Winter-spring transition in the subarctic Atlantic: microbial response to deep mixing and pre-bloom production

Ml Paulsen,K Riisgaard,Tg Nielsen,Tf Thingstad,M St John

doi:10.3354/ame01767

Ml Paulsen, K Riisgaard + Show 3 more

Open Access

https://doi.org/10.3354/ame01767

Copy DOI

Abstract

In temperate, subpolar and polar marine systems, the classical perception is that diatoms initiate the spring bloom and thereby mark the beginning of the productive season. Contrary to this view, we document an active microbial food web dominated by picoand nanoplankton prior to the diatom bloom, a period with excess nutrients and deep convection of the water column. During repeated visits to stations in the deep Iceland and Norwegian basins and the shallow Shetland Shelf (26 March to 29 April 2012), we investigated the succession and dynamics of photo synthetic and heterotrophic microorganisms. We observed that the early phytoplankton production was followed by a decrease in the carbon:nitrogen ratio of the dissolved organic matter in the deep mixed stations, an increase in heterotrophic prokaryote (bacteria) abundance and activity (indicated by the high nucleic acid:low nucleic acid bacteria ratio), and an increase in abundance and size of heterotrophic protists. The major chl a contribution in the early winter−spring transition was found in the fraction 50 μm) were stimulated by deep mixing later in the period, while picophytoplankton were unaffected by mixing; both physical and biological reasons for this development are discussed herein.

Highlights

Much of our conceptual understanding of the marine pelagic food web originates from the pioneer work of Sverdrup (1953), Cushing (1959) and Steele (1974)
We observed that the early phytoplankton production was followed by a decrease in the carbon:nitrogen ratio of the dissolved organic matter in the deep mixed stations, an increase in heterotrophic prokaryote abundance and activity, and an increase in abundance and size of heterotrophic protists
The dominating water masses at each of the 3 localities remained consistent throughout the period

Summary

Introduction

Much of our conceptual understanding of the marine pelagic food web originates from the pioneer work of Sverdrup (1953), Cushing (1959) and Steele (1974). The importance of the heterotrophic components of the microbial loop became recognised (Williams 1981); the role of photosynthetic picophytoplankton in northern ecosystems still received little attention. This was due to the fact that sampling efforts traditionally have been focused on the spring bloom period because the new production of larger-celled species in this period has a strong link to mesozooplankton and fish production (Sverdrup 1953, Steele 1974, Braarud & Nygaard 1978). The microbial food web, including picophytoplankton, has received more attention in recent years in northern systems (Søndergaard et al 1991, Joint et al 1993, Sherr et al 2003, Irigoien et al 2005, Tremblay et al 2009, Seuthe et al 2011a,b)

Methods

Results

Conclusion