Another Flotation Strategy is the Increase in Surface Resistance

‭Another flotation strategy is the increase in surface resistance:‬ ‭For the same body weight, the greater the surface, the greater the friction in movement through the‬ ‭layers of water and the lower the falling speed.‬ ‭There are two ways to achieve this, decrease body size or alter body shape. In any solid the surface area‬ ‭increases according to the square of the linear dimensions but the volume increases according to the cube of‬ ‭the linear dimensions. It follows that the smaller a body is, the greater its surface area compared to its mass.‬ ‭The small body volume of the plankton is a constant given that they are mostly microscopic organisms. The‬ ‭reduction in size is accentuated in warm water plankton‬ ‭tropical: the higher temperature of these waters decreases density and viscosity, making sinking‬ ‭faster and floating more difficult. This is why in these conditions smaller sized forms are found‬ ‭compared to what is generally present in waters at lower temperatures. The second method is the‬ ‭alteration of the shape which allows to obtain a greater increase in surface area for the same‬ ‭volume, compared to a hypothetical spherical shape in which the surface to volume ratio takes on‬ ‭a lower value. For this reason, among colonial algae, the filamentous shape and flattening of some‬ ‭sections or of the entire body are frequent, widespread in both zooplankton and phytoplankton.‬ ‭Many planktonic organisms have evolved modified appendages to offer great strength but have‬ ‭little impact on weight. The most adopted strategies to float better, i.e. reducing body size and‬ ‭developing extrusions and flattened parts, are often associated in planktons and always involve an‬ ‭increase in the ratio between surface area and body volume. Therefore they also represent a‬ ‭notable advantage from the trophic point of view and exchanges with the environment (gas‬ ‭exchange and release of catabolites). A general characteristic of plankton is that it is distributed in‬ ‭patches or patches, which is why it is called patchiness, and consists of alternating areas of‬ ‭rarefaction and densification of the plankton.‬ ‭These aggregates are variable in size and duration: the size of a patch varies from a few meters‬ ‭to many km and then also varies vertically. A patch can disperse quickly or disappear or persist‬ ‭longer: there are variations on time scales that affect the distribution of plankton which can be‬ ‭daily, seasonal or annual. This great spatial and temporal variability makes it difficult to guarantee‬ ‭repeatability conditions when sampling plankton. Unless you have a double screen, you do not‬ ‭make replicates of a plankton sampling, so information obtained from the analysis of a sample‬ ‭will be more robust the more you are able to filter a greater volume of filtered water. Only in this‬ ‭way can the conditions that better reflect the conditions of the water mass be integrated into the‬ ‭collected sample. Patches can be determined by biological and physical factors. The biological‬ ‭factors are for example the trophic relationships between the various components of the‬ ‭plankton: for example the herbivorous zooplankton will follow the distribution of the‬ ‭phytoplankton, determined by the distribution of light. However, physical factors often have a‬ ‭dominant role, especially in relation to the formation of currents: for example, surface waters‬ ‭warm up during the day and cool down at night: this determines variations in density which lead‬ ‭to the formation of convective motions, in which small masses of water sink or rise based on‬ ‭their density. Even upwellings and the large-scale formation of vortices from large ocean currents‬ ‭create inhomogeneities in the body of water and variations in the distribution of plankton.‬ ‭Furthermore, wind blowing on plankton can also produce verticalized movements of plankton‬ ‭due to Langmuir circulation. It represents the result of the interaction between a surface current‬ ‭and wave motion: it occurs when a moderate wind blows in the same direction for an average‬ ‭long time. So, when these conditions are created, the layers of surface water are divided into‬ ‭cylinders of a few meters in diameter and a few hundred meters in length, with an axis parallel to‬ ‭the direction of the wind and rotating in the opposite direction to each other, i.e. each one rotates‬ ‭with a direction of rotation opposite to that of the two adjacent water cylinders. Parallel and‬ ‭alternating zones of convergence and divergence are thus created at a superficial level, in which‬ ‭plankton is passively accumulated which tend respectively to descend downwards in the‬ ‭convergence zones and rise upwards in the divergence zones. Therefore the Langmuir circulation‬ ‭determines a very rapid mixing of the plankton but also contributes to redistributing heat and‬ ‭particulate dissolved substances in the surface layers. Water that moves in an upward direction,‬ ‭creating an upwelling, has a lower density of organisms and the current is slower. Conversely, in‬ ‭the convergence zones where there is downwelling, the organisms reach higher densities and the‬ ‭downward motion is faster. Another important phenomenon in a pelagic environment is the‬ ‭formation of marine snow,a further cause of inhomogeneity. To understand this aspect we must‬ ‭understand how it is formed and what it is. It is amorphous particulate material that derives from‬ ‭the disintegration of the remains of gelatinous organisms, but also from mucous secretions‬ ‭formed by many marine organisms. All this material slowly sinks in a column carried by the‬ ‭currents, enriching itself with other presences such as fecal pellets, diatom frustules, remains of‬ ‭algal walls, coccoliths, fragments of phanerogams, but it also attracts bacteria that degrade the‬ ‭organic substance and flagellates that feed on bacteria : each marine snowflake becomes a small‬ ‭floating microcosm with its own community. The dimensions of marine snow are very variable,‬ ‭generally less than half a cm, and marine snow cannot be collected with plankton nets because it‬ ‭would be destroyed. It was observed for the first time scuba diving by William Beebe in the‬ ‭1930s, when he dived to a depth of 900 m with his bathysphere: it was he who coined the name‬ ‭marine snow.‬