Beschreibung: The Red Sea features a natural environmental gradient characterized by increasing water temperature, nutrient and chlorophyll a concentrations from North to South. The aim of this study was to assess the relationships between ecohydrography, particulate organic matter (POM) and coral reef biota that are poorly understood by means of carbon (δ13C) and nitrogen (δ15N) stable isotopes. Herbivorous, planktivorous and carnivorous fishes, zooplankton, soft corals (Alcyonidae), and bivalves (Tridacna squamosa)were a priori defined as biota guilds. Environmental samples (nutrients, chlorophyll a), oceanographic data (salinity, temperature), POMand biotawere collected at eight coral reefs between 28°31′ N and 16°31′ N. Isotopic niches of guilds separated in δ13C and δ15N isotopic niche spaces and were significantly correlated with environmental factors at latitudinal scale. Dietary end member contributionswere estimated using the Bayesian isotope mixingmodel SIAR. POMand zooplankton 15N enrichment suggested influences by urban run-off in the industrialized central region of the Red Sea. Both δ15N and their relative trophic positions (RTPs) tend to increase southwards, but urban runoff offsets the natural environmental gradient in the central region of the Red Sea toward higher δ15N and RTPs. The present study reveals that consumer δ13C and δ15N in Red Sea coral reefs are influenced primarily by the latitudinal environmental gradient and localized urban runoff. This study illustrates the importance of ecohydrography when interpreting trophic relationships from stable isotopes in Red Sea coral reefs.

Beschreibung: The Eastern Tropical North Atlantic (ETNA) is characterised by a strong east to west gradient in the vertical upward flux of dissolved inorganic nitrogen to the photic zone. We measured the stable nitrogen isotope (δ15N) signatures of various zooplankton taxa covering twelve stations in the ETNA (04°–14°N, 016–030°W) in fall 2009, and observed significant differences in δ15N values among stations. These spatial differences in δ15N within zooplankton taxa exceeded those between trophic levels and revealed an increasing atmospheric input of nitrogen by N2 fixation and Aeolian dust in the open ocean as opposed to remineralised NO3− close to the NW African upwelling. In order to investigate the spatial distribution of upwelling-fuelled versus atmospheric-derived nitrogen more closely, we examined the δ15N signatures in size-fractionated zooplankton as well as in three widely distributed epipelagic copepod species on a second cruise in fall 2010 in the ETNA (02-17°35′N, 015–028°W). Copepods were sampled for δ15N and RNA/DNA as a proxy for nutritional condition on 25 stations. At the same stations, vertical profiles of chlorophyll-a and dissolved nutrients were obtained. High standing stocks of chl-a were associated with shallow mixed layer depth and thickening of the nutricline. As the nitracline was generally deeper and less thick than the phosphacline, it appears that non-diazotroph primary production was limited by N rather than P throughout the study area, which is in line with enrichment experiments during these cruises. Estimated by the δ15N in zooplankton, atmospheric sources of new N contributed less than 20% close to the African coast and in the Guinea Dome area and up to 60% at the offshore stations, depending on the depth of the nitracline. δ15N of the three different copepod species investigated strongly correlated with each other, in spite of their distinct feeding ecology, which resulted in different spatial patterns of nutritional condition as indicated by RNA/DNA. Highlights: ► We studied δ15N and RNA/DNA of eastern tropical Atlantic zooplankton along with nutrients and Chl-α. ► Zooplankton −δ15N was decreasing from east (West African Shelf) to west (oligotrophic open ocean). ► Total integrated Chl-a depended mainly on nutricline depth and was N-limited throughout the area. ► Zooplankton δ15N and nutricline depth were used to estimate atmospheric N sources to the food web. ► Estimated atmospheric nitrogen sources were less than 20% at the shelf slope and up to 60% offshore.

Beschreibung: A sediment core from the high latitude of the Northern Atlantic (Nordic seas) was intensively studied by means of biogeochemical, sedimentological, and micropaleontological methods. The proxy records of interglacial marine oxygen isotope stage (MIS) 11 are directly compared with records from the Holocene (MIS 1), revealing that many features of MIS 11 are rather atypical for an interglaciation at these latitudes. Full-interglacial conditions without deposition of ice-rafted debris existed in MIS 11 for about 10 kyr (∼398–408 ka). This time is marked by the lightest d18O values in benthic foraminifera, indicating a small global ice volume, and by the appearance of subpolar planktic foraminifera, indicating a northward advection of Atlantic surface water. A comparison with MIS 1, using the same proxies, implies that surface temperatures were lower and global ice volume was larger during MIS 11. A comparative study of the ratio between planktic and benthic foraminifera also reveals strong differences among the two intervals. These data imply that the coupling between surface and bottom bioproductivity, i.e., the vertical transportation of the amount of fresh organic matter, was different in MIS 11. This is corroborated by a benthic fauna in MIS 11, which contains no epifaunally-living species. Despite comparable values in carbonate content (%), reflectance analyses of the total sediment (greylevel) show much higher values for MIS 11 than for MIS 1. These high values are attributed to increased corrosion of foraminiferal tests, directly affecting the sediment greylevel. The reason for this enhanced carbonate corrosion in MIS 11 remains speculative, but may be linked to the global carbon cycle.

