From the Coast to the Deep Sea: Carbonate Chemistry of Calcifying Ecosystems

Abstract

Increasing ocean acidification (OA) driven by anthropogenic activities represents a significant threat to calcifying reef ecosystems worldwide. Calcifying reefs, such as coral and oyster reefs, provide valuable ecosystems services including coastal protection, fishery habitat, and recreational and tourism opportunities. However, our understanding of acidification under extreme conditions (i.e., tropical cyclones) and environments (i.e., deep ocean) is currently limited. In the following studies, we focus on the carbonate chemistry of two calcifying ecosystems: coastal oyster reefs in Galveston Bay and deep-sea coral reefs in the Hawaiian Emperor Seamount Chain (HESC).

In Galveston Bay, we show that river discharge exhibits a strong control on spatiotemporal variability of bay and river water carbonate chemistry signals, with enhanced acidification under wet (i.e., high discharge) conditions. We also document the first records of tropical cyclone-induced undersaturation, which persisted for multiple weeks after two storms made landfall. Undersaturation was driven by high volumes of rainfall and runoff from a Category 4 Hurricane and a Tropical Storm. Our results reveal that rainfall rates, rather than storm severity (i.e., wind speed), more accurately predict tropical cyclone effects on coastal acidification. However, levels of low aragonite saturation harmful to oysters was not exclusive to post-storm periods and occurred more than 15% of the time during our study in response to increased local river fluxes.

In the HESC, we characterized the carbonate chemistry signatures of major water masses in the central North Pacific, including intermediate waters (300-800 m) where deep-sea coral reef ecosystems have been found. We hypothesize that the influence of the more northern, acidic Pacific Subpolar Intermediate Water, characterized by hypoxia (low oxygen) and aragonite undersaturation, acts as a biogeochemical barrier for deep-sea reef development in the HESC. Our results also show that seamount topography can affect physical currents and alter the water chemistry of benthic seamount communities. While the average reef depths are currently near or above the aragonite saturation horizon (ASH, the boundary between undersaturated and supersaturated water), long-term shoaling of the ASH due to anthropogenic OA, along with seasonal variability, represent an increasing risk to these reef ecosystems.