Effects of kraft-mill effluents on benthic macrophyte assemblages in a shallow-bay system (Apalachee Bay, North Florida, USA)

Abstract

A 14-month study was carried out to determine the impact of kraft-mill effluents on the offshore benthic macrophyte distribution in a shallow north Florida Bay (USA). A polluted river drainage system was compared to an adjoining (unpolluted) one. The affected area was characterized by elevated levels of color and turbidity. Inshore areas associated with high levels of KME were characterized by severely reduced benthic macrophyte biomass, reduced numbers of species/unit area, and altered species composition when compared to control stations. Areas of chronic impact also had reduced levels of biomass, although the total number of species taken were not significantly different from the controls. In polluted areas, red and brown algae were proportionately more abundant than chlorophytes and spermatophytes. Dominance generally was reduced in areas of the Bay affected by KME. Except for areas of acute impact, there was no significant difference in species diversity (H″) between polluted and unpolluted portions of the Bay. There were uniform increases in the total number of species on either side of the affected drainage system. Such areas were considered to be transition zones between the polluted and unpolluted parts of Apalachee Bay. Benthic macrophyte distribution, in terms of biomass and species (community) composition, was considered an important indicator of the impact of KME on the shallow coastal systems in question. The pattern of macrophyte species composition reflected various water-quality parameters. Species normally inhabiting deeper water were found in areas of increased turbidity and color. It was postulated that reduction of normal dominants such as Thalassia testudinum and Halimeda incrassata allowed the colonization of such areas by opportunistic species. This would explain the maintenance of comparable (total) numbers of species in polluted areas and the lack of effect of KME on species diversity even though biomass was severely restricted. These parameters were analogous to the epibenthic fish distribution in the respective drainage areas. Near-shore coastal systems in Apalachee Bay thus were affected by gradients in water quality in addition to natural (seasonal) fluctuations in key physical and chemical parameters. The benthic plant assemblages reflected variations in dominance, the occurrence of opportunistic species, and ecological diversity that resulted in a continuum of disturbance phenomena ranging from sparsely distributed groups in grossly polluted systems to well developed plant assemblages in areas that remain unaffected by KME.