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

Abstract

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 investigated 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 O 2 m -2 d -1, respectively. Gross photosynthesis in the carbonate and silicate sands accounted for 15 to 23 mmol produced O 2 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 energyrich 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.

The influence of pore‐water advection, benthic photosynthesis, and respiration on calcium carbonate dynamics in reef sands

“…Implications for Heron Reef and global ocean CaCO 3 cycling-Our results confirm previous observations of extremely rapid benthic primary production and organic matter turnover rates in permeable biogenic deposits at Heron Reef (Wild et al 2004;Werner et al 2006). The net sediment trophic status was found to depend on both benthic light flux and pore-water filtration rate, through their combined influence on benthic microalgal photosynthesis and sediment community respiration.…”

“…Water depth at these sites ranged from 20 cm at low tide to 2 m at high tide. Heron Reef sands are generally low in organic carbon (0.24%) and nitrogen (0.042%) content (Wild et al 2004). We were careful to avoid sampling in areas with a high density of macrofaunal structures.…”

“…Furthermore, reef environments are characterized by high gross primary production rates, which are supported by the recycling of nutrients and organic matter from coral mucus, eggs, and phyto-and zooplankton detritus in reef sands (Wild et al 2004;Werner et al 2006). As biogenic carbonate sands have greater specific surface area, porosity, permeability, and buffering capacity than silicate sands, they may also support higher metabolic rates, more biofilm formation, greater bacterial abundance, different microbial populations, and a stronger response to organic substrates than terrestrially derived silicate sands (Wild et al 2005). Therefore, CaCO 3 and organic matter cycling in reef sands present a new challenge to understanding the global carbon (C) cycle and ecosystem dynamics in fragile coral reef environments.…”

Coral sand O2 uptake and pelagic–benthic coupling in a subtropical fringing reef, Aqaba, Red Sea

“…The reef sediments, in particular the permeable calcareous sands with their high abundances of phototrophic and heterotrophic microbes , contribute significantly to primary production, carbon mineralization and nutrient cycling of reef ecosystems (Johnstone et al 1990, Clavier & Garrigue 1999, Wild et al 2004b,c, 2005. Sedimentary production and decomposition processes control O 2 flux across the sediment -water interface, and investigations by Werner et al (2006) suggest that O 2 is the dominant electron acceptor over sulphate in permeable coral reef sands.…”

High sedimentary oxygen consumption indicates that sewage input from small islands drives benthic community shifts on overfished reefs

“…The current study extended existing in situ methods for determining local pollution by exploring the potential for sedimentary oxygen consumption (SOC) incubations to assess local anthropogenic OM input promptly in the field. In shallow-water reef environments with primary benthic cover by permeable sediments, intense benthic-pelagic coupling causes sediments to integrate OM concentration changes in the overlying water (Wild et al 2005). As key organic components of sewage are rapidly recycled in surface sediments (Wild et al 2008), elevated SOC rates should directly reflect recent sewage (OM) input.…”