Linking patterns of freshwater discharge and sources of organic matter within the Río de la Plata estuary and adjacent marshes

We investigated carbon isotopic ratios (δ13C) v. carbon to nitrogen (C : N) ratios for surface sediments throughout a large estuarine system (Río de la Plata, RdlP), combined with sediment cores from adjacent marshes to infer main carbon sources. We also evaluated the influence of the El Niño–Southern Oscillation (ENSO) and associated high freshwater-discharge events on the organic-matter transport within the estuary. The isotopic pattern in surface sediments of the RdlP showed the upper reaches to be influenced by riverine particulate matter (δ13C range: –24 to –26‰). Similarly, in the sediment cores from marshes of the upper reaches, δ13C values decreased from –24‰ in ancient sediments to –28‰ in recent sediments, reflecting an increased contribution of organic matter from land, including C3 plants and freshwater phytoplankton, during the past 50 years. However, the lower reaches represent a depositional environment of marine algae (δ13C range: –21 to –23‰), with no influence of detritus from adjacent marshes, indicating minor erosion of the marshes in the lower reaches operating as carbon-sink habitats. Our isotopic analysis showed that the transport and deposition of terrigenous organic matter within the RdlP and adjacent marsh habitat appear to be both temporally and spatially linked to hydrology patterns.This work was partly funded by PEDECIBA–Geociencias and Agenica Nacional de Investigación en Innovación (SNI–ANII). This paper is part of the M.Sc. Thesis of A. Tudurí. This project (CP1211) was financially supported by a grant of the Cluster of Excellence 80 ‘The Future Ocean’ to Mark Schuerch. ‘The Future Ocean’ is funded within the framework of the Excellence Initiative by the ‘Deutsche Forschungsgemeinschaft’ (DFG) on behalf of the German federal and state governments. L. Bergamino thanks CSIC-Program ‘Contratación de Científicos Provenientes del Exterior’

Reliability of dynamic contrast-enhanced magnetic resonance imaging data in primary brain tumours: a comparison of Tofts and shutter speed models

Purpose To investigate the robustness of pharmacokinetic modelling of DCE-MRI brain tumour data and to ascertain reliable perfusion parameters through a model selection process and a stability test. Methods DCE-MRI data of 14 patients with primary brain tumours were analysed using the Tofts model (TM), the extended Tofts model (ETM), the shutter speed model (SSM) and the extended shutter speed model (ESSM). A no-effect model (NEM) was implemented to assess overfitting of data by the other models. For each lesion, the Akaike Information Criteria (AIC) was used to build a 3D model selection map. The variability of each pharmacokinetic parameter extracted from this map was assessed with a noise propagation procedure, resulting in voxel-wise distributions of the coefficient of variation (CV). Results The model selection map over all patients showed NEM had the best fit in 35.5% of voxels, followed by ETM (32%), TM (28.2%), SSM (4.3%) and ESSM (<0.1%). In analysing the reliability of Ktrans, when considering regions with a CV<20%, ≈25% of voxels were found to be stable across all patients. The remaining 75% of voxels were considered unreliable. Conclusions The majority of studies quantifying DCE-MRI data in brain tumours only consider a single model and whole-tumour statistics for the output parameters. Appropriate model selection, considering tissue biology and its effects on blood brain barrier permeability and exchange conditions, together with an analysis on the reliability and stability of the calculated parameters, is critical in processing robust brain tumour DCE-MRI data

Stable isotope evidence of food web connectivity by a top predatory fish (Argyrosomus japonicus: Sciaenidae: Teleostei) in the Kowie Estuary, South Africa

In this study, food web connectivity within the Kowie Estuary on the south-east coast of South Africa was evidenced by the trophic behaviour of the predominantly piscivorous Argyrosomus japonicus. We examined stable isotopes of carbon (δ 13C) and nitrogen (δ 15N) in the dominant consumers (zooplankton, invertebrates and fishes) and food sources (particulate organic matter, epibionts and benthic microalgae) in the system. An SIAR (Stable Isotope Analysis in R) mixing model was used to interpret the possible food sources for this dominant top predatory fish. Small fishes and large epibenthic invertebrates dominated the diet of A. japonicus. Based on the contrasting diet of these prey fish and invertebrates, we propose that organic matter enters the predatory fish community via two major pathways: (1) a littoral pathway dominated by benthic microalgae production and epibionts, and (2) a channel pathway dominated by suspended particulate organic matter (including phytoplankton). We conclude that the highly mobile A. japonicus consumes both pelagic and benthic fauna from the littoral and channel zones of the estuary, thereby playing a key functional role in linking food webs. This dietary diversity may help explain the success of A. japonicus as a dominant top predator in the system, primarily by increasing the energy available to this species.Keywords: diet diversity, food webs, stable isotopes, trophic connectivityAfrican Journal of Marine Science 2014, 36(2): 207–21

An EXAFS study of the binding of Cd and Pb ions to lipid films

Extended X-ray Absorption Fine Structure (EXAFS) measurements performed on Langmuir-Blodgett (LB) films containing cadmium and lead ions reveal the different coordination structures of the two cations in lipid membranes. We describe the local atomic environment of cadmium and lead in LB films prepared with stearic and 1,2 distearoyl-Lα-phosphatidic acids. The measurements have been performed on films of two different thicknesses, one and seven molecular layers, and at two different values of relative humidity. The local atomic environment of Cd ions in stearate films is consistent with unidentate coordination in which a Cd ion binds two stearate molecules, while that of Pb ions is consistent with a bidentate coordination in which a Pb ion binds one stearate molecule. Furthermore, in lead stearate films, there is Pb-Pb coordination as already observed in Langmuir films. In films of Pb-phosphatidic acid, oxygen atoms of the organic phosphate and oxygen atoms of bound water form two distinct shells

Examining the dynamics of phytoplankton biomass in Lake Tanganyika using Empirical Orthogonal Functions

The spatial and seasonal variations in phytoplankton concentrations can be pronounced in large tropical lakes and can have a direct impact on the regional communities in developing nations. Given the complex hydrodynamics and ecology of the African Great Lakes, there is a clear need for spatial databases to address their inter-annual and seasonal variability. In the past, phytoplankton studies in these extensive ecosystems focused mostly on point stations or lake transects repeated over several years. In the present work, satellite based reflectanceswere used to determine anomalies in chlorophyll related reflectance bands over a 7-year period for the whole of Lake Tanganyika. Empirical Orthogonal Function analysis was used to define regions with similar temporal co-variation of phytoplankton biomass. Using AutoRegressive Integrated Moving Average modelling techniques, it was possible to determine differences in phytoplankton dynamics and link these to climate and lake basin characteristics. An important shift in the phytoplankton seasonalitywas observed at the end of 2000, in concomitance to changes in wind and air temperatures which favoured entrainment of nutrients present in the deeper lake waters. This shift confirms the high sensitivity of the lake to climate change. The combination of extensive satellite based measurements with modelling approaches that consider both spatial and temporal dynamics is an important contribution to the understanding of long term changes in this important freshwater ecosystem