Plankton Dynamics Flashcards

What are the main factors that drive variability in productivity?

Light and nutrients are the main factors that drive variability in productivity.

What generates almost half of global primary production?

ocean phytoplankton generate almost half of global primary production, supporting the marine food web through carbon transfer to higher trophic levels from zooplankton to apex predators

Where does most primary production occur?

in the upper half of the photic zone, in the zone called the euphotic zone.

Euphotic zone

The euphotic zone is where there is enough light so that phytoplankton capture more light energy than they require to merely stay alive. In the euphotic zone they can grow, reproduce, and store excess carbohydrates and sugars.

compensation depth

The depth at which primary production equals respiration. Above this depth, phytoplankton can make a living; below this depth, they cannot and either die or go into a resting stage (cyst) to await better light conditions. The compensation depth coincides with the depth in the ocean where the light level is 0.1% to 1% of the amount of the sunlight entering the surface of the ocean. The compensation depth defines the bottom of the euphotic zone.

Secchi desk

a flat plate about 20 cm in diameter hung from a line. The disk is colored with alternate black and white quadrants to make it easier to see. It is lowered until the observer can no longer see the disk (cannot distinguish between the black and white quadrants), and that depth is recorded. It is then raised until the disk just reappears, and this depth is recorded. The method is semi-quantitative but simple and used frequently.

How does compensation depth vary?

Once sunlight penetrates the water, the compensation depth varies with ocean conditions. In very productive water with an abundance of nutrients and an explosive algae population, sunlight can penetrate to a depth well under a meter. In coastal areas sediment can increase turbidity and reduce light penetration.

PAR (photosynthetically active radiation)

The total range of light wavelengths used in photosynthesis. It is a total count of light energy (in photons) between 400nm and 700nm

What do photosynthetic plankton use?

chlorophyll a, chlorophyll c, and a group of accessory pigments, such as protein-bonded fucoxanthin, beta carotene, and peridinin, to utilize light energy from most of the visible spectrum.

What is intense enough to inhibit photosynthesis by bleaching pigments?

Incident light near the ocean's surface is intense enough to inhibit photosynthesis through bleaching of photosynthetic pigments.

thermocline

area of rapid change in the temperature gradient

pycnocline

area of rapid change in the density gradient

stratified

When the ocean has layers separated by differences in density we call the water column stratified.

surface mixed layer

The layer that is mixed by the wind

Why is stratification critical for phytoplankton?

Stratification between the surface mixed layer and the deep ocean is critical for the success of ocean phytoplankton because it ensures phytoplankton will not be mixed to depths away from the sunlight (panel a). If the pycnocline falls below the compensation depth, mixing can carry algae out of the light and thus away from their energy source (panel b).

important nutrients for photosynthetic plankton

Important nutrients include nitrogen (N), phosphorus (P), iron (Fe), and silicon (Si).

Redfield ratio

In the early 1900s, oceanographer Alfred Redfield found that plankton build their biomass with the elements in a nearly universal stoichiometric ratio for C:N:P of ~106:16:1. We call this the Redfield ratio. Redfield also noted that the inventory of dissolved N:P in the deep ocean is close to the 16:1 ratio of plankton biomass, likely because the primary source is decaying plankton.

Liebig’s Law of the Minimum

Phytoplankton growth limitation has traditionally been interpreted in the context of Liebig's Law of the Minimum, which states that plant growth will be as great as allowed by the least available resource, the "limiting nutrient" that sets the productivity of the system

What happens when plankton die and sink towards the bottom?

Export of material from the surface ocean leads to the accumulation of nutrients in deep waters where there is no light available for photosynthesis. Because of the density difference between surface water and the deep sea across most of the ocean, ocean circulation can only very slowly reintroduce dissolved nutrients to the euphotic zone.

Check your understanding: Efficiency of nutrient cycling in the surface ocean is a function of plankton sizes.

true

Check your understanding: Which most accurately describes a phytoplankton bloom?

• when zooplankton numbers are low

• when phytoplankton reproduce sexually using flowers

• time and place in the ocean with elevated phytoplankton concentrations

• periods of rapid growth in phytoplankton biomass

• time and place in the ocean with elevated phytoplankton concentrations

Check your understanding: The critical depth and critical turbulence hypotheses are top-down models meaning that they focus on resources such as light and nutrients.

false

Check your understanding: The disturbance recovery hypothesis is a top-down model where bloom dynamics are controlled by predator-prey interactions.

true

Check your understanding: What limits production in polar regions?

light

What affects the distribution of phytoplankton and nutrients?

While the availability of light and nutrients control the growth of phytoplankton, grazing by zooplankton in turn affects the distribution of phytoplankton and nutrients.

