Benthic dinitrogen fixation in a Northern Red Sea coral reef under seasonally changing environmental conditions

Tropical coral reefs are among the most productive ecosystems on this planet, despite being surrounded by very oligotrophic waters. Effective recycling processes of the limiting nutrients, particularly nitrogen (N), and input of new bioavailable N via dinitrogen (N2) fixation are essential to sustain such high gross primary production. In fact, several benthic reef organisms and substrates are associated with diverse communities of N2 fixing microbes (diazotrophs), but the respective contribution of the different benthic groups to total benthic N2 fixation and the effect of changing environmental conditions on N2 fixation have not been investigated yet. Therefore, this thesis, through a series of interconnected studies carried out in a seasonally dynamic coral reef system, the Northern Red Sea, and in a number of manipulative experiments, contributes to the understanding of benthic N2 fixation in coral reefs by answering the following key questions: 1) How much N2 is fixed by the dominant benthic groups in the Northern Red Sea? 2) What is the relative contribution of the benthic groups to total benthic N2 fixation within the reef? 3) What is the effect of seasonally changing environmental conditions and of single environmental factors on key metabolic processes, particularly N2 fixation, associated with the different benthic groups? Findings revealed that all investigated benthic groups showed N2 fixation activity, whereof bare coral rock, turf algae, carbonate sands and living hard corals were the main N2 fixing components contributing ~ 90% to benthic N2 fixation in investigated Red Sea coral reefs. Soft corals revealed the lowest N2 fixation activity among all investigated groups and released significantly less organic matter to the surrounding water compared to hard corals indicating that N2 fixation may also influence their role as allogenic ecosystem engineers. N2 fixation by most benthic groups was usually highest when nutrient availability was lowest, and water temperature as well as light intensity highest suggesting that the N2 fixation products fuel the metabolic N requirements of reef organisms, particularly during nutrient-depleted conditions (i.e. summer). The manipulation of single environmental factors revealed a stimulation of N2 fixation activities under global warming conditions and a reduction under ocean acidification scenarios indicating that global climate change will affect the nutrient status of reef organisms. In summary, this thesis underlines the ubiquity of N2 fixation associated with different benthic coral reef organisms and substrates, and highlights its importance in sustaining coral reef productivity. Both, the benthic community structure and the prevailing environmental conditions appear to be important in controlling the amount of N2 fixation in coral reefs. Finally, as coral reefs are increasingly and simultaneously exposed to global and local stressors, it is necessary to understand how important metabolic processes on coral reefs (i.e. organic matter fluxes and N2 fixation activity) will be affected, and this thesis provides first quantitative insights into these processes from physiological, biogeochemical and ecological points of view

Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef

Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m−2 reef d−1, with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m−2 reef d−1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle

Has the GZK suppression been discovered?

The energy spectra of ultra high energy cosmic rays reported by the AGASA, Fly's Eye, Haverah Park, HiRes, and Yakutsk experiments are all shown to be in agreement with each other for energies below 10^{20} eV (after small adjustments, within the known uncertainties, of the absolute energy scales). The data from HiRes, Fly's Eye, and Yakutsk are consistent with the expected flux suppression above 5\times 10^{19} eV due to interactions of cosmic rays with the cosmic microwave background, the Greisen-Zatsepin-Kuzmin (GZK) "supression," and are inconsistent with a smooth extrapolation of the observed cosmic ray energy spectrum to energies > 5\times 10^{19}$ eV. AGASA data show an excess of events above 10^{20} eV, compared to the predicted GZK suppression and to the flux measured by the other experiments.Comment: Editorial changes, including replacing 'cutoff' by 'supression

High energy cosmic-rays: puzzles, models, and giga-ton neutrino telescopes

The existence of cosmic rays of energies exceeding 10^20 eV is one of the mysteries of high energy astrophysics. The spectrum and the high energy to which it extends rule out almost all suggested source models. The challenges posed by observations to models for the origin of high energy cosmic rays are reviewed, and the implications of recent new experimental results are discussed. Large area high energy cosmic ray detectors and large volume high energy neutrino detectors currently under construction may resolve the high energy cosmic ray puzzle, and shed light on the identity and physics of the most powerful accelerators in the universe.Comment: 12 pages, 7 figures; Summary of review talk, PASCOS 03 (Mumbai, India

Studies With Soft Corals – Recommendations on Sample Processing and Normalization Metrics

Soft corals (Octocorallia) often constitute the second most abundant macrobenthic group on many tropical and temperate reefs. However, the gelatinous and leather-like nature of their tissue and their variable hydroskeleton entails a number of problems for tissue homogenization and data normalization. An easy and fast protocol for tissue homogenization, as well as a normalization metric that can be used to perform inter-studies or inter-species comparisons, are thus needed. In this study, we tested whether the tissue sample state before processing (frozen vs. freeze-dried samples) and the media used for tissue homogenization (0.2 μm filtered seawater; FSW vs. Milli-Q water; DI) affect the quantitative measurements of tissue descriptors (chlorophyll, protein, and Symbiodinium concentrations) in the model species Heteroxenia fuscescens. Furthermore, the suitability of dry weight (DW) and ash-free dry weight (AFDW) as size-normalizing metric was investigated across different soft coral species. Our results reveal that freeze-drying the samples and homogenizing them in DI water exhibited several benefits, namely enhancing chlorophyll and protein concentrations up to 50%, saving processing time and providing a more accurate determination of DW and AFDW. Overall, this optimized tissue processing protocol offers a more reliable quantification of tissue descriptors and reduces the chance of underestimating these parameters in soft corals. Finally, since the contribution of sclerites to the total DW of the colony can highly differ between species, we demonstrate that AFDW is a reliable metric for normalizing soft coral data, particularly when inter-species comparisons are made

