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Science Themes

Euphotic zone

This is the upper ocean, from the surface to the twighlight zone about 1000m deep. In the euphotic zone the phytoplankton use sunlight and carbon dioxide to produce energy through photosynthesis. Phytoplankton growth is limited by macronutrients: nitrate, silicate and phosphate and micronutrients such as iron. These nutrients are mixed to the surface in the winter. In the spring when sunlight increases and the water column is stable the phytoplankton 'bloom'. The 'bloom' tends to happen later in the spring as we head northwards due to latitudinal reductions in light intensity. Dissolved gases such as oxygen and carbon dioxide cycle between the ocean surface and the atmosphere. A knowledge of these processes is crucial in understanding how the ocean helps to regulate our climate.

Ocean interior

The twighlight zone marks the transition from a region that receives sunlight during the daytime, to a region that remains in perpetual darkness. In the open ocean it is below 1000m and encompasses the range of winter mixed layer depths in most parts of the oceans. This transition zone is of great ecological significance and it is the region where not only does the light decrease but so too does the abundance of organisms and their total biomass. The fauna living here are poorly described as are the rates of biological and biogeochemical processes. The downward flux of particulate matter decreases greatly with depth. Although there is considerable uncertainty about the magnitude of the supply of material from above, it is likely that between 70 and 90% of the material entering it is remineralised or dissolved within the twighlight zone. Warm surface waters cool in the Atlantic as they move northwards, releasing heat into the atmosphere and eventually they sink. This is the North Atlantic Drift, an important driver of our climate. Deep water masses around the globe, can be distinguished from their temperature, salinity, dissolved gas and nutrient content.

Seafloor

The abyssal seafloor reaches depths of over 5000m in the Atlantic. Many of the species found there feed on phytodetritus, which is the seasonal deposition of dead organic matter on the seabed initially derived from photosynthesis in surface waters. Recent work has shown that different species feed on different types of phytodetritus. Qualitative changes in the downward flux of organic matter to the seafloor may stimulate radical changes in the structure of seabed animal communities. Climate variability in surface waters effects deep-sea communities, even at a depth of 3 miles!