Zooplankton hold a key position in aquatic food webs. They feed on phytoplankton, bacteria, and other zooplankton, and are a major food source for fish, seabirds, and baleen whales. The zooplankton are a diverse group with some that form calcium carbonate structures (foraminifera, pteropods and heteropods) and others that do not calcify. Thus, the zooplankton response to acidification is variable. Direct responses to acidification can include reduced calcification rates and disturbance to acid-base (metabolic) physiology, which can affect other bodily functions. Pteropods, with thin snail-like shells, have been shown to be highly sensitive to decreases in ocean pH. Indirect effects, while more difficult to measure, include changes to the abundance, composition, and quality of their food source which can significantly impact organism function.
Adult non-calcifying zooplankton are thought to be more tolerant of acidification, but negative effects tend to be amplified in their younger life stages. This has been demonstrated with reduced survival and hatching success, and irregular larval development at early developmental stages in several zooplankton species, including copepods and krill. Both direct and indirect responses to acidification can impact zooplankton development, growth, reproduction, and overall population.
Due to acidification response variability and differential effects to various zooplankton life stages, it is difficult to predict how natural zooplankton communities will respond to acidification and how those changes will reverberate through food webs. Furthermore, other stressors including changes in temperature, food source, and levels of dissolved oxygen can also affect zooplankton abundance and community composition and need to be considered simultaneously with acidification.
(Photo: Katey Marancik, NOAA)
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