Microplanktonic diatom assemblages dominated the primary production but not the biomass in an Antarctic fjord

Abstract

The western Antarctic Peninsula (WAP) fjords, currently threatened by climate change, are known as high krill congregation areas, however, the primary productivity in relation to phytoplankton groups in these coastal areas remains uncertain. To test this, we studied the specific composition, diversity and cellular carbon content of phytoplankton communities in relation to primary production (PP) and growth rates in Andvord Bay, its connection to Gerlache Strait and a near station on the continental shelf during spring (2015) and autumn (2016). Based on the microscopic determination of the relative biomass of the different microalgal taxa we determined three main phytoplankton assemblages that differ in their composition, PP, growth, and spatial and temporal distribution: (1) an assemblage dominated by cryptophytes, with high biomass (59.2 ± 28.2 μgC L−1), intermediate PP (63.1 ± 32 mgC m−3 d−1), and low growth (0.2 ± 1.1 d−1) present in the inner part of the fjord during spring, and related to higher surface water temperatures; (2) an assemblage dominated by microplanktonic diatoms (mainly Odontella weissflogii) with intermediate biomass (40.1 ± 17.2 μgC L−1), high PP (151.2 ± 62.5 mgC m−3 d−1), and high growth (0.9 ± 1 d−1) present from the fjord mouth to the Gerlache Strait waters during spring and in shelf waters during autumn, related to high irradiance and microzooplankton biomass; and (3) an assemblage dominated by dinoflagellates, mainly small gymnodinioids (< 15 μm long), with low biomass (3.5 ± 3.5 μgC L−1), low PP (6.9 ± 5.1 mgC m−3 d−1), and intermediate growth (0.5 ± 0.6 d−1) present in the whole fjord and the Gerlache Strait during autumn and infrequently during spring, related to high meltwater fraction. During both seasons, the highest phytoplankton biomass, richness and productivity in Andvord Bay was located in a frontal area at the fjord's mouth. Overall, the results of this study support the notion that WAP fjords are highly productive, highlighting the role of microplanktonic diatom as the main organic carbon producers, even when they occur in low abundances, due to their photosynthetic potential and higher Carbon-to-Chlorophyll a ratio. Our results, together with previous phytoplankton growth rates measured in the WAP nearshore waters from King George Island to Marguerite Bay, are higher than those measured on the WAP shelf and in the Ross Sea suggesting micro- and macronutrient enrichment from glacial melt maintains high productivity throughout the growth season in fjords. Our results support the hypothesis that climate change could highly affect the coastal WAP ecosystems by changing the balance between more productive phytoplankton assemblages (diatoms) to considerably less productive phytoplankton assemblages (cryptophytes).