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Within two coastal shallow lagoons, trophic state was assessed by integrating water and sediment chemical indicators such as the TRIX and the benthic biopolymeric carbon (BPC) trophic indicator, altogether with biological environmental indicators (diatom species characterization). Spatial and temporal behavior of TRIX and BPC indices suggest that water column trophic indicators reflect rather short-term variations in water quality changes, while benthic trophic indicators rather reflect consistent long-term trends which make them useful as enduring indicators of eutrophication. Canonical Correspondence Analysis (CCA) showed that both sediment and transitional water trophic state indices increased eutrophic conditions with a decreasing salinity and increasing total nutrients. Diatom species associated with elevated eutrophic condition such as Staurosirella martyi, Staurosira breviestriata, Amphora copulata, Amphora veneta, Nitzschia sp., and Bacillaria paradoxa, showed a positive correlation with both trophic indices. We highlight the need for considering sediment eutrophication indicators towards in monitoring programmes within shallow coastal lagoons.

The aim of this study is to examine sedimentary organic matter sources, spatial distribution and temporal variability in a large estuarine system, the Rio de la Plata estuary (South America). For this purpose, this work integrates recent and historical carbon and nitrogen isotopes (δ 13 C, δ 15 N), total organic carbon, total nitrogen, C/N ratios and sediment grain size analyses along the Rio de la Plata estuary. Principal component analysis based on biogeochemical variables and salinity revealed two main biogeochemical contrasting zones, corresponding to the upper and the lower estuary. Such zones are derived from the density gradient observed at the maximum turbidity zone acting as a physical barrier by trapping fine sediments and controlling primary productivity. As a consequence, sedimentary total organic carbon and total nitrogen increase from upper reaches towards lower reaches to attain maximum values under the turbidity gradient due to the presence of fine sediments. On the other hand, C/N ratios display an opposite trend, with higher values in the upper reaches due to a higher influence of continental organic matter. Moreover, Stable Isotope Analysis in R (SIAR) based on δ 13 C and δ 15 N reveals that the main organic matter source to the sediment appears to be the continental particulate organic matter and the estimated percentages of terrestrial allochthonous contribution (C3 plants) indicated a decreased contribution towards the lower reaches. The paleoenvironmental analysis shows a change in the sedimentary organic matter composition since 1970 associated to an increased influence of terrestrial organic matter. This trend is related to an increase in the Parana River flow, which in turn is related to climatic variability (i.e., the polarity change of the Pacific Decadal Oscillation). Resumo O objetivo deste estudo e estimar fontes de materia orgânica sedimentar, distribuicao espacial e variabilidade temporal num grande sistema estuarino, o estuario do Rio de la Plata (America do Sul). Este trabalho integra dados atuais e historicos de isotopos de carbono e nitrogenio (δ 13 C, δ 15 N), carbono orgânico total, nitrogenio total, relacao C/N e granulometria do sedimento ao longo do estuario do Rio de la Plata. A analise de componentes principais baseada em variaveis biogeoquimicas e salinidade revelou duas zonas biogeoquimicas contrastantes, correspondentes ao estuario superior e inferior. Tais zonas sao definidas por diferencas no gradiente de densidade na zona de turbidez maxima, o qual atua como uma barreira fisica, causando o aprisionamento de sedimentos finos e influenciando a produtividade primaria. Como consequencia, o carbono orgânico total sedimentar e o nitrogenio total aumentam dos trechos superiores para os trechos inferiores atingindo valores maximos sob o gradiente de turbidez devido a presenca de sedimentos finos. Por outro lado, a razao C/N exibe uma tendencia oposta, atingindo valores mais elevados no estuario superior devido a uma maior influencia da materia orgânica continental. Os resultados isotopicos de δ 13 C e δ 15 N sugerem que a principal fonte de materia orgânica para o sedimento e a materia orgânica particulada continental. A contribuicao aloctone terrestre (plantas C3) diminui no estuario inferior. A analise paleoambiental mostra uma mudanca na composicao da materia orgânica sedimentar a partir de 1970, causada pelo aumento do fornecimento da materia orgânica terrestre. Essa tendencia esta relacionada a um aumento no fluxo do Rio Parana, na sequencia de alteracoes climaticas relacionadas com a mudanca de polaridade da Oscilacao Decadal do Pacifico.

