Dataset: Seawater temperature between Cabritte Horn and White Point on the south shore of St. John, US Virgin Islands, 1996-2018

From publication Limnology and Oceanography

Environmental conditions

Seawater temperature and rainfall were tested for their capacity to account for variation in the dynamics of small corals in the target size range. Seawater temperature was measured every 15–30 minutes using loggers deployed in two locations. Most measurements came from Yawzi Point, where temperature was recorded at 11-m depth using a Ryan Industries thermistor (± 0.3°C) from 1996 to June 1997, and from November 1997 to August 1999. From May 1997 to October 1997, and from August 1999 to August 2001, Optic Stowaway loggers (± 0.2°C) were used, and from August 2001 to the present, Hobo Aquapro loggers (± 0.2°C) were used. Gaps in the time-series of seawater temperature from Yawzi Point were created by periodic equipment malfunction, and these gaps were filled with records from 14-m depth at Tektite (~ 500 m southwest) that were recorded with Hobo Aquapro loggers. A representative year (2015–2016) of temperature records from Yawzi Point and Tektite showed that the daily temperature differed by ≤ 0.1°C between these sites (mean = 0.043 ± 0.003°C, n = 368). Temperature was not consistently measured at the six sites at which small corals were surveyed, but records from 2016–2017 show that the mean (± SE) variation between Cabritte Horn (9 m depth, easternmost site) and White Point (5 m depth, westernmost site) was 0.14 ± 0.01°C (warmer to the west). Among site differences were less than the resolution of the loggers (± 0.2°C). Temperature records from Yawzi Point were averaged by day and used to create two summary statistics for the 12 months between 1 August and 31 July of the subsequent year for 22 years: mean temperature and an annualized summation of thermally extreme days.

The mean temperature was used to characterize the year between samplings. The extent to which temperature potentially was unfavorable to corals was evaluated by calculating the number of upwardly thermally extreme days over 12 months (in units of weeks). In this calculation, hot days were gauged relative to the local coral bleaching threshold for sea surface temperature (29.4°C), as determined by the United States National Oceanic and Atmospheric Administration (http://coralreefwatch.noaa.gov/vs/index.php). Days between 1 August and 31 July of the following year were ranked by the extent to which their daily mean temperature exceeded 29.4°C, with a rank of 0 for ≤ 29.4°, a rank of 1 for > 29.4°C but ≤ 30.4°C, and a rank of 2 for > 30.4 but ≤ 31.4°C (mean daily temperature did not exceed 31.4°C). Annualized summaries of thermal extremes were obtained by summing the daily ranks in each 12 month period and dividing by 7 to provide units of weeks.