Influence ofSpartina alternifloraon Superficial Sediment Characteristics of Tidal Flats in Paranaguá Bay (South-eastern Brazil)

Abstract

Superficial sediment characteristics of salt marshes and adjacent unvegetated sites were compared in Paranaguá Bay, SE Brazil, in order to assess the influence ofSpartina alterniflora. Twenty stations were sampled along a salinity and environmental gradient, from the inner to the outer bay, to provide a first characterization of sediment distribution along the tidal flats. Paranaguá Bay was then divided into three different sectors according to the hydrodynamic regime and sediment characteristics. The mesohaline sector is characterized by typically fluid sediments, with the predominance of medium silt, and high organic and water contents. Deposits of the middle polyhaline sector are poorly sorted, very fine sands, with variable organic matter content. Sediments in the higher-energy euhaline sector are well-sorted fine sands, with low organic content. Four sampling sites were chosen in different sectors to assess whetherS. alternifloramarshes can introduce differences in sediment texture along spatial and seasonal gradients. Differences in sediment parameters between marshes and unvegetated tidal flats are not evident in the inner sector, but are increasingly significant towards the high-energy euhaline sector. With the exception of water and organic content, the latter usually higher in summer, seasonal differences in sediment parameters are not significant. The results suggest that differences between the sediments of salt marshes and adjacent unvegetated tidal flats are positively correlated with the environmental energy gradient along Paranaguá Bay. Sand unavailability and high sedimentation rates lead to a marked sedimentological homogeneity between vegetated and unvegetated flats in the upper reaches of the bay. On the other hand,S. alterniflorais a major source of sediment heterogeneity between vegetated and unvegetated sites in the high-energy euhaline sector, characterized by more mature sediments.