Corals&Waves: Calcification and bioerosion in Large Amplitude Internal Wave affected coral reefs

Abstract

The results of this work are a contribution to the understanding of the influence of dramatic natural variations of the physico-chemical environment in coral reef areas caused by Large Amplitude Internal Waves (LAIW).LAIW are a worldwide oceanographic phenomenon generated in areas with a rich underwater topography, strong density stratification and tidal currents. They deliver sub-thermocline water upslope into shallow shelf regions but their possible biological impact especially on benthic communities and ecosystems such as coral reefs is so far largely unknown. In this work the impact of LAIW on coral reefs was investigated at the Similan Island chain in the Andaman Sea. The unusual coral reef distribution with reefs flourishing only on the sheltered eastern island sides and no true reef framework in the west was shown to be consistent among the islands and related to LAIW. Temperature records revealed that LAIW have their strongest impact on deeper west slopes where they cause frequent and abrupt drops in temperature with peak activity during the north-east monsoon (January through April). LAIW advect deep cold, nutrient rich, suboxic and low-pH waters coming with strong currents into shallow near shore areas. In contrast to low numbers of frame-building species in east, the west slopes harbour loose yet more divers communities of scattered corals. This might be due to the alternating impact of south-west monsoon (May to October) and LAIW from above and below maintaining the coral community in a non-equilibrium state of intermediate disturbances which enhances species diversity.Coral growth of and bioerosion on Porites lutea revealed both, lowest rates at the site of highest LAIW-impact in west deep areas and highest rates within the dense coral reef of the sheltered east shallow. A calculated carbonate budget confirmed that carbonate accretion by reduced coral growth and carbonate erosion by alike reduced grazing and boring organisms are balanced but only barely positive under highly variable LAIW conditions in west deep. Coral recruitment on natural reef substrate revealed significantly higher abundances in shallow west than in east. However with increasing exposure period of experimental settlement tiles, the proportion of juvenile corals increased in east but decreased in west, indicating a failure of recruits to establish permanently on the LAIW-exposed sides in spite of the higher initial spat fall. Early stages of settlement and growth seemed to occur independently of LAIW exposure while the subsequent establishment of juvenile corals appeared to be related to LAIW intensity, reflecting the lack of true reefs on the LAIW-exposed west sides of the islands.