Below are some examples of common algal phenomena observed in the natural world, structured around bloom formation and its various causes. These examples are intended to serve as the foundation for your experimentation. Click one of the buttons below, read through a description of the dynamics at play during the event, and think about how you could model this phenomenon. What questions about the could you ask and answer using our products?
Written by Tristin Rammel (BS Marine Biology, MS Student Marine Biology at Scripps Institution of Oceanography)
(Top) Offshore bloom of Emiliana huxleyi in the Barents Sea, courtesy of the MODIS sensor on NASA's Aqua earth-observing satellite.
(Left) Algal bloom in Lake Erie off the coast of Toledo, courtesy of NASA/USGS Landsat-8.
(Right) Red algal bloom propagating along the front of an internal wave, courtesy of Peter J.S. Franks, SIO (Franks, Peter J. S., (1997), Spatial patterns in dense algal blooms, Limnology and Oceanography, 42, doi: 10.4319/lo.1997.42.5)
Phytoplankton biomass in regions of the ocean varies based on the concentration of nutrients in their region. In certain coastal regions the rotation of the planet combined with coastal winds pushes the top layer of water offshore, bringing nutrient-rich water from the deep ocean in so-called upwelling events. This physical forcing gives rise to high phytoplankton biomass on the coast, but similar forcings are not present in the open ocean. Consequentially, the biomass of pelagic (open ocean) phytoplankton is not as high` as it is on the coastline. In many cases only one nutrient is termed "limiting," because while there may be enough of other nutrients to sustain growth, only one need to be deficient in order to suppress it. Regardless of the normal or average phytoplankton biomass in a region, that most of the concentration can be explained by the limits of the most depleted nutrient raises many questions. Chief among them: what happens when nutrients are suddenly dumped into a nutrient limited environment? Sudden spikes of the limiting nutrient into an environment gives it additional carrying capacity, which leads to a rapid increase in phytoplankton biomass. This now abundant assemblages can cloud the water column to such a degree that these events are called "blooms." Although blooms come in many different colors, they are also colloquially known as "red tides."
Some blooms are seasonal, whilst others can be attributed to abnormal fluctuations in the carrying capacity of the region. There are seasonal upwelling zones on certain coasts across the planet and a moderate band of upwelling is present at the equator. Trade winds blow Saharan dust into iron-limited Atlantic Ocean regions, and swirling kilometers-wide eddies of nutrient-rich surface water can be pushed offshore into oligotrophic waters. One of the largest global biomass increases in the ocean comes in the spring, with large blooms occuring in the open ocean that can be seen from space. The ocean has layers of water with different temperatures, salinities and pressures, which can create a cake-like organization of water slices that do not interact because of those differences. Sitting atop the cake is the mixed layer, which as the name suggests is mixed homogenously by passing storms. The mixed layer is present mainly because of the sunlight that warms it, separating it from the colder and denser layers of the ocean. During winter when heating is minimal, the mixed layer increases in size and interacts more with the colder, nutrient-rich water below it. During winter conditions are not conducive to a bloom, but as spring comes and the water column warms, the mixed layer becomes smaller. This gives phytoplankton more daily sunlight, which when combined with the nutrients that have been pooling throughout winter cause normally sparse regions to bloom in magnificent blooms of life. All of these events and more are seasonal cycles dictated by natural processes, but human activities can also have effects. Some are scientific, like the IRONEX and later iron fertilization experiments, which studied the effects of artificial iron seeding in iron-limited pelagic regions. Others are more systemic and long-lasting pollution, like river runoff carrying agricultural fertilizers into the ocean. These events can cause eutrophication (nutrient levels that are abnormally high), which can lead to dead zones, both of which will be explored in later sections.
