Phytoplankton dynamics in contrasting early stage North Atlantic spring blooms: composition, succession, and potential drivers

Abstract

Abstract. The spring bloom is a key annual event in the phenology of pelagic ecosystems, making a major contribution to the oceanic biological carbon pump through the production and export of organic carbon. However, there is little consensus as to the main drivers of spring bloom formation, exacerbated by a lack of in situ observations of the phytoplankton community composition and its evolution during this critical period.We investigated the dynamics of the phytoplankton community structure at two contrasting sites in the Iceland and Norwegian basins during the early stage (25 March-25 April) of the 2012 North Atlantic spring bloom. The plankton composition and characteristics of the initial stages of the bloom were markedly different between the two basins. The Iceland Basin (ICB) appeared well mixed down to > 400 m, yet surface chlorophyll a (0.27-2.2 mg m −3 ) and primary production (0.06-0.66 mmol C m −3 d −1 ) were elevated in the upper 100 m. Although the Norwegian Basin (NWB) had a persistently shallower mixed layer (< 100 m), chlorophyll a (0.58-0.93 mg m −3 ) and primary production (0.08-0.15 mmol C m −3 d −1 ) remained lower than in the ICB, with picoplankton (< 2 µm) dominating chlorophyll a biomass. The ICB phytoplankton composition appeared primarily driven by the physicochemical environment, with periodic events of increased mixing restricting further increases in biomass. In contrast, the NWB phytoplankton community was potentially limited by physicochemical and/or biological factors such as grazing.Diatoms dominated the ICB, with the genus Chaetoceros (1-166 cells mL −1 ) being succeeded by Pseudo-nitzschia (0.2-210 cells mL −1 ). However, large diatoms (> 10 µm) were virtually absent (< 0.5 cells mL −1 ) from the NWB, with only small nano-sized (< 5 µm) diatoms (i.e. Minidiscus spp.) present (101-600 cells mL −1 ). We suggest microzooplankton grazing, potentially coupled with the lack of a seed population of bloom-forming diatoms, was restricting diatom growth in the NWB, and that large diatoms may be absent in NWB spring blooms. Despite both phytoplankton communities being in the early stages of bloom formation, different physicochemical and biological factors controlled bloom formation at the two sites. If these differences in phytoplankton composition persist, the subsequent spring blooms are likely to be significantly different in terms of biogeochemistry and trophic interactions throughout the growth season, with important implications for carbon cycling and organic matter export.

Phytoplankton temporal dynamics in the coastal waters of the north-eastern Adriatic Sea (Mediterranean Sea) from 2010 to 2017

“…This contradicts the general view of the typical spring diatom assemblages often being dominated by large species, such as Pseudo-nitzschia and large Chaetoceros, (e.g. Ribera d'Alcalà et al 2004;Daniels et al 2015). However, Daniels et al (2015), who investigated the dynamics of the phytoplankton community structure at two contrasting sites in the Icelandic and Norwegian basins, found that the typical large diatoms dominated the spring bloom in the Icelandic basin, while very small (even <5 µm) diatoms dominated in the Norwegian basin.…”

Open‐ocean convection process: A driver of the winter nutrient supply and the spring phytoplankton distribution in the Northwestern Mediterranean Sea

“…Nevertheless the large size range of the diatoms, from nanosized to microsized classes, is not taken into account with the method used in our study to determine the phytoplankton community structure [ Uitz et al ., ]. While previous studies in the NWM observed diatoms bloom of the microphytoplankton size class [ Percopo et al ., ; Rigual‐Hernández et al ., ], it is possible that smaller diatoms taxa become dominant like in the North Atlantic spring bloom because of changes in the environmental conditions [ Daniels et al ., ].…”

Seasonal similarity in rates of protistan herbivory in fjords along the Western Antarctic Peninsula

“…Temperature was recorded at 15 min intervals using Hobo data loggers. To recreate the ambient in situ light environment, the deck incubators were covered with an optical filter (light blue #118 LEE™), which was chosen to best represent the light attenuation and spectral quality within the mixed layer by transmitting light in the 400–570 nm range and removing longer wavelengths (Daniels et al and references within). Based on comparisons between the on‐deck incoming irradiance and the light intensity inside the screened incubator measured with a Biospherical QSL‐2101 light meter, the filter reduced the overall light to ca.…”