Distribution of phytoplankton and other protists in the North Water

Abstract

Specific taxa and community structure of the phytoplankton and other protists in the North Water, a productive polynya ecosystem between Ellesmere Island (Canada) and Greenland, were determined from April to July 1998. Distinct temporal and spatial trends were observed within different geographic regions, influenced by water masses determined by temperature and salinity (TS) characteristics. In April, the northernmost communities were primarily flagellates and dinoflagellates, resembling those of the Arctic Basin, while large centric diatoms dominated in May and persisted through July. Throughout the rest of the polynya in April, centric diatoms and alveolates (ciliates and dinoflagellates) dominated. In the eastern part of the polynya in May and June, centric diatoms, especially Thalassiosira spp., reached very high levels, comprising 50–80% of biomass carbon there, while in July the eastern community was primarily Chaetoceros spp. and large alveolates. The community in the western region in April was more mixed, with Actinocyclus spp./Coscinodiscus spp. and alveolates as the dominant groups. In May and June, pennate-ribbon diatoms were mixed with Thalassiosira and Chaetoceros spp., while in July western biomass was primarily alveolates and Chaetoceros spp. Throughout the polynya and the 4-month sampling period, the community of phytoplankton and other protists was dominated by microplankton (20–200 μm). Records of this size class from the current study were comparable to historical records collected since 1876. The planktonic communities along the southern border of the North Water, which was ice-covered until mid-June and had TS characteristics of the West Greenland Current (WGC), differed from those in the rest of the polynya. They contained few planktonic diatoms and had a higher proportion of nanoplankton size (2–20 μm) flagellates and gymnodinoid dinoflagellates. The WGC microplankton were dominated by thecate dinoflagellates such as Ceratium arcticum. Overall, the diatom-dominated phytoplankton bloom in the North Water was 2–3 months earlier than blooms in the southern region. In addition to blooming sooner, large cells persisted, were a dominant biomass component from May to July, and thus were major contributors to the overall productivity of the North Water.

Introduction

Phytoplankton species have been reported to be associated with different water masses for at least 120 years (Castracane, 1884; Braarud and Klem, 1931; Grøntved and Seidenfaden, 1938; Hasle, 1990). Species composition of protist communities is functionally important because it is thought to influence the export capacity of oceans (Goldman, 1993; Dugdale et al., 1990; Legendre and Rassoulzadegan, 1996) and zooplankton growth rate (Starr et al., 1999). The North Water is a biologically rich polynya, isolated from the temperate oceanic provinces and, due to its high latitude, characterized by extreme seasonal fluctuations. The area is also nutrient-rich and contains a number of distinct physical water masses that intermingle to a greater or lesser extent over the summer growing season and over different regions of the polynya (Bâcle et al., 2002). Grøntved and Seidenfaden (1938) summarized all records of net phytoplankton distribution west of Greenland, between 1850 and 1928. The North Water, however, had not been sampled extensively for phytoplankton and other protists over the past 70 years.

The International North Water Polynya Study (NOW) provided an opportunity to investigate general phytoplankton and other protist distribution over the spring and early summer in 1998. Older studies had emphasized the biogeography of net plankton; no information was available on the relative importance of different groups of diatoms compared to ciliates, dinoflagellates, and flagellates in terms of total standing biomass carbon over the growth season or within different regions. In many oceanic waters picoplankton (0.2–2 μm) and nanoplankton (2–20 μm) contribute substantially more to total particulate organic carbon than do large net plankton (Li and Wood, 1988; Booth et al., 1993; Sanders et al., 2000). In the North Water the relative importance of the larger microplankton or net-plankton (20–200 μm) compared to smaller nanoplankton (2–20 μm) had not been investigated.

Taxonomic details on the distribution of small flagellates, especially prymnesiophytes, prasinophytes, and choanoflagellates, are given in Gammelgaard (2000). A more detailed description of the spatial and temporal distribution of Chaetoceros socialis, a dominant diatom found during 1998, is given in Booth et al. (2002). The aims of the present study were two-fold: to quantify the contribution of different types of large and colony-forming diatoms, reported from net tows taken in the North Water since 1876; and to compare the biomass of these large diatoms to the contribution of alveolates (ciliates and dinoflagellates) and the smaller and more taxonomically diverse nanoplankton for the various regions of the polynya, as influenced by different water masses (Bâcle et al., 2002).