Spatial Variation

Soil carbon (C) losses and soil translocation from tillage operations have been identified as causes of soil degradation and soil erosion. The objective of this work was to quantify the variability in tillage-induced carbon dioxide (CO 2 ) loss by moldboard (MP) and chisel (CP) plowing across an eroded landscape and relate the C loss to soil properties. The study site was a 4 ha wheat (Triticum aestivum L. cv. Marshall) field with rolling topography and five soil types in the Svea-Barnes complex in west central Minnesota (N. Latitude = 45841 0 W, Longitude = 95843 0 ). Soil properties were measured at several depths at a 10 m spacing along north-south (N-S) and west-east (W-E) transects through severely eroded, moderately eroded and non-eroded sites. Conventional MP (25 cm deep) and CP (15 cm deep) equipment were used along the pre-marked transects. Gas exchange measurements were obtained with a large, portable chamber within 2 m of each sample site following tillage. The measured CO 2 fluxes were largest with the MP > CP > not tilled (before tillage). The variation in 24 h cumulative CO 2 flux from MP was nearly 3-fold on the N-S transect and 4-fold on the W-E transect. The surface soil organic C on the transects was lowest on the eroded knolls at 5.1 g C kg À1 and increased to 19.6 g C kg À1 in the depositional areas. The lowest CO 2 fluxes were measured from severely eroded sites which indicated that the variation in CO 2 loss was partially reflected by the degradation of soil properties caused by historic tillage-induced soil translocation with some wind and water erosion.

Glacial outburst floods (jökulhlaups) have a significant role for landscape evolution in NE Iceland. A number of jökulhlaups have routed from the northern margin of Vatnajökull during the Holocene. In this study, reconstruction of the largest Holocene jökulhlaup along Jökulsá á Fjöllum, NE Iceland was undertaken using the HEC-RAS hydraulic modelling and HEC-GeoRAS flood mapping techniques with a Digital Elevation Model (DEM) derived from ERS-InSAR data and field-based wash limit evidence. The largest jökulhlaup produced extensive erosional and depositional landforms across an inundated area of ˜1390 km2 and is calculated to have had a peak discharge of 0.9×106 m3 s-1. Power per unit area within this jökulhlaup varied from 6 to 46,000 W m-2. Jökulhlaup hydraulics are related to geomorphogical evidence at three key sites: in Vaðalda, Upptyppingar and Möðrudalur sub-areas in order to explain the abrupt spatial variation of the flood characteristics on a regional scale and to relate erosional and depositional features to spatial variations in jökulhlaup hydraulics. These process-form relationships of the largest jökulhlaup along the Jökulsá á Fjöllum are compared with large outburst floods elsewhere. The largest Jökulsá á Fjöllum jökulhlaup had a factor of ˜20 times smaller discharge and ˜a factor of 20 times lower power per unit area than Altai palaeoflood—the largest known flood on the Earth.

This study has analysed for the first time fish composition and assemblage structures of three small macrotidal estuaries of the Eastern English Channel (EEC) and has explored the influences of 19 biotic and abiotic variables on the fish assemblages. Fish from Canche, Authie and Somme estuaries were collected during spring (June 2006 and May 2007) and autumn (September 2006) along the estuarine gradients using a 1.5 m beam trawl. Using identical sampling protocols, the study also analysed and compared for the first time taxonomic and functional aspects of the fish assemblages in 15 estuaries located along the Atlantic and English Channel coasts. SIMPER analysis showed high similarities in fish assemblages in the three EEC estuaries and during either spring or autumn periods. However, intraestuary similarities were relatively low, indicating that fish assemblage structures (species richnesses or abundances) were more variable within the estuary (salinity gradient) than between estuaries and/or seasons (spring vs autumn). Although numerous environmental variables were included in the study, only 47% of the variability observed in the fish distribution was explained. Fish spatial variations in the EEC estuaries are mostly driven by abiotic variables as opposed to biological interactions. As indicated by CCA, salinity and muddy sediments were the two most important factors structuring the fish assemblages. The macrobenthos being very abundant in the EEC estuaries (580-1121 ind. m À2 ), the availability of potential prey is probably not a limiting factor in the utilization of estuaries by fish. Contrary to the majority of French estuaries dominated by estuarine species (ES), the fish assemblages of the EEC estuaries are clearly dominated by marine migrant (MM) species (65% on average) with high abundance of juveniles (mostly young-of-the-year). Cluster and SIMPROF's analyses distinguished the functional structure of the 15 estuarine fish assemblages into different clusters. The three EEC estuaries form a similar group with the largest French estuaries (Seine, Loire and Gironde). The latters were characterized by higher MM/ES ratio compared to the other French estuaries. Freshwater flow and tidal range were the main factors associated with this group of estuaries. The results of the present study do not indicate that anthropogenic impacts (chemical contamination) on estuaries affect their ecological functioning as described by the fish ecological guilds. It is suggested that the guild approach may not be useful to provide valuable information on the ecological status of estuaries.