The presence of large amounts of nutrients in water bodies causes excessive growth of which type of planktonic algae leading to dense algal blooms on the surface?

Introduction / Context:
Aquatic ecosystems such as lakes, ponds, and slow flowing rivers are very sensitive to changes in nutrient levels. When excess nutrients enter these water bodies from sewage, fertilisers, or industrial effluents, the growth of microscopic algae and other plankton can increase dramatically. This question asks you to identify which type of algae shows explosive growth under such nutrient rich conditions, leading to the visible phenomenon called algal bloom.

Given Data / Assumptions:

• Excess nutrients mainly refer to compounds containing nitrogen and phosphorus.
• The water body receives these nutrients from human activities such as agriculture and sewage discharge.
• Options include terms such as biomagnification, algal bloom, planktonic algae, eutrophication, and benthic algae.
• We focus on the type of algae that actually grows excessively in the water column.

Concept / Approach:
Algae that float freely in the upper layers of water are called planktonic algae. When nutrient levels are high, these algae multiply rapidly and form dense populations that colour the water green, red, or brown, depending on the species. This sudden increase in algal population is referred to as an algal bloom. Eutrophication is the general process of nutrient enrichment and its ecological effects, while biomagnification refers to accumulation of toxins in food chains. Benthic algae are attached to the bottom or submerged surfaces and are not the main contributors to surface blooms.

Step-by-Step Solution:
Step 1: Recognise that nutrient enrichment primarily stimulates the growth of microscopic algae and other phytoplankton. Step 2: Understand that these algae live suspended in the water, especially near the surface where light is available, and are called planktonic algae. Step 3: When they reproduce rapidly, the water surface becomes covered with a thick layer of algal cells. Step 4: This dense growth reduces light penetration and can lead to oxygen depletion when the algae die and decompose. Step 5: The phenomenon is known as an algal bloom, but the organisms that actually are growing are planktonic algae. Step 6: Therefore, the correct answer to the type of algae that shows excessive growth is planktonic algae.

Verification / Alternative check:
Environmental science texts describe eutrophication as nutrient enrichment that leads first to increased growth of phytoplankton, especially free floating algae and cyanobacteria. Photographs of eutrophic lakes show surface scums formed by these planktonic organisms. Field surveys and microscopic analysis of water samples from such lakes reveal high densities of planktonic algae rather than benthic forms. These observations confirm that planktonic algae are responsible for the visible blooms.

Why Other Options Are Wrong:
Biomagnification describes the increase in concentration of persistent pollutants along a food chain and does not refer to algal growth. Algal bloom is the name of the phenomenon, not the type of algae, so it does not directly answer what kind of algae is growing. Eutrophication is the overall process of nutrient enrichment and ecological change in the water body, not the specific algae type. Benthic algae live attached to rocks or sediments and are less responsible for surface blooms in nutrient rich conditions.

Common Pitfalls:
Students may confuse the terms eutrophication and algal bloom, or they may select these process names when the question asks about the algal type. To avoid this, pay attention to whether the question is asking about the process, the phenomenon, or the organisms involved. Here, the phrase excessive growth of algae clearly points to planktonic algae as the biological group that responds to nutrient enrichment.

Final Answer:
Excess nutrients in water bodies cause rapid growth of Planktonic algae, leading to algal blooms.