Impact of Global Warming on Aquatic Ecosystems
Global warming is a phenomenon associated with the rising of temperature of the Earth’s atmosphere, which is attributed to increased emission of greenhouse gases. As greenhouse gases accumulate in the atmosphere, they warm it at an unprecedented rate, leading to catastrophic events. Some of these events directly or indirectly affects aquatic ecosystems. This paper presents a literature review of the potential and established impact of increased temperature on aquatic ecosystems, from freshwater to marine and coastal ecosystems, with the view of establishing a universal response to global warming. The paper draws attention to the effect of climate change, in particular global warming, on aquatic ecosystems, including shift of species range from lower to higher altitudes and latitudes; seasonal shift in life cycle events; and an increase in the number of small-sized aquatic. Moreover, the author also points out that global warming cannot be considered as the only cause of changes in the aquatic ecosystems. On the contrary, the impact of global warming on aquatic systems is better understood when discussed along with other stressors. Finally, the author recommends taking proactive measures such as legislation to mitigate detrimental effect of global warming on aquatic ecosystems.
Aquatic ecosystem is one of the most vulnerable ecosystems to the phenomenon of climate change. There is increasing evidence suggesting that aquatic ecosystems are strongly impacted by climate change. This suggestion is based on climate change projections as well as on observation records made in the last five decades. Climate change trends as indicated by the level of humidity, run-offs and precipitation point to a negative trajectory in aquatic ecosystems across the world, with far-reaching consequences for societies and other ecosystems. Particular concern causes an unprecedented rate of global warming, with the Earth surface temperature increasing by an average of 0.2°C per decade for at least three past decades (Koch, Bowes, Ross, & Zhang, 2013). At the moment, research findings indicate that the Earth could be warming at an average of 0.8°C every decade, which is very dangerous for all ecosystems. Warming of the water bodies affects the most important habitat for aquatic life and goes further to impact species and their distribution (Barange et al., 2014). The body-size is also affected by the increased warming of the planet. Moreover, water quantity and quality as well as physical habitat and biological assemblages are impacted by the rise in surface temperature. High accumulation of greenhouse gases in the atmosphere poses an additional threat to aquatic ecosystems and continues to be a cause of worry for many scientists, who predict a grim future if decisive actions are not taken to curb climate change. Over the last few decades, numerous global climate change model projections have pointed to possible extinction and redistribution of species as well as large-scale biological invasion that could change the nature of aquatic ecosystems across the globe.
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The current review offers an insight into the most likely impact of global warming on aquatic ecosystems (freshwater, marine and coastal ecosystems), with the focus on a number of aspects of the ecosystem. The review presents information from the existing body of literature on aquatic ecosystems, focusing on both lotic and lentic systems as well as experimental data and long-term surveys availabe for the author. Based on the resources for this review, the author establishes trends that are likely to persist for a very long time if the situation remains unchanged. In the end, a few suggestions about the ways to improve the situation are also discussed in the summary.
Abiotic Drivers of Climate Change
Several factors have an impact on the global warming phenomenon, most importantly, temperature and the level of precipitation. There is a wide difference in the rate of global warming between various regions across the globe. Most significant is the difference in the global warming rate between Western Equatorial Pacific and Eastern Equatorial Pacific. The difference in warming contributes to various climate scenarios affecting aquatic life.
Temperature is a very important abiotic factor when studying aquatic ecosystem’s response to global warming. Global warming is the phenomenon of increasing surface temperature that takes place across the globe due to accumulation of greenhouse gases in the atmosphere. Temperature is important when discussing other abiotic factors and their relation to the consequences of global warming to ecosystems. In a recent research by Jacob, Woodward, and O’Gorman (2012), the researchers established that the surface temperature has been increasing at an approximate rate of 0.2°C each decade, and there was an average increase of 0.8°C in the surface temperature since 2000. The researchers argue that a further increase to 1.0°C above the baseline temperature at the year 2000 will constitute a “dangerous” rise in surface temperature for the 21st century. Additionally, researchers also indicate that the West and the East warm at different rates. Thus, the Western Equatorial Pacific (WEP) is warming faster compared to the Eastern Equatorial Pacific (EEP). The consequences of this difference are quite tangible. All in all, while terrestrial ecosystem reaches higher temperatures than the aquatic ecosystem with higher tolerance, it is likely that increased warming of water bodies is affecting biota in different ways, which will be discussed later.
