Role of Zooplankton in Aquatic and Sea Environments

The importance of zooplankton to food webs makes quantitative and qualitative studies of these organisms relevant to water quality assessment.  Their significance in the aquatic food chain is due to the fact that they serve as both primary consumers and mediators between the major producers of the system—phytoplankton—and higher consumers, such as predatory fish.  Their contributions to the biological output of ecosystems are substantial, and they also play an essential role in the lentic community. 

Environmental Physicochemical Zooplankton Populations Fluctuate, and Water Quality

There can be unexpected changes in population size in the event of a disturbance due to the complex interactions that these organisms maintain with their surroundings throughout their lifetimes.  Which is why changes in their population size, species diversity, or community composition could tell us a lot about environmental health.  This is why they might be useful as a bio-indicator species to measure pollution levels in water.  Environmental physicochemical factors cause zooplankton populations to fluctuate, and water quality variables including temperature, light penetration, nutrient enrichment, herbivores, and heterotrophic bacteria all play a role in determining zooplankton density.  The presence of aroma, taste, turbidity, and visible particles in water may therefore be better comprehended by using plankton analysis values.

 Most zooplankton populations in freshwater habitats are made up of rotifers, cladocerans, and copepods.  Not only do these creatures provide a substantial amount of food for invertebrates and planktivorous fish, but they also play an important part in the energy transfer and nutrient cycling processes that occur within aquatic food webs.  Furthermore, they may be used as bioindicators to assess the level of eutrophication and water quality.  .

Composition, Quantity, and Variety Of Zooplankton Communities

The composition, quantity, and variety of zooplankton communities vary throughout natural and manmade habitats, as well as across different degrees of eutrophication. Natural environments include swamps, rivers, and shallow and deep lakes. manmade ecosystems include reservoirs, created lakes/ponds, and ancient mining lakes. Because of eutrophication-induced habitat degradation, zooplankton diversity and composition are affected by mortality, decreased reproductive ability, and dispersion. Lakes become more eutrophic when nutrients, especially nitrogen and phosphorus, accumulate over time, mostly as a result of human activities in the catchment region. Pesticides, herbicides, fertilisers, and insecticides used in farming may pollute lakes with harmful compounds and increase chemical and nutrient runoffs to the water. Higher nutrient concentrations in the water were also caused by activities inside waterbodies, such aquaculture, which discharged undigested food and fish waste into the water directly.

In the water, aquatic life is negatively affected in many ways, including development, locomotion, immunity, neurological function, and reproduction, as a consequence of toxic algal blooms (HABs) brought about by eutrophication. Another research found that zooplankton's eating, survival, and reproduction are all impacted by toxic algae's secondary metabolites, which are strong enough to reduce their lifespan survival rate. However, certain zooplankton species are able to survive in eutrophic conditions because they are vulnerable to the harmful effects of cyanobacteria toxin and because they can use inducible defences.

1. Water Temperature

The structural, hydrological, geographical, and climatic elements of the catchment area and surrounding areas determine the natural thermal properties of riverine ecosystems.  Water temperatures may be momentarily raised or lowered during precipitation occurrences.  Under normal circumstances, the temperature of moving water was between zero and thirty degrees Celsius.  The water in hot springs and volcanic areas reached temperatures of around 40 degrees Celsius.  From the headwaters to the river's mouth, the water temperature increases at a constant rate.  Rising water temperatures are caused by cooling waters that are released into rivers, such as those that result from power generating or industrial processes.  Ingesting, breathing, and reproductive development are only a few of zooplankton's physiological functions that are very temperature sensitive; in fact, their rates may increase by a factor of two or three for every degree Celsius that the water temperature rises.  Researchers Ishaq and Khan found that in the Indian river Tonnes, rotifera and copepoda were inversely connected to water temperature.  As water temperatures rose, Havens et al. observed that cladocerans and cyclopoids had smaller bodies on average.  Contrarily, water temperature has no discernible effect on the average body size of calanoids.  As water temperature decreased, Gladyshev et al. observed that copepod fatty acid content increased.

2. Dissolved oxygen

Dosage oxygen (DO) in freshwater was theoretically between zero and eighteen milligrammes per litre. When 100% of the sample is saturated with air, the result is also given as a percentage. So, algal photosynthesis causes a high saturation percentage during the day, whereas respiration and breakdown generate a low saturation percentage in the late hours.. There is less dissolved oxygen in warm salt water than in cold fresh water, even when the saturation level is 100%..

3. Oxygen Demand in Biochemistry (BOD5)

“The biochemical oxygen demand, an indirect measure of organic matter, is the quantity of oxygen that microbes in a water sample use over five days at 20°C in order to aerobically decompose the water's contents. The chemical equation for organic matter is ammonia and carbon dioxide. Measurements of biological oxygen demand (BOD5) are typically reported in milligrammes per litre (mg l-1). In order to assess the degree of water contamination, this number is critical. The levels of BOD5 were below 1 mg/L in the cleanest river, whereas those in rivers with moderate pollution varied from 2 to 8 mg l-1. The river is considered very polluted if the value is more than 8 mg l-1. Poorly treated wastewater had a BOD5 level higher than 200 mg l-1, as stated by Hanrahan. The number of zooplankton in rivers is inversely related to the biological oxygen requirement”.

