Implications of Paralia sulcata abundance in Scottish isolation basins

Abstract

Diatom assemblages from eight basins of northwest Scotland, isolated from the sea during the Late Devensian and Holocene, show Paralia sulcata abundant towards the ingression and isolation contacts of the sedimentary sequences. Across the contacts, Paralia sulcata is commonly replaced by, or associated with, salt-tolerant freshwater species, in particular Fragilaria spp. Such a Paralia sulcata-Fragilaria spp. transition indicates a gradual reduction in marine influence and highly fluctuating water salinities. Optimal growth of Paralia sulcata occurred as a result of the favoured conditions in isolation basins, under which many other marine and brackish taxa were disadvantaged. Notwithstanding the typical transition, there are other smaller variations in the diatom assemblages, reflecting morphological features and aquatic and saltmarsh vegetation development. Most importantly. Paradia sulcata was abundant during warmer periods and disappeared when the nutrient supply, water chemistry, vegetation and sediment characteristics were altered under colder climates.