Springer 2006
Hydrobiologia (2006) 559:135–148 DOI 10.1007/s10750-005-1371-2
Primary Research Paper
Spatial subtidal macrobenthic distribution in relation to abiotic conditions in the Lima estuary, NW of Portugal Ronaldo Sousa1,2,*, Se´rgia Dias1 & Jose´ Carlos Antunes1 1
CIIMAR – Centro Interdisciplinar de Investigac¸a˜o Marinha e Ambiental, Rua dos Bragas, 123, 4050-123, Porto, Portugal 2 ICBAS – Instituto de Cieˆncias Biome´dicas de Abel Salazar, Universidade do Porto, Lg. Prof. Abel Salazar 2, 4099-003, Porto, Portugal (*Author for correspondence: E-mail:
[email protected]) Received 5 May 2005; in revised form 27 July 2005; accepted 31 July 2005
Key words: Lima estuary, macrobenthic assemblages, abundance, biomass, diversity, environmental factors
Abstract During the summer of 2002, sampling was carried out in the Lima estuary in order to compare the pattern of the macrobenthic community’s distribution in relation to physical and chemical variables. A total of 54 macrobenthic taxa were identified. Abundance, biomass and specific diversity varied among the twenty stations. Abundance ranged from 212 to 9856 ind./m2, with an average of 1581 ind./m2. Abra alba presented the highest density corresponding to 39.1% of the total specimens gathered, followed by Hediste diversicolor with 31.5%. Biomass ranged from 0.12 to 264.62 g AFDW/m2, with an average of 17.58 g AFDW/m2. Cerastoderma edule and A. alba were the species with a clear predominance in the total biomass, contributing 75.3 and 13.8%, respectively. The multivariate techniques used revealed a macrobenthic community with five distinct groups, particularly related to the sedimentological characteristics and salinity. These results demonstrated significant differences in macrobenthic assemblage’s composition along an estuarine gradient. For the first time the presence of the nonindigenous invasive species Corbicula fluminea was described in this estuary.
Introduction Estuaries are normally recognized as transitional areas of distinct biological importance, where salinity varies between fresh water and seawater (McLusky, 1989; Little, 2000). These estuarine areas are used temporarily as a nursery ground for many commercially and ecologically important species of marine and migrating fishes, shellfish, crustaceans and shorebirds (Day et al., 1989; Raffaelli, 1999). At the same time these productive and complex estuarine ecosystems are frequently located near large cities and are receptacle of all types of contaminants (Allchin et al., 1999; Baudrimont et al.,
2005; Cave et al., 2005; Cheggour et al., 2005). The Lima estuary is not an exception, and it is also subject to various impacts (mainly in the lower part of the estuary) from harbour activity, constant dredging in the first 3 km, discharge of nutrients and other substances deriving from domestic, industrial and agricultural areas. These types of activities are responsible for modifications in the bathymetry with a consequential hydrodynamic change and also for the increasing eutrophication processes in this estuarine system (Alves, 1996; Sousa, 2003). The understanding of the macrobenthic estuarine species’ distribution relative to their habitat is fundamental to the development of estuarine ecology (Ysebaert et al., 2003). In addition, the
136 benthic macroinvertebrates are one of the most useful bioindicators of possible environmental changes (Warwick et al., 2002; Ysebaert et al., 2002). Since there is a complete lack of quantitative data on the relationship between spatial distribution of macrobenthic assemblages and environmental variables in the Lima estuary reference work was carried out, during the summer of 2002, on the subtidal area. The objectives of this work were to compare the pattern of the macrobenthic community’s distribution in relation to physical and chemical variables, to estimate the abundance and biomass of subtidal macrozoobenthos and to identify possible key species in the functioning of the ecosystem.
