The diffusive boundary layer of sediments: oxygen microgradients over a microbial mat

Oxygen microelectrodes were used to analyze the distribution of the diffusive boundary layer (DBL) at the sediment-water interface in relation to surface topography and flow velocity. The sediment, collected from saline ponds, was covered by a microbial mat that had high oxygen consumption rate and well-defined surface structure. Diffusion through the DBL constituted an important rate limitation to the oxygen uptake of the sediment. The mean effective DBL thickness decreased from 0.59 to 0.16 mm as the flow velocity of the overlying water was increased from 0.3 to 7.7 cm s-1 (measured 1 cm above the mat). The oxygen uptake rate concurrently increased from 3.9 to 9.4 nmol cm-2 min-1. The effects of surface roughness and topography on the thickness and distribution of the DBL were studied by three-dimensional mapping of the sediment-water interface and the upper DBL boundary at 0.1-mm spatial resolution. The DBL boundary followed mat structures that had characteristic dimensions > 1/2 DBL thickness but the DBL had a dampened relief relative to the mat. The effective surface area of the sediment-water interface and of the upper DBL boundary were 31 and 14% larger, respectively, than a flat plane. Surface topography thereby increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients.     

Effect of wharves on intertidal assemblages on seawalls in Sydney Harbour, Australia

Worldwide, urbanisation has resulted in extensive replacement of natural habitats with man-made habitats. In Sydney Harbour, Australia, approximately half of the natural foreshore has been replaced by seawalls. Many of these have wharves built over part of their length, which could affect intertidal assemblages on seawalls beneath the wharves. This was tested by sampling and comparing assemblages under and not under wharves in Sydney Harbour. Assemblages differed between the two habitats, with greater cover of macro-algae and abundance of grazing molluscs on seawalls without a wharf and, to a lesser extent, greater cover of sessile invertebrates on seawalls under a wharf. There was, however, considerable spatial variability among locations in composition of assemblages and the species dominating differences between the two habitats. The impact of multiple artificial structures in close proximity and the variability among apparently homogeneous artificial habitats must be considered for the management of urbanised estuaries.     

Quantity, distribution, and impacts of coastal driftwood triggered by a typhoon

Typhoon Morakot pounded Taiwan in 2009 with record-breaking rainfall, washing an unprecedented amount of driftwood into the sea that was partially deposited at the coastal areas. According to the satellite imagery analysis, more than three million trees fell and were washed away to occupy 83.2% of the Taiwanese coastline, including 52 fishing harbors. The amount cleaned-up was only 1/7 of the total coastal driftwood. It was found that the amount of coastal driftwood is not only related to the amount of precipitation but is also related to the distance from the location of the landslide to the river mouth and to the landslide area. The amount of accumulated coastal driftwood demonstrated log-profile declines with increasing distance to the river mouth. Nearshore current and wave motion are the critical factors for driftwood deposition. Much of the driftwood washed into the sea harmed the tourism and fishing industries, endangered navigation and oceanic activities, and impacted the marine environment and ecosystem.     

Carbon isotopic fractionation in lipids from methanotrophic bacteria II: the effects of physiology and environmental parameters on the biosynthesis and isotopic signatures of biomarkers

Controls on the carbon isotopic signatures of methanotroph biomarkers have been further explored using cultured organisms. Growth under conditions which select for the membrane-bound particulate form of the methane monooxygenase enzyme (pMMO) leads to a significantly higher isotopic fractionation than does growth based on the soluble isozyme in both RuMP and serine pathway methanotrophs; in an RuMP type the delta delta 13Cbiomass equaled -23.9% for pMMO and -12.6% for sMMO. The distribution of biomarker lipids does not appear to be significantly affected by the dominance of one or the other MMO type and their isotopic compositions generally track those of the parent biomass. The 13C fractionation behaviour of serine pathway methanotrophs is very complex, reflecting the assimilation of both methane and carbon dioxide and concomitant dissimilation of methane-derived carbon. A limitation in CH4 availability leads to the production of biomass which is 13C-enriched with respect to both carbon substrates and this occurs irrespective of MMO type. This startling result indicates that there must be an additional fractionation step downstream from the MMO reaction which leads to incorporation of 13C-enriched carbon at the expense of dissimilation of 13C-depleted CO2. In these organisms, polyisoprenoid lipids are 13C-enriched compared to polymethylenic lipid which is the reverse of that found in the RuMP types. Serine cycle hopanoids, for example, can vary anywhere from 12% depleted to 10% enriched with respect to the CH4 substrate depending on its concentration. Decrease in growth temperature caused an overall increase in isotopic fractionation. In the total biomass, this effect tended to be masked by physiological factors associated with the type of organism and variation in the bulk composition. The effect was, however, clearly evident when monitoring the 13C signature of total lipid and individual biomarkers. Our results demonstrate that extreme carbon isotopic depletion in field samples and fossil biomarker lipids can be indicative of methanotrophy but the converse is not always true. For example, the hopanoids of a serine cycle methanotroph may be isotopically enriched by more than 10% compared to the substrate methane when the latter is limiting. In other words, hopanoids from some methanotrophs such as M. trichosporium would be indistinguishable from those of cyanobacteria or heterotrophic bacteria on the basis of either chemical structure or carbon isotopic signature.     

Drought, desiccation and discourse: missionary correspondence and nineteenth-century climate change in central southern Africa

This paper examines the role that representatives of the London Missionary Society in central southern Africa during the nineteenth century may have played in the development of geographical debates concerning the long-term desiccation of the African continent. Observations on climate included within missionary documents are used to reconstruct a chronology of intra-decadal climatic variability for the period 1815–1900. This reveals six drought periods and seven wet phases that affected large areas of the region, but identifies no evidence for progressive desiccation. The chronology is then used as a framework within which to view missionary perspectives on drought and desiccation. Major influences upon the development of desiccationist theory appear to include the prevalence of contemporary moral economic explanations of climatic variability, as well as the uptake and acceptance of indigenous understanding of climate change. Significantly, many of the key observations by eminent missionaries used as supporting evidence for progressive desiccation are identified as having been made during periods of severe drought. This is used to suggest that the most widely propagated evidence for desiccation may, therefore, simply be the end-product of periods of short-term drought rather than long-term climatic deterioration.     

Correlations between metal uptake in the soft tissue of Perna perna and gill filament pathology after exposure to mercury

The accumulation of metal in soft tissues, filtration rate and gill filament morphology are correlated in the southern African rock mussel, Perna perna, during exposure to mercury (24 days) and recovery (24 days). The amount of Hg in soft tissues increased from 0.13 to 87.5 μg/g dry weight after 24 days exposure, and declined to 13 μg/g during recovery. Mean filtration rate fell from 3979 to 1818 ml/h/g dry weight by day 2, but recovered slightly through days 4 and 8 (3037 ml/h/g), with a higher average rate (5030 ml/h/g) being maintained over the 24–48 days recovery period. The initial decline in filtration coincided with epithelial cell deterioration presented as interstitial oedema, neural and epithelial cell degeneration and reduced ciliation. Between days 8 and 24, cilia regenerated and there was a general improvement in cell morphology. Gill filament morphology returned to near normal during the metal-free recovery period. The usefulness of P. perna as an indicator of pollution is discussed.