Interannual changes in nutrient and phytoplankton dynamics in the Eastern Mediterranean Sea (EMS) predict the consequences of climate change; results from the Sdot-Yam Time-series station 2018–2022

Abstract

Global climate change is predicted to reduce nutrient fluxes into the photic zone, particularly in tropical and subtropical ocean gyres, while the occasional major storms will result in increased nutrient pulses. In this study the nutrient and phytoplankton dynamics have been determined at a new time-series station in the southeastern Levantine basin of the Eastern Mediterranean Sea (EMS) over 4.5 years (2017–2022). In 2018 and 2019, there was a moderate concentration of residual nitrate and nitrite (N + N) in the photic zone (280–410 nM) in winter, resulting in phytoplankton dynamics dominated by cyanobacteria with relatively few picoeukaryotes (280 ± 90 μgC m⁻²). Winter storm driven mixing was much reduced in 2020 and particularly in 2021, resulting in a lower concentration of N + N in the photic zone, which decreased during summer stratification, such that by August 2021, the N + N was highly depleted (<60 nM) resulting in an integrated phytoplankton biomass of 23 μgC m⁻². A major storm in December 2021 (Storm Carmel) injected high N + N (750 nM; max = 1090 nM) in the upper 100 m, which stimulated pico and nanophytoplankton biomass (∼2400 μgC m⁻²) and according to our inference increased eukaryotes (diatoms). The pattern of measured silica reinforced our conclusion that we sampled 3 different nutrient and ecosystem states. Phosphate was always at or close to limit of detection (LoD) because of rapid uptake by cyanobacteria into their periplasm. These results predict that climate change in the EMS will result in periods of nutrient and phytoplankton depletion (Famine) interrupted by short periods of Mesotrophy (Feast) caused by major storms.