Material and Methods
Study site
The present study was conducted in a fringing reef of the Northern Red Sea (Gulf of Aqaba) during 4 seasonal expeditions (November/December 2006 (fall), August/September 2007 (summer), February/March 2008 (winter) and May 2008 (spring)) to the Marine Science Station (MSS) Aqaba, Jordan (latitude: 29° 27′ N, longitude: 34° 58′ E). According to earlier investigations regarding morphology and zonation, this fringing reef can be partitioned in a reef flat divided into back reef (depth: 0.0–1.8 m), reef crest (0.5–1.0 m) and reef slope (1–6 m) joined by a fore reef facing the open sea (Mergner & Schuhmacher 1974). The fore reef consists of an upper (4–8 m depth), middle (8–20 m) and lower (20–40 m) part distinguished by morphological features and species composition. The fringing reef system extends approximately 1.1 km along its reef crest bordering the coastline in a half-ellipsoid shape (Fig. 1). Investigations carried out by the present study focussed on the reef area framed by the reef flat and the middle fore reef (depth range: 0–20 m; hereinafter called: reef-wide).
Ecosystem assessment
Benthic coverage
To assess the benthic reef assemblage by substrate types and to identify the dominant hermatypic coral genera in the study area, Line-point intercept surveys (total: n = 44), as described in Naumann et al. (submitted), were carried out in 0.5, 1.0, 5.0, 10.0 and 20.0 m water depth during the seasonal expeditions. Analysis of LPI data yielded the percentage coverage by main substrate types including the total live coral coverage and the contribution of dominant hermatypic coral genera (i.e. anthozoans: Acropora, Fungia, Goniastrea, Pocillopora and Stylophora, and the hydrozoan Millepora) (Table 1).
Seawater organic C content
To investigate organic C concentrations in the reef-overlying water column, seawater samples (3000–5000 ml; n = 4) were collected from the fore reef area in 10 m water depth on three dates during each of the seasonal expeditions. Sampling containers were immediately transferred to the laboratory where samples for dissolved organic carbon (DOC) analysis were prepared. Sterile
Fig. 1 Air-borne photography of the study area (highlighted in yellow) representing the area between 0 – 20 m water depth of a fringing reef close to the Marine Science Station Aqaba (MSS) bordering the Jordanian Red Sea coast line. Numbers indicate the respective water depths. Photograph generated by ASEZA GIS Unit (R. Mobiedeen), Aqaba, Jordan.
Table 1 Percentage coverage by reef substrates types within the study site. Abbreviation: LCC = Live hermatypic coral coverage.
Substrate Benthic coverage (%) Live hermatypic coral coverage
Dominant genera (n = 6) Non-dominant genera (n = 62) 40 45 (of LCC) 55 (of LCC) Macro-algae 16
Bare coral rock 12
Sand 28 Other 4
subsequently filtered through 0.2-µm-pore-sized sterile polyethersulfone membrane filters. The initial 5 ml of sample volume were used to wash the filter membrane discarding the filtrate, while the remaining sample volume (5 ml) was filtered at low pressure into pre-combusted (450°C; 4 h) amber glass vials. DOC samples were kept frozen at –20°C until analysis by high-temperature catalytic oxidation (HTCO) using a Rosemount Dohrmann DC-190 TOC analyser. The concentration of non-purgable organic C was measured, after sample acidification with orthophosphoric acid (20 %) to pH < 2 and O2 sparging, using potassium hydrogen phthalate as an elemental standard (standard deviation <3%).
Subsamples (1000 ml) for particulate organic carbon (POC) analysis were filtered onto pre- combusted (450°C; 4 h) GF/F filters (Whatman™, 25 mm diameter), subsequently dried for at least 48 h at 40°C and analysed for POC content using a Thermo™ NA 2500 elemental analyser (elemental standards: atropine, cyclohexanone-2,4-dinitrophenylhydrazone; Thermo Quest; standard deviation < 3 %).
Pelagic metabolism
Three subsamples from each seawater sampling container were used to determine primary production (PP) and microbial oxygen consumption in reef-overlying waters. The initial dissolved oxygen concentration was measured in the first subsample using an optical dissolved O2 sensor (Hach Lange HQ10, accuracy ± 0.05 %). The second and third subsamples were filled into 60 ml gas-proof Winkler glass bottles, of which one was incubated during the day light period (8 to 12 h) at in-situ light and temperature conditions monitored by data loggers, while the other was kept in the dark at in-situ temperature for 16 to 24 h. O2 concentration was measured at the end of the incubation periods as described above and the difference was used to calculate PP and microbial O2 consumption rates.
Benthic metabolism of reef sand communities
Benthic PP and microbial O2 consumption in calcareous reef sands was assessed during 2 expeditions (fall and summer) by measuring O2 dynamics in seawater enclosed by stirred benthic chambers as described in Wild et al. (2005b, 2009). Briefly, between 1 and 4 stirred benthic chambers were used in 7 independent chamber experiments carried out at water depths of 2.5 to 16.5 m. Transparent (PP measurements) and opaque (respiration measurements) acrylic chambers were inserted into calcareous reef sands to a depth of about 12 cm. The volume inside the chambers was 5.7 l and incubations lasted for 5 to 8 h.
At pre-set time intervals (30 to 120 min), water samples (60 ml) were collected from the chambers through sampling ports for later analyses of oxygen concentrations. Fixation of the samples took place within 15 min after collection. O2 concentration was measured by Winkler titration within 1 h after fixation. PP and O2 consumption rates by the benthic communities were determined by linear regression of O2 concentrations over time (4 data points per chamber) and finally related to the water volume and sediment surface enclosed by the chambers.
Sedimentation of particulate organic carbon
To determine the amount of POC settling to the seafloor, sediment traps were deployed in triplicates at 1, 5, 10 and 20 m water depth during each of the seasonal expeditions. Traps were designed and used as described in Wild et al. (2009). All traps (total: n = 48) were placed on reef sands with their opening at a height of 7.5 cm above the seafloor.
Traps were deployed for 48 h, after which the collected contents were prepared for POC analysis by filtering the particulate material onto pre-combusted GF/F filters. Samples were subsequently dried for 48 h at 40°C and exposed to a fuming HCl (1 N) atmosphere for 24 h to remove inorganic C contents, before they were analysed using a Thermo™ NA 2500 elemental analyser. POC contents of all sediment traps were finally related to the trapping area (i.e. trap opening in m2) and time of deployment to generate POC sedimentation rates.