Dissolved organic carbon (DOC) flux dynamics were examined in the context of other biogeochemical cycles in intertidal sediments inhabited by benthic microalgae. In August 2003, gross oxygenic photosynthetic (GOP) rates, oxygen penetration depths, and benthic flux rates were quantified at seven sites along the Duplin River, GA, USA. Sediments contained abundant benthic microalgal (BMA) biomass with a maximum chlorophyll a concentration of 201 mg chl a m −2 . Oxygen microelectrodes were used to determine GOP rates and O 2 penetration depth, which were tightly correlated with light intensity. Baseline and 15 N-nitrate amended benthic flux core incubations were employed to quantify benthic fluxes and to investigate the impact of BMA on sediment water exchange under nitrogen (N)-limited and N-replete conditions. Unamended sediments exhibited tight coupling between GOP and respiration and served as a sink for water column dissolved inorganic nitrogen (DIN) and a source of silicate and dissolved inorganic carbon (DIC). The BMA response to the N addition indicated sequential nutrient limitation, with N limitation followed by silicate limitation. In diel (light-dark) incubations, biological assimilation accounted for 83% to 150% of the nitrate uptake, while denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) accounted for <7%; in contrast, under dark conditions, DNF and DNRA accounted for >40% of the NO 3 − uptake. The N addition shifted the metabolic status of the sediments from a balance of autotrophy and heterotrophy to net autotrophy under diel conditions, and the sediments served as a sink for water column DIN, silicate, and DIC but became a source of DOC, suggesting that the increased BMA production was decoupled from sediment bacterial consumption of DOC.

A comprehensive study of the marine currents of the Gulf of Uraba ´is presented for temporal scales ranging from intertidal to seasonal. The analyses made were based on three-dimensional (3D) numerical modeling and extensive current observations. The hydrodynamic model based on the Delft3D platform included the influence of different forcings such as tide, waves, atmosphere, river discharges, and density gradients. Because field data concerning these variables are scarce for the study area, measuring campaigns, in combination with global models and databases, were used to overcome this condition. The evaluation of the model was attained by the use of unprecedented field data obtained in different climatic seasons through a mobile gauge station. This station registered instantaneous vertical profiles of flow along 1200 km with approximately 50-m spacing. This type of measurement was preferred instead of those obtained from stationary stations, because the spatial gradients of the currents were much greater than their temporal variations. The good agreement of the 3D mathematical model in the reproduction of the observed instantaneous currents supported its use in defining the marine currents in the gulf. A combined analysis of the model results and the measured currents revealed a complex circulation pattern comprising simultaneously typical estuarine circulation, one-to three-layer flows, and even inverse circulation. This flow complexity would hardly have been determined through common measurement methodologies based on stationary gauge stations.

A total of 263 molluscan species (201 of them found alive), collected from 1982 to 1989, is reported from seven coral reef localities of the Santa Marta area, Colombia. Species were arranged according to the reef zones in which they were found, their feeding habits and mode of life. Cluster analysis of faunal similarity between reef zones revealed the ocurrence of three molluscan assemblages (settled respectively in shallow environments, reef-slope zones, and sand-coral rubble zones), plus one containing few species and limited toa small zone dominated by lettuce coral (Agaricia tenuifoiia). There are some differences between reef zones in the distribution of species with respect to feeding habits. Vagrant, carnivore gastropods constitute the most diverse group, but filter-feeding bivalves and herbivore gastropods reach high numbers in some zones. Some species were not recorded previously from the Caribbean coast of Colombia.