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How does the competition between phytoplankton and bacteria for iron alter ocean biogeochemical cycles?

Posted by mmaheigan

· Friday, August 26th, 2022

Free-living bacteria play a key role in cycling essential biogeochemical resources in the ocean, including iron, via their uptake, transformation, and release of organic matter throughout the water column. Bacteria process half of the ocean’s primary production, remineralize dissolved organic matter, and re-direct otherwise lost organic matter to higher trophic levels. For these reasons, it is crucial to understand what factors limit the growth of bacteria and how bacteria activities impact global ocean biogeochemical cycles.

In a recent study, Pham and colleagues used a global ocean ecosystem model to dive into how iron limits the growth of free-living marine bacteria, how bacteria modulate ocean iron cycling, and the consequences to marine ecosystems of the competition between bacteria and phytoplankton for iron.

Figure 1: (a) Iron limitation status of bacteria in December, January, and February (DJF) in the surface ocean. Low values (in blue color = close to zero) mean that iron is the limiting factor for the growth of bacteria; (b) Bacterial iron consumption in the upper 120m of the ocean and (c) Changes (anomalies) in export carbon production when bacteria have a high requirement for iron.

Through a series of computer simulations performed in the global ocean ecosystem model, the authors found that iron is a limiting factor for bacterial growth in iron-limited regions in the Southern Ocean, the tropical, and the subarctic Pacific due to the high iron requirement and iron uptake capability of bacteria. Bacteria act as an iron sink in the upper ocean due to their significant iron consumption, a rate comparable to phytoplankton. The competition between bacteria and phytoplankton for iron alters phytoplankton bloom dynamics, ocean carbon export, and the availability of dissolved organic carbon needed for bacterial growth. These results suggest that earth system models that omit bacteria ignore an important organism modulating biogeochemical responses of the ocean to future changes.

Authors:
Anh Le-Duy Pham (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Olivier Aumont (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Lavenia Ratnarajah (University of Liverpool, United Kingdom)
Alessandro Tagliabue (University of Liverpool, United Kingdom)

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)

Profiling floats reveal fate of Southern Ocean phytoplankton stocks

Posted by mmaheigan

· Tuesday, September 1st, 2020

More observations are needed to constrain the relative roles of physical (advection), biogeochemical (downward export), and ecological (grazing and biological losses) processes in driving the fate of phytoplankton blooms in Southern Ocean waters. In a recent paper published in Nature Communications, authors used seven Biogeochemical Argo (BGC-Argo) floats that vertically profiled the upper ocean every ten days as they drifted for three years across the remote Sea Ice Zone of the Southern Ocean. Using the floats’ biogeochemical sensors (chlorophyll, nitrate, and backscattering) and regional ratios of nitrate consumption:chlorophyll synthesis, the authors developed a new approach to remotely estimate the fate of the phytoplankton stocks, enabling calculations of herbivory and of downward carbon export. The study revealed that the major fate of phytoplankton biomass in this region is grazing, which consumes ~90% of stocks. The remaining 10% is exported to depth. This pattern was consistent throughout the entire sea ice zone where the floats drifted, from 60°-69° South.

Figure Caption: Southern Ocean Chlorophyll a climatology and floats’ trajectories (top panel). Total losses of Chlorophyll a (including grazing and phytodetritus export, left panel). Phytodetritus export (right panel).

This study region comprises two of the three major krill growth and development areas—the eastern Weddell and King Haakon VII Seas and Prydz Bay and the Kerguelen Plateau—so the observed grazing was probably due to Antarctic krill, underscoring their pivotal importance in this ecosystem. Building upon the greater understanding of ocean ecosystems via satellite ocean colour development in the 1990s, BGC-Argo floats and this new approach will allow remote monitoring of the different fates of phytoplankton stocks and insights into the status of the ecosystem.

