Beatriz Modenutti

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.

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.

Grazing impact of two aquatic invertebrates on periphyton from an Andean-Patagonian stream

The mayfly Meridialaris chiloeensis and the snail Chilina dombeiana were observed to coexist and develop abundant populations in several Andean streams. In this study we examined and compared the mouthpart morphologies and the grazing mechanisms of these two species. In addition, through field experiments we analysed the grazing effect on periphyton composition and biomass. Results showed that the herbivores contrasted in their mouthpart morphology and foraging behaviour but would play a similar ecological role, since they both can be considered as scrapers. Experimental results indicated that the individual mayfly effect on chlorophyll-a and - ash free dry mass was lower than that of the individual snail. However, considering the spring and autumn abundances of both populations in a natural environment, their grazing impact might be similar or even higher for the mayfly. In addition, M. chi- loeensis depressed the rosette forming algae and favoured the prostrate ones. As a re- sult, the mayfly grazing produced a community dominated by Nitzschia palea instead - of Achnanthes minutissima that dominated the grazer-free controls, while the snail changed the taxonomic composition very little.

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

Summer population of Hexarthra bulgarica in a high elevation lake of south Andes

A Hexarthra bulgarica population was studied over one summer in a high-elevation lake of the southern Andes. A brief description of specimens is included. The zooplankton is composed of this species and the calanoid copepod Pseudoboeckella gibbosa. The lake lacks vertebrate predators. Analysis of Pseudoboeckella guts showed that its omnivorous diet included Hexarthra. A biogeographical comparison is made between this and other American high-elevation lakes. It is concluded that the Hexarthra population is controlled by predation and by extreme abiotic conditions.

Rapid Enzymatic Response to Compensate UV Radiation in Copepods

Ultraviolet radiation (UVR) causes physical damage to DNA, carboxylation of proteins and peroxidation of lipids in copepod crustaceans, ubiquitous and abundant secondary producers in most aquatic ecosystems. Copepod adaptations for long duration exposures include changes in behaviour, changes in pigmentation and ultimately changes in morphology. Adaptations to short-term exposures are little studied. Here we show that short-duration exposure to UVR causes the freshwater calanoid copepod, Eudiaptomus gracilis, to rapidly activate production of enzymes that prevent widespread collateral peroxidation (glutathione S-transferase, GST), that regulate apoptosis cell death (Caspase-3, Casp-3), and that facilitate neurotransmissions (cholinesterase-ChE). None of these enzyme systems is alone sufficient, but they act in concert to reduce the stress level of the organism. The interplay among enzymatic responses provides useful information on how organisms respond to environmental stressors acting on short time scales.

When prey mating increases predation risk : The relationship between the flatworm Mesostoma ehrenbergii and the copepod Boeckella gracilis

The zooplanktivorous flatworm Mesostoma ehrenbergii and the calanoid copepod Boeckella gracilis were observed to coexist in Patagonian fishless ponds. In laboratory experiments, we studied the vulnerability of B. gracilis to M. ehrenbergii predation, testing the attack rates on copulating pairs and single adults in different abundances. We also determined B. gracilis dimorphism, sex ratio and copulating pair ratio on two occasions in a temporary pond, with and without M. ehrenbergii. Our results indicated that B. gracilis exhibited a male-skewed sex ratio irrespective of the presence of the predator. A marked dimorphism characterized this copepod species (females are about 40 % larger than males) and a large proportion of adults were observed participating in copulating pairs that lasted for days. M. ehrenbergii ate sim- ilar quantities of single males and females of B. gracilis but significantly more copu- lating pairs. The use of mucus threads allowed Mesostoma to ingest both members of the pairs instead of only one in most attacks. Larger prey may create more turbulence in the water while swimming, so the hydrodynamic signals produced by pairs should be greater than those produced by single individuals, making them more vulnerable. Besides, the attack rates obtained in the different prey abundances showed that en- counter rate is the factor that determines M. ehrenbergii predation. We suggest that B. gracilis prolonged mating duration (days) is dangerous because it increases predation risk.