Spatial and Temporal Variability of pCO₂, Carbon Fluxes, and Saturation State on the West Florida Shelf

Abstract

The West Florida Shelf (WFS) is a source of uncertainty for the Gulf of Mexico carbon budget.Data from the synthesis of approximately 135,000 pCO 2 values from 97 cruises from the WFS show that the shelf waters fluctuate between being a weak source to a weak sink of carbon. Overall, the shelf acts as a weak source of CO 2 at 0.32 ± 1.5 mol m À2 yr À1 . Subregions, however, reveal slightly different trends, where surface waters associated with 40-200-m isobath in the northern and southern WFS are generally weak sinks all year, except for summer when they act as sources of CO 2 . Conversely, nearshore waters (<40 m) are a source of CO 2 , particularly the southern shallow waters, which are a source all year round. The pCO 2 of seawater has been increasing at a rate of approximately 4.37 μatm/year as compared to atmospheric pCO 2 which has increased at a rate of about 1.7 μatm per year from 1996 to 2016. The annual CO 2 flux has increased from À0.78 to 0.92 mol m À2 yr À1 on the shelf from 1996-2016. The WFS is emitting 9.23 Tg C/year, with the southern nearshore region emitting the most at 9.01 Tg C/year and the northern region acting as a sink of À1.96 Tg C/year. Aragonite saturation state on the WFS shows seasonal and geographic trends with values ranging from 2 to 5. Lowest values are found in winter associated with subregion <40-m isobath.Plain Language Summary The West Florida Shelf (WFS) is a source of uncertainty for determining the Gulf of Mexico carbon budget and how surface waters are being affected by increasing atmospheric carbon dioxide (CO 2 ) levels. Little is known about the WFS trends of the seawater partial pressure of carbon dioxide (pCO 2 ) over the last decades; much of the uncertainty stems from lack of data. In order to address some of this uncertainty, approximately 135,000 pCO 2 values collected on 97 research cruises between 1996 and 2016 were analyzed and show that the shelf waters have changed from being a weak sink to weak source of CO 2 to the atmosphere. Further, data was divided into four geographical subregions. Offshore surface waters absorb CO 2 , whereas nearshore surface waters emit CO 2 to the atmosphere. Importantly, pCO 2 of the nearshore seawater has been increasing at a rate approximately 2.5 times faster than atmospheric pCO 2 over the past 20 years. These data indicate that factors in addition to the atmosphere CO 2 are influencing increases in nearshore seawater. Additionally, WFS aragonite saturation state, often used to monitor ocean acidification conditions, shows seasonal and geographic trends, with year-round supersaturated values ranging from 2 to 5. Key Points:• WFS surface water shows a change from carbon sink to source from 1996 to 2016 • The WFS emits 9.23 Tg C/year, with the south nearshore emitting the most at 9.01 Tg C/year and the north acting as a sink of À1.96 Tg C/year • Shelf water (<40-m isobath) pCO 2 has increased at a rate approximately 2.6 times faster than the atmospheric rate in the past 20 yearsSupporting Information:• Supporting Information S1

Large Decadal Changes in Air‐Sea CO2 Fluxes in the Caribbean Sea

“…The winters of 2010 and 2011 showed anomalous low SSTs for much of the North Atlantic, including the Caribbean (Buchan et al, 2013). Rivière and Drouard (2015) point out the contrasting phase of the NAO during 2010 and 2014. The negative NAO index in 2010 corresponds to low SST and low fCO 2w while maximum in the NAO in 2014 is associated with higher SSTs and fCO 2w .…”

Reply to review by Niki Gruber

“…A more recent estimate for the west Florida shelf is 4.4±18.0 g C m −2 yr −1 (Robbins et al, 2018). Huang et al (2015) estimated a larger uptake on the northern gulf shelf of 11 ± 44 g C m −2 yr −1 (i.e., about twice the estimate of Robbins et al, 2014) and reported a much larger uncertainty. In an analysis that combines satellite and in situ observations, Lohrenz et al (2018) estimated a similar uptake for the northern Gulf of Mexico of 13 ± 3.6 g C m −2 yr −1 .…”

“…Smaller shelf regions within the gulf differ markedly from this mean flux. The West Florida shelf and western gulf shelf act as sources to the atmosphere, with estimated annual average fluxes of 4.4 ± 1.3 and 2.2 ± 0.6 g C m −2 yr −1 , respectively; the northern gulf acts as a sink, with an estimated flux of 5.3 ± 4.4 g C m −2 yr −1 , and the Mexican shelf is almost neutral, with an estimated uptake flux of 1.1±0.6 g C m −2 yr −1 (Robbins et al, 2014). A more recent estimate for the west Florida shelf is 4.4±18.0 g C m −2 yr −1 (Robbins et al, 2018).…”

“…The West Florida shelf and western gulf shelf act as sources to the atmosphere, with estimated annual average fluxes of 4.4 ± 1.3 and 2.2 ± 0.6 g C m −2 yr −1 , respectively; the northern gulf acts as a sink, with an estimated flux of 5.3 ± 4.4 g C m −2 yr −1 , and the Mexican shelf is almost neutral, with an estimated uptake flux of 1.1±0.6 g C m −2 yr −1 (Robbins et al, 2014). A more recent estimate for the west Florida shelf is 4.4±18.0 g C m −2 yr −1 (Robbins et al, 2018). Huang et al (2015) estimated a larger uptake on the northern gulf shelf of 11 ± 44 g C m −2 yr −1 (i.e., about twice the estimate of Robbins et al, 2014) and reported a much larger uncertainty.…”

Seasonal patterns of surface inorganic carbon system variables in the Gulf of Mexico inferred from a regional high-resolution ocean-biogeochemical model

“…Previous studies have 265 described to some degree surface pCO 2 seasonality within the GoM (e.g. Lohrenz et al, 2010;Robbins et al, 2018), but less has been done to describe seasonal patterns for other inorganic carbon system variables. In the present study, we focused our analysis on the seasonal cycles of surface pCO 2 and Ω Ar , but seasonal patterns of surface DIC and TA were also reported.…”