Biocoenosis and thanatocoenosis of diatoms in a western Galician ria

Abstract

Seasonal variability of diatom abundances was measured in the water column, sediment traps and in the underlying sediment from the Pontevedra ría. Seasonal variations of the diatom species in the water column were indicative of ocean influence, river influx and water column stratification. Additionally, analysis of diatom distribution and other biosiliceous components was studied in 27 surface sediment samples. The marine planktonic assemblage in the surface sediments is located in the outer area of the ría. It is mainly composed by Chaetoceros resting spores (R.S.) together with Thalassionema nitzschioides, Leptocylindrus danicus R.S. and Paralia sulcata. Higher abundances of freshwater assemblages are limited to the inner ría, as well as crysophycean cysts and phytoliths, where the Lérez River has its strongest influence. The benthic group is also restricted to the innermost and shallow areas where shallow water depths allow for growth. Factor analysis distinguished between estuarine-freshwater and ocean-dominated upwelling areas in the Pontevedra ría. A mixed area where both processes take place was also discriminated. A good agreement between the biocoenotic and the thanatocoenotic diatom community was found. Despite the fact that the low preservation of some species obscures the interpretation of the sedimentary record of biogenic silica flux and accumulation, the results are useful in the reconstruction of hydrodynamics and production in the marine paleo-records in the ría and estuarine domains. This is even though the low preservation of some species results in difficulties interpreting the sedimentary record. Biological response, production patterns and diatom succession related with ecological preferences in this ría can also be applied to other rías and temperate coastal environments.