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Apart from temperature, precipitation is another abiotic factor that affects global warming. Most likely, the level of precipitation and patterns in which it occurs affect the quantity and quality of water across different regions. Issues such as water levels and salinity relate to precipitation and are likely to affect distribution of biological life, particularly in the aquatic ecosystem (Cornwall et al., 2012). Apparently, there is a growing level of evidence showing that precipitation is no longer easy to predict in the last few decades, which is a result of global warming. Besides, there are certain catastrophic events related to precipitation that are more likely to occur if the rate of warming of the Earth persists. For example, El Ni?o is a natural phenomenon that is defined as a prolonged heavy downfall which results in run-offs. Water run-offs also have an impact on the aquatic life and the quality and quantity of water in the aquatic ecosystem (Koch et al., 2013). These and other factors related to the level of precipitation will be discussed in details further.
Ecological Consequences of Global Warming
As mentioned in the foregoing section, temperature and precipitation constitute the primary drivers of climate change. When talking about global warming as a phenomenon, evaporative demand is another primary driver. Consequences of climate change and thus global warming on aquatic ecosystems can be grouped into effects on quality and quantity of water and effects on physical habitat and biological assemblages (Dallas & Rivers-Moore, 2014). In short, the consequences of global warming for biological assemblages is the result of a combination of a synergistic relationship between different drivers. Furthermore, it is most likely that global warming and climate change generally affect land use (Jacob, Woodward, & O’Gorman, 2012). The impact of land use patterns on the aquatic ecosystem is very clear, especially when considering water quality as indicated by of presence of sediments. Therefore, land use also ought to be considered while discussing the issue of consequences of climate change for aquatic ecosystems.
The quality of water is defined by a number of factors, including groundwater recharge rate, runoff patterns as well as intensity and frequency of extreme events such as droughts and floods. Apparently, climate change leads to devastating consequences for water resources across the globe as supported by the existing evidence on rainfall patterns (Huntingford, 2013). For example, in some regions, there is low conversion of rainfall into run-off, while the converse is true for other regions. Moreover, the level of year-to-year variability in rainfall also varies from region to region. Even where some patterns have been established, there is an increasingly wide variability from what had been established as patterns of rainfall. Additionally, the response of run-off to rainfall is also nonlinear; more run-off is expected in water catchment areas or in regions that have witnessed heavy downpours for some period of time, resulting in a high soil-water content (Barange et al., 2014).
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Using agrohydrological catchment system of the Agricultural Catchments Research Unit (ACRU), researches have been in a position to project the impact of global warming on hydrological patterns of various regions (Dallas & Rivers-Moore, 2014). As a matter of fact, there is no uniformity in terms of the topography of different regions. However, every region falls into either a quinary or quaternary catchment. Quaternary catchment serves as the prime water management unit and is often measured in terms of run-off per unit area. Principally, drier regions most often have larger quaternary catchments compared to regions experiencing higher run-offs. On the other hand, quinary catchments defined by areas of uniform topography are often characterized by homogeneous physiography and hydrologoly (Dallas & Rivers-Moore, 2014). Using climate values such as maximum and minimum rainfall levels, solar radiation as well as temperature and inputting these data into the ACRU, it is possible to establish run-off patterns, intensity and frequency of extreme weather-related events and groundwater recharge rates. These three factors affecting the quality of water resources are discussed in the following sub-sections with reference to global warming.
