Trophic dynamics in Australian seagrass beds : unravelling linkages from nutrients to fish

University of Technology, Sydney. Faculty of Science.The study of food webs and trophic dynamics has been a major endeavour for biologists for over half a century. The major aim of trophic ecology has been to explain the complex structures and interaction of food webs using simple models and generalities. A key debate in this research field has been the extent to which trophic structures of communities are driven by resource or consumer control. This issue has important implications for the management of marine ecosystems with anthropogenic disturbance rapidly altering bottom-up (e.g. nutrient enrichment) and top-down (e.g. fisheries exploitation) processes. To enhance understanding of bottom up and top-down influences in productive estuaries, this study investigated a prominent four level food chain (epiphytic algae, mesograzers, juvenile fish, and piscivorous fish) in seagrass habitats. Large-scale mensurative field experiments were combined with manipulative experiments to gain an improved understanding of changes in biomass and community structure of trophic levels under different nutrient loading and to determine the type of functional response driving these changes. In general, epiphyte and grazer biomass were greater in developed catchments with higher nutrient loads during long-term field sampling of systems approaching equilibrium. However, a decoupling from the epiphyte trophic pathway was evident in higher trophic levels. The biomass of carnivorous juvenile fish and large piscivores displayed no significant change across the nutrient gradient of this study. This pattern indicates that a ratio-dependent functional response would be most appropriate for modelling the lower trophic levels and their responses to nutrient enrichment. However, more complex models taking into account other factors such as trophic transfer and subsidy are required for models across all four trophic levels. Short-term experiments involving epiphytes, grazers and juvenile fish that manipulated nutrient levels and predation rates often behaved in ways that contradicted the patterns described above for long-term studies. In particular, manipulative studies showed strong evidence for top-down control sometimes resulting in lower or similar levels of epiphytes in enriched plots compared to ambient controls. The structure of fish assemblages was influenced by the nutrient status of the waters they inhabited. Assemblages from developed and undeveloped catchments separated into distinct communities with higher abundances of fish in low nutrient waters. The species that contributed most to these differing assemblages were small pelagic carnivores (Ambassis jacksoniensis and Redigobius macrostoma). These species appeared to be sensitive to poor water quality making them candidates for bio-indicators of anthropogenic disturbance in catchments. Enclosure experiments and mensurative field sampling found that growth, survival and condition of juvenile trumpeter (Pelates sexlineatus) were greater under ambient nutrient levels despite increased prey items in nutrient enriched sites. This suggests that the positive indirect effects of increased trophic support from nutrient enrichment may have been countered by direct negative effects of toxicity either from the nutrients in their dissolved forms of ammonia; nitrate; or from other anthropogenic pollutants such as metals or hydrocarbons. Overall, nutrient enrichment of coastal waters in south-eastern Australia promoted epiphyte growth that may compete with seagrasses for light, causing stress and possible declines in seagrass health and distribution. My study, however found greater seagrass biomass at some sites with nutrient enrichment suggesting moderate increases to nitrogen-limited systems may be beneficial to seagrasses. It also appears that the increase in primary productivity associated with nutrient loading did not translate into an increase in biomass of juvenile fish or piscivores in seagrass habitats, providing no benefit to fisheries production. Given the potential for nutrient enrichment to alter trophic structure in coastal waters and to have negative impacts on seagrass health it is recommended that an integrated system of management be implemented. This should include further targeted research for a better understanding of seagrass trophic systems, monitoring of environmental indicators that alert catchment managers to early signs of eutrophication, remediation of existing eutrophic systems through nutrient reduction mechanisms and planning policies that identify and afford greater protection to ecologically-important habitats such as seagrass; and estuaries with high residence times that are more vulnerable to eutrophication

Evaluation of reference genes for RT-qPCR studies in the seagrass zostera muelleri exposed to light limitation

Seagrass meadows are threatened by coastal development and global change. In the face of these pressures, molecular techniques such as reverse transcription quantitative real-time PCR (RT-qPCR) have great potential to improve management of these ecosystems by allowing early detection of chronic stress. In RT-qPCR, the expression levels of target genes are estimated on the basis of reference genes, in order to control for RNA variations. Although determination of suitable reference genes is critical for RT-qPCR studies, reports on the evaluation of reference genes are still absent for the major Australian species Zostera muelleri subsp. capricorni (Z. muelleri). Here, we used three different software (geNorm, NormFinder and Bestkeeper) to evaluate ten widely used reference genes according to their expression stability in Z. muelleri exposed to light limitation. We then combined results from different software and used a consensus rank of four best reference genes to validate regulation in Photosystem I reaction center subunit IV B and Heat Stress Transcription factor A- gene expression in Z. muelleri under light limitation. This study provides the first comprehensive list of reference genes in Z. muelleri and demonstrates RT-qPCR as an effective tool to identify early responses to light limitation in seagrass

Can mud (silt and clay) concentration be used to predict soil organic carbon content within seagrass ecosystems?

