Trophodynamics and functional feeding groups of North Sea fauna: a combined stable isotope and fatty acid approach

The trophodynamics of pelagic and benthic animals of the North Sea, North Atlantic shelf, were assessed using stable isotope analysis (SIA) of natural abundance carbon and nitrogen isotopes, lipid fingerprinting and compound-specific SIA (CSIA) of phospholipid-derived fatty acids (PLFAs). Zooplankton (z), epi- and supra-benthic macrofauna were collected in the Southern Bight, at the Oyster Grounds and at North Dogger, 111 km north of the Dogger Bank. The study included 22 taxonomic groups with particular reference to Mollusca (Bivalvia and Gastropoda) and Crustacea. Primary consumers (Bivalvia) were overall most 15N enriched in the southern North Sea (6.1‰) and more depleted in the Oyster Grounds (5.5‰) and at North Dogger (2.8‰) demonstrating differences in isotopic baselines for bivalve fauna between the study sites. Higher trophic levels also followed this trend. Over an annual cycle, consumers tended to exhibit 15N depletion during spring followed by 15N enriched signatures in autumn and winter. The observed seasonal changes of ? 15N were more pronounced for suspension feeders and deposit feeders (dfs) than for filter feeders (ffs). The position of animals in plots of ? 13C and ? 15N largely concurred with the expected position according to literature-based functional feeding groups. PLFA fingerprints of groups such as z were distinct from benthic groups, e.g. benthic ffs and dfs, and predatory macrobenthos. ? 13CPLFA signatures indicated similarities in 13C moiety sources that constituted ? 13CPLFA. Although functional groups of pelagic zooplankton and (supra-) benthic animals represented phylogenetically distinct consumer groups, ? 13CPLFA demonstrated that both groups were supported by pelagic primary production and relied on the same macronutrients such as PLFAs. Errors related to the static categorization of small invertebrates into fixed trophic positions defined by phylogenetic groupings rather than by functional feeding groups, and information on seasonal trophodynamic variability, may have implications for the reliability of numerical marine ecosystem models

Seasonal modulation of mesoscale processes alters nutrient availability and plankton communities in the Red Sea

Hydrographic and atmospheric forcing set fundamental constraints on the biogeochemistry of aquatic ecosystems and manifest in the patterns of nutrient availability and recycling, species composition of communities, trophic dynamics, and ecosystem metabolism. In the Red Sea, latitudinal gradients in environmental conditions and primary production have been ascribed to fluctuations in Gulf of Aden Water inflow, upwelling/mixing, and regenerated nutrient utilization i.e. rapidly recycled nitrogen in upper layers. However, our understanding of upper layer dynamics and related changes in plankton communities, metabolism and carbon and nitrogen export is limited. We surmised that stratification and mesoscale eddies modulate the nutrient availability and taxonomic identity of plankton communities in the Red Sea. Based on remote-sensing data of sea level anomalies and high resolution in situ measurements (ScanFish) we selected stations for hydrographic CTD profiles, water sampling (nutrients, seawater oxygen stable isotopes [δ18OSW]), phytoplankton and zooplankton collections. In fall 2014, strong stratification subjected the plankton community to an overall nitrogen and phosphorus shortage. The nutrient deficiency increased numbers of heterotrophic dinoflagellates, microzooplankton, and diazotrophs (Trichodesmium, diatom-diazotroph associations [DDAs]), albeit largely decreased phytoplankton and mesozooplankton abundances. In spring 2015, mesoscale eddies increased the nutrient availability, and the thermohaline characteristics and low δ18OSW point to the interaction of eddies with Gulf of Aden Surface Water (GASW). Cyclonic eddies and, most likely, the availability of nutrients associated with the GASW, increased the abundances of autotrophs (diatoms, Prasinophytes) and supported larger numbers of zooplankton and their larvae. We demonstrate that the interplay of stratification, advection of Gulf of Aden water and mesoscale eddies are key elements to better understand changes in plankton community composition, ecosystem metabolism, and macronutrient export in the Red Sea in space and time

Physical mechanisms routing nutrients in the central Red Sea

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 9032–9046, doi:10.1002/2017JC013017.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°N–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.King Abdullah University of Science and Technology (KAUST); Core Marine Operation Research Lab (CMOR

Nitrogen eutrophication particularly promotes turf algae in coral reefs of the central Red Sea

While various sources increasingly release nutrients to the Red Sea, knowledge about their effects on benthic coral reef communities is scarce. Here, we provide the first comparative assessment of the response of all major benthic groups (hard and soft corals, turf algae and reef sands-together accounting for 80% of the benthic reef community) to in-situ eutrophication in a central Red Sea coral reef. For 8 weeks, dissolved inorganic nitrogen (DIN) concentrations were experimentally increased 3-fold above environmental background concentrations around natural benthic reef communities using a slow release fertilizer with 15% total nitrogen (N) content. We investigated which major functional groups took up the available N, and how this changed organic carbon (C-org) and N contents using elemental and stable isotope measurements. Findings revealed that hard corals (in their tissue), soft corals and turf algae incorporated fertilizer N as indicated by significant increases in delta N-15 by 8%, 27% and 28%, respectively. Among the investigated groups, C-org content significantly increased in sediments (+24%) and in turf algae (+33%). Altogether, this suggests that among the benthic organisms only turf algae were limited by N availability and thus benefited most from N addition. Thereby, based on higher C-org content, turf algae potentially gained competitive advantage over, for example, hard corals. Local management should, thus, particularly address DIN eutrophication by coastal development and consider the role of turf algae as potential bioindicator for eutrophication.Peer reviewe

