A local authority in a region of East Africa manages a freshwater lake that covers 45 km2 and provides drinking water, irrigation and fish for a population ...

(a)(i) Eutrophication is the enrichment of water with excess nutrients, particularly phosphates and nitrates [1], which causes excessive growth of algae and other aquatic plants, leading to reduced oxygen levels and degraded water quality [1]. This process transforms a healthy, balanced aquatic ecosystem into one dominated by algal growth and starved of oxygen.

(a)(ii) Two sources of nutrients that can enter a freshwater lake [1 each, max 2]:

• Sewage/waste water from growing towns flowing untreated into streams.
• Fertiliser runoff from farmland washed into the lake by rainfall.

Other valid answers include: industrial waste/effluent from factories (such as the soap factory and textile plants mentioned), animal waste from farms, or detergents containing phosphates.

(b) Using Table 4:

1. Phosphate concentration increased from 0.02 mg/L in 2010 to 0.58 mg/L in 2020, a 29-fold increase [1].
2. Dissolved oxygen decreased from 8.4 mg/L to 1.6 mg/L over the same period, a decrease of 6.8 mg/L or approximately 81% [1]:

\[\text{Percentage decrease} = \frac{8.4 - 1.6}{8.4} \times 100 = 81.0\%\]

As nutrient concentration increased, dissolved oxygen declined, showing an inverse (negative) relationship [1]. This is the hallmark of eutrophication: excess nutrients drive algal growth, and when algae die, their decomposition consumes dissolved oxygen.

(c)(i) Excess nutrients lead to algal blooms through a step-by-step process:

1. Excess phosphates and nitrates entering the lake from sewage, farmland and factories provide abundant nutrients that algae need for growth [1].
2. With plentiful nutrients, algae reproduce rapidly, forming a dense bloom - a thick mat of green growth that covers the water surface [1].
3. The algal bloom blocks sunlight from reaching underwater aquatic plants. These submerged plants cannot photosynthesise, so they die, further reducing oxygen production [1].

(c)(ii) The death of large numbers of algae reduces dissolved oxygen through:

1. When large numbers of algae die, they sink to the bottom of the lake, accumulating as dead organic material on the lake bed [1].
2. Aerobic bacteria decompose this dead organic matter. During decomposition, these bacteria carry out respiration, which consumes dissolved oxygen from the water [1].
3. The oxygen demand from bacterial decomposition exceeds the rate at which oxygen dissolves into the water from the atmosphere, so dissolved oxygen levels fall rapidly - sometimes to levels where fish cannot survive (the 1.6 mg/L recorded in 2020 is well below the level most fish species need) [1].

(d) Using Fig. 4 to describe the lake food web:

1. The food web shows algae, aquatic plants and phytoplankton as producers at the base, converting sunlight energy into food through photosynthesis [1].
2. These are consumed by primary consumers including zooplankton, insect larvae and snails, which are then eaten by secondary consumers such as small fish and freshwater crabs [1].
3. Catfish and adult tilapia feed on the secondary consumers, and the fish eagle is the top predator (tertiary consumer) feeding on both catfish and adult tilapia [1].

(e)(i) A sewage treatment plant would improve water quality by:

1. Removing phosphates and nitrates from waste water before it is discharged into streams flowing into the lake. Treatment processes break down organic matter and filter out chemical pollutants [1].
2. Reducing the nutrient load entering the lake would slow algal growth and, over time, allow dissolved oxygen levels to recover as less organic matter decomposes [1].

(e)(ii) One other method of reducing nutrient inputs to the lake [1]:

• Reduce fertiliser application on farmland in the catchment area, or create buffer strips of vegetation along streams to filter nutrient-rich runoff before it reaches the lake.

Other valid answers include: regulating industrial discharges from the soap factory and textile plants, or banning phosphate-containing detergents.

(f) Declining water quality affects the local population in three ways:

1. Unsafe drinking water: Declining water quality makes the lake water unsafe for drinking and domestic use without expensive treatment that may not be available [1].
2. Food and income loss: Fish kills and declining fish populations (from 34 species in 2010 to just 7 in 2020) reduce the food supply and income from fishing for the 80,000 people who depend on the lake [1].
3. Health risks: Algal blooms can produce toxins (cyanotoxins) that are harmful to people and livestock who use the lake water for bathing, washing or irrigation [1].

(g) Two methods of monitoring the health of the lake ecosystem [1 each, max 2]:

• Regular water testing: Collect water samples at multiple stations and test for dissolved oxygen, nutrient concentrations (phosphate, nitrate) and pH to track chemical changes over time.
• Biological surveys: Conduct regular surveys to count fish species and populations, or measure the percentage of algal cover across the lake surface using aerial or satellite imagery.

Other valid answers include: monitoring invertebrate species as biological indicators of water quality, or measuring turbidity (water clarity).

(h) Managing freshwater ecosystems requires cooperation because multiple stakeholders contribute to the problem and are affected by it:

1. Multiple polluters: Farmers, factory owners and town authorities all contribute nutrients to the lake through different pathways (fertiliser runoff, industrial discharge, sewage). All groups must cooperate to reduce total nutrient inputs [1].
2. Infrastructure needs: The water authority needs to work with the government to build sewage treatment infrastructure and enforce regulations on industrial discharge. This requires funding and political will [1].
3. Scientific guidance: Scientists are needed to advise on monitoring strategies, set safe nutrient limits based on research, and evaluate whether management actions are working [1].
4. Community involvement: Local communities who depend on the lake for fish and drinking water need to be involved in management decisions. Their needs must be considered, and they must support regulations for the management plan to succeed [1].