National Coral Reef Monitoring Program
Climate Monitoring Brief: St. Thomas and St. John USVI
Diver swaping out a temperature instrument at Newfound Reef
NOAA/Atlantic Oceanographic & Meteorological Laboratory (AOML) Coral
Program
University of Miami Cooperative Institute of Marine and
Atmospheric Studies (CIMAS)
N. Besemer, A. Palacio, T.Gill, M. Studivan, K. Simmons, I. Enochs - March 2024
Mission
The AOML Coral Program tracks the status and trends of coral reef ecosystems of the U.S. Atlantic and Caribbean as part of the National Coral Reef Monitoring Program (NCRMP). This summary brief provides an overview of the most recent climate monitoring efforts in St. Thomas and St. John (US Virgin Islands).
Expedition summary
- The NCRMP Atlantic climate monitoring team recently traveled to St. Thomas and St. John from July 19th to July 29th, 2023
- 27 different sites were visited by four team members and collaborators completing a total of 68 dives.
Data collection summary
Subsurface temperature: Subsurface temperature recorders (STRs) were recovered and redeployed at all seven transects, each one composed by four depths (Fig. 1). In total, more than 10 million temperature observations were collected from 26 instruments (Table 1)
| Site | 1m | 5m | 15m | 25m | Total |
|---|---|---|---|---|---|
| STT North | 148,882 | 138,022 | 626,111 | 439,058 | 1,352,073 |
| STT South | 625,219 | 354,882 | 488,880 | 455,315 | 1,924,296 |
| STJ West | 138,804 | 554,763 | 621,498 | 579,392 | 1,894,457 |
| STJ South | 624,720 | 591,584 | 335,248 | 562,830 | 2,114,382 |
| STJ East | 139,666 | 431,320 | 548,123 | 623,809 | 1,742,918 |
| STT West | NA | 80,756 | 438,556 | 470,210 | 989,522 |
| STJ North | NA | 489,383 | 300,495 | 583,640 | 1,373,518 |
NCRMP Climate fixed sentinel site monitoring: At Brewers Bay - 5m site, located in St. Thomas, short term instruments (72h) were deployed to monitor daily fluctuations in:
- Current: 303 observations
- pH: 307 observations
- Light: 301 observations
- Carbonate Chemistry: 17 samples collected
Habitat persistence: Changes in bioerosion and accretion were monitored:
- Carbonate budget surveys: Benthic cover, sponge, urchin, and parrotfish surveys completed at six transects
- Bioerosion: 70 Bioerosion Monitoring Units (BMUs) collected, 70 redeployed
- Calcification: 35 Calcification Accretions Units (CAUs) collected, 35 redeployed
Figure 1: Study sites and depths in St. Thomas and St. John USVI. All study areas have 4 depth points associated with them except for the north transect of St. John and the west transect in St. Thomas.
Subsurface temperature
The temperatures that marine organisms experience are a function of local oceanographic conditions and vary with depth. To monitor subsurface temperature, seven transects were established around the two islands. Each transect consists of STRs at four depths (1, 5, 15, 25m; Fig.2). Temperature was measured using SeaBird Electronics Subsurface Temperature Recorders (STR)s that collected data at 5-minute intervals. The instruments are usually swapped every 3 years for data collection but the COVID - 19 pandemic delayed field work in 2020. Local contractors were hired to swap the instruments as travel was not allowed. Gaps exist in the data as not all instruments survived the extended deployment time. The full time series of data from 2017 - 2023 at all six locations and all four depths are provided to allow for temporal and spatial comparison of subsurface temperature.
Figure 2: Temperature conditions at seven locations in St. Thomas and St. John (Savana, Inner Brass, Brewers Bay and Southwaters, Stevens Cay, Johnson’s Reef, Tektite) representing a depth gradient (1m, 5m, 15m and 25m). Data were collected from August 2017 to July 2023.
Mean temperature values were similar among the locations and depths. The lowest temperatures generally occurred in February (mean: 26.2\(^\circ\)C, min: 25.0\(^\circ\)C, max: 29.1\(^\circ\)C) and the highest temperatures in September (mean: 29.3\(^\circ\)C, min: 27.1\(^\circ\)C, max: 32.1\(^\circ\)C). Temperature values in the shallow (1m) South transects at STT and STJ were more variable compared to the rest of the stations (Fig. 2).
Diurnal suite deployment
Seawater carbonate chemistry can fluctuate diurnally, due to biological forcing processes such as photosynthesis and respiration, as well as calcification and dissolution. To characterize this, discrete water samples (Fig. 3) were collected at three-hour intervals (n=15) using Subsurface Automatic Samplers (SAS, www.coral.noaa.gov/accrete/sas).
These samples will be analyzed for Total Alkalinity (TA), Dissolved Inorganic Carbon (DIC), and SpectrophotometricpH (SpecpH), which will be used to calculate pCO2 and aragonite saturation state (ΩAragonite). A suite of instruments was deployed for a 72-hour period at the Brewers Bay 5m site. A SeaFET was used to log pH, an EcoPAR measured Photosynthetically Active Radiation (PAR), and a Lowell Tiltmeter measured current speed and direction. Each collected measurements at 15-minute intervals (Fig. 3).
