Effects of nitrate availability on phytoplnakton photophysiology in the Arctic Ocean in summer

Abstract

Nitrogen in the Arctic Ocean is well known as the main limiting nutrient for phytoplankton photosynthesis, so nitrogen flux is essential to understand the distribution of characteristics of phytoplankton physiology and community. However, there have been few studies on the phytoplankton physiology and community structure in responses to nitrate availability in the Arctic Ocean. The aim of this study was 1) to investigate the photophysiological properties of phytoplankton affected by nitrate availability, and 2) to understand the relationship between the photophysiological characteristics and phytoplankton community structure. Four field campaigns were conducted from the Bering Strait to the north of Chukchi Sea every summer of 2015-2018. The photochemical efficiency of photosystem II (Fv/Fm) and photosynthetic parameter (P-E measurement) of phytoplankton were obtained using Fluorescence Induction & Relaxation System (FIRe). In addition, the changes of phytoplankton physiological properties and community size structure (micro: >10 μm, nano: 3-10 μm, pico: <3 μm) were observed through nutrient enrichment experiments. We have found that high photochemical efficiency and high maximum electron transfer rate were associated with micro-sized phytoplankton community in the nitrate-replete waters where chlorophyll-a concentration is relatively high (>0.5 mg m-3). On the orther hand, in the nitrate-deficient waters, pico-sized phytoplankton community showed relatively high photochemical efficiency while maximum electron rate and chlorophyll-a were relatively low. This suggest that the photophysiological properties of phytoplankton could be affected by community size structure depending on the various distributions of nitrates.