Spring bloom

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). 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). 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. This creates a comparatively high nutrient and high light environment that allows rapid phytoplankton growth. 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. Rapid increases in phytoplankton growth, that typically occur during the spring bloom, arise because phytoplankton can reproduce rapidly under optimal growth conditions (i.e., high nutrient levels, ideal light and temperature, and minimal losses from 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 minimize phytoplankton losses. This lag occurs because there is low winter zooplankton abundance and many zooplankton, such as copepods, have longer generation times than 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. During winter, wind-driven turbulence and cooling water temperatures break down the stratified water column formed during the summer. This breakdown allows vertical mixing of the water column and replenishes nutrients from deep water to the surface waters and the rest of the euphotic zone. 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. Historically, blooms have been explained by Sverdrup's critical depth 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 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. 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.