Abstract
Global metrics of land cover and land use provide a fundamental basis to examine the spatial variability of human-induced impacts on freshwater ecosystems. However, microscale processes and site specific conditions related to bank vegetation, pollution sources, adjacent land use and water uses can have important influences on ecosystem conditions, in particular in smaller tributary rivers. Compared to larger order rivers, these low-order streams and rivers are more numerous, yet often under-monitored. The present study explored the relationship of nutrient concentrations in 150 streams in 57 hydrological basins in South, Central and North America (Buenos Aires, Curitiba, São Paulo, Rio de Janeiro, Mexico City and Vancouver) with macroscale information available from global datasets and microscale data acquired by trained citizen scientists. Average sub-basin phosphate (P-PO4) concentrations were found to be well correlated with sub-basin attributes on both macro and microscales, while the relationships between sub-basin attributes and nitrate (N-NO3) concentrations were limited. A phosphate threshold for eutrophic conditions (>0.1 mg L-1 P-PO4) was exceeded in basins where microscale point source discharge points (eg. residential, industrial, urban/road) were identified in more than 86% of stream reaches monitored by citizen scientists. The presence of bankside vegetation covaried (rho = –0.53) with lower phosphate concentrations in the ecosystems studied. Macroscale information on nutrient loading allowed for a strong separation between basins with and without eutrophic conditions. Most importantly, the combination of macroscale and microscale information acquired increased our ability to explain sub-basin variability of P-PO4 concentrations. The identification of microscale point sources and bank vegetation conditions by citizen scientists provided important information that local authorities could use to improve their management of lower order river ecosystems.
Highlights
Anthropogenic stressors endanger more than 65% of fluvial habitats globally [1]
We focused on microscale data that would best describe point sources and processes and macroscale data for diffuse processes and sources
Adjusted Human Water Security (AHWS) was highest in Rio de Janeiro, São Paulo and Mexico
Summary
Increased nutrient loads and reduced ecosystem functioning have led to algal blooms and widespread artificial eutrophication in most freshwater ecosystems. This is evident in both periurban and rural ecosystems, where land management has a strong influence on nutrient fluxes, in comparison to the dominant climate influences in undisturbed areas [2, 3]. In urban and periurban areas, elevated impervious land cover modifies nutrient dynamics [4,5,6] and particulate inputs [7]. Increasingly industrial-scale activities utilise major inputs of mineral based nutrients which have basin-scale (and long term) impacts on the nutrient dynamics of rivers, river sediments and receiving waterbodies [8]. The resulting eutrophication modifies macroinvertebrate and native fish populations, carbon sequestration and instream vegetation diversity (eg. favouring harmful algal blooms), effectively changing the basis of ecosystem functioning [9,10,11,12]
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