Seawater Carbonate Chemistry at the Flower Garden Banks National Marine Sanctuary in the Northwestern Gulf of Mexico Following Hurricane Harvey

The Flower Garden Banks National Marine Sanctuary (FGBNMS) is a vital coral reef ecosystem located on the edge of the continental shelf, ~200 km off the Texas-Louisiana coast. FGBNMS coral reefs remain some of the healthiest reefs in the Caribbean and Gulf of Mexico (GoM). Corals build aragonite (a type of calcium carbonate (CaCO₃)) skeletons that make up the reef structure and support the entire ecosystem. Coral reef calcification declines, and CaCO₃ dissolution increases, as pH and aragonite saturation state (Ω_a) decrease with global and coastal ocean acidification. Here we present carbonate chemistry data from 3 consecutive cruises (Sep, Oct and Nov 2017) following Hurricane Harvey (Aug 2017). Although Harvey did not hit the FGBNMS directly, storm runoff can cause coastal acidification and negatively impact coral reefs. Depth profiles were collected in a cross-shelf transect from Galveston, Texas to the FGBNMS, and in a grid that includes the East and West Bank coral reefs. Surface salinity, pH, and Ω_a increase moving offshore, with decreased salinities indicating Harvey runoff extending ~100 km offshore, but not reaching the FGBNMS, during our Sep cruise. Across all three cruises, average surface salinity, pH, and Ω_a within the FGBNMS were 35.4 ± 2.6, 8.06 ± 0.02, and 4.01 ± 0.21, respectively. pH and Ω_a decrease with depth due to an increase in net respiration, with minimum pH (~) and Ω_a (~1.65) at about 290 m depth. At the depth of the reefs (~20 m), pH and Ω_a are at levels that are supportive for coral reef calcification with averages of 8.06 ± 0.02, and 4.00 ± 0.08, respectively. Following Harvey and before our cruises, there were 2 short periods of time (<48 h) where salinity in the FGBNMS decreased from ~36 to 32. However, our data do not indicate any impact of Harvey runoff on FGBNSM water chemistry during our cruises. Our data indicate that changes in pH and Ω_a from Sep through Nov were primarily driven by decreasing water temperatures. Hurricanes are usually associated with physical damage to coral reef ecosystems, but runoff-induced changes in water chemistry can also have negative consequences for shelf and offshore marine ecosystems.