will affect marine CO₂ uptake.

Authors

Michael R. Stukel (Florida State University)
Lihini I. Aluwihare, Katherine A. Barbeau, Ralf Goericke, Arthur J. Miller, Mark D. Ohman, Angel Ruacho, Brandon M. Stephens, Michael R. Landry (University of California, San Diego)
Hajoon Song (Massachusetts Institute of Technology)
Alexander M. Chekalyuk (Lamont-Doherty Earth Observatory)

A framework for ENSO predictability of marine ecosystem drivers along the US West Coast

Posted by mmaheigan

· Thursday, February 16th, 2017

The US West Coast eastern boundary upwelling system supports one of the most productive marine ecosystems in the world and is a primary source of ecosystem services for the US (e.g., fishing, shipping, and recreation). Long-term historical observations of physical and biological variables in this region have been collected since the 1950s (e.g., the CalCOFI program and now including the coastal ocean observing systems), leading to an excellent foundation for understanding the ecosystem impacts of dominant climate fluctuations such as the El Niño-Southern Oscillation (ENSO). In the northeast Pacific, ENSO impacts a wide range of physical and biotic processes, including temperature, stratification, winds, upwelling, and primary and secondary production. The El Niño phase of ENSO, in particular, can result in extensive geographic habitat range displacements and altered catches of fishes and invertebrates, and impact vertical and lateral export fluxes of carbon and other elements (Jacox et al., this issue; Anderson et al., this issue; Ohman et al., this issue). However, despite empirical observations and increased understanding of the coupling between climate and marine ecosystems along the US West Coast, there has been no systematic attempt to use this knowledge to forecast marine ecosystem responses to individual ENSO events. ENSO forecasting has become routine in the climate community. However, little has been done to forecast the impacts of ENSO on ecosystems and their services. This becomes especially important considering the occurrence of recent strong El Niño events (such as 2015-16) and climate model projections that suggest that ENSO extremes may become more frequent (Cai et al. 2015).

The joint US CLIVAR/OCB/NOAA/PICES/ICES workshop on Forecasting ENSO impacts on marine ecosystems of the US West Coast (Di Lorenzo et al. 2017) held in La Jolla, California, in August 2016 outlined a three-step strategy to better understand and quantify the ENSO-related predictability of marine ecosystem drivers along the US West Coast (Figure 1). The first step is to use a high-resolution ocean reanalysis to determine the association between local ecosystem drivers and regional forcing patterns (RFPs). The identification of ecosystem drivers will depend on the ecosystem indicators or target species selected for prediction (Ohman et al., this issue). The second step is to objectively identify the tropical sea surface temperature (SST) patterns that optimally force the RFPs along the US West Coast region using available long-term large-scale reanalysis products. While the goal of the first two steps is to understand the dynamical basis for predictability (Figure 1, blue path), the final third step (Figure 1, orange path) aims at quantifying the predictability of the RFPs and estimating their prediction skill at seasonal timescales. This third step can be implemented using the output of multi-model ensemble forecasts such as the North America Multi-Model Ensemble (NMME) or by building efficient statistical prediction models such as Linear Inverse Models (LIMs; Newman et al. 2003).

Figure 1. Framework for understanding and predicting ENSO impacts on ecosystem drivers. Blue path shows the steps that will lead to Understanding of the ecosystem drivers and their dependence on tropical Pacific anomalies. Orange path shows the steps that will lead to quantifying the Predictability of marine ecosystem drivers along the US West Coast that are predictable from large-scale tropical teleconnection dynamics.

Important to the concept of ENSO predictability is the realization that the expressions of ENSO are very diverse and cannot be identified with a few indices (Capotondi et al. 2015; Capotondi et al., this issue). In fact, different expressions of sea surface temperature anomalies (SSTa) in the tropics give rise to oceanic and atmospheric teleconnections that generate different coastal impacts in the northeast Pacific. For this reason, we will refer to ENSO as the collection of tropical Pacific SSTa that lead to deterministic and predictable responses in the regional ocean and atmosphere along the US West Coast.

In the sections below, we articulate in more detail the elements of the framework for quantifying the predictability of ENSO-related impacts on coastal ecosystems along the US West Coast (Figure 1). Our focus will be on the California Current System (CCS), reflecting the regional expertise of the workshop participants. Specifically, we discuss (1) the ecosystem drivers and what is identified as such; (2) RFP definitions; and (3) the teleconnections from the tropical Pacific and their predictability.

Ecosystem drivers in the California Current System

The impacts of oceanic processes on the CCS marine ecosystem have been investigated since the 1950s when the long-term California Cooperative Oceanic Fisheries Investigations (CalCOFI) began routine seasonal sampling of coastal ocean waters. The CalCOFI program continues today and has been augmented with several other sampling programs (e.g., the coastal ocean observing network), leading to an unprecedented understanding of how climate and physical ocean processes, such as upwelling, drive ecosystem variability and change (e.g., see more recent reviews from King et al.2011; Ohman et al. 2013; Di Lorenzo et al. 2013).

