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The pejerrey Odontesthes bonariensis is the most important target species in temperate freshwater fisheries of Argentina, and assessment of condition has been a regular practice and common diagnostic tool. Most pejerrey fishery studies have used Fulton's (K) index, without testing whether underlying assumptions or requirements were met. We analyzed and contrasted the applicability of K, Kn and Wr indices to assess condition status in several pejerrey populations inhabiting Pampean lakes. Our results showed that whereas Wr and Kn displayed significant condition changes across length at some study lakes, Kn portrayed a small range of variation. We also noted that pejerrey maximum length and size structure strongly varied among populations probably due to the characteristics of trophic niche changes through lifespan, depending on lake limnological characteristics and zooplankton availability. We conclude that the K index should be disregarded in those cases where populations show allometric growth and size ranges strongly vary. In turn, the Kn index appears to be only appropriate for regular within population assessment, being difficult to apply when comparisons between populations are needed and when they exhibit different weight-length relationship slopes. Finally, the Wr index should be strongly preferred if the objective is to perform comparisons between pejerrey populations, particularly when population structure is not well known, stocking has been used for population recovery, lakes are strongly supported by limnological changes, data are limited to only one or few samplings and metaphoetesis is suspected in pejerrey populations.O peixe-rei Odontesthes bonariensis é a espécie mais importante na pesca de água doce da região temperada da Argentina, e a avaliação de sua condição corpórea tem sido uma prática normal e uma ferramenta de diagnóstico bastante utilizada. Grande parte dos estudos sobre a pesca do peixe-rei utiliza o índice de Fulton (K), sem testar se seus pressupostos são atendidos. Nós analisamos e comparamos a aplicabilidade dos índices K, Kn e Wr para a avaliação do fator de condição em várias populações de peixe-rei em lagos dos Pampas. Nossos resultados mostram que, enquanto o Wr e o Kn dependeram significativamente do comprimento corporal em alguns lagos, o Kn apresentou uma pequena amplitude de variação. Foi também notado que o comprimento máximo e a estrutura de tamanho variaram fortemente entre populações, provavelmente devido às mudanças ontogenéticas no nicho trófico, dependendo das características limnológicas e da disponibilidade de zooplâncton. Concluímos que o índice K deve ser desconsiderado nos casos em que as populações apresentam variações alométricas marcantes no crescimento e na amplitude de tamanhos. O índice Kn parece ser apropriado apenas para avaliações de uma única população, não sendo apropriado para comparações entre populações quando elas exibem diferentes inclinações na relação pesocomprimento. Finalmente, o índice Wr é recomendado se o objetivo for fazer comparações en...

No abstract

Lake eutrophication is a pervasive problem globally, particularly serious in agricultural and densely populated areas. Whenever nutrients nitrogen and phosphorus do not limit phytoplankton growth directly, high growth rates will rapidly lead to biomass increases causing self-shading and light-limitation, and eventually CO 2 depletion. The paradigm of phytoplankton limitation by nutrients and light is so pervasively established, that the lack of nutrient limitation is ordinarily interpreted as sufficient evidence for the condition of light limitation, without considering the possibility of limitation by inorganic carbon. Here, we firstly evaluated how frequently CO 2 undersaturation occurs in a set of eutrophic lakes in the Pampa plains. Our results confirm that conditions of CO 2 undersaturation develop much more frequently (yearly 34%, summer 44%) in these agriculturally impacted lakes than in deep, temperate lakes in forested watersheds. Secondly, we used Generalized Additive Models to fit trends in CO 2 concentration considering three drivers: total incident irradiance, chlorophyll a concentration, and lake depth; in eight multi-year datasets from eutrophic lakes from Europe, North and South America, Asia and New Zealand. CO 2 depletion was more often observed at high irradiance levels, and shallow water. CO 2 depletion also occurred at high chlorophyll concentration. Finally, we identified occurrences of light-and carbon-limitation at the whole-lake scale. The different responses of chlorophyll a and CO 2 allowed us to develop criteria for detecting conditions of CO 2 limitation. For the first time, we provided whole-lake evidence of carbon limitation of phytoplankton biomass. CO 2 increases and eutrophication represent two major and converging environmental problems that have additive and contrasting effects, promoting phytoplankton, and also leading to carbon depletion. Their interactions deserve further exploration and imaginative approaches to deal with their effects.

