A tour through the ocean: understanding the comings and goings of humpback whales

By Daniela Abras

It is immensely challenging to try to understand the mechanisms that move a 15 meter-long and 40 ton organism 9,000 km yearly.

Humpback whales migrate every year from the feeding grounds of Antarctica to the mating grounds of Brazil. The route, which is about 4,500 km each way, is made twice a year and typically takes about 2 months going, and 2 months coming back. By including their 4 month stay in Brazil mating, these whales spend 8 months of the year without food. That’s a long fast! To accomplish this feat, they need to eat a lot during the 4 months in Antarctica, and they need to stock up on energy reserves, in the form of body fat.

Map that shows the migratory corridor of the humpback whales between the feeding
area in Antarctica, and the main reproductive area on Abrolhos Bank.

But what do these whales eat? As the adorable Dory, from Disney/Pixar’s Finding Nemo would say, whales don't eat fish, they eat krill. Krill are small crustaceans, similar to shrimp, that are about 5cm long and live in giant clusters (swarms). Krill are the base of the vertebrate food chain in Antarctica, where most species depend on it, directly or not. Many species of fish, seals, penguins, and whales prey almost exclusively on it. Some species, like Orca whales and Leopard seals, prey on fish or penguins. This is why the food chain in Antarctica has been called by scientists “krill-dependent.”

Krill (Euphausia superba), the main food of Humpback whales
in Antarctica, live in large swarms.

Every year, whales arrive at the Brazilian coast in July and stay there until November. There are times when the population arrives slightly earlier in the year and stay longer, but they can also come later in the season and leave more quickly. In some years, there are more whales than in others. This started to raise some questions: When they stay in Abrolhos longer, is it because they fed better? When they leave the bank earlier than average, is it because of high water temperatures? Or do these things not influence their behavior at all, and they rely mostly on genetic programming? What initiates the migration process?

My Master's research focused on these questions to try to understand the diverse environmental mechanisms influencing the migratory dynamics of humpback whales. I primarily focused on the availability of their main source of energy. To do that, I analyzed parameters such as photoperiod, water temperature in both Abrolhos and in Scotia Sea (where they stay in Antarctica), and the availability of krill during summer. I compared this to 7 years of sighting data collected at a fixed location around the Abrolhos Archipelago. To observe the whales, a piece of topography equipment with 30X zoom, called a theodolite, was used. For the 5 months the whales were in Abrolhos, we observed the whales daily, and found that the population's abundance fluctuates throughout the reproductive season with a gradual increase in July, followed by the peak in August/September, and then a gradual decrease, until no more whales were present by the end of November.

Watching whales with the theodolite, 

from Abrolhos Archipelago.

The results were more than expected. In years when there were more krill available, the whales fed more and had greater energy stores. This allowed them to invest a longer period of time on reproduction and more whales were seen in Abrolhos. The opposite was also true. In years with less krill, fewer whales were seen in Abrolhos and their time at Abrolhos was shortened. The water temperature didn't seem to have significant influence on their migration, however it assisted in indicating the starting moment for the migration – the migratory timing.

The most surprising result was related to the photoperiod (length of daylight in a day). No other research had related the migratory dynamics with photoperiod, perhaps because scientists thought it was too obvious. But, sometimes, it's important to understand the obvious! The photoperiod in Antarctica has a huge difference between summer (18 hours of light) and winter (6 hours), while in Abrolhos, the difference from summer (13 h) and winter (11h) is far smaller.

Therefore, as my dissertation's conclusion, I discovered that the humpback whale's migration starts and is influenced by the sharp lowering of photoperiod when they are in Antarctica. When in Abrolhos, migration is impacted by the sum of 3 factors: the photoperiod (which is more steady than in Antarctica), the sea surface temperature (this slightly increases gradually during the reproductive season) and krill availability while in Antarctica.

It was difficult to analyze such a high volume of data, linking different environmental parameters in order to answer all of my research questions. With these results, we have started to understand complex migratory dynamics and the importance of krill in the maintenance of the humpback's population.

