Geographical And Seasonal Variations In Phytoplankton Production

‭GEOGRAPHICAL AND SEASONAL VARIATIONS IN PHYTOPLANKTON PRODUCTION‬ ‭ fter light and nutrients, another factor limiting primary phytoplankton production is represented‬ A ‭by the mixing motions that occur in the water column. Mixing may be due to wind or density‬ ‭variations and thermohaline circulation. If these motions occur above the compensation depth,‬ ‭plant cells always produce more than they consume, so phytoplankton populations will tend to‬ ‭grow in these conditions. Compensation depth is the depth at which production and respiration of‬ ‭an algal cell equalize.‬ ‭ he relationship between photosynthesis, respiration and depth at the single algal cell level can be described‬ T ‭using this graphical representation. The trend of gross primary production of a single cell shows an‬ ‭exponential decrease with increasing depth. In the graph, gross production is represented in green. However,‬ ‭breathing will always remain at constant values and the amount of breathing is displayed in gray. Then it‬ ‭follows that with increasing depth the magnitude of net primary production will progressively decrease until‬ ‭the compensation depth is reached. We defined the compensation depth as that depth at which 1% of the‬ ‭surface light intensity is present. Above the compensation depth, in what is also called the trophogenic layer,‬ ‭the gross production is always‬ ‭greater than respiration and net production is greater than zero. So every cell is capable of‬ ‭growing and reproducing. At the compensation depth level, gross production and respiration‬ ‭become equal, net production is equal to zero and therefore the cell at this level can maintain itself‬ ‭but cannot reproduce, we only maintain biomass. Below the compensation depth in what we can‬ ‭define as the tropholytic layer, respiration becomes greater than primary production and therefore‬ ‭there will be a net loss on photosynthesis: the cell consumes more than it produces. At this point,‬ ‭if the mixing motions go deeper and push the algal cells below the compensation depth, then light‬ ‭becomes a limiting factor. The algal cells will in fact find themselves circulating between the two‬ ‭zones, the superficial photic one and the underlying dysphotic one. Only in the photic zone above‬ ‭the compensation depth does photosynthesis exceed respiration and therefore a net production of‬ ‭organic substance can occur. At this point, the duration of the period spent by the algal cells in the‬ ‭unlit area becomes decisive; the duration of this period in turn depends on the intensity of the‬ ‭mixing and how deep the vertical motion extends. If the motion is not extensive in depth, the cells‬ ‭can quickly return to the illuminated area, but if the mixing reaches deeper areas, the cells can‬ ‭spend most of the time (for example in 24 hours) just below the compensation depth. and‬ ‭therefore there will be no net production but consumption will exceed production and the algal‬ ‭population will therefore tend to reduce.‬ ‭Due to the particular structure of the water column, the mixing motions may act as a limiting factor in winter,‬ ‭when the water column (as in the figure) appears unstable and homogeneous and is easily remixed. In these‬ ‭conditions, the algal cells will easily be able to reach depths below the compensation depth, despite the high‬ ‭presence of nutrients (also highlighted in the figure) throughout the water column. In winter, the reduced‬ ‭illumination at the surface level will be enhanced by the effect of the mixing motions which take the cells away‬ ‭from the light which is already scarce in the more superficial layers. Therefore light and mixing motions are‬ ‭found to act‬ ‭synergistically in limiting primary phytoplankton production. In summer the column is stratified‬ ‭and very stable, the mixing motions stop above the thermocline, this reduced mobility towards‬ ‭the bottom will ensure that the light, which is otherwise optimal during the summer period, never‬ ‭becomes limiting. Once again, light and mixing motions act synergistically, this time defining a‬ ‭condition that is positive for the phytoplankton, then its growth will be limited in summer only by‬ ‭the lack of nutrients. In fact, the nutrients remain confined below the thermocline, we only have‬ ‭those few nutrients that can derive from the regeneration that occurs above the thermocline. We‬ ‭can then reconsider what happens in the column in terms of respiration and primary production,‬ ‭no longer at the level of a single cell as we did before (single cell positioned at a given depth),‬ ‭but we can go and see what happens when respiration and primary production are considered‬ ‭as integrated processes of the entire water column.