Spatial and temporal changes in the structure of soft-bottom benthic communities in an Amazon estuary (Caeté estuary, Brazil)

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1 Journal of Coastal Research SI ICS2011 (Proceedings) Poland ISSN Spatial and temporal changes in the structure of soft-bottom benthic communities in an Amazon estuary (Caeté estuary, Brazil) R.F. Silva, J.S. Rosa Filho, S.R. Souza and P.W. Souza-Filho Universidade Federal do Universidade Federal do Universidade Federal Universidade Federal Abstract Silva, R.F., Rosa Filho, J.S., Souza, S.R. and Souza Filho, P.W., Spatial and temporal changes in the structure of soft-bottom benthic communities in an Amazon estuary, Caeté estuary, Pará, Brazil. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium), Szczecin, Poland, ISSN Macrobenthos communities were studied at different seasons and areas in the Caeté estuary, Brazil. Samples were collected along the estuary in May 2005 (rainy season) and December 2005 (dry season), using a Petersen grab (20x20x20 cm). After collection, the samples were passed through a 0.3-mm mesh screen. At each station, a sediment sample was taken for organic-matter determination and textural analysis, and the water salinity was measured. Univariate (ANOVA) and multivariate (MDS, ANOSIM and ACC) techniques were used for data analysis. The macrofauna was composed of 83 taxa belonging to six phyla, with annelids dominating; 47 taxa were common to both seasons, 15 were restricted to the rainy season, and 21 to the dry season. In the rainy season, richness and diversity increased towards the estuary mouth. Density did not vary significantly among seasons and estuarine zones, with the lowest values always in the upper estuary. During the rainy season, all community descriptors were highest in the middle estuary. The ANOSIM indicated significant differences among estuarine zones (upper, middle and lower estuary). There was a clear faunistic gradient from the upper to the lower estuary in both seasons. The ACC showed that in the rainy season, salinity and organic matter explained most of the macrofauna variation. During the dry season, salinity and mean particle diameter were the most important parameters. These results showed that changes in salinity, hydrodynamics and substrate characteristics are the main factors causing spatial and temporal changes in the macrobenthic communities in the Caeté estuary. Aditional index words: macrobenthos, variation, salinity gradient INTRODUCTION In estuaries, large changes in salinity, due to freshwater flow and tidal movements, create a characteristically variable environment, where the biota is constantly challenged to maintain osmotic and ionic equilibrium. The estuarine benthic fauna is mainly composed of Polychaeta, Crustacea, and Mollusca, and shows high tolerance to environmental stress and to large spatial and temporal changes in community structure. In estuaries, three physical factors are of primary importance in regulating benthic species composition and abundance: salinity, dissolved oxygen, and substratum composition (McLusky and Elliott, 2004). The northern Brazil coast is 1,200 km long (from São Marcos Bay to the Oiapoque River) and encompasses two geomorphologic world records: the largest mangrove system (Souza-Filho, 2005) and the largest river in length, water flow, and sediment discharge, the Amazon (Souza-Filho et al., 2009). The coastline is extremely irregular, harboring 23 estuaries (Martins et al., 2007). Mangroves are dominated by Rhizophora mangle and Avicenia schaueriana, occupying an area of 8,894 km -2 (Kjefve and Lacerda, 2003). Most of these mangroves are still untouched by humans and represent a unique opportunity to understand the responses of the macrobenthic fauna to changes in environmental characteristics in humid tropical estuaries. This study characterized the macrobenthic communities in different seasons and areas in the Caeté estuary in northern Brazil, describing spatial and temporal changes in community-structure descriptors (taxonomic composition, richness, abundance, diversity, and eveness) along the estuary, and identifying environmental variables responsible for these changes. METHODS The Caeté estuary is situated on the northern coast of Brazil, approximately 450 km east of the mouth of the Amazon River (Figure 1). The climate is equatorial with a rainy season extending from January through June, and a dry season from July through December. Tides are semidiurnal, with a mean range close to 5 m. Based on salinity, the estuary can be divided into three zones: upper, 5 km long and m wide, salinity lower than 10; middle, 15 km long and m wide, salinity between 10 and 20; and lower, 20 km long and km wide, salinity higher than 20 (Barletta et al., 2002). Samples were taken using a Petersen grab (20x20x20 cm) along the estuary in May (rainy season) and December (dry season) of After collection, the samples were passed through a 0.3-mm mesh screen and fixed in 5% buffered formalin. At each station, a 440

