Short-term dynamics of nano- and picoplankton production in an embayment of the southern Benguela upwelling region

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In the southern Benguela ecosystem, regenerated production and high levels of organic matter remineralisation are expected to dominate during periods of relaxation. To study microbial growth and productivity under these remineralising conditions, we measured size fractionated (micro-nanoplankton: 10–200 μm and nano-picoplankton: 0.3–10 μm) net primary production, uptake rates of nitrate, ammonium, and urea, as well as nano- and picoplankton community composition and biomass over five consecutive days in autumn (March 2018). Samples were collected from three depths (1 m, 25 m, and 50 m) at a single station in St Helena Bay and abundances of nanophytoplankton, picophytoplankton and heterotrophic bacteria were determined using flow cytometry. There were differences in productivity among days but depth-differentiation was more apparent in the rates of net primary production and nitrogen uptake, with the highest rates (rate ± SD) (NPP: 3.36 ± 1.82 μmol L−1 d−1; ρNO3-: 0.30 ± 0.18 μmol L−1 d−1; ρNH4+: 2.27 ± 0.75 μmol L−1 d−1; ρUrea: 1.38 ± 0.02 μmol L−1 d−1) measured at the surface. Ammonium and urea uptake rates were two-to six-fold higher than those of nitrate, indicating that most of the biomass was produced through regenerated production with f-ratios of 0.02–0.21. Nano-picoplankton comprised 67% of the carbon biomass and were responsible for 90% of net primary production and 79–85% of total nitrogen (i.e., nitrate + ammonium + urea) uptake. Small cell size likely conferred advantages on nano-picoplankton in the nutrient-deplete euphotic zone and the low-oxygen (<89.3 μmol L−1) conditions at 25 m and 50 m. Nitrite oxidation rates were fastest at deeper depths where heterotrophic bacteria were most abundant. Heterotrophic bacteria also contributed the most (95%) carbon biomass at all depths, suggesting a major role for these microorganisms in carbon and nitrogen cycling throughout the water column of St Helena Bay.