Distribution, abundance and biomass of Chaetognaths off São Sebastião region, Brazil in February 1994

Rev. bras, oceanogr., 50(unico):l-12, 2002 Distribution, abundance and biomass of Chaetognaths off São Sebastião region, Brazil in February 1994 Tsui-Hua Liang 1 & Luz Amelia Vega-Perez^ 1 Universidade Federal do Pará Campus Universitário de Bragança, (Alameda Leandro Ribeiro s/n, 68600-000 Bragança, PA, Brasil) 2 Instituto Oceanográfico da Universidade de São Paulo (Praça do Oceanográfico, 19l Butantã 05508-900 São Paulo, SP, Brasil) • Abstract: The distribution, abundance, biomass, population structure and feeding habits of chaetognaths collected off São Sebastião region, Brazil, in February 1994 are described. Bongo nets were hauled obliquely to collect zooplankton samples. Forty-three samples obtained with the 333 urn mesh were analysed. In this study, 7 chaetognath species belonging to two genera were identified. Sagitta friderici, S. tenuis and S. bipunctata were grouped into the neritic category, and Sagitta enflata, S. hispida, S. minima and Krohnita pacifica into the semi-neritic group. The analysis of the community structure distinguished 3 zones: 1) a nearshore zone evidenced by low richness; 2) an offshore zone evidenced by higher number of species and 3) another offshore zone, located south and south-westward of São Sebastião Island, characterised by higher richness but with dominance of one species. The nearshore zone was dominated by the neritic species S. friderici and S. tenuis, whereas the offshore zone was dominated by S. enflata. Abundance and biomass increase from nearshore toward offshore zones by about two orders of magnitude. Gut content analysis revealed over 90% of empty guts. The chaetognath population was composed mainly of juveniles. The diets among the different chaetognath species was very similar, composed mostly of small copepods and appendicularians. • Resumo: No presente trabalho foram estudados a distribuição, abundância, biomassa, estrutura da população e hábito alimentar dos quetógnatos coletados na região de São Sebastião, Brasil, em fevereiro de 1994. As 43 amostras de zooplâncton utilizadas na elaboração deste trabalho foram obtidas através de arrastos oblíquos usando uma rede Bongô (333 um), providas de fluxômetro. Foram identificadas sete espécies de Chaetognatha pertencentes a dois gêneros. Sagitta friderici, S. tenuis e S. bipunctata foram agrupadas como espécies neríticas, enquanto que Sagitta enflata, S. hispida, S. minima e Krohnita pacifica semi-neriticas. A análise da estrutura da comunidade, mostrou a existência de 3 zonas: 1) zona costeira caracterizada por poucas espécies; 2) zona oceânica com muitas espécies e 3) zona oceânica, ao sul e sudoeste da Ilha de São Sebastião, com muitas espécies, mas predominância de uma espécie. A zona nerítica foi dominada pelas espécies S. friderici e S. tenuis, e a zona oceânica por S.enflata. Abundância e biomassa do grupo aumentou da região costeira para a oceânica. A população foi composta principalmente de jovens. A análise do conteúdo intestinal revelou mais de 90% de indivíduos com o trato digestivo vazio. A dieta das diferentes espécies foi bastante similar, sendo composta principalmente de pequenos copépodos e apendiculários. • Descriptors: Chaetognatha, Distribution, Abundance, Biomass, Feeding habits, Brazil. • Descritores: Chaetognatha, Distribuição, Abundância, Biomassa. Hábito alimentar, Brasil. Contr.no847 do Inst. oceanogr. da Usp. Introduction Like most coastal environments, the São Sebastião region suffers from the effects of human activities such as domestic waste water discharges and intense tourism. Moreover, this region is frequently under effects of oil spills (Lamparelli & Moura, 1999). A multidisciplinary project was designed to investigate the structure and functioning of the coastal ecosystem off São Sebastião region. As part of this project, the present paper focused on chaetognaths, a small and evolutionary isolated group of animals (Hyman, 1959), ubiquitous in the marine plankton. The phylum Chaetognatha comprises about one hundred species of small size, between 2 and 120 mm in length, living in various marine habitats (Casanova, 1999). They are ubiquitous and extremely abundant in the marine plankton, often surpassed by copepods in number (Feigenbaum & Maris, 1984). The distribution and abundance of these organisms are related with local hydrological conditions (Heydorn, 1959) and their food (Alvarino, 1969). Some species are closely associated with specific water masses (Grant, 1991; Liang & Vega-Pérez, 2001), being considered as excellent indicators of water masses movements (Cheney, 1985). Chaetognaths are considered to play a central role in the pelagic food webs, serving as the main link between copepods and higher trophic levels (Alvarino, 1985). Chaetognaths are one of the most active predators among zooplankters, and they can remove a substantial fraction of prey production each day (Kimmerer, 1984). The dominance of chaetognaths over other pelagic predators (Oresland, 1987) in number and their presence throughout the year in the zooplankton samples, indicate the prominence of this group. This study aims to describe the abundance, distribution, biomass, population structure and feeding habits of chaetognaths collected off São Sebastião region, Brazil, in February 1994. Study area The study area, the São Sebastião region (23°30'S - 24°15'S; 45°00'W - 45°45'W) lies on the northern coast of the São Paulo State, Brazil, and is characterised by a broad continental shelf extending over approximately 100 km in length. The coastline has many peninsulas, bays and inlets that, with São Sebastião island as well as other islets, act altogether to attenuate the energy of waves originating at the open sea and hence to lower the hydrodynamism. In February 1994, two types of water masses were described in the region (Castro Filho & Miranda, 1997): one is the Coastal Water (CW), with oligotrophic features, temperatures higher than 20°C and salinity lower than 35 psu. It was found in the surface layer above 25 m depth. The other water mass is the South Atlantic Central Water (SACW), rich in nutrients and characterised by lower temperatures (<18°C) and salinity ranging from 35 to 36 psu. The SACW flows in the deeper layers, below 30 m depth. Material and methods Zooplankton samplings were conducted aboard the R/V Prof. W. Besnard in the São Sebastião region from February 7th to 10th, 1994 over a grid of 43 oceanographic stations (Fig. 1). A total of 86 zooplankton samples were collected with Bongo nets fitted with 333 urn and 505 urn mesh size, provided with digital flowmeters for measurement of filtered water volume. For this study, 43 samples obtained using the 333 urn mesh size net were analysed. Double oblique hauls were made from the surface to 5 m above the bottom of the sea and to surface, during 5-10 minutes, maintaining the velocity of vessel of about 1.5 knots. The collected samples were immediately fixed on board with a solution of 4% (...truncated)