Abstract This study analyses the feeding ecology of Brevortia aurea (Brazilian menhaden) larvae during a reproductive season in the Río de la Plata estuary (RPE), and explores its changes along larval development. Data were obtained from stomach content analyses of 205 individuals collected with a Bongo net during spring 2001. Larvae were discriminated into preflexion (PF), flexion (F) and post-flexion (PsF) stages. Feeding was described using complementary metrics like Feeding incidence (FI), the Relative Importance of different prey types consumed (IRI), the Degree of prey digestion (E) and Diet overlap (D), which were estimated and compared between stages. Also, larval morphometric relationships (body length, BL vs mouth width, MW) and between MW and maximum prey width (Wmax) were established for the range of observed larval sizes. The overall FI was 46.3% and increased with the development stage: 40.4% PF, 63.3% F, 78.6% PsF. Feeding occurred mainly during sunlit hours but no clear daily cycle could be established. Copepods and in particular Acartia tonsa were well represented among ingested preys, followed by invertebrate eggs and nauplii. Prey diversity and overlap between stages tended to decrease as development progressed. B. aurea exhibited isometric growth of MW in relation to BL. Average Wmax was 222.5 µm ± 100.9 SD, and increased non-linearly with both BL and MW. Gape size alone did not seem to be the limiting factor for prey choice (size), and we hypothesize that factors involved in the feeding mechanism other than mouth gape and linked with capture performance substantially influence the feeding ecology of this species in the RPE.

ABSTRACT Salinity is a key variable for ecological processes in estuaries. Acartia tonsa is a typical estuarine copepod whose responses to salinity have been thoroughly studied in the laboratory. However, results cannot be extrapolated to the field, and formal comparisons between lab and field responses to salinity were not attempted. Here we compare lab-based with field copepod egg production rates (EPR) from the Río de la Plata estuary (RPE), with focus on A. tonsa. Field work was conducted between 2009 and 2011 in the mixing zone of the RPE. Water temperature, salinity and chlorophyll-a varied over ample ranges (temperature: 10.54 - 24.56ºC, salinity: 2.83 - 32.99, chlorophyll-a: 0.62 - 7.27 mg m-3). A. tonsa was the strongly dominant species. EPR ranged between 6.7 and 95.7 eggs female-1 day-1, and correlated to salinity, temperature (weakly), but not to chlorophyll. The relationship between A. tonsa’s EPR (EPRAT) and salinity was consistent with that obtained under laboratory conditions: a humped pattern with a maximum at intermediate salinities. However, differences were also evident, e.g., higher EPRAT was measured in the field. We speculate that discrepancies derive from nutritional differences between field and reference (laboratory) data sets. Besides salinity, food quality and quantity may be first order drivers of A. tonsa’s productivity in the RPE.

Gelatinous carnivores are important components of the pelagic system and potentially key organisms in trophic webs, since they prey on several groups of organisms. In the Gullmar fjord many species of gelatinous organisms have been found exerting an important predatory impact on Acartia clausi, a dominant copepod of the mesozooplankton in this ecosystem. This work aims at evaluating the predatory impact of M. leydi and S. setosa on the community of copepods of the Gullmar fjord. These experiments were of functional-response type. In addition, length-weight regressions for predator biomass estimates were obtained. S. setosa showed a functional response Type II. By contrast, M. leidyi showed a linear functional response (Type I) with maximum ingestion (Imax) rates of 7 copepods ind-1 h-1 for individuals of length size between 7 and 15 mm. Sagitta setosa exhibited an Imax of 3.96 (copepods pred-1 h-1) with individuals in the size-range between 6 and 7 mm. The predatory impact of S. setosa and M. leidyi was high, suggesting that these species exert a strong impact on the mesozooplankton community of the Gullmar fjord.

Los organismos gelatinosos carnívoros son importantes componentes del ecosistema pelágico y potencialmente organismos claves en las tramas tróficas pelágicas, depredando sobre varios grupos. En el fiordo Gullmar, muchas especies de gelatinosos han sido encontradas ejerciendo un gran impacto depredador sobre A. clausi, un abundante copépodo del mesozooplancton de este ecosistema. El propósito de este trabajo fue evaluar el impacto depredador de M. leydi y S. setosa sobre la comunidad de copépodos del fiordo Gullmar. Los experimentos fueron de respuesta funcional. Además, regresiones de longitud- peso seco fueron estimadas para determinar la biomasa del depredador. Sagitta setosa exhibió una respuesta funcional tipo II. Mnemiopsis leidyi mostró una respuesta funcional lineal (Tipo I) con tasas de ingestión máximas (Imax) de 7 copépodos ind-1 h-1 para individuos cuyo tamaño osciló entre 7-15 mm. Sagitta setosa exhibió un Imax de 3,96 (copépodos pred-1 h-1) en individuos de tamaños de 6 y 7 mm. El impacto depredador de ambas especies fue alto, por esto se sugiere que ambas pueden ejercer un fuerte impacto sobre la comunidad del fiordo Gullmar.