If you want to know more about my Master's dissertation, contact me via email at daniabras@gmail.com

The humpback whale population was almost driven to extinction in the early 20^th century from intensive commercial hunting. Before commercial whaling, the estimated population was around 25,000 individuals, but it dropped to about 800 individuals while at the peak of whaling. After the whale-hunting moratorium in 1986, the population recovered and is now around 15,000 individuals today! In 2015, humpback whales were officially removed from the endangered species list in Brazil. This is a victory for the whales as well as for those of us that have the privilege of watching them arrive annually, in bigger numbers every time, performing their aquatic ballet. Go meet them! Between July and November, they are concentrated on the Abrolhos region, but they can also be seen from the states of Rio Grande do Norte state to Rio de Janeiro.

Want to know more about humpback whales? Visit the Brazilian Humpback Whale Institute website: www.baleiajubarte.org.br

Humpback whale jumping in Abrolhos region.

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Daniela Abras is from Belo Horizonte, has a bachelor’s degree in Marine Biology from UFRJ, and has a Masters degree in Oceanography from USP. She has loved cetaceans since she was 8 years old, when she did a school project about them. When she was a teenager, she would say that she wanted to work with whales, but was never taken seriously. In the early 90s, she heard the famous National Geographic “Whale Songs” vinyl record and discovered the “Save the whales” project. From all of this obstinacy, her dream to study and protect whales came to life. She is now a researcher for the Brazilian Humpback Whale Institute, dedicating herself daily to studying these magnificent animals.

When to add children to the academic timeline?

By Jana M. del Favero

When starting a research project, it is necessary to establish a project timeline in which all of the activities to be carried out are mapped out to keep on schedule. My question, one I know other women ask as well, is where and how to fit a pregnancy in the academic timeline?

During undergrad, we're too young and have the whole world ahead of us; during a masters', time is short, we have approximately two years in which it is impossible to think of things other than classes and the thesis. Then comes the doctorate. We're more mature, some are already married, but we still only think of research and publications – we know that after the four years of the doctorate we'll face the competition for jobs or need to be able to engage in a postdoctoral position. Therefore, the best option would be to wait for all of that to end, and decide to get pregnant after getting hired, with some professional, financial, and personal stability guaranteed. That stability generally occurs when a woman is around 37 years old, though, long after her fertility peak (Figure 1).

Figure 1. The age at which a scientist builds her career occurs at the same time of peak fertility

(measured by the number of ovarian follicles). Source: Willians & Ceci (2012).

Although it is not difficult to name successful female researchers/professors with kids, the “graduate students that gave up their academic career after getting pregnant” scenario is far more common. As Figure 2 shows, the dropout percentage amongst post-doctorate researchers with no kids or plans to have kids is practically the same for both men and women. However, having a child after starting a post-doc doubles the dropout rate among women, but has no effect for men.

Figure 2. Influence of children and plans to have children in the postdoctoral

careers of men and women. Source: Williams & Ceci (2012).

Of course a child can alter a woman's life path, and also her academic productivity. Leslie (2007) shows that the more children a woman has, the less time she spends on professional activities (Figure 3). Shockingly (although the research dialog doesn't discuss reasons), the same study shows that the effect is reversed in men: more kids equals more working hours! I won't dare to go further in discussing causes for this difference, but I see two possibilities: a man sees it as more responsibility and, seeing himself as the family provider, works more (this is not necessarily his fault, the systemic tradition imposes and teaches women to take care of their home and men to provide for it); or they run from the domestic responsibilities for whatever reasons. A friend told me that when his child was a baby and required all mom's all attention and care, he would prefer to work late to avoid getting home before the baby was asleep, justifying himself by saying he was jealous of all the care his spouse had for the baby and felt he didn’t fit in his own home.

Figure 3. The number of hours worked weekly for men and women compared to the number of dependent children. Source: Leslie (2007).

One way to enhance female representation in universities and to reduce the academic dropouts is to focus on the problems mothers face to take care of a family while studying and researching. Williams and Ceci (2012) made a list of strategies that could be adopted to minimize problems and help families. As an example: universities could offer quality childcare, offer maternity leave for the primary care giver, regardless of sex; they could also instruct selection committees how to ignore curriculum gaps that happened while one was using more time to take care of the family (as an example, the committee would understand why someone didn't publish for some time if that time was used to take care of a newborn), and so on. What is not on the study's list, and what I consider extremely important, is a structural change in people's minds. I heard once that, to be accepted in a certain lab in Spain, the professor in charge would ask women to sign a form, agreeing not to get pregnant during the doctoral program. It's painfully hard to believe many minds still work like that!