## DateTime pH Temperature
## Min. :2023-07-24 09:00:00 Min. :7.368 Min. :29.38
## 1st Qu.:2023-07-25 04:07:30 1st Qu.:7.767 1st Qu.:29.64
## Median :2023-07-25 23:15:00 Median :7.814 Median :29.70
## Mean :2023-07-25 23:15:00 Mean :7.790 Mean :29.71
## 3rd Qu.:2023-07-26 18:22:30 3rd Qu.:7.870 3rd Qu.:29.84
## Max. :2023-07-27 13:30:00 Max. :7.958 Max. :29.98## DateTime pH Temperature Temp.adj
## Min. :2023-07-24 09:00:00 Min. :7.368 Min. :29.38 Min. :7.599
## 1st Qu.:2023-07-25 04:07:30 1st Qu.:7.767 1st Qu.:29.64 1st Qu.:7.667
## Median :2023-07-25 23:15:00 Median :7.814 Median :29.70 Median :7.682
## Mean :2023-07-25 23:15:00 Mean :7.790 Mean :29.71 Mean :7.686
## 3rd Qu.:2023-07-26 18:22:30 3rd Qu.:7.870 3rd Qu.:29.84 3rd Qu.:7.721
## Max. :2023-07-27 13:30:00 Max. :7.958 Max. :29.98 Max. :7.758## DateTime PAR sd
## Min. :2023-07-24 09:25:48.7 Min. : 0.0343 Min. : 0.00000
## 1st Qu.:2023-07-25 04:12:03.7 1st Qu.: 0.0344 1st Qu.: 0.00001
## Median :2023-07-25 22:58:18.7 Median : 15.1437 Median : 0.04935
## Mean :2023-07-25 22:58:18.7 Mean : 96.7526 Mean : 7.34244
## 3rd Qu.:2023-07-26 17:44:33.7 3rd Qu.:134.0962 3rd Qu.: 1.07443
## Max. :2023-07-27 12:30:48.7 Max. :742.1805 Max. :199.70382## date ws wd
## Min. :2023-07-24 13:15:00 Min. :0.290 Min. : 0.41
## 1st Qu.:2023-07-25 08:07:30 1st Qu.:2.045 1st Qu.: 98.39
## Median :2023-07-26 03:00:00 Median :3.270 Median :210.04
## Mean :2023-07-26 03:00:00 Mean :3.367 Mean :184.89
## 3rd Qu.:2023-07-26 21:52:30 3rd Qu.:4.375 3rd Qu.:266.55
## Max. :2023-07-27 16:45:00 Max. :7.370 Max. :359.64
## DateTime ws.adj
## Min. :2023-07-24 09:15:00 Min. : 29.2
## 1st Qu.:2023-07-25 04:07:30 1st Qu.:205.9
## Median :2023-07-25 23:00:00 Median :329.3
## Mean :2023-07-25 23:00:00 Mean :339.1
## 3rd Qu.:2023-07-26 17:52:30 3rd Qu.:440.6
## Max. :2023-07-27 12:45:00 Max. :742.2
Figure 3: Data from Brewers Bay diurnal suite monitoring from July 24th to July 27th. Top panel: pH and temperature from SeaFET. Bottom panel: Photosynthetically Available Radiation (PAR) and current speed from EcoPAR and Tiltmeter. Shading denotes nighttime throughout sequence of the plot. Instruments measured parameters every 15 minutes.
Habitat persistance
Carbonate budget assessments use transect-based surveys (Reef Budget) to quantify the abundance of carbonate producers (e.g., Corals and Crustose coralline algae), and carbonate bioeroders, (e.g., parrotfish and sea urchins). Abundances are multiplied by taxon-specific rates of carbonate alteration to determine if a reef is in a state of net accretion (habitat growth) or net loss (habitat loss; Fig. 4). At Brewers Bay, six transects were surveys in 2017 and 2023 to obtain carbonate budgets. These data show stable carbonate production over the monitoring time points. The transect results showed positive carbonate budgets in 2017 and 2023, which implies that this site supported reef accretion over the past eight years. However, carbonate production significantly declined from 5.08 +- 1.78 sd kg m-2 yr-1 in 2017 to 1.33 +- 1.78sd kg m-2 yr-1. This change was mainly driven by a reduction in coral production from 6.90 +- 2.13 in 2017 to 1.98 +- 0.326 in 2023.
Figure 4: Net carbonate production and the magnitude of carbonate alteration by calcifying or bioeroding functional groups. PF represents parrotfish. The scale of the y-axis varies for each functional group
Calcification Accretion Units (CAUs) and Bioerosion Monitoring Units (BMUs) were used to investigate the balance between calcification and erosion. CAUs and BMUs were collected and redeployed for the next sampling cycle. CAUs are processed by the NCRMP Pacific Climate group and the data will be available within a year. BMUs will be dried and cleaned using a hydrogen peroxide solution. Samples will be weighed and scanned using a CT scanner and then compared to their pre-scans to quantify bioerosion.
Landscape mosaics are used to quantify the benthic community, and to monitor changes in coral cover over time. Thousands of underwater images are digitally stitched together to create a high-resolution archive of the reef at the time of collection.