Flower Garden Banks: Long-Term Changes to the Upper Ocean with Implications for Ecosystem Health
Abstract
The Flower Garden Banks National Marine Sanctuary is home to vibrant coral reef ecosystems that continue to prosper despite global coral reef decline. The health of these reefs is attributed to their advantageous location—offshore and in moderate depths (16-46 m). During the summertime when temperatures are high and bleaching stress manifests, the upper portion of the reef resides within the mixed layer. Although the depth of the mixed layer has been shown to impact the bleaching response, the effects of long-term changes to the mixed layer have yet to be examined particularly as they relate to coral health. Here, by analyzing 29 years of water column observations near the Flower Garden Banks (1993-2021), a significant deepening of the summertime mixed layer at a rate of 2.4±0.96 m/decade was detected along with a significant warming of the upper ocean (15-50 m) by 0.3-0.7 C/decade. There is some evidence to suggest that changes to the local wind speed may have played a role in the observed deepening. During this same period, the pycnocline’s stratification weakened by 1.7±1.3 x10-4 s -2dec-1 due to rising salinity in the mixed layer. These trends may lead to damaging consequences for coral health as a deepening mixed layer would expose a greater portion of the reef to warmer surface conditions during the summer, the season when potential for warming-induced bleaching is already highest. This could put the coral within the mesophotic zone, which is typically located below the mixed layer during the summer, at a greater risk for bleaching. As the upper ocean shapes the thermal environment experienced on reefs, a change in its characteristics may have wide-ranging implications for the future coral ecosystems.
Description
Keywords
coral reefs, Flower Garden Banks