How can a population explosion of phytoplankton kill

Changes in ocean circulation and water temperature both critically influence which phytoplankton live and which die, says Oscar Schofield , oceanography professor at Rutgers University.

The conditions of the ocean might favor a particular toxic species, and that causes it to grow," he says. Known colloquially as "red tides," these harmful algal blooms have been known to devastate fisheries along the Pacific and Atlantic coasts. But as devastating as climate change and harmful blooms can be, the news isn't all bad for marine life, Schofield says. Species have a lot of unknown capacities for adaptation.

They can evolve and change their characteristics—but it happens slowly. The question is, Schofield says, whether organisms can adapt in time with the climate change to ensure their survival. For phytoplankton, marine life, and humans alike, that remains to be seen. California desert town takes back the night, wins rare "Dark Sky" award.

The U. Harmful algal blooms Figures 1 and 2 can cause fish kills, human illness through shellfish poisoning, and death of marine mammals and shore birds. Hypoxia, considered to be the most severe symptom of eutrophication, has escalated dramatically over the past 50 years, increasing from about 10 documented cases in to at least in Hypoxia occurs when algae and other organisms die, sink to the bottom, and are decomposed by bacteria, using the available dissolved oxygen.

The formatioin of dead zones can lead to fish kills Figure 3 and benthic mortality. Because benthic organisms are bottom dwelling and cannot easily flee low-oxygen zones, they are often the most severely impacted. These toxic blooms can kill marine life and people who eat contaminated seafood.

Dead fish washed onto a beach at Padre Island, Texas, in October , following a red tide harmful algal bloom. Phytoplankton cause mass mortality in other ways. In the aftermath of a massive bloom, dead phytoplankton sink to the ocean or lake floor.

The bacteria that decompose the phytoplankton deplete the oxygen in the water, suffocating animal life; the result is a dead zone. Through photosynthesis, phytoplankton consume carbon dioxide on a scale equivalent to forests and other land plants. Some of this carbon is carried to the deep ocean when phytoplankton die, and some is transferred to different layers of the ocean as phytoplankton are eaten by other creatures, which themselves reproduce, generate waste, and die.

Phytoplankton are responsible for most of the transfer of carbon dioxide from the atmosphere to the ocean. Carbon dioxide is consumed during photosynthesis, and the carbon is incorporated in the phytoplankton, just as carbon is stored in the wood and leaves of a tree. Most of the carbon is returned to near-surface waters when phytoplankton are eaten or decompose, but some falls into the ocean depths. Even small changes in the growth of phytoplankton may affect atmospheric carbon dioxide concentrations, which would feed back to global surface temperatures.

Phytoplankton form the base of the aquatic food web. Phytoplankton samples can be taken directly from the water at permanent observation stations or from ships.

Sampling devices include hoses and flasks to collect water samples, and sometimes, plankton are collected on filters dragged through the water behind a ship. Marine biologists use plankton nets to sample phytoplankton directly from the ocean. Samples may be sealed and put on ice and transported for laboratory analysis, where researchers may be able to identify the phytoplankton collected down to the genus or even species level through microscopic investigation or genetic analysis.

Although samples taken from the ocean are necessary for some studies, satellites are pivotal for global-scale studies of phytoplankton and their role in climate change.

Individual phytoplankton are tiny, but when they bloom by the billions, the high concentrations of chlorophyll and other light-catching pigments change the way the surface reflects light.

In natural-color satellite images top , phytoplankton appear as colorful swirls. Scientists use these observations to estimate chlorophyll concentration bottom in the water. These images show a bloom near Kamchatka on June 2, The water may turn greenish, reddish, or brownish. The chalky scales that cover coccolithophores color the water milky white or bright blue.

Scientists use these changes in ocean color to estimate chlorophyll concentration and the biomass of phytoplankton in the ocean. Phytoplankton thrive along coastlines and continental shelves, along the equator in the Pacific and Atlantic Oceans, and in high-latitude areas. Winds play a strong role in the distribution of phytoplankton because they drive currents that cause deep water, loaded with nutrients, to be pulled up to the surface.

These upwelling zones, including one along the equator maintained by the convergence of the easterly trade winds, and others along the western coasts of several continents, are among the most productive ocean ecosystems. By contrast, phytoplankton are scarce in remote ocean gyres due to nutrient limitations. Phytoplankton are most abundant yellow, high chlorophyll in high latitudes and in upwelling zones along the equator and near coastlines. They are scarce in remote oceans dark blue , where nutrient levels are low.