Authors:
Sebastien Moreau (Norwegian Polar Institute, Tromsø, Norway)
Philip Boyd (Institute for Marine and Antarctic Studies, Hobart, Australia)
Peter Strutton (Institute for Marine and Antarctic Studies, Hobart, Australia)

Turning a spotlight on grazing

Posted by mmaheigan

· Thursday, July 23rd, 2020

Microscopic plankton in the surface ocean make planet Earth habitable by generating oxygen and forming the basis of marine food webs, yielding harvestable protein. For over 100 years, oceanographers have tried to ascertain the physical, chemical, and biological processes governing phytoplankton blooms. Zooplankton grazing of phytoplankton is the single largest loss process for primary production, but empirical grazing data are sparse and thus poorly constrained in modeling frameworks, including assessments of global elemental cycles, cross-ecosystem comparisons, and predictive efforts anticipating future ocean ecosystem function. As sunlight decays exponentially with depth, upper-ocean mixing creates dynamic light environments with predictable effects on phytoplankton growth but unknown consequences for grazing.

Figure caption: Rates (d⁻¹) of phytoplankton growth (μ), grazing mortality (g), and biomass accumulation (r) under four mixed layer scenarios simulated using light as a proxy of (a) sustained deep mixing, (b) rapid shoaling, (c) sustained shallow mixing, and (d) rapid mixed layer deepening. Error bars represent one standard deviation of the mean of duplicate experiments. Grazing was measured but not detected in the sustained deep mixing and rapid shoaling conditions, denoted with x.

Using data from a spring cruise in the North Atlantic, authors of a recent study published in Limnology & Oceanography compared the influences of microzooplankton predation and fluctuations in light availability—representative of a mixing water column—on phytoplankton standing stock. Data from at-sea incubations and light manipulation experiments provide evidence that phytoplankton’s instantaneous and zooplankton’s delayed responses to light fluctuations are key modulators of the balance between phytoplankton growth and grazing rates (Figure 1). These results suggest that light is a potential, remotely retrievable predictor of when and where in the ocean zooplankton grazing may represent an important loss term of phytoplankton production. If broadly verified, this approach could be used to systematically assess sparsely measured grazing across spatial and temporal gradients in representative regions of the ocean. Such data will be essential for enhancing our predictive capacity of ocean food web function, global biogeochemical cycles and the many derived processes, including fisheries production and the flow of carbon through the oceans.

Authors:
Françoise Morison (University of Rhode Island)
Gayantonia Franzè (University of Rhode Island, currently Institute of Marine Research, Norway)
Elizabeth Harvey (University of Georgia, currently University of New Hampshire)
Susanne Menden-Deuer (University of Rhode Island)

Tiny, but effective: Gelatinous zooplankton and the ocean biological carbon pump

Posted by mmaheigan

· Wednesday, March 25th, 2020

Barely visible to the naked eye, gelatinous zooplankton play important roles in marine food webs. Cnidaria, Ctenophora, and Urochordata are omnipresent and provide important food sources for many more highly developed marine organisms. These small, nearly transparent organisms also transport large quantities of “jelly-carbon” from the upper ocean to depth. A recent study in Global Biogeochemical Cycles focused on quantifying the gelatinous zooplankton contribution to the ocean carbon cycle.

Figure 1. Processes and pathways or gelatinous carbon transfer to the deep ocean.

Using >90,000 data points (1934 to 2011) from the Jellyfish Database Initiative (JeDI), the authors compiled global estimates of jellyfish biomass, production, vertical migration, and jelly carbon transfer efficiency. Despite their small biomass relative to the total mass of organisms living in the upper ocean, their rapid, highly efficient sinking makes them a globally significant source of organic carbon for deep-ocean ecosystems, with 43-48% of their upper ocean production reaching 2000 m, which translates into 0.016 Pg C yr^-1.

Figure 2. Mass deposition event of jellyfish at 3500 m in the Arabian Sea (Billett et al. 2006).

Sediment trap data have suggested that carbon transport associated with large, episodic gelatinous blooms in localized open ocean and continental shelf regions could often exceed phytodetrital sources, in particular instances. These mass deposition events and their contributions to deep carbon export must be taken into account in models to better characterize marine ecosystems and reduce uncertainties in our understanding of the ocean’s role in the global carbon cycle.