So if the larger phytoplankton are typically blooming in the marine environment, which species or genera are the usual suspects? Diatoms, dinoflagellates and other eukaryotic phytoplankton like cryptophytes mostly, though there are some larger colonial cyanobacteria that can form blooms. The types of blooms spoken of normally typically carry with them some kind of environment-altering characteristic that makes the public aware of their presence, either wonderous or harmful. In San Diego, dinoflagellates named Lingulodinium polyedra bloom right offshore in huge numbers. In 2020 this species recorded historically high abundances (over 9 million cells per liter), mostly due to the equally historic high sea surface temperature and their ability to migrate to deeper waters at night for access to nutrients and up to the surface during the day to photosynthesize. These phytoplankton are bioluminescent, they can glow in the dark when disturbed. Even during a pandemic, San Diegans flocked to the beaches at night to safely watch glowing waves crash into the surf. The species of diatom that caused seabirds to behave erratically in Northern California (and spawn a Hitchcock movie) secreted toxins that were filtered out of the water by bivalves and concentrated in their tissues, which when eaten caused neurological damage and avian psychosis. These types of bloom are termed Harmful Algal Blooms, because they present a danger to the surrounding ecosystem. Even the beautiful dinoflagellates in San Diego died eventually, causing heterotrophic microbes in the area to burn up all the oxygen while consuming this massive biomass input. The resultant dead zone killed fish and other sea life throughout the area. Blooms with effects that are a hindrance but not immediately threatening the ecosystem are termed Noxious Algal Blooms. While most blooms are not at all harmful and indeed promote growth throughout their regional food web, it is important to highlight deleterious effects (both natural and man-made) that blooms can have on fisheries other regions.
(Top) Figure 1 of Heymans et al 2014, https://doi.org/10.1371/journal.pone.0095845
(Left) Schematic of nutrient and energy cycling in the water column, courtesy of the Office of Environmental and Biological Research, US Dept. of Energy's Office of Science
(Right) Examples of kelp forest ecosystems with (left) and without (right) top-down sea otter controls, courtesy of NOAA
Conventional wisdom holds that as energy is transferred through trophic levels, only 10% of the energy present in the prey level is available to the consumer level. Though this may not be true in all regions or situations, it is imperative to understand that the total amount of energy produced by the producers (grass, plants, phytoplankton) decreases as more constituents join its flow through the ecosystem. Therefore, shorter food chains are more efficient at transferring energy to its final consumer, be that a shark, a whale or a human.
Food chains are an acceptable introduction to trophic dynamics (the way things consume and transfer energy), but they can often can be too linear (implying a chain rather than a web), or highlight "bottom-up" effects only. Bottom-up effects are those exerted by primary producers. In the case of phytoplankton (oceanic primary producers), algal blooms may be considered bottom-up controls, as they allow for more energy to be transferred to consumers up the food chain. Food webs can also be controlled by top-down effects, or those exerted on the environment by high trophic level consumers. Predation in a good general example, but the ocean provides much more specific and explanatory ones. Sea otters are at the top of many kelp forest food webs, and heavily predate sea urchins. During the late 19th and early 20th centuries, fur trappers hunted sea otters nearly to extinction. The lack of top consumers relieved the pressure on sea urchins, which rapidly proliferated and decimated kelp beds, completely altering whole ecosystems. Removing sea otters removed predation pressure on organisms that would otherwise completely carpet whole areas, an excellent example of what happens when top-down controls are removed from an ecosystem.
Included in the critiques of food chain theory is their linearity, when in fact ecosystems consist of interconnected links through trophic levels (primary producers, primary and secondary consumers, etc) but also along them, forming a web. Animals of the same trophic level can compete with one another for energy as well as avoid predation from higher levels. Consumers at all sizes and trophic levels compete to predate producers, and the outcome of that competition serves as a foundation for how the ecosystem is structured. This concept can help explain the dominant size classes present during algal blooms.
The circumstances surrounding bloom formation in the ocean depends on a variety of factors, and all blooms are by no means the same. The dynamics at play are dependent on the location of the bloom, which particular environmental variable changes, and the species present. Regardless of formation dynamics, most blooms in the marine environment consist of large phytoplankton like diatoms or dinoflagellates. Even in nutrient-limited pelagic regions (oligotrophic or low-productivity, low nutrient regions) where small phytoplankton like cyanobacteria typically dominate, this phenomenon still occurs. Although algal blooms can be considered a bottom-up control as mentioned above, top-down effects are levied upon the bloom itself, highlighting the complexity of trophic dynamics. This top-down control with differential grazing rates on different sizes of phytoplankton.
