Composition and origin of benthic flocculent layers in Swedish fjords following the spring bloom - contribution of diatom frustules and resting stages
Mc Quoid, Melissa R.; Nordberg, Kjell
Although diatom resting stages are abundant in coastal sediments, their usefulness as paleoindicators is limited by how little we know about their coupling to the overlying phytoplankton community. This study investigates the taxonomic composition of planktonic diatoms during the spring bloom and in sedimented material from four fjords on the Swedish west coast. After the spring bloom, a large proportion of diatoms sink to the benthos. In some fjords, this material accumulates at the sediment-water interface as a brown flocculent layer. This flocculent "bloom layer" was sampled from sediment cores collected in 1994, 1996, and 2002. Using diatom composition of these bloom layers, comparisons were made contrasting the different fjords, different sampling years, and the sedimented layer versus the overlying phytoplankton community. Flocculent layers consisted mostly of diatom resting stages of Chaetoceros debilis, Chaetoceros diadema, Chaetoceros similis, Detonula confervacea, Skeletonema spp., and Thalassiosira nordenskioeldii. The diatom composition of these layers was highly variable over time, but spatial variation was low especially for neighboring basins. Although relative composition of the flocculent layer was somewhat modified compared to the preceding phytoplankton community, samples from the four fjords in 2002 represent a large-scale spring bloom along the Swedish west coast. Because they occur abundantly in the spring bloom and transport readily to the sediments, spore-forming diatoms are useful indicators of phytoplankton history in coastal sediments.