Links:

Jellyfish Database Initiative http://jedi.nceas.ucsb.edu, http://jedi.nceas.ucsb.edu-dmo.org/dataset/526852 )

Authors:
Mario Lebrato (Christian‐Albrechts‐University Kiel and Bazaruto Center for Scientific Studies, Mozambique)
Markus Pahlow (GEOMAR)
Jessica R. Frost (South Florida Water Management District)
Marie Küter (Christian‐Albrechts‐University Kiel)
Pedro de Jesus Mendes (Marine and Environmental Scientific and Technological Solutions, Germany)
Juan‐Carlos Molinero (GEOMAR)
Andreas Oschlies (GEOMAR)

Chasing Sargassum in the Atlantic Ocean

Posted by mmaheigan

· Wednesday, March 25th, 2020

The pelagic brown alga Sargassum forms a habitat that hosts a rich diversity of life, including other algae, crustaceans, fish, turtles, and birds in both the Gulf of Mexico and the area of the Atlantic Ocean known as the Sargasso Sea. However, high abundances of Sargassum have been appearing in the tropical Atlantic, in some cases 3,000 miles away from the Sargasso Sea. This is a new phenomenon. Nearly every year since 2011, thick mats of Sargassum have blanketed the coastlines of many countries in tropical Africa and the Americas. When masses of Sargassum wash ashore, the seaweed rots, attracts insects, and repels beachgoers, with adverse ecological and socioeconomic effects. A new study in Progress in Oceanography sheds light on the mystery.

Figure 1. The hypothesized route of Sargasso Sea Sargassum to the tropical Atlantic and the Caribbean Sea. The solid black lines indicate the climatological surface flow, the dashed black lines indicate areas where there was variability from the average conditions.

The authors analyzed reams of satellite data and used computer models of the Earth’s winds and ocean currents to try to understand why these large mats started to arrive in coastal areas in 2011. A strengthening and southward shift of the westerlies in the winter of 2009-2010 caused ocean currents to move the Sargassum toward the Iberian Peninsula, then southward in the Canary Current along Africa, where it entered the tropics by the middle of 2010 (Figure 1). The tropical Atlantic provided ample sunlight, warmer sea temperatures, and nutrients for the algae to flourish. In 2011, Sargassum spread across the entire tropical Atlantic in a massive belt north of the Equator, along the Intertropical Convergence Zone (ITCZ), and these blooms have appeared nearly every year since. Utilizing international oceanographic studies done in the Atlantic since the 1960s, and multiple satellite sensors combined with Sargassum distribution patterns, the authors discovered that the trade winds aggregate the Sargassum under the ITCZ and mix the water deep enough to bring new nutrients to the surface and sustain the bloom.

Improved understanding and predictive capacity of Sargassum bloom occurrence will help us better constrain and quantify its impacts on our ecosystems, which can inform management of valuable fisheries and protected species.

Authors:
Elizabeth Johns (NOAA AMOL)
Rick Lumpkin (NOAA AMOL)
Nathan Putman (LGL Ecological Research Associates)
Ryan Smith (NOAA AMOL)
Frank Muller-Karger (University of South Florida)
Digna Rueda-Roa (University of South Florida)
Chuanmin Hu (University of South Florida)
Mengqiu Wang (University of South Florida)
Maureen Brooks (University of Maryland Center for Environmental Science)
Lewis Gramer (NOAA AMOL and University of Miami)
Francisco Werner (NOAA Fisheries)

Hurricane-driven surge of labile carbon into the deep North Atlantic Ocean

Posted by mmaheigan

· Thursday, February 27th, 2020

Tropical cyclones (hurricanes and typhoons) are the most extreme episodic weather event affecting subtropical and temperate oceans. Hurricanes generate intense surface cooling and vertical mixing in the upper ocean, resulting in nutrient upwelling into the photic zone and episodic phytoplankton blooms. However, their influence on the deep ocean is unknown.