Beschreibung: On the basis of various lithological, mircopaleontological and isotopic proxy records covering the last 30,000 calendar years (cal kyr) the paleoenvironmental evolution of the deep and surface water circulation in the subarctic Nordic seas was reconstructed for a climate interval characterized by intensive ice-sheet growth and subsequent decay on the surrounding land masses. The data reveal considerable temporal changes in the type of thermohaline circulation. Open-water convection prevailed in the early record, providing moisture for the Fennoscandian-Barents ice sheets to grow until they reached the shelf break at ∼26 cal. kyr and started to deliver high amounts of ice-rafted debris (IRD) into the ocean via melting icebergs. Low epibenthic δ18O values and small-sized subpolar foraminifera observed after 26 cal. kyr may implicate that advection of Atlantic water into the Nordic seas occurred at the subsurface until 15 cal. kyr. Although modern-like surface and deep-water conditions first developed at ∼13.5 cal. kyr, thermohaline circulation remained unstable, switching between a subsurface and surface advection of Atlantic water until 10 cal. kyr when IRD deposition and major input of meltwater ceased. During this time, two depletions in epibenthic δ13C are recognized just before and after the Younger Dryas indicating a notable reduction in convectional processes. Despite an intermittent cooling at ∼8 cal. kyr, warmest surface conditions existed in the central Nordic seas between 10 and 6 cal. kyr. However, already after 7 cal. kyr the present day situation gradually evolved, verified by a strong water mass exchange with the Arctic Ocean and an intensifying deep convection as well as surface temperature decrease in the central Nordic seas. This process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal. kyr and, eventually, to today's steep east–west surface temperature gradient. The time discrepancy between intensive vertical convection after 5 cal. kyr but warmest surface temperatures already between 10 and 6 cal. kyr strongly implicates that widespread postglacial surface warming in the Nordic seas was not directly linked to the rates in deep-water formation.

Beschreibung: Highlights: • The natural ecohydrographic gradient of the Red Sea translates into an isoscape. • The Red Sea isoscape features increasing zooplankton δ15 N values towards the South. • Isotopic baseline variations propagate through the pelagic food web. • Eddy-induced upwelling modifies the natural ecohydrographic North-South gradient. Abstract: Although zooplankton occupy key roles in aquatic biogeochemical cycles, little is known about the pelagic food web and trophodynamics of zooplankton in the Red Sea. Natural abundance stable isotope analysis (SIA) of carbon (δ13C) and N (δ15N) is one approach to elucidating pelagic food web structures and diet assimilation. Integrating the combined effects of ecological processes and hydrography, ecohydrographic features often translate into geographic patterns in δ13C and δ15N values at the base of food webs. This is due, for example, to divergent 15N abundances in source end-members (deep water sources: high δ15N, diazotrophs: low δ15N). Such patterns in the spatial distributions of stable isotope values were coined isoscapes. Empirical data of atmospheric, oceanographic, and biological processes, which drive the ecohydrographic gradients of the oligotrophic Red Sea, are under-explored and some rather anticipated than proven. Specifically, five processes underpin Red Sea gradients: (a) monsoon-related intrusions of nutrient-rich Indian Ocean water; (b) basin scale thermohaline circulation; (c) mesoscale eddy activity that causes up-welling of deep water nutrients into the upper layer; (d) the biological fixation of atmospheric nitrogen (N2) by diazotrophs; and (e) the deposition of dust and aerosol-derived N. This study assessed relationships between environmental samples (nutrients, chlorophyll a), oceanographic data (temperature, salinity, current velocity [ADCP]), particulate organic matter (POM), and net-phytoplankton, with the δ13C and δ15N values of zooplankton collected in spring 2012 from 16°28′ to 26°57′N along the central axis of the Red Sea. The δ15N of bulk POM and most zooplankton taxa increased from North (Duba) to South (Farasan). The potential contribution of deep water nutrient-fueled phytoplankton, POM, and diazotrophs varied among sites. Estimates suggested higher diazotroph contributions in the North, a greater contribution of POM in the South, and of small phytoplankton in the central Red Sea. Consistent variation across taxonomic and trophic groups at latitudinal scale, corresponding with patterns of nutrient stoichiometry and phytoplankton composition, indicates that the zooplankton ecology in the Red Sea is largely influenced by hydrographic features. It suggests that the primary ecohydrography of the Red Sea is driven not only by the thermohaline circulation, but also by mesoscale activities that transports nutrients to the upper water layers and interact with the general circulation pattern. Ecohydrographic features of the Red Sea, therefore, aid in explaining the observed configuration of its isoscape at the macroecological scale.