Plankton 

Plankton, an adaptation of the Greek word "planktons," or "drifter" or "wanderer," refers to a diverse group of organisms that exist in vast bodies of water and are unable to swim against the currents. A great number of food webs-aquatic also marine-basically exist on plankton. Plankton can be broadly categorized in several ways, depending on the size, function, and taxonomy. These include phytoplankton, which resemble plankton similar to plants, zooplankton with characteristics of animals, and bacterioplankton that are bacteria-like. Despite the small size, plankton is part and parcel of ecosystems on Earth.

1.0Types of Plankton

Phytoplankton or Plant Plankton :

Phytoplanktons are autotrophic, meaning they synthesize their own food by photosynthesis. They contain chlorophyll and are thought to produce approximately 50% of the Earth's oxygen, thus being an essential part of living on this planet. Among the most common types of phytoplankton are:

(i) Dinoflagellates : This is a single-celled organism with two whiplash flagella. They are bioluminescent and can create "glowing" waves at night.

(ii) Cyanobacteria : Known to man as blue-green algae, these are some of the oldest photosynthetic organisms on Earth. They are important for nitrogen fixation, making atmospheric nitrogen available to other forms of life.

Phytoplankton is the main source of nutrition for most marine animals, ranging from small zooplankton to big fish and even whales.

Zooplankton (Animal Plankton) :

Zooplankton are heterotrophic. They acquire energy from other organisms by consuming them. Zooplankton vary from very tiny microscopic protozoans to much larger animals like jellyfish. Generally speaking, zooplankton can be broadly categorized on the basis of their life cycle as under:

(i) Holoplankton: These organisms remain plankton throughout their life cycle. Some copepods are tiny crustaceans, while others are jellyfish.

(ii) Meroplankton : These are organisms that are plankton at some stage in their life cycle. This is primarily when they are larvae. Some examples include fish larvae, mollusks, and sea urchin larvae.

This way, zooplankton feed on other phytoplankton, bacterioplankton, and even on smaller zooplankton, thus playing a very important role as a link between the primary producers, the phytoplankton, and the larger marine predators.

Bacterioplankton (Bacterial Plankton) :

Bacterioplankton includes all types of bacteria and archaea that take part in a substantial nutrient cycle within aquatic environments. They involve the decomposition of organic material, recycling it, as well as regulating the energy flow throughout the ecosystem. Bacterioplankton assists in the decomposition of dead organisms and detritus, which releases important nutrients to other marine life.

2.0Plankton Ecology

Plankton ecology refers to the study of plankton, the tiny organisms floating within aquatic environments, and their role within ecosystems. Plankton represent an important component that impacts the health and proper functioning of marine as well as freshwater ecosystems since they are located at the bottom of the aquatic food web.

Plankton ecology aims at determining the interaction between plankton and its environment. Its factors would include:

A. Nutrient availability : Phytoplankton need easy nutrients like nitrogen, phosphorus, and iron that will limit the growth of phytoplankton.

B. Light and temperature : These control the growth of phytoplankton, imposing seasonal cycles that may result in blooms or declines.

C. Predation : Zooplankton are preyed upon by phytoplankton, while larger organisms preyed on them due to population dynamics.

D. Mixing and currents : Ocean circulation and vertical mixing determine distribution and nutrient availability for plankton.

E. Climate change : Changes in plankton populations are being forced through sea surface and ocean acidification; these changes have an influence on global fisheries, carbon cycling, and the health of marine ecosystems.

3.0Role in the Ecosystem

A. Base of the Food Web :

Plankton, largely phytoplankton, are at the base of aquatic food webs. They harness sunlight for energy through photosynthesis, which works its way up the food chain from zooplankton to larger animals such as fish, seabirds, and marine mammals. All this is the foundation of the entire marine ecosystem.

B. Carbon Sequestration :

Thus, plankton plays a crucial role in the Earth's carbon cycle. In the process of photosynthesis, phytoplankton absorbs carbon dioxide. When they become dead, some of their biomass sinks into the ocean floor, trapping carbon in the deep ocean and it does not come back into the atmosphere. Thus, this is often called the "biological pump," which helps reduce the effects of climate change.

C. Oxygen Production :

Phytoplankton contains high oxygen levels, and subsequently it greatly contributes to the Earth's atmosphere; it is approximated that they produce about 50% of the oxygen we breathe. Phytoplankton through photosynthesis transform carbon dioxide into organic compounds, and in this process, oxygen is produced as a byproduct.

D. Biodiversity and Habitat :

Plankton support marine life diversity. Many aquatic animals use plankton for either direct or indirect means of survival. Zooplankton are actually the food of most small fishes, which are thereafter preyed upon by larger predators. The plankton distribution and abundance influence general ecosystem health and biodiversity. 

4.0Environmental Importance

A. Environmental Indicators :

These are sensitive to changes in salinity and nutrient levels, water temperature, thus being excellent indicators of environmental change. The monitoring of plankton populations provides some very important information on the health status of marine ecosystems and impacts of climate change.

For example, the shift in the plankton community brought about by the rising ocean temperature may cascade down and even have impacts on the whole food web. Phytoplankton blooms mainly tend to be regulated by nutrient availability and water temperature; in some cases, such blooms lead to HABs. Such toxic substance-releasing organisms are harmful to marine life and humans.

B. Adaptation to Global Warming :

Climate change has deep impacts on plankton populations. Warmer waters reduce the population of certain phytoplankton species, which in turn lowers their potential productions of oxygen and upset food webs. Oceans are becoming more acidic as they absorb excess atmospheric CO ₂, causing changes to plankton, particularly those with calcium carbonate shells, such as some diatoms and coccolithophores.

5.0Human Impacts and Concerns

A. Pollution :  Human activities such as industrial pollution, agricultural runoff, and oil spills dump toxins and excess nutrients into marine environments. It leads to a situation such as eutrophication - nutrient overloading that causes dense blooms of phytoplankton. Then the blooms die, decompose, consume oxygen in water, and create "dead zones" where marine life cannot live.

B. Overfishing : However, this can tip the delicate balance of marine ecosystems. Many fish species targeted by humans for fishing feed on plankton or are part of a food chain that starts with plankton. Therefore, depletion of these fish populations may have significant effects on the plankton and the organisms dependent on them.