Algal bloom
An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in fresh water or marine water systems. It may be a benign or harmful algal bloom.
Algal bloom is often recognized by the discoloration in the water from the algae's pigments. The term algae encompasses many types of aquatic photosynthetic organisms, both macroscopic multicellular organisms like seaweed and microscopic unicellular organisms like cyanobacteria. Algal bloom commonly refers to the rapid growth of microscopic unicellular algae, not macroscopic algae. An example of a macroscopic algal bloom is a kelp forest.
Algal blooms are the result of a nutrient, like nitrogen or phosphorus from various sources, entering the aquatic system and causing excessive growth of algae. An algal bloom affects the whole ecosystem.
Consequences range from benign effects, such as feeding of higher trophic levels, to more harmful effects like blocking sunlight from reaching other organisms, causing a depletion of oxygen levels in the water, and, depending on the organism, releasing toxins into the water. Yet, algae also play a crucial role by producing about 70 % of Earth's oxygen, which supports terrestrial life. Blooms that can injure animals or the ecology, especially those blooms where toxins are secreted by the algae, are usually called "harmful algal blooms", and can lead to fish die-offs, cities cutting off water to residents, or states having to close fisheries. The process of the oversupply of nutrients leading to algae growth and oxygen depletion is called eutrophication.
Algal and bacterial blooms have persistently contributed to mass extinctions driven by global warming in the geologic past, such as during the end-Permian extinction driven by Siberian Traps volcanism and during the biotic recovery following the mass extinction.
Description
The term algal bloom is defined inconsistently depending on the scientific field, and can range from a "minibloom" of harmless algae to a large, harmful bloom event. Since algae is a broad term including organisms of widely varying sizes, growth rates, and nutrient requirements, there is no officially recognized threshold level as to what is defined as a bloom. Because there is no scientific consensus, blooms can be described and quantified in several ways: measurements of new algal biomass, the concentration of photosynthetic pigment, quantification of the bloom's negative effect, or relative concentration of the algae compared to the rest of the microbial community. For example, definitions of blooms have included:- concentration of chlorophyll exceeding 100 μg/L,
- concentration of chlorophyll exceeding 5 μg/L,
- concentration of the species considered to be blooming in excess of 1000 cells/mL, and
- algae species concentration simply deviating from its normal growth.
Algal blooms, especially large algal bloom events, can reduce the transparency of the water and can discolor it. The photosynthetic pigments in the algal cells, like chlorophyll and photoprotective pigments, determine the color of the algal bloom. Depending on the organism, its pigments, and the depth in the water column, algal blooms can be green, red, brown, golden, or purple. Bright green blooms in freshwater systems are frequently a result of cyanobacteria such as Microcystis. Blooms may also consist of macroalgal species. These blooms are recognizable by large blades of algae that may wash up onto the shoreline.
Once the nutrient is present in the water, the algae begin to grow at a much faster rate than usual. In a mini bloom, this fast growth benefits the whole ecosystem by providing food and nutrients for other organisms.
Of particular note are the harmful algal blooms, which are algal bloom events involving toxic or otherwise harmful phytoplankton. Many species can cause harmful algal blooms. For example,
- Gymnodinium nagasakiense can cause harmful red tides,
- dinoflagellates Gonyaulax polygramma can cause oxygen depletion and result in large fish kills,
- cyanobacteria Microcystis aeruginosa can make toxins, and
- diatom Chaetoceros convolutus can damage fish gills.
Freshwater algal blooms
The reduction of phosphorus inputs is required to mitigate blooms that contain cyanobacteria. In lakes that are stratified in the summer, autumn turnover can release substantial quantities of bio-available phosphorus potentially triggering algal blooms as soon as sufficient photosynthetic light is available. Excess nutrients can enter watersheds through water runoff. Excess carbon and nitrogen have also been suspected as causes. Presence of residual sodium carbonate acts as catalyst for the algae to bloom by providing dissolved carbon dioxide for enhanced photosynthesis in the presence of nutrients.
