Response of a natural Antarctic phytoplankton assemblage to changes in temperature and salinity

Highlights

• Experimental response of phytoplankton to increased temperature and reduced salinity

• Phytoplankton assemblage characterization by HPLC, light and electron microscopy

• Dominance of nano-diatoms and phytoflagellates on high temperature and low salinity

• Sub-polar species previously undetected in Antarctica favoured by climate change

• The first report of Shionodiscus gaarderae (sub-Antarctic species) in Antarctica

Abstract

The climate around the Western Antarctic Peninsula (WAP) is rapidly changing and dramatically affecting marine coastal waters. Increases in air and seawater temperatures, not matter how small, can alter coastal biological communities due to both temperature increases as well as salinity reduction from glacier melting. The aim of this study was to evaluate the individual and combined effects of elevated sea surface temperature (+4 °C) and decreased salinity (−4) on growth and assemblage composition of natural summer phytoplankton from Potter Cove (King George Island, South Shetlands, northern WAP), using an outdoor microcosm experiment. Pigment composition was analyzed by high performance liquid chromatography (HPLC/Chemtax) and species composition by light and electron microscopy. Increases in phytoplankton biomass during the first 3 days at elevated-temperatures coincided with an increase in the abundance and the specific growth rate of small centric diatoms (Chaetoceros socialis and Shionodiscus gaarderae, mostly observed in temperate waters) and unidentified small phytoflagellates <5 μm. In contrast, pennate diatoms significantly decreased. At the end of the experiment on day 7, under nitrate and phosphate limitation, chlorophytes abundances increased under low salinity whereas prasinophytes decreased in all treatments. This study suggests that climate change could notably affect Antarctic phytoplankton composition by favouring temperate-water species previously undetected in Antarctic waters, such us S. gaarderae. Moreover, the observed changes in phytoplankton structure, associated with an increase of nano- over micro-size taxa, could have important implications for future Antarctic food webs.

Introduction

Over the last five decades, the West Antarctic Peninsula (WAP) has experienced strong atmospheric warming, with the highest heating rates recorded worldwide (Vaughan et al., 2003; Turner et al., 2005, Turner et al., 2014; Gutt et al., 2015). The consequent increases in air and surface seawater temperatures are responsible for the melting of glaciers that drain large amounts of freshwater and terrestrial particulate materials to coastal waters. The resulting lowering of surface seawater salinity strengthens the stratification of the water column while increased turbidity decreases light availability in surface waters (Schloss et al., 2012; Meredith et al., 2018). Since the mid-1950's, the glacial area of King George Island (KGI) in the South Shetland Archipelago (WAP) has lost 89 km², a 7% reduction of the total area (Simões et al., 1999). This makes KGI an ideal region for the study of climate change and its impact on marine ecosystem (Meredith et al., 2017). Planktonic organisms living in extreme environments, such as Antarctica, tend to be more sensitive to changes in environmental stressors than their temperate counterparts (Clarke et al., 2007). Because marine phytoplankton contribute ~50% to total global primary production and CO₂ fixation (Falkowski et al., 1998), they provide critical energy to the different components of the marine food web and ultimately to the top consumers (Waite et al., 1997). Therefore, it is critical to understand the responses of phytoplankton to temperature and salinity stress since variations in these abiotic factors can have profound consequences for marine food webs (Gleitz et al., 1995; Trathan et al., 2007; Lewandowska et al., 2014a) and biogeochemical cycles in the Southern Ocean (Nelson et al., 1991; Smetacek et al., 2004; Tagliabue et al., 2009).

Variations in seawater salinity and temperature have been shown to cause changes in Antarctic phytoplankton physiology under experimental conditions (Ralph et al., 2005; Hernando et al., 2015, Hernando et al., 2018) as well as in the taxonomic structure of natural phytoplankton assemblages (Moline et al., 2001; Montes Hugo et al., 2009; Schofield et al., 2017). Previous studies demonstrated that reduced salinity, associated with meltwater input, was responsible for the increase in the relative abundance of cryptophytes (Moline et al., 2001; Garibotti et al., 2003; Mendes et al., 2013). However, little is known about the expected changes on Antarctic phytoplankton biomass and composition to the combined effect of these two stressors.

Located in KGI, Potter Cove (PC) (Fig. 1) was historically considered a low chlorophyll-a (Chl-a) area compared to other regions of the WAP, with a mean value below 1 mg Chl-a m⁻³ during 25-years (Kim et al., 2018). Previous to this study, Chemtax analyses of PC plankton samples showed a dominance of diatoms during the growing season, with a relatively low contributions of other taxonomic groups such as haptophytes and chlorophytes (van de Poll et al., 2011). More recently, anomalously large phytoplankton blooms were observed in the summer of 2010 (up to 20 mg Chl-a m⁻³) and it was associated with the common large diatom species from PC, such as Porosira glacialis and Thalassiosira antarctica (Schloss et al., 2014).

After this unusual bloom, two microcosm experiments were carried out at PC in order to study the effect of decreased salinity (Hernando et al., 2015) and the combined effects of decreased salinity and increased temperature (Hernando et al., 2018) on phytoplankton stress responses, changes in assemblage composition and fatty acid profiles. These studies conducted in the summers of 2011 and 2014, respectively, exposed assemblages dominated by large diatoms (Hernando et al., 2015, Hernando et al., 2018) to changes in these stressors. In contrast, the PC phytoplankton assemblage from the summer of 2016 used in this study was characterized by the dominance of nano-size diatoms. This is the first report of the responses of a coastal Antarctic phytoplankton assemblage dominated by small diatoms to increased temperature and decreased salinity in PC using microcosm experiments. Moreover, dominant species found at the beginning of the present experiment are mostly known to occur in subpolar waters and therefore it is relevant to analyse their potential response to increased temperature and reduced salinity conditions induced by climate change in coastal waters of the WAP. We expect their response to forecast their ability to colonize other Antarctic environments.