Phytoplankton dynamics, growth and microzooplankton grazing across the subtropical frontal zone, east of New Zealand

Elevated but variable phytoplankton biomass and productivity is often associated with the subtropical front (STF) where nitrogen-limited subtropical and iron-limited subantarctic waters mix. To understand variability within the STF east of New Zealand, we assessed phytoplankton community structure, growth, and grazing dynamics in relation to physico-chemical conditions across 23 stations distributed along the Chatham Rise region during late autumn-early winter. Serial dilution experiments were coupled with size-fractionated chlorophyll a (Chla) analysis (Total and <20 μm) and flow-cytometry (Synechococcus and picoeukaryote numbers, <2 μm) to estimate phytoplankton growth and microzooplankton grazing rates. Within the broad STF zone, subantarctic influenced waters (SAIW), frontal zone (FZ), and subtropical influenced waters (STIW) were delimited based on salinity, temperature and nutrient gradients. The chlorophyll a biomass (TChla) of phytoplankton and the abundance of larger sized cells (>20 μm chlorophyll a) peaked in FZ waters but declined steadily southwards into the colder SAIW, and rapidly reduced north into the STIW. Chlorophyll a <2 μm peaked in the northern STIW. Phytoplankton growth (TChla) was higher in warmer STIW (μ = 0.49 ± 0.07 day⁻¹) than in iron limited SAIW (μ = 0.29 ± 0.06 day⁻¹) but was on average moderate (μ = 0.42 ± 0.05 day⁻¹) when compared to previous studies in the region. Microzooplankton grazing on TChla was lower (m = 0.17 ± 0.04 day⁻¹) than growth and accounted for half of daily primary production (m:μ, 0.47 ± 0.06). Growth in the <20 μm Chla size fraction was higher (μ = 0.52 ± 0.06 day⁻¹) but a larger proportion was consumed by microzooplankton (m:μ = 0.64 ± 0.06). Picoeukaryotes showed the fastest growth (1.49 ± 0.13 day⁻¹) and grazing (1.43 ± 0.11 day⁻¹) rates on average, which peaked in the FZ but remained closely balanced across different waters (m:μ = 1.00 ± 0.02). Conversely, Synechococcus rates peaked in STIW and decreased southwards, with growth (μ = 0.42 ± 0.08 day⁻¹) generally exceeding grazing (m = 0.28 ± 0.06 day⁻¹) across all regions. Our results indicate differences in grazing together with nutrient (likely iron) availability were the primary factors controlling phytoplankton dynamics in the STF zone. These factors also affected the accumulation of larger phytoplankton biomass in the FZ, including the potential for export or transfer to higher trophic levels.