Investigation of Changing Seawater Chemistry by Deep-Sea Coral Beds Due to Climate Change

Abstract

Anthropogenic impacts on the global ocean, including increasing carbon dioxide levels and sea surface temperatures, have significantly changed the chemistry of the seawater itself, by acidifying waters, creating low oxygen zones and changing nutrient levels. Deep-sea corals are vulnerable to these environmental changes, as calcification of their skeletons becomes more difficult under acidified conditions. Nutrient availability from the surface may also change and affect the food supply to deep-sea corals. This study analyzed the current chemistry of seawater around deep-sea coral beds in the Northwestern Hawaiian Islands with data from two cruises conducted in 2014 and 2015 and compared it to historical measurements from the 1990s from the same region taken from the World Ocean Circulation Experiment (WOCE). The possibility of shoaling calcium carbonate (CaCO3) saturation horizons due to climate change was assessed. The saturation horizons of two forms of CaCO3—calcite and aragonite—were found to have shoaled since the 1990s. Aragonite saturation horizons shoaled from 747 to 652 m and calcite saturation horizons shoaled from 1006 to 854 m. The shoaling of aragonite saturation horizons over time was consistent for two different methods for calculating horizon depth while shoaling of calcite saturation horizons was only shown for one of the two methods. Shoaling rates from the 1990s to the 2010s has also accelerated, compared to rates from the preindustrial era to the 1990s. The aragonite saturation horizon is shoaling 28 times faster and the calcite saturation horizon 32 times faster than shoaling rates determined in the 1990s. This study found saturation horizons of calcite and aragonite are becoming shallower faster, which means that deep-sea corals that were previously in saturated waters are now experiencing undersaturation.