Plankton are microscopic organisms that drift in the water column, forming the base of almost every aquatic food web and playing a role in global ecosystems. They include plant-like phytoplankton and animal-like zooplankton, existing in vast numbers in marine and freshwater environments. A plankton bloom is a short-lived, explosive increase in the population of these organisms. This proliferation can be so dense that it visibly discolors the water, creating patches sometimes visible even from space.
The Engines of a Bloom
The rapid population increase of a plankton bloom is triggered by environmental conditions that favor exponential growth. A primary requirement is an abundance of dissolved nutrients, particularly inorganic nitrogen and phosphorus, which act as fertilizer for the phytoplankton. These nutrients enter surface waters through natural processes like upwelling or human-driven runoff from agriculture and wastewater discharge.
Warmer water temperatures accelerate the plankton’s reproductive cycles, which is why blooms often occur in spring and summer. Sunlight is also necessary for photosynthetic phytoplankton, meaning the bloom must occur in the photic zone near the water surface. Finally, a stable water column, known as stratification, keeps the phytoplankton concentrated, allowing the population to reach peak density.
The Essential Role of Plankton
Most plankton blooms support life on Earth. Phytoplankton, through photosynthesis, generate approximately half of the breathable oxygen in the atmosphere. They also form the foundation of the marine food web as primary producers that convert sunlight into organic matter. This organic matter is consumed by zooplankton, which are then preyed upon by small fish, shellfish, and larger marine mammals.
Furthermore, plankton play a significant part in the global carbon cycle. They absorb carbon dioxide from the atmosphere and sequester it in the deep ocean when they die and sink, a process known as the biological pump.
When Blooms Turn Toxic
A bloom becomes a public concern when classified as a Harmful Algal Bloom (HAB). This occurs when certain species of phytoplankton produce toxins or cause physical harm to the ecosystem. While not all massive blooms are toxic, a small number of species, such as specific dinoflagellates and cyanobacteria, can produce toxins, including neurotoxins. These toxins accumulate in shellfish and fish, causing severe human illnesses like paralytic, neurotoxic, or amnesic shellfish poisoning when contaminated seafood is consumed.
The other primary mechanism of harm is the creation of hypoxic zones, commonly called “dead zones.” When a dense bloom dies off, bacterial decomposition consumes large amounts of dissolved oxygen in the water. This rapid oxygen depletion can suffocate fish, crabs, and other aquatic life that cannot escape the area. These effects lead to economic impacts, including the closure of commercial fisheries, losses in tourism revenue, and increased costs for monitoring and water treatment.
Tracking and Forecasting Water Events
Government agencies and research institutions use technologies to monitor and predict the occurrence of plankton blooms. Satellite imagery is a powerful tool, detecting elevated levels of chlorophyll-a, the green pigment in phytoplankton, from space. Scientists analyze changes in the ocean’s color and surface reflectance to map the location and intensity of a bloom in near real-time.
In addition to remote sensing, researchers conduct regular water sampling to measure nutrient concentrations, temperature, and the presence of specific plankton species. The collected data are integrated into complex hydrodynamic models that simulate water currents and flow. These models forecast how a bloom is likely to move, grow, and spread, providing communities with advance warning to protect public health and manage coastal resources.