And back to Brazil, where are we? USP, one of the largest universities in Brazil, has a childcare center that is praised by the parents, but just got at least 117 spots suspended for lack of funds invested in them (read more here). Not all funding agencies provide paid maternity leave for those with grants. Some progress can be seen, but many setbacks are still noticed. Even though some universities have adopted measures to help families' lives, a lot still need to be done.

I'll not be able to give an answer to the question I posed in the text's title here, mostly because I believe it's a personal decision and not just a cake recipe. Personally, I've been married for 3 years and will finish my PhD in the middle of 2016, with no intentions of expanding the family by then.

Nonetheless, I will not end the post with this matter. The blog will have testimonials of “women who are warriors,” that managed to study and have children; “altruistic women,” that gave up their academic career to dedicate themselves fully to their family and feel good about it; “scrappy women,” that stepped out of the university for a while to take care of kids and suffered many obstacles to get back in. My testimonial of an “indecisive woman” you already have.

What about you, have something to share? We welcome you to comment or contact us.

References

Goulden, M.; Frasch, K.; Mason, M. 2009. Staying competitive: Patching America’s leaky pipeline in the sciences. Center for American Progress,

https://www.americanprogress.org/issues/technology/report/2009/11/10/6979/staying-competitive/

Leslie, D.W. 2007. The reshaping of America’s academic workforce. Research Dialogue 87. https://www.tiaa-crefinstitute.org/public/pdf/institute/research/dialogue/87.pdf

Willians, W.M.; Ceci, S.J. 2012. When Scientists Choose Motherhood. American Scientist, Volume 100. http://www.americanscientist.org/issues/pub/when-scientists-choose-motherhood

Ocean fertilization and climate change

By Yonara Garcia

Have you heard of geoengineering? It’s a tool becoming increasingly used, but is often controversial because, in some cases, the result can be completely unexpected!

Today we’ll talk about a polemic experiment carried out in July 2012 by Russ George, an American businessman who dumped approximately 100 tons of iron sulphate in the Pacific Ocean as part of a geoengineering project off the west coast of Canada (http://www.nature.com/news/ocean-fertilization-project-off-canada-sparks-furore-1.11631).

Ocean fertilization by iron sulfate. Source: http://officerofthewatch.com/2012/11/05/canada-iron-fertilization-incident/

Iron is considered an essential element, often limiting, for phytoplankton growth. Phytoplankton perform photosynthesis, a process in which sunlight is used as an energy source and absorbs carbon dioxide (CO2) and water to produce organic matter in the form of carbohydrates. Phytoplankton cells are formed from these carbohydrates with the addition of other substances such as proteins, amino acids, and other molecules.

In 1980, oceanographer John Martin proposed that certain regions of the ocean (the areas called HNLC - High Nutrient, Low Chlorophyll), although rich in nutrients, would be poor in primary production due to lack of iron. Thus, the addition of iron should increase the production of phytoplankton and hence affect the carbon cycle, reducing CO2 levels in the atmosphere. His famous phrase “Give me half a tanker full of iron and I’ll give you an Ice Age” caused great excitement because he believed that if certain areas of the ocean were fertilized, the effects of global warming could be reversed, cooling the Earth.

Thus arose the idea that the American businessman put into practice. Russ and his team released a certain amount of iron into the sea, believing it would promote photosynthetic activity and thus increase the efficiency of the carbon sequestration processes in the ocean. Just like the process to fertilize a crop for it to go grow faster! This issue has generated much controversy because it conflicts with ethical and political questions about the effects that an intervention like this would bring to a complex ecosystem. We still know relatively little about the ocean. To better understand why the idea of this project is so controversial, let’s first talk about some important processes in the “wonderful world ocean.”

Have you ever heard of “physical pump”? Or a “biological pump”? No, it’s not a kind of weapon of war to decimate an enemy population. The physical pump is the process related to the solubility of CO2 in the ocean (solubility = maximum amount of a substance that can be dissolved in a liquid). The biological pump takes into account what happens to the CO2 after it is dissolved in the ocean, when a fraction of dissolved carbon is absorbed through photosynthesis, in the surface layers of the ocean, and transported to the bottom. The diagram below explains how carbon is transported in the ocean.

