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Striebel M.,University of Oslo | Singer G.,WasserCluster Lunz | Singer G.,University of Vienna | Stibor H.,Ludwig Maximilians University of Munich | And 2 more authors.
Ecology | Year: 2012

Diversity-productivity relationships at the primary producer level have been extensively studied, especially for terrestrial systems. Here, we explore whether the diversity of aquatic primary producers (phytoplankton) has effects on higher trophic levels (zooplankton). We investigated the effect of phytoplankton diversity on an artificial zooplankton community in a laboratory experiment where phytoplankton biomass and elemental composition (carbon-to-phosphorus ratio) were kept constant. Phytoplankton diversity increased the means of both zooplankton growth rate and abundance while suppressing their variability, and sustained higher zooplankton diversity. Likely explanations include resource complementarity effects among phytoplankton species as food entities, as well as niche complementarity effects among Daphnia species as competitors. By affecting the productivity as well as the variability of the next trophic level, biodiversity of primary producers may have far-reaching consequences in aquatic food webs. © 2012 by the Ecological Society of America.

Biton E.,Weizmann Institute of Science | Gildor H.,Hebrew University of Jerusalem | Gildor H.,Weizmann Institute of Science | Trommer G.,University of Tübingen | And 6 more authors.
Paleoceanography | Year: 2010

We used an oceanic general circulation model to evaluate the sensitivity of the hydrography and circulation of the Red Sea in response to reduced sea level and modified atmospheric conditions during the Holocene. With Holocene sea level close to the modern level, the Red Sea was sensitive to changes in atmospheric conditions, and it only shows a relatively mild response to sea level change. Changes in the monsoon system influence the exchange flow through the Strait of Bab el Mandab, the meridional overturning circulation of the Red Sea, and its hydrography. Forced by humid conditions the (modeled) Red Sea temperature increased by ∼1.5C, while when arid conditions were imposed, the temperature decreased by ∼2.5C. Similar heating and cooling events during the early and late Holocene are seen in a sea surface temperature record from the northern Red Sea (derived from the temperature sensitive TEX86 molecular biomarker), which suggests that humid conditions prevailed during the early Holocene and more arid conditions prevailed during the late Holocene. The gradual decline in Red Sea temperature between these two time periods suggests a gradual decline in the summer monsoon strength. This monsoon trend and the resulting changes in the Red Sea circulation are supported by the distribution of crenarchaea fossil lipids in Red Sea sediments from this period. Monsoon-driven changes in the exchange flow through the Strait of Bab el Mandab affected the crenarchaea population structure, and therefore, their molecular fossil distribution in the sediments of the Red Sea potentially provides an index for the summer monsoon strength during the Holocene. © 2010 by the American Geophysical Union.

Diehl S.,Ludwig Maximilians University of Munich | Diehl S.,Umeå University | Berger S.A.,Ludwig Maximilians University of Munich | Berger S.A.,Leibniz Institute of Freshwater Ecology and Inland Fisheries | And 4 more authors.
ICES Journal of Marine Science | Year: 2015

Sverdrup's critical depth hypothesis, which has had an almost canonical status in biological oceanography, has recently been challenged as a universal explanation for the formation of oceanic spring blooms, and several alternative hypotheses have been proposed. Arguments pro and contra alternative explanations have so far relied on theoretical considerations and purely observational data. In this paper, we propose that mesocosm experiments with natural plankton communities could make important contributions to the resolution of the issue. We first briefly review the foundations of the critical depth concept and derive an approximate relationship that relates optically scaled critical depth (="critical optical depth", i.e. the product of the light attenuation coefficient and the critical depth) to light-dependent phytoplankton production in the mixed surface layer. We describe how this relationship can be used to scale experimental mesocosms such that they reproduce ambient light conditions of natural water columns from the surface down to the critical depth and beyond. We illustrate the power of the approach with a mesocosm study in which we experimentally controlled the onset of the spring bloom of a lake plankton community through the manipulation of optically scaled mixed-layer depth. This experiment may be the first experimental demonstration of the critical depth principle acting on a natural plankton community. Compensation light intensity (=minimum average mixed-layer light intensity required to trigger a bloom of the ambient plankton community) could be constrained to be somewhat above 3.2 moles PAR m-2 d-1, corresponding to a critical optical depth of 10.5. We compare these numbers to estimates from marine systems and end with a discussion of how experiments could be designed to (i) more accurately determine the critical depth in a given system and (ii) resolve among competing hypotheses for vernal bloom onset. © 2015 International Council for the Exploration of the Sea. All rights reserved.