When nutrients are spiked into a region, phytoplankton of all sizes begin reproducing at higher rates. This is true for their predators as well, however as size increases from the smaller unicellular zooplankton that graze smaller algae to the larger and more complex multicellular zooplankton graze on diatom and dinoflagellate-type size classes, predator reproduction time increases. Copepods, shrimp, salps, krill and other large grazers have complex life cycles, and cannot as rapidly increase their biomass in response to a bloom of their prey as the grazers of smaller, unicellular phytoplankton can. So when bloom-forming pressures are exerted, smaller phytoplankton's rapid biomass rise is checked by their grazers while the larger phytoplankton are more free to take advantage of the situation. In areas with high seasonal or semi-constant productivity, the dominance of larger phytoplankton size classes can be filtered by certain fish, which shortens the trophic pathway and partially explains why coastal regions are such productive fisheries. This example highlights the way that competition between primary consumers of different types can structure trophic dynamics in certain regions.
(Top): Runoff containing soil and fertilizer from an agricultural field flows into a forming stream, courtesy of iStock and JJ Gouin
(Right) Satellite view of sediment and algae at Mississippi river outflows in 2007, courtesy of Phil Degginger/LANDSAT/NASA/NYTimes
As mentioned above humans pollute nearly all of the oceanic water column. As can be seen through the examples above however, not all alter the phytoplankton community nearly as directly or intensely as others. Some alter the composition of the heterotrophic assemblage, like a sudden influx of carbon based fuels as in the Deepwater Horizon spill. Agricultural runoff directly impacts the , and must therefore be examined and alleviated due to its direct effects on primary production in the ocean.
In the early 20th German chemists Fritz Haber and Carl Bosch created the appropriately named Haber-Bosch Process, which created ammonia from nitrogen gas and hydrogen gas. This accomplishment, which would later earn Haber a Nobel Prize, solved a problem that farmers the world over had begun to feel acutely. With rising populations in the industrial age came an increased demand for food production, something that natural nutrients in global arable lands could not sustain. Soils were particularly nitrate limited, but the Haber-Bosch Process now provided a method to capture inactive nitrogen gas from the atmosphere and transform it into ammonia, which can be taken up by crops and other plants. Use of this process in fertilizer production greatly increased global agricultural production, but it came with unforeseen consequences to the coastal regions of the world.
Rainfall washes this nitrogen-rich fertilizer off of farm fields, through the regional water table, and into the rivers that feeds it. These rivers eventually flow out into the ocean, which provides the coastal regions with huge inputs of nutrients that promote phytoplankton growth. Because the river outputs freshwater, this runoff stays at the surface because saltwater is more dense, which keeps the fertilizer in areas where phytoplankton photosynthesize longer than if the input came from saline water. Coastal ecosystems with rivers that flow through major agricultural land experience these effects cyclically, as crops go in and out of season throughout the year.
Other human pollutants in the coastal ocean include wastewater disposal, which drains both human feces and fecal-associated bacteria from human gut microbiomes into the water column. The nutrients included in the feces can roughly mimic those of the agricultural runoff created in the Haber-Bosch Process. Fecal bacteria can pose their own problems, as microbes from humans infected with certain pathogens can be washed into the surf after heavy rain or flooding. Vibrio cholerae is responsible for the disease cholera, with part of infection cycle depending on water-based transmission that is intensified after these heavy rain events.
