292 research outputs found
A new way to look at mesoscale zooplankton distributions: an application at the Antarctic Polar Front
No full textDuring the austral summer of 1996 an intensive multidisciplinary survey was carried out in the Antarctic Polar Front. In this paper, we examine the relationship between hydrographic features and the acoustic backscatter signal from a vessel-mounted acoustic Doppler current profiler. We introduce two new analysis methods: mapping of depth-integrated scattering cross section and a model of the vertical zooplankton distribution through idealised population dynamics. Three distinct layers are characterised by a different balance between diel migration behaviour and direct/indirect forcing by mesoscale physical processes in the ocean
Fluxes of nutrients in a three-dimensional meander structure of the Antarctic Polar Front
No full textThe horizontal and vertical advection and the vertical diffusion of two plant nutrients (nitrate and silicate) are estimated at the Antarctic Polar Front (APF) in the Atlantic sector using quasi-synoptic, high-resolution physical and chemical measurements. Our results suggest that the routes of nutrient supply are more complex than indicated by existing large-scale views. The vertical advection associated with mesoscale upwelling events is shown to be between two and three orders of magnitude larger than the diffusion, and to potentially amount to the phytoplankton uptake rate locally. Averaged over the survey area, however, the vertical nutrient transport is downward and concords with the front acting as a barrier to the northward export of surface nutrients by the Ekman drift. This poses significant constraints on the global cycles of nutrients and may have an impact in the sediment record
A 3-D mesoscale map of primary production at the Antarctic Polar Front: results of a diagnostic model
No full textA diagnostic model is established to estimate synoptically the mesoscale distribution of primary production at the Antarctic Polar Front (APF). The model domain is a 3-D box, centred at roughly 50 S and 10 E, of about 1 degree latitude and 2 degree longitude horizontal extent, and of 300 m depth. The box was surveyed in high resolution during austral summer
1995/1996 with a towed undulating vehicle and by complementary ship-based measurements. Measurements of global solar radiation, of the underwater light field, and of the chlorophyll concentration from the survey are used as input variables for the model. The model is based on photosynthesis-light relationships, with parameters taken from in vitro
incubations performed during the survey. The model results show mesoscale patches of elevated primary production along a meander of the APF, and lowest production in a cold cyclonic eddy south of the front. Production is confined to a shallower depth range in the front than outside, due to self-shading effects from generally higher mixed-layer chlorophyll concentrations. Self-shading effects account for variations of the percent light depths, and of the saturation light depth, by a factor of 2 within the survey area. Primary production at the surface varies horizontally between 7 and 56 mg C m^-3 d^-1, with a mean of 26 mg C m^-3 d^-1, and vertically integrated production ranges from 295 to 975 mg C m^-2 d^-1, with an areal mean of 585 mg C m^-2 d^-1. Changes by a factor of 2 in integrated production occur on horizontal scales as small as 10 km. Production rates also differ significantly between days as a result of changes in global solar radiation
Significance of the Polar Frontal Zone for large-sized diatoms and new production during summer in the Atlantic sector of the Southern Ocean
No full textThe chlorophyll a (chl a) biomass and primary production of three phytoplankton size fractions were estimated in the Atlantic sector of the Southern Ocean from 12 December 1995 to 20 January 1996. Elevated concentrations of chl a, primary production, and contribution of microplankton (>20 ?m) coincided with dominance by large or long-chained diatoms (Thalassiothrix spp., Pseudonitzschia lineola, and Chaetoceros spp.) in the Polar Frontal Zone (PFZ, 49.5°S–52°S). Vertically resolved assimilation numbers (i.e. primary production normalized to chl a) and intrinsic growth rates of microplankton were much lower at high-biomass stations of the Northern Polar Frontal Zone (NPFZ) than in adjacent waters. Silicic acid appeared to be the proximal factor limiting the growth and yield of resident diatoms in the NPFZ. A carbon budget showed that, during the sampling period, diatom dominance at high-biomass sites was associated with near-steady-state conditions. This system exhibited a low POC sinking flux relative to total primary production, despite strong dominance by microplankton. South of 52°S, total chl a and primary production were generally low, but increased sharply in a mixed Phaeocystis-diatom bloom that extended from 61°S to 65°S in the Seasonal Ice Zone (SIZ). We estimated that the PFZ contributes from 37% to 67% of the total open-water new production in the Southern Ocean near the Greenwich meridian, depending on whether blooms occur or not in the SIZ.<br/
Mesoscale frontal dynamics: shaping the environment of primary production in the Antarctic Circumpolar Current