‬ ‭The integrated production in the column is in this case represented by the area of the green triangle ABC,‬ ‭while the integrated respiration in the column is in this case represented by the area of the ADEC rectangle.‬ ‭Critical depth is therefore defined as that depth in which an equality is achieved between the area of ABC, i.e.‬ ‭the area of the triangle which represents production, and the area of the rectangle which represents breathing.‬ ‭Which is equivalent to saying that the critical depth is that depth at which production and respiration of the‬ ‭entire column are equal. Above the critical depth, phytoplankton cells can circulate freely, at a given time‬ ‭interval, without consumption prevailing over photosynthetic production in the column. This allows us to‬ ‭define the condition of limitation of primary production imposed by the presence of mixing motions. Therefore‬ ‭we will have that the mixing motions constitute a limiting factor‬ ‭if they extend below the critical depth. In any case, it must be remembered that even the‬ ‭presence of phytoplankton itself influences the value of the compensation depth and‬ ‭therefore that of the critical depth.‬ ‭The value of the compensation depth depends on the transparency of the water, in more transparent ocean‬ ‭waters we will have greater light penetration and the compensation depth, like the critical depth, will be found‬ ‭at greater depths. Conversely, in coastal waters with large quantities of suspended particles (phytoplankton‬ ‭but not only), the compensation depth is found at more superficial levels. In each geographical area the‬ ‭compensation depth and consequently the critical depth can then vary on a daily and seasonal basis,‬ ‭reflecting the changes in light, water transparency and nutrient availability.‬ ‭A fundamental element, an important biotic factor in defining the trend of phytoplankton biomass,‬ ‭is the grazing of herbivorous zooplankton. Zooplankton herbivores are essentially made up of‬ ‭protozoa that feed on the smallest components of small and nanoplankton, both autotrophic and‬ ‭heterotrophic, then there is a vast array of herbivorous planktonic crustaceans and finally there are‬ ‭the many larval forms of organisms that as adults they then become benthic or nektonic. However,‬ ‭speaking of herbivores, it should be underlined that a whole series of benthic herbivores also‬ ‭exploit phytoplankton as a food resource, even if often you do not do so exclusively, just think of‬ ‭barnacles and bivalves that capture food material in suspension, therefore not only microalgae but‬ ‭also bacteria, small protozoa and organic particles. The impact of the consumption of benthic‬ ‭herbivores on phytoplankton is much greater in coastal waters, where the depth is reduced and‬ ‭the bottom is still illuminated or in any case very close to the photic zone. Grazing can occur‬ ‭through different methods ranging from the direct capture of single phytoplankton cells, as occurs‬ ‭in protozoa, to actual filtration processes as occurs for example in larvaceans and caries, thus‬ ‭selecting the food particles within a specific dimensional range depending on the characteristics of‬ ‭the filter. Some authors also consider copepods among filter feeders because they highlight the‬ ‭filter role played by the bristles placed on the appendages around the mouth. More recently,‬ ‭however, a different vision has emerged because the appendages equipped with bristles move‬ ‭more like balls than sieves and with the movement they create water currents that transport single‬ ‭phytoplankton cells towards the animal, as close as possible in a that can capture them. Some‬ ‭copepods would be able to perceive the presence of single phytoplankton cells partly thanks to‬ ‭sight but above all by receiving olfactory and mechanical stimuli. In this way they can also select‬ ‭food particles and adjust the intake rate based on the quality and quantity of the particles‬ ‭themselves. After having considered the main factors that limit phytoplankton primary production,‬ ‭we move on to putting together all our knowledge and therefore defining the variations in primary‬ ‭production as a function of latitude and the different seasons. Light, transparency and nutrients‬ ‭interact with each other determining the seasonal and latitudinal variations in productivity‬ ‭observed in the oceans. In temperate waters the amount of solar