40 research outputs found
Response of macroinvertebrates to experimental nutrient and fish additions in European localities at different latitudes
The sedimentary and plant-associated macroinvertebrate communities have been examined in a series of mesocosm experiments carried out in five locations, in Finland, Sweden, UK, Northwest Spain and Mediterranean Spain (the International Mesocosm Experiment). The experiments were designed to investigate how shallow lake ecosystems respond to the addition of nutrients and planktivorous fish along a climate gradient. Nutrient additions increased sedimentary benthos densities and total biomass to a plateau at intermediate levels of nutrients. The effect was greater in the warmest localities. Beyond the saturation point a further increase in nutrient loading had different effects depending on locality. Only in the southernmost site (in Mediterranean Spain), did it induce a shift to a turbid phase and macrophyte disappearance. Thus there are breakpoints in the relationship with nutrients, leading to alternative sediment communities subsequent to the water alternative states. Presence of fish increased the densities and biomass of sediment macroinvertebrates, more clearly in the more southern locality, where the fish Gambusia did not prey in the bottom animals. A stabilising role of macrophytes was observed by cutting them, samples taken three weeks after cutting indicated a higher effect of fish on the sediment community when macrophytes were not present. Nutrients tended also to increase densities of plant associated macroinvertebrates at low levels with a flattening of response at intermediate levels and a decline at high levels. At high fish level the numbers and biomass of plant-associated macroinvertebrates were lowered in all cases, because of increased predation. However fish addition at low level in most cases had the opposite effect, because their predation pressure was not high and regeneration of nutrients through their activities might increase food availability and control invertebrate predators. Fish influenced especially community composition, because they prey selectively. Addition of fish at low densities increased macroinvertebrate diversity. A diel migratory cycle was observed in plant-associated macroinvertebrates, but only in the southerly locations. Latitudinal influences on the structure of the macroinvertebrate communities occur but that they operate indirectly. There was a prominent effect of nutrients on these communities so that starting trophy is very important. Fish effects are layered on these and depend to a large extent on the particular range of diet of the species involved.Se ha estudiado la comunidad de macroinvertebrados bentónicos del sedimento así como la de los asociados a los macrófitos en una serie de mesocosmos concebidos para un experimento múltiple simultáneo llevado a cabo en cinco localidades europeas: Finlandia, Suecia, Reino Unido (UK), Noroeste de España y Levante Español. El experimento fue diseñado para investigar como responden los ecosistemas de lagos someros a los aportes de nutrientes y a la abundancia de peces planctívoros a lo largo de un gradiente climático. La adición de nutrientes incrementó la densidad del bentos del sedimento y su biomasa hasta estabilizarse a niveles intermedios de nutrientes, siendo este efecto mayor en las latitudes más bajas y cálidas. Más allá del punto de saturación, un aumento adicional de la carga de nutrientes tuvo efectos diferentes dependiendo de la localidad y solo en el lugar de menor latitud, (en el Levante Español) indujo el cambio hacia una fase turbia, con desaparición de los macrófitos, esto es, hay puntos de ruptura relacionados con la carga de nutrientes que conducen a comunidades alternativas en el sedimento, subsiguientes a los mencionados estados alternativos en las aguas someras. La presencia de peces aumentó la densidad y biomasa de los macroinvertebrados del sedimento, ello también más claramente en la localidad situada más al Sur, ya que el pez planctívoro ensayado (Gambusia) por lo general no depreda en los animales del sedimento. Se observó también el papel estabilizador de los macrófitos comparando las muestras tomadas antes y después de que estos fueran cortados, por ejemplo se observó un efecto mayor de los peces en las comunidades del sedimento cuando los macrófitos no estaban presentes. Los nutrientes tendieron también a incrementar las densidades de los macroinvertebrados asociados a los macrófitos cuando se aportan a bajos niveles, con una estabilización de la respuesta para los valores intermedios y un declive para los valores más altos. Cuando se adicionaron densidades altas de peces, el número y biomasa de los macroinvertebrados asociados a los macrófitos descendieron en todos los casos, como resultado del aumento de la depredación. Sin embargo, las adiciones a baja densidad de peces, en la mayoría de los casos, tuvo el efecto contrario, ya que su presión de depredación no es alta y pueden contribuir a la regeneración de los nutrientes a través de la vía detrítica que incrementa la disponibilidad de alimento, así como al control de otros invertebrados depredadores. La presencia de peces influenció especialmente la composición de la comunidad, ya que su depredación es selectiva. La adición de peces en bajas densidades aumentó la diversidad de los macroinvertebrados. También fue observado que el ciclo diario de migración en los macroinvertebrados asociados a los macrófitos, es más importante en las localidades ubicadas en las latitudes de más al Sur. Las influencias latitudinales en la estructura de las comunidades de macroinvertebrados, si bien sucede, opera indirectamente. Hay un efecto dominante de los nutrientes en estas comunidades, y el nivel trófico de las condiciones iniciales es muy importante. Los efectos de los peces se subordinan al de los nutrientes y dependen en gran medida de la dieta de las especies involucradas
