Multispecies algal bioassay to study the contamination of coastal waters in the estuarine mixing area

Coastal marine environments are likely to be the ultimate sink for the anthropogenic pollutants, deliberately or purposely discharged into the environment. While the river inputs are recognized as the major contamination source of the coastal systems, marine and freshwater quality studies are separately undertaken, according to a traditional dichotomy existing between methodological approaches in water quality assessment. Currently separated methods are possibly limiting the correct river-water management, which should be better integrating the information from land-based impact studies of coastal marine waters. Biological assays simultaneously investigating the responses of marine and fresh-water organisms are rarely used as toxicity or trophic assessment methods to study the contamination of river-coastal water systems. Particularly, estuarine mixing waters, where major physicochemical changes can likely affect the bioavailability of pollutants, are most difficult to be tested by biological assays because it should require the use of euryhaline organisms capable of tolerating the entire range of estuarine salinities, from fresh to marine. The aim of this work is to evaluate the suitability of three selected algal species to be simultaneously used as test organisms in biological assays to investigate the trophic contamination over the entire salinity gradient from 0 to 37 psu, linking the algal responses to the river, estuarine and marine water trophic status. While the joint use of two species, the freshwater {Pseudokirchneriella subcapitata} and the marine (15 to 37 psu) {Phaeodactylum tricornutum} was previously applied to the study of the trophic potential of the Po River - Adriatic Sea system, the suitability of {Dunaliella tertiolecta}, as a further, more tolerant species, was evaluated to cover the estuarine mixing area in the salinity range 6 to 15 psu. Like the other two species, {Dunaliella}, which is a motile green alga, is a fast-growing unicellular species particularly easy to cultivate and to count. By contrast, its large difference in cell biomass makes its growth potential, in terms of cell number, hardly comparable within the multispecies assay. The use of a Multisizer electronic particle counter coupled to the gravimetric analysis of the dried biomass of the three algal species enabled to calculate the specific yield conversion factor starting from the cell number. The responses to laboratory enrichment bioassays performed using the three species showed a good correlation linking phosphorus concentration to biomass, revealing the same yield of all species per phosphorus unit. The joint use of the three species revealed effective, enabling to define the trophic gradient over the entire salinity mixing (0 to 35 psu) of river with seawaters in the Po - Adriatic estuarine area.