DOI: 10.5061/dryad.n0066
title: Data from: Evidence of small-scale spatial structuring of phytoplankton alpha- and beta-diversity in the open ocean

description: 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 beta-diversity patterns. Until now,
 no empirical evidence has been presented to support this hypothesis. 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. 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. 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.
description: nab2008_species_dataPhytoplankton taxonomic data from 30 samples collected in the North Atlantic Ocean, south of Iceland, in spring 2007. Samples were preserved with acidified Lugol’s solution (approximately 2% final concentration). Phytoplankton taxa were determined to the lowest taxonomic level at which an accurate identification could be made using light microscopy and identified by Orbicon A/S, Aarhus, Denmark. Enumeration and calculation of carbon content followed the protocol used in the Danish National Water and Nature Monitoring Program. Axial dimensions of a subset of each taxon in each sample were measured and used to calculate the biovolume following standard taxon (group) specific equations. Biovolume was then converted to carbon equivalents using group specific conversions.