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Abstract: The phytoplankton and zooplankton assemblages in the Guadiana estuary (south Portugal) were compared in two contrasting hydrologic years, characterized by high and low freshwater discharges. Changes in salinity, nutrients and turbidity, which were mainly due to the changes in freshwater input, had an important influence on the structure of the planktonic assemblages. Nevertheless, coastal upwelling can also influence the productivity of the lower estuarine area during short periods both in winter and in summer. In 2000, a short-term freshwater pulse during May (average 126.46m3 s−1) was markedly beneficial; phytoplanktonic diversity increased, and this caused an increase in zooplanktonic diversity. As phyto-zooplankton coupling in these systems is an important regulator of processes in the trophic web, changes in the dominant groups can have consequences on water quality, especially on the occurrence of toxic plankton blooms. Modelling simulations indicate that a day discharge of 50m3 s−1, every week during critical periods, such as summer/autumn, could be enough to avoid cyanobacteria blooms. Since freshwater “pulses” can be managed by operating the freshwater release from hydrotechnical structures, understanding the relationships between the periodicity and magnitude of inflow pulse events and the estuarine ecosystem structure and healthy functioning is a crucial step towards the development of management modelling tools, as proposed in this study. [Copyright &y& Elsevier]

Abstract: The objective of this study was to evaluate how inter-annual changes in the volume of freshwater input and water parameters (salinity, temperature, major dissolved nutrients, seston and chlorophyll a) affect fish assemblages in the Guadiana estuary (South Portugal). During the sampling period (two distinct hydrological years), 56 fish species were identified. Anchovies (Engraulis encrasicolus) and barbells (Barbus species) dominated the abundances in the high inflow year (2001), but Pomatoschistus species were the most important taxa in the low inflow year (2002). Barbells and Portuguese toadfish (Halobatrachus didactylus) dominated the biomass in both years under different inflow conditions, but a reduction in barbells'' biomass occurred during the low inflow year. Multivariate analysis indicated a persistent spatial structuring of the estuarine community for both years and in different seasonal periods. Changes in salinity and seston, which were mainly due to changes in freshwater input, had an important influence on the structure of the fish assemblages. In 2002, the increased salinity in the upper estuary allowed colonization by marine species of an area that usually contains freshwater, decreasing even more the habitat for indigenous freshwater species in the downstream area of the Guadiana River. There was also a decrease in the abundances of planktivorous and omnivorous fishes and an increase in carnivorous fishes during the low inflow year. As fishes in these systems are important regulators of processes in the trophic web, changes in the dominant feeding groups can have consequences on water quality, particularly in relation to the occurrence of plankton blooms. [Copyright &y& Elsevier]

Responses of the clam Spisula solida to stress imposed by dredging were analyzed in terms of changes in chronic indices of biochemical conditions (RNA/DNA ratio and neutral/polar (N/P) lipid ratio). Cumulative stress on undersized (<25 mm) S. solida from repeated habitat disturbance by dredging was simulated in the laboratory and measured with in situ studies off the southern coast of Portugal, in April and July 1999. Laboratory simulations on undersized bivalves indicated decreases in RNA/DNA and N/P lipid ratios. Responses were sublethal; however, even though survival was not directly threatened, decreases in condition suggest that bivalves are more susceptible to predation when they have been left in the seabed after the dredging activity. Moreover, the in situ study revealed that this effect could be especially critical during spawning. [Copyright &y& Elsevier]

Anthropogenic impact can alter food web structure through changes in species interactions. In this study, we explored the food web of three Mediterranean stream reaches (two seasonal and one permanent) along an anthropogenic impact gradient to test the hypothesis that increasing impact simplifies food webs. To test this, we applied the isotopic (δ13C and δ15N) niche concept to compare reaches using isotopic metrics (isotopic richness, divergence, dispersion, evenness, and redundancy). The isotopic indices were useful to identify differences in food web architecture among the three reaches. The least impacted site had the highest isotopic richness, dispersion, and isotopic redundancy, suggesting higher ecological resilience at this site. The effect of disturbance in the remaining two sites was masked by the presence of invasive crayfish, which increased isotopic divergence and was responsible for higher food-chain length in the most impacted reach, but not in the moderately impacted reach. Consumers displayed generalistic feeding habits, with Bayesian mixing models indicating that they relied primarily on a mixture of periphyton, other macroinvertebrates, and to a lesser extent, detritus. Some taxa displayed changes in their dietary habitats depending on the site, indicating that the same type of taxa fed on distinct foods at each stream reach. [ABSTRACT FROM AUTHOR]