Removal of bacteria and nutrient dynamics within the coral reef framework of Curaçao (Netherlands Antilles)

Abstract

The authors studied removal rates of bacteria and the regeneration of inorganic nutrients in coral reef cavities in the reef slope of Curacao, Netherlands Antilles. We found that in cavities the hard substratum surface area (=ca 68% of cavity surface area) is 65% covered with sessile filter feeders. The cryptic cavity surface area exceeds the projected surface area of the reef by 1.5–8 times. Consequently, the organisms living in these cryptic habitats have potentially a large impact on pico- and nano-plankton densities and are important in reef water nutrient dynamics. We closed cavities (±70 l volume, 15 m depth) in seven experiments to study changes in bacterial densities and dissolved inorganic nutrients (DIN, DIP, and silicate) over time. Water samples were taken from the middle of the cavity at 5-min intervals, for 30 min, and analyzed for heterotrophic bacterial abundance and nutrient concentrations. After closure, bacterial abundance dropped rapidly. Of the initial bacterial concentration in the cavities, 50–60% had disappeared after 30 min, an average disappearance rate of 1.43×104 bacteria ml−1 min−1 (0.62 mg C l−1 d−1; or 30.1 mg C m−2 cavity surface area d−1). NOx concentrations increased significantly during the time of closure. Efflux rates varied between 1.02–9.77 mmol m−2 cavity surface area d−1. NH4+ and PO43− concentrations were variable and did not show a consistent change over time in the experiments. Comparison of bacterial organic nitrogen disappearance rates and DIN (NOx+NH4+) release rates suggests that on average only 30–40% of additional sources of N besides bacteria were required to balance the nitrogen budget. This highlights the importance of heterotrophic bacterioplankton as food for cryptic filter feeders on coral reefs. Silicate concentrations significantly decreased after closure with 0.50 mmol m−2 cavity surface area d−1, suggesting the net deposition of SiO42− in spicules of cryptic filter feeding sponges. We conclude that coral reef cavities are a major sink for heterotrophic bacteria, a sink for dissolved silicon (DSi), and a source for NOx. That reef cavities are a source for NOx suggests strong remineralization and nitrification in cavities with a potential role for sponge-symbiotic microbial nitrification.