The dominant physical oceanographic drivers of ecosystem variability occur on seasonal, interannual, and decadal timescales and are associated with changes in (1) SST; (2) upwelling velocity; (3) alongshore transport; (4) cross-shore transport; and (5) thermocline/nutricline depth (see Ohman et al., this issue). This set of ecosystem drivers emerged from discussions among experts at the workshop. Ecosystem responses to these drivers include multiple trophic levels, including phytoplankton, zooplankton, small pelagic fish, and top predators, and several examples have been identified for the CCS (see summary table in Ohman et al., this issue).

While much research has focused on diagnosing the mechanisms by which these physical drivers impact marine ecosystems, less is known about the dynamics controlling the predictability of these drivers. As highlighted in Ohman et al. (this issue), most of the regional oceanographic drivers (e.g., changes in local SST, upwelling, transport, thermocline depth) are connected to changes in large-scale forcings (e.g., winds, surface heat fluxes, large-scale SST and sea surface height patterns, freshwater fluxes, and remotely forced coastally trapped waves entering the CCS from the south). In fact, several studies have documented how large-scale changes in wind patterns associated with the Aleutian Low and the North Pacific Oscillation drive oceanic modes of variability such as the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation (Mantua et al. 1997; Di Lorenzo et al. 2008; Chhak et al. 2009; Ohman et al., this issue; Jacox et al., this issue; Anderson et al., this issue; Capotondi et al., this issue) that influence the CCS. However, these large-scale modes only explain a fraction of the ecosystem’s atmospheric forcing functions at the regional-scale. Thus, it is important to identify other key forcings to gain a more complete mechanistic understanding of CCS ecosystem drivers (e.g., Jacox et al. 2014; 2015).

Atmospheric and oceanic regional forcing patterns

The dominant large-scale quantities that control the CCS ecosystem drivers are winds, heat fluxes, and remotely forced coastally trapped waves (Hickey 1979). Regional expressions or patterns of these large-scale forcings have been linked to changes in local stratification and thermocline depth (Veneziani et al. 2009a; 2009b; Combes et al. 2013), cross-shore transport associated with mesoscale eddies (Kurian et al. 2011; Todd et al. 2012; Song et al. 2012; Davis and Di Lorenzo 2015b), and along-shore transport (Davis and Di Lorenzo 2015a; Bograd et al. 2015). For this reason, we define the regional expressions of the atmospheric and remote wave forcing that are optimal in driving SST, ocean transport, upwelling, and thermocline depth as the RFPs. To clarify this concept, consider the estimation of coastal upwelling velocities. While a change in the position and strength in the Aleutian Low has been related to coastal upwelling in the northern CCS, a more targeted measure of the actual upwelling vertical velocity and nutrient fluxes that are relevant to primary productivity can only be quantified by taking into account a combination of oceanic processes that depend on multiple RFPs such as thermocline depth (e.g., remote waves), thermal stratification (e.g., heat fluxes), mesoscale eddies, and upwelling velocities (e.g., local patterns of wind stress curl and alongshore winds; see Gruber et al 2011; Jacox et al. 2015; Renault et al. 2016). In other words, if we consider the vertical coastal upwelling velocity (w) along the northern CCS, a more adequate physical description and quantification would be given from a linear combination of the different regional forcing functions w = Σ_n∝_n * RFP_n rather than w = ∝*Aleutian Low.

The largest interannual variability in the Pacific that impacts the RFPs is ENSO, which also constitutes the largest source of seasonal (3-6 months) predictability. During El Niño and La Niña, atmospheric and oceanic teleconnections from the tropics modify large-scale and local surface wind patterns and ocean currents of the CCS and force coastally trapped waves.

ENSO teleconnections and potential seasonal predictability of the regional forcing patterns

While ENSO exerts important controls on the RFPs in the CCS, it has become evident that ENSO expressions in the tropics vary significantly from event to event, leading to different responses in the CCS (Capotondi et al., this issue). Also, as previously pointed out, the CCS is not only sensitive to strong ENSO events but more generally responds to a wide range of tropical SSTa variability that is driven by ENSO-type dynamics in the tropical and sub-tropical Pacific. For this reason, we define an “ENSO teleconnection” as any RFP response that is linked to ENSO-type variability in the tropics.

