Ivan González-Bergonzoni

Impacts of climate warming on lake fish community structure and potential effects on ecosystem function

Fish play a key role in the trophic dynamics of lakes, not least in shallow systems. With climate warming, complex changes in fish community structure may be expected owing to the direct and indirect effects of temperature, and indirect effects of eutrophication, water-level changes and salinisation on fish metabolism, biotic interactions and geographical distribution. We review published and new data supporting the hypotheses that, with a warming climate, there will be changes in: fish community structure (e.g. higher or lower richness depending on local conditions); life history traits (e.g. smaller body size, shorter life span, earlier and less synchronised reproduction); feeding mode (i.e. increased omnivory and herbivory); behaviour (i.e. stronger association with littoral areas and a greater proportion of benthivores); and winter survival. All these changes imply higher predation on zooplankton and macroinvertebrates with increasing temperatures, suggesting that the changes in the fish communities partly resemble, and may intensify, the effects triggered by eutrophication. Modulating factors identified in cold and temperate systems, such as the presence of submerged plants and winter ice cover, seem to be weaker or non-existent in warm(ing) lakes. Consequently, in the future lower nutrient thresholds may be needed to obtain clear-water conditions and good ecological status in the future in currently cold or temperate lakes. Although examples are still scarce and more research is needed, we foresee biomanipulation to be a less successful restoration tool in warm(ing) lakes without a strong reduction of the nutrient load.

Environmental Warming in Shallow Lakes: A Review of Potential Changes in Community Structure as Evidenced from Space-for-Time Substitution Approaches

Abstract Shallow lakes, one of the most widespread water bodies in the world landscape, are very sensitive to climate change. Several theories predict changes in community traits, relevant for ecosystem functioning, with higher temperature. The space-for-time substitution approach (SFTS) provides one of the most plausible empirical evaluations for these theories, helping to elucidate the long-term consequences of changes in climate. Here, we reviewed the changes at the community level for the main freshwater taxa and assemblages (i.e. fishes, macroinvertebrates, zooplankton, macrophytes, phytoplankton, periphyton and bacterioplankton), under different climates. We analyzed data obtained from latitudinal and altitudinal gradients and cross-comparison (i.e. SFTS) studies, supplemented by an analysis of published geographically dispersed data for those communities or traits not covered in the SFTS literature. We found only partial empirical evidence supporting the theoretical predictions. The prediction of higher richness at warmer locations was supported for fishes, phytoplankton and periphyton, while the opposite was true for macroinvertebrates and zooplankton. With decreasing latitude, the biomass of cladoceran zooplankton and periphyton and the density of zooplankton and macroinvertebrates declined (opposite for fishes for both biomass and density variables). Fishes and cladoceran zooplankton showed the expected reduction in body size with higher temperature. Life history changes in fish and zooplankton and stronger trophic interactions at intermediate positions in the food web (fish predation on zooplankton and macroinvertebrates) were evident, but also a weaker grazing pressure of zooplankton on phytoplankton occurred with increasing temperatures. The potential impacts of lake productivity, fish predation and other factors, such as salinity, were often stronger than those of temperature itself. Additionally, shallow lakes may shift between alternative states, complicating theoretical predictions of warming effects. SFTS and meta-analyses approaches have their shortcomings, but in combination with experimental and model studies that help reveal mechanisms, the “field situation” is indispensable to understand the potential effects of warming.

Small birds, big effects: the little auk (Alle alle) transforms high Arctic ecosystems

In some arctic areas, marine-derived nutrients (MDN) resulting from fish migrations fuel freshwater and terrestrial ecosystems, increasing primary production and biodiversity. Less is known, however, about the role of seabird-MDN in shaping ecosystems. Here, we examine how the most abundant seabird in the North Atlantic, the little auk (Alle alle), alters freshwater and terrestrial ecosystems around the North Water Polynya (NOW) in Greenland. We compare stable isotope ratios (δ15N and δ13C) of freshwater and terrestrial biota, terrestrial vegetation indices and physical–chemical properties, productivity and community structure of fresh waters in catchments with and without little auk colonies. The presence of colonies profoundly alters freshwater and terrestrial ecosystems by providing nutrients and massively enhancing primary production. Based on elevated δ15N in MDN, we estimate that MDN fuels more than 85% of terrestrial and aquatic biomass in bird influenced systems. Furthermore, by using different proxies of bird impact (colony distance, algal δ15N) it is possible to identify a gradient in ecosystem response to increasing bird impact. Little auk impact acidifies the freshwater systems, reducing taxonomic richness of macroinvertebrates and truncating food webs. These results demonstrate that the little auk acts as an ecosystem engineer, transforming ecosystems across a vast region of Northwest Greenland.

