Watershed geomorphology modifies the sensitivity of aquatic ecosystem metabolism to temperature

The regulation of aquatic carbon cycles by temperature is a significant uncertainty in our understanding of how watersheds will respond to climate change. Aquatic ecosystems transport substantial quantities of carbon to the atmosphere and ocean, yet we have limited understanding of how temperature modifies aquatic ecosystem metabolic processes and contributions to carbon cycles at watershed to global scales. We propose that geomorphology controls the distribution and quality of organic material that forms the metabolic base of aquatic ecosystems, thereby controlling the response of aquatic ecosystem metabolism to temperature across landscapes. Across 23 streams and four years during summer baseflow, we estimated variation in the temperature sensitivity of ecosystem respiration (R) among streams draining watersheds with different geomorphic characteristics across a boreal river basin. We found that geomorphic features imposed strong controls on temperature sensitivity; R in streams draining flat watersheds was up to six times more temperature sensitive than streams draining steeper watersheds. Further, our results show that this association between watershed geomorphology and temperature sensitivity of R was linked to the carbon quality of substrates that changed systematically across the geomorphic gradient. This suggests that geomorphology will control how carbon is transported, stored, and incorporated into river food webs as the climate warms.

Boreal river impoundments caused nearshore fish community assemblage shifts but little change in diversity: a multiscale analysis

River flow regulation, fragmentation, and changes in water quality caused by dams have varying effects on aquatic biodiversity and ecosystem functions, but are not clearly resolved in boreal ecosystems. We adopted a multiscale approach to quantify fish community trajectories over 20 years using a network of sites spread across four reservoirs in two hydroelectricity complexes in northern Quebec, where other anthropogenic factors have been negligible. Across three spatial scales, we found little evidence of directional temporal trends in diversity relative to reference sites. Using beta-diversity analyses, we also detected a high degree of stability in fish composition over time and space at the complex and reservoir scales. However, changes in species assemblage following impoundment were detected at the scale of the sampling station. At this scale, we found that some species consistently benefited (coregonids and pikes) from impoundment, whereas others were detrimentally affected (suckers and one salmonid). Overall, we conclude that examining different scales is key when trying to understand the impacts of humans on biodiversity and in formulating management recommendations.

EEM-PARAFAC-SOM for assessing variation in the quality of dissolved organic matter: simultaneous detection of differences by source and season

Environmental contextDissolved organic matter (DOM) is a highly diverse mixture of interacting compounds, which plays a key role in environmental processes in aquatic systems. The quality and functionality of DOM are measured using fluorescence spectroscopy, but established data analysis assumes linear behaviour, limiting the effectiveness of characterisation. We apply self-organising maps to fluorescence composition to improve the assessment of DOM quality and behaviour by visualising the interdependent nature of its components. AbstractSelf-organising maps (SOMs) were used to sort the excitation–emission matrices (EEMs) of dissolved organic matter (DOM) based on their multivariate ‘fluorescence composition’ (i.e. each parallel factor analysis (PARAFAC) component loading, viz. ‘Fmax’ value was expressed as a proportion of all Fmax values in each EEM). This sorting provided a simultaneous organisation of DOM according to differences in quality along a 125-km stretch of a large boreal river, corresponding with both source and season. The information provided by the SOM-based spatial organisation of samples was also used to assess the likelihood of PARAFAC model overfitting. Changes in fluorescence composition caused by changing salinity were also assessed for multiple sources. Seasonal and source-based differences were readily apparent for the main stem of the river and tributaries, and source-based differences were apparent in both fresh and saline groundwaters. Proportions of humic-like components were positively correlated with the amounts of bog, fen and swamp in tributary watersheds. Proportions of six PARAFAC components were negatively correlated with the proportions of all wetland types, and positively correlated with the proportions of open water and other land cover. Ancient saline groundwaters contained >50% protein-like DOM. There was no change in DOM quality from upstream to downstream in August or October. Increasing salinity was associated with additional protein-like fluorescence in all sources, but source-based differences were also apparent. The application of SOM to fluorescence composition is highly recommended for assessing and visualising transformations and differences in DOM quality, and relating them to associated properties.