Marine Biology

1. Light

One of the most obvious variable factors influencing primary production is the amount of solar energy reaching the surface of the sea. It is dependent on the altitude of the sum and the changing weather patterns. The light that penetrates the water is rapidly absorbed by inorganic and dead organic matter present in it. Thus, nearly 80% of the total solar radiation is absorbed in the upper 10 metres, and only 1% of incident visible light reaches 120 meters in clear tropical waters and 10 to 20 meters in turbid inshore waters of the incident light that falls over the sea, only 0.02 to 2.0% or 0.1% on an average alone is utilized for the production of organic matter.

Since photosynthesis is primarily dependent on light energy, the nature of penetration of sunlight in the seawater is of prime importance in governing the productivity of an area. That is when the light strikes the surface of the water, a certain amount of light is reflected back. The amount of light refection depends upon the angle at which the light strikes the surface of the water. If the angle of incidence is low, large amount of light will be reflected. On the other hand, if the angle is to nearly 90^o (perpendicular to the horizontal surface of the water) the greater will be the penetration and the lesser will be reflection. Light that is reflected is lost to the system and hence maximum penetration is the most desirable for maximum production. In the tropical regions of the earth, the sun is directly overhead at midday (or) virtually perpendicular to the sea surface, giving an angle for maximum penetration of light into the water column. In the temperate regions, the sun may be directly overhead during the summer months, but may be far from this position at the other times of the year. Also in the polar regions, the sun is absent during the winter or is so low to the horizon that no light can penetrate the water. The presence of ice in these areas also reduces light penetration into the water.

The portion of light that enter the water column is subject to further reduction from two additional processes acting on it within the water. The first is reflection from various suspended particles in the water column. Suspended living or dead particles intercept the light and either absorb it (or) reflect it back to the surface. This light is unavailable for use and further reduces available light. Secondly, water itself absorbs light, making it unavailable for the plants. This absorption of light by water is the reason that vast majority of the water masses of the ocean is dark below a certain level. Because of this absorption of light by water, photosynthesis is automatically restricted to the thin, upper most lighted layer. Water, however, does not absorb all wavelengths of light equally. Sunlight spectrum includes all the visible colours ranging from violet to red or wavelengths from about 400 - 700 mm. As these wavelengths enter into the seawater, the violet and red components are very quickly absorbed by the water. The green and blue components are absorbed less rapidly and hence penetrate most deeply. Even though blue and green light penetrate deeper into the water column, the intensity decreases with depth, and it is intensity that is needed by plants. Intensity is measured by the extinction co-efficient, which is the ratio between the intensity at a given depth and intensity at surface. For pure water, it is 0.035.

Where there are large numbers of particles in the water, such as in coastal waters, the depth of light penetration may be greatly reduced and hence the amount of light insufficient for photosynthesis below a few meters.

The rate of photosynthesis is high in high light levels and decreases as the light intensity decreases. On the other hand, the rate of respiration of the phytoplankton cells is essentially constant at all depths. That is, as the algal cells go deeper in the water column, the rate of photosynthesis declines as the light intensity decreases, until at some point, the photosynthetic rate equals the respiration rate. At this point, there is net production of organic material and this depth is called Compensation depth and it is the depth to which 1% of the incident radiation penetrates. The compensation depth also changes with season, due to the change in the position of the sun and it may be absent during the winter months in high latitudes. This depth marks the lower limit of the euphotic zone and varies geographically from a few meters in very turbid in shore waters to depths of 120 m or more in the open waters of tropical oceans.