Spring phytoplankton bloom dynamics in Norwegian coastal waters : Microbial community succession and diversity

International audienceMost studies of spring bloom succession in Norwegian waters have employed light microscopy and accounted for species composition of phyto- and zooplankton. Flow cytometry and molecular tools enable us to extend such investigations to include smaller organisms like bacterio- and virioplankton. Here, we describe succession and diversity of algae, bacteria, and viruses in relation to environmental changes from 15 February to 27 April. The spring succession started with an increase in autotrophic picoeukaryotes and Synechococcus sp. The diatoms bloomed around the middle of March and caused nutrient depletion in the upper part of the water column. Upwelling in the beginning of April gave rise to a second bloom, consisting of diatoms and Phaeocystis pouchetii. Numerically, autotrophic picoeukaryotes and Synechococcus sp. dominated the periods between and after these two major blooms. Heterotrophic bacterial abundance increased throughout the experimental period and reached peak values during and after phytoplankton blooms. These bacteria were succeeded by viruses having low DNA fluorescence, whereas viruses with medium DNA fluorescence bloomed during or after blooms of autotrophic picoeukaryotes. High-DNA fluorescence viruses reached maximum concentrations during and after the diatom and Phaeocystis blooms. The diversity of the bacterial community remained relatively stable, whereas viral diversity varied more and increased after major phytoplankton blooms. Our investigation thus demonstrates how virioplankton are important elements of the total microbial diversity and how they are intimately linked to the rest of the microbial community and possibly act as an internal driving force in spring bloom successions

UV (280 to 400 nm) optical properties in a Norwegian fjord system and an intercomparison of underwater radiometers

The depth to which solar ultraviolet (UV) radiation penetrates water columns is highly variable. The range in diffuse attenuation coefficients observed in the clearest ocean waters versus small oligotrophic lakes can be as much as 3 orders of magnitude. In this study, we investigated the variability of UV penetration (and its sources) in a typical Norwegian fjord system (Samnanger fjord, 60°N, 5° E) after the main spring diatom bloom had passed, so that the primary production in the fjord was low, with chl a concentrations of 1 to 2 mg m–3 at 10 m depth (April 1999). At 320 nm, diffuse attenuation coefficients varied between 1.2 and 5.0 m–1 from the outer to the inner part of the fjord. At 305 nm, the variation was between 2.3 and 5.4 m–1. Less variability was found at longer wavelengths (340 and 380 nm). River run off, containing high concentrations of colored dissolved organic matter (CDOM), provided a low salinity layer of highly UV-absorbing water at the surface. There was a strong correlation between diffuse attenuation coefficients in the UV and CDOM, but only a weak correlation with chl a. Uncertainty in measuring diffuse attenuation coefficients are seldom addressed. Thus, we undertook an intercomparison of diffuse attenuation coefficients calculated from data produced using 3 different UV radiometers: a spectroradiometer, a narrowband filter radiometer and 2 moderate-bandwidth filter radiometers. The deviations in attenuation ranged from 20 to 40%, being most pronounced at the shortest wavelengths (305 and 320 nm) at 3 different stations. This intercomparison illustrates that there has not been significant improvement in the uncertainties of measuring diffuse attenuation coefficients since a similar intercomparison was performed in 1994