Limnology

Unit 9- Benthos

9.1 Classification of benthic regions, beach zones, periphyton,distribution
Benthos
The term benthos includes all bottom dwelling organisms, comprising the great assemblages of plants and animals. Benthos includes the organisms of the bottom from uppermost water bearing portion of a body of water right up to the greatest depths.
Classification of benthic regions
The benthic habitat is not uniform and it can be classified into different zones, they are.
1. Lake zone:
There are usually 3 zones and in exceptionally in deep lakes there are 4 zones, they are Littoral, Sublittoral, Profundal and Abyssal zones.
i) Littoral Zone : Extends from water’s edge to the lakeward limit of rooted vegetation.
ii) Sublittoral Zone : From lakeward limit of vegetation down to about the level of the upper limit of the hypolimnion.
iii) Profundal zone : The entire lake floor that bounds the hypolimnion
iv) Abyssal : Present only in lakes of depths greater than 600 m.
1.Beach zones
Benthic region was formerly regarded as water’s edge extending to the deepest region. The biota is truly aquatic, living in water at beach and has direct continuity with the main body of the lake or stream and the organisms involved are benthos and the environment are considered as benthic region. The exposed sandy beaches maintain 3 parallel environments, they are :
a. The inner beach
b. The middle beach
c. The outer beach
The inner beach extends from water’s edge, during periods of calm water up to the slope to the place where the surface of sand ceases to be with water and shows the first signs of drying. This zone is narrow and exposed to the slight wash of gentle wave of calm weather.
The middle beach occupies the space just beyond the inner beach to the waves during the rough weather. The outer beach extends from middle beach to the outer limits of beach proper. It is washed by waves during storms or during times of highest water levels.
2.Sandy Beach (Psammolittoral)
This is divided into three categories, they are as follows :
i. Hydropsammon : Submerged sandy bottom
ii. Hygropsammon : Zone immediately landward from water, edge – always saturatd with water roughly corresponds to the inner beach.
iii. Eupsammon : Zone above hygropsammon – corresponds to middle beach.
Benthic Communities of Inland waters
The benthos of inland waters are vast assemblage of flora and fauna. However, it must be remarked that, “no vertebrates are represented as true Benthos”. The benthic organisms may be classified as follows :
i) Based on size
a) Macrobenthos (more than 0-5 mm) eg. Crab, insects and their larvae, other crustaceans, molluscs such as gastropods, polychaete worms.
b) Meiobenthos (Size 0.1 – 0.5 mm) eg. Ciliates, annelids, copepods
c) Microbenthos (size smaller than 0.1 mm) eg. Bacteria, flagellates, ciliates.
ii) Based on mode of nutrients
a)Phytobenthos : Autotrophic life, include varieties of algae, flowering plants and other angiosperms, sea grasses etc.
b)Zoobenthos : Heterotrophic life, range from microscopic to macroscopic organisms.
iii) Based on mode of life and area of dwelling
a)Epibenthos : Live on top of the substratum regardless of whether the bottom sediment is soft or hard rock. eg. benthic molluscs, crabs, prawn, macrophtes, worms, micro algae.
b)Endobenthos : Live within the substratum regardless of its type. Interstitial fauna may be included under endobenthos, mainly borers and burrowers.
iv)Based on Mobility
a)Sessile : They do not possess any form of mobility but remain attached to the substrate.
eg. Fresh water sponges, macro phytic and microvegetation
b)Vagrant : They possess locomotive powers and can move either rapidly or slowly eg. shelled macro – invertebrates, worms etc.
v) Based on ooze film
a) Ooze film assemblage : Ooze forming / secreting organisms.
b) Associated ooze film assemblage : organisms associated with ooze film.
Periphyton
Periphyton are a miscellaneous assemblage of organisms growing upon free surfaces of objects submerged in water frequently appearing as a brownish green slimy, slippery layer. It commonly found on plants, wood, stones and various other objects. It seems to develop in littoral and sub-littoral regions. Periphyton may not always be regarded as true benthos as they tend to grow on any solid support and likely to contain plankton and benthic organisms
Distribution of benthos
Qualitative distribution
At different bottom conditions and exposure to wave action diversify the littoral zone and makes differences in fauna and supports different kinds if animals.
- Depth and quality related distribution
- Decreasing diversity and numbers with increase in depth
- Mostly of uniform distribution
The following animation shows the qualitative depth relations of two American lakes

The littoral and sub-littoral populations, insects and molluscs comprise as much as 70% of total number of different components. Increasing depths beyond the littoral zone, the number of benthic organisms may diminish namely Bryozoans, Platyhelminthes, Snails, Bivalves, Nematodes, Annelids, insects etc. especially at first 18 m depth. There is no constancy of composition of different types of lakes because lakes do not have profundal benthic animal population. Even within a single lake n different composition of population occur at different places.

The nature of the bottom has selective influence upon the quality of the fauna. Baker (1981) classified bottom materials in the littoral zone of lake Oneida into 6 different types namely mud, sand, clay, gravel, boulders and sandy clay. Microfauna exists in the bottom deposits is also established and hence microorganisms are considered as bottom dwellers and appear as :

1) free swimming or
2) free swimming only in some stage of life cycle or
3) always spend most of time on the bottom materials
Bigelow (1928) divided littoral benthic microorganisms of lake Nipigon, Canada into two ecological groups :
i) the ooze-film assemblage and
ii) the associated ooze–film assemblage
The former group comprises those microorganisms living in and on that film of ooze at upper surface of the lake bottom contain certain algae, including diatoms, protozoa, rotifers and cladocerans, while the latter directly dependent upon the ooze film and swim very close to it eg, Cladocerans and rotifers .