Effects of a bloom of Planktothrix rubescens on the fish community of a Spanish reservoir

Abstract

During the spring of 2000 and 2001 there were two cyanobacterial bloom episodes in El Atazar reservoir (central Spain). They were attributed to the toxic cyanobacteria Planktothrix rubescens. The goal of the present study was to determine the possible effects and toxicity of these blooms on the fish community of the reservoir. The main objectives were to analyse the presence and concentration of toxins in fish liver and to carry out histopathological analyses of liver tissue. In addition, recent data on water quality of the reservoir were inspected to ascertain the likely causes of the blooms. The analysis of water quality data and weather conditions showed that there was not a single key cause of the P. rubescens bloom. Toxin analyses showed that microcystins were absent or in very low concentrations in fish liver extracts obtained from the reservoir. This finding should not be considered as definitive since liver pathologies were detected in the fish studied, which were probably due to exposure to hepatotoxins. Histological analyses revealed the occurrence of pigmented macrophage aggregates (MAs) in the liver sections of nearly all fish exposed to the bloom. MAs are known to change in number, size and pigment content in relation to fish health and environmental degradation. The average MAs per specimen showed a great variability among samples, and their distribution within the liver tissue was highly heterogeneous. However, further research is needed to give an insight into the pathogenesis for the cyanobacteria-associated liver pathology.