Question

runoff leads to algal blooms.
as algae die, consume oxygen during respiration.
low oxygen levels stress and disrupt food webs.
this demonstrates links between energy flow, cycling, and population
effects.

Explanation:

1. The first blank: Nutrient (specifically like nitrogen and phosphorus) runoff from sources like fertilizers, sewage etc. enriches water bodies, leading to excessive growth of algae (algal blooms). So "Nutrient" (or more specifically "Nitrogen and Phosphorus" but generally "Nutrient") fits here.
2. The second blank: When algae die, decomposers (like bacteria and fungi) break down the dead algal matter. During this decomposition (respiration - like process for decomposers), they consume oxygen. So "decomposers (bacteria/fungi)" is appropriate.
3. The third blank: Low oxygen levels (hypoxia) in water stress aquatic organisms (like fish, invertebrates, aquatic plants that need oxygen) and disrupt the food webs as these organisms are part of the food chain/ web. So "aquatic organisms (fish, invertebrates etc.)" fits.
4. The fourth blank: This eutrophication process shows links between energy flow, nutrient (like carbon, nitrogen, phosphorus) cycling (as nutrients are cycled through the system - from runoff, to algae, to decomposers, affecting nutrient availability) and population effects (on the organisms in the water body). So "nutrient" (specifically the nutrients involved in the cycle like N, P, C) is correct.

Answer:

1. Nutrient (or Nitrogen - Phosphorus)
2. decomposers (bacteria / fungi)
3. aquatic organisms (fish, invertebrates, etc.)
4. nutrient (e.g., carbon, nitrogen, phosphorus)