Run-off patterns. A run-off pattern is defined by the variability of flow, its timing and duration. Recent projections for most regions predict increases in stream flows for drier regions and reduced flows for water catchment regions regardless of whether it is a year of low, median or high flows (Cornwall et al, 2012). Similarly, inter-annual variability is also expected to increase with the exception of drier regions. While more studies are still needed in this area, it is clear from the available data that the observed evidence are likely to point to reduced persistence of rivers and reduced permanence of wetlands. In this scenario, perennial rivers are likely to become non-perennial or rather, seasonal and turn into permanent or temporary wetlands. Additionally, rivers that rely on surface run-offs will most likely be susceptible to global warming compared to those rivers whose major source of flow is groundwater.
Extreme events: intencity and frequency. Over the last decades, global warming has been a prime cause of extreme weather conditions including droughts and floods. Hurricanes are a result of unprecedented levels of global warming in the Northern Hemisphere and desertification in parts of Africa, Asia, Middle East and South America are just some of the examples of extreme effects of drought in the regions (Saleem & Shapewi, 2015). Floods and storm flows are expected to be on the rise as global warming continues to wreck a havoc on many regions of the globe. Most important in this context, however, is the fact that the increasing frequency of these extreme events is likely to affect the quality of aquatic ecosystems across the globe. However, the extent to which various aquatic ecosystems are affected depends on other factors. These factors include background conditions, such as catchment hardening or the degree of canalisation, and the response humans give to increased flooding through non-structural management of floods (Goldman, Kumaqai, & Robarts, 2013).
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Groundwater recharge rate. Groundwater is a significant advantage to aquatic life during the periods of low flows and dry seasons. However, the frequency and timing of extreme weather events as well as their amplitudes have an impact on the recharge of groundwater (Dallas & Rivers-Moore, 2014).
Higher temperatures due to global warming, higher intensity of precipitation, and longer periods of low flows result in the exacerbated status of pollution of the water bodies (MacLennan, 2015). For instance, higher intensity of precipitation may culminate into increased run-offs that sweep more sediments into water bodies. There is a whole range of other kinds of water pollution that are preceded by the abovementioned factors, including dissolved organic carbon, pesticides, thermal pollution and even pathogens among others. These climate drivers are a real threat to already compromised aquatic ecosystems. Changes in the quality of water are factors that exert a significant impact on the solubility of oxygen in other gases, chemical reactions, and consequently the level of toxicity, and activity of microbes (Dallas & Rivers-Moore, 2014). Synergistic effect of high temperatures and reduced flows is the reduction in the rate of absorption of oxygen in water, which is deleterious for aquatic life. This area has been significantly researched, and there is a rich body of literature documenting variables that affect the quality of water. One of the variables examined is the temperature, which correlates with global warming as a phenomenon of climate change.
A combination of various effects by different researchers in recent studies has facilitated developing tools to assess the effects of elevated water temperatures in aquatic ecosystems. The studies done in the laboratory as well as those from the field studies make it possible for scientists to predict possible scenarios that elevated surface temperatures have on the water quality and generally on the aquatic ecosystems. While studies of the correlation between air and temperature could be useful in these predictions, Marguerite Koch and her colleagues (2013) determined that such prediction may be hampered because such scenarios are also dependent on other factors, including buffering and insulation from groundwater input, shading, and solar radiation. Consequently, the question that puzzles many researchers is how the lapse rate is likely to change over the periods of scenario prediction as this could dynamically change the final results.
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There are other variables apart from surface temperature that has received focus in the wake of discussion of the impact of global warming. Thus, intensity of rainfall and the response of the aquatic ecosystems are other important variables that affect the quality of water (Jeppesen et al., 2015). More specifically, intense events of rainfall increase the level of sediments and nutrients washed to water bodies, which raises concern over the quality of water. According to studies carried in the UK, there is increasing eutrophication of surface water, especially in freshwater ecosystems, which is associated with an increase in run-offs that carry phosphorus to water bodies (Koch et al., 2013). The water becomes enriched with this mineral and nutrients, which leads to the growth of freshwater algae at an unprecedented rate, which in turn deprives water of oxygen needed to support aquatic life. As aquatic ecosystems become highly eutrophic, they become less likely to support biota, which may result in redistribution and extermination of some species. Along with nutrients there are other pollutants such as metals, pesticides, and pathogens that are mobilized together and compromise the quality of water in rivers, lakes, and oceans across the globe.
