Feeding ecology of flatfish juveniles (pleuronectiformes) in Mar Chiquita coastal lagoon (Buenos Aires, Argentina)

During a two-year period in Mar Chiquita coastal lagoon (Buenos Aires, Argentina), the diet of juvenileParalichthys orbignyanus andOncopterus darwinii sorted according to size, area, and season was analyzed. Sampling was carried out using a beach seine net in three zones of different salinity. The small size of the individuals collected and the high percentage of full stomachs, corresponding with high food availability in the lagoon, indicated that Mar Chiquita is used as a nursery ground. The diet ofP. orbignyanus consisted of crustaceans and polychaetes, whereasO. darwinii fed predominantly on small crustaceans (gammarid amphipods, harpacticoid copepods, mysids, and ostracods); polychaetes were less important. Diet of both flatfish species varied with length. Smaller individuals were microphagous, while bigger specimens included larger prey in their diet. Prey taxa number increased for both species, seawards to the mouth of the lagoon, probably related to the salinity gradient. Prey diversity forP. orbignyanus was highest in autumn and spring, and the lowest percentage of stomachs with food occurred in winter. Prey diversity was lowest forO. darwinii in spring and the highest percentage of empty stomachs was found in autumn.     

Malaria reinfestation on the Northern Border of Argentina

Until 1950 malaria was a major public health hazard in Argentina affecting a large number of people in the northern territory. In that year a Malaria Eradication Programme began and this initiated a dramatic fall in incidence especially in the north east in subsequent years.In 1970 there was an increase in the incidence in the north west of the country. Than, in 1989 malaria was diagnosed along the northeastern border. The earlier outbreak was associated with the growth of border traffic and increasing immigration. Ecological changes consequent upon the building of large dams in the Paraná basin have been blamed for the outbreak in 1989.The reinfestation of malaria of northern Argentina is analysed, from the point of view of the trends in incidence during the period 1937–89 and especially the increase after 1970. Control measures are also proposed.     

Explicit meshfree solution for large deformation dynamic problems in saturated porous media

In this paper a new methodology to simulate saturated soils subjected to dynamic loadings under large deformation regime (locally up to 40% in equivalent plastic strain) is presented. The coupling between solid and fluid phases is solved through the complete formulation of the Biot’s equations. The additional novelty lies in the employment of an explicit time integration scheme of the \(u-w\) (solid displacement–relative fluid displacement) formulation which enables us to take advantage of such explicit schemes. Shape functions based on the principle of maximum entropy implemented in the framework of Optimal Transportation Meshfree schemes are utilized to solve both elastic and plastic problems.     

Spatial distribution and vertical migrations of fish larvae communities off Northwestern Iberia sampled with LHPR and Bongo nets

The spatial distribution and diel vertical migration of fish larvae were studied in relation to the environmental conditions off NW Iberia during May 2002. Larvae from 23 families were identified, the most abundant were the Clupeidae, Gobiidae, Callionymidae, Blenniidae, Sparidae and Labridae. Sardina pilchardus was the most abundant species, mean concentrations 1 order of magnitude higher than the other fish larvae species. Larval horizontal distribution was mainly related to upwelling-driven circulation, resulting in an offshore increase of larval abundance while the vertical distribution was closely associated to the Western Iberia Buoyant Plume. Despite this general trend, taxon-specific relationships between the distribution of larvae and environmental variables were observed, and temperature was an important regressor explaining the distribution of most taxa. A comparison between ichthyoplankton samples collected alternatively with the LHPR and Bongo nets resulted in captures of larvae ≈1 order of magnitude higher for the LHPR, probably related to its higher towing speed. The spatial distribution and relative composition of larvae were also different for both nets, although the most frequent/abundant groups were the same. A fixed station sampled for 69-h showed diel vertical migrations performed by the larvae, with the highest larval concentrations occurring at surface layers during the night and most larvae being found in the neuston layer only during that period.     

Impact of predation on the polychaete Hediste diversicolor in estuarine intertidal flats

In estuarine sediment flats benthic macroinvertebrates are intensively consumed by a variety of predators, such as aquatic birds and nekton (mostly fish and crustaceans). However, there is still a lack of conclusive studies that evaluate if this predation has a relevant impact on the populations of those invertebrates, which are a key element of the estuarine food chain. In the Tagus estuary we experimentally tested and quantified the impact of predation on the polychaete Hediste diversicolor, one of the most important prey for a variety of predators in many estuaries. Using an exclusion experiment, we compared the seasonal variation in the densities of H. diversicolor from February to November in sediment plots (1) available to both bird and nekton predators, (2) just to nekton, and (3) without predators. We also followed changes in the abundance of potential predators throughout the study. The lowest densities were systematically observed in the plots accessible to all predators, followed by those which excluded just birds, and finally by those that excluded all predators. The exclosures were in place for 9 months, at the end of which the average density of H. diversicolor in the plots protected from all predators was eight times greater than in those without any protection. These results demonstrate that predation had a major impact on the densities of H. diversicolor. The relative importance of bird and nekton predation varied along the study, and this seems to be determined by different peaks of abundance of the two types of predators. However, when present in high densities, birds and nekton seem to have a similar impact on H. diversicolor. Our results suggest that predation is a key factor on the population dynamics of H. diversicolor. In addition, the levels of predation that we observed suggest that this polychaete can be a limited resource, and this could have major ecological consequences for predators for which it is a key prey.     

Growth and photosynthetic responses to copper stress of an invasive cordgrass, Spartina densiflora

Spartina densiflora Brongn. is found in coastal marshes of south-west Spain, growing in sediments with between 300 and 3000mg Cu kg(-1) total soil DW (450-4500mg Cu kg(-1) supposing that the soil porosity is 0.5). An experiment was designed to investigate the effect of copper from 0 to 5000mgkg(-1) (64mmoll(-1)) on the photosynthetic apparatus and the growth of S. densiflora. We also determined total ash, copper, calcium, magnesium and phosphorous concentrations, as well as C/N ratio. S. densiflora survived to concentrations as high as 320mg Cukg(-1) DW in leaves, although excess of Cu diminished water use efficiency and Ca-, Mg- and P-uptake. Also, quantum efficiency of PSII, net photosynthetic rate, stomatal conductance and pigment concentrations declined with increasing external Cu. Finally, the decline in the photosynthetic function resulted in a biomass reduction of between 50 and 80% (for 600 and 5000mg Cu kg(-1), respectively).     

Galaxy cluster mergers

We present the results of an Eulerian adaptive mesh refinement (AMR) hydrodynamical and N -body simulation in a Λ cold dark matter (ΛCDM) cosmology. The simulation incorporates common cooling and heating processes for a primordial gas. A specific halo finder has been designed and applied in order to extract a sample of galaxy clusters directly obtained from the simulation without considering any resimulating scheme. We have studied the evolutionary history of the cluster haloes, and classified them into three categories depending on the merger events they have undergone: major mergers, minor mergers and relaxed clusters. The main properties of each one of these classes and the differences among them are discussed. The collisions among galaxy clusters are produced naturally by the non-linear evolution in the simulated cosmological volume; no controlled collisions have been considered. We pay special attention to discuss the role of merger events as a source of feedback and reheating, and their effects on the existence of cool cores in galaxy clusters, as well as in the scaling relations.