Wilfredo Y. Licuanan 1,2 * and Sandra Enn D. Bahinting 1 1 Br. Alfred Shields FSC Ocean Research Center; 2 Biology Department De La Salle University 1004 Manila, Philippines Changes in Benthic Cover in the South Atoll of Tubbataha Reefs due to Possible Eutrophication Keywords: benthic cover, coral reefs, eutrophication, monitoring, seabirds The Tubbataha Reefs Natural Park is of global and local importance and is a benchmark and aspirational model for the condition of reefs within an MPA. Recently, small but significant overall declines in hard coral cover (HCC) and increases in algae were detected in Tubbataha. Changes were most consistent in Site 4 at the South Atoll, where lagoon waters drain through during ebb tide. Space available to corals here was covered by cyanobacteria in 2018, and then by sponges in 2019. These sponges have begun to overgrow coral. These changes in Site 4 are likely due to an increase in guano-derived nutrients in the lagoon waters of the South Atoll. They may initiate a continuing decline in corals and, thus, require enhanced monitoring. *Corresponding Author: wilfredo.licuanan@dlsu.edu.ph The Tubbataha Reefs Natural Park, a UNESCO World Heritage Site, is the largest and best-managed no-take marine protected area in the Philippines (Dygico et al. 2013). The 970.3-km 2 park consists of 100 km 2 of reefs and is situated in the Sulu Sea, which itself is globally known for its high diversity of corals (de Vantier and Turak 2017). The coral reefs in the marine park appeared to be resilient to thermal stress since no significant changes in HCC and diversity were detected despite ocean warming (Licuanan et al. 2017). HCC and diversity in Tubbataha were so high and stable from 2012–2015 that the park was declared a benchmark and aspirational model for the condition of reefs within an MPA (Licuanan et al. 2017). Average HCC and diversity there were, thus, used as bases to define assessment scales for reef status [i.e. an HCC Category B reef has more than the then average HCC of Tubbataha, which was 33% (Licuanan et al. 2017, 2019)]. However, there was an annual 1.1% decrease in HCC from 2012–2019 in four monitoring sites of the North and South Atolls in Tubbataha (Figure 1; two-level repeated measures ANOVA year: F 7 = 2.54, p < 0.05; site x year: F 21 = 2.09, p < 0.05). Each monitoring site is made up of two 75 m x 25 m monitoring stations, and each station is sampled using five randomly deployed 50 m x 1 m photo-transects [methods are described in Licuanan et al. (2017)]. In contrast, the cover of algal assemblages (mainly turf algae) in the same four sites increased 1.9% over the same period (year: F 7 = 52.461, p < 0.0001; site x year: F 21 = 3.064, p < 0.01). The significant interaction term indicates the patterns in HCC and algal assemblages varied per site per year (Figure 2a). Much of these patterns were driven by changes in Site 3 (South Atoll), where there was a 23% decline in HCC between 2015 and 2017 due to damage from drifting logs and metal buoys. Of concern in this report are the smaller changes since 2017, particularly in monitoring Site 4 (South Atoll), where consistent losses in HCC of 1.1% per year were measured since 2012 (F 79 = 6.64, p < 0.05) and where the cover of algal assemblages is highest (Figure 2a). These shifts in the transect data of Site 4 are mirrored in the changes in cover within a fixed 4 m x 4 m plot (geographic coordinates 8.80656 N, 119.82169 E) in Site 4, Station B that is used to monitor minute changes with high statistical power [at least 80% power Philippine Journal of Science 150 (1): 139-143 February 2021 ISSN 0031 - 7683 Date Received: 30ww Jun 2020 139 RESEARCH NOTE