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Primary producer dynamics and deep convective overturn in the Mediterranean Sea

Atualizado: 27 de jan. de 2021

A 3D high resolution 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).

About 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.

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