Benjamin Kürten

Latitudinal environmental gradients and diel variability influence abundance and community structure of Chaetognatha in Red Sea coral reefs

The Red Sea has been recognized as a unique region to study the effects of ecohydrographic gradients at a basin-wide scale. Its gradient of temperature and salinity relates to the Indian Ocean monsoon and associated wind-driven transport of fertile and plankton-rich water in winter from the Gulf of Aden into the Red Sea. Subsequent evaporation and thermohaline circulation increase the salinity and decrease water temperatures toward the North. Compared with other ocean systems, however, relatively little is known about the zooplankton biodiversity of the Red Sea and how this relates to Red Sea latitudinal gradients. Among the most abundant zooplankton taxa are Chaetognatha, which play an important role as secondary consumers in most marine food webs. Since Chaetognatha are sensitive to changes in temperature and salinity, we surmised latitudinal changes in their biodiversity, community structure and diel variability along the coast of Saudi Arabia. Samples were collected at nine coral reefs spanning approximately 1500 km, from the Gulf of Aqaba in the northern Red Sea to the Farasan Archipelago in the southern Red Sea. Thirteen Chaetognatha species belonging to two families (Sagittidae and Krohnittidae) were identified. Latitudinal environmental changes and availability of prey (i.e. Copepoda, Crustacea) altered Chaetognatha density and distribution. The cosmopolitan epiplanktonic Flaccisagitta enflata (38.1%) dominated the Chaetognatha community, and its abundance gradually decreased from South to North. Notable were two mesopelagic species (Decipisagitta decipiens and Caecosagitta macrocephala) in the near-reef surface mixed layers at some sites. This was related to wind-induced upwelling of deep water into the coral reefs providing evidence of trophic oceanic subsidies. Most Sagittidae occurred in higher abundances at night, whereas Krohnittidae were more present during the day. Chaetognatha with developing (stage II) or mature ovaries (stage III) were more active at night, demonstrating stage-specific diel vertical migration as a potential predator avoidance strategy.

Physical mechanisms routing nutrients in the central Red Sea

Mesoscale eddies and boundary currents play a key role in the upper layer circulation of the Red Sea. This study assesses the physical and biochemical characteristics of an eastern boundary current (EBC) and recurrent eddies in the central Red Sea (CRS) using a combination of in situ and satellite observations. Hydrographic surveys in November 2013 (autumn) and in April 2014 (spring) in the CRS (22.15 − 24.1°N) included a total of 39 and 27 CTD stations, respectively. In addition, high-resolution hydrographic data were acquired in spring 2014 with a towed undulating vehicle (ScanFish). In situ measurements of salinity, temperature, chlorophyll fluorescence, colored dissolved organic matter (CDOM), and dissolved nitrate: phosphorous ratios reveal distinct water mass characteristics for the two periods. An EBC, observed in the upper 150 m of the water column during autumn, transported low-salinity and warm water from the south toward the CRS. Patches of the low-salinity water of southern origin tended to contain relatively high concentrations of chlorophyll and CDOM. The prominent dynamic feature observed in spring was a cyclonic/anticyclonic eddy pair. The cyclonic eddy was responsible for an upward nutrient flux into the euphotic zone. Higher chlorophyll and CDOM concentrations, and concomitant lower nitrate:phosphorous ratios indicate the influence of the EBC in the CRS at the end of the stratified summer period.

RV PELAGIA Fahrtbericht / Cruise Report 64PE350/64PE351 – JEDDAH-TRANSECT ;08.03. – 05.04.2012 Jeddah - Jeddah, 06.04 - 22.04.2012 Jeddah - Duba

The multidisciplinary research cruise 64PE350 was conducted in the central part of the Red Sea to strengthen our understanding of volcanism and tectonics and related hydrothermal processes in this young rifting system. Therefore we extended our intense multibeam mapping campaign, which had started during the PO408 cruise in 2011, by mainly focusing on the hydrothermally active “Multi Deep” area. Detailed geological mapping was performed by selective rock sampling (dredging, coring) in areas of special interest (i.e. fresh basalt flows, or zones with signals of recent volcanic activities). Moreover, a geochemical sampling campaign (i.e. gravity coring, Niskin water sampling) was conducted in brine-filled Red Sea deeps where variable hydrothermal activity is expected. The geochemical characteristics of sediment (pore water and mineral composition) and brine will help to identify sources and sinks relative to hydrothermal activity (e.g. hydrocarbon inflow, heavy metal concentrations in brine and sediment, etc.). Transport and degradation processes (biogeochemical cycles) at the brine- seawater interfaces were investigated by high-resolution water sampling and subsequent onboard membrane inlet mass spectrometry, gas chromatography, and major and trace element sampling. Biomarker studies are being performed by sampling/ filtering water column and brine to decipher allochthonous from autochthonous organic matter input to the Red Sea Deeps. Moreover, microbiological studies will be performed on selected brine and sediment samples. Hydrocarbon seepage at the pockmark area was investigated by hydroacoustic and geophysical methods (water column imaging, sparker reflection seismics). Additionally, sediment cores were retrieved from selected pockmark structures to investigate gas/ fluid seepage activities during Holocene. The recently described subsea salt glacier phenomenon was investigated by seismic reflection studies and comparative multibeam profiling (salt glacier flow velocities will be calculated).

The Red Sea

Abstract The Red Sea is one of the warmest and saltiest seas of the world. Water exchanges occur only in the south, and there is moderate nutrient variability at macroecological scales. The Red Sea contains extensive areas of coral reefs, supporting high levels of diversity with numerous endemic species. Seagrass meadows and mangrove stands are also common. Despite the relatively low population density along most of the Red Sea shores, coastal development has intensified over the last few decades and is not expected to slow down. Marine communities face increasing pressures from fishing, shipping, oil exploration, aquaculture, desalination discharges, growing human population, terrestrial run-off, plastic waste, and climatic changes. Projections of increasing tourism add further pressures and, if sound marine spatial planning tools are not in place, could result in adverse effects for the sustainable management of resources. Current levels of contamination are overall low but coral reefs show signs of overfishing and there are also signs of homogenization of coral reef communities along the entire latitudinal gradient, probably due to anthropogenic pressures. Ecosystem-based management approaches are urgently needed for sustainable management.

Characterization of Optical Attenuation by Colored Dissolved Organic Matter (CDOM) in the Red Sea

This study is supported by King Abdullah University of Science and Technology (KAUST), Kingdom of Saudi Arabia.