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Free keywords:
nutrients; coral reef; seasonality; porewater; reef framework; coral sand
Abstract:
Inorganic nutrients and chlorophyll a concentrations were measured bi-weekly in a transect across a coral reef in the Gulf of Aqaba over a period of 1 yr. The nutrient and chlorophyll concentrations were compared to those in adjacent offshore waters (400 m depth), In reef and offshore waters, nutrient (ammonium, nitrite, nitrate, phosphate and silicate) and chlorophyll a data showed seasonal changes, with high concentrations in winter and low concentrations in summer. However, throughout the summer, nutrient concentrations in the coral reef waters significantly exceeded those in the offshore waters, while this difference was less pronounced in winter. This difference was caused by nutrient release from regenerative spaces in the reef framework and coral sand, In the reef framework water (i.e. cavity water), nutrient concentrations were 1.2- to 2.3-fold higher than those in the surrounding waters, corresponding to fluxes of 14.5 mmol m(-2) d(-1) for ammonium, 7.7 mmol m(-2) d(-1) for nitrate, 0.9 mmol m(-2) d(-1) for nitrite, and 1.3 mmol m(-2) d(-1) for phosphate, In the less permeable reef sediments, nutrient concentrations exceeded those of the free-stream waters by factors of 15 to 80. Here, the calculated diffusive fluxes were 0.06 mmol m(-2) d(-1) for ammonium, 0.03 mmol m(-2) d(-1) for nitrate, 0.01 mmol m(-2) d(-1) for nitrite, 0.01 mmol m(-2) d(- 1) for phosphate, and 0.07 mmol m(-2) d(-1) for silicate. Our results highlight the importance of the reef framework and coral sand for the trapping and mineralization of particulate organic matter and the regeneration of nutrients in oligotrophic coral reef waters.
