Growth of Odontesthes bonariiensis (Atherinidae) larvae feeding on suboptimal zooplankton densities

Synopsis In this study we investigate the effect of food availability (zooplankton biomass) on the growth of Odontesthes bonariensis (Atherinidae) larvae. The larvae were stocked in four 45 m² outdoor tanks at relatively high densities (100 and 200 larvae m^–2). Because of the high stocking densities, the zooplankton biomass was depleted in all tanks. However, the patterns of food limitation, and particularly periods of severe food shortage, differed in tanks stocked at different densities. We could therefore, observe the effect of food limitation in larvae that differed in weight and age. The effects of variables suspected to influence O. bonariensis growth rates (age and weight of larvae, available zooplankton biomass, mean individual weight of available preys, total ingested prey weight, and mean weight of ingested preys) were investigated using standard multiple regression methods, and a model assuming: (1) an allometric relationship between maximum growth rates and weight of larvae, and (2) an inverse relationship between growth depression and the available zooplankton biomass. Both methods were consistent in showing that only the weight of larvae, and the availability of zooplankton prey had significant effects on the growth of O. bonariensis. The model's results additionally suggest that, if the observed growth rates are scaled by the maximum growth rate corresponding to the larva weight, the effect of zooplankton biomass is largely independent of age and weight of larvae.     

UV-B-induced damage and photoreactivation in three species of Boeckella (Copepoda, Calanoida)

Solar ultraviolet (UV) radiation poses a threat to most livingorganisms. Aquatic organisms have evolved three basic mechanismsto cope with harmful levels of radiation. Two mechanisms, avoidance(e.g. vertical migration) and photoprotection (e.g. productionof photoprotective compounds that act as filters, antioxidants,etc.), serve to minimize the dose of UV radiation that reachesthe organism's vital structures (DNA, membranes, etc.). Thethird mechanism, repair (e.g. dark repair mechanisms, such asnucleotide excision repair; or photoreactivation mechanisms,such as photoenzymatic repair), serves to repair the damagefollowing UV exposure. Here, we compare the vulnerability toUV-B radiation of three copepod species (Boeckella brevicaudata,Boeckella gibbosa, and Boeckella gracilipes) that occur in lakesthat differ in UV-B penetration and depth. Our aim was to gaininsight into the significance of each of the three mechanismsin different UV-B environments. Results from a 3-day ‘insitu’ incubation in ultra-oligotrophic Lake Toncek showedthat B.gracilipes is highly vulnerable to UV-B and UV-A radiation.In contrast, virtually no mortality was observed in B.gibbosaand B.brevicaudata during the same period. In order to discriminatethe contribution of photoprotection and photoreactivation, thethree species were subsequently exposed in the laboratory toan artificial source of UV-B radiation, both in the presenceand absence of visible radiation (recovery radiation). The photoprotectionpotential (i.e. resistance to UV-B in the absence of recoveryradiation) of B.gracilipes and B.gibbosa was lower than thatof B.brevicaudata. On the other hand, photoreactivation (higherresistance to UV-B in the presence of recovery radiation) wasobserved in B.brevicaudata and B.gibbosa, but not in B.gracilipes.To cope with damaging UV-B levels in nature, B.gracilipes dependsexclusively on the attenuation by the external media (i.e. avoidance).Although B.gibbosa tends to avoid the surface waters of lakes,it also occurs in shallow transparent pools. Most likely itsability to survive in these shallow, high UV environments isdue to its photoreactivation potential. Finally, despite itsoccurrence in highly turbid lakes, B.brevicaudata seems extremelywell suited to cope with UV-B radiation thanks to a combinationof photoreactivation and photoprotection.     

Predator-induced reaction patterns of landlocked <Emphasis Type="Italic">Galaxias maculatus</Emphasis> to visual and chemical cues

Anti-predator behaviour often represents a trade-off between the benefits of reducing predation risk and the drawbacks of limiting access to resources (e.g. food availability, mating and nesting sites). The effectiveness of avoidance behaviour relies on the ability to detect predator cues, which may provide reliable information on predation risk. Using controlled laboratory experiments, we studied the relative importance of visual and chemical cues in the triggering of anti-predator responses in Galaxias maculatus, where Oncorhyncus mykiss was used as the predator. Metabolic cost was also estimated, measured as oxygen consumption. Exposure to different types of predator cue induced diverse behavioural responses in G. maculatus. Detection of the exotic predator, using both visual and chemical stimuli, resulted in reduced G. maculatus swimming activity and changes in respiratory rate.     

Effect of land use on the phytoplankton community of Pampean shallow lakes of the Salado River basin (Buenos Aires Province,Argentina)

Aquatic Ecology - The Salado River basin, Buenos Aires, Argentina, contains numerous permanent shallow lakes under anthropic pressure due to different land uses/land covers (LULC) in their...     

How Much Does Ultraviolet Radiation Contribute to the Feeding Performance of Rainbow Trout, Oncorhynchus mykiss, Juveniles under Natural Illumination?

