Plankton are pelagic organisms as primary production providing food for marine mammals and commercially important fish. However, nowadays, it is widely accepted that global warming is occurring, and it is inevitable to impact on the marine pelagic realm. Any decline or increase in abundance, growth and trophic efficiency of phytoplankton and zooplankton communities will lead to decline or increase in higher tropic levels, even the entire ecosystems. The only way to reduce these effects is to reduce CO2 emissions to atmosphere. Further, the consideration of research should be including long-term changes in plankton biomass and community structure.
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Plankton are organisms that have limited locomotive ability relative to the water where they live. These organisms are ranging in size from viruses to large jellyfish. In tropical scale, plankton communities are highly diverse, containing organisms from almost all phyla and families. Furthermore, these organisms use their environment, its resources, and each other, in a wide variety of ways. The way to classify planktonic organisms is based on their size, which affects sinking, light utilization, mobility and tropic status. In addition, they have particular functional roles (grazers and nitrogen-fixers) in the ecosystem as well (McKinnon et al. 2007).
However, nowadays, more and more marine scientists have paid attention on climate change which has strong impacts on these organisms in the ocean. For example, increased water temperature and ocean acidification have impacts on those tiny organisms in biological and physical ways (Richardson et al. 2004; Riebesell et al. 2000; Beaugrand et al. 2003; Lynam et al. 2005).
The role of plankton in the ocean
Phytoplankton account for approximately half the global primary production Richardson et al. 2004), and consequently play an important role in cycling of atmospheric carbon dioxide (CO2). Micro- and Macrozooplankton are the basis of food webs supporting oceanic and many coastal fisheries (Richardson et al. 2004). In addition, they are also playing an important role in linking pelagic and benthic environment (McKinnon et al. 2007).
Critical factors regulating plankton communities
To date, a number of studies have demonstrated that the abundance and growth of plankton are affected by several climate stressors that will respond to climate change, including water temperature, ocean chemistry, light, ultraviolet radiation (UVR) and nutrient enrichment (McKinnon et al. 2007). Although there are still having a limited understanding of how climate change will affect planktonic organisms, more studies have done that trying to find out profound meaning.
Vulnerability
Planktonic organisms all have short life cycles: hours to days for phytoplankton, seven to ten days for copepods, and weeks to months for macrozooplankton. This means that plankton respond quickly to changes in their physical environment. Therefore, they respond more rapidly than longer-lived animals such as fish, and mammals (McKinnon et al. 2007).
Changes in water temperature
All plankton are poikilothermic. A number of studies have shown that plankton growth rate, abundance, distribution, and timing of bloom are all influenced by temperature (Beaugrand et al. 2002; Edwards and Richardson 2004; Kirby et al. 2007; Richardson and Schoeman 2004). Besides, studies have shown that plankton species changes in temperature are more likely to directly affect metabolic processes rather than the whole community biomass, especially if plankton communities are resource limited. Moreover, changes in phytoplankton community composition and productivity will have flow-on effects on the productivity of zooplankton grazers (McKinnon et al. 2007).
Ocean acidification and increased dissolved CO2
The direct effect of ocean acidification on zooplankton will be to dissolve their shells, increasing shell maintenance costs and reducing growth (Hallegraeff 1984). Furthermore, the declining pH may also change the growth rates of photosynthetic organisms. This means changes in pH will affect nutrient taking and thus alter rates of growth and photosynthesis (McKinnon et al. 2007). Changes may also occur in phytoplankton cell composition, which could affect their nutritional value for higher trophic levels (Richardson et al. 2004).
Ultraviolet radiation (UVR)
Studies have found that UVR impacts growth, mobility, and the relative dominance of many phytoplanktonic organisms (McKinnon et al. 2007). These effects compromise the ability of phytoplankton to adapt to changing environmental conditions (Hader and Hader 1989; Hader and Liu 1990). They also result in changes in cellular elemental stoichometry including increased cellular carbon content and decreased chlorophyll content (Hessen et al 1997). Further, in large-scale, UVR can cause changes in phytoplankton community structure because small cells are more prone to effects of UVR than large cells, and have comparatively high metabolic costs to screen out damaging UVR (Raven and Gilmartin 1982). Consequently, these negative effects of such changing can propagate to zooplankton (Keller et al. 1997).
Linkages with other ecosystem components
Some studies have shown that any decline or increase in abundance, growth and trophic efficiency of phytoplankton and zooplankton communities that is influenced by climate change is likely to lead to the decline or increase in higher trophic levels (Hunter 1981; Richardson et al. 2004; McKinnon et al. 2007). For example, fish larvae feed on plankton, and variations in the timing and extent of plankton reproduction could influence patterns of recruitment of fishes and invertebrates (Hunter 1981; Lynam et al. 2005).
Management strategies
The large-scale oceanographic, weather and climate processes are driving climate change impacting on plankton. Furthermore, due to the enhanced levels of CO2 in the atmosphere and rates of fossil burning, the process of ocean acidification is deterioration inevitable over next several centuries. To re-equilibrate the pH is not practical, and this will take a long time for ocean chemistry to return to a condition before industrial times. The only way to reduce these effects is to reduce CO2 emissions to the atmosphere.
Conclusion
The lack of information on the state of specific regions of plankton communities currently hinders biologists from being able to address the impacts of climate changes on those areas. Therefore, in the future, the consideration should be given to the inclusion of more plankton monitoring sites in that specific region to track long-term changes in plankton biomass and community structure, particularly for those few organisms that are at risk from ocean acidification.
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