The influence of landuse and seasonality on the concentration, generation and transport of dissolved organic matter (DOM) in the Conwy catchment, North Wales was addressed. The peaty moorland headwaters were shown to be the major source of dissolved organic carbon to the entire Conwy catchment. However, substantial variations in DOM concentration were apparent amongst the spatially proximal upland streams that drain catchments with varied landscape properties. Even streams draining catchments classified as containing in excess of 94% peat exhibited considerable variation in both water colour (CDOM440) and dissolved organic carbon (DOC) concentration.
The water chemistry of the headwater streams displayed a pronounced seasonal cycle. Water colour (CDOM440) and dissolved organic nitrogen (DON) concentrations were significantly correlated to temperature and were greatest during summer whereas DOC peaked during late summer I autumn. Episodic variations were superimposed onto the seasonal cycle and were linked primarily to rainfall events. The flushing of organic rich surface soils appeared to release DOC and DON from peaty moorland catchments. Rainfall has such a profound effect on DOC release that stormflow induced the flushing and transport of DOC throughout the entire Conwy catchment, apparent from the headwaters to the estuary. The composition of DOM also varied with season and was associated with rainfall events suggesting that variations in the proportions of dissolved organic substances reaching the stream maybe somewhat associated with discharge.
The role of epilithic autotrophs in generating autochthonous DOC were considered but significant (P<0.05) correlations could not be found between stream water DOC concentrations and autotrophic biomass (chlorophyll a). This implied that the primary source of DOC to the headwater streams was of allochthonous origin. This was confirmed using dual isotope analysis (o13C and 14C) which enabled further understanding of the source and age of DOC to be
determined in different catchment types. The 813C ranges of soil and stream waters would suggest that the DOC originated from terrestrial vegetation and that the matter being decomposed was a product of plant material fixed via the C3 photosynthetic pathway. The decomposition of soil organic matter was assessed through the measurement of the major carbon decomposing soil enzymes. Sub optimal enzyme activity was apparent for all seasons, with the exception of summer, leading to greater concentrations of extractable soil DOC. This suggested that the peatlands were the major source of DOC because the cool temperatures, periodic water logging and anaerobic conditions typical of northern peatlands prevented the complete decomposition of organic material. The carbon isotope composition of organic matter varied between catchment types with greater similarities observed between peats and gleys (i.e. organic rich soils) compared to farmland, woodland and montane dominated catchments (i.e. mineral rich soils). Stream water DOC draining peatland and peaty gley catchments was predominantly modern suggesting that the material originated from the recent degradation of plant material. This was even more apparent when considering that the age of carbon contained within the soil was dated in excess of thousand years. However streams draining both montane and farmed grassland catchments, sampled during low flow contained DOC with an approximate average age in the order of 350 to 700 years BP suggesting that it originated from a much older source, potentially from the degradation of old soil carbon stores.