Phosphorus flux from wetland ditch sediments

The accumulation of phosphorus (P) in the bottom sediment of field drainage ditches poses a threat to the ecology both of the ditch water and downstream water courses. We investigated the amounts, forms and internal loading of sediment-bound P along two drainage ditches that regulate water levels in a basin fen (~ 200 ha) supporting a mixture of restored wetland and drained agricultural fields. Water levels in the Lady's Drove Rhyne are currently managed to enhance the biodiversity of the wetland (Catcott Lows Reserve — an area formerly cultivated for arable crop production); whereas, the East Ditch is managed to drain adjoining land that remains under arable and livestock production. Laboratory-based chemical fractionation schemes were used to characterise the forms and potential mobility of the sediment-bound P, whilst pore-water equilibrators were employed in situ to evaluate the diffusive flux of P through the sediment–water column, and to characterise the corresponding redox conditions. Along both ditches, sediment pore-water profiles indicated conditions ranging from weakly to very reducing conditions with increasing depth, and net fluxes of P from the sediment to overlying water. P flux values ranged from 0.33 to 1.30 mg m− 2 day− 1. Both the degree of P saturation (DPS) of the sediment and NaOH extractable (Fe/Al-bound) P correlated significantly (P < 0.05) with P flux. Both in the wetland and agricultural ditches, by far the highest values for P flux were recorded at sites closest to points of drainage water entry from the corresponding, adjoining land. Although the P flux data were obtained from only a single sampling event, this study highlights the contribution of historical as well as ongoing agricultural land use on the sustained elevated P status of ditch sediments in lowland catchments.

Further evidence for variable synchrotron emission in XTE J1118+480 in outburst

We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst, supported by simultaneous X-ray observations. The variability is dominated by short timescales, ~10 s, although a weak superhump also appears to be present in the optical. The optical rapid variations, at least, are well correlated with those in X-rays. Infrared JHKs photometry, as in the previous outburst, exhibits especially large-amplitude variability. The spectral energy distribution (SED) of the variable infrared component can be fitted with a power law of slope α=-0.78+/-0.07, where F_ν~ν^α. There is no compelling evidence for evolution in the slope over five nights, during which time the source brightness decayed along almost the same track as seen in variations within the nights. We conclude that both short-term variability and longer timescale fading are dominated by a single component of constant spectral shape. We cannot fit the SED of the IR variability with a credible thermal component, either optically thick or thin. This IR SED is, however, approximately consistent with optically thin synchrotron emission from a jet. These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission, which is impossible, based on the average spectral energy distribution alone.