Spring bloom
Encyclopedia
The spring bloom is a strong increase in phytoplankton
abundance (i.e. stock) that typically occurs in the early spring and lasts until late spring or early summer. This seasonal event is characteristic of temperate North Atlantic, sub-polar, and coastal waters. The magnitude, spatial extent and duration of a bloom depends on a variety of environmental conditions, such as light availability, nutrients, temperature, and stratification
of the water column. The initial phytoplankton stock size is also important.
. However, vertical mixing also causes high losses, as phytoplankton are carried below the euphotic zone (so their respiration
exceeds primary production). In addition, reduced illumination (intensity and daily duration) during winter limits growth rates.
In the spring, more light becomes available and stratification of the water column occurs as increasing temperatures warming the surface waters (referred to as thermal stratification). As a result, vertical mixing is inhibited and phytoplankton and nutrients are held at the surface. This coupling of nutrients and phytoplankton promotes exponential increases in photosynthetic activity, and, thus, primary production.
Along with thermal stratification, spring blooms can be triggered by salinity stratification due to freshwater input, from sources such as high river runoff. This type of stratification is normally limited to coastal areas and estuaries, including Chesapeake Bay. Freshwater influences primary productivity in two ways. First, because freshwater is less dense, it rests on top of seawater and creates a stratified water column. Second, freshwater often carries nutrients that phytoplankton need to carry out processes, including photosynthesis.
Drastic increases in phytoplankton growth, such as that which occurs during the spring bloom, take place because phytoplankton can reproduce rapidly under optimal growth conditions (i.e. high nutrients, ideal illumination and temperature, and minimal losses due to grazing and vertical mixing). In terms of reproduction, many species of phytoplankton can double at least once per day, allowing for exponential increases in phytoplankton stock size. For example, the stock size of a population that doubles once per day will increase 1000-fold in just 10 days. In addition, there is a lag in the grazing response of herbivorous zooplankton
at the start of blooms, which further promotes rapid growth of phytoplankton. This lag occurs because there is lower zooplankton abundance coming out of winter and many zooplankton, such as copepods, require weeks to reproduce rather than the hours or days it takes phytoplankton.
Spring blooms typically last until late spring or early summer, at which time the bloom collapses due to nutrient depletion in the stratified water column and increased grazing pressure by zooplankton. The most limiting nutrient in the marine environment is typically nitrogen
(N). This is because most organisms are unable to fix atmospheric nitrogen into usable forms (i.e. ammonium, nitrite, or nitrate). However, with the exception of coastal waters, it can be argued, that iron
(Fe) is the most limiting nutrient because it is required to fix nitrogen, but is only available in small quantities in the marine environment, coming from dust storms and leaching from rocks. Phosphorus
can also be limiting, particularly in freshwater environments and tropical coastal regions.
Hypothesis, which says blooms are caused by shoaling of the mixed layer. Similarly, Winder and Cloern (2010) described spring blooms as a response to increasing temperature and light availability. However, new explanations have been offered recently, including that blooms occur due to:
At high latitudes, the shorter warm season commonly results in one mid-summer bloom. These blooms tend to be more intense than spring blooms of temperate areas because there is a longer duration of daylight for photosynthesis to take place. Also, grazing pressure tends to be lower because the generally cooler temperatures at higher latitudes slow zooplankton metabolism.
growth rate becomes limited when the supply of silicate
is depleted. Since silicate is not required by other phytoplankton, such as dinoflagellates, their growth rates continue to increase.
For example, in oceanic environments, diatoms (cells diameter >10 to 70 µm or larger) typically dominate first because they are capable of growing faster. Once silicate is depleted in the environment, diatoms are succeeded by smaller dinoflagellates. This scenario has been observed in Rhode Island, as well as Massachusetts and Cape Cod Bay. By the end of a spring bloom, when most nutrients have been depleted, the majority of the total phytoplankton biomass
is very small phytoplankton, known as ultraphytoplankton (cell diameter <5 to 10 µm). Ultraphytoplankton can sustain low, but constant stocks, in nutrient depleted environments because they have a larger surface area to volume ratio
, which offers a much more effective rate of diffusion
. The types of phytoplankton comprising a bloom can be determined by examination of the varying photosynthetic pigments found in chloroplasts of each species.
Links have been found between temperature and spring bloom patterns. For example, several studies have reported the earlier onset of the spring bloom to be correlated with temperature increases over time. Furthermore, in Long Island Sound and the Gulf of Maine, blooms begin later in the year, are more productive, and last longer during colder years, while years that are warmer exhibit earlier, shorter blooms of greater magnitude.
Temperature may also regulate bloom sizes. In Narragansett Bay, Rhode Island, a study by Durbin et al. (1992) indicated that a 2°C increase in water temperature resulted in a three week shift in the maturation of the copepod, Acartia hudsonica, which could significantly increase zooplankton grazing intensity. Oviatt et al. (2002) noted a reduction in spring bloom intensity and duration in years when winter water temperatures were warmer. Oviatt et al. suggested that the reduction was due to increased grazing pressure, which could potentially become intense enough to prevent spring blooms from occurring altogether.
Miller and Harding (2007) suggested climate change (influencing winter weather patterns and freshwater influxes) was responsible for shifts in spring bloom patterns in the Chesapeake Bay. They found that during warm, wet years (as opposed to cool, dry years) the spatial extent of blooms were larger and were positioned more seaward. Also, during these same years, biomass was higher and peak biomass occurred later in the spring.
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
abundance (i.e. stock) that typically occurs in the early spring and lasts until late spring or early summer. This seasonal event is characteristic of temperate North Atlantic, sub-polar, and coastal waters. The magnitude, spatial extent and duration of a bloom depends on a variety of environmental conditions, such as light availability, nutrients, temperature, and stratification
Stratification
Stratification is the building up of layers. Stratified is an adjective referring to the arranging of layers, and is also the past form of the verb stratify, to separate or become separated into layers...
of the water column. The initial phytoplankton stock size is also important.
The standard spring bloom mechanism
While other mechanisms have been described (see below), the standard explanation is that during winter, wind-driven turbulence (often derived from storms) and cooling water temperatures break down the stratified water column formed during the summer. This breakdown allows vertical mixing of the water column. This mixing replenishes nutrients from depth to the surface waters and the rest of the euphotic zone. Phytoplankton use these nutrients to perform photosynthesisPhotosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
. However, vertical mixing also causes high losses, as phytoplankton are carried below the euphotic zone (so their respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
exceeds primary production). In addition, reduced illumination (intensity and daily duration) during winter limits growth rates.
In the spring, more light becomes available and stratification of the water column occurs as increasing temperatures warming the surface waters (referred to as thermal stratification). As a result, vertical mixing is inhibited and phytoplankton and nutrients are held at the surface. This coupling of nutrients and phytoplankton promotes exponential increases in photosynthetic activity, and, thus, primary production.
Along with thermal stratification, spring blooms can be triggered by salinity stratification due to freshwater input, from sources such as high river runoff. This type of stratification is normally limited to coastal areas and estuaries, including Chesapeake Bay. Freshwater influences primary productivity in two ways. First, because freshwater is less dense, it rests on top of seawater and creates a stratified water column. Second, freshwater often carries nutrients that phytoplankton need to carry out processes, including photosynthesis.
Drastic increases in phytoplankton growth, such as that which occurs during the spring bloom, take place because phytoplankton can reproduce rapidly under optimal growth conditions (i.e. high nutrients, ideal illumination and temperature, and minimal losses due to grazing and vertical mixing). In terms of reproduction, many species of phytoplankton can double at least once per day, allowing for exponential increases in phytoplankton stock size. For example, the stock size of a population that doubles once per day will increase 1000-fold in just 10 days. In addition, there is a lag in the grazing response of herbivorous zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
at the start of blooms, which further promotes rapid growth of phytoplankton. This lag occurs because there is lower zooplankton abundance coming out of winter and many zooplankton, such as copepods, require weeks to reproduce rather than the hours or days it takes phytoplankton.
Spring blooms typically last until late spring or early summer, at which time the bloom collapses due to nutrient depletion in the stratified water column and increased grazing pressure by zooplankton. The most limiting nutrient in the marine environment is typically nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(N). This is because most organisms are unable to fix atmospheric nitrogen into usable forms (i.e. ammonium, nitrite, or nitrate). However, with the exception of coastal waters, it can be argued, that iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(Fe) is the most limiting nutrient because it is required to fix nitrogen, but is only available in small quantities in the marine environment, coming from dust storms and leaching from rocks. Phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
can also be limiting, particularly in freshwater environments and tropical coastal regions.
Alternative mechanisms of spring blooms
Historically, blooms have been explained by Sverdrup's Critical DepthCritical Depth
In biological oceanography, Critical Depth is defined as a hypothesized surface mixing depth at which phytoplankton growth is precisely matched by losses of phytoplankton biomass within this depth interval. -History:...
Hypothesis, which says blooms are caused by shoaling of the mixed layer. Similarly, Winder and Cloern (2010) described spring blooms as a response to increasing temperature and light availability. However, new explanations have been offered recently, including that blooms occur due to:
- Coupling between phytoplankton growth and zooplankton grazing. In this scenario, blooms occur when physical processes (seasonally variable) influence this balance and favor phytoplankton growth.
- Mixing of the water column, rather than stratification
- Low turbulence
- Increasing light intensity (in shallow water environments).
Northward progression of spring blooms
At greater latitudes, spring blooms take place later in the year. This northward progression is because spring occurs later, delaying thermal stratification and increases in illumination that promotes blooms. A study by Wolf and Woods (1988) showed evidence that spring blooms follow the northward migration of the 12°C isotherm, suggesting that blooms may be controlled by temperature limitations, in addition to stratification.At high latitudes, the shorter warm season commonly results in one mid-summer bloom. These blooms tend to be more intense than spring blooms of temperate areas because there is a longer duration of daylight for photosynthesis to take place. Also, grazing pressure tends to be lower because the generally cooler temperatures at higher latitudes slow zooplankton metabolism.
Species succession during spring blooms
The spring bloom often consists of a series of sequential blooms of different phytoplankton species. Succession occurs because different species have optimal nutrient uptake at different ambient concentrations and reach their growth peaks at different times. Shifts in the dominant phytoplankton species are likely caused by biological and physical (i.e. environmental) factors. For instance, diatomDiatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
growth rate becomes limited when the supply of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
is depleted. Since silicate is not required by other phytoplankton, such as dinoflagellates, their growth rates continue to increase.
For example, in oceanic environments, diatoms (cells diameter >10 to 70 µm or larger) typically dominate first because they are capable of growing faster. Once silicate is depleted in the environment, diatoms are succeeded by smaller dinoflagellates. This scenario has been observed in Rhode Island, as well as Massachusetts and Cape Cod Bay. By the end of a spring bloom, when most nutrients have been depleted, the majority of the total phytoplankton biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
is very small phytoplankton, known as ultraphytoplankton (cell diameter <5 to 10 µm). Ultraphytoplankton can sustain low, but constant stocks, in nutrient depleted environments because they have a larger surface area to volume ratio
Surface area to volume ratio
The surface-area-to-volume ratio also called the surface-to-volume ratio and variously denoted sa/vol or SA:V, is the amount of surface area per unit volume of an object or collection of objects. The surface-area-to-volume ratio is measured in units of inverse distance. A cube with sides of...
, which offers a much more effective rate of diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
. The types of phytoplankton comprising a bloom can be determined by examination of the varying photosynthetic pigments found in chloroplasts of each species.
Spring bloom variability and the influence of climate change
Variability in the patterns (e.g. timing of onset, duration, magnitude, position, and spatial extent) of annual spring bloom events has been well documented. These variations occur due to fluctuations in environmental conditions, such as wind intensity, temperature, freshwater input, and light. Consequently, spring bloom patterns are likely sensitive to global climate change.Links have been found between temperature and spring bloom patterns. For example, several studies have reported the earlier onset of the spring bloom to be correlated with temperature increases over time. Furthermore, in Long Island Sound and the Gulf of Maine, blooms begin later in the year, are more productive, and last longer during colder years, while years that are warmer exhibit earlier, shorter blooms of greater magnitude.
Temperature may also regulate bloom sizes. In Narragansett Bay, Rhode Island, a study by Durbin et al. (1992) indicated that a 2°C increase in water temperature resulted in a three week shift in the maturation of the copepod, Acartia hudsonica, which could significantly increase zooplankton grazing intensity. Oviatt et al. (2002) noted a reduction in spring bloom intensity and duration in years when winter water temperatures were warmer. Oviatt et al. suggested that the reduction was due to increased grazing pressure, which could potentially become intense enough to prevent spring blooms from occurring altogether.
Miller and Harding (2007) suggested climate change (influencing winter weather patterns and freshwater influxes) was responsible for shifts in spring bloom patterns in the Chesapeake Bay. They found that during warm, wet years (as opposed to cool, dry years) the spatial extent of blooms were larger and were positioned more seaward. Also, during these same years, biomass was higher and peak biomass occurred later in the spring.