Substantial Sub-Surface Chlorophyll Patch Sustained by Vertical Nutrient Fluxes in Fram Strait Observed With an Autonomous Underwater Vehicle

Sandra Tippenhauer,Thorben Wulff,Markus Janout,Allison Fong,Manita Chouksey,Sinhue Torres-Valdes

doi:10.3389/fmars.2021.605225

Abstract

We present results from a coordinated frontal survey in Fram Strait in summer 2016 using an autonomous underwater vehicle (AUV) combined with shipboard and zodiac-based hydrographic measurements. Based on satellite information, we identified a front between warm Atlantic Water and cold Polar Water. The AUV, equipped with oceanographic and biogeochemical sensors, profiled the upper 50 m along a 10 km-long cross-front oriented transect resulting in a high-resolution snapshot of the upper ocean. The transect was dominated by a 6 km-wide, 10 m-thick subsurface patch of high chlorophyll, located near the euphotic depth within a band of cold water. Nitrate was depleted in the surface, but abundant below the pycnocline. Potential vorticity and Richardson number estimates indicate conditions favorable for vertical mixing, which indicates that the high chlorophyll patch may have been sustained by upward nitrate fluxes. Our observations underline the complex hydrographic and biogeochemical structure in a region featuring fronts and meanders, and further underline the patchy and small-scale nature of subsurface phytoplankton blooms potentially fueled by submesoscale dynamics, which are easily missed by traditional surveys and satellite missions.

Highlights

Sea ice retreat in the Arctic Ocean is dramatically changing the ocean’s light regime with drastic implications for the ecosystem
This paper presents results from a 10 km-long high-resolution AUV transect across a front separating warm AW from cold PW in Fram Strait as part of a coordinated frontal experiment carried out during an RV-Polarstern campaign in 2016
The euphotic depth (i.e., 1%-photosynthetically active radiation (PAR) level) as determined with the AUV, was located within the phytoplankton patch around 30 m. These observations provide an example of an Arctic subsurface chlorophyll-maximum, featuring substantial chlorophyll concentrations in a thin (5–10 m) and narrow (5 km) band, that is missed by satellites or traditional ship-board surveys

Summary

Introduction

Sea ice retreat in the Arctic Ocean is dramatically changing the ocean’s light regime with drastic implications for the ecosystem. Model projections on the future Arctic ecosystem predict that primary production increases may occur along the Atlantic- and Pacific-water inflow pathways and along the continental slopes, where stratification weakens due to reductions in sea ice melt (Slagstad et al, 2015). These projections further indicate, that central Arctic regions away from the boundaries may not become significantly more productive. This is especially true under a changing Arctic icescape, that is characterized by increased mobility and variability (Serreze et al, 2007; Stroeve et al, 2007)

Methods

Results

Discussion

Conclusion