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

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.

The Pan‐Arctic Continental Slope as an Intensifying Conveyer Belt for Nutrients in the Central Arctic Ocean (1985–2015)

“…The boundary current, as a jet, is also conveying kinetic energy that is eventually transferred to the mesoscale (Spall et al., 2008; Wang, Wekerle, et al., 2020). The prominent role for mesoscale eddies and submesoscale processes on shaping the AO biogeochemical environment has been documented only very recently (Schourup‐Kristensen et al., 2021; Tippenhauer et al., 2021).…”

“…Recent advances therefore not only documented changes in the circulation but also in the mesoscale activity. A variety of recent observations and modeling experiments support the idea that (sub)mesoscale turbulence influences phytoplankton growth by affecting both the nutrient and light environments (e.g., D'Ovidio et al, 2010;Lévy et al, 2018;Schourup-Kristensen et al, 2021;Tippenhauer et al, 2021). Nutrient supply occurring at the scale of eddies (the mesoscale) and at the scale of frontogenesis (the submesoscale) contribute significantly to the primary production budget (Lévy, 2008;Lévy et al, 2012).…”

Contrasting phytoplankton-zooplankton distributions observed through autonomous platforms, in-situ optical sensors and discrete sampling

“…Stations with deep mixing (50 m) had higher phytoplankton and ciliate abundances, whilst nutrient drawdown was noticed throughout the upper 60 m, suggesting that access to nutrients from the deep might have boosted primary production and the growth of primary consumers (ciliates). Deep mixing caused, for instance, by strong winds or fronts, has been suggested to sustain SCM in seasonally-stratified shelf seas [ 40 ] and in frontal regions of the Fram Strait [ 41 ]. This study provides evidence that an episodic wind surge during summer temporarily disrupted the vertical stratification of the water column.…”

The power of multi-matrix monitoring in the Pan-Arctic region: plastics in water and sediment

“…The filter-holder should be deployed upstream against the current and sampling should be carried out with the engine off (Norén & Naustvoll, 2011;Granberg et al, 2019). Another approach could be to sample water for microplastic analyses along pre-set vertical or horizontal paths of autonomously operated vehicles (AUVs) which would allow assessments along specific sub-mesoscale features such as fronts or the marginal ice zone (Wulff et al, 2016;Choy et al, 2019;Tippenhauer et al, 2021) and avoid the research vessel footprint. Sample replication will be vital for the robustness of this approach in accounting for variation in the sample medium, thus further research and 15 development is required before AUV techniques can be recommended for monitoring microplastics.…”