Stable isotope analysis confirms substantial differences between subtropical and temperate shallow lake food webs

Differences in trophic web structure in otherwise similar ecosystems as a consequence of direct or indirect effects of ambient temperature differences can lead to changes in ecosystem functioning. Based on nitrogen and carbon stable isotope analysis, we compared the food-web structure in a series of subtropical (Uruguay, 30–35°S) and temperate (Denmark, 55–57°N) shallow lakes. The food-web length was on average one trophic position shorter in the subtropical shallow lakes compared with their temperate counterparts. This may reflect the fact that the large majority of subtropical fish species are omnivores (i.e., feed on more than one trophic level) and have a strong degree of feeding niche overlap. The shapes of the food webs of the subtropical lakes (truncated and trapezoidal) suggest that they are fuelled by a combination of different energy pathways. In contrast, temperate lake food webs tended to be more triangular, likely as a result of more simple pathways with a top predator integrating different carbon sources. The effects of such differences on ecosystem functioning and stability, and the connection with ambient temperature as a major underlying factor, are, however, still incipiently known.

Potential drivers of seasonal shifts in fish omnivory in a subtropical stream

Abstract The trophic structure of fish assemblages often varies seasonally, following the changes in food availability and supposedly water temperature. To unveil potential drivers of trophic shifts, we studied changes in fish trophic structure at both whole-assemblage and species levels at contrasting food availability and water temperatures in a subtropical stream. We analysed the diet of the most abundant omnivorous species (Bryconamericus iheringii) monthly along the year, searching for relationships with environmental variables changing seasonally (i.e. temperature and water level) and with fish reproductive stage. We ran a single-species food choice field experiment with fixed animal and vegetal food availability in contrasting seasons to test food availability as driver of diet shifts. At the assemblage level, we found a higher consumption of vegetal during summer, reflecting the increased proportion of vegetal in the diet of omnivores, which dominated the assemblage. At the species level, the enhanced vegetal consumption was related to increases in temperature and reduction in water level. Moreover, fish selected for vegetal during summer and for animal food in winter under experimental conditions. Our findings support the role of temperature driving food web dynamics by increasing fish herbivory towards warmer scenarios, with potential strong implications for whole-assemblage trophic structure.

Sound production and pectoral spine locking in a Neotropical catfish (Iheringichthys labrosus, Pimelodidae)