radiation penetrating the sea‬ ‭varies seasonally, the thermal structure of the water column also varies seasonally as the water‬ ‭temperature varies.In winter the sun is low on the horizon, the solar energy penetrating the sea is‬ ‭at a minimum, thermal stratification is absent and there is strong mixing. The mixing easily brings‬ ‭the phytoplankton below the critical depth, resulting in lower productivity. As the first spring‬ ‭approaches, the days get longer, solar radiation increases, the surface layers of the sea tend to‬ ‭warm up little by little and thermal stratification begins in the water column. The waters are also‬ ‭rich in nutrients, redistributed in the column by winter mixing. Therefore light and nutrients act‬ ‭positively on phytoplankton, triggering a significant recovery in phytoplankton production and also‬ ‭algal blooms. In the summer months the light is in optimal quantity, the warmer and lighter‬ ‭superficial layers stratify on the colder and heavier deeper ones and there is no mixing, therefore in‬ ‭the mixed layer the nutrients are scarce because they have been abundantly consumed since the‬ ‭beginning of the thermal stratification and then were only limitedly replaced by regeneration‬ ‭phenomena that occur above the thermocline. Nutrients therefore play a fundamental role even‬ ‭though the light is optimal and productivity, precisely due to the low availability of nutrients,‬ ‭remains low. In autumn, however, the intensity of solar radiation decreases, the days become‬ ‭shorter, the surface layers cool and stratification is reduced. A mixing process begins which‬ ‭gradually extends deeper, aided in this by the first autumn squalls and storms. The mixing motions‬ ‭begin to bring to the surface the nutrients that have been confined to the bottom throughout the‬ ‭summer, but at the same time this mixing does not yet bring the phytoplankton below the critical‬ ‭depth. Nutrients and light once again play in synergy in autumn, leading to an increase in‬ ‭productivity and even small blooms.the warmer and lighter superficial layers stratify on the colder‬ ‭and heavier deeper ones and there is no mixing, therefore in the mixed layer the nutrients are‬ ‭scarce because they have been abundantly consumed since the beginning of the thermal‬ ‭stratification and then have been only limitedly replaced by regeneration phenomena that occur‬ ‭above the thermocline. Nutrients therefore play a fundamental role even though the light is optimal‬ ‭and productivity, precisely due to the low availability of nutrients, remains low. In autumn,‬ ‭however, the intensity of solar radiation decreases, the days become shorter, the surface layers‬ ‭cool and stratification is reduced. A mixing process begins which gradually extends deeper into‬ ‭the depths, aided in this by the first autumn squalls and storms. The mixing motions begin to bring‬ ‭to the surface the nutrients that have been confined to the bottom throughout the summer, but at‬ ‭the same time this mixing does not yet bring the phytoplankton below the critical depth. Nutrients‬ ‭and light once again play in synergy in autumn, leading to an increase in productivity and even‬ ‭small blooms.the warmer and lighter superficial layers stratify on the colder and heavier deeper‬ ‭ones and there is no mixing, therefore in the mixed layer the nutrients are scarce because they‬ ‭have been abundantly consumed since the beginning of the thermal stratification and then have‬ ‭been only limitedly replaced by regeneration phenomena that occur above the thermocline.‬ ‭aided in this by the first autumn squalls and storms. The mixing motions begin to bring to the‬ ‭surface the nutrients that have been confined to the bottom throughout the summer, but at the‬ ‭same time this mixing does not yet bring the phytoplankton below the critical depth. Nutrients and‬ ‭light once again play in synergy in autumn leading to an increase in productivity and even small‬ ‭flowering.‬ ‭What has been described for temperate seas is summarized and focused on this slide where you can find at‬ ‭the top what the water looks like in the column at the turn of the seasons, at the bottom left the trend of‬ ‭primary production is represented, therefore with two peaks a much more pronounced spring one and an‬ ‭autumnal one and then two minimums, the summer one and the winter one. The different limiting conditions‬ ‭are also highlighted: from late autumn to early spring we have light as the prevalent limiting factor, from late‬ ‭spring to mid-autumn the prevalent, if not only, limiting factor is nutrients. In the figure on the right, the trends‬ ‭of phytoplankton biomass (in blue) and zooplankton biomass (black hatching) are compared. For now we can‬ ‭limit ourselves to recording how the trend of zooplankton follows that of phytoplankton equally, albeit with a‬ ‭time lag.