2 Spatio-temporal changes in benthic communities in an Amazon estuary sediment sample was taken for organic-matter determination and textural analysis, and the water salinity was measured. For each biological sample, density, richness, diversity, and evenness were calculated. Biotic and abiotic variables were compared among estuarine zones and seasons using one-way ANOVA (data log (x+1) transformed). To compare communities between estuarine zones and seasons, Analysis of Similarity (ANOSIM) was applied. The most important environmental variables related to faunal patterns were identified using Canonical Correspondence Analysis. The significance level of 5% was used for all analysis. RESULTS Environmental characteristics All the environmental factors studied differed significantly among sampling occasions and estuarine zones, except for sediment sorting. There was a clear salinity gradient from the upper to the lower estuary on both sampling occasions. Bottom sediments were composed of well-sorted fine sand or well-sorted very fine sands. Coarser sediments were observed in the upper estuary and during the dry season. The amount of organic matter in the sediments was significantly higher during the rainy season. In the dry season, organic matter slowly decreased from the upper to the lower estuary. In the rainy season, maximum amounts of organic matter were observed in the middle estuary (Table 1 and Table 2). Benthic fauna Along the estuary and between seasons, there were pronounced changes in species composition. In the dry season, in the upper estuary Namalycastis terrestris, Nephtys fluviatilis, and Tubificidae were dominant. Sigambra grubii, Mediomastus sp., and Peloscolex sp. dominated in the middle estuary; while in the lower estuary Nephtys simoni, Boccardiella ligerica, and Bivalvia were the most abundant taxa. In the rainy season, N. terrestris and N. fluviatilis were again the most abundant taxa in the upper estuary. In the middle estuary, Mediomastus, Ostracoda, and Sigambra grubii were dominant. In the lower estuary, Armandia sp. and Aricidea philbinae became the most abundant taxa. Figure 1 Map of study site (Caeté River estuary, Pará, Brazil). Density and richness were significantly higher in the dry season. In both seasons, there were no significant changes in densities along estuary. In general, the number of taxa significantly increased down the estuary (Table 1 and Table 3). Significantly higher diversity was observed in the dry season. Evenness, although somewhat higher in the dry season, did not change significantly between seasons. Both of these descriptors increased from the upper to the lower estuary (Table 1 and Table 3). Table 1 Mean values (±SD) of environmental and biological variables. Dry season Rainy season Upper estuary Middle estuary Lower estuary Upper estuary Middle estuary Lower estuary Environment Macrobenths Salinity 5.95± ± ± ± ± ±0.46 MPD 3.7± ± ± ± ± ±1.8 Sorting 3.65± ± ± ± ± ±0.15 % OM 3.18± ± ± ± ± ±0.27 Abundance (ind.m -2 ) 492± ± ± ± ± ±240.1 Richness (no. taxa) Diversity (H ) 1.06± ± ± ± ± ±0.12 Eveness (J) 0.82± ± ± ± ± ±

3 Silva et al. Table 2: Summary of ANOVA used to investigate spatial and temporal variability of environmental variables in the Caeté estuary. 1 Zone, 2 Season, MPD Mean particle diameter, % OM percentage of organic matter in sediments. Source MPD Sorting % OM Salinity of F P F p F p F P change x In the dry season, CCA results revealed that the first axis explains % of the variation, and the second axis explains % of the variation. The CCA graph illustrates the separation of the lower-estuary samples from the others (Figure 2). Salinity, organic matter, and grain size explained most of the variation along axis one; and along axis two, the percentage of gravel and sand, and sediment sorting had higher loads. ANOSIM results showed significant differences in macrobenthos communities among samples from the upper, middle, and lower estuary (Table 4). In the rainy season, the first two CCA axes explained 77.45% of the variation. In the CCA graph it is possible to identify two main groups of samples. The first group