Multi-Year Observations of Fluorescence and Backscatter at the Southern Ocean Time Series (SOTS) Shed Light on Two Distinct Seasonal Bio-Optical Regimes

Christina Schallenberg,Thomas W Trull,Peter Jansen,Diana M Davies,James W Harley

doi:10.3389/fmars.2019.00595

Abstract

This work presents insights from 6 years of chlorophyll-a fluorescence and backscatter (700 nm) data at the Southern Ocean Time Series (SOTS) moorings, located in the Subantarctic Zone southwest of Tasmania. Using local calibrations from available voyage data, the fluorescence and backscatter records were related to chlorophyll-a (Chl) and particulate organic carbon (POC), allowing us to estimate and interpret carbon:Chl ratios. Surprisingly, observed carbon:Chl ratios were higher in winter than in summer, indicating that photo-acclimation of phytoplankton to decreased light levels in the deep winter mixed layer is not the main signal. Instead, the data suggest a seasonal succession of two trophodynamic regimes at SOTS: a phytoplankton-dominated community in summer, while in winter the proportion of “non-phytoplankton” POC increases. The two regimes can also be differentiated in an optical index based on fluorescence and backscatter, indicating two distinct bio-optical populations. Seasonal iron limitation and deep winter mixing in the SAZ, reaching as deep as 600m, likely play key roles in setting the stage for the observed ecological succession of the two trophodynamic regimes.

Highlights

The Subantarctic Zone (SAZ) plays an important role for the marine biological carbon pump because it lies at the interface between two contrasting regions: the nutrient rich polar seas to the south and the nutrient poor subtropical gyres to the north
The regression for P3 excluding the two highest points yields a slope of 1.5 (Figure 5A), which is lower than the factor 2.3 reported by Trull et al (2019) for samples taken in March at the Southern Ocean Time Series (SOTS) site, but it diverges in the same direction from the average slope of 3.12
The presented analysis of the fluorescence and backscatter records from SOTS has shed new light on the system in several ways. Both the FChl:bbp optical index and the derived C:Chl ratio indicate that the there is a clear distinction between the summer and winter microplankton community

Summary

Introduction

The Subantarctic Zone (SAZ) plays an important role for the marine biological carbon pump because it lies at the interface between two contrasting regions: the nutrient rich polar seas to the south and the nutrient poor subtropical gyres to the north. Changes in the efficiency of its nutrient consumption appear to have modulated atmospheric CO2 levels on glacial/interglacial timescales (Sigman and Boyle, 2000). The efficiency of this pump affects ocean productivity outside the Southern Ocean via the control of nutrient delivery in the upper limb of the overturning circulation (Sarmiento et al, 2004). The SAZ is characterized by both iron and light limitation (Sedwick et al, 1999; Boyd et al, 2001). Deposition of aerosols can relieve iron limitation in summer, while deep mixing and Ekman transport provide an initial winter reserve of iron (Ellwood et al, 2008; Bowie et al, 2009).

Methods

Results

Conclusion