Trommer G.,University of Tübingen | Trommer G.,European Institute for Marine Studies | Siccha M.,University of Tübingen | Siccha M.,Hebrew University of Jerusalem | And 6 more authors.
Climate of the Past | Year: 2011

This study investigates the response of Red Sea circulation to sea level and insolation changes during termination II and across the last interglacial, in comparison with termination I and the Holocene. Sediment cores from the central and northern part of the Red Sea were investigated by micropaleontological and geochemical proxies. The recovery of the planktic foraminiferal fauna following high salinities during marine isotopic stage (MIS) 6 took place at similar sea-level stand (∼50 m below present day), and with a similar species succession, as during termination I. This indicates a consistent sensitivity of the basin oceanography and the plankton ecology to sea-level forcing. Based on planktic foraminifera, we find that increased water exchange with the Gulf of Aden especially occurred during the sea-level highstand of interglacial MIS 5e. From MIS 6 to the peak of MIS 5e, northern Red Sea sea surface temperature (SST) increased from 21 °C to 25 °C, with about 3 °C of this increase taking place during termination II. Changes in planktic foraminiferal assemblages indicate that the development of the Red Sea oceanography during MIS 5 was strongly determined by insolation and monsoon strength. The SW Monsoon summer circulation mode was enhanced during the termination, causing low productivity in northern central Red Sea core KL9, marked by high abundance of G. sacculifer, which ĝ€" as in the Holocene ĝ€" followed summer insolation. Core KL11 records the northern tip of the intruding intermediate water layer from the Gulf of Aden and its planktic foraminifera fauna shows evidence for elevated productivity during the sea-level highstand in the southern central Red Sea. By the time of MIS 5 sea-level regression, elevated organic biomarker BIT values suggest denudation of soil organic matter into the Red Sea and high abundances of G. glutinata, and high reconstructed chlorophyll-a values, indicate an intensified NE Monsoon winter circulation mode. Our results imply that the amplitude of insolation fluctuations, and the resulting monsoon strength, strongly influence the Red Sea oceanography during sea-level highstands by regulating the intensity of water exchange with the Gulf of Aden. These processes are responsible for the observation that MIS 5e/d is characterized by higher primary productivity than the Holocene. © Author(s) 2011.

Berger S.A.,Ludwig Maximilians University of Munich | Berger S.A.,University of Bergen | Diehl S.,Ludwig Maximilians University of Munich | Diehl S.,Umeå University | And 4 more authors.
Global Change Biology | Year: 2010

In deep temperate lakes, the beginning of the growing season is triggered by thermal stratification, which alleviates light limitation of planktonic producers in the surface layer and prevents heat loss to deeper strata. The sequence of subsequent phenological events (phytoplankton spring bloom, grazer peak, clearwater phase) results in part from coupled phytoplankton-grazer interactions. Disentangling the separate, direct effects of correlated climatic drivers (stratification-dependent underwater light climate vs. water temperature) from their indirect effects mediated through trophic feedbacks is impossible using observational field data, which challenges our understanding of global warming effects on seasonal plankton dynamics. We therefore manipulated water temperature and stratification depth independently in experimental field mesocosms containing ambient microplankton and inocula of the resident grazer Daphnia hyalina. Higher light availability in shallower surface layers accelerated primary production, warming accelerated consumption and growth of Daphnia, and both factors speeded up successional dynamics driven by trophic feedbacks. Specifically, phytoplankton peaked and decreased earlier and Daphnia populations increased and peaked earlier at both shallower stratification and higher temperature. The timing of ciliate dynamics was unrelated to both factors. Volumetric peak densities of phytoplankton, ciliates and Daphnia in the surface layer were also unaffected by temperature but declined with stratification depth in parallel with light availability. The latter relationship vanished, however, when population sizes were integrated over the entire water column. Overall our results suggest that, integrated over the entire water column of a deep lake, surface warming and shallower stratification independently speed up spring successional events, whereas the magnitudes of phytoplankton and zooplankton spring peaks are less sensitive to these factors. Therefore, accelerated dynamics under warming need not lead to a trophic mismatch (given similar grazer inocula at the time of stratification). We emphasize that entire water column dynamics must be studied to estimate global warming effects on lake ecosystems. © 2010 Blackwell Publishing Ltd.