was composed of samples from the upper estuary, and the second group assembled middle- and lower-estuary samples. Again, salinity and organic matter best explained sample separation along axis one, and percentage of gravel and sand along axis two. As in the dry season, ANOSIM results showed significant differences in communities from the upper, middle, and lower estuary (Table 5). DISCUSSION The macrofauna in the Caeté estuary is composed of typically estuarine species, with a predominance of annelids, as commonly occurs in tropical estuaries (Flint and Younk, 1983; Dittman, 1995; Gambi et al., 1997). The dominance of annelids was a consequence of the substratum, composed mainly of fine sands rich in organic matter (Manino and Montagna, 1997). Organic-rich sediments favored dense populations of deposit feeders including Namalycastis terrestris, Mediomastus sp., Nephtys fluviatilis, and Tubificidae. (Glasby, 1999; Beesley et al., 2000; Bemvenuti, 1994; Erséus, 2002). Polychaeta are also known for their tolerance to drastic daily and seasonal changes in environmental characteristics (salinity, temperature, dissolved oxygen), allowing them to reach high abundances in estuaries (Gambi et al, 1997; Dittman, 2000; Rosa Filho et al., 2005). The species richness found in this study is low compared to other estuaries (Dittman, 2000; Currie and Small, 2005; Lu, 2005). This is due to the high physiological stress to which benthic communities are subject in the Amazon estuaries, as a result of certain environmental characteristics prevailing in this area. Table 3: Summary of ANOVA used to investigate spatial and temporal variability of macrobenthic fauna in the Caeté estuary. 1 Zone, 2 Season. Dry Rainy season season CCA case scores M.P.D. % Gravel 1.5 Salinity % O.M. Axis Sea % Sand Vector scaling: 5,42 Sea % Sand Axis 1 CCA case scores Selection River River U M L Axis M.P.D. Salinity % Gravel -2.2 % O.M Selection Vector scaling: 4,86 Axis 1 Figure 2: CCA ordination plots. The first two axis explained 70.23% and 76.45% of variation in the dry season and in the rainy season respectively. The coast of Pará is located in an area of semi-diurnal macrotidal regime, with a maximum tide range of 6.0 m, which exposes the substrate to air twice a day for periods of up to six hours, causing abrupt changes in physical and chemical characteristics of the substrate; and because of its geographical position close to the equator, in the course of the year, water salinity may change from 0-5 during the rainy season (December to May) to in the remaining months of the year (dry season) (Flambor, 1988; Rosa Filho et al., 2005). Source of Abundance Richness Diversity Eveness change F P F p F p F p x

4 Spatio-temporal changes in benthic communities in an Amazon estuary Tabela 4: Summary of CCA results. The bold numbers indicate variables with highest loads at each axis. Dry season Wet season Axis 1 Axis 2 Axis 1 Axis 2 Eigenvalues 0,44 0,26 0,40 0,12 Cum.Constr.Percentage 44,17 70,23 58,76 76,45 Factor load Dry season Wet season Axis 1 Axis 2 Axis 1 Axis 2 Salinity -84,5-16,9-84,6-16,5 Mean particle diameter 50,5-14,4 39,7-27,2 Sorting 21,7-40,5 22,0-30,5 % gravel 1,7-20,2-28,1 48,6 % sand -47,4 36,1-35,5 39,0 % organic matter 35,8-36,0 45,9-18,7 Salinity was the most important environmental factor in the establishment of the macrobenthic community in the Caeté estuary. Kinne (1971) mentioned the salt content in water as the most influential environmental characteristic in structuring estuarine benthic faunas. Generally, the decrease in river inflow and the proximity to the ocean have a favorable effect on most macrofaunal taxa, increasing their density and diversity. We observed the formation of a gradient of organism distribution, following the salinity gradient. The genus Nephtys clearly demonstrated this salinity-dependent distribution pattern. Nephtys fluviatilis and N. simoni, although of the same genus, have different salinity preferences. The former species is able to survive in places where the salinity is near zero (Bemvenuti, 1994), and the latter is marine (Lana, 1986). In the rainy season, N. fluviatilis occurred from the upper to the middle estuary, whereas N. simoni reached its greatest abundance in the lower estuary. In the dry season, with high salinities along the entire estuary, N. simoni had high densities throughout the area, and abundances of N. fluviatilis decreased toward the estuary mouth. In the Caeté estuary, even the sandy sediments are composed mainly of fine grains, explaining the higher density, richness, and diversity of