Abstract
For reefs in South East Asia the synergistic effects of rapid land development, insufficient environmental policies and a lack of enforcement has led to poor water quality and compromised coral health from increased sediment and pollution. Those inshore turbid coral reefs, subject to significant sediment inputs, may also inherit some resilience to the effects of thermal stress and coral bleaching. We studied the inshore turbid reefs near Miri, in northwest Borneo through a comprehensive assessment of coral cover and health in addition to quantifying sediment-related parameters. Although Miri’s Reefs had comparatively low coral species diversity, dominated by massive and encrusting forms of Diploastrea, Porites, Montipora, Favites, Dipsastrea and Pachyseris, they were characterized by a healthy cover ranging from 22 to 39%. We found a strong inshore to offshore gradient in hard coral cover, diversity and community composition as a direct result of spatial differences in sediment at distances <10 km. As well as distance to shore, we included other environmental variables like reef depth and sediment trap accumulation and particle size that explained 62.5% of variation in benthic composition among sites. Miri’s reefs showed little evidence of coral disease and relatively low prevalence of compromised health signs including bleaching (6.7%), bioerosion (6.6%), pigmentation response (2.2%), scars (1.1%) and excessive mucus production (0.5%). Tagged colonies of Diploastrea and Pachyseris suffering partial bleaching in 2016 had fully (90–100%) recovered the following year. There were, however, seasonal differences in bioerosion rates, which increased five-fold after the 2017 wet season. Differences in measures of coral physiology, like that of symbiont density and chlorophyll a for Montipora, Pachyseris and Acropora, were not detected among sites. We conclude that Miri’s reefs may be in a temporally stable state given minimal recently dead coral and a limited decline in coral cover over the last two decades. This study provides further evidence that turbid coral reefs exposed to seasonally elevated sediment loads can exhibit relatively high coral cover and be resilient to disease and elevated sea surface temperatures.
Highlights
Turbid reefs are commonly regarded as marginal reefs living near their environmental limits (Kleypas, McManus & Menez, 1999; Guinotte, Buddemeier & Kleypas, 2003; Perry & Larcombe, 2003; Palmer et al, 2010; Goodkin et al, 2011)
Site differences in potential sediment load were observed during the dry season with higher and more variable turbidity recorded at the nearshore Eve’s Garden (EG) reef than at Siwa Reef (SW) located 10 km further south from the large Baram river mouth (Fig. S3)
Low recently dead coral cover and potentially limited decline in coral cover over the last two decades indicate these reefs are stable despite elevated sediment inputs and regular exposure to thermal stress events
Summary
Turbid reefs are commonly regarded as marginal reefs living near their environmental limits (Kleypas, McManus & Menez, 1999; Guinotte, Buddemeier & Kleypas, 2003; Perry & Larcombe, 2003; Palmer et al, 2010; Goodkin et al, 2011). There is growing evidence that turbid reefs may be more resilient to future climate change effects (Goodkin et al, 2011; Morgan et al, 2017) and serve as refugia for surviving corals (Cacciapaglia & van Woesik, 2015, 2016; Morgan et al, 2016) This has been demonstrated on turbid reefs that experience significant sediment and nutrient inputs, yet are still characterized by high coral cover, low bleaching and rapid recovery rates from bleaching and cyclonic events (Larcombe, Costen & Woolfe, 2001; Browne, Smithers & Perry, 2010; Richards et al, 2015; Morgan et al, 2016). Sedimentation rates are high with SE Asian coastal systems experiencing the highest siltation loads globally (Kamp-Nielsen et al, 2002; Syvitski et al, 2005)
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