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Tramas tróficas en sistemas frontales del Mar Argentino: estructura, dinámica y complejidad analizada mediante isótopos estables

Sun, 01 Jan 2012 00:00:00 GMT

Title: Tramas tróficas en sistemas frontales del Mar Argentino: estructura, dinámica y complejidad analizada mediante isótopos estables Authors: Gaitán, E.N. Abstract: Two of the most important frontal systems of the Argentine Sea (SW Atlantic Ocean) were compared to obtain evidences on the regulatory mechanisms of food web length, incorporating both spatial (areas within fronts) and temporal (presence or absence of frontal structure) variability. The stable isotopes of Carbon and Nitrogen were used to evaluate the origin and composition of the organic matter used by primary consumers, the structure of food webs, the trophic levels of several taxa and the benthic-pelagic coupling within each system. In the Río de la Plata estuarine front, different sources of organic matter would support the food webs occurring in the three areas of the system. The particulate organic material (POM) would be the main resource supporting the food web in the mixohaline area, although with different composition between surface and bottom layers. The similarity of the ~d¹³C signature between the fluvial detritus and the surface POM revealed a strong river-estuary coupling. On the contrary, the salt marsh area did not contribute significantly to POM composition. Consumers of the three areas used both in situ generated and alocthonous resources. A clear division between those species that used planktonic (estuarine or river species) and benthic resources (sea-originated) was observed. Invertebrates such as Acartia tonsa and Neomysis americana were the base of the planktonic food web and canalized most part of the energy present in the system (detritus and POM). The salt marsh contribution to the estuarine system was restricted to crabs from the tidal plain. The trophic level of fish community mostly corresponded to secondary consumers. In the tidal front, there were differences in the POM composition between homogeneous and stratified sectors. Notwithstanding that, the trophic structure was similar between sectors, with variations in the composition of primary consumers. The different signatures of lower consumers were integrated by higher order consumers (fishes). Two trophic pathways (herbivorous and microbial) may dominate in different areas and periods. The fish community was characterized by a higher trophic level (mostly third consumers). The productivity, environmental compartmentation and species diversity affected the trophic structure differently in each front. The structure of the estuarine trophic food web was based on a few dominant species in the lower levels that canalized most part of the energy, a high trophic dependence of these species to fish community and, as a corollary, a classic and short trophic food web stable through time. In the tidal front, the trophic food web presented spatio-temporal fluctuations in the POM composition, a strong benthic-pelagic coupling with a great proportion of fish using benthic preys and a larger food web than in the estuary. This study indicates that high resource availability alone is not an indicator of a longer food web.

Hábitats esenciales de condrictios (Chondrichthyes) costeros, y su relación con los procesos oceanográficos

Sun, 01 Jan 2012 00:00:00 GMT

Title: Hábitats esenciales de condrictios (Chondrichthyes) costeros, y su relación con los procesos oceanográficos Authors: Cortés, F. Abstract: The relationship between environmental variables (depth, temperature and salinity) and the abundance of sharks (Mustelus schmitti and Squatina guggenheim) and skates (Sympterygia bonapartii, Rioraja agassizi and Atlantoraja castelnaui) was analyzed in the Southwestern Atlantic coastal waters (between 34~'S and 42~'S at depths of less than 50 m from the coast line). Data were collected during research Cruises carried out by the National Institute of Fisheries Research and Development (INIDEP) during winter and late spring. Due to differences in the continental water discharges, salinity spatial distribution and in the environmental condition seasonality, data from the Río de la Plata and El Rincón were independently analyzed. Shark and skate species shows different relationships with the environmental variables, showing differences in the spatial distribution patterns and in the abundance seasonality between species. M. schmitti and S. guggenheim showed ontogenetic differences in the habitat use and in the abundance seasonality. Juveniles occupy shallow waters (< 25 m depth), defining nursery areas along the coast. Adults occupied deep waters (> 40 m depth) during no reproductive season (winter) and they migrate to shallow waters during the reproductive season (late spring). S. bonapartii, R. agassizi and A. castelnaui not showed ontogenetic differences in habitat use or in the abundance seasonality. Both study areas showed inter-annual variations in the temperature and salinity spatial distributions, which explains the abundance and spatial distribution variations of the coastal chondrichthyans. In these areas, zones with high environmental variability can be identified and associated to the edges of the oceanographic regimes (e.g. Punta Rasa, Punta Médanos, the coastal area among Montevideo and Punta del Este, coastal area among Monte Hermoso y Claromecó). The abundance and availability of M. schmitti, S. guggenheim, S. bonapartii, R. agassizi and A. castelnaui in the SW Atlantic coastal waters were affected by the seasonal and inter-annual variations in the geographical distribution of the environmental conditions. The viviparous chondrichthyans shows a strong seasonality in the habitat use, migrating from deep to shallow waters (M. schmitti, S. guggenheim). Habitat use seasonality in oviparous chondrichthyans studied here are not clearly defined being S. bonapartii the skate which showed higher abundance seasonality. Considering habitat use, the species that forms coastal aggregations as part of their reproductive migrations (M. schmitti, S. guggeheim and S. bonapartii) would be the most vulnerable to environmental variations and localized exploitation. Therefore, in the SW Atlantic coastal waters, the chondrichthyans which use specifics habitat for egg laying (S. bonapartti) or parturition (M. schmitti y S. guggenheim) would be more vulnerable in this regard than those which have no preference for specific habitats (R. agassizi y A. castelnaui). These findings represent an essential knowledge to these species and provide valuable information concerning the conservation programmes and management strategies of the chondrichthyan fishes.

