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 pCO2 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 CO2 at 0.32 ± 1.5 mol m−2 yr−1. 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 CO2. Conversely, nearshore waters (<40 m) are a source of CO2, particularly the southern shallow waters, which are a source all year round. The pCO2 of seawater has been increasing at a rate of approximately 4.37 μatm/year as compared to atmospheric pCO2 which has increased at a rate of about 1.7 μatm per year from 1996 to 2016. The annual CO2 flux has increased from −0.78 to 0.92 mol m−2 yr−1 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 −1.96 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 (CO2) levels. Little is known about the WFS trends of the seawater partial pressure of carbon dioxide (pCO2) over the last decades; much of the uncertainty stems from lack of data. In order to address some of this uncertainty, approximately 135,000 pCO2 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 CO2 to the atmosphere. Further, data was divided into four geographical subregions. Offshore surface waters absorb CO2, whereas nearshore surface waters emit CO2 to the atmosphere. Importantly, pCO2 of the nearshore seawater has been increasing at a rate approximately 2.5 times faster than atmospheric pCO2 over the past 20 years. These data indicate that factors in addition to the atmosphere CO2 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 −1.96 Tg C/year
Shelf water (<40‐m isobath) pCO2 has increased at a rate approximately 2.6 times faster than the atmospheric rate in the past 20 years
