Recently there has been evidence of a shift in the distribution of some species. Look at the maps below. We can see that in 1960- 69 Calanus finmarchicus had quite an extensive range in the North Atlantic and the North Sea. The map showing distribution for 1990-99 shows that Calanus finmarchicus is moving out of the North Sea and is being replaced by its cousin Calanus helgolandicus. Could this be evidence for global warming?

Marine Food Chains
Phytoplankton photosynthesise like terrestrial plants. They absorb carbon dioxide from the atmosphere and release oxygen in vast quantities. Plankton are at the start of almost all oceanic food chains. Phytoplankton are grazed upon by zooplankton, which in turn are food for fish, basking shark, shellfish and whales. This means that phytoplankton are primary producers and the zooplankton are primary consumers.

Each link in the food chain represents a trophic level, which helps us determine the energy flow and the biomass. Food chains make up larger more complex food webs which are linked to food webs in other habitats. Food chains are rarely made up of more than 5 trophic levels becasue there is so little energy at the top of a food chain. The shorter a food chain the more energy efficentity is.

Primary production in the Marine Environment

Primary productivity is often used when describing the process of photosynthesis. However the two are subtly different. Photosynthesis is the chemical process, whereas primary production is the resulting amount of organic material. The rate of photosynthesis affects the amount of primary production.

Factors affecting primary production

LIGHT Photosynthesis can only occur if the light intensity reaching the autotrophic cell is adequate. This means that light is a limiting factor in primary production. We already know that phytoplankton can only survive in the photic zone, where light penetrates the sea, and as you go deeper and deeper the light intensity decreases, until no photosynthesis can take place there. See diagram

NUTRIENTS
These are important to all living organisms and even green plants/ autotrophs have to start somewhere. Phytoplankton like terrestrial plants require the inorganic compounds nitrogen (as nitrate or ammonium) and phosphorus (as phosphate). These are essential ingredients for phytoplankton growth and reproduction. Diatoms and silicoflagellates also require silica (as silicate) in large amounts to make their cell walls.
Seawater contains very little of these compounds and as a result nutrients are by far the most limiting factor for photosynthesis.
Phytoplankton rely on the physical and chemical composition of the sea at various times of the year to trigger their production.
For more information see Calendar of Phytoplankton Growth

Thermoclines in temperate seas

Thermoclines are areas or depths in the sea where the temperature of the water suddenly decreases. This stratifies the water column into warmer surface waters and colder deeper waters. A thermocline usually develops in the summer months in temperate seas and it influences the phytoplankton calendar of growth.
In the winter months storms and strong winds churn the sea and mix deep cold water with warmer surface water. This brings nutrients to the surface and increases the availability in the water column. In the winter months it is light intensity that limits phytoplankton growth.
In the spring and early summer, the days grow longer and weather conditions improve. With no mixing of the water column, phytoplankton take advantage of the favorable conditions and the nutrients available. This causes a serge in their growth and spring ‘blooms’ are common.
During the summer a thermocline develops separating cold and warm water. After the first bloom of phytoplankton, nutrients start to be depleted and the diatoms are replaced by dinoflagellates.
In the autumn strong winds and shorter days limit phytoplankton growth. Winter sets in, churning the seas and breaking up the thermocline. Nutrients are brought to the surface ready for the following spring.

Productivity of the Oceans

As a direct result of the limitations of phytoplankton to nutrients (and less so to light), there are distinct areas of the worlds oceans which are more productive than others. These areas include Arctic sea, shallow shelf seas around Europe, Labrador sea, Gulf of Mexico and sea around Alaska and Canada. The image below shows the worlds chlorophyll levels.

Image source: www.nodc.noaa/gov/OC5/WOAF/chsearch.html

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