While observations made above apply mostly to areas that support intense agricultural production, semi-arid and arid areas are not any exception when it comes the effects of global warming on water quality. In these regions, elevated temperatures are also a concern as they increase evaporative demand. A higher rate of evaporation taking place from surface and shallow ground waters leads to high salinity in arid and semi-arid areas. Salinity, in turn, is a characteristic of poor quality of water that affects a range of phytoplanktons among other aquatic organisms. In summary, the synergistic and sometimes antagonistic ways in which various water quality variables interact make it particularly difficult to make predictions about the effects of the variables on aquatic life. However, given the fact that aquatic systems are already in a devastating state after many decades of climate deterioroation, it is likely that the interactions among the variables will exacerbate the current quality of water in significant ways.
Frequency and intensity of precipitation determine the flow of germophology and hence longitudinal and lateral connectivity with the aquatic habitat. The reason behind this assertion is that the seasonal distribution of rainfall, its amount and its intensity affect the generated run-offs as well as their direction (Saleem & Shapewi, 2015). For instance, according to Dallas and Rivers-Moore (2014), increased discharge rises instability of channels while also enlarging them and initiating incision. On the other hand, decreased discharge is associated with increased stability of channels, encroachment of vegetation, shrinking of channels and possibly, increased quantity of sediments in side channels (Dallas & Rivers-Moore, 2014). As the researchers rightfully observe, the most affected of the physical habitat in the aquatic ecosystem are “fine-grained alluvial streams” (Dallas & Rivers-Moore, 2013, p. 4).
Turning to longitudinal and lateral connectivity, loss of this kind of connectivity among aquatic species is the reason for isolation and even extinction locally. On the other hand, maintenance of connectivity among species facilitates maintenance of integrity of aquatic ecosystems and their species. Functional habitat units, which are defined as natural settings that are capable of supporting the life-cycle of species, are often threatened by structural modifications that may be undertaken to redirect run-offs (Leakey & Lau, 2012). When global warming makes such modifications such as channelization necessary to protect human lives, it is most likely that aquatic species lose their connectivity.
Finally, the flow is another important determiner of the physical habitat of aquatic organisms. Flow-related studies show that a decreased flow leads to the loss of connectivity of aquatic species as a result of broken biotic composition of riparian and in-stream habitats. There is an inverse relationship between the flow rate and fragmentation, in which a reduced flow leads to more fragmented communities of aquatic species.
It is now clear from the foregoing discussion that flow and temperature are very important when examining the impact of global warming on aquatic ecosystems. Temperature is an important driver of global warming, and it plays a major role in the quality of water (Jeppesen et al., 2015). Flow, on the one hand, is determined by the frequency and intensity of precipitation. On the other hand, it determines the biotic composition of a physical habitat in aquatic ecosystems. These assemblages have a far-reaching impact on the aquatic ecosystems that are highly susceptible to climate change and thus to global warming. However, the extent to which aquatic life is affected by the assemblages depends on the species as well as their biological survival characteristics (Leakey & Lau, 2012). Thus, global warming has far-reaching biological consequences, including ranges in aquatic diversity, shifts in species ranges and distribution, seasonal shifts in life-patterns, community changes, and sometimes extinction of certain species (MacLennan, 2015).
Aquatic biodiversity. Generally speaking, three factors that affect biodiversity of freshwater and marine aquatic ecosystems are climate change, biotic exchange and land use. Although the focus here should be on global warming as a part of climate change, the other two factors are also closely related to it. For example, over-exploitation of fisheries and flow modification are issues that have been in existence for a long time and have degraded aquatic ecosystems. Global warming aggravates the situation further since these ecosystems are highly susceptible to climate change (Goldman, Kumaqai, & Robarts, 2013). The most affected niche is the freshwater ecosystem, which hosts a rich aquatic biodiversity and is more susceptible to over-exploitation, fluctuation from different flow patterns and increasing surface temperatures.
