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Energy, Climate change, Environment

Map of the Week – Chlorophyll concentration unlocks marine information

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Monthly global surface ocean chlorophyll-a concentration

The vibrant growth of vegetation in summer has, in our sensory experiences, associated the colour green to the natural world. However, contrary to our instinctive association, the higher concentrations of the green pigment chlorophyll in sea surface water do not indicate the hot summer months. Chlorophyll concentration reveals the dynamic interplay of multiple factors, including nutrient availability, sunlight, and the delicate balance of biomass productivity. Elevated chlorophyll levels often indicate nutrient-rich waters, where microscopic single-celled aquatic plant-like organisms, known as phytoplankton , thrive. Like plants on land, phytoplankton are the primary producers of the ocean. They contain chlorophyll that captures the energy of sunlight to undergo photosynthesis . Because of the distinct green colour of the chlorophyll pigment, we can use optical satellite sensors to visualise the distribution of chlorophyll and thus the phytoplankton in the ocean.

Phytoplankton’s growth relies on temperature, nutrient levels and the availability of sunlight in the illuminated zone of the surface of the ocean. Since cold water, which upwells from below, tends to carry more nutrients from deeper ocean, chlorophyll tends to be more abundant in cold water as phytoplankton flourish [1]. The map of this week shows the average chlorophyll-a concentration (in mg per cubic meter) at the ocean surface during the last month in regions which were not covered by clouds. Provided by Copernicus, the map is derived from multiple optical satellite sensors (SeaWiFS, MODIS-Aqua, MERIS, VIIRSN and OLCI-S3A) and calculated using the Copernicus GlobColour processor (this information is available in the map layer description which is accessible directly in the map and in the Atlas overview).

By analysing chlorophyll patterns, not only can scientists study sea surface temperatures (SSTs), but they also gain insights into ocean currents  and mixing processes. For instance, chlorophyll features can be used to trace oceanographic currents [2], such as the chlorophyll concentration anomalies associated with the El Niño–Southern Oscillation (ENSO ) [3]. Chlorophyll data also support the assessment of ecosystem health, advise fisheries management, and contribute to our understanding of the Earth's broader biogeochemical processes as well as impacts by human activities. On the map, it can be seen that the phytoplankton concentrations are commonly higher along the coastal regions than offshore marine water. Such elevated levels may be explained by anthropogenic nutrient enrichment [4], leading to eutrophication, which can cause loss of biodiversity, ecosystem degradation, harmful algae blooms and oxygen deficiency in bottom waters [5]. Another clear phenomenon shown on the map is the excessive phytoplankton concentration in the Baltic Sea. This overgrowth is due to the current overabundance of nutrients, and eutrophication is further enhanced by the shallow depth and limited water exchange in the Baltic Sea [6,7]. Eutrophication is one of the main threats to the biodiversity of the Baltic Sea.

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The data in this map are provided by Copernicus.