The influence of predator community composition on photoprotective traits of copepods.

Trait expression of natural populations often jointly depends on prevailing abiotic environmental conditions and predation risk. Copepods, for example, can vary their expression of compounds that confer protection against ultraviolet radiation (UVR), such as astaxanthin and mycosporine-like amino acids (MAAs), in relation to predation risk. Despite ample evidence that copepods accumulate less astaxanthin in the presence of predators, little is known about how the community composition of planktivorous fish can affect the overall expression of photoprotective compounds. Here, we investigate how the (co-)occurrence of Arctic charr (Salvelinus alpinus) and threespine stickleback (Gasterosteus aculeatus) affects the photoprotective phenotype of the copepod Leptodiaptomus minutus in lake ecosystems in southern Greenland. We found that average astaxanthin and MAA contents were lowest in lakes with stickleback, but we found no evidence that these photoprotective compounds were affected by the presence of charr. Furthermore, variance in astaxanthin among individual copepods was greatest in the presence of stickleback and the astaxanthin content of copepods was negatively correlated with increasing stickleback density. Overall, we show that the presence and density of stickleback jointly affect the content of photoprotective compounds by copepods, illustrating how the community composition of predators in an ecosystem can determine the expression of prey traits that are also influenced by abiotic stressors

The effects of hypoxia on zooplankton population estimates and migration in lakes

Many zooplankton species typically exhibit diel vertical migration (DVM), where zooplankton migrate from the hypolimnion to the epilimnion of lakes at night. Zooplankton exhibit this behavior to avoid visual predators and UV radiation by remaining in the bottom waters during the day and ascending to the surface waters to feed on phytoplankton at night. However, hypoxic conditions in the hypolimnion of lakes mayinterfere with DVM and force zooplankton to increase diel horizontal migration (DHM) to find predation refuge in littoral zones. Climate change and eutrophication are expected to increase the prevalence and severity of hypoxic conditions worldwide and thereby possibly alter zooplankton migration patterns. We hypothesize that hypoxia will force zooplankton to shift their migration patterns from predominantly DVM to DHM to avoid oxygen-depleted bottom waters. To test our hypothesis, we are conducting a standardized global sampling program to test whether pelagic, full water column estimates of zooplankton are greater at night versus the day under hypolimnetic hypoxic versus oxic conditions. Participants are aiming to sample at least one lake with an oxic hypolimnion and one lake with a hypoxic hypolimnion during the thermally-stratified period at midday and midnight. With our global dataset (currently expecting about 60 lakes in 22 countries), our goal is to improve our understanding of how global change may alter zooplankton migration behavior and patterns in lakes.info:eu-repo/semantics/publishedVersio

UV radiation and freshwater zooplankton: damage, protection and recovery

While many laboratory and field studies show that zooplankton are negatively affected when exposed to high intensities of ultraviolet radiation (UVR), most studies also indicate that zooplankton are well adapted to cope with large variations in their UVR exposure in the pelagic zone of lakes. The response mechanisms of zooplankton are diverse and efficient and may explain the success and richness of freshwater zooplankton in optically variable waters. While no single behavioural or physiological protection mechanism seems to be superior, and while several unexplained and contradictory patterns exist in zooplankton UVR ecology, recent increases in our understanding are consistent with UVR playing an important role for zooplankton. This review examines the variability in freshwater zooplankton responses to UVR, with a focus on crustacean zooplankton (Cladocera and Copepoda). We present an overview of UVR-induced damages, and the protection and recovery mechanisms freshwater zooplankton use when exposed to UVR. We review the current knowledge of UVR impact on freshwater zooplankton at species and community levels, and discuss briefly how global change over the last three decades has influenced the UVR milieu in lakes

Phytoplankton and zooplankton response to ultraviolet radiation in a high-altitude Andean lake: Short- versus long-term effects

Exclusion experiments on global UV (A and B) radiation and global UVB were performed in 460 1 mesocosms with plankton communities from the oligotrophic Andean lake Laguna Negra (33°35'S-70°04'W; 2700 m a.s.1.). The experiments were run for 30 days during the summers of 1991-1992 and 1992-1993, and for 48 days in 1993-1994. When UVB radiation was allowed to enter into the mesocosms (full sun), the population of Ankyra judayi (Chlorophyta) reached the highest density, suggesting that this species can endure high levels of UV radiation. Concurrently, an increase in chlorophyll a concentration was observed in this treatment. The cladoceran Chydorus sphaericus and the rotifer Lepadella ovalis were strongly inhibited by UVB. Conversely, UVB radiation had no effect on the survival of the different life stages of the calanoid copepod Boeckella graalipes, suggesting a species-specific difference in the sensitivity to solar UVB radiation. Moreover, no reduction in the number of copepod eggs per female and the number of nauplii produced was observed. Apparently, herbivory does not strongly affect phytoplankton abundance. Moreover, the phytoplankton species composition changed in the different treatments over the time. Fragilaria construens and Fragilaria crolonensis were dominant in those mesocosms where UVB was excluded. Populations fluctuated depending on their life cycles and the period of time they were exposed to UVB radiation. It is important to define the time scale of exclusion experiments, because different conclusions about the influence of UVB irradiance result from short-, medium- or long-term exposure

Biological Weighting Function for the Mortality of Boeckella gracilipes (Copepoda, Crustacea) Derived from Experiments with Natural Solar Radiation

We performed in situ experiments during the austral summer of 1998 to quantify the mortality of the freshwater copepod Boeckella gracilipes as a function of the UV dose. The copepods were exposed to solar radiation at the water‐surface for ∼24–34 h. Long‐pass cut‐off filters (Schott) were used in the exposure experiments. UV radiation and PAR were measured with an IL‐1700 (International Light Inc.) and a PUV‐500 radiometer (Biospherical Instruments Inc.). A biological weighting function for UV‐induced mortality was calculated by fitting a model based on a logistic curve. Our results show that UV damage in this species is strongly wavelength‐ and dose‐dependent. B. gracilipes was highly vulnerable to both UV‐B (290–320 nm) and UV‐A radiation (< 360 nm). The shape of the BWF obtained for B. gracilipes resembles more closely the action spectra (AS) † for UV‐induced erythema, than the AS for naked DNA.Fil: Tartarotti, Barbara. University of Innsbruck; AustriaFil: Cravero, Walter Ruben. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; ArgentinaFil: Zagarese, Horacio Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; Argentin

Survivorship of Cyclops abyssorum tatricus (Cyclopoida, Copepoda) and Boeckella gracilipes (Calanoida, Copepoda) under ambient levels of solar UVB radiation in two high-mountain lakes

We performed in situ experiments during the summer of 1995 and 1996 to assess the potential effect of solar ultraviolet B (UVB) radiation (290-320 nm) on the survival of Cyclops abyssorum tatricus Kozminski and Boeckella gracilipes Daday. These species are numerically dominant within the crustacean zooplankton living in two high-mountain lakes, one located in the Austrian Alps [Gossenkollesee (GKS), 2417 m above sea level, maximum depth 9.9 m] and another in the Chilean Andes (Laguna Negra, 2700 m above sea level, maximum depth 320 m). The copepods were incubated in quartz tubes (11) or in quartz tubes wrapped with Mylar D® to exclude most of the UVB radiation. The organisms were exposed at 0.5 m depth for 10-72 h on cloudless days. Both lakes were very transparent to UVB and 10% of the surface radiation at the nominal wavelength of 305 nm was still present at 9.6 m in GKS and at 12.8 m in Laguna Negra. These species migrate vertically and have a maximum daytime distribution close to