The Photophysiological Response of Nitrogen-Limited Phytoplankton to Episodic Nitrogen Supply Associated With Tropical Instability Waves in the Equatorial Atlantic

Jonathan Sherman,Maxim Y Gorbunov,Peter Brandt,Ajit Subramaniam,Paul G Falkowski,Ana Fernández-Carrera,Rainer Kiko

doi:10.3389/fmars.2021.814663

Abstract

In the Equatorial Atlantic nitrogen availability is assumed to control phytoplankton dynamics. However, in situ measurements of phytoplankton physiology and productivity are surprisingly sparse in comparison with the North Atlantic. In addition to the formation of the Equatorial cold tongue in the boreal summer, tropical instability waves (TIWs) and related short-term processes may locally cause episodic events of enhanced nutrient supply to the euphotic layer. Here, we assess changes in phytoplankton photophysiology in response to such episodic events as well as short-term nutrient addition experiments using a pair of custom-built fluorometers that measure chlorophyll a (Chl a) variable fluorescence and fluorescence lifetimes. The fluorometers were deployed during a transatlantic cruise along the Equator in the fall of 2019. We hypothesized that the Equatorial Atlantic is nitrogen-limited, with an increasing degree of limitation to the west where the cold tongue is not prominent, and that infrequent nitrate injection by TIW related processes are the primary source alleviating this limitation. We further hypothesized phytoplankton are well acclimated to the low levels of nitrogen, and once nitrogen is supplied, they can rapidly utilize it to stimulate growth and productivity. Across three TIW events encountered, we observed increased productivity and chlorophyll a concentration concurrent with a decreased photochemical conversion efficiency and overall photophysiological competency. Moreover, the observed decrease in photosynthetic turnover rates toward the western section suggested a 70% decrease in growth rates compared to their maximum values under nutrient-replete conditions. This decrease aligned with the increased growth rates observed following 24 h incubation with added nitrate in the western section. These results support our hypotheses that nitrogen is the limiting factor in the region and that phytoplankton are in a state of balanced growth, waiting to “body surf” waves of nutrients which fuel growth and productivity.

Highlights

The Atlantic Ocean is the best studied ocean basin
To decrease the potential impact of pressure-related stress on phytoplankton and their photophysiology as they pass through the intake pump, the underway system was connected to a membrane type pump which results in less shear (Cetinicet al., 2016)
Our results strongly suggest that dissolved nitrogen is the primary limiting nutrient for phytoplankton across the Equatorial Atlantic (EA) in the boreal summer and that tropical instability waves (TIWs) related processes are critical to its supply and ensuing new production

Summary

Introduction

The Atlantic Ocean is the best studied ocean basin. the vast majority of research on phytoplankton dynamics have been focused on the North Atlantic. In situ data in the EA is sporadic, and is based mostly on data collected during Equatorial crossings of European ships of opportunity such as the Atlantic Meridional Transect (AMT) program transiting to and from the Southern Ocean (Rees et al, 2015) These cruises mostly occur between April–June and September–November, and as such, generally miss the most productive boreal summer months when, based on satellite observations, phytoplankton bloom (Longhurst, 1993; Pérez et al, 2005; Grodsky et al, 2008). Phytoplankton dynamics in this region are poorly understood This sampling disparity is interesting, since satellite primary productivity assessments estimate that the EA (between 10◦N and 10◦S) contributes more to global carbon fixation than the entire spring bloom region of the North Atlantic (Longhurst, 1993). Studies have shown that most of the tropical Atlantic, including the EA where equatorial upwelling occurs, can be considered at times as a low-nutrient low-chlorophyll (LNLC) region in which nitrogen is hypothesized to be the limiting factor for phytoplankton growth and photosynthetic yields (Mills et al, 2004; Davey et al, 2008; Moore et al, 2008, 2013)

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Results

Discussion

Conclusion