Pasturage of the Sea
The vast ocean water is about 99%, of which usually up to 200 m depth or beyond (up to 300 m, depending on the latitudes, seasons, and water transparency) phytoplankton grow constituting “pasturage of the sea”.
Their photosynthetic activity contributes about 80% oxygen in the atmosphere and absorbs about the same amount of CO2 reducing global warming factors.
Phytoplankton productivity along the coasts and oceans
Though in the vast seas and oceans the light is plenty and almost uniform, the phyto-plankton productivity is highly variable and this is due to variable nutrient containing waters. Coastal waters are usually with high nutrients and gradually decreases towards open ocean waters and phytoplankton growth decreases accordingly [501 or more mg C/m2/day near the coast (red), to 100 or less in the centre (pink colour) of the ocean].
On an average, the productivity is high in the higher latitude, where the temperature and light intensity though are low, turn over of the ocean water results nutrient circulation causing high productivity. The central ocean water nutrients around equator are limited in the upper portion of water but high in the aphotic deeper water. As the water is stratified with lighter surface water; turn over does not occur resulting oligotrophic water, thus it shows low productivity.
Nitrate and phosphate concentration in the ocean waters is low in upper 200 m or more depth (photic zone) due to their utilization by the phytoplankton. Though the concentration is high at about 500 to 1000 m depth, the nutriens are not available due to lack of turn over in the tropical and sub-tropical waters resulting low productivity.
A Three Step Food Chain
Phytoplankton is the primary producer at the base in all types of water bodies, particularly in the oceans. They grow by taking nutrients from water and used as food by zoo-plankton (here Krills), which are called primary consumers. The zoo-plankton will be consumed by fishes or whales, called secondary consumers or top consumers (here).
- In the ocean, the water mass moves in great surface spiral patterns called Gyres. Gyres of both the hemispheres hardly cross the equator
- The gyres mix cold and warm waters and profoundly affect the climate on adjacent land mass.
- The gyres also affects the distribution of phytoplankton and seaweeds.
Corethron criophylum, Antarctic species can be found up to 4º South latitude in the Peru Current.
Virus and bacteria of microbial loop work on detritus pool (DOM) and release nutrients like N and P.
Without the loop, the energy in the DOM would go largely unused and deposit in the ocean floor. As much as half of the primary production in the epipelagic is channeled through the microbial loop.
- Nano- and pico- phytoplankton, protozoa, bacterioplankton and virioplankton constitute a microbial loop in food webs.
- The microbial loop in simplified form, refers to the flow of energy through the series phytoplankton → DOM → bacteria → virus → protozoans → zooplankton.
A Complex Food Chain
Phytoplankton is the primary producer (1st trophic level) in all types of water bodies, particularly in the oceans. They grow taking nutrients from water and are used as food by protozoans and zooplankton. Here, the protozoans are considered as primary (2nd trophic level) and zooplankton are secondary (3rd trophic level) consumers. The bacteria in the system decompose organic matter, release nutrients that are recycled through phytoplankton.
- Zooplankton will be consumed by small and large fishes, called tertiary consumers (4th trophic level). The tertiary consumers will be eaten by top consumers.
- The food chain will take the shape of a pyramid, in terms of number, biomass and energy, i.e. number, biomass and energy at each trophic level are high at the bottom and lower in the upper levels. The organisms at each level use their biomass as energy for their growth and some energy is lost as heat energy, thus only about 10% remain that enters in to the next level.
The energy flow here follows the two laws of thermodynamics:
- 1st law of thermodynamics: Energy can not be created nor destroyed but is transformed from one form to the other.
- 2nd law of thermodynamics: The transfer of energy is not 100% efficient, i.e. some amount of energy is released as heat energy.
The structural units/components formed in the food chain function as energy flow which altogether is called Ecosystem. The connection between organisms and non-living environment occurs through nutrient cycling and energy flow.
Penetration of Light Rays in Sea Water
As mentioned earlier light quantity in ocean water changes with depth, so does the light rays. In the open ocean waters blue light penetrates to the greatest depth, on the other hand in the turbid coastal waters the green light penetrates to the highest.
Changes in light quantity in relation to latitude(zone)/bloom/turbidity
Light quantity in ocean water changes with latitude(zone)/ bloom/turbidity.
In clear tropical open ocean waters light can penetrate up to 300 m, on the other hand in turbid coastal waters the penetration may be up to 50 m or less.
Therefore, the compensation depth (photic zone) increases as we proceed towards the open ocean water, on the other hand photic zone decreases at higher latitudes i.e. temperate (up to 200 m) to polar (up to 100 m) waters
Effects of light quantity on algal photosynthesis
Maximum photosynthesis by different groups of algae differ in different light quantity: green algae had max. at low light, diatoms require medium light while dinoflagellates need high light quantity for maximum photosynthesis or growth.
Primary productivity in seas at three latitudes round a year
It was general concept that primary productivity in a sea varies with seasons. But in tropics the productivity remains same round the year because of nutrient limitations in the euphotic zone. In the tropics the water is stratified with lighter surface water resulting no upwelling and hence no mixing of surface and deeper nutrient rich waters- low productivity.
In the temperate and polar regions however, the water is heavier at the end of the winter over which strong winds blow that cause water to move downward at 45º pushing the lower nutrient rich lighter water upward called Spring Upwelling/Turnover. In the temperate zone a Fall-upwelling also occurs, thus a large Spring and small Fall blooms are found. The summer is nutrient limited due to uptake by the bloom.
In the Polar zone upwelling occurs at the end of winter but light limited mostly. Bloom occurs in late Spring and early Summer.
Interaction of physical, chemical and biological factors on the productivity of a shallow water body or temperate oceans occurs during a year giving undulating nature of all the factors.
In the temperate zone of the world, phytoplankton growth is lowest during January when light, temperature and consumers levels were also lowest though the nutrients were at the highest concentrations.
As the light and temperature increased phytoplankton growth is accelerated using nutrients from the surrounding (showing peak during April and May). Zooplankton also show growth following the phytoplankton, grazing on them giving a peak in June.
Nutrient depletion by phytoplankton peak and grazing by zooplankton result decrease in phytoplankton biomass by late May.
Under this situation faecal pellets of zooplankton and organic debris are acted upon by the microbial loop and release nutrients.
Under reduced phytoplankton condition, the grazers move to another place for grazing resulting decreased zooplankton in the area.
Low grazers and nutrient replenishment by fall turn over cause increased phytoplanktron giving a second small peak in September.
The increased phytoplankton is again followed by second small zooplankton peak in October
Microbial loop again releases nutrients in the surrounding. This time of a year the light and temperature are low, so low phytoplankton resulting gradual accumulation of nutrients with peak in winter.
In the temperate region there is a stratification with heavier water on the top of water column and nutrient upwelling or turn over in Spring supply nutrients in the eupohotic water and the cycle continues.