ENSO can influence the upwelling and circulation in the CCS region through both oceanic and atmospheric pathways. It is well known that equatorial Kelvin waves, an integral part of ENSO dynamics, propagate eastward along the Equator and continue both northward (and southward) along the coasts of the Americas as coastally trapped Kelvin waves after reaching the eastern ocean boundary. El Niño events are associated with downwelling Kelvin waves, leading to a deepening of the thermocline, while La Niña events produce a shoaling of the thermocline in the CCS (Simpson 1984; Lynn and Bograd 2002; Huyer et al. 2002; Bograd et al. 2009; Hermann et al. 2009; Miller et al. 2015). The offshore scale of coastal Kelvin waves decreases with latitude, and the waves decay while propagating northward along the coast due to dissipation and radiation of westward propagating Rossby waves. In addition, topography and bathymetry can modify the nature of the waves and perhaps partially impede their propagation at some location. Thus, the efficiency of coastal waves of equatorial origin in modifying the stratification in the CCS is still a matter of debate. To complicate matters, regional wind variability south of the CCS also excites coastally trapped waves, which supplement the tropical source.

In the tropics, SST anomalies associated with ENSO change tropical convection and excite mid-troposphere stationary atmospheric Rossby waves that propagate signals to the extratropics, the so-called atmospheric ENSO teleconnections (Capotondi et al., this issue). Through these atmospheric waves, warm ENSO events favor a deepening and southward shift of the Aleutian Low pressure system that is dominant during winter, as well as changes in the North Pacific Subtropical High that is dominant during spring and summer, resulting in a weakening of the alongshore winds, reduced upwelling, and warmer surface water. These changes are similar to those induced by coastal Kelvin waves of equatorial origin, making it very difficult to distinguish the relative importance of the oceanic and atmospheric pathways in the CCS. In addition, due to internal atmospheric noise, the details of the ENSO teleconnections can vary significantly from event to event and result in important differences along the California Coast (Figure 2).

Figure 2. Schematic of ENSO teleconnection associated with different flavors of tropical SSTa. (a) Atmospheric teleconnections of the canonical eastern Pacific El Niño tend to impact the winter expression of the Aleutian Low, which in turn drives an oceanic SSTa anomaly that projects onto the pattern of the Pacific Decadal Oscillation (PDO). (b) Atmospheric teleconnections of the central Pacific El Niño tend to impact the winter expression of the North Pacific High, which in turn drives an oceanic SSTa anomaly that projects onto the pattern of the North Pacific Gyre Oscillation (NPGO). The ENSO SSTa maps are obtained by regressing indices of central and eastern Pacific ENSO with SSTa. The other maps are obtained by regression of SSTa/SLPa with the PDO (a) and NPGO (b) indices.

El Niño events exhibit a large diversity in amplitude, duration, and spatial pattern (Capotondi et al. 2015). The amplitude and location of the maximum SST anomalies, whether in the eastern (EP) or central (CP) Pacific, can have a large impact on ENSO teleconnections (Ashok et al. 2007; Larkin and Harrison 2005). While “canonical” EP events induce changes in the Aleutian Low (Figure 2b), CP events have been associated with a strengthening of the second mode of North Pacific atmospheric variability, the North Pacific Oscillation (NPO; Figure 2a; Di Lorenzo et al. 2010; Furtado et al. 2012). In addition, it is conceivable that EP events have a larger Kelvin wave signature than CP events, resulting in different oceanic influences in the CCS.

In summary, while the ENSO influence on the CCS physical and biological environments is undeniable, several sources of uncertainty remain about the details of that influence. This uncertainty arises in the physical environment on seasonal timescales from many sources, including the diversity of ENSO events, the intrinsic unpredictable components of the atmosphere, and the intrinsic unpredictable eddy variations in the CCS. We also need to distinguish between physically forced ecosystem response versus intrinsic biological variability, which is potentially nonlinear and likely unpredictable. Skill levels need to be quantified for each step of the prediction process (i.e., ENSO, teleconnections, local oceanic response, local ecosystem response) relative to a baseline—for example the persistence of initial condition, which is also being exploited for skillful predictions of the large marine ecosystem at the seasonal timescale (Tommasi et al., this issue). The target populations should be exploitable species that are of interest to federal and state agencies that regulate certain stocks. Models are currently being developed to use ocean forecasts to advance top predator management (Hazen et al., this issue). The implementation of this framework (Figure 1) for practical uses will require a collaborative effort between physical climate scientists with expertise in predicting and understanding ENSO and biologists who have expertise in understanding ecosystem response to physical climate forcing.

Authors

Emanuele Di Lorenzo (Georgia Institute of Technology)
Arthur J. Miller (Scripps Institution of Oceanography)

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Veneziani, M., C. A. Edwards, J. D. Doyle, and D. Foley, 2009: A central California coastal ocean modeling study: 1. Forward model and the influence of realistic versus climatological forcing. J. Geophy. Res.-Oceans, 114, doi:10.1029/2008jc004774.

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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.

Archive for modeling – Page 7

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Ecosystem drivers in the California Current System

Atmospheric and oceanic regional forcing patterns

ENSO teleconnections and potential seasonal predictability of the regional forcing patterns

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