Limnology (2+1)

Unit 9- Benthos

9.2. Quantitative and Qualitative movements, seasonal changes and migrations of benthos
Quantitative distribution
In shallow lakes, the entire bottom is uniformly productive. The growth of bottom dwellers, whose quantity of produce at different depths especially in shallow lake, is not same.
The deeper inland lakes can be divided in to 2 groups 1) Those which do not develop chemical stratification during summer and 2) Those which become chemically stratified and remain so throughout the summer period.
In non-stratified deeper lakes, the benthic forms extend much deeper into the basin resulting in greater mass production. However, in stratified deeper lakes during summer, the bottom region exposed to hypolimnic conditions lead to less production quantitatively than the regions above them.
Movements and migration of benthos
Under certain circumstances, the benthos exhibits movements within or on the bottom. Certain other organisms move up and down in streams. Moffett (1943) observed movements of bottom organisms on large wave swept shoal and found some changes in space due to bottom shifting.
Moon (1935) found that :
(a) the littoral fauna in lake Windermere is in a continual state of movements
(b) this fauna is sensitive to changes is surface level of the lake
(c) more active elements of the fauna move quickly into the inundate portions of the beach and
(d) a rise in surface level only 2.5cm is sufficient to produce a movement of the fauna.
Seasonal changes in benthos
Little is known about the seasonal changes in benthic organisms. In very shallow lakes, the seasonal changes in the whole benthic region are essentially the same as those which prevail in the littoral zone of deeper lakes. In the abyssal regions of deeper lakes which never overturn and hence the benthic conditions remain same throughout the year. Benthos are known to show different morphological changes with respect to different seasons (spring, summer, autumn and winter) at different zones (littoral, sub-littoral and profundal). In some lakes, the organisms may compose the overwhelming bulk of entire population and significant fact that each profundal species manifests similar and corresponding seasonal increase and decrease magnitudes, which are characteristics of the species.
Origin and permanence of profoundal bottom fauna
The preformed bottom fauna occurs in inland lakes is composed of forms which occur in neither peculiar nor restricted to the profundal region, they seem to occupy also the sub-littoral and some extent the littoral zones. The bottom fauna is not qualitatively unique but is composed of representative of a few species belonging to larger littoral and sub-littoral population. During spring and fall overturns, these population spread down the lake bottom into the profundal region bringing into not only the grater numbers of individuals but certain additional species characteristics. However, the gradual development of stagnation at the bottom bring about a condition which some of the species cannot under go. Occasionally in unusually deep lakes, certain animals occur which never come to shallow depth (above 50m). Hence, it appears that profundal bottom forms are not truly a permanent profundal fauna but may themselves be eliminated completely or party by exposing to stagnation.
Vertical distribution of profundal bottom fauna
Evidently it has been assumed that the profundal bottom fauna largely confined to the uppermost layer of the bottom deposits. Information secured by Lenz (1931) who by using constructed bottom sampler was able to bring virtually undisturbed, vertical samples of bottom deposits to the surface and then isolating and examining horizontal strata in their unaltered relations. Various horizontal levels have been studied down to a depth of 24cm. The largest part of the fauna occurred in the upper half of the sampler. Some difference appeared in the vertical distribution of various organisms for eg, Tubificidae concentrated in the upper position, while Chironomus were distributed from the surface of the mud to a depth of 20cm or more. Moore (1939) indicated and noted that there was an obvious similarity in the vertical distribution of micro and macro benthos.