|1. Phytoplankton assemblages in the open ocean are usually assumed to be mixed on local scales
unless large semi-permanent density discontinuities separating water masses are present. Recent modelling
studies have, however, suggested that ephemeral submesoscale oceanographic features leading
to only subtle density discontinuities may be important for controlling phytoplankton alpha- and betadiversity
patterns. Until now, no empirical evidence has been presented to support this hypothesis.
2. Using hydrographic and taxonomic composition data collected near Iceland during the period of
the 2008 spring bloom, we show that the distribution of phytoplankton alpha- and beta-diversity
was related to submesoscale heterogeneity in oceanographic conditions. Distinct phytoplankton communities
as well as differences in richness were identified on either side of a front delimiting surface
waters of slightly different (~0.03) salinities.
3. Alpha-diversity was significantly higher on the high salinity side of the front compared to the
low salinity side. This difference was primarily driven by the presence of several large diatom species
in the high salinity region, especially of the genus Chaetoceros which dominated the biomass
here. By investigating beta-diversity in relation to environmental and spatiotemporal variables, we
show that the regional distribution of phytoplankton taxa was influenced by both different environmental
conditions on either side of the front and dispersal limitation across the front. Changes in
beta-diversity were primarily driven by turnover rather than nestedness and were apparently controlled
by different processes in each region.
4. Synthesis. This study shows that small-scale and ephemeral density discontinuities created by submesoscale
frontal dynamics can play a major role in structuring patterns of phytoplankton diversity.
Evidence is presented that they can generate changes in environmental conditions (leading to environmental
filtering) and act as physical (dispersal) barriers for phytoplankton transport. The study
suggests that dispersal barriers are potentially of much greater importance for phytoplankton diversity
at local scales than currently recognized and indicates that drivers of marine phytoplankton
diversity are similar to those structuring diversity of land plants.|