The marine areas of Uruguay consist of the Río de la Plata estuary and the adjacent shelf and slope, part of the Subtropical Convergence Ecosystem. In this paper, the main advances in the knowledge of marine life in these areas (the known) are reviewed in order to discuss future lines of research (the unknown). Information has been separately analysed for the plankton, nekton, and benthos in each of 3 areas-the littoral, the shelfs and the "open ocean". Current knowledge of marine life is uneven among the groups and areas. In the case of the plankton, research has concentrated on the near-shore waters and focused on taxonomy and distribution. Little is known about the responses of organisms to environmental variability and about biological processes. The nekton of coastal and estuarine areas is better known, but, with shelf and slope assemblages, research has focused on exploited species. The main unknowns for the nekton are how trophodynamics, reproduction, and recruitment processes are linked to environmental variability and the effect of fisheries on community structure. Littoral benthos, in particular the macroinfauna of sandy beaches, is much better studied and spatial patterns of community distribution have been identified at different scales and in relation to environmental variables. Also, at the population level, there is information about distribution, responses to disturbances, dynamics, and the roles of biotic and abiotic factors in modulating population variability. Information is mainly lacking for the sub-tidal fringe and regarding the macro-ecology of population dynamics, the dispersive abilities of larval phases, and the effects of toxic blooms on suspension feeders. Knowledge of the benthos of estuaries and more so of the shelf and slope environments is rather scarce. For the latter two, faunal inventories are far from complete. Topics identified for future research include taxonomy, macro-scale community structure and its temporal variability in relation to environmental gradients, diverse aspects of population dynamics trophodynamics and the effects of human intervention on ecosystems. The incorporation of both experimental and modelling approaches is considered important for future investigations.

Las aguas uruguayas comprenden el estuario del Río de la Plata y la plataforma y talud contiguos, parte del Ecosistema de Convergencia Subtropical. Esta contribución revisa los principales avances en el conocimiento de la vida marina en esta zona (lo conocido), para discutir futuras líneas de investigación (lo desconocido). Se analiza separadamente la información del plancton, necton y bentos de tres áreas: litoral, plataforma y ``océano profundo''. El conocimiento resulta disparejo entre grupos y entre áreas. En el plancton, la investigación se concentró en el litoral y enfocada hacia aspectos taxonómicos y de distribución, existiendo fuertes deficiencias en el conocimiento de las respuestas de los organismos a la variabilidad ambiental y de procesos biológicos. El necton de áreas estuarinas y costeras es mejor conocido que el de plataforma y talud, donde la investigación ha sido dirigida principalmente a especies explotadas. En este grupo, los principales aspectos desconocidos son los trofodinámicos, los procesos reproductivos y de reclutamiento en función de la variabilidad ambiental, y el efecto de las pesquerías sobre la estructura comunitaria. El bentos litoral, en particular la macroinfauna de playas arenosas, es el componente mejor conocido: se han establecido patrones de distribución comunitaria en diferentes escalas espaciales y en relación a variables ambientales. A nivel poblacional existe información sobre distribución, repuesta a perturbaciones, dinámica, y sobre el papel de factores bióticos y abióticos en la modulación de la variabilidad poblacional. La información faltante se refiere a la franja submareal, así como a aspectos macro ecológicos de dinámica poblacional, dispersión larval y el efecto de las floraciones tóxicas sobre organismos suspensívoros. El bentos de estuarios, y en particular aquél de plataforma y talud, es muy poco conocido; para los últimos los inventarios faunísticos son extremadamente incompletos. Los tópicos identificados para próximas investigaciones incluyen taxonomía, la estructura de las comunidades y su variabilidad temporal en gradientes ambientales, diversos aspectos de dinámica poblacional, trofodinámica y el efecto de la intervención humana en los ecosistemas. Se considera muy importante la incorporación de enfoques experimentales y modelización.