User:HannahW1414/Planktivore

A planktivore is an aquatic organism that feeds on plankton: this includes either zooplankton or phytoplankton or a combination of these two plankton types.

Forms of Plankton
Phytoplankton are usually photosynthetic one-celled organisms. They tend to be found near the surface of water since they utilize light energy in their photosynthetic processes and most light energy hits the surface level of water sources. Phytoplankton provide most of the oxygen that is in a body of water and provide a large amount of food for other organisms in their environment.

Zooplankton are heterotrophic plankton, which are animals that ingest organic matter rather than producing it via chemical reactions. Zooplankton can provide an important food source for juvenile fish.

Forms of Planktivory
Most fish are planktivores for at least part of their life cycle- especially when they are larvae.

Obligate Planktivory
Obligate planktivores feed exclusively on plankton. Examples include fish such as alewife and blueback herring. Obligate planktivores have historically been studied less than facultative planktivores on account of less species existing in this category (as far as the scientific community currently knows) and most of the known species are sensitive to handling. Obligate planktivores live in the open waters of oceans (Pelagic zone) or lakes because it is the only place that can consistently supply a high amount of plankton for obligate planktivores to thrive.

Facultative Planktivory
Facultative planktivores do not feed exclusively on plankton; they are opportunistic feeders and are also referred to as generalist species. Facultative planktivores are the most common type of planktivore and facultative planktivory is the most studied method of plankton predation.

Feeding Technique
The method fish employ to consume plankton is categorized as either particulate feeding or filter feeding- or a mix between these two methods. Particulate feeders visually target plankton to consume them: as such, the success rate of this predation method is impacted by environmental factors. Some of these environmental factors include aquatic plant (like macrophytes) density and prey size. A real life example of the impacts of this dynamic is seen in Lake Blanca Chica, a eutrophic lake in Argentina, filled with planktivorous particulate feeders: smaller zooplankton were found to exist in higher numbers than larger zooplankton since particulate feeders had a more difficult time spotting and catching the smaller zooplankton.

The second main way planktivorous fish can catch plankton is through filter feeding. Planktivorous fish do this through intaking and processing large volumes of water and straining nutritious items. There are multiple types of filter feeders. One type that does not employ visual selection is called a "Tow-net" filter feeder; fish in this category swim rapidly with their mouths open and simultaneously filter water for food particles. Pumping filter feeding is a second non-visual method used by some fish in which water is slowly collected while the fish does not move or moves slowly (compared to quick "tow-net feeding"). An example of pumping filter feeders are corals.

"Gulping" describes a method some fish employ to capture plankton that is a mixture between particulate feeding and filter feeding. It is considered a mixture of both methods because it is very close mechanically to pump filter feeding, but is not technically a filter feeding (non-visual) method because gulping is a visual method (like particulate feeding) that is affected by visual stimuli. An example of a fish that uses this method is the alewife (Alosa pseudoharengus).

Lifecycle Variance in Feeding Habits
As noted above, many fish have changing plankton dietary needs throughout their lifecycle. One example is the gizzard shad, which is an obligate planktivore during its larval stage. This is the case in part due to its very small mouth size. As the gizzard shad grows in size, it becomes a facultative planktivore. Gizzard shad tend to thrive in nutrient- abundant eutrophic environments due to their opportunistic feeding tendency. This fish species can make eutrophic conditions worse by feeding on sediment and releasing more nutrients (as a result of this behavior) into their aquatic environment.

Planktivore Ecology
Phytoplankton make up the lowest trophic level of aquatic food chains. Planktivores affect the food chain by altering different plankton populations in various ways through top-down control: this concept is visually illustrated in trophic cascade diagrams. . More specifically, top-down control of plankton occurs when planktivorous fish consume zooplankton which in turn affects the zooplankton population as well as the local phytoplankton population. An overabundance of planktivorous fish can greatly impact an aquatic ecosystem through accelerating the process of eutrophication occurring in lakes via limiting zooplankton populations. Lower zooplankton populations can then cause a rise in phytoplankton population, eventually causing algal blooms and, if the system becomes too overloaded by nutrients, potentially make the aquatic system eutrophic.

Evolution of Planktivory
Titanichthys is thought to be the first massive vertebrate pelagic planktivore; it lived in the Famennian Stage of the Devonian Era (it is now extinct). It had a lifestyle similar to that of the modern basking, whale, or megamouth sharks.

Planktivore Management Applications
Planktivores significantly impact the aquatic ecosystems they live in. Here are some examples of how planktivore management occurs in practice.

Top-down impact of Planktivory
A recent study analyzed the changing plankton composition in Lake Blanca Chica, Argentina during the past 250 years through various palaeoecological indicators. They found that planktivorous fish did seem to impact the local zooplankton population, and by extension phytoplankton population, by their preferred consumption of bigger zooplankton. This preference of planktivores to consume larger species caused smaller zooplankton species, like Bosmina spp., to be selectively favored over larger zooplankton over time.

Biomanipulation
Since the 1950s, biomanipulation experiments have been done in small and shallow lakes to test if biomanipulation theories in practice could improve the health of water sources by lessening their eutrophic state. One research team compiled a list of these biomanipulation studies and categorized them by method (such as adding more piscivorous species to a water system or reducing the amount of planktivorous and/or benthivorous species in a lake) and success level (did it make the water source less eutrophic and, if so, for how long) to compile some generalizations of knowledge on the subject. The conclusion was that all methods of biomanipulation have the potential to limit eutrophic conditions occurring in lakes: but some methods have a higher probability of succeeding than others. Partial fish removal was found to be the most successful method employed in regards to biomanipulation in multiple studies. Partial fish removal is a method of biomanipulation used where concentrated removal of individuals from a particular planktivorous fish species occurs in the affected water source in the hopes of reducing eutrophic conditions there. Biomanipulation has been effective on relatively small-scale aquatic environments, but has not been tested in real-life on a huge aquatic environmental scale: this possibility is currently being explored through virtual simulations before being implemented.