their fauna. Similar pattern were demonstred by Netto and Lana (1995) that have shown that substrata rich in fine particles (silt and clay) are favorable for benthic organisms, because they contain, among other advantages, a high proportion of organic matter (which can be used as food by deposit feeders); are associated with low-hydrodynamic environments; and are generally soft, favoring the movements of infaunal organisms. Currie and Small (2006) and Shin and Thompsom (1982) stated that the great complexity of sandy substrata and the difficulty of colonization of muddy sediments due to their fluidity, and the risk of clogging siphons and gills, reduce benthic diversity in sediments with more than 80% silt and clay. Sediment characteristics, mainly grain size, also strongly influenced the macrobenthos distribution. Since Thorson (1957) described the concept of parallel level-bottom communities, observing that differences in species composition between communities were related to the sediments that they inhabit, many investigators have demonstrated that sediment characteristics are key factors in determining and controlling benthic communities in marine and estuarine environments (e.g. Bacescu 1972, Snelglove and Buttman 1994, McLusky and Elliott, 2004). Table 3: Summary of ANOSIM (r-values) used to investigate spatial and temporal variability of macrobenthic fauna in the Caeté estuary. Asterisk denotes significant result Dry season Rainy season UE ME LE UE ME LE UE - 0,114 * 0,469 * - 0,287 * 0,401 * ME 0,114 * - 0,217 * 0,287 * - 0,066 * LE 0,469 * 0,217 * - 0,401 * 0,066 * - Along the Caeté estuary in this study, the highest densities were recorded in sandy sediments. Currie and Small (2006) and Shin e Thompsom (1982) stated that the great complexity of sandy substrata and the difficulty of colonization of muddy sediments due to their fluidity, and the risk of clogging siphons and gills, reduce benthic diversity in sediments with more than 80% silt and clay. Conversely, other studies have shown that substrata rich in fine particles (silt and clay) are favorable for benthic organisms, because they contain, among other advantages, a high proportion of organic matter (which can be used as food by deposit feeders); are associated with low-hydrodynamic environments; and are generally soft, favoring the movements of infaunal organisms (Levinton, 2001; Pejrüp 1988; Litle 2000). In the Caeté estuary, even the sandy sediments are composed mainly of fine grains, explaining the higher density, richness, and diversity of their fauna. There were significant differences in communities between the dry and rainy seasons. Generally, abundance, richness, and diversity were higher in the dry season. In contrast to temperate estuaries, where changes in the structure of benthic communities result predominantly from modifications in temperature and photoperiod (Neves et al., 2007), on the Amazon coast the major factor responsible for temporal variations in biotic communities is the low salinity in the rainy months. Similar results were obtained by Ansell et al. (1972) in India, who reported that large changes observed in the benthic fauna were related to the high rainfall during the monsoon months. In the Salgado region, where the Caeté estuary is located, the maximum rainfall occurs between January and June and may reach up to 1000 mm.month -1 (Moraes et al., 2005). This abundance of rain causes strong reductions of salinity, leading to the restriction or exclusion of strictly marine species or of species that are poorly adapted to low salinities (Kinne, 1971). CONCLUSION The macrofauna in the Caeté estuary is composed of typically estuarine species, with a predominance of annelids. Salinity is the most important environmental factor in the establishment of the macrobenthic community, and we observed the formation of a gradient of organism distribution, following the salinity gradient. Abundance, richness, and diversity were higher in the dry season. Along estuary, in the dry season highest richness, abundance diversity and eveness were higher in the middle estuary, and in the rainy seasons all descriptors increase down estuary. ACKNOLEDGEMENT This work was funded by the National Council for Scientific and Technological Development (CNPq), Brazil - Institutes of the Millennium Programme (Milênio RECOS). We are very grateful to Afonso Quaresma de Lima and Wagner Alberto Ramos Vieira, who assisted in the field work and to Dr. Janet Reid for the English revision of the original manuscript.. 443

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