Assessment of water quality in the Alqueva Reservoir (Portugal) using bioassays

Alqueva Reservoir is the biggest artificial freshwater reservoir in Europe and is an important water supply for human and agricultural consumption in the Alentejo region (Portugal). Pollution can impair environmental and human health status, and to assure water quality and ecological balance, it is crucial to frequently monitor water supplies. In this study, we used an ecotoxicological test battery to identify the potential toxicity of water from this reservoir. Water samples from the Alqueva aquatic system were collected bimonthly in 2006 from 11 different water points within the reservoir. Several bioassays were carried out: a 72-h growth test with Pseudokirchneriella subcapitata, a 6-day growth test with Chironomus riparius larvae, and the luminescence inhibition test with Vibrio fischeri (MicrotoxA (R)). Algae growth was significantly inhibited in several sampling points and periods throughout the year, mainly due to the presence of pesticides. Although in some sampling points pesticide concentrations (single and sum) were still below the maximum permissible concentrations, water samples showed high toxicities to algae, especially during the summer months. In addition, several sampling points showed pesticide concentrations above the permissible level which can pose a significant risk to humans and the environment. Chironomids showed less sensitivity to the water samples, possibly due to the low concentrations of insecticides present. V. fischeri showed no sensitivity when exposed to all the water samples collected throughout the year of 2006. Standardized laboratory bioassays can be useful tools to assess water quality from aquatic systems and can valuably complement chemical analysis evaluation. The results obtained in this study demonstrated that the most sensitive species used in this test battery was the microalgae P. subcapitata. The growth of C. riparius was less affected, which is probably due to the fact that low insecticide concentrations were measured and, furthermore, since this species lives in the sediment and not in the water column and is, therefore, usually more resistant to pollutants. On its own, chemical analysis is not enough to derive conclusions on the water quality and/or status, which can be valuably complemented by laboratory bioassays. Single chemical, maximum permissible values, and the sum of pesticide concentrations do not take into account possible patterns of synergism, antagonism, dose level dependencies, or even the dominance of several chemicals within a mixture. In addition, several species from different levels in trophic chains are recommended due to differences in species' sensitivities to chemical compounds that are present.