The Vertical Migration of Zooplankton
Writer’s comment: When Jared Haynes assigned a literature review project in English 104E (Scientific Writing), I immediately knew which particular topic I wanted to research. My interest in the vertical migration of zooplankton began when I first learned about this important phenomenon in my limnology class, Spring Quarter 1997. In this class, I learned that although many species of zooplankton exhibit some form of diel vertical migration, scientists could not explain the phenomenon very well. This literature review assignment gave me the excuse, and, more importantly, the motivation to research current knowledge about the vertical migration of zooplankton. After a perusal of several limnology and oceanography journals, I found sufficient information on this topic to write a literature review.
- Lisa Dicke
Instructor’s comment: Lisa wrote this review article for my Advanced Composition course, English 104E: Scientific Writing. Like all good review articles, this one on vertical migration of zooplankton brings together recent research to demonstrate what is known on an area of research. Lisa’s introduction gives us an insight into the variety of migratory behaviors exhibited by zooplankton. It also sets us up for the body of the paper, which is organized around the various hypotheses as to why species of zooplankton migrate vertically. Even though this organization is simple, the real work lay in negotiating her way through the evidence from the articles, evidence that is at times contradictory and at times subtly complex. A subtheme that Lisa brings out well here is that there doesn’t seem to be a single explanation that accounts for all the observations.
- Jared Haynes, English
Vertical migration occurs within a variety of both marine and freshwater species. The normal pattern of this migration involves a descent within the water columns of oceans and lakes during the day and an ascent to near surface waters at night (Hays et al. 1994). These migratory behaviors vary significantly among and within species of zooplankton with some species exhibiting a reverse migration or no migration at all (Nesbitt et al. 1996). For example, in Lake Maarsseveen, The Netherlands, Ringelberg and Flik (1994) found that the Daphniapopulation migrated for only five to six weeks in June and July when large shoals of juvenile perch were present in the open waters. In another study, Dagg et al. (1997) did not observe a vertical migration pattern in a large fraction of the Calanus pacificusfemale population in Dabob Bay, Washington. Despite the absence of vertical migration in some species, the presence of this migration in so many taxa suggests that it has some adaptive value (Nesbitt et al. 1996). Since a substantial amount of variation exists within these migratory patterns, the same adaptive value may not hold true for each classification of zooplankton. Although it is difficult to explain the near ubiquity of the vertical migration of zooplankton, a number of hypotheses for this phenomenon have gained considerable support.
Hypotheses for vertical migration
The most widely accepted hypothesis as to why zooplankton migrate vertically in water columns is the predator-evasion hypothesis (Dagg et al. 1997). This hypothesis explains the migration as an antipredator defense in which zooplankton typically descend to dimly lit areas during the daylight to avoid visual predators. This zooplankton migration is stimulated by the release of kairomones, or chemical signals, by the predators (Nesbitt et al. 1996). A great deal of compelling evidence exists in support of this predator-evasion hypothesis (Nesbitt et al. 1996, Hays et al. 1997, Dodson et al. 1997, Dini and Carpenter 1992). Nesbitt et al. (1996) studied the response of Daphnia pulexwhen in the presence of two predators, Chaoboruslarvae and the backswimmer Notonecta,in Ranger Lake, Ontario. They observed that D. pulexmodified their migratory behavior in order to avoid portions of the water columns where predation pressure was high. Since the Chaoborusoccupied the lower stratum of Ranger Lake and Notonectaresided near the surface, D. pulexmigrated during the day to a middle stratum where the risk of visual predation was low. Nesbitt et al. (1996) conclude that D. pulexis capable of sophisticated antipredator behavior that reduces predation risk in order to increase individual fitness.
Dodson et al. (1997) and Dini and Carpenter (1992) also concluded that the migratory behaviors of selected zooplankton species are affected by visual predators. Dodson et al. (1997) observed an increased migratory response of Daphniazooplankton when in the presence of the predator cyprinid fish, Leucaspius delineatus.Similarly, Dini and Carpenter (1992) observed a more intense migration of Daphniain mesocosm enclosures with fish than in those enclosures without fish.
Hays et al. (1994) concluded that the risk of predation is the most important factor influencing the taxonomic differences in the migratory behaviors of zooplankton. They observed that the marine copepods most susceptible to visual predation due to conspicuous coloration and large body size exhibited a greater vertical migration response. The opposite was also observed: copepods that could easily escape detection due to camouflage coloration and small body size remained in surface waters during the day, exhibiting no migratory response to predation (Hays et al. 1994).