Materials and methods Study area The River Lima originates in Serra de Sa˜o Mamede, in the province of Orense–Spain, at about 950 m altitude. It is 108 km long, of which 67 km are located in Portuguese territory and drains ENE–WSW into the Atlantic Ocean. Its hydrological basin is located between 41 35¢ N and 42 15¢ N of latitude and 07 35¢ W and 08 55¢ W of longitude, with an area of 2480 km2, 1303 km2 in Spain (53%) and 1177 km2 in Portugal (47%). The estuary is located in the NW of Portugal (Fig. 1) and the influence of spring tides extends approximately 20 km upstream. The Lima estuary has a maximum width of just over 1 km, being
significantly larger with the flooding of the banks during high tide. This mesotidal estuary is partially mixed, however, during the period of high floods it tends to evolve towards a salt wedge estuary (Alves, 1996). This estuarine ecosystem includes biotypes with mobile and rocky substrata, favourable to the occurrence of various types of organisms. Rocky substrata are, essentially, represented by sea walls in the lower part of the estuary. Biotypes with mobile substrata occur in intertidal and subtidal areas and comprise areas close to the riverbanks, inlets which form small bays and as marsh existing on the various islands found in the lower part of the estuary and on the north bank (Sousa, 2003). The few biological studies carried out in this estuary (performed in the 1980s and the beginning of the 1990s) described the ecology of some species and characterized the main physical and chemical, biological and human processes which modify the structure and dynamic of the biological communities (Fontoura, 1984; Fontoura & Moura, 1984; Guimara˜es & Galhano 1987, 1988, 1989; Valente & Alexandrino, 1988; Da Silva, 1990; Coelho, 1997). Sampling and laboratory analysis Samples were collected in the summer of 2002 in the subtidal areas of the Lima estuary at high tide (Fig. 1). Six replicates per sampling station (one for sediment analysis and five for biological analysis) were gathered with a Van Veen grab with an area of 500 cm2 and a maximum capacity of 5000 cm3.
Figure 1. Map of the Lima estuary showing location of the twenty sampling stations.
137 During fieldwork, the following water column information was collected: temperature, salinity, dissolved oxygen and pH. This information was obtained in situ, close to the bottom, using a multiparametrical sea gauge YSI 820. Particle size and organic content analyses of the sediment at every station were carried out. Sediment granulometry was assessed following drying at 60 C for 72 h. Dried sediment was sieved through a column of sieves corresponding to integer values of the Wentworth scale and the frequency of each class was expressed as the percentage of total weight. The quantity of organic matter contained in the sediment was determined, after combusting for 24 h at 550 C, in a muffle furnace. Values are expressed in percentage, relatively to the weight loss on ignition of each sample analysed. Grab samples were sieved on a 1 mm mesh and fixed with 4% formalin in seawater. Macrofauna was sorted and, whenever possible, identified to species level. Biomass was calculated using the Ash Free Dry Weight Method – AFDW (Kramer et al., 1994).
Data analysis To compare the similarity between stations (data pooled over five biological replicates for each site) in terms of species composition (abundance and biomass), univariate measures and multivariate analyses were applied using the PRIMER package (Clarke & Warwick, 2001). Univariate measures included abundance, biomass, number of species, and diversity (H¢) and evenness (J¢) indices. Classification (CLUSTER) and ordination by nonmetric multidimensional scaling (MDS) based on the Bray–Curtis similarity matrix were used to analyse spatial distribution of benthic assemblages. In order to establish correlations between biological parameters and abiotic characteristics, indeces of biotic and abiotic similarity were compared using PCA (Principal Components Analysis) and BIOENV (using the Spearman coefficient) (Clarke & Ainsworth, 1993). Biological information was previously transformed (square root transformation; abundance per unit of area), however, for the abiotic factors only granulometry and the percentage of organic matter were transformed )log(1+y).
Results The physical and chemical parameters are presented in Table 1. The water temperature was higher in upstream stations and ranged between 17.8 C (station 18) and 13.2 C (stations 5 and 6). Salinity values varied significantly with the distance to the river mouth, with the highest value recorded in station 1 (34.8 psu) and the lowest in station 20 (0.4 psu). The dissolved oxygen values varied between 3.7 mg/l (station 14) and 7.8 mg/l (station 10) and the pH values between 7.0 (station 6) and 8.2 (stations 13 and 14). There was a large variation in the cumulative curves of distribution of the sediment throughout the different sampling stations. The stations located in the river mouth (stations 1, 2, 3 and 4) contained abundant fine matter and the other stations, located upstream, contained coarser sediment with a preponderance of sands and gravels. Organic matter in the sediment ranged from 0.5% (station 19) to 10.8% (station 3). There were large spatial variations, related to the type of substrate existent in the different sampling stations and a significant correlation (r=0.98, p