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Based on the discussion on temperature as a key driver of climate change whose intensity varies between WEP and EEP, it is evident why the US is currently experiencing more devastation. Thus, WEP experiences higher annual heating than EEP and thus is likely have its aquatic systems affected more by rising surface temperatures (Collier, Uthicke, & Waycott, 2011). Some South African countries also report significant threats to inland freshwater ecosystems as the mostly endemic species are affected by rising surface temperatures and agricultural activities destroy biodiversity in catchment areas. As agriculture exceeds a 30 to 50 percent mark, the threat to freshwater ecosystems is exacerbated due to global warming (Dallas & River-Moore, 2014). As an example, dams and channels cause discontinuities, disrupt downstream flow in natural aquatic habitats and thus result in changes in temperatures. These changes affect chemical activities, turbidity, level of toxicity and even energy flow down freshwater bodies. In turn, the changes obviously affect competitive capabilities of species differently; therefore, some species become more vulnerable than others (Goldman, Kumaqai, & Robarts, 2013). Consequently, the biotic composition of such ecosystems is likely to change.
Phenology. Life-history parameters enable organisms to survive in their environment more conveniently. A study of genetic characteristics and environmental variants helps to create an understanding of how individual species adapt to their physical habitats. Abiotic factors in the environment of species, such as temperature and flow, play major roles in how organisms manage life-history patterns such as reproduction, growth and general survival in aquatic habitats (Barange et al., 2014). The more appropriately an organism responds to changes in the ecological system, the higher the likelihood of its survival and passing of survival traits to offspring. A study on the response of insect to a changing ecological landscape in South Africa revealed that life histories such as development of eggs, growth of nymphs and oviposition are significant in understanding the subtle nature of response to changes in temperature and water flow (Goldman, Kumaqai, & Robarts, 2013). Most importantly, however, was the discovery that the degree of flexibility in navigating life histories by species is important for survival of aquatic life.
Communities. Changes in temperature due to the effects of global warming have put pressure on aquatic communities to devise ways to survive the dynamic abiotic regime, daily as well as seasonally. However, species respond differentially to the changes in temperatures, which is usually reflected in distribution of various species. Dallas and Rivers-Moore (2014) report that an onset of floods that is accompanied by rising temperature affects distribution of communities of invertebrates. According to the researchers, some species (tagged “winners”) develop resilient traits against changes in abiotic regimes, while others (tagged “losers”) decrease in abundance (Dallas & Rives-Moore, 2014, p. 5). Therefore, the major observation at the community level is that community shifts relate to thermal tolerance of each individual species taxa. On the other hand, flow affects riparian species whose response to changes in abiotic factors is advance compared to non-riparian species. Non-riparian species are reported to decrease in abundance as flow increases (Jacob, Woodward, & O’Gorman, 2012). During increased flow, elevated sites are often isolated, which leads to difficulty in upward migration. The flow, in turn, affects temperature of the aquatic ecosystems, leading to either loss or shift of species.
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Species range and distribution. Studies on distributional range of species across various geographical regions show that the patterns in shifts of species are species-specific. That is, based on the difference in temperature tolerance, cold-water species are affected negatively by elevated temperatures, while their warm-water counterparts are positively affected by the same elevation. While this observation sets foundation for further studies on this subject, the challenge is that most of the existing literature is on species of economic importance, mostly fish types (Barange et al., 2014). Despite this observation, a recent study by Koch et al. (2013) on the effect of climate change and ocean acidification on seagrasses and marine macroalgae reach a similar conclusion. The researchers conclude that some species of ectothermic organisms are less tolerant to elevated temperatures than endothermic organisms. The most important conclusion on the correlation between temperature and distributional range of various species is the fact that using the knowledge of thermophily, it is easier to predict the shifts in distributional ranges (Gao & Zheng, 2010). With the increasing surface temperature of oceans and seas, aquatic organisms that are less temperature tolerant will shift to lower altitudes. while those more tolerant to elevated temperatures shift to higher altitudes. Such distributional range shifts are likely to impact biodiversity of aquatic ecosystems.