On the Pampa plain, one of the most productive modified areas of Argentina, important changes in land uses have drastically altered the landscape during the last decades. This has led to an increased deterioration of surface waters affecting fish that inhabit them. We provide a list of fish species inhabiting an unsurveyed prairie stream of this region. Environmental variables were measured and fish samplings were conducted in 3 sites of the Langueyú stream. A total of 15 species belonging to 10 families and 6 orders were collected. Characiformes and Siluriformes were the richest orders. Characidae was the most representative family. Species richness was highest when compared with other similar regional environments without connection with the Salado river basin. Most of the fish species collected are typical of the region but others are species which typically do not progress beyond the Salado river basin. The role of human intervention in fish species distribution is discussed.

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A myriad of factors affect stream fish assemblages, and most of them are intimately regulated by surrounding land uses. One of the most pernicious impacts on streams is the release of urban effluents. Accordingly, understanding the fish assemblage patterns as well as the environmental drivers that modulate them in the context of urban streams which also are exposed to other land uses is mandatory. We evaluated the longitudinal patterns of fish assemblages in an urbanised Neotropical prairie stream exposed to downstream patches of different surrounding land uses. Several variables of water quality, in‐stream habitat and riparian corridors were measured, and their relationships with fish metrics were explored. A fish metric selection protocol was implemented, and several multivariate analyses were used. Three main patterns of fish response were observed. Assemblage‐level attributes as diversity, richness, biomass, number of families and trophic guilds and proportion of Characiformes and intolerant species were responsive to the natural gradient in habitat (pools) and riparian (riparian width) conditions expected in non‐impacted lotic ecosystems. Conversely, a downstream continuum of recovery in water quality (dissolved oxygen, conductivity, NO3:NH4 and bacteriological loads) and habitat structure (sediment depth) would influence the distribution of fish species regarding their environmental tolerance and habitat preferences. Finally, local habitat aspects (nitrites, submerged macrophytes, bare soil cover and bank incisions by livestock) seemed relevant for the trophic structure (omnivorous species) of fish assemblages. A complex natural and anthropic longitudinal scenario together with local disruptions imposed by surrounding land uses were directive for fish.

Los usos del suelo generan profundos cambios en los ecosistemas acuáticos, sus organismos y las comunidades que los parasitan. En este trabajo analizamos los ensambles parasitarios de Corydoras paleatus bajo condiciones contrastantes de calidad de agua y hábitat en un arroyo pampeano impuestas por vertidos urbanos y ganadería. Los posibles efectos de estos aspectos sobre algunos atributos de las comunidades parasitarias de C. paleatus son discutidos. Se identificaron 136 parásitos pertenecientes a cuatro grupos taxonómicos: Phylocorydoras platensis (monogenos), Heterophyidae (digeneos larvales), Protocephalidea (cestodes lavales), y protozoos ciliados, Trichodinidae gen. sp. e Ichthyophthirius cf. multifiliis. Bajo condiciones de impacto urbano se registró una menor riqueza y diversidad. Contrariamente, el tramo ganadero mostró una mayor riqueza, diversidad y equitatividad parasitaria, y presencia de monogenos (P. platensis) y ciliados (Trichodinidae gen. sp.). Las poblaciones de digeneos registraron una mayor abundancia y la población de cestodes y I. cf. multifiliis fueron más prevalentes. El mayor deterioro en la calidad de agua y en el hábitat del tramo más cercano a la ciudad, podría explicar el empobrecimiento de las comunidades parasitarias. Una mejor calidad de agua con aporte de sustrato orgánico, menor velocidad de corriente y mayor desarrollo de macrófitas observadas bajo influencia del ganado, mejorarían las condiciones para el establecimiento de monogenos y ciliados en branquias, digeneos en aletas y cestodes en tracto digestivo. Este estudio brinda una primera aproximación al conocimiento de la ecología parasitaria de C. paleatus y los factores que podrían modelar o influir en la estructuración de las comunidades parasitarias de esta especie