Revising upper-ocean sulfur dynamics near Bermuda: new lessons from 3 years of concentration and rate measurements

Abstract

Environmental context. Microscopic marine organisms have the potential to influence the global climate through the production of a trace gas, dimethylsulfide, which contributes to cloud formation. Using 3 years of observations, we investigated the environmental drivers behind the production and degradation of dimethylsulfide and its precursor dimethylsulfoniopropionate. Our results highlight the important role of the microbial community in rapidly cycling these compounds and provide an important dataset for future modelling studies.Abstract. Oceanic biogeochemical cycling of dimethylsulfide (DMS), and its precursor dimethylsulfoniopropionate (DMSP), has gained considerable attention over the past three decades because of the potential role of DMS in climate mediation. Here we report 3 years of monthly vertical profiles of organic sulfur cycle concentrations (DMS, particulate DMSP (DMSPp) and dissolved DMSP (DMSPd)) and rates (DMSPd consumption, biological DMS consumption and DMS photolysis) from the Bermuda Atlantic Time-series Study (BATS) site taken between 2005 and 2008. Concentrations confirm the summer paradox with mixed layer DMS peaking ,90 days after peak DMSPp and ,50 days after peak DMSPp : Chl. A small decline in mixed layer DMS was observed relative to those measured during a previous study at BATS (1992)(1993)(1994), potentially driven by long-term climate shifts at the site. On average, DMS cycling occurred on longer timescales than DMSPd (0.43 AE 0.35 v. 1.39 AE 0.76 day À1 ) with DMSPd consumption rates remaining elevated throughout the year despite significant seasonal variability in the bacterial DMSP degrader community. DMSPp was estimated to account for 4-5 % of mixed layer primary production and turned over at a significantly slower rate (,0.2 day À1 ). Photolysis drove DMS loss in the mixed layer during the summer, whereas biological consumption of DMS was the dominant loss process in the winter and at depth. These findings offer new insight into the underlying mechanisms driving DMS(P) cycling in the oligotrophic ocean, provide an extended dataset for future model evaluation and hypothesis testing and highlight the need for a reexamination of past modelling results and conclusions drawn from data collected with old methodologies.

Sea-surface dimethylsulfide (DMS) concentration from satellite data at global and regional scales

“…Note also the irregular occurrence of high DMS at the BATS station in late summer in different years ( Fig. 9d; Levine et al, 2016), and that a BATS-like seasonality is not observed at other sites with late summer macronutrient limitation (Archer et al, 2009;Belviso et al, 2012;Galí and Simó, 2015;Vila-Costa et al, 2008). Altogether, these observations suggest that regional macronutrient stress responses are difficult to generalize.…”

“…This exercise shows our limited capacity to account for relevant biogeochemical pro- (Asher et al, 2017;Levasseur et al, 2006;Royer et al, 2010). BATS data are comprised of the monthly 2005-2008 time series (Levine et al, 2016) and cruise data from the vicinity of BATS station in July 2004 (Bailey et al, 2008;Gabric et al, 2008). cesses and explain their interannual variation using satellite data, and identifies knowledge gaps that need to be tackled to improve diagnostic and prognostic modeling of oceanic DMS(P).…”

“…Yet examination of the BATS and OSP time series does not support this explanation. At both sites, the summer MLD is stable at about ≤ 20 m and deepens slowly in late summer (Levine et al, 2016;Steiner et al, 2012), such that PAR MLD declines faster than sea-surface PAR SAT (Eq. 1).…”

“…Satellite time series were compared to the L11 climatology and to in situ DMS and DMSPt. These in situ data, kindly provided by the BATS (Levine et al, 2016) and OSP (https://www.waterproperties. ca/linep/, last access: 29 November 2017) teams, were not used in algorithm development.…”

Foreword to the special issue on ‘Biological and environmental chemistry of DMS(P) and related compounds'

“…Lee et al [16] conduct experiments in two contrasting open-ocean settings and show that addition of vitamin B 12 often results in decreased DMS and DMSP concentrations; potential biochemical links between vitamin B 12 and dimethylated sulfur are discussed. Levine et al [17] revisit the most emblematic study site for DMS(P) cycling seasonality in the oligotrophic ocean: Bermuda; in a new, 3-year time series, they show that microbial consumption is the main driver of DMSP and DMS turnover throughout most of the year, whereas photolysis drives DMS loss in the mixed layer during the summer. Once again, this study stresses the importance of the (costly) efforts to maintain time series studies in the open ocean.…”