Effects of Water Column Mixing and Stratification on Planktonic Primary Production and Dinitrogen Fixation on a Northern Red Sea Coral Reef

The northern Red Sea experiences strong annual differences in environmental conditions due to its relative high-latitude location for coral reefs. This allows the study of regulatory effects by key environmental parameters (i.e., temperature, inorganic nutrient, and organic matter concentrations) on reef primary production and dinitrogen (N2) fixation, but related knowledge is scarce. Therefore, this study measured environmental parameters, primary production and N2 fixation of phytoplankton groups in the water overlying a coral reef in the Gulf of Aqaba. To this end, we used a comparative approach between mixed and stratified water column scenarios in a full year of seasonal observations. Findings revealed that inorganic nutrient concentrations were significantly higher in the mixed compared to the stratified period. While gross photosynthesis and N2 fixation rates remained similar, net photosynthesis decreased from mixed to stratified period. Net heterotrophic activity of the planktonic community increased significantly during the stratified compared to the mixed period. While inorganic nitrogen (N) availability was correlated with net photosynthesis over the year, N2 fixation only correlated with N availability during the mixed period. This emphasizes the complexity of planktonic trophodynamics in northern Red Sea coral reefs. Comparing mixed and stratified planktonic N2 fixation rates with those of benthic organisms and substrates revealed a close seasonal activity similarity between free-living pelagic and benthic diazotrophs. During the mixed period, N2 fixation potentially contributed up to 3% of planktonic primary production N demand. This contribution increased by ca. one order of magnitude to 21% during the stratified period. Planktonic N2 fixation is likely a significant N source for phytoplankton to maintain high photosynthesis under oligotrophic conditions in coral reefs, especially during stratified conditions

Spatio-temporal patterns in the coral reef communities of the Spermonde Archipelago, 2012–2014, II: Fish assemblages display structured variation related to benthic condition

The Spermonde Archipelago is a complex of ~70 mostly populated islands off Southwest Sulawesi, Indonesia, in the center of the Coral Triangle. The reefs in this area are exposed to a high level of anthropogenic disturbances. Previous studies have shown that variation in the benthos is strongly linked to water quality and distance from the mainland. However, little is known about the fish assemblages of the region and if their community structure also follows a relationship with benthic structure and distance from shore. In this study, we used eight islands of the archipelago, varying in distance from 1 to 55 km relative to the mainland, and 3 years of surveys, to describe benthic and fish assemblages and to examine the spatial and temporal influence of benthic composition on the structure of the fish assemblages. Cluster analysis indicated that distinct groups of fish were associated with distance, while few species were present across the entire range of sites. Relating fish communities to benthic composition using a multivariate generalized linear model confirmed that fish groups relate to structural complexity (rugosity) or differing benthic groups; either algae, reef builders (coral and crustose coralline algae) or invertebrates and rubble. From these relationships we can identify sets of fish species that may be lost given continued degradation of the Spermonde reefs. Lastly, the incorporation of water quality, benthic and fish indices indicates that local coral reefs responded positively after an acute disturbance in 2013 with increases in reef builders and fish diversity over relatively short (1 year) time frames. This study contributes an important, missing component (fish community structure) to the growing literature on the Spermonde Archipelago, a system that features environmental pressures common in the greater Southeast Asian region

Simulating the atmospheric response to the 11-year solar cycle forcing with the UM-UKCA model: the role of detection method and natural variability

The 11-year solar cycle forcing is recognised as an important atmospheric forcing; however, there remain uncertainties in characterising the effects of solar variability on the atmosphere from observations and models. Here we present the first detailed assessment of the atmospheric response to the 11-year solar cycle in the UM-UKCA (Unified Model coupled to the United Kingdom Chemistry and Aerosol model) chemistry–climate model (CCM) using a three-member ensemble over the recent past (1966–2010). Comparison of the model simulations is made with satellite observations and reanalysis datasets. The UM-UKCA model produces a statistically significant response to the 11-year solar cycle in stratospheric temperatures, ozone and zonal winds. However, there are also differences in magnitude, spatial structure and timing of the signals compared to observational and reanalysis estimates. This could be due to deficiencies in the model performance, and so we include a critical discussion of the model limitations, and/or uncertainties in the current observational estimates of the solar cycle signals. Importantly, in contrast to many previous studies of the solar cycle impacts, we pay particular attention to the role of the chosen analysis method in UM-UKCA by comparing the model composite and a multiple linear regression (MLR) results. We show that the stratospheric solar responses diagnosed using both techniques largely agree with each other within the associated uncertainties; however, the results show that apparently different signals can be identified by the methods in the troposphere and in the tropical lower stratosphere. Lastly, we examine how internal atmospheric variability affects the detection of the 11-year solar responses in the model by comparing the results diagnosed from the three individual ensemble members (as opposed to those diagnosed from the full ensemble). We show overall agreement between the responses diagnosed for the ensemble members in the tropical and mid-latitude mid-stratosphere to lower mesosphere but larger apparent differences at Northern Hemisphere (NH) high latitudes during the dynamically active season. Our UM-UKCA results suggest the need for long data sets for confident detection of solar cycle impacts in the atmosphere, as well as for more research on possible interdependence of the solar cycle forcing with other atmospheric forcings and processes (e.g. Quasi-Biennial Oscillation, QBO; El Niño–Southern Oscillation, ENSO)