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coral reef; mucus release; quantification experiments; compositional analyses; nutrient fluxes; carbonate sands
Abstract:
Mucus release by hard corals of the genus Acropora under submersed and naturally occurring air exposure was quantified at Heron Island/Great Barrier Reef. These measurements were conducted with beaker and in situ container incubation techniques. Mucus release rates for A. millepora, normalized to the coral surface area, were 10 ± 5 mg C and 1.3 ± 0.8 mg N m-2 h-1 for submersed corals, and 117 ± 79 mg C and 13 ± 8 mg N m-2 h-1 after exposure to air at low tide. This corresponds to increases by factors of 12 for C and 10 for N. The main monosaccharide components of freshly released Acropora mucus were arabinose and glucose, accounting for 14 to 63% and 13 to 41% of the carbohydrates. A protein content of 13 to 26 mg l-1 caused a low C:N ratio of 8 to 14. The chlorophyll content of 7 to 8 µg l-1 in the mucus compared to 0.6 ± 0.004 µg l-1 in the surrounding seawater revealed mucus contamination with zooxanthellae. A low pH value of 7.7 compared to 8.3 in the surrounding seawater indicates the existence of acidic components in fresh coral mucus. Concentrations of most measured inorganic nutrients were highly increased in coral mucus, reaching values of 3 to 4 µM for silicate, 19 to 22 µM for phosphate and 20 to 50 µM for ammonium concentration. Phosphate concentrations were 130-fold higher in coral mucus compared to the surrounding seawater, underlining the role of coral mucus as a carrier of nutrients. Addition of coral mucus to stirred benthic chambers resulted in a shift of phosphate, ammonium and nitrate/nitrite fluxes towards the sediments, confirming the transport of nutrients via coral mucus into permeable reef sands.
