Budget of coral-derived organic carbon in a fringing coral reef of the Gulf of Aqaba, Red Sea

Abstract

The continuous release of organic C-rich material by reef-building corals can contribute substantially to biogeochemical processes and concomitant rapid nutrient recycling in coral reef ecosystems. However, our current understanding of these processes is limited to platform reefs exhibiting a high degree of ecosystem closure compared to the globally most common fringing reef type. This study carried out in the northern Gulf of Aqaba (Red Sea) presents the first quantitative budget for coral-derived organic carbon (COC) in a fringing reef and highlights the importance of local hydrodynamics. Diel reef-wide COC release amounted to 1.1± 0.2 kmol total organic carbon (TOC) representing 1-3% of gross benthic primary production. Most COC (73%) was released as particulate organic C (POC), the bulk of which (34-63%) rapidly settled as mucus string aggregates accounting for approximately 28% of total POC sedimentation. Sedimentation of mucus strings, but also dilution of suspended and dissolved COC in reef waters retained 82% of diel COC release in the fringing reef, providing a potentially important organic source for a COC-based food web. Pelagic COC degradation represented 0.1-1.6% of pelagic microbial respiration recycling 32% of diel retained COC. Benthic COC degradation contributed substantially (29-47%) to reef-wide microbial respiration in reef sands, including 20-38% by mucus string POC, and consumed approximately 52% of all retained COC. These findings point out the importance of COC as a C carrier for different reef types. COC may further represent a source of organic carbon for faunal communities colonising reef framework cavities complementing the efficient retention and recycling of COC within fringing reef environments.

Influence of coral and algal exudates on microbially mediated reef metabolism

“…Concomitantly, the fleshy algae fixed more inorganic carbon during photosynthetic processes (estimated as higher DO production) and also exuded significantly higher amounts of DOC into their surrounding environment. DOC release rates of the different primary producers (8–57 µmol dm -2 d -1 ) were comparable to release rates previously published from reef locations around the world (Naumann et al, 2012, Red Sea corals: 6–56 µmol dm -2 d -1 ; Haas et al, 2010b, Mexican Caribbean algae: 28–208 µmol dm -2 d -1 , Haas et al, 2011, Central Pacific coral and macroalgae: 48–336 µmol dm -2 d -1 ). In the present study, over a full diurnal cycle, benthic primary producers released about 10% of their daily fixed carbon as DOC in the surrounding waters.…”

“…It has further been established that fleshy macroalgae and especially small (<2 cm) filamentous algal turfs generally have noticeably higher DOC release rates than calcifying primary producers including hermatypic corals. These results are consistent over a wide range of reef systems, comprising the Red Sea (Haas et al, 2010a; Naumann et al, 2012), the Caribbean (Haas et al, 2010b), and the Central Pacific (Haas et al, 2011). …”

Monitoring of coastal coral reefs near Dahab (Gulf of Aqaba, Red Sea) indicates local eutrophication as potential cause for change in benthic communities

“…Overall hard coral contribution to benthic cover in 2010 is within the range previously reported for reefs near Dahab (33-37 %; Hasler and Ott 2008;Tilot et al 2008) and for the wider Gulf of Aqaba (35-43 %, Kotb et al 2008;Naumann et al 2012), even for the lowest hard coral cover measured at the site TP (31±2 %). Spatial variability in hard coral cover may result from an array of distinct biotic (e.g.…”

Species-specific control of external superoxide levels by the coral holobiont during a natural bleaching event

“…In addition to superoxide, corals also release dissolved organic carbon4243 and hydrogen peroxide44 to seawater, both of which could in theory contribute to the indirect formation of superoxide. Yet, the observation that coral-derived superoxide is detectable only several millimetres beyond the coral surface reflects the rapid decay kinetics of superoxide, which is inconsistent with the higher residence times and transport distances for dissolved organic carbon and hydrogen peroxide in relation to the coral surface4445. Further, organic carbon fluxes from corals and within coral reefs have a pronounced diel nature4647, which is not observed here for superoxide production, suggesting that dissolved organic carbon-derived superoxide is not a dominant contributor to the superoxide concentrations reported herein.…”