Pond scum is a general term for a dense surface layer of aquatic microorganisms and plants that multiply rapidly in freshwater environments. This proliferation, often called a bloom, is typically composed of diverse photosynthetic organisms, primarily various types of algae and cyanobacteria. It creates a slimy, discolored film that can cover large areas. Managing and preventing this common issue requires understanding the organisms and the conditions that trigger their growth.
The Primary Biological Components
Pond scum is a mixture dominated by two main categories: filamentous algae and cyanobacteria. Filamentous algae are simple plants that grow in long, thread-like chains of cells. These strands intertwine to form dense, mat-like accumulations that often float to the surface, resembling wet cotton or wool.
A common type is Spirogyra, which feels slimy and whose mats are often buoyed by trapped oxygen bubbles produced during photosynthesis. The other dominant component is cyanobacteria, a type of photosynthetic bacteria often mistakenly called blue-green algae. Cyanobacteria blooms generally appear as a powdery, paint-like sheen on the water’s surface that can be easily stirred.
Environmental Factors Driving Excessive Growth
Pond scum formation is driven by nutrient availability, sunlight, and water movement. The most important factor is the loading of limiting nutrients, specifically phosphorus and nitrogen, which act as fertilizer for these organisms. These nutrients enter the water body through stormwater runoff, carrying residue from agricultural fertilizers, septic discharge, decaying organic matter, or animal waste.
The addition of these nutrients, known as eutrophication, allows algae and cyanobacteria to multiply rapidly. Warm temperatures and ample sunlight accelerate this growth by increasing the organisms’ metabolic rate and reproductive cycles. Cyanobacteria thrive when water temperatures exceed 77°F (25°C), making blooms most common during the late spring and summer months.
A lack of water circulation also contributes to surface scum formation. Still or slow-moving water allows organisms to aggregate without being dispersed or diluted. This stagnation prevents the natural mixing of the water column, which contributes to the formation of thick, visible mats.
Identifying Hazardous Blooms
While many forms of pond scum are merely unsightly, blooms dominated by cyanobacteria carry a risk of toxicity and are known as Harmful Algal Blooms (HABs). Certain species produce cyanotoxins, including neurotoxins that affect the nervous system and hepatotoxins that target the liver. Exposure to these toxins can cause serious illness or death in humans, pets, and wildlife.
Differentiating a hazardous bloom from harmless filamentous algae is important for public safety. Cyanobacteria blooms often look like spilled paint, pea soup, or a thick surface film that may be bright green, blue-green, or reddish-brown. In contrast, filamentous algae form distinct, long, stringy mats that can be physically pulled out of the water.
Since the presence of cyanotoxins cannot be confirmed by sight alone, laboratory testing is the only definitive way to assess the risk level. When a bloom is suspected, the safest course of action is to assume it is toxic and avoid all contact with the water.
Strategies for Controlling Pond Scum Formation
Controlling pond scum requires a multi-faceted approach that addresses the underlying environmental causes, primarily the excess nutrient supply. Preventing runoff from entering the water body is a long-term strategy. This involves reducing the use of nitrogen and phosphorus fertilizers on surrounding lawns and gardens and establishing vegetative buffer zones that filter stormwater before it reaches the pond.
Physical management techniques can also be employed to disrupt the conditions that favor excessive growth. Installing an aeration system increases oxygen levels and promotes constant water circulation, which prevents the water from becoming stagnant. Aeration also helps to break down organic waste, which further reduces the available nutrient load. Limiting the amount of direct sunlight reaching the water can inhibit photosynthesis. This can be achieved by adding aquatic plants that provide shade or by using pond dye, which filters light penetration.