BENTHIC DIVERSITY OF THE RÍO DE LA PLATA ESTUARY AND ADJACENT MARINE WATERS DIEGO A. GIBERTO SUPERVISOR: CLAUDIA S. BREMEC PNUD Project/Gef RLA/99/G31 2003 1 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ BENTHIC DIVERSITY OF THE RÍO DE LA PLATA ESTUARY AND ADJACENT MARINE WATERS Report __________________________________________________________________ AUTHOR: DIEGO A. GIBERTO SUPERVISOR: CLAUDIA S. BREMEC __________________________________________________________________ Summary The main objectives of this report are to analyze the infralitoral benthic assemblages of the Río de la Plata estuary and adjacent marine zones, and establish broad patterns of species diversity throughout the study area. Species richness was utilized as a measure of diversity. The report was divided in two major approaches: the analysis of an historical background and the EH-09-99 survey. The taxonomical groups that contributed with the major number of species were the mollusks, crustaceans, polychaetes, echinoderms and coelenterates. A classical gradient of increasing species richness from estuarine to marine waters was found. Also, few species were found inhabiting both estuarine and marine waters. The species richness values were high at the continental shelf and the coastal marine environments. The highest number of species was recorded in mussel beds and coarse-sandy bottoms marine assemblages. The irregular pattern of species richness found in marine waters could be due to the presence of different heterogeneous bottom types all over the study area. It is concluded that the available information about benthic communities inhabiting the study area is scarce. In addition, a large portion of the studies were developed mainly on the marine zones < 50 m, while the estuarine zones and the marine zones > 50 m are underestimated. Basic information related with diversity like abundance, biomass, dispersal and temporal variations of macrobenthic species are needed. 2 Index I. Introduction…………………………………………………………………………..1 II. Methodology……………………………………………………………………........1 III. Results………………………………………………………………………………..3 IV. Concluding remarks…………………………………………………………….......5 V. Literature cited……………………………………………………………………….7 VI. Tables……………………………………………………………………………….10 VII. Figures………………………………………………………………………………13 VIII. Appendix…………………………………………………………………………….22 1 Introduction Species diversity is a community attribute influenced by historical events and geographical phenomena; it is also an emergent consequence of local ecosystems-level processes. Changes in diversity reflect changes in ecosystem processes, such as productivity, pathways of energy and material flow, disturbance regime, abiotic stress, and biological interactions (Brown et al., 2001). Species richness, or the number of species, is currently the most widely used diversity measure, together with indexes that also consider the relative species abundance in the community like H’ (Shannon Wiener) o J’ (Pielou’s evenness) (Stirling & Wilsey, 2001). The main objectives of this work are to analyze the infralitoral benthic assemblages of the Río de la Plata estuary and adjacent marine zones, and establish broad patterns of species diversity throughout the study area. Methodology Despite the fact that diversity can change without any change in species richness (species richness is not generally the common cause of variation in either H’ o J’) (Stirling & Wilsey, 2001), the species richness was utilized as a measure of diversity in this study, following Gray (2000) who suggested the species richness as a basic estimation of diversity. At the most elementary level, species richness is the total number of species in a given area (Gray, 2000). Following Gray (2000), we calculated for each defined sector the point species richness SRP (number of species in a single sampling unit from a given area) and the sample species richness. The sample species richness can be calculated as the total number of species, SRS, based on a given number of sampling units. Samples species richness is equivalent to the diversity of samples within a habitat, usually called alpha diversity. For the entire study area the SRL or large area species richness was calculated. This is defined as the species richness of a large area which includes a variety of habitats and assemblages. 