Alexandra Baeta

Δ15N and δ13C in the Mondego estuary food web : Seasonal variation in producers and consumers

Assessments of temporal variation in stable carbon and nitrogen ratios were used to examine seasonal trends of the water column and benthic food webs in the Mondego estuary (Portugal). There was a marked seasonality in weather and water column conditions, including nutrient supply and chlorophyll concentrations. In spite of the pronounced environmental changes, we found little evidence of seasonal variation in delta(13)C and delta(15)N of producers and consumers in the Mondego estuary, with a few notable exceptions. Nitrogen isotope ratios in macrophytes (Zostera noltii, Ulva sp., Enteromorpha sp., and Gracilaria sp.), and in two grazers (Idotea chelipes and Lekanesphaera levii) increased during late summer, with the highest delta(15)N values being measured in July, during a period of elevated temperatures and drought, which may have favored high rates of denitrification and heavier delta(15)N values. The results suggest that stable-isotope values from macrophytes and selected grazers are useful as tracers of seasonal changes in nitrogen inputs into estuaries, and that those of consumers reflect other factors beyond seasonal variations in N and C sources.

Feeding ecology of the green crab, Carcinus maenas (L., 1758) in a temperate estuary, Portugal

[The feeding ecology of Carcinus maenas was studied between June 2003 and June 2004 in three areas in the Mondego estuary, Portugal. Samples were collected monthly, during the night, at high water of spring tides using a 2 m beam trawl. Among 837 stomachs examined, 25 different food items were identified. Crangon crangon, Hediste diversicolor, and Teleostei were the most important food items by occurrence, numbers, and weight. As an opportunistic feeder, differences in diet found between areas and seasons reflected prey availability. No differences were found in prey selection according to sex and carapace width of individuals. The feeding intensity of moulting and ovigerous crabs was lower. The incidence of cannibalism was higher than reported in the literature. C. maenas seems to be a main top predator in the food web of the Mondego estuarine ecosystem. A ecologia alimentar de Carcinus maenas foi estudada entre Junho de 2003 e Junho de 2004 em tres locais no estuario do Mondego, Portugal. As amostras foram colhidas mensalmente, durante a noite, na vazante de mares vivas usando um arrasto de vara com 2 m. Entre os 837 estomagos examinados, 25 itens alimentares diferentes foram identificados. Crangon crangon, Hediste diversicolor e Teleostei foram os itens alimentares mais importantes por ocorrencia, numero e peso. Sendo um consumidor oportunista, as diferencas na dieta encontradas entre locais e estacoes do ano refectem a disponibilidade das presas no meio. Nao foram encontradas diferencas na seleccao de presas de acordo com o sexo e a largura da carapaca dos individuos. A intensidade alimentar dos caranguejos em muda e femeas ovigeras foi baixa. A incidencia de canibalismo foi mais elevada do que reportado na literatura. C. maenas parece ser um predador topo na teia alimentar do ecossistema estuarino do Mondego., The feeding ecology of Carcinus maenas was studied between June 2003 and June 2004 in three areas in the Mondego estuary, Portugal. Samples were collected monthly, during the night, at high water of spring tides using a 2 m beam trawl. Among 837 stomachs examined, 25 different food items were identified. Crangon crangon, Hediste diversicolor, and Teleostei were the most important food items by occurrence, numbers, and weight. As an opportunistic feeder, differences in diet found between areas and seasons reflected prey availability. No differences were found in prey selection according to sex and carapace width of individuals. The feeding intensity of moulting and ovigerous crabs was lower. The incidence of cannibalism was higher than reported in the literature. C. maenas seems to be a main top predator in the food web of the Mondego estuarine ecosystem. A ecologia alimentar de Carcinus maenas foi estudada entre Junho de 2003 e Junho de 2004 em tres locais no estuario do Mondego, Portugal. As amostras foram colhidas mensalmente, durante a noite, na vazante de mares vivas usando um arrasto de vara com 2 m. Entre os 837 estomagos examinados, 25 itens alimentares diferentes foram identificados. Crangon crangon, Hediste diversicolor e Teleostei foram os itens alimentares mais importantes por ocorrencia, numero e peso. Sendo um consumidor oportunista, as diferencas na dieta encontradas entre locais e estacoes do ano refectem a disponibilidade das presas no meio. Nao foram encontradas diferencas na seleccao de presas de acordo com o sexo e a largura da carapaca dos individuos. A intensidade alimentar dos caranguejos em muda e femeas ovigeras foi baixa. A incidencia de canibalismo foi mais elevada do que reportado na literatura. C. maenas parece ser um predador topo na teia alimentar do ecossistema estuarino do Mondego.]

Distribution, habitat and trophic ecology of Antarctic squid Kondakovia longimana and Moroteuthis knipovitchi: inferences from predators and stable isotopes

Cephalopods have a key role in the marine environment though knowledge of their distribution and trophic ecology is limited by a lack of observations. This is particularly true for Antarctic species. Toothfish species are key predators of cephalopods and may be viewed as ideal biological samplers of these species. A total of 256 cephalopod lower beaks were identified from the stomachs of Patagonian toothfish (Dissostichus eleginoides) and Antarctic toothfish (Dissostichus mawsoni), captured in fisheries of South Georgia and the South Sandwich Islands in the South Atlantic between March and April 2009. Long-armed octopus squid (Kondakovia longimana) and smooth-hooked squid (Moroteuthis knipovitchi) were the main cephalopod prey and both were predated upon wherever toothfish were captured, though this cephalopod species appear to inhabit deeper waters at the South Sandwich Islands than at South Georgia. Measurements of δ13C from beak material indicated a clear segregation of habitat use comparing adult and sub-adult sized K. longimana. Variation in δ15N with size indicated an ontogenetic shift in the diet of cephalopods and also suggested some trophic plasticity among years. This study provides new insights into the private life of some elusive Antarctic cephalopods in an underexplored region of the South Atlantic.

