Brown algae is a common name applied to various organisms, ranging from microscopic, single-celled life to massive seaweeds forming underwater forests. Whether it presents a problem depends entirely on its specific form and environment. The characteristic brown or golden-brown color comes from the pigment fucoxanthin, which masks the green chlorophyll. While some brown algae are fundamental to ocean health, others can become a significant nuisance or ecological threat when conditions allow for uncontrolled growth.
Identifying the Two Main Forms of Brown Algae
The term “brown algae” generally refers to two distinct groups: the class Phaeophyceae and the microscopic organisms known as diatoms. Phaeophyceae are the true brown algae, which are exclusively multicellular and include the large seaweeds, like kelp and Sargassum. These species are found almost entirely in marine environments, predominantly in temperate and polar regions.
Diatoms are single-celled organisms belonging to the class Bacillariophyceae, and they are the form most frequently encountered in freshwater and new aquarium setups. They are structurally unique because their cell walls, called frustules, are encased in silica, essentially a form of glass. Both diatoms and Phaeophyceae are grouped together due to their similar photosynthetic pigments.
Brown Algae as an Aquatic Nuisance
Diatoms are considered a nuisance when they appear as a fine, dusty, brown or golden-brown film coating surfaces in controlled aquatic environments, such as home aquariums and ponds. This bloom is especially common in newly established tanks during the initial cycling phase. The primary reason diatoms proliferate is the availability of silicates, which they require to construct their silica-based cell walls.
Silicates are often introduced through tap water, new substrates, or certain decorative elements within the tank. Diatoms are also opportunistic, utilizing other nutrients like nitrates and phosphates that are common in new or unbalanced systems. While generally harmless to fish, a dense diatom layer is aesthetically unpleasing and can smother the leaves of slow-growing aquatic plants, blocking light and hindering their growth. A persistent diatom bloom beyond the initial cycling phase usually indicates a high silicate level in the source water or an ongoing nutrient imbalance.
Large Scale Marine Impacts
In the marine environment, the impact of brown algae is characterized by a stark duality. Large Phaeophyceae species like kelp are foundational to coastal ecosystems, forming extensive kelp forests that provide shelter, food, and habitat for hundreds of marine species. These forests are highly productive, contributing to biodiversity, sequestering carbon, and helping to buffer coastlines against erosion.
Conversely, massive, uncontrolled blooms of true brown algae, such as the free-floating Sargassum, have become an environmental problem on a global scale. While Sargassum provides important habitat while floating offshore, unprecedented volumes are now washing ashore in annual inundation events across the Caribbean and Gulf of Mexico. These large influxes of seaweed lead to the fouling of beaches, which damages tourism and local economies.
As the massive mats decompose on shore, they release hydrogen sulfide gas, which smells like rotten eggs and can pose a public health concern. Near the coast, dense Sargassum rafts can smother sensitive habitats like coral reefs and seagrass beds, blocking the sunlight they need to survive. The increased scale of these blooms is strongly linked to elevated nutrient runoff, particularly nitrogen, from human activities.
Strategies for Algae Control and Removal
For small-scale aquatic nuisance issues caused by diatoms, management strategies focus on both mechanical removal and environmental control. The brown film is easily removed by gently wiping tank glass with a scraper and siphoning it off substrate during regular water changes. This manual removal prevents the film from overwhelming plants and equipment.
Long-term prevention depends on addressing the root causes of the bloom: silicates and excess nutrients. If the source water contains high levels of silicates, using reverse osmosis or deionized water for tank refills can significantly reduce the diatoms’ primary food source. Limiting light exposure to between eight and ten hours per day, along with ensuring proper filtration, helps to keep the environment less favorable for diatom growth. Introducing algae-eating species, such as certain snails and shrimp, can also provide continuous biological control.