Many land-based nutrient inputs would not be transferred to oceanic regions if not for floods. During dry or planting seasons, fertilizer accumulates on fields. With winter and spring comes heavy rains that wash this material into waterways and aquifers, which eventually make their way to the ocean. During flooding around farmlands, the entire flood water mass becomes inundated with fertilizer that directly washes into the ocean when the flood plain drains. When the Tijuana River floods during heavy rains, fecal matter and its associated bacteria are washed into the waters around Imperial Beach, San Diego and can be detected as far north as La Jolla or Carlsbad. These ecosystem-altering events are entirely dependent on heavy rains and freshwater flooding that makes its way to the coast, then enhanced by the freshwater mass persisting in that region due to its lower density.
Rodriguez-R, L. M., Overholt, W. A., Hagan, C., Huettel, M., Kostka, J. E., & Konstantinidis, K. T. (2015). Microbial community successional patterns in beach sands impacted by the Deepwater Horizon oil spill. The ISME Journal, 9(9), 1928–1940. https://doi.org/10.1038/ismej.2015.5
Yergeau, E., Maynard, C., Sanschagrin, S., Champagne, J., Juck, D., Lee, K., & Greer, C. W. (2015). Microbial Community Composition, Functions, and Activities in the Gulf of Mexico 1 Year after the Deepwater Horizon Accident. Applied and Environmental Microbiology, 81(17), 5855–5866. https://doi.org/10.1128/aem.01470-15
Amaral-Zettler, L. A., Zettler, E. R., & Mincer, T. J. (2020). Ecology of the plastisphere. Nature Reviews Microbiology, 18(3), 139–151. https://doi.org/10.1038/s41579-019-0308-0
(Left) Plot of Dissolved Oxygen (DO) off the Ellen Browning Scripps Pier, courtesy of Jen Smith and Samantha Clements (UCSD/SIO, SOAR Program)
(Right) A mass of dead fish in the Bayou Chaland area of Louisiana, courtesy of P.J. Hahn, National Geographic
Photosynthesis requires inputs of water, carbon dioxide, and light energy, producing oxygen and glucose. When algae bloom, they take up more carbon dioxide and produce more oxygen than would be regionally standard for that time of the year. But that is only one half of the story. Suspended in the plankton with algae are heterotrophs, microbes that do not photosynthesize and instead consume organic sources of carbon for energy. In the simple equation outlined above, these organisms take the glucose from phytoplankton and respire it, using oxygen to convert it back into carbon dioxide. This can be sourced from direct predation on live phytoplankton or grazing the "marine snow" of organic material that floats down in the water column. In this way both sides of the equation are continuously explaining the cycle of photosynthesis and respiration.
When the organisms on one side of the equation experience rapid growth, it must be balanced on the other side. We can see the effects of blooms like the one in La Jolla during 2020 through measurements of dissolved oxygen in the region. Measurements from the Smith Lab at Scripps Institution of Oceanography's SOAR (Scripps Ocean Acidification Real-Time) program show that during the period of the bloom, oxygen concentrations increased substantially due to the the rapid increase in primary production. After nutrients were depleted and the bloom died, heterotrophic life began to consume the dead assemblage. As they ate they respired, consuming oxygen until regional levels were below that capable of sustaining fish and animal life. Animals across San Diego began washing up on shore, dead of hypoxia (low oxygen). The phytoplankton bloom itself contributed to that respiration at night, as can be seen by the daily oscillations of oxygen concentration in the figure above.
What had happened was the formation of a "dead zone," or body of water with oxygen levels too low to sustain life. When a system becomes eutrophic and eventually dies, the heterotrophic organisms performing decomposition become overwhelmed and die after using up the available oxygen. This bloom was temporary, and not all blooms have to severely inhibit life. But some dead zones are huge and growing larger, threatening the ecosystems they impact as well as the fisheries that feed nearby coastal regions. Large and more cyclically forming dead zones exist in regions across the world, with the second-largest being offshore the US in the Gulf of Mexico. It spans from Texas through Louisiana and kills millions of organisms each year. This phenomenon and its intensity is relatively new for the region. Human pollution from agricultural activity flows through rivers and established a persistent presence offshore, spurring a eutrophic event and subsequent depletion of oxygen by the organisms attempting to decompose all that growth.