2 The report was divided in two major approaches, as follows: Historical background: the prevalent information available in the bibliography of benthic assemblages of the Río de la Plata estuary and adjacent marine zones is the number of species. These studies were done with highly variable sampling methodologies that make unfeasible the building of a detailed species inventory with standardized abundance data (see table I). Systematic publications that focused only on one taxonomical group (see for example Bernasconi & D’Agostino, 1977; Tablado & Maytía, 1988; Boschi et al., 1990) and other ecological ones that do not provide information on the presence of species in every sampling station (Bastida et al., 1989, 1992; Bremec et al. 1998; Bremec and Lasta, 2002; Roux et al., 1993) were not used in the analysis, because it would introduce a bias throughout the species richness pattern of certain taxa at a given area. The study area was divided in four main sectors, as follows: 1) an estuarine zone (mixohaline waters, is equivalent to the sector 1 of EH-09-01 survey), 2) a marine zone in front of the Argentine coast (depth < 50 m, is equivalent to sector 2 of EH-09-01 survey), 3) a marine zone in front of the Uruguayan coast (depth < 50 m, is equivalent to sector 2 of EH-09-01 survey) , and 4) a marine zone with depths > 50 m (is equivalent to sectors 3 and 4 of the EH-09-01 survey) (Fig. 1). These division was made considering oceanographic (salinity, bathymetry) and bottom (sediment composition) features (Urien, 1972; Guerrero et al., 1997, Mianzan et al., 2001), which probably influence the spatial distribution of benthic assemblages. Also, each publication was assigned to the following benthic assemblages: mussel beds (N1, N5 and N7, dominance of Mytilus edulis platensis), scallops beds (N8, dominance of Zygochlamys patagonica), coarse-sandy bottoms (N4, N6, N9 and N10), muddy-bottoms (N6 and N9) and Uruguayan rocky-sandy beaches (N2 and N3) (Table II and Fig. 1). EH-09-01 survey: The study area was divided in four main sectors, as follows: 1) an estuarine environment (< 25, mixohaline waters), 2) a coastal environment (> 25, depths < 50 m), 3) a continental shelf environment (depths 50-200 m) and 4) a 3 shelf-break environment (depths > 200 m) (Table II and Fig. 2). This division was made largely following Acha and Lo Nostro (2002). Results Historical background A total of 420 species (SRL) and 14 taxa were recorded in the study area (see list in Appendix 1). Marine waters were characterized by higher number of species than estuarine waters (373 vs. 47 respectively). SRP from the sampling stations of the estuarine waters ranged between 1 and 20, while stations of the marine waters ranged between 1 and 65 (Fig. 3). The taxonomical groups that contributed with the major number of species were the Mollusks (153 species), Crustaceans (88 species), Polychaetes (81 species), Echinoderms (29 species) and Cnidarians (27 species) (Appendix 1). The estuarine zone was characterized by 6 taxa (SRS= 47), with mollusks, crustaceans and polychaetes contributing with more than 90 % of specific richness (Table III and Fig. 4). The marine waters < 50 m reached the highest number of species and taxa (SRS= 273 and SRS= 138 at Uruguay and Argentine respectively), while deeper stations (> 50 m) displayed the lowest values of specific richness in marine waters (SRS= 51) (Table III and Fig. 4). The contribution of all different taxa recorded at each zone is indicated in Fig. 4. The marine zone > 50 m presented the higher percentages of “exclusive species” (species that were only recorded in the defined area) (84.31%), followed by the marine Uruguayan zone (73.26%) and the marine Argentine zone (54.35%). The lowest value was recorded in the estuarine zone (34.04%). Only ten species were recorded in estuarine and marine waters: Angulus gibber, Corbula patagonica, Mactra isabelleana, Nucula puelcha, Pitaria rostrata, Ancinus depressus, Artemesia longinaris, Pagurus exilis, Pinnixa brevipollex and Encope emarginata. Benthic assemblages that reached the highest values of SRS were those inhabiting mussel beds (SRS= 213) and coarse-sandy bottoms (SRS= 184) (Table IV). 4 The scallop beds presented the higher values of “exclusive species” (81.58%), followed by the coarse-sandy bottoms (77.17%), mussel beds (76.53%), Uruguayan beaches (53.57%) and the muddy bottoms (34.29%) assemblages. Only two species were recorded at three of the defined assemblages: Mytilus edulis platensis and Pseudoechinus magellanicus (see Appendix 1). EH-09-01 survey A total of 145 species (SRL) and 12 taxa were recorded in the study area (see list in Appendix 1 and 2). Mollusks were the dominant group, with 43 species, followed by polychaetes (35 species), crustaceans (32 species) and echinoderms (14 species) (Fig. 5). The highest value of SRS (sample species richness) was found in the continental shelf environment, while the estuarine environment displayed the lowest SRS value (Fig. 6). The contribution of the different taxa recorded at each environment is indicated in Fig. 4. Mollusks and polychaetes reached the highest richness at the continental shelf environment, while crustaceans richness