Extinction. Local extinction becomes a reality as global warming continues to be a menace to acquatic ecosystems. The most vulnerable are aquatic species, especially those in the freshwater ecosystems (Dallas & Rivers-Moore, 2014). The reason for this assertion is that, according to recent studies, freshwater species, especially those endemic to their regions, are less tolerant to high temperatures compared with other ecosystems (Goldman, Kumaqai, & Robarts, 2013). A study of South African fish species, for example, found out that temperature elevation in high altitude and fast-flowing rivers is likely to affect endemic species in the region. Thus, these species are likely to lose their thermal refuge as surface temperature continues to rise and its effects are felt more in such ecosystems.
Invasion. As it has been discussed in this section, shifts in species are a compromise on the community equilibrium and a chance for alien species to launch attacks on the vulnerable species. Factors that increase chances for invasion are the inter-basin transfer and emergence of more open aquatic ecosystems (Huntingford, 2013). In one scenario, alien species introduced into an aquatic ecosystem are more competitive than the indigenous species, while in another one, warm-water species become more tolerant to a temperature rise compared to cold-water species.
From the foregoing it is clear that global warming is an additional factor that amplifies system variability along with other stressors. It would, therefore, be less prudent to review effects of global warming on aquatic ecosystems in isolation from the other system stressors. Such stressors in the review literature include land use, over-exploitation, and structural modifications, which combined with abiotic factors such as flow and temperature, collectively influence aquatic ecosystems. Hence, it may seem irresponsible and unfounded to claim that changes that are currently witnessed in aquatic ecosystems are solely a result of global warming. However, temperature elevation that is currently experienced is the most significant factor impacting aquatic life because aquatic ecosystems are highly susceptible to global warming. Some of the issues which have not been covered in this review, such as ocean acidification, have a strong impact on aquatic ecosystems (Anthony, Kline, Diaz-Pulido, & Hoegh-Guldberg, 2008; Arnold et al., 2012; Cornwall et al., 2012). Moreover, while water has higher tolerance to high temperatures than the terrestrial ecosystem, aquatic ecosystem has a quite diverse biotic composition whose tolerance to elevated temperatures ranges widely.
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Recommendations and Conclusion
Based on the findings from the existing body of literature on the impact of global warming on aquatic ecosystems, it is apparent that some action is needed to remedy some aspects of change brought by global warming. Generally, two measures that apply to the scenario are proactive management, which involves taking preventive measures, and reactive management, which involves managing outcomes. However, the latter measure – reactive management – can not produce tangible results in the long-term. Moreover, this management approach may result in higher costs for repairing damage or mitigating ongoing impacts. Therefore, the recommendations are based on proactive management.
As groundwork to establishing proactive management of the impact of global warming on aquatic ecosystems, legislative efforts are required to establish policies and a shared vision when it comes to saving the vulnerable ecosystems. Such a policy will help to direct efforts and resources and make mitigation of global warming a common objective pursued collectively and individually. For example, by extending the science of climate change into a public policy, it is will be easier to make climate change a national issue that must be pursued collectively. The goal is to share knowledge on management options to the public and to work in conjunction with communities in managing aquatic ecosystems.
Another way to ensure proper management of the quality of water is to integrate this aspect of water management into global climate change. Local authorities will thus pursue global climate change management by focusing on water as an important resource that needs first priority. In the meantime, the goal to save freshwater ecosystems as well as marine ecosystems will be attained.
In conclusion, there is enough evidence to show that aquatic ecosystems are threatened by synergy of abiotic regime of elevated temperatures. In essence, global warming has a detrimental effect to aquatic organisms. The situation is likely to persist unless proactive measures are taken at the earliest opportunity.