4. Hydrogen Ion Concentration (pH)

Most creatures can't survive in waters that are very acidic or alkaline. The majority of aquatic creatures have evolved to thrive in environments where the pH ranges from 6 to 8. Water plants and aquatic life are often negatively impacted by pollution because it changes the pH of water. Suárez-Morales distinguished between the euryionic, acidic, and mesoionic, or neutral, groups of copepod species according to their pH tolerance. “The first group consists of species that are able to thrive in environments with pH values ranging from 4.0 to 10.5: Macrocyclops albidus, Tropocyclops prasinus, and Paracyclops fimbriatus. The second group consists of acid-tolerant species, as Diacyclops languidus and D. nanus, which should not exceed an acidity value of 8.0. The third group includes species such as Cyclops furcifer, C. vicinus, Metacyclops minutus, and M. gracilis, which inhabit freshwater environments with a neutral pH ranging from 6.5-8. The zooplankton composition of the Betwa River decreases when the pH value drops, according to research by Vishwakarma et al., which is associated with the flow of industrial effluents into the river”.

5. Electrical Conductivity (EC)

If k is between 0.55 and 0.75, then TDS is equal to k times EC.  Having said that, the constant's value often varies depending on where it's placed.  The main cause of variations in conductivity is the concentration of charged ions in water.   Furthermore, ionic composition and temperature do not play a significant role.   Conductivity is measured in microsiemens per centimetre (μS cm~-1).   Most significantly, a river's conductivity is affected by the local geology.   Rivers that run over granite bedrock appear to have poorer conductivity. This is because the inert elements that make up granite do not ionise into the water.  The conductivity of rivers is often greater in regions where clay soils are present.  The standard range for conductivity in fresh water is less than 325 μS cm-1, while in brackish water it is between 975 and 3.250 μS cm-1, and in salt water it is more than 3.250 μS cm-1.  Rotifer density in the Tigris River declined as BOD5 levels increased, according to research by Abed et al., which is associated with the direct discharge of residential sewage into the river.

6. Salinity

Usually given as ppt, or parts per thousand.  Numerous studies have shown that salinity has an effect on riverine zooplankton. For example, Nguyen et al. and Majeed et al. discovered that when salinity rose, the density of Copepoda increased, while the density of Rotifera declined.  Salinity has a significant impact on the zooplankton composition in Zamantı Rive, as shown by Kaya et al.  In addition, when salinity rose, the Richness index declined.

1.     To study on Composition, Quantity, and Variety Of Zooplankton Communities

2.     To study on role of zooplankton in aquatic and sea environments

Preserving Earth and its ecosystems is crucial because ecosystem services rely on natural ecosystem functioning. As an example, according to Austen et al. (2011), the most exploited ecosystem service is the supply of seafood.

Mussels are one kind of benthonic creature that has several industrial applications; for instance, the anti-inflammatory compound Lyprinol was derived from a green-lipped mussel that was native to New Zealand (Benkendorff, 2009). Research in the future and the creation of novel human services may benefit from the abundance of bioactive chemicals found in sponges and coral .

Marine resources provide a wide variety of ecosystem services. As an example, mangrove ecosystems play a crucial role in protecting and providing a nursery and breeding ground for several economically valuable fish species that may be farmed (Barbier, 2017).

The gathering and utilisation of natural maritime resources is also closely tied to cultural, religious, and touristic pursuits. Both commercial fishing and marine tourism rely heavily on ecosystems, however research has shown that the former has a less ecological footprint and more economic benefit (Bryhn et al., 2020). The cultural ecosystem services provided by marine resources are advantageous to many groups: Aesthetic experience, which includes taking in sights of beautiful things in nature; (2) recreation, since beaches and the ocean are often thought of as vacation spots where a variety of water sports, including swimming, snorkelling, and scuba diving, are available; (3) cultural heritage and identity, as many artists find inspiration in the ocean; and (4) local connections and family ties, among other things (Fletcher et al., 2014). Degradation of habitat, pollution, overfishing, and other human-caused problems all have an effect on our ecosystems. Ecosystem services are reduced as a result of humans' unsustainable use of natural resources (Gentry et al., 2020).

Therefore, it is critical to conserve and implement sustainable practices in order to prevent the total degradation of marine ecosystems and to ensure the continued provision of functioning ecosystem services. In order to aid in the restoration process, the Marine Strategy Framework Directive proposes using biological indicators to identify habitats that are stressed.

It was believed that plankton were equally scattered in space, and in 1884 Hensen used the word "plankton" to describe creatures that were dispersed in space (Hensen, 1884). (Lussenhop, 1974). Limnologists have long observed that zooplankton inhabit all environments according to a vertical and horizontal distribution pattern; nevertheless, Hutchinson distinguished between clumps, swarms, and aggregation in the 1950s (Hutchinson, 1953).. Daytime activity for Kreffrichthys anderssoni occurs 300–400 meters below the surface, but during the night, when zooplankton are present in the surface water, the species moves to the top 50–100 meters (Perissinotto and McQuaid, 1992). The impacts of zooplankton vertical migration on fish movement and food availability are evident.