Characterization of the sedimentary organic matter (SOM) within an intermittently open coastal lagoon in SE Uruguay was carried out using optical properties of dissolved organic matter (DOM), stable isotope analyses (δ13C and δ15N) and mineralogical properties of the sedimentary pool were assessed using Fourier transform infrared spectroscopy (FTIR). Additionally, chemical and biological variables including total and dissolved nitrogen and phosphorus, chlorophyll-a in water and sediments, and benthic diatoms were analyzed. Potential sources of SOM (e.g., marine and riverine phytoplankton, terrestrial plant material) and fauna were collected during summer, autumn, and winter from seaward (South) to landward (North) zones. In the pore water, the high contribution of fluorescent, protein-like components, the δ13C values of sediments (−22‰ to −16‰), and the low C/N ratios of sediments (∼7), indicate the dominant source of SOM is marine microalgae (benthic/planktonic, tychoplanktonic). Most of the diatom species recorded in the sediments were benthic marine/brackish species including the polyhaline species Pseudopodosira echinus. Furthermore, FTIR spectra showed higher contributions of quartz, carbonates, and feldspar towards the seaward zones. Clay minerals increase towards landward zones, indicating changes in the hydrodynamic energy within the lagoon with greater overall rates of sediment deposition in landward zones. In shallow systems, active resuspension and mixing conditions may help to explain spatial similarities in the composition of the sedimentary pool. The combined use of stable isotopes, FTIR, diatoms and nutrient measurements (rather than single approaches) represents an alternative comprehensive tool to infer information on the composition and sources contributing to the sedimentary pool.

La Laguna de Rocha forma parte de la cadena de lagunas costeras que se extienden por la costa atlántica uruguaya y se continúa por el sur de Brasil. Presenta una dinámica ecológica compleja debido al gradiente marino-salobre. La macrofauna bentónica es un componente importante de estos ambientes estuariales que se ve afectada por la compleja dinámica del mismo. Analizamos cómo afectan las lenguas de arena (acumulaciones de arena perpendiculares a la barra) a la fauna macrobentónica. Se buscó establecer la relación del macrobentos con variables ambientales, a una escala vertical (estratos dentro del sedimento), y horizontal (a un lado y al otro de las lenguas), además de evaluar la influencia de la distancia a la intrusión marina. En todos los puntos de muestreo los parámetros ambientales (materia orgánica, fósforo total, oxígeno disuelto, clorofila-ay feopigmentos) presentaron una tendencia vertical (disminución hacia el fondo) y el macrobentos se relacionó con estos cambios. Se observó la dominancia de Heleobia australis, Nephtys fluviatilisy Heteromastus similisen diferentes estaciones y estratos. Existe una partición espacial de los recursos reflejada por estos cambios de dominancia, mientras quela distribución vertical de la comunidad macrobentónica responde tanto a las condiciones bióticas como a las ambientales. Rocha Lagoon is Part of the chain of coastal lagoons extended along Uruguayan Atlantic coast, which continues through southern Brazil. It presents a complex ecological dynamic due to the marine-brackish gradient. The benthic macrofauna is an important component of these estuarine environments which is affected by its complex dynamics. In this work, we analysed how sand-tongues (sand accumulation perpendicular to the sandbar) affect the macrobenthic fauna. We stablished the relationship between macrobenthos and environmental variables on a vertical (different depths within the sediment) and horizontal (both sides of the sand-tongues) scales, as well as evaluating the influence of the distance to the marine intrusion. In all the sampling points, the environmental parameters (organic matter, total phosphorus, dissolved oxygen, chlorophyll-a and phaeopigments) showed a vertical trend (decreasing towards the bottom), and the macrobenthos responded to these changes. The dominance of Heleobia australis, Nephtys fluviatilis and Heteromastus similis was observed in different sampling points and sediment depths. There is a spatial partition of resources reflected by these changes in dominance, while the vertical distribution of the macrobenthic community responds to both biotic and environmental conditions.

El análisis de isótopos estables en sedimentos, vegetales y animales representa una herramienta de importancia para estudios ecológicos, reconstrucciones paleoclimáticas y paleoambientales. Con base en la diferenciación isotópica entre productores primarios, esta técnica ha tenido un gran impacto en la identificación de flujos de energía entre ecosistemas terrestres y acuáticos adyacentes y en la estructura trófica. Se sintetizan aquí la nomenclatura y los principios básicos para la aplicación de isótopos estables en estudios de ambientes acuáticos. Además, se muestra su utilidad describiendo tres ejemplos recientes en ambientes costeros de Uruguay con diferentes objetivos: 1) evaluar el origen de la materia orgánica en sedimentos del Río de la Plata, 2) determinar la importancia trófica de una especie de diatomea en playas arenosas de Uruguay, y 3) evaluar la influencia de la materia orgánica antropogénica en la Bahía de Montevideo. La composición isotópica de las fuentes de materia orgánica permitió inferir los mecanismos involucrados en la transferencia de materia orgánica en los ecosistemas costeros. En esta revisión se subrayan las ventajas de este marcador isotópico de carbono que permite discriminar fuentes de materia orgánica. Asimismo, la combinación con otros análisis complementarios como la espectrofluorometría o los biopolímeros resulta importante en investigaciones de funcionamiento ecosistémico.

We investigated carbon isotopic ratios (δ13C) v. carbon to nitrogen (C : N) ratios for surface sediments throughout a large estuarine system (Rio de la Plata, RdlP), combined with sediment cores from adjacent marshes to infer main carbon sources. We also evaluated the influence of the El Nino–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., Facultad de Ciencias Naturales y Museo