Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula

Abstract

The spatial distribution of phytoplankton cell abundance, carbon (C) biomass and chlorophyll a (Ch.l a) concentration was analysed during three summers (1996, 1997 and 1999) in a seasonal sea-ice area, west of the Antarctic Peninsula. Tie objective of the study was to assess interannual variability in phytoplankton spatial distribution and the mechanisms that, regulate phytoplankton accumulation in the water column. Phytoplankton C biomass and Ch.l a distributions were consistent, from year to year, exhibiting a negative on/offshore gradient.. Tie variations in C concentration had a close and non-linear relationship with, the upper mixed layer depth, suggesting that, the vertical mixing of the water column is the main factor regulating phytoplankton stock. Tie magnitude of C gradients was 5-folcl higher during 1996 than during 1997 and 1999. This was ascribed to interannual variations in the concentration of diatom blooms in the region influenced by sea-ice melting. Vertical distribution of the phytoplankton, as estima ted from Ch.l a profiles, also varied along an on/offshore gradient.: Ch.l a was distributed homogeneously in the upper mixed layer in coastal and mid-shelf stations and concentrated in the deep layer (40-100 m) occupied by the winter waters (11 J t. remnants of the Antarctic surface waters during summer) in more offshore stations. Tie region with, a deep Ch.l a maximum layer (DCM layer) was dominated by a phytoplankton assemblage characterized by a relatively high, concentration of diatoms. Tie extent, of this region varied from year to year: it. was restricted to pelagic waters during 1996, extended to the shelf slope during 1997 and occupied a major portion of the area during 1999. It. is hypothesized that, iron depletion in near surface waters clue to phytoplankton consumption, and a higher concentration in Illi, regulated this vertical phytoplankton distribution pattern. Furthermore, we postulate that, year-to-year variations in the spatial distribution of the DCM layer were related to interannual variations in the timing of the sea-ice retreat.. Tie similarity between our results and those reported in literature for other areas of the Southern Ocean allows us to suggest, that, the mechanisms proposed here as regulating phytoplankton stock in our area may be applicable elsewhere.

Austral fall−winter transition of mesozooplankton assemblages and krill aggregations in an embayment west of the Antarctic Peninsula

“…The upper end of this daily consumption within such a super-aggregation is not realistic compared with the upper end of the summer primary production estimated to range from 1.4 to 12.5 g C m −2 d −1 near Palmer Station (Moline & Prézelin 1996, Garibotti et al 2005, Ducklow et al 2007). The formation, maintenance and fate of this super-aggregation of krill at such a scale remain unanswered.…”

Temporal and spatial variation of phytoplankton in Admiralty Bay, South Shetlands: the dynamics of summer blooms shown by pigment and light microscopy analysis

“…In January 2010, fresh water inflow from melting ice and increased water temperatures caused low surface salinities (\34 PSU) in sites located in Ezcurra Inlet, which created suitable conditions for a shallow mixed layer of \2 m and increased water column stability. Throughout the region of the Western Antarctic Peninsula, many studies have found significant correlations between water column stability (largely resulting from freshwater lenses produced by sea-ice melt and glacial runoff) and the diatom concentration in this region (Kang and Lee 1995;Smith and Stammerjohn 2001;Garibotti et al 2003bGaribotti et al , 2005. The shallow mixed layer may allow algal cells to remain in the upper portion of the water column and to grow under a propitious light regime (Garibotti et al 2005).…”

“…Throughout the region of the Western Antarctic Peninsula, many studies have found significant correlations between water column stability (largely resulting from freshwater lenses produced by sea-ice melt and glacial runoff) and the diatom concentration in this region (Kang and Lee 1995;Smith and Stammerjohn 2001;Garibotti et al 2003bGaribotti et al , 2005. The shallow mixed layer may allow algal cells to remain in the upper portion of the water column and to grow under a propitious light regime (Garibotti et al 2005). The euphotic zone depth of 24-98 m, observed in 2010 in Admiralty Bay, also suggests good light conditions for phytoplankton bloom.…”

Foraging strategy switch of a top marine predator according to seasonal resource differences

“…Seal search intensified and foraging success increased in response to elevated plankton densities not only in summer, but also during other times of the year including winter. This is despite the well-documented seasonal decline in phytoplankton biomass from the bloom period during early spring and summer to oligotrophic conditions during winter (Behrenfeld and Falkowski, 1997;Garibotti et al, 2005;Thomalla et al, 2011). These biological changes do not appear to affect the ability of seals to locate elevated plankton densities where prey availability increases.…”