Figure 1. (a) Particulate organic carbon (POC) flux and percentage of the total mass flux (yellow) (top panel); fluxes (middle panel) and POC-normalized concentrations (bottom panel) of diagnostic lipid biomarkers for phytoplankton-derived and labile material, zooplankton, bacteria, and other (see legend); (b) Lipid concentrations (left panel) and POC-normalized concentrations (right panel) of diagnostic lipid biomarkers for the same sources as in (a) (see legend) measured two weeks after Nicole’s passage (25-29 Oct. 2016). Shown for reference are total lipid concentration profiles in April 2015 (dark gray, typical post spring bloom conditions) and Nov 2015 (light gray, typical minimum production period).

In October 2016, Category 3 Hurricane Nicole passed over the Bermuda time-series site (Oceanic Flux Program (OFP) and Bermuda Atlantic Time-Series site (BATS)) in the oligotrophic NW Atlantic Ocean. In a recent study published in Geophysical Research Letters, authors synthesized multidisciplinary data from hydrographic and phytoplankton measurements and lipid composition of sinking and suspended particles collected from OFP and BATS, respectively, after Hurricane Nicole in 2016. After the hurricane passed, particulate fluxes of lipids diagnostic of fresh phytodetritus, zooplankton, and microbial biomass increased by 30-300% at 1500 m depth and 30-800% at 3200 m depth (Figure 1a). In addition, mesopelagic suspended particles were enriched in phytodetrital material, as well as zooplankton- and bacteria-sourced lipids (Figure 1b), indicating particle disaggregation and a deep-water ecosystem response.

These results suggest that carbon export and biogeochemical cycles may be impacted by climate-induced changes in hurricane frequency, intensity, and tracks, and, underscore the sensitivity of deep ocean ecosystems to climate perturbations.

Authors:
Rut Pedrosa-Pamies (Marine Biological Laboratory)
Maureen H. Conte (Bermuda Institute of Ocean Science and Marine Biological Laboratory)
JC Weber (Marine Biological Laboratory)
Rodney Johnson (Bermuda Institute of Ocean Science)

Upwelled hydrothermal Fe stimulates massive phytoplankton blooms in the Southern Ocean

Posted by mmaheigan

· Tuesday, July 9th, 2019

Joint feature with GEOTRACES

Figure 1a: Southern Ocean phytoplankton blooms showing distribution, biomass (circle size) and type (color key).

In a recent study, Ardyna et al combined observations of profiling floats with historical trace element data and satellite altimetry and ocean color data from the Southern Ocean to reveal that dissolved iron of hydrothermal origin can be upwelled to the surface. Furthermore, the activity of deep hydrothermal sources can influence upper ocean biogeochemical cycles of the Southern Ocean, and in particular stimulate the biological carbon pump.

Authors:
Mathieu Ardyna
Léo Lacour
Sara Sergi
Francesco d’Ovidio
Jean-Baptiste Sallée
Mathieu Rembauville
Stéphane Blain
Alessandro Tagliabue
Reiner Schlitzer
Catherine Jeandel
Kevin Robert Arrigo
Hervé Claustre

Dramatic Increase in Chlorophyll-a Concentrations in Response to Spring Asian Dust Events in the Western North Pacific

Posted by mmaheigan

· Tuesday, October 23rd, 2018

According to Martin’s iron hypothesis, input of aeolian dust into the ocean environment temporarily relieves iron limitation that suppresses primary productivity. Asian dust events that originate in the Taklimakan and Gobi Deserts occur primarily in the spring and represent the second largest global source of dust to the oceans. The western North Pacific, where productivity is co-limited by nitrogen and iron, is located directly downwind of these source regions and is therefore an ideal location for determining the response of open water primary productivity to these dust input events.

