*3.4. Temperature and the Prevalence of Bleaching*

The average temperature difference between the 5 and 10 m temperature loggers across the three sites was 0.3 ◦C (±0.1 ◦C) and thus the loggers were pooled by site for further testing of site differences. The monthly means of the temperature recorded by the in situ loggers were not significantly different by site [one-way ANOVA: χ2(2) = 0.1277, *p* = 0.9382] (Figure 7).

**Figure 7.** Mean temperature by month ± 2 standard error of the daily maximum temperature for remotely sensed sea surface temperature (5 km2 pixels) from Coral Reef Watch Timor-Leste (CRWTL) and in situ temperature loggers pooled by depth (*n* = 6). Loggers sampled every 30 min between November 2015 and July 2017, the period between benthic surveys. The gray shading indicates the annual monsoon season from November to May. The dashed blue line is the maximum monthly mean (MMM, 29.5 ◦C) from the CRWTL data and the solid blue line is MMM + 1 ◦C (30.5 ◦C), the bleaching threshold for accumulation of degree heating weeks.

Comparison of the monthly means of all in situ temperature loggers with the monthly means of the CRWTL SSTs showed they were significantly different by a season and method interaction [two-way ANOVA: χ2(3) = 8.2054, *p* = 0.0420]. Pairwise tests for this interaction revealed that the CRWTL satellite-derived SST values were not significantly higher than the in situ logger temperatures within the same season (Summer CRWTL:Summer in situ, *p* = 0.4991; Fall CRWTL:Fall in situ, *p* = 0.5562; Spring CRWTL:Spring in situ, *p* = 0.2291; Winter CRWTL:Winter in situ, *p* = 0.0712). However, the elevated CRTWL temperatures during the austral summer (Jan–Mar, CRWTL = 30.7 ± 0.2 ◦C and in situ = 29.1 ± 0.1 ◦C)

and austral spring (Oct–Dec, CRWTL = 30.5 ◦C and in situ = 29.7 ◦C) were ecologically significant, 1.6 ◦C and 0.8 ◦C warmer, respectively, than in situ temperatures for the same seasons. During the summer, the 1.6 ◦C difference between methods meant that CRWTL SSTs were above the MMM + 1 ◦C bleaching threshold while in situ temperatures were below the MMM. Thus, according the CRWTL, DHWs were accumulated over this period while in situ temperature remained below the bleaching threshold with no DHW accumulation (Figure 8).

**Figure 8.** Plot of the significant season × method interaction between the Coral Reef Watch Timor-Leste virtual station (CRWTL) remotely sensed sea surface temperature and in situ temperature logger data collected between November 2015 and July 2017. Divergence in temperature data between the two methods was greatest during the austral summer. The dashed blue line indicates the maximum monthly mean (MMM, 29.5 ◦C) and the solid blue line is MMM + 1 ◦C (30.5 ◦C), the bleaching threshold for the accumulation of degree heating weeks. CRWTL was above the MMM + 1 ◦C in both the austral summer and spring seasons while the in situ temperature remained below this threshold. Error bars represent 95% confidence intervals.

During the 639 days between surveys, major heat stress events occurred. CRWTL indicated there were 190 days (30.2%) of bleaching warning (0 < DHW < 4) and 161 days (25.2%) of bleaching alert 1 (4 <= DHW < 8). The accumulation of DHWs was limited to November 2015 through July 2016 (224 days) and November 2016 through March 2017 (119 days). This corresponded to the months where the CRWTL monthly averaged temperatures were greater than the maximum monthly mean (MMM; Figure 7), the warmest month of the twelve-monthly mean SST climatology values from the CRWTL data. The accumulation of DHWs during 2015–2016 was almost 8 months, nearly twice as long as the DHW accumulation during 2016–2017. The in situ temperature data, however, never reached the MMM + 1 ◦C threshold for bleaching, and based on these data there would be no accumulation of DHWs.

There was a three-way interaction between site, depth, and year on bleaching prevalence [three-way repeated-measures ANOVA, χ2(3) = 19.662, *p* = 0.0002] (Figure 5). All sites at each depth showed a decrease in the prevalence of coral bleaching from one survey to the next (*p* < 0.05; Figure 5), which is expected as the second survey was conducted at the onset of austral winter. However, only Rural-E at 10 m (13.4 ± 0.7% and 2.7 ± 1.2% in 2015 and 2017, respectively) and Urban-W at 5 m (17.4 ± 1.6% and 1.8 ± 1.3%) had significant decreases in the prevalence of bleaching.
