Mesozooplankton structure and functioning during the onset of the Kerguelen phytoplankton bloom during the Keops2 survey

Abstract

Abstract. This study presents results on the zooplankton response to the early phase of the northeastern Kerguelen bloom during the KEOPS2 survey (15 October–20 November 2011). The campaign combined a large coverage of the eastern part of the shelf and the adjacent oceanic regions with 2 quasi-perpendicular transects oriented south to north (between 49°08' and 46°50' S) and west to east (between 69°50' and 74°60' E) aiming to document the spatial extension of the bloom and its coastal-off shore gradient, and a pseudo-lagrangian survey located in a complex recirculation zone in a stationary meander of the Polar front nearly centered at the crossing of the 2 initial transects. In addition, 8 stations were performed for 24 h observations, distributed in key areas and some of them common with the KEOPS1 cruise (January–February 2005). The mesozooplankton biomass stocks observed at the beginning of the KEOPS2 cruise were around 2 g C m−2 both above the plateau and in oceanic waters. Zooplankton biomasses in oceanic waters were maintained in average below 2 g C m−2 over the study period, except for one station in the Polar Front Zone (FL), whereas zooplankton biomasses were around 4 g C m−2 on the plateau at the end of the cruise. Taxonomic composition and stable isotope ratios of size-fractionated zooplankton indicated the strong domination of herbivores. The most remarkable feature during the sampling period was the stronger increase in the integrated 0–250 m abundances in the oceanic waters (25 × 103 to 160 × 103 ind m−2) than on the plateau (25 × 103 to 90 × 103 ind m−2). The size structure and taxonomic distributions revealed a cumulative contribution of various larval stages of dominant copepods and euphausiids particularly in the oceanic waters, with clearly identifiable stages of progress during the Lagrangian survey. These different results during KEOPS2 suggested that the zooplankton community was able to respond to the growing phytoplankton blooms earlier on the plateau than in the oceanic waters. The reproduction and early stage development of dominant species were sustained by mesoscale-related initial ephemeral blooms in oceanic waters but individual growth was still food-limited and zooplankton biomass stagnated. On the contrary, zooplankton abundances and biomasses on the shelf were both in a growing phase, with slightly different rates, due to sub-optimal conditions of growth and reproduction conditions. Combined with the KEOPS1, the present results deliver a consistent understanding of the spring changes in zooplankton abundance and biomass in the Kerguelen area.