Dependence of juvenile reef fishes on semi‐arid hypersaline estuary microhabitats as nurseries

The differences between fish assemblages in three microhabitat types, in relation to vegetation and sediment characteristics of a hypersaline estuary located in an semi-arid zone in north-eastern Brazil, were investigated. Fishes were collected using a beach seine during the rainy and dry seasons in 2012. A total of 78 species were recorded, with the most common families being Gerreidae, Lutjanidae and Tetraodontidae. The majority of species were represented by juveniles, with Eucinostomus argenteus, Ulaema lefroyi and Sphoeroides greeleyi being the dominant species. The fish assemblage structures differed significantly among microhabitat types, with the narrow intertidal flat adjacent to the mangrove fringe supporting the most diverse fish fauna. In addition, only 27 species were common to all of the microhabitats. The results support the hypothesis that hypersaline estuaries serve as important nursery areas for various reef fish species, due to the structural complexity provided by their macroalgae beds and mangroves.

Interactions between seagrasses and seaweeds during surge nitrogen acquisition determine interspecific competition

Seagrasses dominate shallow coastal environments where nitrogen (N) availability in the water column is often sporadic and mainly in the form of pulses. We investigated the N uptake competition between seagrasses and seaweeds through a series of 15N surge uptake experiments combining single-species and mixed incubations across ammonium concentrations. N surge uptake rates of seagrasses were 2 to 14-fold higher than those of seaweeds in the majority of combinations, showing that seagrasses are generally in a competitive advantage over seaweeds in N-poor environments with N-pulses. No threshold concentration of ammonium was found beyond which seaweeds performed better than seagrasses. Mixed incubations revealed interspecific interactions that affected rates positively and negatively. Uptake rates obtained in single-species incubations, therefore, cannot always be used to predict the outcome of uptake competition. Only two (Zostera marina vs. Ulva rotundata and Zostera marina vs. Codium decorticatum) of the nine combinations tested (Z. marina, Z. noltei and Cymodocea nodosa vs. U. rotundata, C. decorticatum and Dictyota dichotoma) were found to enhance macroalgal uptake. Our results showed that the surge uptake capacity of seagrasses represents an important mechanism in their N acquisition strategy that justifies their dominance in shallow oligotrophic environments.

Atmospheric CO 2 enrichment effect on the Cu-tolerance of the C 4 cordgrass Spartina densiflora

A glasshouse experiment was designed to investigate the effect of the co-occurrence of 400 and 700ppm CO2 at 0, 15 and 45mM Cu on the Cu-tolerance of C4 cordgrass species Spartina densiflora, by measuring growth, gas exchange, efficiency of PSII, pigments profiles, antioxidative enzyme activities and nutritional balance. Our results revealed that the rising atmospheric CO2 mitigated growth reduction imposed by Cu in plants grown at 45mM Cu, leading to leaf Cu concentration bellow than 270mgKg-1 Cu, caused by an evident dilution effect. On the other hand, non-CO2 enrichment plants showed leaf Cu concentration values up to 737.5mgKg-1 Cu. Furthermore, improved growth was associated with higher net photosynthetic rate (AN). The beneficial effect of rising CO2 on photosynthetic apparatus seems to be associated with a reduction of stomatal limitation imposed by Cu excess, which allowed these plants to maintain greater iWUE values. Also, plants grown at 45mM Cu and 700ppm CO2, showed higher ETR values and lower energy dissipation, which could be linked with an induction of Rubisco carboxylation and supported by the recorded amelioration of N imbalance. Furthermore, higher ETR values under CO2 enrichment could lead to an additional consumption of reducing equivalents. Idea that was reflected in the lower values of ETRmax/AN ratio, malondialdehyde (MDA) and ascorbate peroxidase (APx), guaiacol peroxidase (GPx) and superoxide dismutase (SOD) activities under Cu excess, which could indicate a lower production of ROS species under elevated CO2 concentration, due to a better use of absorbed energy.

Are vegetated areas more attractive for juvenile fish in estuaries? A comparison in a tropical estuary

Seagrass beds are some of the essential nursery areas for fish, due to the great structural complexity found in these environments. Despite their importance, they are among the most threatened coastal ecosystems and their influence on fish assemblages is still poorly studied. Therefore, this study aimed to compare the composition and structure of fish assemblages found in vegetated and unvegetated areas in a tropical estuary. Samples were taken in two areas with seagrass (Halodule wrightii) and two where this vegetation is absent (tidal flats), observing variations in the hydrological regime between the dry and rainy seasons of 2014. A total of 86 species were recorded, of which 11 occurred only in vegetated areas and 31 only in the unvegetated regions. No difference was found in diversity between vegetated and unvegetated areas, but the values of abundance and biomass were higher in unvegetated areas. Thus, the environmental characteristic of unvegetated areas proved to be a major factor in determining the biomass and richness patterns. Analysis of the abiotic data set indicated that temperature and dissolved oxygen are the variables that act as environmental filters in the temporal differentiation of the structure of fish assemblages, while salinity and total dissolved solids are related to variation between seagrass beds and tidal flats. In addition, our results show that these areas are used by the species in different stages of their ontogenetic development, which may be linked to differences related to the availability of food resources and the possibility of refuge from predation.