Structure and dynamics of the pelagic microbial food web of the Subtropical Convergence region east of New Zealand

Abstract

The Subtropical Convergence (STC) region to the east of New Zealand was sampled in both winter and spring to establish the food web structure and dynamics in 3 water masses. Subtropical (ST) waters to the north were warm with high salinity, and in spring macronutrient concentrations were potentially limiting to phytoplankton growth. Subantarctic (SA) waters to the south in both seasons were cold, with lower salinity and high macronutrient concentrations with chlorophyll a concentrations of 0.12 to 0.19 pg 1-'. The frontal zone between these water masses (STC) had maximum chlorophyll a concentrations ranging from 0.39 to 3.42 pg I-'. In SA waters in both seasons and ST waters in spring the pelagic food web in the mixed layer was dominated by the microbial components with up to 98% of the phytoplankton biomass in the <20 pm size fraction and the bacterial C/phytoplankton C ratio ranging from 0.9 to 1.9. In comparison, in STC waters in both seasons and subtropical waters in winter the >20 pm phytoplankton dominated the phytoplankton biomass with ratios of 0.2 to 0.3 bacterial C/phytoplankton C. Despite significant differences in the food web structure, primary grazers of the phytoplankton in all water masses in both seasons were microzooplankton with 78 to 118 % of the primary production being grazed on a daily basis. This suggests that the food web was dominated by recycling processes, and phytoplankton export fluxes from the upper mixed layer were low in all water masses at the time of sampling.

Temporal coupling between surface and deep ocean biogeochemical processes in contrasting subtropical and subantarctic water masses, southwest Pacific Ocean

“…Phytoplankton are iron‐limited and dominated by picophytoplankton [ Bradford‐Grieve et al , 1997, 1999; Boyd et al , 1999]. The biota are mainly components of the microbial food web in this pelagic ecosystem [ James and Hall , 1998; Hall et al , 1999, 2004] with correspondingly low export rates of biogenic material [ Nodder and Northcote , 2001].…”

Mixotrophic and heterotrophic nanoflagellate grazing in the convergence zone east of New Zealand

“…Previous investigations in winter and spring reported that during both seasons in the subantarctic waters, and in the subtropical waters during spring, the microbial food web was the dominant pathway for carbon and energy flows (James et al 1996, Hall et al 1999. During these periods the majority of phytoplankton biomass was found to be in the 1 2 0 pm size class and bacterial biomass dominated phytoplankton carbon (Hall et al 1999). In this late summer study chl a was generally low, reflecting oligotrophic conditions (Table l ) , and the majority of phyto- Table 3.…”

Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on &lt;sup&gt;234&lt;/sup&gt;Th: &lt;sup&gt;238&lt;/sup&gt;U disequilibria

“…In the present study, the most plausible reason causing the little enhancement of the export flux would be related to microzooplankton grazing. Indeed, Hall et al (1999) demonstrated that >78 % of daily PP can be grazed by microzooplankton in the STF in austral spring and winter. In contrast, mesozooplankton grazing is likely to be in the order of only 1-2 % of daily PP (Bradford-Grieve et al, 1998).…”