Seasonal and Spatial Patterns of Distribution of Subtidal Benthic Invertebrate Communities in the Mondego River, Portugal – A Poikilohaline Estuary

Abstract

The use of benthic assemblages to assess ecological quality of estuarine environments is a major tool for the implementation of the Water Framework Directive (2000/60/CE) for European aquatic ecosystems. Benthic communities show spatially heterogeneous distributions and experience seasonal variations due to both natural and anthropogenic stresses. The major goal of this study was to quantify the relationships between environmental gradients and the spatial and temporal patterns of the benthic communities along a Portuguese estuary. Seasonal and spatial variations relating macrobenthic communities and measures of water and sediment quality along the northern branch of the Mondego River estuary were examined at seven sampling stations from July 2000 to June 2001. Cluster analysis of biological data indicated three major groups of communities based on spatial distribution patterns: (1) a lower sector with stronger marine influence and dominated by Streblospio shrubsolii and Cerastoderma glaucum; (2) a middle sector with dominance of S. shrubsolii and Corophium multisetosum; (3) and an upper sector where C. multisetosum dominates a community characterized by a lower number of species. Canonical correspondence analyses of biological and environmental data determined a major salinity gradient influencing benthic communities. Seasonal changes of benthic communities were mainly determined by freshwater input and salinity changes that imposed a strong decrease in densities and number of species during winter, followed by a recovery during spring. Benthic ecological structure and contaminant levels indicated that the Mondego northern branch is moderately disturbed, although opportunistic species dominated the benthic community, suggesting that natural and anthopogenic sources of stress may be acting together. The Mondego River estuary, a poikilohaline-type estuary, characterized by strong seasonal changes in water flow and salinity, cannot be consistently stratified into salinity regions based upon the Venice classification system. Biotic communities, exemplified here by the benthic communities, are seasonally displaced, compared to a homiohaline-type estuary where the Venice system can be applied without modification. Future identification of reference conditions and design of monitoring programs cannot be accomplished without understanding how interactions between biotic and physico-chemical dynamics differ between homiohaline and poikilohaline estuaries. Results obtained in this study could be used to assist future assessments in other Portuguese estuaries.

Factors affecting the distribution of epibenthic biodiversity in the Cávado estuary (NW Portugal)

“…Winter and spring were the wettest seasons compared to summer, which was generally drier. This follows the pattern of most temperate estuaries, and is characterised by having a significant influx of freshwater during the winter, which has an impact on salinity variation (Elliott & McLusky 2002;Chainho et al 2006;Teixeira et al 2008). The presence of gravel at upstream stations and the accumulation of muddy sediments in the stations near of mouth of Cávado estuary detected in the present study has also been observed in other Portuguese estuaries Chainho et al 2007;Sousa et al 2008b;Teixeira et al 2008).…”

“…The sediment type was also important in spatial distribution of epibenthic communities, being the abundance, biomass and the diversity indexes better correlated with the presence of fine sediment (<0.125mm). The differences found were expected since the hydrodynamic fluctuations have an important effect on the cycles of sediment erosion and deposition and, hence, on the colonisation of benthic communities (Chainho et al 2006).…”

The effect of high flow events on mussels (Mytilus edulis) in the Conwy estuary, North Wales, UK

“…The response of an estuary to these events can reveal a great deal about its resilience and the role of disturbance in shaping its structure and function (Davis et al, 2004). In addition, an understanding of the effects of natural events on estuarine organisms is a basic tool in discriminating between natural changes and human disturbances (Chainho et al, 2006).…”

“…However, less is understood about their effects in estuaries (Matthews & Constable, 2004). The use of benthic invertebrate assemblages to monitor the ecological quality of estuaries is a major tool in the implementation of the Water Framework Directive (2000/60/CE) but it is vital to identify the impact of extreme natural events so that they can be discriminated from human impacts (Chainho et al, 2006). It was predicted that the high flow events would cause changes in the diversity of the associated epi-and in-fauna of the estuarine mussel beds due to mortality of more sensitive species, colonisation by opportunistic species and/or physical transportation of species from further up the estuary (e.g.…”

Spatial variation of intertidal macroinvertebrates and environmental variables in Robbins Passage wetlands, NW Tasmania

“…Many studies have investigated the distribution (Chainho et al, 2006;Rodrigues et al, 2006), abundance Dittman, 2002b), diversity (Boehs et al, 2004) and biomass (Masero et al, 1999;Silva et al, 2006) of invertebrates on intertidal flats. There have also been many studies that examine the effect of environmental variables on invertebrate abundance and community composition (Anderson, 1972;Castel et al, 1989;Edgar & Barrett, 2002).…”