Carbon movement in the ocean system. 1) Using solar energy, carbon dioxide is fixed by phytoplankton in the photic zone (where there is light). 2) Part of this organic matter is consumed by zooplankton and some heterotrophic microorganisms. 3) Other organic matter is exported from the photic zone toward the mesopelagic zone (about 1000 m deep), and a fraction of this organic matter is remineralized while the rest goes to the bottom of the ocean, where it will take thousands of years to return to the surface. Adapted from United States Joint Global Ocean Flux Study.

CO2 is a gas capable of dissolving in the surface of the oceans. This solubility mechanism is related to the concentration of this gas in the atmosphere and the water temperature: the more CO2 in the atmosphere and the lower the temperature, the greater the amount of gas dissolved in the ocean surface. Once dissolved in water, the CO2 passes to a further stage of the cycle, where it can be absorbed by photosynthetic marine organisms.

Part of the organic matter formed during photosynthesis is used in cellular respiration and released back into the seawater as CO2. The other fraction, which was used in the formation of the cell, is consumed by zooplankton (primary consumers in marine food webs - read more here) and/or transported by gravity to the bottom of the ocean through  “marine snow,” particles made up of food debris and fecal pellets coming from feeding zooplankton, shells, and dead microorganisms. This carbon transfer process to the deep ocean decreases the amount of carbon in the photic zone (zone that receives enough sunlight for photosynthesis to occur), sequestering (removing) billions of tons of carbon from the atmosphere each year. Some studies have estimated that the biological pump is responsible for removing about 5-15 gigatons of carbon per year (Henson et al., 2011).

Marine Phytoplankton. Source: http://www.smithsonianmag.com/science-nature/vanishing-marine-algae-can-be-monitored-from-a-boat-with-your-smartphone-2785190/?no-ist

You can probably imagine how important this removal is when looking at the large amount of carbon that our industrial activities, cars, and planes have emitted into the atmosphere over the last few years. It is important to remember that the much discussed global warming, among other issues, is largely caused by an excess of carbon in the atmosphere. According to the IPCC (Intergovernmental Panel on Climate Change) 2014,  in 2010 alone, 49 gigatons of carbon were released into the atmosphere by human activities. And that is precisely why these experiments with iron have gained so much popularity.

Sounds simple, right? Okay, solved the problem of global warming! Let's fertilize the oceans! But it is not so simple. Interfering in natural ecosystems is an extremely sensitive subject, which can cause incalculable and irreparable damage.

Some researchers performed similar experiments as the American businessman and concluded that despite the fertilization increasing the rate of photosynthesis, it can trigger changes in ocean chemistry by changing the operation of the entire system. For example, increased photosynthetic rates by phytoplankton are directly proportional to the amount of dimethylsulfide (DMS - volatile sulfur in reduced form) secreted by these microalgae in water, which is vaporized and form condensation particles in the air (i.e. more photosynthesis by the phytoplankton = more dimethylsufide into the air). In the atmosphere, these particles facilitate the formation of clouds, which would be great, because with the increased formation of clouds there is increased reflection of solar radiation and thus greater cooling of the planet. However, not all types of clouds have the property to cool the planet. Recent studies suggest that other climatic factors may also affect the distribution and properties of clouds, which could increase the temperature of the planet. Furthermore, it was observed that fertilization also increases the production of nitrous oxide (N2O), a molecule that heats 320 times more than CO2.

Another study, published in April 2014 in Geophysical Research Letters, showed that more than 66% of the carbon sequestered by the ocean returns to the atmosphere in 100 years. That is, the biological pump may lessen the temperature of the Earth, sequestering carbon from the atmosphere, but we do not know what will happen when this carbon returns. Controversial enough for you?

Image obtained by NASA, satellite view of a phytoplankton bloom.

Thus, although the processes that occur in the ocean are responsible for reducing the concentration of CO2 in the atmosphere, altering the system may not be the best solution because there are many chemical, physical, and biological processes that are not fully understood. While we did not reach a more integrated understanding of these processes, the reduction of CO2 emissions would be much more efficient and safer than trying to remedy a problem by manipulating a process so complex and poorly understood.