When phosphates are introduced into water systems, higher concentrations can cause increased growth of algae and plants. Algae tend to grow very quickly under high nutrient availability, but each alga is short-lived, and the result is a high concentration of dead organic matter which starts to decompose. Natural decomposers present in the water begin decomposing the dead algae, consuming dissolved oxygen present in the water during the process. This can result in a sharp decrease in available dissolved oxygen for other aquatic life. Without sufficient dissolved oxygen in the water, animals and plants may die off in large numbers. This may also be known as a dead zone.
Blooms may be observed in freshwater aquariums when fish are overfed and excess nutrients are not absorbed by plants. These are generally harmful for fish, and the situation can be corrected by changing the water in the tank and then reducing the amount of food given.
Natural causes of algal blooms
Algal blooms in freshwater systems are not always caused by human contamination and have been observed to occur naturally in both eutrophic and oligotrophic lakes. Eutrophic lakes contain an abundance of nutrients such as nitrogen and phosphates which increase the likelihood for blooms. Oligotrophic lakes don't contain much of these nutrients. Oligotrophic lakes are defined by various degrees of scarcity. The trophic state index measures nutrients in freshwater systems and a TSI under 30 defines oligotrophic waters. Algal blooms in oligotrophic bodies of water have also been observed. This is a result of cyanobacteria which cause blooms in eutrophic lakes and oligotrophic lakes despite the latter containing a lack of natural and man-made nutrients.Nutrient uptake and cyanobacteria
A cause for algal blooms in nutrient-lacking environments come in the form of nutrient uptake. Cyanobacteria have evolved to have better nutrient uptake in oligotrophic waters. Cyanobacteria utilize nitrogen and phosphates in their biological processes. Because of this, cyanobacteria are known to be important in the nitrogen and phosphate fixing cycle in oligotrophic waters. Cyanobacteria can fix nitrogen by accessing atmospheric nitrogen that has been dissolved into water and transforming it into nitrogen accessible to other organisms. This higher amount of nitrogen is then able to sustain large algae blooms in oligotrophic waters.Cyanobacteria are able to retain high phosphorus uptake in the absence of nutrients which help their success in oligotrophic environments. Cyanobacteria species such as D. lemmermannii are able to move between the hypolimnion which is rich in nutrients such as phosphates and the nutrient-poor metalimnion which lacks phosphates. This causes phosphates to be brought up to the metalimnion and give organisms an abundance of phosphates, exacerbating the likelihood for algal blooms.
Upwelling of nutrients
Upwelling events happen when nutrients such as phosphates and nitrogen are moved from the nutrient dense hypolimnion to the nutrient poor metalimnion. This happens as result of geological processes such as seasonal overturn when lake surfaces freeze or melt, prompting mixing due to changing water densities mixing up the composition of limnion layers and mixing nutrients around the system. This overabundance in nutrients leads to blooms.Marine algal blooms
Turbulent storms churn the ocean in summer, adding nutrients to sunlit waters near the surface. This sparks a feeding frenzy each spring that gives rise to massive blooms of phytoplankton. Tiny molecules found inside these microscopic plants harvest vital energy from sunlight through photosynthesis. The natural pigments, called chlorophyll, allow phytoplankton to thrive in Earth's oceans and enable scientists to monitor blooms from space.Satellites reveal the location and abundance of phytoplankton by detecting the amount of chlorophyll present in coastal and open waters—the higher the concentration, the larger the bloom. Observations show blooms typically last until late spring or early summer, when nutrient stocks are in decline and predatory zooplankton start to graze. The visualization on the left immediately below uses NASA SeaWiFS data to map bloom populations.
The NAAMES study conducted between 2015 and 2019 investigated aspects of phytoplankton dynamics in ocean ecosystems, and how such dynamics influence atmospheric aerosols, clouds, and climate.
In France, citizens are requested to report coloured waters through the project PHENOMER. This helps to understand the occurrence of marine blooms.
Wildfires can cause phytoplankton blooms via oceanic deposition of wildfire aerosols.