Factors That Affect Primary Phytoplankton Production

‭FACTORS THAT AFFECT PRIMARY PHYTOPLANKTON PRODUCTION‬ ‭•‬ ‭Light‬ ‭It is the promo factor that we analyze that influences phytoplankton PP. The conditions that lead to‬ ‭a variation in the availability of light energy for photosynthetic organisms concern the amount of‬ ‭incident light at surface level, the inclination of the incident ray, the extent of reflection and‬ ‭effective absorption in the mass of water. Variations will occur in depth penetration and in the‬ ‭quality of the transmitted radiation based on the optical characteristics of the water mass. Factors‬ ‭external and internal to the water mass that determine variations in the quantity of light radiation‬ ‭available for phytoplankton act on a time scale, therefore on a daily and seasonal level, and on a‬ ‭geographical scale (with variations in latitude). We can make some considerations on the‬ ‭interactions between phytoplankton and light. In the photic zone the rate of photosynthesis of‬ ‭phytoplankton depends on the amount of light present, in general the rate of photosynthesis is‬ ‭high at intermediate levels of brightness but decreases both if the light intensity increases and‬ ‭decreases. High light intensities can have an inhibitory effect on photosynthesis or simply the lack‬ ‭of increase in the photosynthetic rate can be due to saturation of the photosynthetic apparatus‬ ‭beyond a certain light intensity value. As we have seen at different depths the light present is‬ ‭different not only from a quantitative point of view but also qualitatively. Organisms can be‬ ‭adapted thanks to the presence of particular pigments to use the light available at the depth at‬ ‭which they live. This type of phenomenon is called chromatic adaptation. For this reason, algae‬ ‭that live at low light intensities can present high values in the quantity of pigment/cell volume ratio.‬ ‭On the contrary, organisms that live in waters very close to the surface and therefore subject to‬ ‭strong radiation may have protective pigments that prevent the onset of damage to DNA and‬ ‭chloroplasts (especially from UV rays). An example of adaptation in this sense has been observed‬ ‭in some Antarctic benthic cyanophyceae, these algae grow forming a kind of felt and a layer of‬ ‭cells rich in red carotenoids is typically formed above the photosynthetically active ones. Different‬ ‭algal species present different light optima (explains the different distribution in the column:‬ ‭sciophilous species deeper down and photophilous species closer to the surface). Differences in‬ ‭light preferences explain the seasonal succession of different algal taxa. The preference for‬ ‭different intensities and wavelengths of light is accompanied by the presence of differences in‬ ‭pigments.‬ ‭In this slide you can see the absorption spectrum of chlorophyll, common to all plant cells. The two‬ ‭absorption peaks are at 430 nm (in the violet) and at 665 nm (in the red). The wavelengths of light‬ ‭corresponding to the two peaks of maximum absorption of chlorophyll a are extinguished at levels sufficient‬ ‭surface in the water column and this becomes an essential aspect that leads to the need for phytoplanktons‬ ‭to equip themselves with accessory pigments that allow them to enhance the absorption of chlorophyll by‬ ‭efficiently capturing other wavelengths that penetrate deeper.‬ ‭Here in the table is a list of pigments found in algae. In diatoms,‬ ‭primesiophyceans and chrysophyceans, fucoxanthin is present,‬ ‭while peridinin is present in dinoflagellates (absorption peaks‬ ‭around 500 nm or in the blue- green radiation which is more‬ ‭penetrating in the sea).‬ ‭Here we can better see how each taxon can be described through a specific absorption spectrum that‬ ‭expresses the relationship between the wavelength of light and the light that is absorbed by that particular‬ ‭algal taxon. Similarly, we could construct action spectra that correlate the amount of carbon fixed in‬ ‭photosynthesis and the wavelength.‬ ‭This image draws attention to the relationship between‬ ‭photosynthesis and light expressed on the basis of different‬ ‭latitudes. In the Northern Hemisphere at low latitudes the light‬ ‭is always abundant, as the latitude increases the seasonal‬ ‭effect is increasingly affected and productivity shows very‬ ‭marked variations going from summer to autumn and then to‬ ‭winter.‬