Introduction
The copepod is a small crustacean with a segmented body and eight legs. It is the most abundant type of zooplankton in the oceans and can be found at almost any depth. Copepods play a vital role in the food web, acting as primary grazers of nanoplankton and microplankton species (Hussain et al., 2020, p.3). Copepods developed in oceanic water from smaller crustaceans related to crabs and lived on the sea bed. They survived by scavenging organic materials and hiding from predators. The nocturnal copepod got larger over time, laying more eggs and allowing for more coping mechanisms in order to survive. Thus, they act as a link between primary producers and consumers on the higher levels of marine food webs, like fish. Since they are mainly herbivores, they primarily feed on plants, not meat. They feed on dead organisms, microbes, algae, and phytoplankton that lie at the bottom of the food chain, meaning they start the marine food web. Furthermore, they provide food for larger marine organisms such as baleen whales, fish, cnidarians, krill, and seabirds. Moreover, they play a significant role in matter and energy fluxes in the aquatic ecosystem by recycling nutrients in coastal waters. They also offer a biological carbon pump deep in the sea by changing the plankton population’s composition (Molinero et al., 2005, p.640). Based on these ecological functions and significance, it is important to understand the impact of climate on copepods’ variability. Thus, this paper highlights the effects of increasing ocean temperature and pH on copepod populations. It presents research findings on how increased ocean temperature and decreased pH affect Copepods populations.
Climate change is a significant issue in global oceans, which harbors vital ecosystems and sustains billions of people. Ocean temperatures are rising due to an increase in solar energy reaching the ocean that is absorbed by the water. The increased temperatures, in turn, lead to the thawing out of glacial algae, which can form large algae blooms which may be toxic to most marine life(Vehmaa et al., 2013p 4549). It also increases ocean pH because of the deterioration of calcium carbonate in shelled organisms and their increased vulnerability to damage. This increased ocean pH and algae bloom significantly affect copepods’ growth, life history, and reproductive success, affecting their population. As the ocean absorbs carbon dioxide from the atmosphere, it acidifies. This paper begins by discussing the importance of copepod species in the marine ecosystem and food web and how they are affected by increasing ocean water temperatures. Next, it covers how increasing ocean acidification affects copepod populations, followed by how these organisms have coped with these variations in temperature and pH. Finally, the paper’s findings and recommendations are summarized, emphasizing the key takeaways, implications of its findings, and steps that can be taken to help reduce climate change and increase acidification in the oceans.
Significance of the Marine Food Web
Algae and phytoplankton are at the bottom of the marine food chain but are equally essential food sources for most ocean life. Algae and phytoplankton do not differ from one another; they are considered two different types of organisms because they both rely on sunlight and photosynthesis to survive. However, not every phytoplankton is algae; some marine organisms called diatoms use a different mechanism for photosynthesis than algae. Phytoplankton mostly floats on the sea surface in order to obtain sunlight energy and use it to manufacture food through photosynthesis. The essential food produced is important to themselves and zooplankton. Thus, they are the primary food source for copepods which in turn become the main source of food for larger marine organisms (McGinty et al., 2021). While the phytoplankton is a primary food source for copepods, they, in turn, supply it with nitrogen and phosphorus, thus creating a perfect cycle for recycling nutrients.
Copepods play a significant role in the marine ecosystem. Aquatic zooplankton comprises various metazoan consumers occupying different trophic phylogenetic and working levels (Hussain et al., 2020). They mainly function as herbivores, thus providing a link between primary producers and consumers belonging to a higher level in the aquatic food chain (McGinty et al., 2021). Additionally, they make significant changes in the marine ecosystem and play a significant role in the carbon and nutrient cycle. Thus, changes in their population and abundance may significantly affect various processes at the ecosystem level, quality of water, and trophic cascades. Also, changes in their epibenthic copepod population affect the availability of other animals living in the benthic region (Rahman et al., 2022).
Copepods are the most common subclass of metazoan in the universe, and they account for 80 percent of the mesozooplankton biomass, thus dominating it (Hussain et al., 2020). They are the primary food source for many fishes and are significant in controlling fish recruitment because their eggs, nauplii, and copepodite stages are major prey for larval and adult fish. They account for 50 percent of the gut contents of fish larvae (Hussain et al., 2020). It is known that some ocean fish larvae like Lutjanidae and Serranidae cannot survive on ordinary live commercial food products like rotifers Branchionus sp and Artemia sp., interfering with aquaculture productivity rate. Big diapausing copepods with high lipid contents are a significant food source for whales (McGinty et al., 2021).
