Tracing the flow of organic matter from primary producers to filter feeders in Ria Formosa lagoon, Southern Portugal

The flow of organic matter along the main navigation channel of Ria Formosa, Portugal, was assessed using determinations of suspended particulate matter (SPM), particulate organic matter (POM), and chlorophyll a (chla) concentrations in conjunction with stable isotope values of primary producers, particulate matter, and two filter feeders. SPM in the lagoon is dominated by inorganic particles comprising 80% of total weight with organic matter averaging about 20%. The algal component of the POM averaged about 5% with the remainder comprised of detritus. The δ¹³C values of primary producers ranged from ?9.1‰ in the intertidal seagrassZostera noltii to ?30.7‰ in the red seaweedBostrychia scorpioides revealing underlying differences in the mechanisms of carbon uptake. The δ¹³C value ofB. scorpioides, which develops entangled on the salt marsh speciesSpartina maritima, suggests that its main source of inorganic carbon is atmospheric CO₂. The δ¹³C values of the high marsh macrophyteSarcocornia perennis significantly increased with distance from the ocean while δ¹³C values ofZ. noltii decreased, probably because higher decomposition of organic matter at inner stations lowers the δ¹³CO₂ value in the water. The δ¹⁵N values of Ulvales, seagrasses, and marsh plants significantly increased from outer stations to inner stations. This increase may be due either to recycling of nitrogen (N) within the marsh (with loss of light N₂ or NH₄) or to inputs of isotopically heavy N from sewage. The δ¹⁵N values of particulate matter showed an opposite trend, which indicates higher microbial degradation of organic matter at the inner lagoon. The data demonstrate that the seston in the lagoon is a mixture of detritus from lagoon primary producers with a minor contribution of microalgae. The filter feeders are most likely assimilating a mixture of phytoplankton and microphytobenthos. Digestion of lagoon seston is selective. The δ¹⁵N values of both muscle and digestive gland of filter feeders showed the opposite gradient of particulate matter indicating that the depleted δ¹⁵N of SPM at inner stations was not assimilated or even ingested. Stable isotopes values did not differ between the filter-feeders—the musselMytilus galloprovincialis collected on buoys and the clamTapes decussatus collected in the sediment—suggesting a considerable mixture of benthic-pelagic organic matter throughout the water column. Assessment of the changes in isotopic decomposition of detritus as it decays is required to refine our understanding of organic matter transfers in detrital food webs.     

Node-centered finite volume discretizations for the numerical simulation of multiphase flow in heterogeneous porous media

Three node-centered finite volume discretizations for multiphase porous media flow are presented and compared. By combination of these methods two additional discretization methods are generated. The ability of these schemes to describe flows at textural interfaces of different geologic formations is investigated. It was found that models with nonzero-entry pressures for the capillary pressure-saturation relationship in conjunction with the Box discretization may give rise to spurious oscillations for flows around low permeable lenses. Furthermore, the applicability and sensitivity of the discretization methods with regard to the used computational grids is discussed. The schemes are used for the numerical study of two-phase flow in porous media with zones of different material properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Freshwater reserves of mid-atlantic coast barrier Islands

A multiple regression model was constructed for the purpose of predicting barrier island hydrology from easily measureable island characteristics. The model was developed using data obtained from 17 sites on the Outer Banks of North Carolina. The accuracy of the model for predicting key hydrologic variables was evaluated by statistical and graphic procedures. In general, agreement between observed and predicted values of the hydrologic variables was very good, suggesting that the quantity of potable water at various island sites can be estimated without resorting to extensive field investigations. The model was then applied to Assateague Island, a barrier island located off the coasts of Maryland and Virginia. Results indicate that the original model developed for the Outer Banks may be applied to other barrier islands but that errors involved may necessitate corrections in detailed studies. Correction for bias in predictions for Assateague was shown to be possible with limited field data from surface resistivity surveys.     

Direct counts of bacteria in the sediments of a North Carolina salt marsh

The number of bacteria in sediments of a North Carolina salt marsh was determined by direct counts with epifluorescent illumination and acridine orange stain. Cell number decreased from 8.36–10.90×10⁹ cells/cm³ of sediment at the surface to 2.19–2.58×10⁹ cells/cm³ of sediment at a depth of 20 cm. No significant difference was found among four stations located on a transect which crossed the marsh and spanned subtidal to intertidal sediments.     

Stochastic groundwater modeling of a sedimentary aquifer: evaluation of the impacts of abstraction scenarios under conditions of reduced recharge

A transient finite difference groundwater flow model has been calibrated for the Nasia sub-catchment of the White Volta Basin. This model has been validated through a stochastic parameter randomization process and used to evaluate the impacts of groundwater abstraction scenarios on resource sustainability in the basin. A total of 1500 equally likely model realizations of the same terrain based on 1500 equally likely combinations of the data of the key aquifer input parameters were calibrated and used for the scenario analysis. This was done to evaluate model non-uniqueness arising from uncertainties in the key aquifer parameters especially hydraulic conductivity and recharge by comparing the realizations and statistically determining the degree to which they differ from each other. Parameter standard deviations, computed from the calibrated data of the key parameters of hydraulic conductivity and recharge, were used as a yardstick for evaluating model non-uniqueness. All model realizations suggest horizontal hydraulic conductivity estimates in the range of 0.03–78.4 m/day, although over 70 % of the area has values in the range of 0.03–14 m/day. Low standard deviations of the horizontal hydraulic conductivity estimates from the 1500 solutions suggest that this range adequately reflects the properties of the material in the terrain. Lateral groundwater inflows and outflows appear to constitute significant components of the groundwater budgets in the terrain, although estimated direct vertical recharge from precipitation amounts to about 7 % of annual precipitation. High potential for groundwater development has been suggested in the simulations, corroborating earlier estimates of groundwater recharge. Simulation of groundwater abstraction scenarios suggests that the domain can sustain abstraction rates of up to 200 % of the current estimated abstraction rates of 12,960 m³/day under the current recharge rates. Decreasing groundwater recharge by 10 % over a 20-year period will not significantly alter the results of this abstraction scenario. However, increasing abstraction rates by 300 % over the period with decreasing recharge by 10 % will lead to drastic drawdowns in the hydraulic head over the entire terrain by up to 6 m and could cause reversals of flow in most parts of the terrain.     

Genetic structure of bay anchovy (Anchoa mitchilli) populations in Chesapeake Bay

To determine the genetic structure of the bay anchovy (Anchoa mitchilli) within Chesapeake Bay, 16 isozyme systems encoding 21 loci for 20 population were examined using horizontal starch gel electrophoresis. Contingency Chisquare analysis revealed significant allelic frequency differences at nine loci (AAT-1, AAT-2, ALD-1, CPK-2, GAP-1, GLY-1, LDH-1, MDH-1, and MDH-2). Two loci, ALD-1 and MDH-1, were responsible for nine of 14 tests not conforming to Hardy-Weinberg expectations, with some of these deviations attributed to possible scoring and/or sampling error. Estimates for mean average heterozygosity were relatively high, ranging from 0.40 to 0.096, with 33–57% of the loci polymorphic. A low F_st value (0.041) along with high genetic identity estimates (I=0.997) indicated little substructuring of bay anchovy populations within Chesapeake Bay.