Biophysical structure and particle dynamics of an algal bloom within a mesoscale eddy

Abstract

Mesoscale eddies create dynamically distinct environments where thermohaline fronts partition the biological and optical structure of the upper water column. How these fronts organize the vertical and lateral structure of spring blooms is a longstanding observational challenge, largely because the bloom environment evolves rapidly across multiple spatial and temporal scales that are difficult to capture with conventional ship‐based sampling. This study presents a novel in situ dataset, combined with model and satellite‐derived products, examining how the thermohaline front of an anticyclonic eddy in the central Labrador Sea controls the biophysical structure of a spring bloom. The Air–Sea Interaction Profiler (ASIP), an autonomous upwardly rising float, collected 112 high‐resolution vertical profiles to 100 m depth during May 2024. Satellite observations of sea surface temperature, salinity, and chlorophyll provided complementary surface context. A well‐defined thermohaline front, marked by the 3°C isotherm and 33.8 g kg ⁻¹ isohaline, separated a cool, fresh, productive eddy core from warmer, saltier, lower biomass surrounding waters. Within the eddy core, a shallow mixed layer of 10–40 m overlay a sharp pycnocline, below which chlorophyll fluorescence and optical backscatter organized into persistent subsurface maxima tightly coupled to the density structure. Turbulence dissipation decreased from 10 ⁻⁷ –10 ⁻⁶  W kg ⁻¹ near the surface to 10 ⁻⁹ –10 ⁻⁸  W kg ⁻¹ at the pycnocline, where moderate shear may have promoted large‐particle aggregation while stratification limited dispersion. Physical and biological characteristic depths shifted across the frontal boundary, demonstrating that the thermohaline front exerts control over the organization of the spring bloom.