A field experiment of the physical and chemical effects of two oils on mangroves

The effects of two different types and five levels of oil on some physical and chemical characteristics of sediments and plants were investigated in four mangrove species under field conditions. The sediments in which four mangrove species (Rhizophora stylosa, Rhizophora mucronata, Ceriops tagal and Avicennia mar ina) were growing, were treated with Kuwait Crude Oil (KCO) and North West Shelf Condensate (NWSC) at application rates of 0, 1375, 2750, 5500 and 11 000 g m(-2). The effects varied with species. The oil treatments usually increased concentrations of most nutrient elements in a. stylosa and R. mucronata, had mixed effects on C, tagal and mostly decreased concentrations in A. marina. KCO significantly decreased concentrations of every element examined in A. marina. The effects on sediments also varied with oil type. NWSC decreased concentrations of about half of the elements tested in the sediments and had no significant effect the others. In contrast, KCO increased concentrations of about half of the elements tested in the sediments and had no significant effect on most of the others. The concentrations of hydrocarbons in leaves increased with increasing oil application to the sediments, but the effects varied in the different species. A. marina accumulated between two and six times more hydrocarbons in leaves than the other species. The concentration of hydrocarbons in sediments treated with NWSC decreased by about an order of magnitude in the 24 weeks after application, but the rate of decrease was less for KCO than for NWSC. Sediment pH, dissolved oxygen concentration, redox potential and the salinity of interstitial water were decreased by oil treatments, causing the oiled sediments to become poorly oxygenated with strongly reducing conditions. It was concluded that these experimental oil treatments in the field caused changes in sediment characteristics and uptake of oil components by mangroves, depending on the characteristics of the oil and the mangrove species. (C) 1999 Elsevier Science B.V. All rights reserved.