Coral Communities, in Contrast to Fish Communities, Maintain a High Assembly Similarity along the Large Latitudinal Gradient along the Saudi Red Sea Coast

The Saudi Arabian Red Sea coast is characterized by a strong environmental gradient from north (28.5°N) to south (16.5°N) with challenging conditions for coral growth particularly in the south (high temperature and nutrient input). We investigated whether assemblies of reef-building corals and the distribution of functional groups follow a latitudinal pattern in the Red Sea, and whether these changes affect the assembly structure of coral associated organisms (e.g. fishes). Functional groups were defined based on life-history traits and functional role. 13 reefs along the north-south gradient, including 5 potentially polluted reefs were investigated. Results showed a substantially weaker latitudinal shift in the assembly structure of coral communities than of fishes communities and of other benthic reef taxa. Competitive fast growing branching and tabular species (mainly Acropora), as well as rather stresstolerant slow growing bulky species (e.g. Porites, Goniastrea, Favites, Favia) were fairly evenly distributed along the north-south axis despite strong changes of environmental conditions. This seems on the one hand attributable to the high species richness within a given functional group (functional redundancy) and on the other hand to a high acclimatization / adaptation potential of some Red Sea coral species. The prime ecosystem service of the coral community, the provision of a habitat complex, is thereby maintained throughout the gradient. In contrast to the coral community, the assembly of the fish community shifts along the environmental gradient with higher abundances of small wrasses and butterfly fishes in the north, and overall higher abundance of fishes including large fishes in the south. This shift seems linked to higher food availability in the south. Altered assembly structures of coral communities were found in reefs close to a source of pollution with either an increased relative abundance of stresstolerant species or a general decrease of coral abundance, latter case accompanied by a substantial reduction in fish abundance

Synergistic HNO3_{3}–H2_{2}SO4_{4}–NH3_{3} upper tropospheric particle formation

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)$^{1,2,3,4}$. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO$_{3}$–H$_{2}$SO$_{4}$–NH$_{3}$ nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere

Role of iodine oxoacids in atmospheric aerosol nucleation

Iodic acid (HIO₃) is known to form aerosol particles in coastal marine regions, but predicted nucleation and growth rates are lacking. Using the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we find that the nucleation rates of HIO₃ particles are rapid, even exceeding sulfuric acid–ammonia rates under similar conditions. We also find that ion-induced nucleation involves IO₃⁻ and the sequential addition of HIO₃ and that it proceeds at the kinetic limit below +10°C. In contrast, neutral nucleation involves the repeated sequential addition of iodous acid (HIO₂) followed by HIO₃, showing that HIO₂ plays a key stabilizing role. Freshly formed particles are composed almost entirely of HIO₃, which drives rapid particle growth at the kinetic limit. Our measurements indicate that iodine oxoacid particle formation can compete with sulfuric acid in pristine regions of the atmosphere

Tracking seasonal changes in North Sea zooplankton trophic dynamics using stable isotopes

Trophodynamics of meso-zooplankton in the North Sea (NS) were assessed at a site in the southern NS, and at a shallow and a deep site in the central NS. Offshore and neritic species from different ecological niches, including Calanus spp., Temora spp. and Sagitta spp., were collected during seven cruises over 14 months from 2007 to 2008. Bulk stable isotope (SI) analysis, phospholipid-derived fatty acid (PLFA) compositions, and δ 13CPLFA data of meso-zooplankton and particulate organic matter (POM) were used to describe changes in zooplankton relative trophic positions (RTPs) and trophodynamics. The aim of the study was to test the hypothesis that the RTPs of zooplankton in the North Sea vary spatially and seasonally, in response to hydrographic variability, with the microbial food web playing an important role at times. Zooplankton RTPs tended to be higher during winter and lower during the phytoplankton bloom in spring. RTPs were highest for predators such as Sagitta sp. and Calanus helgolandicus and lowest for small copepods such as Pseudocalanus elongatus and zoea larvae (Brachyura). δ 15NPOM-based RTPs were only moderate surrogates for animals’ ecological niches, because of the plasticity in source materials from the herbivorous and the microbial loop food web. Common (16:0) and essential (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA) structural lipids showed relatively constant abundances. This could be explained by incorporation of PLFAs with δ 13C signatures which followed seasonal changes in bulk δ 13CPOM and PLFA δ 13CPOM signatures. This study highlighted the complementarity of three biogeochemical approaches for trophodynamic studies and substantiated conceptual views of size-based food web analysis, in which small individuals of large species may be functionally equivalent to large individuals of small species. Seasonal and spatial variability was also important in altering the relative importance of the herbivorous and microbial food webs