Fiorentini M.L.,University of Western Australia | Bekker A.,University of Manitoba | Rouxel O.,European Institute for Marine Studies | Wing B.A.,McGill University | And 2 more authors.
Economic Geology | Year: 2012

We report here high-precision multiple sulfur and iron isotope compositions for a series of mineralized samples from Ni-Cu-(PGE) sulfide deposits in the Archean Tati greenstone belt and the Phikwe Complex of eastern Botswana. Mineralized samples from the Phoenix and Selkirk Ni-Cu-(PGE) deposits in the Tati greenstone belt display slightly positive δ 34S isotope values, ranging from 0.2 to 0.8% V-CDT. Δ 33S values of sulfides at Phoenix and Selkirk are -0.01 to -0.08% V-CDT, suggesting either a dominantly mantle sulfur source or effective eradication of a crustal Δ 33S anomaly through equilibration with large amounts of silicate melt. In the Selebi-Phikwe belt, a granite-gneiss terrane with abundant amphibolite lenses of either volcanic and/or intrusive nature, mineralized lower grade samples from the Phikwe, Phokoje, and Dikoloti Ni-Cu-(PGE) deposits have more variable δ 34S values ranging from -3.1 to +0.3% and display significant mass independent anomalies (Δ 33S values ranging from -0.89 to -0.27%), suggesting that barren sulfides associated with distal or lowtemperature sea-floor hydrothermal activity contributed sulfur to these deposits. Iron isotopes of sulfides from these deposits show a relatively small range of negative Δ 56Fe values (-0.29 to -0.04%), consistent with hightemperature fractionations in magmatic systems, with the exception of one sample from the Dikoloti Ni-Cu-(PGE) deposit of the Selebi-Phikwe greenstone belt, which shows a more negative δ 56Fe value of -0.61%, consistent with assimilation of sedimentary or hydrothermal sulfides rather than fractionations in high-temperature magmatic systems. Data from this study highlight the complexity and variability that characterize oreforming processes in magmatic systems. We suggest that the presence of sulfur-bearing lithologic units in host rocks of mafic and ultramafic intrusions may not be essential toward the assessment of the prospectivity of a province to host orthomagmatic nickel sulfides. Geologic settings without any or little sulfur in the stratigraphy, which have been traditionally neglected in terms of their prospectivity, should thus be revisited and possibly reassessed considering the potential importance of external source of sulfur to generate ore deposits. © 2012 Society of Economic Geologists, Inc.

Leye P.O.,European Institute for Marine Studies | Tarits P.,European Institute for Marine Studies
Radio Science | Year: 2015

At and near the Earth surface, electromagnetic (EM) fields radiated from VLF transmitters are commonly used in geological exploration to determine the shallow Earth conductivity structure. Onboard satellites such as DEMETER, the electric and magnetic sensors detect the VLF signal in altitude. While we know for surface measurement that the VLF EM field recorded at some distance from the transmitter is a function of the ground conductivity, we do not know how this relationship changes when the field is measured at satellite altitude. Here we study the electromagnetic field radiated by a vertical electric dipole located on the Earth surface in the VLF range and measured at satellite altitude in a free space. We investigate the EM field as function of distance from the source, the height above the Earth surface, and the electrical conductivity of the Earth. The mathematical solution in altitude has more severe numerical complications than the well-known solutions at or near the Earth surface. We test most of the solutions developed for the latter case and found that direct summation was best at several hundred kilometers above the Earth. The numerical modeling of the EM field in altitude shows that the field remains a function of Earth conductivity. The dependence weakens with altitude and distance from the transmitter. It remains more important for the electric radial component. Key Points Investigating the Earth conductivity from VLF signal at satellite altitude Accounting the Earth sphericity effect on the calculation Solution of the numerical problem at high altitude ©2015. American Geophysical Union. All Rights Reserved.

