Physiological responses and scope for growth of Ruditapes decussatus from Ria Formosa, southern Portugal, exposed to increased ambient ammonia

Abstract

In this study we investigate the effects of exposure to increased ambient ammonia-N concentrations as a cause of decreased scope for growth and mortality of R.decussatus during the summer months in Ria Formosa, southern Portugal. The physiological measurements were performed at 20 ºC and average total ammonia-N concentrations (TA-N) were 9, 48, 85, 134 and 260 µmol l^-1 (corresponding to 0.2, 1.1, 2.0, 3.2 and 6.2 µmol l^-1 NH₃). No mortality was observed. Our results show that during short term exposure to TA-N concentrations >48 µmol l^-1 (1.1 µmol NH₃), R.decussautus survives at the expense of energy reserves. No energy is available for growth and reproduction. Though it is uncertain how intermittent exposures during tidal cycles will affect the long-term survival of R.decussatus in the Ria Formosa, high ammonia concentrations together with unfavourable physical and chemical summer gradients in the sediment can clearly contribute to the high mortality that has been registered in recent years (Ferreira et al. 1989). The sensitivity of the physiological energetics approach and the need to assess tolerance of infaunal bivalves addressing endpoints that can provide insight into the condition for growth and reproduction of the population is stressed.