Bendtsen J.,ClimateLab |
Hansen J.L.S.,University of Aarhus
Ecological Modelling | Year: 2013
A bifurcation point implied a high benthic biomass sensitivity to spawning events. The life cycle of many benthic macro-invertebrates is characterised by a planktonic life stage where larvae are transported by ocean currents and mixing and after some time they reach a stage where they settle onto the ocean bottom and, provided a suitable habitat is available, initiate their benthic life stage as juvenile organisms and complete their life cycle when they mature and spawn as adults. Such age-specific behaviour is in general difficult to include in large scale ocean models unless dispersal is considered on an individual basis, i.e. individual based model coupled with a Lagrangian description of the flow field. However, here we address this issue in an Eulerian framework and develop and apply a new life cycle and population dynamical model of benthic fauna and implement the model in a high-resolution three-dimensional circulation model of the North Sea/Baltic Sea transition zone. The model explicitly describes specific life stages of the population and considers the different processes affecting the organisms during their life cycle, e.g. spawning, dispersion and settling. The model considers different life stages of an idealised marine organism, representing a typical benthic macro-invertebrate species in the area. Populations of juvenile and adult benthic organisms are maintained by spawning, occurring regularly every spring, and subsequent settling of larvae. The pelagic larval stages are simulated by a larval concentration distribution function, i.e. discrete age-classes of the total larval concentration, and age-specific physiological processes, as the onset of their settling behaviour, is explicitly accounted for in the model. Model simulations show, in general, a large connectivity between habitats in the northern and southern part of the area but also that self-recruitment is sufficient to sustain the two populations independently. A sensitivity study were carried out with the spawning rate as a control parameter and two non-trivial quasi-stationary steady states of benthic biomass distributions were identified, characterised by a high and low distribution of organisms in the area, respectively. A stability diagram identifies a bifurcation point when the spawning rate is reduced by 65% and where lower spawning rates implies two different stable equilibria. The existence of multiple quasi-stationary steady states can be explained by the general circulation in the area: when spawning into the surface layer takes place from recruitment areas close to the North Sea where the out-flowing Baltic Sea surface water hinder the southward transport of pelagic larvae. The existence of multiple equilibria support the hypothesis of regime shifts in coupled physical-biological systems where modest changes in critical processes causes rapid and extensive structural changes in the ecosystem. Such changes could occur if a system is close to a bifurcation point such that small changes in critical internal biotic dynamics or in environmental conditions force the system into a new equilibria, for example due to hypoxia or changes in temperature or salinity. Finally, it is shown that model simulations of periods with hypoxic bottom water masses reduces the total benthic biomass distribution in the area significantly. © 2013 Elsevier B.V.
Hansen J.L.S.,University of Aarhus |
Marine Ecology Progress Series | Year: 2013
An organic carbon budget for the Kattegat and Belt Seas in the North Sea-Baltic Sea transition zone was used to parameterise the pelagic and benthic respiration in a new oxygen model, OXYCON, which describes the influence of temperature-dependent pelagic and benthic respiration on bottom water oxygen conditions. The significance of respiration versus physical mixing and advection processes for the bottom water oxygen concentration was analysed through a sensitivity study where the OXYCON model was implemented in 2 transport models: A Lagrangian model of bottom water transport, based on an age-tracer of the bottom water transittime through the area, and a 3-dimensional circulation model of the transition zone. The solutions of both models were in accordance with the observed spatial and temporal distribution of oxygen in the area during the period 2001 to 2003. In particular, the temporal and spatial dynamics of a severe hypoxic event in 2002 were well described. The inter-Annual variability of hypoxia during this period could therefore be explained by changes in physical mixing and ventilation of the bottom layer with oxygen-rich surface water and by the bottom water temperature. Variability in the sources of organic material available for remineralisation in the bottom water seems to have less influence on the inter-Annual variation in hypoxia but instead determines the background conditions and the long-term trend in oxygen dynamics. © Inter-Research 2013.
Mortensen J.,Greenland Institute of Natural Resources |
Bendtsen J.,ClimateLab |
Bendtsen J.,University of Aarhus |
Lennert K.,Greenland Institute of Natural Resources |
And 3 more authors.
