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The influence of natural particulate organic matter (POM) input on sedimentary oxygen consumption (SOC) in permeable carbonate and silicate sediments close to a coral reef was investi- gated in front of the Marine Science Station in Aqaba, Jordan (northern Red Sea). We conducted 7 in situ experiments in stirred benthic chambers. Without additional POM input, SOC rates were similar and not significantly different (p > 0.5) in carbonate and silicate sands, with average rates of 20 ± 4 (n = 10) and 16 ± 2 (n = 3) mmol O2 m -2 d -1 , respectively. Gross photosynthesis in the carbon- ate and silicate sands accounted for 15 to 23 mmol produced O2 m -2 d -1 , characterising both sands as largely independent of allochthonous carbon input. SOC of unamended carbonate sands showed no significant variation in 5 in situ experiments conducted within a period of 19 d. Addition of 2 energy- rich sources of naturally occurring POM (coral mucus and clam eggs) resulted in significantly (p < 0.0001) increased SOC rates in the carbonate sands, but not in the silicate sands. Addition of a suspension containing high concentrations of zooxanthellae did not result in higher SOC in the carbonate sands, indicating that zooxanthellae cannot easily be degraded in reef sediments. Our results highlight the short cut between coral mucus production and degradation in the adjacent reef sands. Suspended particles are initially trapped by the cohesive mucus on the coral surface, and ensuing mucus strings sink to the seafloor at a short distance from the mucus-producing coral. Carbonate sands as porous filter systems obviously harbour more active heterotrophic microbial communities than silicate sands, and thus may constitute a major site of organic matter degradation in the reef ecosystem.

Benthic metabolism and degradation of natural particulate organic matter in carbonate and silicate reef sands of the northern Red Sea