No full textThe frontal regions of the Antarctic Circumpolar Current (ACC) differ from other parts of the ACC due to higher phytoplankton concentrations and primary production rates. We hypothesise that the enhancement of primary production results from the mesoscale frontal dynamics, in particular the cross-front circulation related to baroclinic instability.The hypothesis is corroborated by data collected in austral summer 1995/1996 at the Antarctic Polar Front between 1 W and 12 E during quasi-synoptic surveys with a measuring system combining towed and vessel-mounted instruments. Further confirmation is obtained from moored current meters and an earlier section worked across the front in austral winter 1992.The quasi-synoptic surveys cover a meander structure of the front and a cold cyclonic eddy located to its south. The highest chlorophyll concentrations, correlating with enhanced primary production rates, are found in a band of mesoscale patches aligned with the front and in a tongue extending southward from the front along the leading edge of a meander ridge. The increased chlorophyll concentrations at the meander edge are explained by confluence of surface water, which carries with it the phytoplankton grown in favourable light conditions. While mesoscale surface confluence structures the synoptic chlorophyll distribution pattern, the mean enhancement of primary production at the front can be attributed to the influence of cross-front circulation on stratification. Ageostrophic cross-front circulation related to mesoscale upwelling and downwelling was identified at a site of eddy/front interaction. Consistent with the
principle of potential vorticity conservation, upwelling was found to occur on the anticyclonic, equatorward side of the jet and downwelling on the cyclonic, poleward side in the frontogenetic situation. The associated cross-front circulation is characterised by poleward motion of light water at the surface and a reversed flow of dense water at greater depth;
thus it contributes to stratification and thereby to a more favourable photic environment for the phytoplankton growing in the shallower mixed layer. While the cross-front circulation varies on horizontal scales of <10 to >100 km and time scales of days to weeks, it is constrained to sites of available potential energy, i.e. fronts marked by sloping isopycnals
Bestuurdersgeheimen: Over samenhang tussen leiderschapsrollen van bestuurders, strategische profielen en prestaties van woningcorporaties
No full textIn dit onderzoek staat samenhang tussen leiderschapsrollen van bestuurders, strategische profielen en prestaties van woningcorporaties centraal. We hebben onderzocht of sprake is van samenhang tussen deze drie variabelen, waarbij we uit zijn gegaan van de veronderstelling dat strategische profielen en leiderschapsrollen als onafhankelijke variabelen de afhankelijke variabele prestaties beïnvloeden. De centrale vraag in het onderzoek luidde als volgt; Is er samenhang tussen de mate waarin bestuurders van woningcorporaties leiderschapsrollen hanteren, strategische profielen van woningcorporaties en prestaties van woningcorporaties? En (hoe) kunnen we de gevonden samenhangen verklaren?Real Estate and HousingArchitecture and The Built Environmen
Balanceren tussen uitvoering en bewuste afwijking van beleid: De implementatie van strategisch voorraadbeleid door woningcorporaties
No full textIn hun strategisch voorraadbeleid beschrijven woningcorporaties welke aanpassingen ze in hun woningbezit willen doorvoeren. Tegelijkertijd kan het in de praktijk soms verstandig zijn dat zij van hun beleid afwijken, bijvoorbeeld omdat er nieuwe inzichten zijn, of omdat er beren op de weg komen die je beter kunt mijden. In het proefschrift wordt ingegaan op de vraag hoe het implementatieproces van strategisch voorraadbeleid verloopt. Het bevat uitgebreide casestudies, uitgevoerd bij vier woningcorporaties, waarin meerdere uitvoeringsprojecten van begin tot eind zijn gereconstrueerd. Daarbij is in beeld gebracht welke middelen corporaties gebruiken om beleid en uitvoering met elkaar te verbinden. Ook is gezocht naar de balans tussen enerzijds het uitvoeren van voorgenomen beleid en anderzijds het bewust afwijken van voorgenomen beleid op basis van nieuwe ontwikkelingen en veranderende inzichten. De belangrijkste uitkomsten van deze studie komen samen in een balansmodel, waarmee gereflecteerd kan worden op implementatieprocessen bij woningcorporaties, andere maatschappelijke organisaties en overheden.DWKOTB Research Institute for the Built Environmen
Uitdagen en ontmoeten
No full textReal Estate & Housing is een leerstoel rijker met de benoeming van Gruis als hoogleraar. Interview met Vincent Gruis over zijn nieuwe taak, de functie van woningbouwcorporaties, duurzaamheid en zijn toekomst visie voor housing management.Real Estate and HousingArchitectur
Omdenken van creatie naar re-creatie: Inspiratie ontlenen aand de randvoorwaarden die de bestaande omgeving dicteert
No full textReal Estate & HousingArchitectur
Stedenbouwkundige waardestelling van industrieel erfgoed
No full textUrbanismArchitecture and The Built Environmen
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