Spring Bloom - The Standard Spring Bloom Mechanism

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 photosynthesis. 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.