![]() ![]() If we assume that the river contains negligible phytoplankton, then river water dilutes phytoplankton densities in the lake at a rate equivalent to the river flow rate divided by lake volume. For example, riverine water inputs to a lake are matched by outflows from the lake. In a water body with constant volume, any water inputs must be accompanied by an equivalent water loss. Photo credit, Chowan/Edenton Environmental Groupĭilution. Surface scum of the cyanobacteria, Microcystsis sp. Cyanobacteria are notorious for accumulating by flotation into dense surface scums (Figure 2).įigure 2. ![]() Many phytoplankton, particularly bloom-forming taxa, avoid settling losses by having either flagella that allow them to swim or ballast mechanisms that provide buoyancy. According to Stokes’ law, larger and more dense phytoplankton tend to sink faster (0.1-1 m/d) while settling velocities are negligible for the smallest phytoplankton (0.001 m/d). Consequently, most phytoplankton are constantly sinking and require turbulent mixing to stay in the upper mixed layer where light levels are appropriate for growth. Phytoplankton are typically 3 to 5 percent denser than their surrounding environment. High cell densities of a single-species bloom favor the spread of infections during blooms, and can result in rapid bloom termination. Infections by viruses, fungi, bacteria, and protists can also contribute substantially to phytoplankton mortality. In shallow systems with a relatively low ratio of volume to benthic surface area, grazing by benthic bivalves can be substantial and dominate grazing losses. Grazing by protistan zooplankton is often the dominant source of mortality for phytoplankton in marine waters, while grazing by crustacean zooplankton is often more important in freshwaters. Consumption by organisms at higher trophic levels generally constitutes the largest source of mortality for phytoplankton. In estuaries, salinity differences between upper and lower layers can enhance vertical stratification, creating similar favorable conditions for phytoplankton growth in the upper layer. Strong density gradients resist mixing by wind or currents and can confine phytoplankton to a shallow upper mixed layer where there is enough light for phytoplankton growth. Additionally, heating of surface waters can create a surface layer that is less dense than cooler bottom waters, separated by a region of strong density gradient called the thermocline. During summer, higher light levels and higher water temperatures promote phytoplankton growth. ![]() The amount of light available for phytoplankton growth varies according to time of year, extent of cloud cover, and water depth and clarity. While anthropogenic loads increase the probability that a bloom will occur, there must also be sufficient light and low enough loss rates (mortality, sedimentation and dilution) for a bloom to develop. Humans contribute to nitrogen and phosphorus loads primarily through wastewater inputs and runoff from agricultural, urban, and residential land. However, many exceptions to these trends exist. Nitrogen supplies tend to be limited in marine waters, and phosphorus supplies tend to be limited in freshwaters. Nitrogen and phosphorus are the nutrients most likely to be in short supply relative to demand, and by Liebig’s law of the minimum, are the primary growth-limiting nutrients. ![]() Limited supplies of light or nutrients can slow or stop cell division, preventing bloom formation. Like terrestrial plants, phytoplankton require sunlight and inorganic nutrients to produce new biomass. The total rate of cell loss is driven by three mechanisms: mortality, dilution, and sedimentation. The division rate, also called the intrinsic growth rate, is the rate of new cell production. The net growth rate can be described asĮquation 1: Net Growth = Division – Mortality – Sedimentation – Dilution. Phytoplankton come in many shapes and sizes.įor a phytoplankton bloom to occur, the net growth rate of a population must be positive for enough time to build a high biomass level. ![]()
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