Behl S.,Ludwig Maximilians University of Munich | Donval A.,European Institute for Marine Studies | Stiborb H.,European Institute for Marine Studies
Limnology and Oceanography | Year: 2011

We conducted laboratory experiments with 85 assembled phytoplankton communities composed of species from four predefined functional groups (chlorophytes, diatoms, cyanobacteria, chrysophytes) to distinguish the relative importance of species diversity from functional group diversity on carbon uptake. We separated the observed diversity effects on carbon uptake into those caused by species with particularly important traits (selection effect) and those caused by positive interactions among species (e.g., complementary resource use or facilitation [complementarity effect]). Additionally, we measured the composition of photosynthetically active pigments and light absorbance in communities and monocultures, and related them to species and functional diversity effects on carbon accrual. Biodiversity effects were weak or even absent in pure cyanobacterial and diatom communities compared to strong effects in chlorophytes. Complementarity effects and light absorbance increased as functional (i.e., phylogenetic) diversity increased. There was a positive correlation between complementarity effects on carbon accrual and light absorbance. These findings support hypotheses regarding biodiversity- productivity relationships in phytoplankton communities based on niche separation along spectral light gradients.

Stockenreiter M.,Ludwig Maximilians University of Munich | Graber A.-K.,Ludwig Maximilians University of Munich | Haupt F.,Ludwig Maximilians University of Munich | Stibor H.,European Institute for Marine Studies
Journal of Applied Phycology | Year: 2012

Current research investigating the importance of diversity for biofuel lipid production remains limited. In contrast, the relationship between diversity and productivity within terrestrial and algal primary producers has been well documented in ecology. Hence, we set out to investigate, experimentally, whether diversity may also affect lipid production in micro-algae. We investigated the growth and lipid production of micro-algae using species from all major algal groups. Algae were grown in a large number of treatments differing in their diversity level. Additionally, we compared the growth and lipid production of laboratory communities to natural lake and pond phytoplankton communities of different diversity. Our results show that lipid production increased with increasing diversity in both natural and laboratory micro-algal communities. The underlying reason for the observed 'diversity-productivity' relationship seems to be resource use complementarity. We observed higher lipid production of highly diverse algal communities under the same growth and resource supply conditions compared to monocultures. Hence, the incorporation of the ecological advantages of diversity-related resource-use dynamics into algal biomass production may provide a powerful and cost effective way to improve biofuel production. © 2011 Springer Science+Business Media B.V.

Stockenreiter M.,Ludwig Maximilians University of Munich | Stockenreiter M.,Michigan State University | Haupt F.,Ludwig Maximilians University of Munich | Graber A.-K.,Ludwig Maximilians University of Munich | And 5 more authors.
Journal of Phycology | Year: 2013

Currently, very few studies address the relationship between diversity and biomass/lipid production in primary producer communities for biofuel production. Basic studies on the growth of microalgal communities, however, provide evidence of a positive relationship between diversity and biomass production. Recent studies have also shown that positive diversity-productivity relationships are related to an increase in the efficiency of light use by diverse microalgal communities. Here, we hypothesize that there is a relationship between diversity, light use, and microalgal lipid production in phytoplankton communities. Microalgae from all major freshwater algal groups were cultivated in treatments that differed in species richness and functional group richness. Polycultures with high functional group richness showed more efficient light use and higher algal lipid content with increasing species richness. There was a clear correlation between light use and lipid production in functionally diverse communities. Hence, a powerful and cost-effective way to improve biofuel production might be accomplished by incorporating diversity related, resource-use-dynamics into algal biomass production. © 2013 Phycological Society of America.

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