Moreover, copepods are significant for marine fish larvae’s success by acting as an effective dietary supplement to optimize their growth and survival rate. Copepods, specifically calanoid and harpacticoids, are the most important natural prey for ocean fish larvae as they have nutrients and can produce and induce stimulatory appetitive impacts on the larvae as well as maximize their digestive enzyme production (Hussain et al., 2020). Furthermore, they are significant food sources for commercial fish species during significant phases of their life cycle (McGinty et al., 2021). However, their use in commercial feeds is currently limited because of the hardships and challenges in intensive culture.
Dead organisms in the marine system get recycled by copepods and other microorganisms. Besides phytoplankton, the primary food source for copepods, they can break down dead materials and ingest carbon from them. These dead materials are referred to as zooplankton detritus and include dead algal cells, animal flesh, and decaying plant materials. In turn, they help in carbon cycling by converting these items into significant fatty acids. Bacteria, flagellates, and copepods play a significant role here as they help recycle carbon back to dead organic matter. Particulate matter made up of detrital remains of terrestrial plants and macrophytes is a crucial source of estuaries’ organic matter and thus has the capability to support the energy requirements of the marine food web at the pelagic level. Owing to these ecological significances and considerations, it is important to understand the impact that ocean temperature and pH have on the interannual variability of copepods and how they could affect the functioning of the marine ecosystem.
Effect of Increasing Water Temperature on Copepod Populations
The melting of glaciers can lead to increased glacial thawing, increasing sea surface temperatures. This leads to an increase in the concentration of nutrients on the sea floor. This organic matter provides nutrients such as nitrogen and phosphorus to local algal blooms—thus increasing the amount of thawed algae and microbial populations. The increased temperatures also increase stratification, which promotes the bloom-forming of dangerous algal species like cyanobacteria (Veehna et al., 2013, p. 4549). The increased cyanobacteria population has a negative impact on herbivores because they produce high toxins and have poor nutritional qualities. They also suffocate primary consumers out of space and locomotion. Thus, the high levels of toxic cyanobacteria do not affect zooplankton grazers and the working of the entire marine ecosystem, especially when combined with warming and acidification. According to Rebstock (2002), enhanced upper sea temperatures and increased stratification are examples of mechanisms that help explain the climatic regime shifts in aquatic ecosystems. These processes are hypothesized to decrease the supply of nutrients to the mixed layer through wind-driven upwelling resulting in a decreased zooplankton biomass (Rebstock, 2002). The increased temperatures affect various copepod congeners differently. For instance, increased temperatures promote the growth of M.leidyi, which preys on and kills copepods, particularly the Acartia tonsa (Sullivan et al., 2007). Temperature is also a significant stimulator for the induction and termination of diapause, and zooplankton with different diapause mechanisms react differently to increased temperatures and changes in temperature (Zhang et al., 2018). For instance, rotifer prey species contain specific temperature needs and utilize limited temperature windows specific to the species to recruit from the sediment. Contrarily, cyclopoid copepods are capable of recruiting from adult and sub-adult resting phases, thus capable of responding to short-term temperature changes (Zhang et al., 2018). Therefore, predatory cyclopoid copepods whose generation periods are longer may greatly profit from heat waves because they can “wake up” rapidly even with small temperature increases, thus suppressing fast reproducing prey species like rotifers. Moreover, an increase in ocean temperatures shifts copepods’ latitudinal range in an attempt to find suitable conditions resulting in species redistribution (Molinero et al., 2005).
As temperatures rise in the ocean, marine organisms experience increased thermal stress. These stressors include eutrophication, sedimentation, and pollution and are associated with climate change. These stressors significantly impact marine organisms’ reproduction, recruitment, and larvae survival. High aquatic temperatures caused by climate change may result in thermal stress in sea animals, leading to growth reduction. Specifically, high temperatures affect copepods’ oxidative status and lead to a reduction in egg production, egg viability, and the development of nauplii (Vehmaa et al., 2021). Periodical temperature changes have directly impacted reproduction in that while high temperatures initiate reproduction in spring spawners, low ones initiate reproduction in autumn-spawning species. For instance, Acartia hudsonica releases resting eggs when temperatures are more than 15oC causing them to disappear between June and July and reappear between December and January. Contrarily, Acartia tonsa hatches resting eggs when the temperature is more than 15 oC and produces them during winter (Sullivan et al., 2007). Considering the temperature variation in these species, it is predicted that A. tonsa becomes dominant during warmer winters or earlier springs. On the other hand, A. hudsonica reduces in dominance as the spring becomes warmer, causing it to begin its resting egg production sooner. Additionally, increased temperatures increase the risk of disease that affect copepod populations. Climate change affects the spread of parasites and illnesses in marine ecosystems. Various diseases tend to have increased virulence at high temperatures, often due to decreased resistance associated with stress, enhanced stimuli of virulence, or increased disease transmission. This disease affects copepods and other commercial activities that depend on them as natural resources. Furthermore, the stress resulting from increased temperatures inhibits shell formation for copepod species. Stress usually induces expensive metabolic countermeasures that greatly impact copepod shell production. Increased temperatures increase metabolic processes (Zhang et al., 2017). Suboptimal temperature and salinity may result in stressors that shift energy from shell formation to cellular procedures like osmoregulation or damaged tissue repair. Also, increased temperatures reduce food supply forcing copepods to rely on their internal energy instead of forming shells. Climate change often results in temperature increases and salinity decreases (Hussain et al., 2020). These changes result in increased stress for copepods species adapted to relatively cool aquatic temperatures and high saline conditions. Other than this, increased ocean temperature results in increased pH due to reduced CO2 absorption levels by water, thus increasing CO2 uptake and ocean acidification. As the amount of carbon dioxide in the atmosphere increases, so does the amount of hydrogen ions and a decrease in carbonate ions. The pH of seawater decreases, reducing its buffering capacity; this change leads to increased variability in pH levels(Almen et al., 2014).
