Variability across a Filaments Edge off Mauritania: Insights from a Field Study into Plankton Community and Molecular Characteristics of the Uppermost Ocean Layer

Abstract:

Eastern Boundary Upwelling Systems (EBUS) are highly productive and dynamic regions creating hydrographic fronts, which potentially translate into submesoscalic biogenic patterns. We investigated the molecular and microbial composition of the sea surface microlayer (SML) by following a filament off Mauritania. Data were acquired in early spring 2018 from board the RV POSEIDON by means of a screen-sampler, resolving the uppermost 0.5mm of the ocean in comparison to reference samples taken from 2m depth. Stratified waters outside (≥17.1°C) opposed colder and upwelled waters inside the filament (≤16.6°C). Identification of the filament was based on remote sensing of sea surface temperature and Chla (MODIS-Aqua), provided by the NASA Ocean Biology Processing Group, and microstructure profiles across the front, exhibiting abrupt changes within 5km. Several classes of organisms and substances were discriminated including flow cytometer resolved size classes of planktonic cells, total organic carbon, total amino acids (TAA) and combined carbohydrates (TCCHO). In general, a greater SML enrichment of autotrophic and heterotrophic organisms was found outside the filament (e.g. enrichment of nano-eukaryotic phytoplankton cells outside 1.2 versus 0.7 inside, Mann-Whitney-U Test, p<0.028, phytopigment Chla) accompanied by an advanced state of microbial degradation (indices derived from the relative molecular fractions of single TAA’s and its contribution to total nitrogen). Phytoplankton community drastically changed across the filament’s front as pico-prokaryotic cells (phytopigment Phycoerythrin) increased by one order of magnitude. Glucose contributed a fraction of 35 and 64% in the morning and afternoon respectively to TCCHO, matching precisely the course of the day and exhibiting significant enrichment within the SML (paired Mann-Whitney-U Test, p<0.001). This study provides valuable insight into a complex uppermost ocean plankton community and biogenic substance dynamics. We aim to explore and discuss possible approaches to make use of our spatial and temporal carefully resolved, discrete field data set. Upscaling of ocean surface properties will help to address urgent questions in climate science such as the global effect of dampening by surface active substances on air-sea gas exchange.