ICAR AIEEA PG Fisheries Science 14 July 2023 Shift-1 Paper

In physical limnology, Holomictic lakes are lakes where the water column undergoes complete circulation and destratification (overturn) from the surface to the maximum bottom depth at least once a year, establishing complete isothermal and uniform density conditions throughout. This contrasts with Meromictic lakes, where a dense bottom saline layer (monimolimnion) never mixes with upper waters.

Option analysis:

• Polymictic Lakes: Mix frequently or continuously due to shallow depth and wind.
• Dimictic Lakes: Specific subgroup of holomictic lakes that mix twice a year (spring and autumn).
• Monomictic Lakes: Subgroup of holomictic lakes that mix once a year.
• Holomictic Lakes: Correct, standard broad limnological classification for lakes that achieve completely uniform temperature and density throughout the entire water column during mixing periods.

Biofertilizers are preparations containing living microorganisms that enhance nutrient availability (especially nitrogen fixation and phosphate solubilization). Rhizobium fixes atmospheric nitrogen symbiotically in legumes, Nostoc is a free-living and symbiotic nitrogen-fixing cyanobacterium, and Azolla is a water fern harboring the nitrogen-fixing cyanobiont Anabaena azollae. Salmonella is a pathogenic enterobacterium that causes foodborne enteritis and typhoid, having zero biofertilizer value.

Option analysis:

• Salmonella: Correct, enteric pathogen, definitely not a biofertilizer.
• Rhizobium: Symbiotic nitrogen-fixing bacterial biofertilizer.
• Nostoc: Blue-green algal biofertilizer.
• Azolla: Aquatic biofertilizer widely used in agriculture and aquaculture ponds.

Water hardness is caused by multivalent metallic cations (primarily $Ca^{2+}$ and $Mg^{2+}$). It is divided into:

• Temporary (carbonate) hardness: Caused by calcium and magnesium bicarbonates ($Ca(HCO_3)_2$ and $Mg(HCO_3)_2$), which decompose and precipitate as insoluble carbonates upon simple boiling.
• Permanent (non-carbonate) hardness: Caused by chlorides ($CaCl_2, MgCl_2$), sulfates, and nitrates/nitrites, which cannot be precipitated by boiling and require chemical softening (such as soda ash or ion-exchange).

Transparency or light penetration (turbidity) of pond and lake water is universally measured using a Secchi disc (devised by Pietro Angelo Secchi in 1865). It is a $20\text{ cm}$ diameter circular metal plate divided into alternating black and white quadrants. The average depth at which the disc disappears and reappears upon lowering and raising gives the Secchi disc visibility depth.

Option analysis:

• Secchi disc: Correct, standard limnological instrument for measuring water transparency.
• Salinometer: Instrument used to measure salinity/salt concentration of water.
• Hydrometer: Instrument used to measure specific gravity or density of liquids.
• Anemometer: Instrument used to measure wind speed and velocity.