A study on sediment surface diatom composition in relation to both protein/carbohydrate content and salinity was carried out in Montevideo Bay and the adjacent coastal zone. Samples were seasonally taken during one year along a human impact gradient, and the trophic conditions were assessed from the biochemical composition of the surface sediment organic matter. The co-occurrence of the marine-brackish (Actinocyclus curvatulus, Actinocyclus gallicus, Coscinodiscus excentricus, Coscinodiscus radiatus, Hyalodiscus subtilis, Paralia sulcata) and freshwater diatom flora (Aulacoseira granulata, Aulacoseira italica, Aulacoseira muzzanensis, Actinocyclus normanii, Amphora copulata, Cyclotella meneghiniana, Nitzschia linearis), suggests myxohaline estuarine conditions in the study area. The sampling stations located within the bay, close to several contamination sources, exhibited hypertrophic conditions as indicated by the highest levels of protein content, but also by the high relative abundances of Actinocyclus normanii and Cyclotella meneghiniana, as both diatom species are very well established cosmopolitan proxies of aquatic degradation. The levels of both protein and carbohydrate content, but also photosynthetic pigments, recorded in the adjacent coastal zone suggest meso-eutrophic conditions as contamination levels from the inner bay might become diluted off shore. Multivariate diatom species-environmental data relationships, indicate that the trophic gradient is hierarchically more important than salinity in explaining the diatom distribution. The present data imply an improvement in our ability of assessing organic contamination in the study area, since both biological and biopolymer variables were introduced for the first time as a tool for assessing the benthic health/degradation in one of the most populated regions of the Río de la Plata.

The Uruguayan continental margin (UCM), located in the Southwestern Atlantic margin’s subtropical region, is positioned in a critical transitional region regarding the global ocean circulation (Río de la Plata (RdlP) outflow and Brazil-Malvinas Confluence), as also reflected in seafloor features (northernmost distribution of a large depositional contourite system and RdlP paleovalley). This complex oceanographic scenario occurring in a relatively small area highlights the advantage of considering the UCM as a natural laboratory for oceanographic research. The present work provides the first conceptual “control” model of the physical drivers (i.e., climate, geomorphology) and main oceanographic processes (i.e., hydrodynamics, sediment, and carbon dynamics) occurring along the UCM, reviewing and synthesizing available relevant information based on a functional integrated approach. Despite the conspicuous knowledge gaps on critical processes, a general picture of the system’s functioning is emerging for this complex biophysical setting. This includes conceptualizations of the actual controls, main processes, feedbacks, and interactions responsible for system dynamics. The structure adopted for developing our conceptual models allows permanent improvement by empirical testing of the working hypothesis and incorporating new information as scientific knowledge advances. These models can be used as a baseline for developing quantitative models and, as representations of relatively “pristine” conditions, for stressors models by identifying sources of stress and ecological responses of key system attributes under a transboundary approach.

The Uruguayan Continental Margin is located in a strategic location of great relevance to understand the dynamics of the regional and global climate and to study the sedimentary processes. The Rio de la Plata represents the largest fluvial discharge of the Southwest Atlantic (23.000 m(3)/s of water and 57.000.000 m(3)/year of sediment for the South Atlantic Ocean). The sub-surface and deep circulation are characterized by the confluence of water masses with contrasting thermohaline characteristics that determine the formation of well-marked hydrological fronts. In the continental shelf, this characteristic is represented by the Subtropical Shelf Front, as a shallow extension of the Brazil-Malvinas Confluence that occurs in deep water off the shelf. Although the knowledge of the regional circulation is acceptable, there are significant gaps in the knowledge related to the influence of this complex hydrological system on sedimentation processes. The objective of this work is to synthesize the present knowledge about the morpho-sedimentary characteristics of the Uruguayan Continental Margin and the processes involved in its formation. It also aims to discuss some knowledge gaps that are the subject of current research in the region. The knowledge reflects the scientific-technological evolution at a global level that is translated in the increase of recent works on the continental slope. Nowadays, most of the available information is associated with the inner shelf and upper slope. Starting from the basis of the close relationship between bathymetry and sedimentological distribution it is clear that efforts should be concentrated on the better understanding of the interaction between the shelf and the slope. The scientific knowledge regarding sedimentology and geochemistry along the Uruguayan Continental Margin is still, scarce and heterogeneous. However, it indicates complexity in the sedimentary coverture present both in the continental shelf and slope. The Uruguayan Continental Margin can be considered as an in situ laboratory to improve the understanding of the influence of oceanic hydrodynamics on the margin configuration, a key point to achieve better paleoceanographic interpretations. In this sense, due to the complexity of both modern hydrology and morphological configuration of the margin, it would be interesting to understand the relative importance of the control of each environment on surface sedimentation. A better understanding of the processes that link the near bottom circulation and continental slope features is needed. At the same time, this will allow deepening a new field of research in Uruguay, considering the joint and interrelated vision between sedimentary and oceanographic processes.

