Algae are diverse aquatic organisms that perform photosynthesis, similar to plants. They range from microscopic single-celled forms, like diatoms, to large multicellular seaweeds, such as kelp. Naturally present in nearly all water bodies, they form the base of aquatic food webs, producing much of Earth’s oxygen and serving as food for various aquatic life. While a natural and essential component of healthy aquatic ecosystems, specific environmental conditions can lead to their rapid, excessive growth, often called algal blooms.
Key Environmental Triggers
Sunlight is essential for algae to grow and reproduce. Increased light penetration, whether direct or reflected, significantly accelerates algal growth. Even indirect sunlight can provide enough energy for algae to proliferate in covered pools. However, excessive exposure can be detrimental, as ultraviolet radiation can damage algal cells.
Water temperature influences algal growth rates. Warmer water speeds up metabolic processes, leading to faster reproduction and increased biomass. Many problematic bloom species, particularly cyanobacteria (blue-green algae), thrive in warmer conditions, some preferring temperatures above 25°C. Elevated temperatures can also prevent water mixing, allowing algae to grow thicker and faster near the surface where light is abundant.
Water movement affects algal accumulation. Still or slow-moving water bodies, like ponds, lakes, or uncirculated pools, provide an ideal environment for algae to settle, multiply, and form dense blooms. Strong currents or turbulent water disperses algae, making it challenging for them to concentrate. Stagnant conditions also allow nutrients to accumulate, further promoting growth.
Role of Nutrient Enrichment
The most significant factor contributing to excessive algae growth, especially harmful algal blooms, is nutrient enrichment. Phosphorus and nitrogen are primary nutrients that act as fertilizers, enabling rapid proliferation. In freshwater systems, phosphorus is often the limiting nutrient, largely dictating algal growth. If phosphorus is abundant, even small human activities can increase algal concentrations.
Agricultural runoff is a major contributor of excess nutrients, carrying nitrogen and phosphorus from fertilizers and animal manure into waterways. When not fully utilized by crops, these nutrients can leach into groundwater or be washed into streams and rivers during rainfall.
Wastewater discharge, including untreated or inadequately treated sewage from residential and industrial sources, is a significant source of nutrient pollution. Improperly functioning sewer and septic systems can release substantial nitrogen and phosphorus directly into aquatic environments.
Urban runoff also contributes to nutrient enrichment. Precipitation on hard surfaces in cities, such as rooftops and roads, collects pollutants including nitrogen and phosphorus from sources like pet waste, detergents, and lawn fertilizers. This nutrient-laden runoff then flows into storm drains and ultimately into local waterways.
Natural processes also release nutrients into water bodies. Decomposition of organic matter, such as decaying leaves, dead plants, and animals, releases phosphorus and nitrogen back into the water. This provides a natural nutrient source for algae, contributing to the overall nutrient load that can trigger excessive growth.