Ecología trófica del gatuzo, Mustellus schmitti (Springer 1939), en el Mar Argentino

Sun, 01 Jan 2012 00:00:00 GMT

Title: Ecología trófica del gatuzo, Mustellus schmitti (Springer 1939), en el Mar Argentino Authors: Belleggia, M. Abstract: The narrownose smoothhound shark Mustelus schmitti is a benthic shark distributed from Florianópolis (27°S, Brazil) to Deseado River Estuary (47º45´S, Argentina) and occurs from shallow waters to 120m. This Thesis describes the feeding ecology of narrownose smoothhound shark M. schmitti in northern Argentinean Continental Shelf and is structured in four chapters. Chapter 1 contains an introduction and includes purpose, theory and concepts. In Chapter 2, the food habits of M. schmitti were studied based on analysis of stomach contents from specimens collected during the years 2008-2011. Of a total of 959 stomachs sampled during this period, 944 (98,43%) contained food. Prey items were identified to the lowest possible taxonomic level, counted and weighted. The analyses revealed that the species fed mostly on polychaetes according to %F, %N, %IRI and %PSIRI, followed by Brachyura crustaceans, fishes and hermit crabs. Moreover, Generalized Linear Models revealed ontogenetic and spatiotemporal shift in the diet of the narrownose smoothhound shark M. schmitti. The trophic level is the position of an organism within the food web; it was 3,6 during 2008-2011. In Chapter 3 stomach contents data collected from fifteen research cruises carried out by INIDEP from 1986-1994 were analyzed. Of the 1009 stomachs analyzed during the older period, 902 (89,39%) contained prey items. The narrownose smoothhound shark M. schmitti fed mostly on Brachyura crustaceans, followed by polychaetes and fishes. Generalized Linear Models, merging stomach contents data from contemporary and older periods, revealed that the consumption of the main prey groups varied over the last 25 years. The trophic level was 3,85 during 1986-1994. As fishing is a major form of ecological disturbance to marine communities throughout the world, these changes in the trophic relationships are probably consequence of trawl fisheries and the disturbance of bottom trawls through the study period. Chapter 4, explores qualitative and quantitatively the dentition of the narrownose smoothhound shark M. schmitti by sex and ontogenetic development. The values of tooth replacement rates were estimated and a brief discussion of the presumed relationship between dentition and the feeding behavior previously analyzed is included. Chapter 5, includes a brief final conclusion of the whole Thesis that aims to provide a global vision about the feeding ecology of M. schmitti. Finally, an appendix provides an illustrated guide to identify the different species of polychaetes that might be encountered in the stomachs contents of benthic species from the northern Argentinean Continental Shelf (35°S -41°S).

El rol del frente de mareas de Península Valdés en el control de la comunidad zooplanctónica

Sun, 01 Jan 2012 00:00:00 GMT

Title: El rol del frente de mareas de Península Valdés en el control de la comunidad zooplanctónica Authors: Derisio, C. Abstract: The Península Valdés tidal front was studied in its rol in the control of the spatial distribution, abundance, assemblages, diversity and life histories of the zooplankton community using samples from two oceanographic surveys. Although the front showed in general high zooplankton abundances, highest values, mainly due to the presence of immature stages of copepods, were recorded in the transitional and stratified zones coinciding with highest values of chlorophyll a. The zooplankton groups showed a distinctive spatial distribution across the front. The results showed that this front did not exert any control on zooplankton composition and diversity, but did on the abundance. In the homogeneous zone the harpacticoid copepods were more abundant, followed by barnacle nauplii and decapod larvae. The transition zone was largely dominated copepod nauplii and high abundances of cyclopoid, calanoid and harpacticoid copepods. Other taxa such as appendicularians, cladocerans, cumacea and mysids also showed higher abundances in this zone. In the stratified zone copepodites I-III, calanoid copepods, euphausiids, jellyfish, ctenophores and hyperiid amphipods were more abundant. The hydrographic characteristics of the front would be controlling the zooplankton community. Nictimeral migrations were observed in almost all zooplankton groups mainly in the stratified zone. The high phytoplankton and zooplankton concentrations suggest a predominance of control "bottom up" in this frontal system.