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, diatom 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.
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