AQUATIC PLANTS IN RELATION TO ENVIRONMENTAL-FACTORS IN NORTHERN SPAIN

Different statistical techniques were applied in an attempt to relate the physical and chemical variables of river water to the distribution of aquatic plants. The results show a low frequency of occurrence of aquatic plants (only 13 species were found) distributed over 53 sampli ng sites. A few plants were found in river headwaters with pronounced slopes: these had turbulent, non-polluted waters with high oxygen conc entration, but with high mineral content (minimum conductivity value 2 00 mu S/cm). Plants were more abundant and frequent in sites with high organic loads in sampling sites located downstream of a small dam. Th e range of eutrophic conditions or mineral concentration over which pl ants were found was different from other European rivers, suggesting t hat the possible indicative nature of the species may differ. The pres ence of aquatic plants is related to chloride, which favours plant gro wth, and ammonium, which has a negative effect. A characteristic of th e rivers studied, especially in middle and downstream zones, is the hi gh chloride concentration associated with high nutrient content and co rrelated with the occurrence of aquatic macrophytes. However, the conc entration of ammonium, an ion which is derived mostly from wastes (pol lution), is associated with the absence of plants. This may indicate t hat the ammonium concentration is toxic to aquatic plants. In this sen se, the presence of aquatic plants indicates an absence of toxic effec ts from pollution (due to urban and industrial dumping). It is possibl e to relate the presence of aquatic plants to the conditions of eutrop hy and alteration of the environment and, hence, to use these plants, to a limited degree, as indicators of the degree of contamination or d egradation of the rivers of Biscay. The following indicative character for the listed species are proposed: Apium nodiflorum is a species ch aracteristic of zones with high nutrient concentrations; Groenlandia d ensa, Zannichellia palustris and Alisma plantago-aquatica are all indi cative of clean and more or less stagnant waters; the presence of Lemn a minor is generally associated with water of greater depth, less curr ent velocity and high nutrient content, generally places affected by h uman intervention; Veronica beccabunga and Potamogeton pectinatus are characteristic of back-flowing and mesoeutrophic streams. Nasturtium a fficinale in our study inhabits clean and oxygenated waters and would therefore be indicative of such areas. (C) 1996 Academic Press Limited