Jean-Pierre Descy

Effects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium)

We report a data-set of CO2, CH4, and N2O concentrations in the surface waters of the Meuse river network in Belgium, obtained during four surveys covering 50 stations (summer 2013 and late winter 2013, 2014 and 2015), from yearly cycles in four rivers of variable size and catchment land cover, and from 111 groundwater samples. Surface waters of the Meuse river network were over-saturated in CO2, CH4, N2O with respect to atmospheric equilibrium, acting as sources of these greenhouse gases to the atmosphere, although the dissolved gases also showed marked seasonal and spatial variations. Seasonal variations were related to changes in freshwater discharge following the hydrological cycle, with highest concentrations of CO2, CH4, N2O during low water owing to a longer water residence time and lower currents (i.e. lower gas transfer velocities), both contributing to the accumulation of gases in the water column, combined with higher temperatures favourable to microbial processes. Inter-annual differences of discharge also led to differences in CH4 and N2O that were higher in years with prolonged low water periods. Spatial variations were mostly due to differences in land cover over the catchments, with systems dominated by agriculture (croplands and pastures) having higher CO2, CH4, N2O levels than forested systems. This seemed to be related to higher levels of dissolved and particulate organic matter, as well as dissolved inorganic nitrogen in agriculture dominated systems compared to forested ones. Groundwater had very low CH4 concentrations in the shallow and unconfined aquifers (mostly fractured limestones) of the Meuse basin, hence, should not contribute significantly to the high CH4 levels in surface riverine waters. Owing to high dissolved concentrations, groundwater could potentially transfer important quantities of CO2 and N2O to surface waters of the Meuse basin, although this hypothesis remains to be tested.

Effects of human land use on the terrestrial and aquatic sources of fluvial organic matter in a temperate river basin (The Meuse River, Belgium)

The impact of human activities on the concentrations and composition of dissolved organic matter (DOM) and particulate organic matter (POM) was investigated in the Walloon Region of the Meuse River basin (Belgium). Water samples were collected at different hydrological periods along a gradient of human disturbance (50 sampling sites ranging from 8.0 to 20,407xa0km2) and during a 1.5xa0year monitoring of the Meuse River at the city of Liège. This dataset was completed by the characterization of the DOM pool in groundwaters. The composition of DOM and POM was investigated through elemental (C:N ratios), isotopic (δ13C) and optical measurements including excitation emission matrix fluorescence with parallel factor analysis (EEM–PARAFAC). Land use was a major driver on fluvial OM composition at the regional scale of the Meuse Basin, the composition of both fluvial DOM and POM pools showing a shift toward a more microbial/algal and less plant/soil-derived character as human disturbance increased. The comparison of DOM composition between surface and groundwaters demonstrated that this pattern can be attributed in part to the transformation of terrestrial sources by agricultural practices that promote the decomposition of soil organic matter in agricultural lands and subsequent microbial inputs in terrestrial sources. In parallel, human land had contrasting effects on the autochthonous production of DOM and POM. While the in-stream generation of fresh DOM through biological activity was promoted in urban areas, summer autochthonous POM production was not influenced by land use. Finally, soil erosion by agricultural management practices favored the transfer of terrestrial organic matter via the particulate phase. Stable isotope data suggest that the hydrological transfer of terrestrial DOM and POM in human-impacted catchment are not subject to the same controls, and that physical exchange between these two pools of organic matter is limited.

Phytoplankton of the River Meuse (1994–1996): form and size structure analysis

REYNOLDS (1984) separates phytoplankton species on ecological grounds and described chree cacegories of adaptacive strategies: the C-strategists (colonist species which are the fast-growing, small algae), the S-strategiscs (stress-tolerant species, which are large, slow-growing, biomass-conserving units) and che Rstrategists (ruderal species, tolerant ofhigh frequency disturbances particularly concerning light in che water column). These strategies can be related to morphological features, scudied in detail by LEWIS (1976) who has shown conservation of surface to volume ratio (SV-), despite the large range o f sizes and volumes among algal species. REYNOLDS (1988, 1996) proposed a synthesis of these two concepts and showed a strong relacionship becween algal morphology and algal strategy. We attempted to test this hypothesis for river phytoplankton through an analysis of changes of size (Greatest Axial Linear Dimension, GALD) and morphology (SV-) parameters and we tried to relate them to the river environmental variations. Phytoplankton of the lowland eurrophic River Meuse (Belgium) were monitored for 3 years (19941996) on a weekly basis throughouc the plankton growing season. Counting of the tora! algal communities was carried out using a modified version of Hamiltons sofcware, which enables the recording of numbers and dimensions of phytoplankton units in each sample. Centric diatoms were determined to the species level in a second step. Measurements of different in sicu environmental parameters, recorded daily (discharge, temperacure, sunlight) or weekly (extinction coefficient, nurrients, ete.) have been used. In situ grazing by zooplankton, using a Haney grazing chamber, and primary production (C method) have been measured biweekly. Mean values of GALD and sv- have been validaced through values reported in che licerature. Changes in size (GALD) and form (SV-), as well as diversiry, were followed and seem to respond primarily to the physical environment (flow, temperature and light climate events) and chen to chemical factors (e.g. silica content). Nevercheless, our results show that they are also secondarily influenced by biotic factors, namely zooplankton grazing, in particular during che summer period. Taking into account the dominant taxa for the 3 years of investigation, it can be seen that these are restricted to Cand mainly R -strategists, as expected by the theories ofREYNOLDS (1997).