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KEYWORDS: Copepods; UVB; Visible light; Life cycle; Feeding - page 2 / 16

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JOURNAL OF PLANKTON RESEARCH

j

VOLUME 32 j

NUMBER 4 j

PAGES 441456 j 2010

It has long been known that light may regulate animal behavior, and many zooplankton are light- dependent (Clarke, 1934; Duval and Geen, 1974; Pagano et al., 1993; Atkinson et al., 1996). Ultraviolet (UV) radiation, especially with a wavelength of 280 to 315 nm, negatively affects many zooplankton species (Hunter et al., 1981; Kouwenberg et al., 1999). Some animals, for example, larvae of Coregonidaf fishes, use skin pigmentation and avoidance behavior to protect themselves against UV radiation (Ylognen et al., 2005).

Larvae of some marine benthic deep-dwelling shrimp species escape

animals and the negative

impact of UV radiation by Widder, 1994; Adams, 2001). 2000; Rhode et al., 2001) and

migrations (Frank and Copepod (Martin et al., cladoceran (Johnsen and

Widder, 2001) species avoid UV stress migrations. Some crustaceans have evolved

by vertical biochemical

methods of avoiding pigmentation (Rhode

the UV-induced et al., 2001); more

stress, including colored shrimps

tend to occur in Kaartvedt, 2009).

deeper water layers Different responses

(Vestheim and of Cladoceran

species

to

UV

exposure

in

various

freshwater

lakes

have

been et al.,

documented 2005).

(Leech

and

Williamson,

2000;

Leech

Unfortunatel , most publications on light-dependent behavior of zooplankton involve freshwater species. Almost nothing is known about the effect of visible light on marine zooplankton. Because most of these animals are transparent, a significant impact of UV-induced stress may be expected. There are currently few publi- cations on behavioral responses of zooplankton to different light wavelengths. Evidence for color vision has been found in some Crustaceans (Stomatopoda) (Marshall et al., 1996). However, it is still unknown if other groups can distinguish different wavelengths (Menzel, 1979; Frank and Case, 1988), except for a single publication by Forward and Cronin (Forward and Cronin, 1979).

On the other hand, many investigators discuss verti- cal migration of zooplankton in close relationships with their diel feeding rhythms. Possible interrelations between the diel feeding patterns, the light dynamics and the migrations for these animals have been ana- lyzed, but only in the field (Bautista et al., 1988; Atkinson et al., 1992, 1996; Pagano et al., 1993; Øresland, 2000). The most recent publications involve experiments on freshwater organisms, usually Cladocera (Stutzman, 2000; Leech and Williamson, 2001; Boeing et al., 2004; Fischer et al., 2006), and almost nothing is known about the response of marine copepods, main- tained in various food environments, to different light conditions (Karanas et al., 1979). The intensity of UV light is assumed to decrease in a very upper layer of the

water column, whereas red and yellow light may pene- trate deeper down to 20 m depth (Jerlov, 1976; Burenkov et al., 2004). We assume that these two wave- lengths may become a sign of the ’upper’ and ’lower’ borders of the photic layer for the organisms. Thus, it is interesting to compare the impact of these wavelengths separately on the zooplankton behavior, because UV light is proposed to have damaging effect, and visible light does not.

It is known that the feeding rhythmicity of zooplank- ton is intimately linked with the diurnal distribution of their populations, with some minor exceptions (Pasternak, 1995; Wang et al., 1998; Torgersen, 2003). The diurnal feeding rhythms are also pronounced at the physiological level. For example, the tissue mor- phology of the gut in Acartia tonsa changes during the day (Hassett and Blades-Eckelbarger, 1995). Also chemi- cal factors may affect the diurnal vertical migrations of zooplankton: for example, daphnids exhibit a clear avoidance of kairomones from some predatory fish (Loose et al., 1993). Other chemical factors of migrations also include the destructive effects of reactive oxygen species caused by UV light (Holm-Hansen et al., 1993). Furthermore, many species of phytoplankton synthesize both attractive and repellent substances for the zoo- plankton (Chaudron et al., 1996; Dutz, 1998; Ianora et al., 1999; Miralto et al., 1999). Some authors suggest visual predator pressure as one of the major factors con- trolling downwards migrations of zooplankton during daylight (Gliwicz and Pijanowska, 1988; Bollens and Frost, 1989; Neill, 1990; Bollens et al., 1993; Loose and Dawidowicz, 1994). However, diel vertical migrations may be observed in the high latitudes during polar day independently of the light regime (Fortier et al., 2001), or the copepods may stop migrating altogether (Blachowiak-Samolyk et al., 2006). It is obvious that the diurnal distribution patterns are crucial for many of the crustaceans that inhabit the upper water layers and are susceptible to light, predators and various chemical factors.

The White Sea, situated near to the North Polar Circle, shares many characteristics with other polar seas but is unique in a number of features (Berger et al., 2001). In the summertime, it has two pronounced water layers, separated by the thermocline. The upper layer, extending 15–50 m in depth, is the productive area, and can warm up to 188C on the surface. The water masses situated under the thermocline are characterized by low productivity and temperatures below 0 Celsius. The UVB radiation in the White Sea does not pene- trate to a depth of more than 3 m, whereas yellow light of 560 nm has relatively low extinction coefficient com- paring to UVB light and goes down to 15–20 m depth

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