M. L. Chaves

Seasonal and spatial patterns of distribution of subtidal benthic invertebrate communities in the Mondego River, Portugal : a poikilohaline estuary

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 classi- fication 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.

Use of multimetric indices to classify estuaries with different hydromorphological characteristics and different levels of human pressure.

The assessment of estuaries based on benthic communities is widely used to determine impacts caused by human pressure and is one of the required tools for the implementation of the European Water Framework Directive (WFD). Our study compared multimetric approaches (B-IBI and TICOR) to assess the benthic condition of three Portuguese estuaries (Mondego, Tejo, and Mira rivers) with different levels of natural and human induced stress. Benthic community condition was classified into quality status categories of the WFD and compared for consistency with a priori status categories based upon physical-chemical criteria. Both multimetric indices discriminated equally well between locations classified above or below the good status category but were unable to provide good separation between other quality classes (high/good, moderate, poor/bad). Metrics included in these indices are greatly affected by natural stress and we recommend the development of habitat-specific thresholds to increase the discriminatory ability of any benthic condition index.

Taxonomic sufficiency as a useful tool for typology in a poikilohaline estuary

Taxonomic sufficiency has been used mainly to assess benthic condition, based on the assumption that taxa can be identified to a taxonomic level higher than the species level without losing the ability to detect changes related to pollution stress. Identifying taxa to a higher level reduces the expertise and time needed to identify organisms and consequently allows increased spatial and temporal replication. The usefulness of taxonomic sufficiency for typology (identification of water body types) was examined using the benthic communities of the Mondego River estuary (Portugal). Benthic samples were collected seasonally along the Northern branch of the Mondego River estuary from July 2000 to June 2001 and several environmental parameters were measured simultaneously. Cluster analysis of species data indicated three major ecological groups, mainly related to a saline gradient along the estuary. The same groups were found when taxa were aggregated to higher taxonomic levels (genus, family, order, class), except for the phylum level. The overall spatial pattern was driven by: (1) the dominance of bivalves and the occurrence of rare marine species in the Lower Estuary; (2) the dominance of polychaetes in the Middle Estuary; (3) and the dominance of arthropods in the Upper Estuary. The ability of different taxocenes to discriminate the three ecological groups was also examined. Mollusca and Bivalvia were the only taxocenes producing the same groupings, although other taxocenes (Annelida, Polychaeta, Spionidae, Arthropoda) showed a significant ability to discriminate between all three groups. Compared to using all taxa identified to the lowest possible taxonomic level, our results indicate that for typology (1) several higher taxonomic levels were sufficient (2) while few taxocenes alone were sufficient.

Impact of freshwater inputs on the spatial structure of benthic macroinvertebrate communities in two landlocked coastal lagoons

Landlocked lagoons are naturally stressed environments. They are strongly influenced by freshwater input which not only varies naturally, but which is also impacted by anthropogenic activities. This study investigated the direct influence of freshwater discharges on the distribution patterns and abundance of benthic communities in two neighbouring landlocked coastal lagoons, assessing the whole system and the confluence area of each tributary. Sampling occurred in the wet and dry seasons of 2011 at two distances from freshwater discharge locations. Both lagoons were colonized by species from two different pools, freshwater and marine. Freshwater flow rates had a direct influence on the spatial structure of the benthic communities of brackish-water/freshwater interface areas, where also specific taxa can act as early indicators of freshwater input variations. The intensity of this influence is highly dependent on lagoon size, creating spatial heterogeneity or affecting the entire system. The benthic fauna at the confluence of the tributary that depends almost exclusively on groundwater showed the lowest variability, suggesting that the biogeochemical nature of the groundwater may be a central cause for setting specific ecotones. The results suggest that benthic communities of landlocked coastal lagoons can be highly impacted by flow reduction from freshwater aquifers under drought conditions or water abstraction activities.