Figure 1. Daily aerosol index values (black squares) and chlorophyll-a concentrations (mg m-3, circles) during the spring (a) 2010 (weak dust event), (b) 1998 (strong dust event) in the western North Pacific. Color scale represents difference between mixed layer depth (MLD) and isolume depth (Z0.054) that indicates conditions for typical spring blooms; water column structures of MLD and isolume were identical in the spring of 1998 and 2010. Dramatic increases in chlorophyll-a (pink shading, maximum of 5.3 mg m-3) occurred in spring 1998 with a lag time of ~10 days after the strong dust event (aerosol index >2.5) on approximately April 20 compared to constant chlorophyll-a values (<2 mg m-3) in the spring of 2010.

A recent study in Geophysical Research Letters included an analysis of the spatial dynamics of spring Asian dust events, from the source regions to the western North Pacific, and their impacts on ocean primary productivity from 1998 to 2014 (except for 2002–2004) using long-term satellite observations (daily aerosol index data and chlorophyll-a). Geographical aerosol index distributions revealed three different transport pathways supported by the westerly wind system: 1) Dust moving predominantly over the Siberian continent (>50°N); 2) Dust passing across the northern East/Japan Sea (40°N‒50°N); and 3) Dust moving over the entire East/Japan Sea (35°N‒55°N). The authors observed that strong dust events could increase ocean primary productivity by more than 70% (>2-fold increase in chlorophyll-a concentrations, Figure 1) compared to weak/non-dust conditions. This result suggests that spring Asian dust events, though episodic, may play a significant role in driving the biological pump, thus sequestering atmospheric CO₂ in the western North Pacific.

Another recent study reported that anthropogenic nitrogen deposition in the western North Pacific has significantly increased over the last three decades (i.e. relieving nitrogen limitation), whereas this study indicated a recent decreasing trend in the frequency of spring Asian dust events (i.e. enhancing iron limitation). Further investigation is required to fully understand the effects of contrasting behavior of iron (i.e., decreasing trend) and nitrogen (i.e., increasing trend) inputs on the ocean primary productivity in the western North Pacific, paying attention on how the marine ecosystem and biogeochemistry will respond to the changes.

Authors:
Joo-Eun Yoon (Incheon National University)
Il-Nam Kim (Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)

When marine-terminating glaciers ‘pump’ the ocean

Posted by mmaheigan

· Wednesday, October 10th, 2018

How will increasing meltwater from Greenland affect the biogeochemistry of the ocean? Release of meltwater into the ocean has physical and biogeochemical effects on the local water column. With respect to nutrient availability, meltwater supplies the bioessential nutrients iron and silicic acid but is deficient in nitrate and phosphate. However, despite very low meltwater nitrate and phosphate concentrations, pronounced summertime phytoplankton blooms are observed in many, though not all, of Greenland’s large fjord systems. These unusual summertime blooms are associated with meltwater from marine-terminating glaciers. So if the meltwater itself is not supplying nitrate and phosphate that these blooms require, what is the source of the nutrients that support these blooms?

An illustration of how changing the depth of a glacier affects downstream productivity

A recent study published in Nature Communications shows that when meltwater is released below sea level under large marine-terminating glaciers, it rises rapidly towards the surface in buoyant discharge plumes. As these plumes rise, they entrain large quantities of deep, nutrient-rich seawater. This vertical transport, or ‘pumping’, of these nutrients to the surface sustains unusually high summertime productivity in Greenland’s fjords. Conversely, when meltwater is released at the ocean surface, primary production is reduced because the meltwater itself lacks the nitrate and phosphate required to fuel phytoplankton blooms. Consequently, the inland retreat of Greenland’s large marine-terminating glaciers is ultimately bad news for summertime marine phytoplankton communities. As the depth of the marine-terminating glaciers shoals, their associated nutrient ‘pumps’ collapse, which will likely have negative effects on primary production and associated inshore fisheries.

Authors:
M.J. Hopwood (GEOMAR)
D. Carroll (Jet Propulsion Laboratory)
T.J. Browning (GEOMAR)
L. Meire (Royal Netherlands Institute for Sea Research and Greenland Climate Research Centre)
J. Mortensen (Greenland Climate Research Centre)
S. Krisch (GEOMAR)
E.P. Achterberg (GEOMAR)

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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