What is heterotrophy dominated by?

heterotrophy is often dominated by single-celled microzooplankton of ~1 to 200 µm and by bacteria of ~0.3 to 1 µm, the latter carrying out most of the organic carbon decomposition in the ocean.

Why are microzooplankton thought to be highly effecient grazers?

Because of their relative physiological simplicity, microzooplankton are thought to be highly efficient grazers that strongly limit the biomass accumulation of their prey. For example, in the nutrient-poor tropical and subtropical ocean, small cyanobacteria tend to be numerically dominant. The microzooplankton effectively graze these small cells, preventing their biomass from accumulating and sinking. These single-celled microzooplankton lack a digestive tract, so they do not produce the fecal pellets that represent a major mechanism of export. Instead, any residual organic matter remains in the upper ocean, to be degraded by bacteria. Microzooplankton grazing of phytoplankton biomass leads to the remineralization of most of its contained nutrients and carbon in the surface ocean.

Examples of large multicellular plankton vs phytoplankton dynamics

We see examples of these dynamics in the nutrient-rich polar ocean. Large phytoplankton, such as diatoms, grow so rapidly they are able to outstrip the grazing rates of multicellular zooplankton. The diatoms accumulate to high concentrations sink as undigested phytodetritus. What is consumed by zooplankton exits the surface ocean repackaged as dense fecal pellets. Both processes export carbon and nutrients very efficiently to the deep sea.

diel vertical migration

Many of the zooplankton that graze on the phytoplankton concentrated at the surface of the ocean spend the daylight hours at depth and migrate to the surface to feed under the cover of darkness in a phenomenon known as the diel vertical migration. This is the biggest animal migration on the planet, and it is vertical.

deep scattering layer (DSL)

In 1942, a U.S. Navy research vessel, the USS Jasper, was testing new sonar technology off California’s coast when it reported sound waves being deflected (or scattered) from a mysterious layer more than 1,000 feet below the surface. Incredibly, this dense layer stretched 300 miles long, leading researchers to think at first that it might be the sea floor itself. Other sonar pioneers soon found similar layers all across the Pacific, the Atlantic and even in lakes worldwide. Yet the cloudlike layers remained an enigma—and a peril for Navy sonar operators, for whom the layers might hide an enemy submarine.

Three years later, in 1945, Martin Johnson of the Scripps Institution of Oceanography used crude plankton nets to conduct nighttime surveys of marine life at various depths and thus became the first to report that the dense clouds were masses of living creatures that moved up and down nightly. The masses of life in what’s called the “deep scattering layer” (DSL) can be hundreds of feet thick and extend for hundreds of miles at various depths across the world’s oceans.

plastic

Different species have different strategies and behaviors are often plastic, meaning they change over time. Different species have different strategies and behaviors are often plastic, meaning they change over time. Some species appear to reduce predation pressure by schooling at depth. Some populations appear to use temperature and salinity gradients as cues. Others choose not to migrate and remain at depth if prey is abundant or can be caught while passing in migration. Some animals may travel only a few meters on their migration, while others travel several hundred. The result is said to be like overlapping ladders of migration.

biological pump

Zooplankton consume phytoplankton at the surface, head back down to depth, and excrete fecal pellets. Other individuals slightly lower down consume the zooplankton or the pellets and excretes, and so on into the depths. The collective effect can move nutrients from the surface as much as 53 percent faster than would happen by gravity alone

phytoplankton bloom

In some cases, blooms have been described as periods of rapid growth in phytoplankton biomass. However, this definition is unsatisfactory because, although some blooms happen quickly, others develop over long periods and have rates of biomass accumulation equivalent to only one or two doublings per month.

More generally, blooms are understood as a condition of elevated phytoplankton concentration. From the satellite view of global ocean chlorophyll concentrations shown in the figure, it is clear that bloom-forming regions correspond to waters enriched in nutrients by convective and wind-driven mixing (e.g., regions 1–4), riverine input (e.g., the Amazon plume), or coastal upwelling (e.g., eastern boundary currents). Satellite data also illustrate both the prevalence of blooms at higher latitudes and the absence of such blooms where a lack of nutrients curtails significant phytoplankton accumulation (e.g., regions 5 and 6).