Changes in light intensity hypothesis
Although scientists have spent a considerable amount of time researching the role of predators in the vertical migration of zooplankton, the importance of a stimulus-response system should not be overlooked. It has been hypothesized that relative changes in light intensity trigger a migratory response in zooplankton (Nesbitt et al. 1996, Ringelberg and Flik 1994, Dodson et al. 1997). Ringelberg and Flik (1994) observed a rapid descent by Daphniapopulations in the early morning when relative light intensities increased. Dodson et al. (1997) observed similar results; when the lights were turned on in a mesocosm-scale plankton tower, the D. hyalinapopulation migrated downward to dimly lit areas. When the lights were turned off, D. hyalinamigrated upward to warmer surface waters of the plankton tower (Dodson et al. 1997).
Zooplankton responses to relative changes in light intensity may be enhanced by the kairomones of predators (Ringelberg and Flik 1994, Manuel and O’Dor 1997). Ringelberg and Flik (1994) observed a greater morning descent when the predator perch was present. Similarly, Manuel and O’Dor (1997) observed an enhanced downward migration of the scallop veliger Placopecten magellanicusin the mornings when visual predators were present.
Zooplankton may also migrate vertically within water columns to avoid the harmful UV light that penetrates the surface waters in daylight hours (Manuel and O’Dor 1997). Similarly, Ringelberg (1980) found that the pigmented copepods are less sensitive to visible light and do not migrate downward during the day. However, Hays et al. (1994) observed different results in their study. They found that the taxa of copepods with the highest pigmentation levels were the only groups that migrated downward during the day. Therefore, the pigmentation of zooplankton may serve a different purpose than protecting against harmful light.
Dagg et al. (1997) suggest that the migratory behavior of zooplankton may also be attributed to the availability of food within the strata of oceans and lakes. In this study, Dagg et al. (1997) concluded that the female Calanus pacificuszooplankton did not migrate up to the surface layer of Dabob Bay at night because there was enough phytoplankton in lower strata to support high feeding rates. The results of Dini and Carpenter’s (1992) study agree with this food availability hypothesis. They observed an increased migratory descent of Daphniawhen food was scarce in the surface waters of Peter Lake, Michigan. On the other hand, when nutrients were added to Peter Lake, Dini and Carpenter (1992) observed decreased migratory responses of Daphniapopulations. However, Ringelberg and Flik (1994) did not find positive correlations between low food concentrations and increased migratory responses or between high food concentrations and no migratory responses in Daphniapopulations.
Vertical migration is an important phenomenon within many taxa of both marine and freshwater zooplankton. The relative complexity and diversity of vertical migration makes it difficult to find a unifying theory to explain the different migration patterns exhibited by zooplankton species. However, a number of hypotheses have gained considerable support. Many researchers have observed increased migratory responses in zooplankton populations when in the presence of predators (Nesbitt et al. 1996, Hays et al. 1997, Dodson et al. 1997, Dini and Carpenter 1992). Relative changes in light intensity have also been shown to act as stimuli in vertical migration (Nesbitt et al. 1996, Ringelberg and Flik 1994, Dodson et al. 1997). Other studies suggest that zooplankton migrate in order to protect themselves against UV radiation (Manuel and O’Dor 1997) or to avoid starvation in strata of water columns where food is scarce (Dagg et al. 1997, Dini and Carpenter 1992).
Although much is known regarding the vertical migration of zooplankton, a number of questions are still unanswered. For example, the role of kairomones in the migratory responses of zooplankton is not completely understood. Some researchers suggest that the release of these kairomones stimulates zooplankton to migrate vertically in order to reduce the risk of predation (Nesbitt et al. 1996). However, other studies indicate that marine zooplankton species are not affected by the release of kairomones (Dodson et al. 1997). It is also not clear whether genetic selection occurs within zooplankton to select for individuals that respond to visual predation by migration (Ringelberg and Flik 1994). Another controversial topic in vertical migration is the role of pigmentation in zooplankton. Some researchers propose that pigmented zooplankton are less susceptible to harmful UV radiation and do not need to migrate vertically (Manuel and O’Dor 1997), while others suggest that the pigments may serve as an energy store (Hays et al. 1994). Since a substantial amount of variation occurs within the migratory behaviors of zooplankton, it may not be possible to explain this variation through a theory that considers only one type of response. Perhaps a more realistic concept is that each species evolved the ability to use a complex of responses to avoid visual predation.
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