Literature:

http://www.nature.com/ngeo/journal/v6/n9/full/ngeo1921.html

http://www.nature.com/nature/journal/v446/n7139/full/nature05700.html

https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch7s7-3.html

http://onlinelibrary.wiley.com/doi/10.1002/2013GL058799/full

https://www.ipcc.ch/pdf/assessment-report/ar5/syr/AR5_SYR_FINAL_SPM.pdf

Henson, S. A., R. Sanders, E. Madsen, P. J. Morris, F. Le Moigne, and G. D. Quartly (2011), A reduced estimate of the strength of the ocean's biological carbon pump, Geophysical Research Letters, 38

Diving for life in the darkness: a survey of the deep

By Camila Negrão Signori

Just being involved in a scientific expedition aboard the R/V Atlantis (managed by the prestigious Woods Hole Oceanographic Institution, WHOI) was itself an enriching experience. I was no stranger to ship research, having crossed the South Atlantic from Africa to Brazil, been to the continental shelf of the southern and southeastern coasts of Brazil, and sailed three times in the waters of the Southern Ocean surrounding the Antarctic Peninsula, but my experience on the Atlantis with the submersible Alvin was quite a different experience. 

Photo 1: Alvin being prepared for its decent, with two divers on top and a boat in the water to offer support. (Photo by Camila)

This experience was only possible by an invitation by my collaborator Dr. Stefan Sievert who had helped develop part of my PhD research with polar samples in Woods Hole (funded by CAPES-Training Coordination of Higher Education Personnel). Stefan was the scientific coordinator of this cruise with a project funded by the US National Science Foundation (NSF) entitled “Integrated Study: metabolic energy, carbon sequestration, and colonization mechanisms in chemosynthetic microbial communities in deep hydrothermal vents.” My job was to help Stefan and Jesse McNichol (my friend and doctoral student in the MIT-WHOI joint program) in all on-board tasks. 

There are many reasons this was such a different experience from my other times at sea. This was my first time in the Pacific Ocean. It was my first time aboard a ship run by a research institute, and it had a greatly reduced crew of about 25 (the other ships I have been on have been run by the Navy of Brazil, manned by 50-60). This was an international ship, with 23 researchers from countries including the United States, Canada, Germany, Italy, Spain, Japan, China, and myself from Brazil. 

Instead of navigating to different oceanographic stations (to spatially explore physical, chemical, biological, and geographical oceanographic features), we remained in the same sample area of 9 degrees N for almost an entire month. Our landscape was an expansive ocean without an end in sight, and we were a 4-5 days steam from the nearest land. The objectives of the project were all related to the deep ocean, at hydrothermal vent sites. 

Photo 2: Camila Negrão Signori observing life in the deep ocean through one of Alvin’s five portholes (Photo by Stefan Sievert).

Typically, water is collected from different depths, selected according to differences in water mass through the layers of the ocean, using a Niskin bottle, usually coupled to a CTD-rosette system. However, for this journey, we used the famous submersible Alvin, diving daily to more than 2500 m deep to collect our samples. With the help of two robotic arms and a “biological basket” able to carry more than 180 kg of bottom material, we collected samples such as fluid from the vents, microorganisms associated with the sources, invertebrate worms, and near-vent settlers. 

Instead of using water collected by Niskin bottles on board the ship, we collected fluids for chemical and microbiological analysis with a special piece of equipment known as an Isobaric Gas Tight sampler (IGTs). These IGTs were developed by WHOI to maintain pressure and environmental conditions of the deep when brought to the surface. 

Photo 3: A “IGT” sampler collecting fluid with the help of a robotic arm. The fluid here was 25C at 2500 m depth, collected at the “Crab Spa” location. Crabs, bivalves, annelids, and microbial mats can be seen here. (Photo by Camila, C WHOI).

Despite calm seas, work in the ship’s lab with the samples was not a trivial task. When removing fluids from the IGTs, we needed to be extremely careful with the high-pressure samples when opening and closing the system. Work was done with tools I had not seen before, and this was often morning and night work (after the Alvin returned to the ship). It was very difficult to draw out 150 mL of hydrothermal fluid and then continue with traditional laboratory protocols such as DNA extraction of microorganisms, gas measurement (such as Hydrogen sulfide), measurements of chemosynthesis processes, counts and cultivation of microorganisms, and incubation experiments using different temperatures and nutrient additions. 

Having the chance to dive so deep was one of my dreams (I thought impossible), but it became a reality on November 14th, 2014. 

