Community structure and grazing impact of mesozooplankton during late spring/early summer 2004/2005 in the vicinity of the Crozet Islands (Southern Ocean)

Fielding, Sophie, Ward, Peter, Pollard, Raymond T., Seeyave, Sophie, Read, Jane F., Hughes, J. Alan, Smith, Tania and Castellani, Claudia (2007) Community structure and grazing impact of mesozooplankton during late spring/early summer 2004/2005 in the vicinity of the Crozet Islands (Southern Ocean). Deep Sea Research Part II: Topical Studies in Oceanography, 54 (18-20), 2106-2125. (doi:10.1016/j.dsr2.2007.06.016).

Abstract

Net sampling within the vicinity of the Crozet archipelago, in the Indian Ocean sector of the Southern Ocean, was conducted during late spring/summer (November 2004–January 2005) to describe the composition, distribution and grazing impact of mesozooplankton, and to investigate their relationships with the prevailing oceanographic regime in the area. The mesozooplankton community was intimately linked with the large-scale physical circulation in the region. To the west and north of the Islands, the sub-Antarctic Front (SAF) presented a strong biogeographic boundary between sub-tropical and sub-Antarctic species. South and east of the SAF, the mesozooplankton community was dominated by Polar Frontal Zone (PFZ) copepod species. Cluster analysis of mesozooplankton abundance data identified two main communities in the PFZ, termed here Island and Oceanic. Island stations, representing the proposed iron-fertilised productive region north of the archipelago, contained an abundance (mean of 2269 ind m?3) of the neritic copepod Drepanopus pectinatus, whose presence indicated that the water had interacted with the Crozet Island shelf at some point. D. pectinatus was present in samples north of Crozet up to the SAF, confirming that water passing the Crozet Islands could be transported throughout the region to the north. The Oceanic stations, south of the Islands and within the SAF, contained similar mesozooplankton abundances and biovolume to the Island stations suggesting little enhanced impact of the iron-fertilised phytoplankton bloom through the mesozooplankton food web.

Copepod community grazing pressure, in both Island and Oceanic stations, during November and December was small (<7% of chlorophyll-a standing stock per day, <35% primary production per day). By January, a phytoplankton bloom had developed at some of the Island stations (up to 3000 mg C m?2 d?1) and grazing pressure was <1% of chlorophyll-a standing stock per day. At the oceanic stations, primary productivity had reduced from 460 to 200 mg C m?2 d?1, typical values for high-nutrient low-chlorophyll (HNLC) waters, and the copepod community grazing pressure had increased to 90% of daily primary productivity. This suggests that a combination of grazing and micronutrient availability controls phytoplankton biomass in HNLC waters to the south of Crozet, while grazing had little impact on the “iron-fertilised” bloom north of the Crozet Islands. The intense seasonal phytoplankton bloom around Crozet may therefore be exported to the sea floor rather than fuelling the higher trophic levels.

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