Esteban Balseiro

Impact of exotic rainbow trout on the benthic macroinvertebrate community from Andean-Patagonian headwater streams.

: Introduction of salmonids is a common and widespread practice in rivers and lakes of Patagonia, but their impacts remain poorly understood. We analyse the effect of exotic introduced salmonids (rainbow trout Oncorhynchus mykiss) on the benthic macroinvertebrate community of low order streams. We conducted a field survey in three headwater streams (Challhuaco, Cascada and Pescadero streams) in the northern Andean-Patagonian region (around 41 ° S and 1500m above sea level). The streams are canopied by deciduous Nothofagus pumi-lio forest. On each system, we established fishless and contiguous fish sites separated by waterfalls that limit fish access to upper sections. At each site we determined benthic macroinvertebrate size, taxonomic and functional structure in addition to trout gut contents. In the presence of trout, we observed significant shifts in invertebrate body size towards smaller individuals, thus a decrease in total macroinvertebrate biomass was observed. We found that large taxa (Klapopteryx kuscheli, Tipula sp.) and active swimming species (Metamonius anceps and Hyalella curvispina) were reduced in abundance or were absent in reaches with trout. At the same time, we found that trout positively selected large size classes of invertebrates. As a consequence of species losses, benthic community structure changed drastically in the presence of trout : shredders were the feeding group most affected negatively (-68 %) followed by scrapers. The reduction of shredders may suggest a potential effect on Nothofagus leaf litter breakdown and hence, supply of FPOM to downstream reaches.

Daphnia distribution in Andean Patagonian lakes: effect of low food quality and fish predation

Food quality in terms of carbon (C):phosphorus (P) ratio can constrain the success of highly demanding P herbivores as Daphnia. North Andean Patagonian lakes are ultraoligotrophic with low nutrient concentrations and well-developed euphotic zones. We investigated the distribution of the large Daphnia commutata in relation with food quality (sestonic C:P ratio) and predation risk in these lakes. The predation risk was estimated based on the fish species present and their relative eye diameter and transparency of the lake. The C:P ratios in the lakes were high, varying from 350 to >1,200. The lakes with D. commutata had significantly lower C:P ratio than those without these daphnids. On the other hand, those lakes where Daphnia is present have the lower predation risk than those were Daphnia is absent. In addition, we carried out growth experiments with neonates and natural seston of three lakes with different C:P ratio. The growth rates were inversely related with C:P of the food. Food quality and predation risk together determined the success or failure of large Daphnia populations in these Andean clear ultraoligotrophic lakes.

C-N mismatch in the leaf litter-shredder relationship of an Andean Patagonian stream detritivore

Abstract Aquatic shredders depend mostly on terrestrial leaf litter as a food resource, and differences between the C:N ratio of their food resource and their bodies may affect nutrient excretion and the composition of their feces. Laboratory experiments were used to test how the stonefly Klapopteryx kuscheli rebalances the high C:N ratio of its food. An experiment was designed in which K. kuscheli was fed 5 different leaf litters across a gradient of C:N ratios and the elemental ratios of the food, insect bodies, and feces were analyzed. Klapopteryx kuscheli regulated its internal elemental composition by changing its excretion of N depending on the N content of its food. An inverse relationship was found between N content of most foods and excreted NH4+. However, K. kuscheli feces were rich in N and the insect excreted very small amounts of NH4+ when fed Nothofagus pumilio, probably because of refractory N-based compounds in the leaves of this species. Homeostatic regulation of N content by invertebrate shredders may influence N dynamics in small nutrient-poor streams.

Impact of different zooplankton structures on the microbial food web of a South Andean oligotrophic lake

Abstract In oligotrophic Andean lakes, omnivorous calanoid copepods are the dominant zooplankters and, remarkably, phototrophic nanoflagellates and mixotrophic ciliates are the prevailing primary producers. In Lake Rivadavia (Patagonia, Argentina), the centropagids Boeckella michaelseni and Parabroteas sarsi coexist with the large cladoceran Daphnia middendorffiana. The particular feeding modes of these zooplanktonic species probably impact differentially on the microbial community. To determine the effect of predation on the pelagic microbial food web in this lake, we conducted a series of field experiments manipulating different zooplankton structures in 2 l enclosures. The results showed that the presence of B. michaelseni and rotifers depressed ciliates and nanoflagellates, but did not affect autotrophic picoplankton and total bacteria abundances. In contrast, the presence of Daphnia was decisive in decreasing autotrophic picoplankton abundances. P. sarsi was observed to prey on B. michaelseni copepodites and adults and a weak cascading effect on the microbial fraction could be detected.

Colony size in Conochilus hippocrepis: defensive adaptation to predator size

Conochilus hippocrepis colonies were analysed in relation to the presence and size of the predaceous calanoid copepod Parabroteas sarsi. Conochilus colonies increase in size throughout the season from May to August and then disappear from the lake. Simultaneously, Parabroteas developed from CI to CV and adults. We observed that when the predaceous copepod begins to prey on Conochilus, colony size increases in relation to maxilliped length of the predator. Our results show that the increasing size of the colony of Conochilus is an effective defense against Parabroteas predation.