Once the Alvin was released into the water from the giant cable it had been suspended from off of the Atlantis, we felt a slight swing in the surface waters of the Pacific. After a last check by two divers on top of the submersible and a brief goodbye and good luck wave through the portholes, we started our descent to the deep sea. 

The first 100 m of the water column were a beautiful turquoise color, but shortly after crossing the 300 m depth, everything became completely dark and quiet. As we passed the Oxygen Minimum Zone (300-800 m), bioluminescent organisms appeared floating in contrast to the black water. After a very gentle hour and a half descent (it felt like I was sitting on a sofa!), the pilot, Phil Forte, turned on the Alvin LED spotlight and a new world appeared under my eyes. 

Photo 4: Hydrothermal source with black smokers in full swing, under the sea. 2514 m deep, observed through the Alvin porthole. (Photo by Camila, C WHOI)

We landed on the seafloor, which was made up of ocean bottom ~200 million years old and some basaltic rock that shone brighter, indicating a more recent formation of a typically more active area. And so, with the help of our GPS, we began to explore the study area for six hours. After another hour, we had returned to the surface.

From all of the scientific papers, pictures, videos on the internet, and stories from those who have plunged to these hydrothermal vents in the Pacific, I expected I would find a bounty of life. But deep down, we always have that nagging in our heads…is this real, did these people actually see these things? 

And yes! We did see an abundance of life in the deep ocean: many small white crabs that justify the name “Crab Spa”; invertebrates including the annelid Tube worm, a species of giant tube worm that can reach nearly 2 meters in height with reddish color at the tips from the hemoglobin complex adapted to the sulfides present, toxic for us humans; 30 cm long, blind, albino fish swimming about resembling eels with their lack of scales (called Eel pout). We also saw yellow bivalves, small shrimp, and lobsters in the area in addition to the famous microbial mats. 

Photo 5: Dr Stefan Sievert (left), pilot Phil Forte (center), and Camila Negrao Signori (right) in Alvin’s sphere 2514 m deep. (Photo by Camila).

I must confess however, that, although a researcher of microbial oceanography, what impressed me the most was the geological structure that seemed artistically carved, surrounded by black smokers rich in metal sulfides. It was simply stunning to see this “step” in the ocean crust, where the Earth was being newly formed, and life abounded. 

How did I feel after the dive? Well, aside from my amazement at the excess of life and beauty, appreciation for the technology we have developed to explore these new frontiers, and how blessed I felt to have experienced this opportunity with such a great international group of competent people, I felt very little. As small as a drop of water in the vast ocean or a tiny bacterium shining under the microscope! We still have much to learn about the mysteries of the sea. 

Photo 6: A group of scientists embarking on the R/V Atlantis, opposite the Alvin garage. (Photo by Camila).

Dive 4769: an experience I will never forget! Sometimes when I find myself thinking about this dive, it pains me to believe that I was there at one time. I am extremely grateful to Dr. Stefan Sievert, who trusted in my work and gave me this chance to ride and learn on board the Atlantis and Alvin. I also thank all of my fellow scientists and competent crew, for sharing this experience with me and for all of the efforts and hard work put in to break into life in the dark. 

For more information, check out the links below:

Expedition blog “Dark Life” to the hydrothemal vents of the East Pacific Rise: http://web.whoi.edu/darklife/
About Woods Hole Oceanographic Institution: http://www.whoi.edu/
An overview of my research career: http://agenciasn.com.br/arquivos/3010

About Camila Negrão Signori:
Oceanographer, Master in Biological Sciences/Zoology, and PhD in Sciences/Microbiology, with periods of comings and goings to WHOI (USA). Born in Campinas (Sao Paulo), but has been enchanted by the sea since a childhood spent in Ubatuba Bay. In her spare time, she loves sports and dance, is always surrounded by family, her boyfriend, and wonderful friends. Today she is a Post Doctoral researcher at the Oceanographic Institute at Sao Paulo (USP) and a member of the microbial ecology laboratory where she researches the effects of climate change on microbial communities of the Southern Ocean. 
Contact: camisignori@hotmail.com

Primary producer dynamics and deep convective overturn in the Mediterranean Sea: a 3D high resolution modeling approach

By Fayçal Kessouri

My work focuses on modeling plankton ecosystems using a physical-biogeochemical coupled model. This kind of modeling is a 3D virtual representation of the main constituents of the lowest trophic level of a marine ecosystem. It includes plankton, bacteria, and the nutrients that support them under realistic hydrological conditions and atmospheric forcing.

