Description: Protists (microbial eukaryotes) are diverse, major components of marine ecosystems, and are fundamental to ecosystem services. In the last 10 years, molecular studies have highlighted substantial novel diversity in marine systems including sequences with no taxonomic context. At the same time, many known protists remain without a DNA identity. Since the majority of pelagic protists are too small to identify by light microscopy, most are neither comprehensively or regularly taken into account, particularly in Long-term Ecological Research Sites. This potentially undermines the quality of research and the accuracy of predictions about biological species shifts in a changing environment. The ICES Working Group for Phytoplankton and Microbial Ecology conducted a questionnaire survey in 2013–2014 on methods and identification of protists using molecular methods plus a literature review of protist molecular diversity studies. The results revealed an increased use of high-throughput sequencing methods and a recognition that sequence data enhance the overall datasets on protist species composition. However, we found only a few long-term molecular studies and noticed a lack of integration between microscopic and molecular methods. Here, we discuss and put forward recommendations to improve and make molecular methods more accessible to Long-term Ecological Research Site investigators.

Description: Ocean iron fertilization experiments enable the quantitative study of processes shaping the structure and functioning of pelagic ecosystems following perturbation under in situ conditions. EIFEX was conducted within a stationary eddy adjacent to the Antarctic Polar Front over 38 days in February/March 2004 and induced a massive diatom bloom. Here, we present the responses in abundance and biomass of all identifiable protozooplankton taxa (heterotrophic protists ranging from 2 to 500 µm) during the bloom. Acantharia, dinoflagellates and ciliates together contributed 〉90% of protozooplankton biomass in the upper 100 m throughout the experiment with heterotrophic nanoflagellates, nassellaria, spumellaria, phaeodaria, foraminifera and the taxopodidean Sticholonche zanclea providing the remainder. Total protozooplankton biomass increased slightly from 1.0 to 1.3 g C m−2 within the fertilized patch and remained at 0.7+0.04 g C m−2 outside it. However, distinct trends in population build-up or decline were observed within the dominant taxa in each group. In general, smaller less-defended groups such as aloricate ciliates and athecate dinoflagellates declined, whereas the biomass of large, spiny and armoured groups, in particular acantharia, large tintinnids and thecate dinoflagellates increased inside the patch. We attribute the higher accumulation rates of defended taxa to selective, heavy grazing pressure by the large stocks of copepods. Of the defended taxa, acantharia had the lowest mortality rates and the highest biomass. Large stocks of tintinnid loricae in the deep water column identify this group as a relevant contributor to deep organic carbon export. Highest accumulation rates (0.11 day−1) were recorded in S. zanclea.