Nutrient supply controls picoplankton community structure during three contrasting seasons in the northwestern Mediterranean Sea

Abstract

We investigated the influence of ocean mix- ing and nutrient supply dynamics on picoplankton com- munity composition in the context of Margalef's Mandala (Margalef 1978). Simultaneous measurements of micro- turbulence, nutrient concentration, and autotrophic and heterotrophic picoplankton properties, were collected during 3 cruises carried out in the northwestern Medi- terranean Sea in March (F1), April/May (F2) and Sep- tember (F3) 2009. The 3 cruises sampled different oceanographic conditions, starting with early stages of the late winter-early spring bloom, followed by the late stage of the bloom, and finally summer stratification. As a result of the variability in vertical diffusivity and the ni- trate gradient across the nitracline, nitrate vertical fluxes were higher during F1 (23 ± 35 mmol m �2 d �1 ), compared to F2 (0.4 ± 0.2 mmol m �2 d �1 ) and F3 (0.09 ± 0.09 mmol m �2 d �1 ). Prochlorococcus abundance was low when ni- trate supply was high, Synechococcus exhibited the highest abundances at intermediate levels of nitrate sup- ply and highest irradiance during F2, and large and small picoeukaryotic groups increased their abundance under high nutrient supply in F1. No significant relation- ships between the abundance of high and low nucleic acid heterotrophic bacteria and nitrate supply were found. In agreement with Margalef's model, our results show different responses of picophytoplankton groups to nitrate supply (probably reflecting differences in nu- trient uptake abilities), and that the ratio of prokaryotic to picoeukaryotic photoautotrophic biomass decreases with increasing nitrate supply.