Catfishes may have two sonic organs: pectoral spines for stridulation and swimbladder drumming muscles. The aim of this study was to characterize the sound production of the catfish Iheringichthys labrosus. The I. labrosus male and female emits two different types of sounds: stridulatory sounds (655.8 + 230 Hz) consisting of a train of pulses, and drumming sounds (220 + 46 Hz), which are composed of single-pulse harmonic signals. Stridulatory sounds are emitted during abduction of the pectoral spine. At the base of the spine there is a dorsal process that bears a series of ridges on its latero-ventral surface, and by pressing the ridges against the groove (with an unspecialized rough surface) during a fin sweep, the animal produce a series of short pulses. Drumming sound is produced by an extrinsic sonic muscle, originated on a flat tendon of the transverse process of the fourth vertebra and inserted on the rostral and ventral surface of the swimbladder. The sounds emitted by both mechanisms are emitted in distress situation. Distress was induced by manipulating fish in a laboratory tank while sounds were recorded. Our results indicate that the catfish initially emits a stridulatory sound, which is followed by a drumming sound. Simultaneous production of stridulatory and drumming sounds was also observed. The catfish drumming sounds were lower in dominant frequency than stridulatory sounds, and also exhibited a small degree of dominant frequency modulation. Another behaviour observed in this catfish was the pectoral spine locking. This reaction was always observed before the distress sound production. Like other authors outline, our results suggest that in the catfish I. labrosus stridulatory and drumming sounds may function primarily as a distress call. Bagres podem apresentar dois orgaos sonoros: o espinho peitoral para o som peitoral ou estridulatorio e o musculo sonoro da bexiga natatoria. O objetivo deste trabalho foi caracterizar a producao de som no bagre Iheringichthys labrosus. Essa especie emite dois sons diferentes, o som peitoral ou estridulatorio (655,8 + 230 Hz) que consiste numa serie de pulsos, e o som de tamboril (220 + 46 Hz) que e composto por sinais harmonicos de pulso simples. O som peitoral e emitido com o movimento do espinho da nadadeira peitoral. A base do espinho possui um processo dorsal que suporta uma serie de cristas na sua superficie lateroventral, e ao pressionar as cristas contra o sulco (com uma superficie rugosa nao especializada) durante o movimento de abertura da nadadeira, se produz uma serie de pulsos curtos. O som de tamboril e produzido por um musculo sonico extrinseco, originado em um tendao plano preso ao processo transversal da quarta vertebra e inserido nas superficies rostral e ventral da bexiga natatoria. Os sons sao emitidos por ambos os mecanismos em situacao de estresse, a qual foi induzida atraves da manipulacao dos peixes em um tanque no laboratorio, enquanto os sons eram gravados. Nossos resultados indicam que o bagre emite primeiro o som peitoral e em seguida o som de tamboril. Tambem foi observada a producao simultânea dos sons estridulatorio e de tamboril. O som de tamboril mostra uma frequencia dominante mais baixa do que o som peitoral, e tambem apresenta um grau menor de modulacao de frequencia dominante. Isso pode estar relacionado com um som de cortejo, como verificado em outros peixes teleosteos. Outro comportamento observado nessa especie de bagre e a capacidade de travar o espinho peitoral. Essa reacao foi observada imediatamente antes da producao do som. Como descrevem outros autores, nossos resultados sugerem que em I. labrosus o som de estridulacao e de tamboril funcionam como uma chamada de alarme.

Riparian forest modifies fuelling sources for stream food webs but not food-chain length in lowland streams of Denmark

Several studies have shown that the origin of carbon fuelling food webs in streams depends on riparian cover type. In forested stream sites allochthonous resources fuel food webs, whereas autochthonous resources support biomass in grassland (open-canopy) stream sites. However, some studies suggest that autochthonous carbon (of highest quality) is preferentially assimilated regardless of riparian cover and that the food-chain length (FCL) may be larger in grassland than in forested sites. We used stable isotopes of carbon and nitrogen in adjacent grassland and forested reaches to compare the contribution of autochthonous vs. allochthonous resources to the biomass of the whole macroinvertebrate assemblage and to the most abundant taxa. Moreover, we compared the FCL between forested and grassland sites by estimating the trophic position of brown trout, Salmo trutta. Autochthonous support to macroinvertebrate biomass was higher in grassland than in forested sites, often changing from a dominantly autochthonous to an allochthonous-generated biomass from grassland to forested. This held true for the whole macroinvertebrate assemblage and for specific species. FCL remained similar between reach types. Our study suggests that autochthonous resources are assimilated to a higher extent when their availability increases with canopy openness but allochthonous carbon sustain macroinvertebrate biomass in forested reaches.

Geographic and seasonal variation analysis of digestive morphology in the catfish Iheringichthys labrosus along lower Río Uruguay

The study of geographic variation of individual traits is an important component of evolutionary research, in that individual morphological features can be subjected to multiple selective pressures. The present work is aimed to analyze the geographic and seasonal varia- tion in several digestive traits in Iheringichthys labrosus along three localities in the lower Rio Uruguay. Statistically significant differences among locations and between seasons were found for all the variables assessed, the most noticeable of differences being observed were intestinal length, intestine weight, and liver weight. In part, these differences could be an adaptive response to changes in food availability and/or in the energetic costs of reproduction. Results obtained herein also suggest that individuals of this species spend enough time in each locality as to show

Sound production in four species of the Loricariidae family

Tellechea, JS.*, Teixeira-De-Mello, F., Gonzalez-Bergonzoni, I. and Vidal, N. Laboratorio de Fisiología de la Reproducción y Ecología de Peces, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay. Grupo de Ecología y Rehabilitación de Sistemas Acuáticos, Departamento de Ecología y Evolución, Facultad de Ciencias, Maldonado, Uruguay Department of Bioscience, Aarhus University, Vejlsøvej 25, 8600 Silkeborg, Denmark *e-mail: jstellechea@gmail.com