This map shows the average chlorophyll concentration in the global oceans from July —May View animation: small 5 MB large 18 MB. Like plants on land, phytoplankton growth varies seasonally.

During summer, higher light levels and higher water temperatures promote 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. 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. In estuaries, salinity differences between upper and lower layers can enhance vertical stratification, creating similar favorable conditions for phytoplankton growth in the upper layer.

Phytoplankton Mortality. Consumption by organisms at higher trophic levels generally constitutes the largest source of mortality for phytoplankton. Grazing by protistan zooplankton is often the dominant source of mortality for phytoplankton in marine waters [4] , while grazing by crustacean zooplankton is often more important in freshwaters [5]. 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 [6].

Infections by viruses, fungi, bacteria, and protists can also contribute substantially to phytoplankton mortality. High cell densities of a single-species bloom favor the spread of infections during blooms, and can result in rapid bloom termination [7] [8]. Phytoplankton are typically 3 to 5 percent denser than their surrounding environment.

Consequently, most phytoplankton are constantly sinking and require turbulent mixing to stay in the upper mixed layer where light levels are appropriate for growth. Many phytoplankton, particularly bloom-forming taxa, avoid settling losses by having either flagella that allow them to swim or ballast mechanisms that provide buoyancy [10].

Cyanobacteria are notorious for accumulating by flotation into dense surface scums Figure 2. In a water body with constant volume, any water inputs must be accompanied by an equivalent water loss. For example, riverine water inputs to a lake are matched by outflows from the lake. 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.

If the dilution rate is higher than net growth rate, then the physical effect of flushing will prevent bloom development. The reciprocal of dilution rate is the water residence time which is the average amount of time that a parcel of water spends within a body of water. Generally, water bodies with residence times of a few days or less will not develop phytoplankton blooms. In water bodies with long residence times e.

In addition to natural flow conditions, residence time and bloom development can be affected by water control infrastructure [11] [12] [13].

Dams and other flow manipulating structures e. Phytoplankton blooms threaten the health of aquatic organisms and the health of humans, pets, or livestock that use affected waters for drinking or recreation.

High concentrations of phytoplankton during bloom conditions colors and clouds the water limiting the transmission of light in the water column. In shallow systems, light levels along the bottom may become insufficient to support beneficial submerged aquatic vegetation SAV that provide habitat, remove nutrients from the water column, and stabilize bottom sediments. Once the SAV is gone, suspension of destabilized sediments causes an increase in turbidity, which in turn often prevents the SAV from returning.

The nutrients that were previously consumed by SAV, are consumed by phytoplankton instead, further perpetuating blooms. These feedback mechanisms can trap a water body in this undesirable alternative stable state [14]. Of all the negative impacts of phytoplankton blooms, production of toxins by some bloom-forming species represents the most direct threat to human health. Cyanobacteria and dinoflagellates are the most common toxin producing group of phytoplankton in fresh and marine waters, respectively.

Cyanobacteria produce a wide variety of cyanotoxins including hepatotoxic liver-damaging microcystins, nodularins, and cylindrospermopsins, neurotoxic nerve-damaging saxitoxins and anatoxins, and dermatoxic skin-damaging lyngbya toxins [15].

Ingestion of toxins in drinking water and contact during recreation activities are the two most common exposure pathways to humans, pets and livestock. Some dinoflagellate blooms also produce the neurotoxin saxitoxin which can bioaccumulate in shellfish and cause paralytic shellfish poisoning in humans or other shellfish eating animals.

Red tide dinoflagellate blooms of the genus Karenia produce brevetoxins that kill fish and other marine life and, when aerosolized by wave action, can cause respiratory irritation in humans [17]. Habitat loss is another potential consequence of phytoplankton blooms. Although phytoplankton photosynthesis produces oxygen, the decomposition of the dead phytoplankton organic matter can deplete dissolved oxygen in the water to levels too low for fish and other animals.

The result is restricted habitat availability due to these dead zones and occasionally mass mortality events i. Hypoxia low oxygen or anoxia no oxygen is particularly common in bottom waters that are disconnected from the atmosphere by a temperature gradient thermocline in lakes or salinity gradient halocline in estuaries.