The biogeochemical model I use shows the impact of current dynamics on the nutrients that support the marine plankton including nitrate, phosphate, and silicate. How are they distributed in the ocean? How are they consumed? Who consumes them?

The biogeochemical model shows only a part of the complex feedbacks between different components of the ecosystem. Some examples are as follows and are shown in figure 1 below: inorganic matter feeds the phytoplankton when certain abiotic conditions are available (enough light, stratified ocean layer), phytoplankton feed zooplankton, both of them produce organic matter. Organic matter is mineralized to inorganic matter, which then feed bacteria, which release dissolved inorganic and organic matter, and the cycle continues.

Figure 1: These biogeochemical feedbacks between compartments take place in the ocean. (POM= particulate organic matter, DOM= dissolved organic matter, DIM: dissolved inorganic matter or nutrients).

Marine plankton are the foundation of all marine life. They influence fisheries, the world economy, and world health, and they have an important role of maintaining biodiversity. Plankton are composed of:

1- The phytoplankton: which contain the largest mass of marine plants in the world. Some estimates show that marine vegetation produces more than half of the oxygen we breathe on Earth.

2- The zooplankton: which feed on phytoplankton (see also Catarina’s post). They represent the largest diurnal animal migration in the world.

3- The bacterioplankton and virioplankton: making up the largest biomass on the planet, prokaryotes and viruses are often a forgotten aspect of classic marine food webs.

The Mediterranean Sea lies between three continents (Europe, Africa and Asia) and therefore undergoes physical pressures from river discharge and atmospheric deposits of inorganic and organic matter, which has two levels of impacts: (1) overall balance of organic and inorganic matter in the whole sea, (2) eutrophication of coastal waters.

One of our most important findings using this modeling is the quantification of all imports and exports of matter during the last ten years between the Mediterranean and the surrounding environments (continents and Atlantic Ocean). We have estimated that the Mediterranean enriches the Atlantic by more than 140 X 109 moles of nitrogen every year through the Strait of Gibraltar.

The Mediterranean Sea has a common feature with the North Atlantic Ocean and the Antarctic Ocean: deep convection zones. In the Mediterranean, intense mixing is observed almost every winter for two months. Imagine a drop of water moving from the bottom of the Mediterranean at a depth of 2300 m and rising to the surface in a single day. This convective overturn from the gradients created by exchanges in surface heat and freshwater fluxes is the engine of global oceanic thermohaline circulation. This density-gradient driven circulation is estimated to be on the timescale of 70 years in the Mediterranean and 1000 years in the world ocean.

The deep-water masses contain high concentrations of nutrients, which are propagated to the surface during the deep mixing events. When the mixing stops at the end of winter, some of these nutrients are trapped in the surface waters, and a huge plankton bloom occurs over an area of 5 000 to 20 000 km2 (figure 2). Phytoplankton blooms can be so large, many can be observed and estimated from space by Satellites, and are thus well modeled. The phytoplankton bloom takes directly above the site of deep convection, which is referred to as the northern gyre of the NW Mediterranean Sea. They gyre is surrounded by strong cyclonic currents (counterclockwise in the Northern hemisphere).

Figure 2: Modeled phytoplankton concentration (chlorophyll-a [mg/m3]) in the NW Mediterranean Sea. a- Winter condition, the center of the basin is phytoplankton deplete because of winter vertical mixing. b Spring phytoplankton bloom (Kessouri et al., in prep).

Fayçal Kessouri

I am currently a postdoc in the Ocean and Atmospheric department at the University of California in Los Angeles, CA, USA, and my Ph.D. was developed at Toulouse University in France (Laboratoire d’Aerologie). My field of work is oceanic biogeochemistry and 3D modeling of plankton ecosystems, especially oceanic physical forcing. I worked on the deep convection of the Mediterranean Sea impact on the plankton ecosystem, and currently I am working on the upwelling of the California Current System and its impact on acidification and hypoxia of the western US coast. My desire to get training in numerical modeling motivated me to work with a team of physicists to acquire a more integrated vision of ecosystem functioning and impacts. It has helped me to study dynamic processes such as the deep convection that has always fascinated me. I am convinced that modeling is the perfect tool to complement the networks of observations currently being made, especially if one wants to study different time and space scales.