© Author(s) 2016. The emerging field of blue carbon science is seeking cost-effective ways to estimate the organic carbon content of soils that are bound by coastal vegetated ecosystems. Organic carbon (Corg) content in terrestrial soils and marine sediments has been correlated with mud content (i.e., silt and clay, particle sizes <63μm), however, empirical tests of this theory are lacking for coastal vegetated ecosystems. Here, we compiled data (n Combining double low line 1345) on the relationship between Corg and mud contents in seagrass ecosystems (79 cores) and adjacent bare sediments (21 cores) to address whether mud can be used to predict soil Corg content. We also combined these data with the ?13C signatures of the soil Corg to understand the sources of Corg stores. The results showed that mud is positively correlated with soil Corg content only when the contribution of seagrass-derived Corg to the sedimentary Corg pool is relatively low, such as in small and fast-growing meadows of the genera Zostera, Halodule and Halophila, and in bare sediments adjacent to seagrass ecosystems. In large and long-living seagrass meadows of the genera Posidonia and Amphibolis there was a lack of, or poor relationship between mud and soil Corg content, related to a higher contribution of seagrass-derived Corg to the sedimentary Corg pool in these meadows. The relatively high soil Corg contents with relatively low mud contents (e.g., mud-Corg saturation) in bare sediments and Zostera, Halodule and Halophila meadows was related to significant allochthonous inputs of terrestrial organic matter, while higher contribution of seagrass detritus in Amphibolis and Posidonia meadows disrupted the correlation expected between soil Corg and mud contents. This study shows that mud is not a universal proxy for blue carbon content in seagrass ecosystems, and therefore should not be applied generally across all seagrass habitats. Mud content can only be used as a proxy to estimate soil Corg content for scaling up purposes when opportunistic and/or low biomass seagrass species (i.e., Zostera, Halodule and Halophila) are present (explaining 34 to 91% of variability), and in bare sediments (explaining 78% of the variability). The results obtained could enable robust scaling up exercises at a low cost as part of blue carbon stock assessments

Timing anthropogenic stressors to mitigate their impact on marine ecosystem resilience

© 2017 The Author(s). Better mitigation of anthropogenic stressors on marine ecosystems is urgently needed to address increasing biodiversity losses worldwide. We explore opportunities for stressor mitigation using whole-of-systems modelling of ecological resilience, accounting for complex interactions between stressors, their timing and duration, background environmental conditions and biological processes. We then search for ecological windows, times when stressors minimally impact ecological resilience, defined here as risk, recovery and resistance. We show for 28 globally distributed seagrass meadows that stressor scheduling that exploits ecological windows for dredging campaigns can achieve up to a fourfold reduction in recovery time and 35% reduction in extinction risk. Although the timing and length of windows vary among sites to some degree, global trends indicate favourable windows in autumn and winter. Our results demonstrate that resilience is dynamic with respect to space, time and stressors, varying most strongly with: (i) the life history of the seagrass genus and (ii) the duration and timing of the impacting stress

Nucleation of a sodium droplet on C60

We investigate theoretically the progressive coating of C60 by several sodium atoms. Density functional calculations using a nonlocal functional are performed for NaC60 and Na2C60 in various configurations. These data are used to construct an empirical atomistic model in order to treat larger sizes in a statistical and dynamical context. Fluctuating charges are incorporated to account for charge transfer between sodium and carbon atoms. By performing systematic global optimization in the size range 1<=n<=30, we find that Na_nC60 is homogeneously coated at small sizes, and that a growing droplet is formed above n=>8. The separate effects of single ionization and thermalization are also considered, as well as the changes due to a strong external electric field. The present results are discussed in the light of various experimental data.Comment: 17 pages, 10 figure

A fresh look on three-loop sum-integrals

In order to prepare the ground for evaluating classes of three-loop sum-integrals that are presently needed for thermodynamic observables, we take a fresh and systematic look on the few known cases, and review their evaluation in a unified way using coherent notation. We do this for three important cases of massless bosonic three-loop vacuum sum-integrals that have been frequently used in the literature, and aim for a streamlined exposition as compared to the original evaluations. In passing, we speculate on options for generalization of the computational techniques that have been employed.Comment: 19 page

A Search for Dark Higgs Bosons

Recent astrophysical and terrestrial experiments have motivated the proposal of a dark sector with GeV-scale gauge boson force carriers and new Higgs bosons. We present a search for a dark Higgs boson using 516 fb-1 of data collected with the BABAR detector. We do not observe a significant signal and we set 90% confidence level upper limits on the product of the Standard Model-dark sector mixing angle and the dark sector coupling constant.Comment: 7 pages, 5 postscript figures, published version with improved plots for b/w printin

Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV

The performance of muon reconstruction, identification, and triggering in CMS has been studied using 40 inverse picobarns of data collected in pp collisions at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection criteria covering a wide range of physics analysis needs have been examined. For all considered selections, the efficiency to reconstruct and identify a muon with a transverse momentum pT larger than a few GeV is above 95% over the whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4, while the probability to misidentify a hadron as a muon is well below 1%. The efficiency to trigger on single muons with pT above a few GeV is higher than 90% over the full eta range, and typically substantially better. The overall momentum scale is measured to a precision of 0.2% with muons from Z decays. The transverse momentum resolution varies from 1% to 6% depending on pseudorapidity for muons with pT below 100 GeV and, using cosmic rays, it is shown to be better than 10% in the central region up to pT = 1 TeV. Observed distributions of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO

Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV

The performance of muon reconstruction, identification, and triggering in CMS has been studied using 40 inverse picobarns of data collected in pp collisions at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection criteria covering a wide range of physics analysis needs have been examined. For all considered selections, the efficiency to reconstruct and identify a muon with a transverse momentum pT larger than a few GeV is above 95% over the whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4, while the probability to misidentify a hadron as a muon is well below 1%. The efficiency to trigger on single muons with pT above a few GeV is higher than 90% over the full eta range, and typically substantially better. The overall momentum scale is measured to a precision of 0.2% with muons from Z decays. The transverse momentum resolution varies from 1% to 6% depending on pseudorapidity for muons with pT below 100 GeV and, using cosmic rays, it is shown to be better than 10% in the central region up to pT = 1 TeV. Observed distributions of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO