thumb|Spring bloom in the currents off the coast of [[New Zealand. Especially bright blue areas may indicate the presence of phytoplankton called coccolithophores, which are coated with calcium carbonate scales that are very reflective. The duller greenish-brown areas of the bloom may be diatoms, which have a silica-based covering.]]
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. Phytoplankton blooms occur when growth exceeds losses, however there is no universally accepted definition of the magnitude of change or the threshold of abundance that constitutes a bloom. The magnitude, spatial extent and duration of a bloom depends on a variety of abiotic and biotic factors. Abiotic factors include light availability, nutrients, temperature, and physical processes that influence light availability, and biotic factors include grazing, viral lysis, and phytoplankton physiology. The factors that lead to bloom initiation are still actively debated (see Critical depth).
Classical mechanism
In the spring, more light becomes available and stratification of the water column occurs as increasing temperatures warm the surface waters (referred to as thermal stratification). As a result, vertical mixing is inhibited and phytoplankton and nutrients are entrained in the euphotic zone. 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. However, new explanations have been offered recently, including that blooms occur due to:
- Coupling between phytoplankton growth and zooplankton grazing.
- The onset of near surface stratification in the spring.
- Mixing of the water column, rather than stratification
- Low turbulence
- Increasing light intensity (in shallow water environments).
The role of eddies in the onset of the North Atlantic spring bloom
A 2012 study showed that the onset of the North Atlantic bloom is due to eddies. Eddies, or circular currents of water, are ubiquitous throughout the world’s ocean and play an important role in ocean mixing. In the North Atlantic, surface water is colder and denser farther north and warmer and lighter in the south. This sets up a horizontal density gradient. Earth’s rotation maintains this gradient by preventing the dense water from slipping underneath the light water. Eddies, however, can mix dense water underneath the lighter water, setting up a vertical stratification that limits the depth of vertical mixing (leading to a shallower mixed layer).
Mechanisms that limit the depth of vertical mixing can be referred to as ‘restratifying mechanisms’ (e.g. eddies, solar heating), which compete against mechanisms that increase vertical mixing (and deepen the mixed layer). This includes convection and down-front winds. Convection is strongest in the winter when surface cooling is strongest. Convection increases the depth of vertical mixing, which can move phytoplankton away from the light they need to grow.
When convection weakens and wind switches direction in the spring, the re-stratifying effect of eddies becomes dominant. Phytoplankton are trapped closer to the surface, increasing their exposure to light. This spurs phytoplankton growth, leading to the onset of the North Atlantic spring bloom 20-30 days earlier than would occur with thermal stratification alone.
Northward progression
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 promote 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. 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 greater than 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. 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).
Links have been found between temperature and spring bloom patterns. For example, several studies have reported a correlation between earlier spring bloom onset and temperature increases over time. 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) 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 was larger and was positioned more seaward. Also, during these same years, biomass was higher and peak biomass occurred later in the spring.
See also
- Algal bloom
- Critical depth
- Gordon Arthur Riley
- Plankton