Impact of an acid, polyhumic river on estuarine zoobenthos and vegetation in the Baltic Sea, Finland

The nutrient-rich, polyhumic River Kyrönjoki, which flows into the low-saline Bothnian Bay, receives a highly acidic runoff from the sulphureous soils common on the west coast of Finland. Due to this runoff, the pH values in the inner estuary remain at between 4 and 5 during the acid periods, which usually occur in spring and autumn. During these pH depressions the concentrations of monomeric inorganic Al are very high in the lower reaches of the river and the inner estuary, ranging from 200 to 2300 μg/1. The inner estuary, including the most acidic areas, supports a luxuriant aquatic vegetation mainly consisting of large helophytes such as Phragmites australis, Schoenoplectus lacustris and the water lilies Nymphaea candida and Nuphar lutea. Some other species are also common, e.g. Warnstorfia mosses, with Utricularia fairly abundant in the shallow, acidic areas. It is concluded that the high nutrient status, particularly with regard to phosphorus, probably reduces the toxic effect of monomeric inorganic aluminium and other metals on the vegetation in the inner estuary. The factors leading to an absence of some common species of freshwater plants in the inner estuary are discussed. Benthic invertebrates were studied in various areas of the inner, middle and outer estuary. Two water mites, Unionicola aculeata and Arrenurus sinuator, are new species to the Finnish fauna, and several species of water mites and chironomids are reported for the first time in the Baltic Sea. The inner estuary harboured a badly depleted fauna and a low biomass of both macrobenthos and meiobenthos. Due to the low salinity in the middle estuary, the shallow water communities are mainly composed of freshwater species, many of which, e.g. all the gastropods, mussels, leeches and mayflies were absent from the inner estuary. It is concluded that the periodic acidity of the river water and interrelated factors, especially the high concentrations of monomeric inorganic Al, are mainly responsible for the poverty of the fauna in the inner estuary, although it is evident that the high concentration of humus in the river water serves to lessen the toxicity of the aluminium