Limnology

Unit 6- Planktonic organisms

6.2. Distribution of plankton
In all natural waters irrespective of latitude, longitude and physic-chemical characters majority of normal supporters are of plankton and differ in great many respects. Occasionally they occur in certain thermal waters, subterranean waters, spring-fed streams, transient pools etc. The presence of plankton in natural waters is itself an indication of the significant position which it occupies in the aquatic environment.
General geographical distribution of plankton
It has been claimed by some of the Oceanographers that the polar seas support abundant plankton than do the tropical ones. Polar and tropical inland water are still little known limnologically plankton production cannot be compared satisfactorily. Certain northern low lying lakes in countries bordering on the Baltic sea, plankton production in the inland water of North American is little known.
Horizontal distribution of plankton
There is irregularity in horizontal distribution of plankton in water. One of the most common causes of irregularity is the wind acting upon surface waters. Plankton drifts due to wind action are always temporary and is common in many inland waters. This drifting leads to meeting towards the shore becoming so thick that the whole water colour is altered and changes the general appearance. The general effect of drift is to concentrate more or less upper waters throughout one part of a water body with corresponding thinning on the opposite side.
Other important factors which are responsible for horizontal distribution of the plankton are :
a) inflowing streams
b) irregularity of shore line
c) depth of water
d) flowage areas
e) water current etc.
Wind is one of the most common causes of irregularity in horizontal distribution of plankton. Since wind causes waves but in addition may produce an actual drift of the upper waters. But under certain conditions of drifting water, plankton organisms become concentrated temporarily in the vicinity of the shore which faces into the wind at that time.
Vertical distribution of plankton
The vertical distribution of plankton is a complicated matter. In deeper region, plankton may show little or no resemblance to that of upper waters. In lakes, a uniform vertical distribution of plankton occurs only during Spring and Fall overturns but certain plankton are distributed in minimal quantities from the surface to the lowermost limits of habitability in the range of concentration that they occur.
Distribution of phytoplankton
The upper most waters are the home of the chlorophyll bearing plankton and perhaps the light plays an important role indicates the distribution of phytoplankton. Considering the general mass distribution, few stalemates are commonly taken such as :
a. The blue green and green phytoplankton (Myxophyceae and Chlorophyceae) concentration is maximum than diatoms and this has been thought to be due to the heavier weight.
b. Maximum populations of chlorophyll bearing phytoplankton are at some level below the surface waters.
c. The blue green algae as a group tend to concentrate towards the surface.
Distribution of zooplankton
It is difficult to generalize on the vertical distribution of zooplankton as a whole and the same is true of the various taxonomic groups composing the zooplankton. Certain tendencies in vertical distribution appear such as :
a. Greater occurrence of Sarcodina in lower waters
b. Preference of Dinoflagellata for upper water
c. General scattering of Ciliatei and
d. Selection of different levels by the young and adult stages of certain crustacean
The distribution behavior may be very different in different kinds of water. Under conditions of well developed physic-chemical stratification, the levels of maximum population of the Crustacea and Rotifera often correspond closely, although such a statement cannot be dependent upon.
Conditions influencing vertical distribution
Among the influences which may operate in the production of various forms of vertical distribution, the following are important a) Light b) Food c) Dissolved gases d) Temperature e) Wind f) Gravity g) Age of individuals of a species.
1. Light is the most important factor in vertical distribution of plankton and is well established. Its presence for various reasons tends to removal or sometime disappearance of plankton. The annual and diurnal variation of light, qualitative and quantitative variation influence the migration of plankton Differences is light reaction of young or adult stages of certain plankters lead to a different vertical distribution of the life history stages in the same species.
2. Food - The abundance and availability of zooplankton also depends to same extent by the distribution of food. It has been wall established that the concentration of protozoa, micro crustacea and rotifers have been correlated with the presence of settling suspended materials.
3. Dissolved gasses and other substances - Dissolved oxygen is an important part in rendering deeper waters partially or wholly uninhabitable for most of common zooplankters. Some zooplankters are more sensitive than others to the accumulating results of decomposition in underlying waters so that all which do not retreat upward at the same rate and certain stratification may result. But nevertheless, the chemical changes progress to the lethal point for pelagic plankton organisms eventually eliminating from underlying waters.
4. Temperature
Direct effects :
a) selection by motile plankters of favorable temperatures or
b) inability of non-motile forms to exist in levels having certain temperatures.
Such effects appear to apply only to those plankters which manifest sensitively to differences in temperature while many plankters are not influenced at all by any of the vertical temperature differences within a lake.
Indirect effects : changes in density and viscosity of water altering the floatation levels of those plankters which are adjusted to floatation.
5. Wind – its effect is significant with the season. During summer, directly influence the epilimnion and during hard blows vertical distribution of plankton is seen. During Autumn and Spring overturns, wind disturbs plankton because of same density and viscosity throughout but during the period of ice cove, the disturbing influence of wind is eliminated
6. Gravity – reduction of specific gravity makes certain phytoplankton such as Gloeotrichia to congregate at surface waters. Plankton especially the pelagic crustacean are heavier than water and sink with appreciable speed when inactive. Daphnia maintain their position in water.
7. Age of individual of a species – as a general rule, young individuals occur near surface, adults tend to sink into deeper waters.