Increasing Ocean Acidification Effect on Copepod Populations
Increasing ocean pH causes detriment to ocean sediment. The increased CO2 concentration in the atmosphere increases ocean surface temperatures resulting in ocean acidification. Even though warm and acidification are unique phenomena, they interact with the marine ecosystems’ detriment. These variations to the marine ecosystem are not co-occurring everywhere. They vary across gradients of temperature, depth, and latitude. As water temperature increases, its rate of CO2 absorption reduces. This means polar areas such as Alaska, where seawater is quite cold, absorb more carbon dioxide than in warmer tropics. Thus, waters in these regions are acidifying at a faster rate compared to those in different latitudes. Averagely, warmer ocean areas release carbon dioxide into the atmosphere rather than absorb it. The impact of ocean circulation patterns can also explain the regional variation in sea acidification. Due to existing patterns of wind and other natural events, the sea upwells waters that are full of nutrients and more acidic or corrosive. Naturally, these nutrient-rich, cool, and corrosive waters are infused into the coastal ecosystems’ upper layer, where it proves useful. However, in areas with acidifying waters, the incorporation of these cooler deep waters, which are highly acidic, amplifies the impact of available acidification. In other latitudes, particularly the tropics, increased surface water temperatures decrease carbon exchange between deep and surface waters. The wind mixes water from the deep and surface ends and carries water saturated with carbon dioxide to deeper ocean sections. However, when temperatures increase, the wind finds it difficult to mix these layers, which become highly stratified (Vehmaa et al., 2013). Thus, in regions with warmer waters, the upper layer becomes highly saturated with CO2 making it difficult for it to absorb more, and lower sections have minimal oxygen levels. The resulting acidification could also lead to the dissolving of rocks and minerals that can build up calcium, magnesium, and sodium in seawater and lead to the uptake of C02 from bicarbonate. The increasing ocean acidification is highly detrimental to copepod species because they build their skeletons and shells from calcium carbonate, thus threatening their survival. Specifically, acidification reduces the presence of carbonate ions in seawater which offers building blocks needed by copepods to build their shells and skeletons, thus decreasing the survival chance of their eggs and offspring. The reduced calcium carbonate levels also reduce their growth rate.
