Phytoplankton and microzooplankton variability between the Subtropical and Polar Fronts south of Australia: Thriving under regenerative and new production in late summer

Abstract

Phytoplankton, microzooplankton were studied along a 42°–55°S, 141°–143°E transect in March 1998 and compared with production‐related parameters (carbon biomass, chlorophyll a, nitrogen and carbon uptake, and ƒ ratios). The transect crossed the Subtropical Front (STF), the Subantarctic Front (SAF), and the Polar Front (PF). Phytoplankton assemblages were dominated by nano‐ and pico‐sized flagellates; their peak numbers (nanoflagellates: 8.2×105 cells L−1) occurred in the areas of STF and within the Subantarctic Zone (SAZ). North of the SAF, dinoflagellates were next in abundance. Diatoms exceeded dinoflagellates in the PF area (maximum 1.26×105 cells L−1). Dinoflagellates were dominated by nano‐sized gymnodinioid forms with microplanktonic species increasing in numbers in SAZ and STF. Diatoms contained mainly Fragilariopsis pseudonana and Pseudonitzschia lineola; several abundant species exibited a latitudinally restricted distribution. Phytoplankton carbon biomass was dominated by dinoflagellates (including >20 μm heterotrophs) representing 48 to 84% of total cell carbon. Maxima of 18–26 μg C L−1 occured both at STF and PF. Heterotrophic dinoflagellates and ciliates showed similar distributions. Their peaks of cell densities and carbon in STF and SAZ were associated with phytoplankton maxima. Microzooplankton cell distribution and biomass suggest they are major grazers and contributors to carbon flow. Phytoplankton assemblages represented at least three stages with different relative contributions of regenerative and new production. Production related parameters (e.g., low ƒ ratio and high NH4+ uptake) point to the presence of regenerative community at the STF. It attracted the highest concentration of microzooplankton. The phytoplankton community associated with a frontal feature (46°–47°S) within the SAZ, thrived under increased new production (e.g., relatively higher ƒ ratio and NO3− uptake). The community along 47°–55°S was characterized by intermediate ƒ ratios, with slight predominance of regenerated production. Southward of 47°S, the relative contribution of new production increased.