Global biological condition of rivers

Rivers suffer from multiple stressors acting simultaneously on
their biota, but the consequences are poorly quantified at the global
scale. We evaluated the biological condition of rivers globally, including
the largest proportion of countries from the Global South published to
date. We gathered macroinvertebrate- and fish-based assessments from
72,275 and 37,676 sites, respectively, from 64 study regions across six
continents and 45 nations. Because assessments were based on differing
methods, different systems were consolidated into a 3-class system: Good,
Impaired, or Severely Impaired, following common guidelines. The
proportion of sites in each class by study area was calculated and each
region was assigned a Köppen-Geiger climate type, Human Footprint score
(addressing landscape alterations), Human Development score (addressing
social welfare), % rivers with good ambient water quality, %
protected freshwater key biodiversity areas; and % of forest area net
change rate. We found that 50% of macroinvertebrate sites and 42%
of fish sites were in Good condition, whereas 21% and 29% were Severely
Impaired, respectively. Poorest biological conditions
occurred in Arid and Equatorial climates and the best conditions
occurred in Snow climates. Severely Impaired conditions were
associated (Pearson correlation coefficient) with higher
Human Development Index scores, poorer physico-chemical water quality, and
lower proportions of protected freshwater areas. Good biological
conditions were associated with good water quality and increased forested
areas. It is essential to implement statutory bioassessment programs in
Asian, African and South American countries, and continue them in Oceania,
Europe and North America. There is a need to invest in assessments based
on fish, as there is less information globally and fish were strong
indicators of degradation. Our study highlights a need to increase the
extent and number of protected river catchments, preserve and restore
natural forested areas in the catchments, treat wastewater discharges, and
improve river connectivity.