The diatom Paralia sulcata as an environmental indicator species in coastal sediments

Paralia sulcata is a diatom commonly found in both the plankton and benthos of coastal environments. This species is heavily silicified and thus preserves well in sedimentary records, making it a potentially useful paleoindicator species. However, its tychopelagic nature and association with a wide range of environmental conditions have made detailed paleoecological interpretations complicated. The abundance and size of P. sulcata varies in sediments along the Swedish west coast. While most cells have diameters from 12 to 18 lm, cells greater than 20 lm can constitute over 50% of the population. High-resolution stratigraphies in sediment cores from two Swedish fjords reveal changes in the abundance and valve diameter of P. sulcata over a 200 year time span. Paralia sulcata is abundant in non-laminated sediment sequences. Cells with large diameters (>20 lm) are however more common in laminated sections. The connections between environmental parameters and P. sulcata abundance and size were further investigated from surface sediments along the Swedish west coast. Multivariate analyses highlight the importance of surface water properties and water column stability in the abundance and size distribution of P. sulcata along the coast. Water columns that are vertically mixed have more upwelling of nutrient-rich, high salinity water to the surface; and these conditions result in large populations of small-sized P. sulcata. In contrast, stratified water columns with low surface water salinity and low phosphorus concentrations show reduced quantities of P. sulcata, but a higher percentage of cells greater than 20 lm. These results help clarify the potential of P. sulcata as a paleoindicator species in coastal environments.