Limnology (2+1)

Practical No. : 12

Enumeration and biomass estimation of benthos from different freshwater bodies
Collection and preparation of sample: Since benthos dwell at or below the surface of the bottom sediments of littoral and profundal zones, they are collected with the help of mud samplers.
Requirements : Mud sampler, Oven, Muffle furnace, Crucibles, Sieve (2mm and 0.5mm mesh size), Bucket, Hand-lens, collection bottles, forceps, 70% alcohol (preservative), Whatman ash free filter paper, etc.
Method

• Collect sediment sample by suitable mud sampler
• Take the sample in a bucket, note its volume and add water to it.
• Sieve out the suspension through 2mm and 0.5mm mesh size sieves, one after the other.
• Collect the micro and macro invertebrates from the sieve with the help of forceps and transfer them to a wide mouth bottle containing 70% alcohol.
• Transport residual organic matter retained in the sieve to the laboratory and 50 to 100ml of water to it.
• Dissolve 5gm of sucrose in it. (This helps in easy collection of benthic fauna. Because they now float on the surface due to change in the density of the medium).
• Alternatively, the colouring of dyes of the organism (by treating residual matter with 0.1- 0.2% eosin, erythrosin or acid fuchsin in 70% alcohol) helps in easy collection of smaller organism like Oligocheats, Chironomids, Nematodes, etc. which pose a greater problem in comparison to Sponges, Mollusks, Arthropods and others.
• Fix the organism in 70% alcohol
• Qualitative and Quantitative analysis
• Identify the organism using keys and monographs.
• Count their numbers species-wise in the sample and note.
• Keep the organism species-wise on pre-weighted ash free filter paper and transfer to a crucible of known weight.
• Deduce the fresh weight of the material and transfer to a dust free oven at 105 2^oC till you get a constant weight.
• Cool in desiccators, weigh and deduce the dry weight.
• Keep the crucible with the contents in a muffle furnace at 550oC for 6-8 hours.
• Cool in a desiccator, find out ash content and note.
• Calculate the number of organisms per unit area and the biomass species-wise and for total benthos.

Observations :
1. Site of collection :
2. Locality :
3. Depth (m) :
4. Date of collection :
5. Type of sampler :
6. Area of sampler (cm²) :


Calculations:

Individual m^-2 = (N/a) x 10⁴

Where N = Average number of organism per sample

a = area of sampler (cm²)

Result: Express number of total benthic organism per m² and total benthic biomass as OM g/m².
Biomass of benthic animals
For collecting benthic animals belonging to deeper muddy areas (15-20 cm deep), corers of 71cm² are ideal. After operating the corers in the specific areas, the corers should be washed through a screen with openings of 6 mm square and the organisms are separated by hand from the material that the screen retained. This screen could retain all the organisms weighing 0.2 gm. The smaller farms would also be entangled in the debris on the screen. The organisms are separated into taxonomic groups, counted and weighed intact after blotting with paper. These live weights, which may include inorganic shells and the water trapped within the body cavities, should be multiplied by appropriate factors to convert them to weights of dry tissue. The factors are obtained by drying representative species at 105^oC for about 24 hrs. prior to drying, molluscs, crustaceans etc. should also be decalcified by immersing them in 20% HCL until all hard parts are dissolved.