Indirect enhancement of large zooplankton by consumption of predacious macroinvertebrates by littoral fish

Food webs in the littoral zones of shallow lakes are inherently complex. Submerged macrophytes are considered refuge areas although they host potential pred- ators for zooplankton. During spring : summer of 2000/2001, we carried out two meso- cosm experiments in Los Padres Lake, a shallow macrophyte dominated lake of the Argentine Pampa plain. We investigated the effect of littoral fish predation on zoo- plankton and the role of submerged macrophytes and benthic macroinvertebrates in the zooplankton-fish relationship. Treatments differed in macrophyte cover (0-65 % PVI) and fish presence. In both experiments, we determined zooplankton abundance and body size distribution, as well as macroinvertebrate abundance. The addition of the small littoral fish Astyanax eigenmanniorum (Characidae, Tetragonopterinae) to the enclosures had either a positive or a negative effect on large zooplankton, especially on calanoid copepods, depending on macroinvertebrate densities and availability. Fish did not impact on small size Cladocera. Instead, littoral fish preferred macroinverte- brates to zooplankton, and thus, the impact of fish predation on plankton community depended largely on the macroinvertebrate abundance. Therefore, our results suggest that macrophyte-fish complex could enhance or depress zooplankton abundance and that the effect is related with predatory macroinvertebrates associated with vegetation.

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.

Effect of UVR on lake water and macrophyte leachates in shallow Andean-Patagonian lakes: bacterial response to changes in optical features.

The aim of this study was to identify bacterial responses in two shallow lakes from Patagonia to UV‐irradiated dissolved organic matter (DOM) coming from different sources. We carried out laboratory experiments in which natural lake water and Potamogeton linguatus leachates were irradiated (UVA‐340 fluorescent tubes Q‐Panel) or kept in darkness. Natural bacterial assemblages were then incubated in four treatments: natural lake water, irradiated lake water, macrophyte leachate and irradiated macrophyte leachate. We estimated bacterial abundance, composition and activity, and changes in the optical features of DOM. Our results showed that the addition of leachates caused an increase in the DOM mean molecular size. After UV exposure, a high bacterial activity was observed in lake water treatments. On the contrary, carbon uptake by bacteria was reduced in the irradiated leachate treatment. The degree of aromatization in the leachate treatments increased and thus may contribute to a dissolved carbon less available for bacterial activity. Regarding the bacteria assemblage we observed that β‐Proteobacteria outcompete the other groups in the leachate treatments, this group being more efficient at utilizing the high molecular weight DOM. These results highlight the importance of UVR interacting with different DOM sources in bacteria responses of shallow lakes.

Grazing impact on autotrophic picoplankton in two south Andean lakes (Patagonia, Argentina) with different light:nutrient ratios

Los lagos andino-patagonicos son ambientes con una alta relacion luz:nutrientes. En estos lagos se ha descrito una red trofica particular constituida por grandes ciliados mixotroficos que comparten y compiten por recursos alimentarios con nanoflagelados y cladoceros. A traves de experimentos de pastoreo se compararon las tasas de limpieza sobre picoplancton autotrofico de nanoflagelados; el ciliado Ophrydium naumanni y cladoceros de los lagos Moreno Oeste y Rivadavia. Estos lagos presentan diferencias significativas en cuanto a su relacion luz:nutrientes y tienen tambien una composicion de crustaceos del zooplancton diferente. En el lago Moreno Oeste, el metalimnion esta incluido dentro de la zona eufotica, determinando un estrato iluminado en el que se desarrolla un maximo de clorofila en profundidad. En el lago Rivadavia, en cambio, los estratos iluminados estan restringidos al epilimnion y no se observo ningun maximo de clorofila en profundidad. En el lago Moreno se observo que a 30 m de profundidad se producia un incremento en la proporcion de la bacterivoria ejercida por el ciliado O. naumanni y el cladocero Ceriodaphnia dubia debido fundamentalmente a la distribucion vertical de ambas especies. Por el contrario, las tasas de limpieza del conjunto de nanoflagelados, dominado por la especie mixotrofa Chrysochromulina parva, no presentaron cambios a lo largo de la columna de agua y fueron un orden de magnitud mayores que aquellas de O. naumanni y C. dubia. En el lago Rivadavia, las tasas de pastoreo de los nanoflagelados fueron menores y comparables con las obtenidas para O. naumanni y Daphnia cf. commutata. Las diferencias observadas en las tasas de limpieza de nanoflagelados probablemente reflejan un incremento de la fagotrofia donde la energia luminica es alta en relacion con el fosforo. De esta manera, en el lago Moreno Oeste, donde la luz no es limitante, el aumento observado en la bacterivoria de los protistas podria deberse a un mayor requerimiento de nutrientes limitantes

Food Quality, Larval Consumption, and Growth of Klapopteryx kuscheli (Plecoptera: Austroperlidae) from a South Andes Stream

ABSTRACT The effect of food quality, measured as leaf resistance and carbon:nitrogen ratio, on the consumption and growth of the plecopteran Klapopteryx kuscheli was analyzed through an in situ experiment. Leaves from seven different tree species were used in feeding trials with intermediate sized K. kuscheli larvae. We used leaves from both deciduous and evergreen species, which exhibited a wide range in C:N ratio as well as leaf resistance. After 30 days, leaf consumption and larval growth were estimated for each of the 119 experimental units. Results showed that K. kuscheli was unable to consume tough leaves (leaf resistance higher than 300 g mm−2). Among deciduous species, consumption was proportional to C:N ratio showing a compensative strategy on leaves with higher C:N ratio. Nevertheless, consumption rate did not compensate for growth as ingestion was inversely related to food quality. Native deciduous plant species were of low quality for larval growth and efficient food conversion, and this may have ecological implications for both secondary production and life cycle length in detritivore-shredders inhabiting Andean low-order canopied streams.