Cecilia Laspoumaderes

Effect of Ultraviolet Radiation on Acetylcholinesterase Activity in Freshwater Copepods

We analyzed the effects of UV radiation (UVR) effects on acetylcholinesterase (AChE) activity in two calanoid copepods, Boeckella gibbosa and Parabroteas sarsi that inhabit Patagonian shallow lakes. We studied the effect of experimental UVR (UV‐B and UV‐A) exposure on AChE activity in relation to basal antioxidant capacities of both copepods. Our experiments showed that UVR can effectively depress AChE activity, although with differences between species. In both copepods AChE was affected by UV‐B, whereas UV‐A only affected AChE in B. gibbosa. Both copepods also differed in body elemental composition (C:N:P), photoprotecting compound content (carotenoids and mycosporine‐like amino acids) and enzymatic antioxidant capacity (glutathione S‐transferase [GST]). Our results suggest that when exposed to UVR, AChE activity would depend more on the antioxidant capacity (GST) and P availability for enzyme synthesis than on the photoprotective compounds.

Does the stoichiometric carbon:phosphorus knife edge apply for predaceous copepods?

Recent work has indicated that stoichiometric food quality in terms of the carbon:phosphorus (C:P) ratio affects consumers whether the imbalance involves a deficit or an excess of nutrients; hence, organisms exist on a “stoichiometric knife edge”. While previous studies have focused primarily on autotroph–herbivore trophic transfer, nutritional imbalances may also affect the interactions between species at higher trophic levels. Since the foods of carnivores are normally stoichiometrically similar to the body compositions of those carnivores, they may be more severely affected than herbivores if imbalances become pronounced. We analysed the response of the predatory copepod Parabroteas sarsi to monospecific diet treatments consisting of high and low C:P prey items. These dietary treatments strongly affected the predator’s elemental composition and growth, although prey selection, excretion, egestion, and respiration rates were not affected. We suggest that, due to their low threshold elemental ratio and a narrow C:P stoichiometric knife edge, these predators are highly vulnerable to stoichiometric imbalances, whether an excess or a deficit of nutrients is involved. Our results demonstrating this high sensitivity to prey C:P ratio show that the stoichiometric knife edge may apply to not only herbivores but also higher trophic levels. Thus, predators such as P. sarsi, with a much narrower range of food quality, may also be strongly affected by fluctuations in the quality of their prey, with negative consequences for their secondary production.

Glacier melting and response of Daphnia oxidative stress

We analysed the antioxidant response of Daphnia commutata in an oligotrophic North-Patagonian lake (Lake Mascardi) that receives inputs of glacial clay in one extreme, which creates a plume with a consequent gradient in underwater light intensity (including ultraviolet radiation) and suspended solid material. This gradient in light intensity also affects the light:nutrient ratio and hence the C:P ratio of the food for planktonic herbivores. In the field, along a 9 km transparency gradient, we measured the activities of glutathione S-transferase (GST) and catalase (CAT) enzymes involved in protection against UVR. Through laboratory experiments, we tested the possible role of suspended sediment particles as an additional stressor for a filter feeding zooplankter. Our results indicate that the inputs of glacial clay into the lake have antagonistic effects on Daphnia. Glacial clay was a stress mitigating factor to UVR (decrease in the antioxidant response of GST activity), but was also a source of stress that generated feeding interference, increased respiration rates and consequently increased CAT activity. This light gradient also affected the C:P ratio of food and the maximum response in GST is also modulated by food quality (C:P ratio) that limits its activity in the transparent end of the gradient.