Journal of Geophysical Research F: Earth Surface | Year: 2014
Many tidewater outlet glacier fjords surround the coast of Greenland, and their dynamics and circulation are of great importance for understanding the heat transport toward glaciers from the ice sheet. Thus, fjord circulation is a critical aspect for assessing the threat of global sea level rise due to melting of the ice sheet. However, very few observational studies describe the seasonal dynamics of fjord circulation. Here we present the first continuous current measurements (April-November) from a deep mooring deployed in a west Greenland tidewater outlet glacier fjord. Four distinct circulation phases are identified during the period, and they are related to exchange processes with coastal waters, tidal mixing, and melt processes on the Greenland Ice Sheet. During early summer, warm intermediate water is transported toward the glacier at an average velocity of about 7 cm s-1. In late summer, the average velocity decreases to 3 cm s-1 during a period with significant subglacial freshwater discharges. During this period, a large variability in current velocities is also observed. The associated average heat transport in an intermediate-depth range corresponds to 568 GW in early summer and is reduced to 287 GW in late summer. These heat fluxes are at the higher end of previously reported fluxes. Our measurements show that the intermediate heat transport varies over time and during summer provides a major contribution to the heat budget and, thereby, potentially to glacial melt. We suggest that intermediate heat transport may play a similar important role in other fjords around Greenland ©2014. American Geophysical Union. All Rights Reserved.
Bendtsen J.,ClimateLab |
Bendtsen J.,Copenhagen University |
Hilligsoe K.M.,University of Aarhus |
Hansen J.L.S.,University of Aarhus |
Richardson K.,Copenhagen University
Progress in Oceanography | Year: 2015
Organic carbon (OC) synthesised by plankton is exported out of the surface layer as particulate (POC) and dissolved (DOC) organic carbon. This "biological pump" constitutes a major pathway in the global marine carbon cycle, each year exporting about 10PgC into the ocean interior, where it is subsequently remineralised via biological decomposition. Remineralised inorganic nutrients and carbon are, ultimately, again brought to the surface by advection and turbulent mixing which closes the OC-cycle in the upper ocean. Thus, remineralisation rates of OC are a critical component of the biological pump. These rates are regulated by the lability of the material and the environmental conditions in the ambient water. Temperature is particularly important in regulating the rate of microbial respiration and, thus, remineralisation rates. A significant temperature dependence of the microbial metabolic activity in the ocean interior is expected, as this is a feature observed elsewhere in the biosphere. Such temperature dependence of microbial remineralisation of POC and DOC will alter the amount of material available for transport by the biological pump to the deep ocean. Very few studies on the lability of OC and temperature sensitivity of microbial degradation processes in the mesopelagic zone (~100-1000m) have, to date, been carried out. Here, we present a comprehensive new experimental data set from all major ocean basins and quantify remineralisation rates of OC and their temperature sensitivity in long-term incubations of water from the upper 350m. Microbial respiration was measured by non-invasive oxygen optodes and oxygen consumption was used as a constraint for determining the remineralisation rates and temperature sensitivity by two complementary methods. First, we analysed the oxygen consumption from a multi-component OC-model where the concentration, remineralisation rates and temperature sensitivity of two bio-available (labile) pools of organic carbon were fitted to the data via a non-linear fitting procedure. Thereafter, a continuous OC-model was fitted to the data through an inverse method and information about lability, temperature sensitivity and structural composition of the OC-pool was analysed together with the results from the two-pool solutions. Median values of remineralisation rates from all experiments on material characterising sinking POC were found to be 0.6 and 0.05days-1 for the two decomposable pools corresponding to turnover times of 2 and 21days, respectively. Accordingly, solutions from the continuous model resulted in median turnover times between 6 and 11days. Similar analyses were carried out for the OC-pool characterising DOC. A significant bio-available OC-pool was found to be present in the surface layer with a median value from all experiments of 30μM TOC. The median values of all remineralisation rates from the two bio-available OC-pools were found to be 0.2 and 0.02days-1, corresponding to turnover times of 5 and 56days, respectively, in good agreement with previous studies. The corresponding temperature sensitivities, characterised by a Q10-value, were found to be about 1.9 for the POC-fraction whereas the DOC fraction was characterised with values above 2.8 for the continuous OC-models. The analysis of the structural composition indicated that the TOC distribution in the surface layer was characterised by more heterogeneous material in terms of lability compared with the POC material sampled from the upper 350m. Finally, we analyse the potential impact of the calculated temperature sensitivity on the general OC-cycling in the upper ocean and show that the implied reduction in OC-flux in a warmer ocean may have significant impact on nutrient cycling in general and also tends to reduce future ocean carbon uptake significantly. © 2014 Elsevier Ltd.