Life-Cycle Switching and Coexistence of Species with No Niche Differentiation
The increasing evidence of coexistence of cryptic species with no recognized niche differentiation has called attention to mechanisms reducing competition that are not based on niche-differentiation. Only sex-based mechanisms have been shown to create the negative feedback needed for stable coexistence of competitors with completely overlapping niches. Here we show that density-dependent sexual and diapause investment can mediate coexistence of facultative sexual species having identical niches. We modelled the dynamics of two competing cyclical parthenogens with species-specific density-dependent sexual and diapause investment and either equal or different competitive abilities. We show that investment in sexual reproduction creates an opportunity for other species to invade and become established. This may happen even if the invading species is an inferior competitor. Our results suggests a previously unnoticed mechanism for species coexistence and can be extended to other facultative sexual species and species investing in diapause where similar density-dependent life-history switches could act to promote coexistence
Differential Levels of Stress Proteins (HSPs) in Male and Female Daphnia magna in Response to Thermal Stress: A Consequence of Sex-Related Behavioral Differences?
In two independent experiments, we compared: (1) water depth selection (and accompanying temperature selection) by male and female Daphnia magna under different kinds of environmental stress, including the presence of filamentous cyanobacteria, the risk of predation from fish, and the presence of toxic compounds; and (2) sex-dependent production of heat shock proteins (HSP60, 70, and 90) in response to a sudden change in temperature. Male D. magna selected deep water strata, which offer a relatively stable environment, and thereby avoided the threat of predation and the presence of toxic compounds in surface waters. Correlated with this behavior, males reduce their molecular defenses against stress, such as the production of heat shock proteins (HSPs), and do not maintain the physiological machinery that triggers an increase in HSP levels in response to stress. In contrast, female D. magna actively select habitats that offer optimal conditions for growth and production of offspring. Consequently, females are exposed to variable environmental conditions that may be associated with increased stress. To permit survival in these different habitats, D. magna females require molecular mechanisms to protect their cells from rapid changes in stress levels. Thus, they maintain high constitutive levels of the heat shock proteins from HSP 60, 70, and 90 families, and they have the potential to further enhance the production of the majority of these proteins under stress conditions. The results of this study indicate that the separate habitats selected by male and female D. magna result in different patterns of HSP production, leading us to hypothesize that that male and female Daphnia magna adopt different strategies to maximize the fitness of the species
Strong differences in the clonal variation of two Daphnia species from mountain lakes affected by overwintering strategy
<p>Abstract</p> <p>Background</p> <p>The population structure of cyclical parthenogens such as water fleas is strongly influenced by the frequency of alternations between sexual and asexual (parthenogenetic) reproduction, which may differ among populations and species. We studied genetic variation within six populations of two closely related species of water fleas of the genus <it>Daphnia </it>(Crustacea, Cladocera). <it>D. galeata </it>and <it>D. longispina </it>both occur in lakes in the Tatra Mountains (Central Europe), but their populations show distinct life history strategies in that region. In three studied lakes inhabited by <it>D. galeata</it>, daphnids overwinter under the ice as adult females. In contrast, in lakes inhabited by <it>D. longispina</it>, populations apparently disappear from the water column and overwinter as dormant eggs in lake sediments. We investigated to what extent these different strategies lead to differences in the clonal composition of late summer populations.</p> <p>Results</p> <p>Analysis of genetic variation at nine microsatellite loci revealed that clonal richness (expressed as the proportion of different multilocus genotypes, MLGs, in the whole analysed sample) consistently differed between the two studied species. In the three <it>D. longispina </it>populations, very high clonal richness was found (MLG/N ranging from 0.97 to 1.00), whereas in <it>D. galeata </it>it was much lower (0.05 to 0.50). The dominant MLGs in all <it>D. galeata </it>populations were heterozygous at five or more loci, suggesting that such individuals all represented the same clonal lineages rather than insufficiently resolved groups of different clones.</p> <p>Conclusions</p> <p>The low clonal diversities and significant deviations from Hardy-Weinberg equilibrium in <it>D. galeata </it>populations were likely a consequence of strong clonal erosion over extended periods of time (several years or even decades) and the limited influence of sexual reproduction. Our data reveal that populations of closely related <it>Daphnia </it>species living in relatively similar habitats (permanent, oligotrophic mountain lakes) within the same region may show strikingly different genetic structures, which most likely depend on their reproductive strategy during unfavourable periods. We assume that similar impacts of life history on population structures are also relevant for other cyclical parthenogen groups. In extreme cases, prolonged clonal erosion may result in the dominance of a single clone within a population, which might limit its microevolutionary potential if selection pressures suddenly change.</p
Climate change and freshwater zooplankton: what does it boil down to?