According to Bruce et al. (2006), zooplankton have an effect on how ecosystems function. Zooplankton play a dual role in the food web, both as grazers of algae and bacteria and as providers of nitrogen and phosphorus to phytoplankton . A perfect nutrient recycling cycle is achieved.

Won et al. (2009) and Parekh et al. (2006) discovered that zooplankton are crucial to the effectiveness of the Biological Carbon Pump (BCP), which regulates atmospheric carbon dioxide levels.  According to Cavan et al. (2017), zooplankton are crucial to the BCP.  Through feeding, zooplankton regulates particle export, separates big, quickly sinking particles into slower ones, and uses diel vertical migration to transport Particulate Organic Carbon (POC) to depth..

Other planktonic groups' biomass stocks are also significantly impacted by zooplankton; in fact, it may change the concentration of predator and prey populations through ingestion, which in turn affects fish biomass (Vanni, 2002)..

To this day, bottom trawling is the most pervasive human activity that disturbs fish populations. Nonetheless, it significantly affects nutrient and carbon biogeochemical cycle. According to Pilskaln et al. (1998), algal blooms may be facilitated by the transport of dissolved and particulate nutrients from benthic to pelagic systems. Marine ecosystems, including zooplankton, may be put under stress as a result of this enhanced primary production, which may lead to water column eutrophication (Dounas et al., 2007).

In fisheries management, long time-series studies may be used to evaluate or forecast fish stocks in connection to zooplankton populations. In their 2013 study, Mackas et al. examined zooplankton records from 1990 to 2010. Locations in the Strait of Georgia that are abundant in herring and salmon are where the samples were taken. Numerous non-crustacean gelatinous predators, including chaetognaths, hydromedusae, and siphonophores, as well as numerous big copepods, euphausiids, and amphipods, made up the zooplankton group. The time-series analysis conducted by Mackas et al. (2013) revealed significant fluctuations in zooplankton biomass over the course of the 20 years. The highest biomass was observed in the early 1990s and from 1999 to 2002, while the lowest biomass was observed in 1994–1995, 2004–2007, and 2008–2010. According to Mackas et al. (2013), young salmon and herring had stunted development and survival between 2005 and 2007 due to reduced zooplankton biomass. There is a possible relationship between the number of zooplankton and the abundance of fish stocks.

Some zooplankton species are bioindicators because of how they react to changes in their native habitats. According to Beaugrand et al. (2010), Legendre and Rivkin (2005), and Richardson (2008), they are also referred to as "sentinels of environmental changes and pressures" because of this. Anthropogenic invasions of nature, including chemical additions, are presumed to be detectable by zooplankton according to the Marine Strategy Framework Directive (Serranito et al., 2016). In order to assess the potential of copepods as bioindicators for environmental pollutants, the study set out to measure their biomass abundance. According to the findings, there was a high concentration of Oithonidae species in the bay. These creatures might serve as bioindicators for the level of pollution caused by humans in the bay (Serranito et al., 2016).

To prevent the overfishing and incorrect fisheries policies that lead to fish populations collapsing, fishery management practices must be environmentally friendly and long-term.

 Think about how changes and stresses in the environment might affect stock recruitment.  According to Pershing et al. (2005), integrating recruitment models with biological indicators might be beneficial for future stock levels.  The number of predators like fish may be inferred from zooplankton populations, which in turn reflect the state of the ecosystem and the living component (Sherman et al., 2002).  Because of their essential role in the food web, zooplankton species can influence fish populations and the processes that bring new fish into the water.  A domino effect that affects fish populations may be set in motion by changes in zooplankton biomass abundance.

Ecosystems are a kind of system that consists of interdependent biological populations and the settings in which they live. Due to the interaction between the biotic and abiotic components, this dynamic system is constantly undergoing structural and functional changes. There are intricate webs of life in aquatic ecosystems, with species relying on water for resources like refuge and nutrients like nitrogen and phosphorus. An aquatic ecosystem is one that is mostly or entirely supported by water, whether it be in the form of a lake, river, pond, or ocean. changes and stresses in the environment might affect stock recruitment.  Integrating recruitment models with biological indicators might be beneficial for future stock levels.  The number of predators like fish may be inferred from zooplankton populations, which in turn reflect the state of the ecosystem and the living component.  Because of their essential role in the food web, zooplankton species can influence fish populations and the processes that bring new fish into the water.  A domino effect that affects fish populations may be set in motion by changes in zooplankton biomass abundance. The biotic component consists of living organisms in the water (such as fish, planktons, annelids, etc.) and how they interact with their environment. The a biotic component is everything else. Since they cover about 70% of Earth's surface, aquatic ecosystems are easily the most noticeable features. In addition to housing a diverse array of species, these ecosystems display complex interplay between physical, chemical, and biological components”.