The Uruguayan continental shelf is characterised by a unique hydrographic system, composed of the Río de la Plata buoyant plume (RdlP-BP), and by water masses of contrasting thermohaline characteristics. Below the RdlP-BP the southward-flowing Subtropical Shelf Water and the northward-flowing Subantarctic Shelf Water converge at the Subtropical Shelf Front, which is the shelf extension of the Brazil-Malvinas Confluence. Three main sedimentary environments can be described associated with; I) “Atlantic coastal sands” (i.e. onshore from the palaeovalley); II) the SW-NE running “RdlP palaeovalley” and; III) “Relict sands” (i.e., offshore of the RdlP palaeovalley). Three exposed sedimentary units (U1 to U3) identified from acoustic profiles and sediment cores (sedimentary characteristics of stratigraphic units) almost entirely restricted to the palaeovalley. The transect here studied intersects these three environments contributing thus with the description of the morphological setting and sedimentary coverture. Physical and geochemical data were integrated and used to characterise the sedimentary facies previously described in the inner shelf and to understand environmental control on the development of these facies.Sediments from U1 show the terrestrial imprint of the RdlP and drier regional conditions, while sediments that characterised U3 indicate a sandy facies (quartz and bioclasts: whole and fragmented shells and polychaetes tubes) corresponding to an ancient coast. This last (with approximately 11 m height), is probably related to sea-level stabilization, between 20 and 25 m occurring during the Upper Pleistocene and Holocene for the South-Western Atlantic. The sediment from U3 reflects the colder and drier conditions prevailing in the region during the formation of this facies (13.7 and 9.7 cal ka BP). Eroded sediments (e.g., from U3; ancient coast) are deposited inside the palaeovalley and on the onshore region (between the palaeovalley and coastal sands). Also, U1 extends from the palaeovalley covering the onshore region. The outcrop of warmer oceanic shelf waters was probably a consequence of the geomorphology of the palaeovalley (edge of the palaeovalley) and related to the still fall presence of waters typical of the austral warm season when higher advection of Brazil Current occurs over the shelf. In this regard, the mound-like feature should induce bottom water to rise, operating as a ramp. Productivity proxies (Si/Ti, Ba/Ti, Ca/Ti and P/Ti) present the highest values in these stations (S16-S18), reflecting the imprint of the upwelling in the sediment.The information reported in this work is particularly important to better understand sedimentological dynamics in the Uruguayan inner shelf and the Southwestern Atlantic region. It is also important for elaborating more precise paleoenvironmental and palaeoceanographic reconstructions. ResumoA plataforma continental uruguaia é caracterizada por um sistema hidrográfico único, composto pela pluma túrbida do Rio de la Plata (RdlP-BP) e por massas de água com características termohalinas contrastantes. Abaixo da RdlP-BP, a água subtropical da plataforma que flui para sul e a água subantártica da plataforma que flui para norte convergem para a frente subtropical da plataforma, a qual é a extensão da Confluência Brasil-Malvinas.Três principais ambientes sedimentares podem ser descritos associados: I) a "areias costeiras atlânticas" (com ocorrência em direção a terra a partir do “Paleovale do RdlP”); II) o “Paleovale do RdlP” com direção SW-NE e; III) a "Areias relíquia" (para offshore do “Paleovale do RdlP”). Três unidades sedimentares estratigráficas expostas (U1 a U3) identificadas a partir de perfis acústicos e testemunhos de sedimentos quase que totalmente restritas ao paleovale. Dados físicos e geoquímicos foram integrados e utilizados para caracterizar as fácies sedimentares encontradas na plataforma continental interna e para entender o controle ambiental sobre o desenvolvimento dessas fácies.Os sedimentos da unidade U1 representam o registo terrestre da RdlP e condições regionais mais secas, enquanto os sedimentos que caracterizam a unidade U3 indicam fácies arenosas (quartzo e bioclastos: conchas inteiras e fragmentadas e tubos de poliquetas) correspondente a uma costa antiga. Este último (com aproximadamente 11 m de altura) está provavelmente relacionado à estabilização do nível do mar, entre 20 e 25 m durante o Pleistoceno Superior e o Holoceno, no Atlântico Sudoeste. O sedimento da unidade U3 reflete as condições mais frias e secas da região durante a formação desta fácies (entre 13,7 e 9,7 cal ka BP). Sedimentos erodidos (por exemplo, de U3; costa antiga) são depositados dentro do paleovale e na região onshore (entre o paleovale e as areias costeiras). Além disso, a unidade U1 estende-se do paleovale até ao continente.O afloramento de águas oceânicas mais quentes, foi provavelmente uma consequência da geomorfologia do paleovale e está relacionado à presença de águas típicas da estação quente austral, quando ocorre maior advecção da corrente do Brasil na plataforma. A estrutura semelhante a um monte deve induzir a subida da água no fundo, operando como uma rampa. Os proxies de produtividade (Si/Ti, Ba/Ti, Ca/Ti e P/Ti) apresentam os maiores valores nessas estações (S16-S18), refletindo a impressão da ressurgência no sedimento.As informações obtidas através deste trabalho são particularmente importantes para melhor compreender a dinâmica sedimentar na plataforma interna uruguaia e na região do...