Increased Frequency of Sediment Heatwaves in a Virginia Seagrass Meadow

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Department of Biological Sciences


Coastal marine heatwaves have destructive and lasting impacts on foundational species and are increasing in frequency, duration, and magnitude. High atmospheric temperatures are often associated with marine heatwaves (MHW) which are defined as 5 days of water temperatures above a seasonally varying 90th percentile threshold. In this study, we consider the prevalence of MHW propagation into surficial sediments to cause sediment heatwaves (SHW). Within a shallow, subtidal seagrass meadow in Virginia, USA, sediment temperature was measured at hourly intervals at a depth of 5 cm between June 2020 and October 2022 at the meadow edge and central meadow interior. The observed sediment temperature along with a 29-year record of water temperature and water level was used to develop a sediment temperature model for each location. Modeled sediment temperatures were used to identify sediment heatwaves that may thermally stress belowground seagrass. At both meadow locations, sediment heatwave frequency increased at a rate twice that of MHWs in the average global open ocean, coinciding with a 172% increase in the annual number of SHW days, from 11 to 30 days year−1 between 1994 and 2022. Sediment heatwaves at both meadow locations co-occurred with a MHW 79–81% of the time, with nearly all SHWs having a zero day lag. The top 10% most extreme MHWs and SHWs occurred between November and April when thermal stress to seagrass was unlikely. In June 2015, a SHW co-occurred with an anomalously long duration MHW that was associated with a 90% decline in seagrass from this system, suggesting that SHWs may have contributed to the observed seagrass loss. These results document heatwave propagation across the pelagic-sediment interface which likely occur broadly in shallow systems with impacts to critical coastal ecosystem processes and species dynamics.

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Estuaries and Coasts