Most research on environmental effects of ultraviolet radiation (UVR) has focused on its potential negative consequences. However, natural UVR can also be beneficial to living organisms (e.g., vitamin D synthesis, UV vision, germicide activity). UV vision has been demonstrated in a variety of animals including several invertebrates and vertebrates. Juvenile rainbow trout, Oncorhynchus mykiss, has a retinal photoreceptor, which is sensitive to UVR between 360 and 370 nm. Among other functions, UV vision has been proposed to contribute to prey detection by enhancing the contrast between the prey and its background. We performed a series of feeding experiments with juvenile rainbow trout and several zooplankters as prey. The fish were allowed to feed either under full solar radiation, or under solar radiation from which the UV component had been removed using a long-pass cut off filter. We found that the removal of UV wavelengths had no effect on the number of prey eaten or on the preference for particular food items. This is contrary to published studies reporting prey detection enhancement mediated by UV vision in rainbow trout. This disparity in the results may be due to our use of natural radiation instead of artificial UV sources, in which the visible component is poorly represented. Although our results do not disproof the presence of UV vision in juvenile rainbow trout, they do cast doubts about its significance in enhancing feeding performance in a natural light environment.     

Phytoplankton and primary production in clear-vegetated,inorganic-turbid,and algal-turbid shallow lakes from the pampa plain (Argentina)

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on ¹⁴C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K _d 4.5–7.7 m⁻¹), had high DOC concentrations (>45 mg l⁻¹), low phytoplankton Chl a (1.6–2.7 μg l⁻¹) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (α ~ 0.03 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²) were relatively low. Inorganic-turbid lakes showed highest K _d values (59.8–61.4 m⁻¹), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 μg l⁻¹. Phytoplankton-turbid lakes showed, in general, high K _d (4.9–58.5 m⁻¹) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2–125.7 μg l⁻¹), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla ⁻¹ h⁻¹ W⁻¹ m²). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 × 10⁵–41.7 × 10⁵ cells ml⁻¹). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America. Handling editor: J. Padisak     

Mycosporine-like amino acids in planktonic organisms living under different UV exposure conditions in Patagonian lakes

Mycosporine-like amino acids (MAAs) were studied in zooplanktonfrom 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 andshinorine) were found in the copepods Boeckella gibbosa, B.gracilipes, B. meteoris and Parabroteas sarsi, and in the ciliateStentor amethystinus, while MAAs were undetectable in the cladoceranDaphnia middendorffiana. Among the different copepods, maximumMAA concentrations accounted for 0.25–1.31% of the dryweight, and contents were generally about three to seven times(up to 43 times) higher in the animals living in the clearestlakes compared to those occurring in low-UV systems. This variabilityin the content of MAAs was related to the lake altitude (r²= 0.71), and the fraction of the water column to which 1% ofthe surface UV radiation at 320 nm penetrated (r² = 0.57). Ourdata therefore underscore the role of MAAs as sunscreens todecrease the potential negative effects of solar radiation,but they also indicate that other environmental factors besidesUV transparency play a role in determining MAA concentrations.One lake was selected to obtain additional information on thequalitative composition of MAAs in seston of <100 µmbetween two sampling sites and over a 2 month study period (australsummer). Six different MAAs were detected in the samples, withporphyra-334 and palythine being predominant. In the copepodscollected simultaneously, there was low variation in MAA concentrationsbetween the two sites and over time. Thus, our results suggestthat under similar UV exposure conditions MAA contents of planktonicorganisms show low temporal variation.     

Effects of ultraviolet radiation on the eggs of landlocked Galaxias maculatus (Galaxiidae, Pisces) in northwestern Patagonia

• 1 Ultraviolet radiation (UVR) damages early life stages of several fish species. Galaxias maculatus is a small catadromous fish, with landlocked forms occurring in many lakes within the Nahuel Huapi National Park (Patagonia, Argentina). In this work, the vulnerability of G. maculatus eggs exposed to both natural and artificial UVR was investigated in relation to water transparency.
• 2 Field experiments were performed in two lakes differing in UVR attenuation. Galaxias maculatus eggs were exposed to in situ levels of UVR in quartz tubes incubated at various depths. For laboratory experiments, the eggs were exposed to five levels of artificial UVB radiation.
• 3 Exposure to natural UVR causes various degrees of egg mortality depending on water transparency and incubation depth. In the less transparent lake (K_d320 = 3.08 m^‐1), almost complete mortality was observed near the surface. At a depth of 43 cm the observed mortality was only 22%, but was still significantly different from the dark control. In the most transparent lake (K_d320 = 0.438 m^‐1), almost total mortality was observed in tubes incubated at 2.56 m or shallower. A gradual decline in mortality was recorded from that depth to 3.78 m where the values approached those in the dark control treatments.
• 4 A monotonic relationship between mortality and UV exposure could be observed both in field and laboratory experiments. Using the results from field incubations, a LD₅₀ of 2.5 J cm^‐2 nm^‐1 was estimated. In a few mountain lakes, this value would be exceeded even if the eggs were laid at the maximum depth of the lake. Thus UVR seems a sufficient cause to explain the absence of G. maculatus populations in some mountain lakes. For most lakes, however, UVR is probably one of several important environmental factors, which together determine the habitat suitability.

     

Mycosporines in carotenogenic yeasts

The ability to produce mycosporines (MYCs) in 157 pigmented yeast strains (eight genera, 25 species) isolated from natural environments of Patagonia (Argentina) was assessed. The strains belong to four taxonomic groups: the Sporidiobolales and Erythrobasidium clade of the class Urediniomycetes, and Cystofilobasidiales and Tremellales of the class Hymenomycetes. Induction of MYCs did not occur in all yeast strains tested and appeared to be an exclusive trait of members of the Erythrobasidium clade and Tremellales. This is the first report on the production of MYCs by pigmented species from the latter group, as well as the first extensive screening of mycosporinogenic yeasts. The consistent occurrence of MYCs in some specific phylogenetic groups suggests this trait bears evolutionary significance and that the presence/absence of MYCs may have potential applications in yeast systematics.