Browsing by Subject "carbon"
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- ItemRestrictedCross-shelf biogeochemical characteristics of sediments in the central Benguela and their relationship to overlying water column hypoxia(2007) Van der Plas, A K; Monteiro, P M S; Pascall, AData from two cross-shelf sediment sampling cruises were used to explain reasons for the sediment biogeochemical variability in respect of carbon, nitrogen and sulphur, and how the cycling of these elements governs the biogeochemistry of the overlying water through their control of the redox conditions. The spatial extent of this benthic–pelagic flux link is limited to the innershelf mud belt system on the Namibian shelf. The inshore mud belt is the primary deposition area of the carbon and nitrogen new production export flux. The offshore organic-rich zones are thought to be relict particulate organic matter originating from the inshore mud belt rather than from an overlying pelagic source. These data were used to set up a multi-layer sediment model that was used through sensitivity analyses to elucidate the input characteristics that result in the most significant feedbacks on hypoxia in the overlying water. The analyses showed that, although the new production flux is a requirement to drive an oxygen demand in the sediments, the onset and persistence of anoxia may depend critically on a low-oxygen boundary condition threshold. This is thought to be a key differentiating factor between systems that, despite comparable carbon export fluxes, are characterised by a persistent hypoxia/anoxia signal and those that are characterised by episodic hypoxia events. It was concluded that sediment oxygen demand and methane and 'sulphide' emissions from the central Benguela sediments are responses to external hypoxia boundary conditions rather than the local drivers of oxygen variability.
- ItemOpen AccessSeasonal rainfall at long-term migratory staging sites is associated with altered carry-over effects in a Palearctic-African migratory bird(2016) Sorensen, Marjorie C; Fairhurst, Graham D; Jenni-Eiermann, Susanne; Newton, Jason; Yohannes, Elizabeth; Spottiswoode, Claire NAbstract Background An understanding of year-round habitat use is essential for determining how carry-over effects shape population dynamics in long-distance migratory songbirds. The recent discovery of long-term migratory staging sites in many species, prior to arrival at final wintering sites, adds complexity to efforts to decipher non-breeding habitat use and connections between sites. We investigated whether habitat conditions during migratory staging carry over to influence great reed warbler (Acrocephalus arundinaceus) body condition at final wintering sites in Zambia. We asked whether the presence/absence and strength of such carry-over effects were modified by contrasting rainfall conditions during 2 years. Results First, we found that individuals staging in a dry year had higher corticosterone (CORT f ) and stable nitrogen isotope values (suggesting higher aridity) than birds staging in a wet year, indicating that regional weather affected staging conditions. Second, we found that carry-over effects from staging habitat conditions (measured via carbon and nitrogen isotopes) to final winter site body condition (measured via scaled mass index and β-hydroxybutyrate) were only present in a dry year, suggesting that environmental factors have consequences for the strength of carry-over effects. Our results also suggest that wet conditions at final winter sites may buffer the effects of poor staging conditions, at least in the short term, since individuals that staged in a dry year had higher scaled mass indices in Zambia than individuals that staged in a wet year. Conclusions This study provides a first insight into the connections between long-term migratory staging sites and final wintering sites, and suggests that local environmental factors can modify the strength of carry-over effects for long-distance migratory birds.