Shifts in microphytoplankton species and cell size at Admiralty Bay, Antarctica

Abstract

Phytoplankton (> 15 µm) was investigated in three shallow coastal areas at Admiralty Bay (AB) between the summers of 2002-03 and 2008-09. Phytoplankton abundance was low (10 3 cells l -1 ) and, over time, the prevailing cell size decreased due to a shift in phytoplankton dominant species from diatoms to dinoflagellates. In situ and remote sensing data showed that oscillations in sea surface temperature, precipitation, ice formation/melting, irradiance (cloud cover) and bottom circulation (indexed by the Antarctic Oscillation Index; AAO) were shown to govern the structure of the phytoplankton. Under negative AAO, diatoms prevailed, with the dominance of large (> 80 µm) benthic diatoms (e.g. Corethron pennatum and Navicula directa) in periods of low production (10 2 cells l -1 in 2002-03), and medium-sized (31-80 µm) centrics (e.g. Thalassiosira spp. and Stellarima microtrias) when the abundance was higher (10 4 cells l -1 in 2003-04). Conversely, positive AAO led to the co-dominance of dinoflagellates and planktonic diatoms (e.g. Pseudo-nitzschia spp.) in the summers of 2007-08 and 2008-09. These results suggest that the AAO can be a good predictor of phytoplankton in coastal areas around the western Antarctic Peninsula, and may help our understanding of changes in other trophic levels of the food web.

Composition, density and size-structure of the autotrophic plankton community in a shallow coastal zone at King George Island, West Antarctic Peninsula (WAP), during early summer 2010

“…Early summer of 2010/2011 was characterized by positive water temperatures at AB (> 0.3ºC), which were on average higher than the temperature described for December during the decades of 1990s and 2000s (< 0.0ºC) (Lange et al, 2007, Kopczynska, 2008Tenenbaum et al, 2011;Lange et al, 2014). This observation is in line with the raising summer water temperatures observed for KGI coastal waters during the last two decades (Schloss et al, 2012;Lange et al, 2014;Schloss et al, 2014).…”

“…This observation is in line with the raising summer water temperatures observed for KGI coastal waters during the last two decades (Schloss et al, 2012;Lange et al, 2014;Schloss et al, 2014). In a long-term study, Boyce et al (2010) reported that phytoplankton biomass in the global ocean, including Arctic and Antarctic regions, declined by 1%, which strongly correlates with the increase in sea surface temperatures over the past century.…”

“…increasing turbidity and attenuating light) effects on the phytoplankton community (Martin et al, 1991;Brandini & Rebello, 1994;Nedzarek and Rakusa-Suszczewski 2004;Lannuzel et al, 2007;Becquevort et al, 2009;Lannuzel et al, 2013). In the present study we observed higher midand large size phytoplankton cell densities (10 3 -10 4 cells L -1 ) than those observed for the same region in previous studies conducted during the austral summers of 2002/03, 2003/04, 2007/08, 2008/09 and 2009Lange et al, 2014). Nevertheless, 10-fold higher (10 5 cells L -1 ) cell densities were commonly observed at Admiralty Bay during the decades of the 1970s, 1980s and 1990s (i.e.…”

“…As observed in a previous study conducted during the late summer 2009/2010 , the small cells represented the main portion of the autotrophic plankton community in Admiralty Bay. Among the mid-size phytoplankton, the centric diatom Thalassiosira anguste-lineata (MDL ~ 32.5 µm) was responsible for the highest abundances, as observed by Lange et al (2014) during the summer 2003/04 and Schloss et al (2014) in January 2010. This diatom dominance is characteristic of AB microphytoplankton.…”

“…Since human occupation has increased in recent years (Montone et al, 2013), the island can also be considered as a field laboratory to study the effects of climate change and human disturbances. The phytoplankton community of Admiralty Bay showed a decline in diatom contribution during the 1990's to 2000's in relation to those observed in the continental shelf region (Kopczynska, 2008;Lange et al, 2014).…”