How do conditions initiate phytoplankton blooms

There are three contemporary schools of thought regarding how conditions initiate phytoplankton blooms in the ocean:

• Critical depth hypothesis

• Critical turbulence hypothesis

• Disturbance-recovery hypothesis.

critical depth hypothesis

a bottom-up model that proposes a bloom starts when there is both sufficient sunlight and the surface mixing layer is becoming shallower. This change induces stratification, allowing phytoplankton to remain in the surface layer, such that their growth rates overcome their losses. According to this hypothesis, deep winter mixing and low light levels in temperate to polar seas cause phytoplankton division rates in the surface layer to be less than total loss rates from respiration, grazing, sinking, and other forms of mortality (figure). As spring proceeds, the decreasing mixed layer depth and increasing sunlight allow more rapid phytoplankton division. Eventually, improvements in the mixed layer light conditions allow division rates to equal loss rates, and conditions are set for bloom initiation. This tipping point represents an alignment of the compensation depth and the mixed layer depth. Any further increase in light or decrease in mixed layer depth causes division to outpace losses, and a bloom commences.

critical turbulence hypothesis

it also presumes that phytoplankton loss rates are constant and that blooms do not occur during deep mixing because phytoplankton division is light limited (i.e., a “bottomup” constraint). In winter, heat is removed from the surface of the ocean to the colder atmosphere. As the water cools, it sinks and convectively mixes the upper ocean. As the seasons change, the atmosphere and the upper ocean warm and convective mixed-layer deepening ends. When this happens, the depth of active turbulent mixing can become shallower before any notable change in vertical density structure occurs. In this modle, bloom initiation is determined by the balance between two opposing processes: light driven photosynthesis which tends to create a near-surface layer of increasing biomass and turbulence which tends to erode this layer by distributing biomass over depth. Under quiescent weather conditions when winds are light or absent, adequate surface sunlight, and minimal nighttime convection, the Critical Turbulence Hypothesis predicts that near-surface phytoplankton growth can outpace the dissipatory effects of turbulence as soon as net heat flux into the ocean becomes positive.

disturbance recovery hypothesis

based on top-down biotic drivers. This theory proposes that a disturbance, such as deep winter mixing, freshwater input, upwelling, or polar night, disrupts zooplankton-phytoplankton predator-prey interactions. The disruption allows the prey (phytoplankton) to grow rapidly, creating a bloom. Later, when the predator-prey interactions recover, the bloom ends as the losses by predation overwhelm the gains in prey biomass. This theory first was proposed for the North Atlantic where deep-water mixing provides the disturbance that disrupts predator-prey interactions in the ecosystem.

The role played by bottom-up and top-down drivers in phytoplankton spring blooms is still a source of debate.

seasonal blooms

in the winter, polar regions are dark so no photosynthesis can occur. In spring sunlight begins to increase and because there are ample nutrients in the water column, phytoplankton begin to grow. In the fall, the sunlight grows weak and productivity continues to drop. In the wintertime, strong storms and the sea ice cycle allow time for nutrients to build in the surface ocean so that when the sun returns in the spring the production cycle can restart.

polar ocean grazing pressure

As the quantity of photosynthetic organisms increase grazers begin to eat them and to reproduce. There is a lag in the peak of biomass in zooplankton compared to the peak in phytoplankton. Grazing starts to limit the numbers of phytoplankton at the same time the phytoplankton begin to reduce nutrient concentrations and light levels begin to drop.

polar ocean productivity

When sea ice melts it creates a fresher layer of water at the surface helping to prevent phytoplankton from being mixed downward out of the sunlight. Although short, the growing season is extremely intense so many animals migrate to polar waters to feed during phytoplankton blooms.

blooms in temperate regions

Phytoplankton blooms in the temperate ocean are a bit more complicated. Temperate regions are highly seasonal so during winter light limits production as it did in the polar regions (shaded blue). There are also times in the year when nutrients are limiting (shaded yellow).

HAB (harmful algal blooms)

occur when plankton grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals, and birds. The human illnesses caused by HABs, though rare, can be debilitating or even fatal.

brevetoxin

This bloom is caused by Karenia brevis, a dinoflagellate that produces a potent neurotoxin called brevetoxin.

Brevetoxins are toxic to fish, marine mammals, birds and humans, but not to shellfish. They can accumulate in shellfish and cause Neurotoxic Shellfish Poisoning in humans when consumed.

The Gulf of Mexico experiences a variety of additional HABs.

• Shellfish harvesting closures have occurred in Texas due to blooms of Dinophysis, a dinoflagellate which can produce toxins causing Diarrhetic Shellfish Poisoning.

• In Florida, closures have occurred due to a diatom called Pseudo-nitzschia, which can produce toxins causing Amnesic Shellfish Poisoning.

• In addition, recreational harvesting of puffer fish is banned in some estuaries in Florida because of the threat of saxitoxins produced by Pyrodinium (also a dinoflagellate).

• Lastly, Ciguatera Fish Poisoning can be an issue in South Florida and off Texas, due to toxins produced by Gambierdiscus, a type of dinoflagellate associated with macroalgae on coral reefs.