Recently, major advances in the climate–zooplankton interface have been made some of which appeared to receive much attention in a broader audience of ecologists as well. In contrast to the marine realm, however, we still lack a more holistic summary of recent knowledge in freshwater. We
discuss climate change-related variation in physical and biological attributes of lakes and running waters, high-order ecological functions, and subsequent alteration
in zooplankton abundance, phenology, distribution, body size, community structure, life history parameters, and behavior by focusing on community level responses. The adequacy of large-scale climatic indices in ecology has received considerable support and provided a framework for the interpretation of community and species level responses in freshwater zooplankton. Modeling perspectives deserve particular consideration, since this promising stream of
ecology is of particular applicability in climate change
research owing to the inherently predictive nature of
this field. In the future, ecologists should expand their
research on species beyond daphnids, should address
questions as to how different intrinsic and extrinsic
drivers interact, should move beyond correlative
approaches toward more mechanistic explanations,
and last but not least, should facilitate transfer of
biological data both across space and time
Prevalence of kairomone-induced diapause in Daphnia magna from habitats with and without fish
Photosynthetic characteristics of the benthic diatom species Nitzschia frustulum (Kützing) Grunow isolated from a soda pan along temperature-, sulfate- and chloride gradients
The distribution of chydorids (Branchiopoda, Anomopoda) in European shallow lakes and its application to ecological quality monitoring
This study describes the chydorid (Branchiopoda, Anomopoda) assemblages from 66 European shallow lakes, and presents data relating the assemblages to lake type and ecological quality. Forty species, out of a total recorded European fauna of 60 species, were found in the study sites. No significant differences were found between chydorid assemblages associated with rock and plant substrata. Patterns of distribution were best explained primarily by latitude and pH. Chlorophyll-a, total phosphorus, water temperature and Secchi depth were also correlated with assemblage descriptors. Alonopsis elongata, Alona rectangula, Alonella excisa and Pleuroxus uncinatus were shown to have higher prevalence in certain lake types. The dominance of Chydorus sphaericus in a third of the study sites was linked to eutrophication and high levels of chlorophyll-a. The relationship between chydorids and lake ecological quality was more apparent at species rather than community level. This study identifies important typological factors affecting chydorid distribution, and confirms that patterns of chydorid distribution previously reported from regional studies hold true across Europe.
Response of zooplankton to nutrient enrichment and fish in shallow lakes: a pan-European mesocosm experiment
1. Responses of zooplankton to nutrient enrichment and fish predation were studied in 1998 and 1999 by carrying out parallel mesocosm experiments in six lakes across Europe. 2. Zooplankton community structure, biomass and responses to nutrient and fish manipulation showed geographical and year-to-year differences. Fish had a greater influence than nutrients in regulating zooplankton biomass and especially the relative abundances of different functional groups of zooplankton. When fish reduced the biomass of large crustaceans, there was a complementary increase in the biomasses of smaller crustacean species and rotifers. 3. High abundance of submerged macrophytes provided refuge for zooplankton against fish predation but this refuge effect differed notably in magnitude among sites. 4. Large crustacean grazers (Daphnia, Diaphanosoma, Sida and Simocephalus) were crucial in controlling algal biomass, while smaller crustacean grazers and rotifers were of minor importance. Large grazers were able to control phytoplankton biomass even under hypereutrophic conditions (up to 1600 g TP L1) when grazer biomass was high (>8090 g dry mass L1) or accounted for >30% of the grazer community. 5. The littoral zooplankton community was less resistant to change following nutrient enrichment in southern Spain, at high temperatures (close to 30 °C), than at lower temperatures (1723 °C) characterising the other sites. This lower resistance was because of a greater importance of nutrients than zooplankton in controlling algal biomass. 6. Apart from the reduced role of large crustacean grazers at the lowest latitude, no consistent geographical patterns were observed in the responses of zooplankton communities to nutrient and fish manipulation. [KEYWORDS: eutrophication ; fish predation ; littoral zooplankton ; mesocosm experiments ; nutrient enrichment