Large variability in the concentration of mycosporine-like amino acids among zooplankton from lakes located across an altitude gradient Abstract-The qualitative and quantitative composition of mycosporine-like amino acids (MAAs), a family of intracellular UV-absorbing compounds, were investigated in zooplankton from 15 lakes located in the Central Alps between 913 and 2,485 m above sea level. The lakes differed in their UV water transparency (1% attenuation depth, Z 1% , at 320 nm: 1.1-25.6 m) and maximum depth (Z max : 3-133 m), thus offering the possibility to test the influence of different UV exposure conditions of zooplankton on the concentration of MAAs. Seven distinct MAAs were detected, but shinorine (maximum absorption: 334 nm) was the predominant compound. In the copepods Cyclops abyssorum, C. abyssorum tatricus, and Acanthodiaptomus denticornis, the total MAA concentration ranged from 0.01 to 3.1% of the dry weight. In the rotifers Keratella cochlearis and Polyarthra dolichoptera, MAAs were also found; however, these compounds were undetectable in Asplanchna priodonta as well as in the cladocerans Daphnia hyalina, D. longispina, Bosmina longispina, and Chydorus sphaericus. The total concentration of MAAs in populations of Cyclops spp. and phytoplankton collected simultaneously was not associated (r 2 ϭ 0.09, P Ͼ 0.05), suggesting a different dynamic in the accumulation of these compounds. The variability in the concentration of MAAs, however, was related with the diffuse attenuation coefficient at 320 nm (r 2 ϭ 0.74, P Ͻ 0.001) and the fraction of the water column to which 1% of the surface irradiance at 320 nm (Z 1% : Z max ) penetrated (r 2 ϭ 0.86, P Ͻ 0.001). These relationships suggest that the prevailing UV exposure condition in the lakes is a major determinant of the concentration of MAAs found in zooplankton. Our data support the hypothesis that MAAs, together with other photoprotective compounds, play a major role in minimizing the damaging effects of solar UV radiation in zooplankton from transparent lakes.

We studied the quantitative and qualitative mycosporine-like amino acid (MAA) composition in phytoplankton and the copepod Cyclops abyssorum tatricus from an alpine lake over a 15-month period. Up to eight MAAs were identified in the samples, with shinorine being predominant. The MAAs occurred year round and showed a strong seasonal pattern. Compared with ice-cover periods, concentrations during the summer were on average 3.6 and 3.0 times higher in phytoplankton and C. abyssorum tatricus, respectively. During the summer, the contents of MAAs in phytoplankton decreased with depth, suggesting their photoprotective role. Chlorophyll a-specific concentrations of MAAs in phytoplankton correlated significantly with the incident solar radiation and ultraviolet (UV) water transparency (r 2 # 0.36), however, the strongest relationship was found with water temperature (r 2 5 0.67). In zooplankton, highest contents of MAAs were found in eggs, nauplii, and young copepodids, presumably providing a high level of photoprotection for progeny. Proportions of the dominant MAAs in the copepod showed seasonal and ontogenetic variations, which were consistent with relative changes in the predominant MAA, but not other abundant MAAs, in phytoplankton. Considering a time lag of approximately 1 month between the synthesis and subsequent accumulation of these compounds, MAA concentrations in late copepodid to adult life stages were significantly correlated to those in phytoplankton. Annual patterns in MAAs with high concentrations during periods of elevated environmental stress are consistent with the idea that these compounds play an important role in protecting aquatic organisms against UV damage.

Mycosporine-like amino acids (MAAs) were studied in zooplankton from 13 Argentinian lakes covering a broad range in altitude, maximum depth and physico-chemical properties of the water. Four to nine different MAAs (predominantly porphyra-334 and shinorine) were found in the copepods Boeckella gibbosa, B. gracilipes, B. meteoris and Parabroteas sarsi, and in the ciliate Stentor amethystinus, while MAAs were undetectable in the cladoceran Daphnia middendorffiana. Among the different copepods, maximum MAA concentrations accounted for 0.25-1.31% of the dry weight, and contents were generally about three to seven times (up to 43 times) higher in the animals living in the clearest lakes compared to those occurring in low-UV systems. This variability in the content of MAAs was related to the lake altitude (r(2) = 0.71), and the fraction of the water column to which 1% of the surface UV radiation at 320 nm penetrated (r(2) = 0.57). Our data therefore underscore the role of MAAs as sunscreens to decrease the potential negative effects of solar radiation, but they also indicate that other environmental factors besides UV transparency play a role in determining MAA concentrations. One lake was selected to obtain additional information on the qualitative composition of MAAs in seston of <100 μm between two sampling sites and over a 2 month study period (austral summer). Six different MAAs were detected in the samples, with porphyra-334 and palythine being predominant. In the copepods collected simultaneously, there was low variation in MAA concentrations between the two sites and over time. Thus, our results suggest that under similar UV exposure conditions MAA contents of planktonic organisms show low temporal variation.

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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.