Understanding the Lacustrine Zone in Reservoirs

Reservoirs are bodies of water created by damming a river. They often exhibit a gradient from river-like conditions at the inflow to lake-like conditions near the dam. This gradient is typically divided into three zones: the riverine zone, the transition zone, and the lacustrine zone.

The question asks about the characteristics of the lacustrine zone. Let's analyze each statement provided:

1. is broad deep lake-like basin: The lacustrine zone is the part of the reservoir closest to the dam. Here, the water velocity slows down significantly, allowing suspended particles to settle. This zone is typically the deepest and widest part of the reservoir, resembling a natural lake. This statement is generally true.
2. is autochthonous: Autochthonous production refers to organic matter produced within the ecosystem itself, primarily through photosynthesis by phytoplankton or aquatic plants. The lacustrine zone often has significant phytoplankton populations in its upper layers (photic zone) due to clearer water and slower flow, leading to considerable primary production within this zone. While reservoirs also receive allochthonous inputs from the riverine zone, the lacustrine zone is characterized by substantial autochthonous production, making this statement true in this context.
3. has P > R: P > R means that the rate of primary production (P) is greater than the rate of respiration (R). This leads to a net production of organic matter. In the photic (sunlit) layer of the lacustrine zone, where photosynthesis occurs, primary production by phytoplankton is often high, exceeding the respiration rates of organisms present. Therefore, the lacustrine zone, especially in its upper part, frequently exhibits P > R. This statement is generally true for the euphotic layer.
4. has turbid water: Turbidity refers to the cloudiness or haziness of the water caused by suspended particles. While some level of turbidity can exist in any part of a reservoir, the highest turbidity is usually found in the riverine zone due to the inflow of sediment-laden river water. As the water moves towards the lacustrine zone, the slower velocity allows suspended particles to settle out, leading to lower turbidity compared to the upstream zones. Therefore, describing the lacustrine zone as having "turbid water" is generally less accurate than describing the riverine or even transition zone. This statement is likely false or at least less characteristic than the others.

Based on the analysis:

• Statement (i) is true.
• Statement (ii) is true.
• Statement (iii) is true.
• Statement (iv) is false (or less characteristic).

Therefore, the correct combination of statements is (i), (ii), and (iii).

Revision Table: Lacustrine Zone Characteristics

Additional Information on Reservoir Zonation

Reservoirs are dynamic environments that bridge the gap between rivers and lakes. The zonation reflects the gradient in physical and biological conditions from the riverine inflow to the dam area. Understanding these zones is crucial for managing reservoir ecosystems, including water quality, fisheries, and biodiversity.

• Riverine Zone: Uppermost zone, influenced strongly by river inflow. Characterized by relatively high flow velocity, high turbidity, rapid sedimentation, and heterotrophic conditions (R > P). Plankton is often dominated by riverine forms washed in.
• Transition Zone: Middle zone, where riverine influences decrease and lake-like conditions begin to develop. Flow slows down, allowing some settling of particles. Light penetration increases compared to the riverine zone. Autochthonous production starts to become more important.
• Lacustrine Zone: The deepest, most lake-like zone near the dam. Very slow flow allows maximum settling of suspended material, leading to clearer water (relative to upstream zones). Primary production by phytoplankton is often significant, leading to P > R in the photic zone. This zone can stratify thermally like a lake.

The exact boundaries and characteristics of these zones can vary greatly depending on the reservoir's size, shape, inflow rate, water level fluctuations, and surrounding landscape.