How Copepods Are Coping With the Changes in Temperature and pH
Copepods are coping with the changes in temperature and pH by migrating to colder temperatures to avoid the warmer once and pH as well as prey selection. Populations can react to climate variation differently, including shifting to new environments or staying in their current environment and adapting to it as conditions change (Hinder et al., 2014). According to Alment et al. (2014), copepods have been adapted to the short-term temperature and pH variability. To determine if and how copepods are coping with climate change, Almeen et al. (2014) examined the vertical profiles of various environmental variables and common copepods’ vertical distribution in a shallow coastal region in the Baltic Sea. The study samples were obtained monthly in June, July, and August every 6 hours for 24 hours. The researchers found that copepods migrated mainly in August and July and experienced a pH variation of 0.5 units and a temperature change of 0.5oC during this migration. Therefore, these species undergo various changes in their physicochemical environment that could be bigger than the estimated climate change scenes. The study showed that copepods could perceive and avoiders with low pH. When the copepods become limited to shallower depths when pH is very low in deep waters, low temperatures near the deeper ends may reduce their metabolic activity, enabling them to survive at low pH levels (Almeen et al., 2014). Additionally, copepods do not produce resting eggs during summer; instead, they produce subcutaneous ones capable of undergoing dominance when the environmental conditions become anoxic or lack oxygen. Therefore, copepods do not produce resting eggs as soon as they experience low pH. As ocean acidification increases, copepods’ response to temperature is modulated, and the increase in temperature enhances their sensitivity to acidification. This is exemplified by Acartia spp., which exhibits increased sensitivity to temperature changes as ocean acidification increases. Thus, the study proves that copepods adapt to predominant pH situations or are capable of tolerating an extensive range of pH. This study’s findings are supported by Sullivan et al. (2007), which showed how different copepod congeners, including Acartia tonsa and Acartia hudsonica, adapt to climate change. Sullivan et al. (2007) conducted a study examining plankton abundance and disposition data from Narragansett Bay, RI, USA, to determine if other zooplankton can exhibit altered phenologies. Specifically, they examined their seasonal succession patterns for proof of alteration between 1950 and 2004. The authors expected that increased temperatures would limit A. hudsonica’s springtime abundance, a temperate-boreal type of copepod that releases resting eggs when the weather is warm. They also expected that high temperatures would promote A. tonsa abundance over A. hudsonica, enabling a change to earlier appearance in spring, hence maintaining the window between the predator and prey that has a for an extended period enabled it to have a high production period before the ctenophore, Mneimiopsis leidyi numbers increase in late summer. However, contrary to their expectation, the researchers found that A. hudsonica has grown to be the dominant species between the two congeners. A. tonsa populations have not undergone any seasonal advancement, and their numbers have greatly reduced because of the increased predator-prey relationship it has with M. leidyi. Contrarily, A. hudsonica’s seasonal advancement is seen in a sustained increase in population earlier in the spring, but there is a decrease in its prevalence in the late spring. The later shift occurs due to M. leidyi predation (Sullivan et al., 2007). Thus, the study effectively shows the complexity of foretelling individual species’ reactions to increased temperatures, even for those species whose season and geographic disposition are well known. The findings from these two studies prove that copepods have the ability to adapt to extreme conditions whenever they occur.
Conclusion
Although copepods are still susceptible to climate change, there is clear evidence that they can adapt to them. Climate change does have its effects on copepods, but under many different circumstances, these effects seem reversible through evolution or acclimation. Adaptation here seems a more likely path to survival than evolution. Copepods can survive in marine environments by avoiding warmer ocean areas, a phenomenon known as thermal avoidance. This strategy allows copepods to select preferred prey, such as tunicate larvae and polychaete worms, from more abundant and easier-to-eat species as indicated by the predator-prey relationship between M.leidyi and A. tonia. However, if increasing temperatures continue to rise, the copepods will have a difficult time migrating to preferred waters.
Several strategies can be taken to help reduce ocean temperature and acidification. First, minimizing greenhouse- gas emissions by specifically focusing on carbon dioxide to reduce ocean acidification. This can be achieved by improving efficiency in the energy sector and promoting energy conservation, particularly in industry and households. Second, expanding the exploitation of fossil fuels with carbon capture and storage technology to reduce carbon dioxide emissions by 50 percent. Third, exploring other technologies remove carbon dioxide directly from the atmosphere, such as finding ways to increase the growth rates of marine plants (mostly phytoplankton) that absorb atmospheric CO2 in oceans. Additionally, excess nutrient runoff should be reduced as it would help lower temperature as well as reduce the rate of algae growth. Ocean-centered solutions should also be infused into the worldwide decarbonization effort. This entails minimizing shipping emissions to zero, beginning with short-term measures like slow steaming and keeping up with long-term measures like shifting to zero-carbon propulsion systems. Also, climate change can be prevented by increasing ocean alkalinity. If we stopped carbon emissions today, the seas would finally be able to absorb a high percentage of emitted CO2. However, the acidification would, in turn, be neutralized by carbonate sediments dissolution. Ocean chemistry can also be artificially altered to minimize climate change impact. An increase in ocean alkalinity can be attained by directly dissolving minerals and rocks into the sea or using engineered systems. This would result in the accumulation of calcium, magnesium, or sodium ion in ocean waters leading to increase CO2 uptake, forming bicarbonate ions, a process referred to as carbon sequestration. Several technological techniques can be taken to help increase ocean alkalinity. These include enhancing weathering, which involves applying rock powder to terrestrial, coastal, and open sea environments; accelerating limestone weathering, which entails increasing limestone dissolution in a reactor with ocean water and gas rich in CO2 and promoting mineral dissolution via electrochemistry in which acidic environments are established around the node. These efforts will help increase and preserve the population of copepods and other marine organisms that depend on carbonate ions for growth, reproduction, and shell formation. In turn, other organisms on a higher food chain level are protected, thus preserving the ocean ecosystem.
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