‬ ‭In tropical waters the lighting is optimal all year round, the heating of the surface layers is constant with low‬ ‭density values and the consequent presence of thermal stratification throughout the year. Stratification‬ ‭prevents the transport of nutrients from deep water, so even if light conditions are optimal, production is low‬ ‭because the photic zone is nutrient-poor. Although low, production remains almost constant throughout the‬ ‭year. The low production typical of tropical waters causes transparency, which is a typical connotation of‬ ‭tropical waters. Here we have a summary picture for tropical seas: constant conditions of thermal‬ ‭stratification in the column, always limiting nutrients and an irregular oscillation of both phytoplankton and‬ ‭zooplankton biomass.‬ ‭Finally, the situation for the polar seas: here primary production is practically restricted to a single short‬ ‭period, the polar summer, in the Arctic essentially between July and August, there is never thermal‬ ‭stratification and nutrients never represent a limiting factor. So production is limited almost all year round by‬ ‭the amount of light available. Even for the polar seas we can see the last summary picture, which shows an‬ ‭unstable column all year round with a slight hint of temperature rise in the surface layers during summer.‬ ‭Productivity shows a summer peak and then remains low during the rest of the year with a limiting effect‬ ‭given by the light. Biomass‬ ‭phytoplankton and zooplankton also show a similar trend in this case but the peak of zooplankton has‬ ‭slipped forward in time compared to that of phytoplankton.‬ ‭The latitudinal and seasonal variations that we have just described refer to oceanic conditions,‬ ‭therefore open sea conditions far from the coasts. In coastal waters there are several factors that‬ ‭contribute to creating a difference compared to open sea conditions. First of all, there are‬ ‭continuous and sometimes even reduced supplies of nutrients of various origins, which is‬ ‭precisely why coastal sea areas never suffer from nutrient deficits. In shallow waters, a thermocline‬ ‭rarely establishes enough to confine nutrients at depth throughout the summer and beyond.‬ ‭Another aspect is that coastal waters are often shallower than the critical depth so phytoplankton‬ ‭will never fall below this threshold and production will always be greater than respiration, even in‬ ‭winter. One factor acts negatively on the productivity of coastal waters and is the high presence of‬ ‭terrigenous material which leads to a decrease in the thickness of the photic layer. Given all these‬ ‭general conditions in temperate seas, coastal areas present production levels in summer that are‬ ‭constantly high, we never have that minimum that we saw occurring in the summer in the open‬ ‭sea. As a result, average annual production in coastal waters is higher than in the open sea.‬ ‭Coastal production would be even higher if it were not for the presence of areas with large‬ ‭quantities of suspended particles, especially debris, which determines a significant absorption of‬ ‭light radiation and therefore the consequent limitation of the photic production zone. While in the‬ ‭open sea the photic zone can easily extend up to 50 meters‬ ‭depth, in coastal waters it can even be reduced to the first 5-10 meters. The difference in productivity‬ ‭between coastal and open sea areas is particularly strong in the tropics where coastal production can be up‬ ‭to 10 times that of the open sea. Another general consideration that we can make is that coastal‬ ‭productivity is also increased by the presence of benthic algae and seagrasses and sometimes also by the‬ ‭presence of marsh plants which are typical of brackish marshes. In general, coastal productivity is less‬ ‭stable from year to year than that of the open sea, so there are years that present very different productivity‬ ‭conditions compared to previous ones or those that follow. In nearby areas, productivity can vary‬ ‭considerably even in the same year or at the same time, based on different factors. These are factors that‬ ‭act locally, such as the morphology of the coast, the presence of rainfall, the prevailing winds, the currents‬ ‭or the tide. The variability of all these factors, but also other factors, affects coastal productivity.‬