What type of algae is described as microscopic plants usually suspended in the upper few feet of water and can reach bloom proportions?

Planktonic algae are indeed microscopic plants that thrive in aquatic environments, typically suspended in the upper layers of water, where sunlight penetration is optimal for their growth. These algae are a significant component of aquatic ecosystems and are often involved in the process of primary production, meaning they convert sunlight into energy through photosynthesis.

When conditions such as nutrient availability, temperature, and light are right, planktonic algae can proliferate rapidly, leading to blooms. These blooms can substantially impact water quality and aquatic life, influencing oxygen levels and possibly leading to harmful algal blooms that can be detrimental to fish and other organisms within the ecosystem.

The other options do not fit the description provided in the question. Filamentous algae are characterized by their long, thread-like structures, whereas Eurasian watermilfoil and curly-leaf pondweed are types of aquatic plants rather than algae. This highlights why planktonic algae is the correct answer, as it encompasses both the microscopic nature and the capability of blooming in water bodies.