Linking spatial patterns of chlorophyll <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si74.svg" display="inline" id="d1e4191"> <mml:mi>a</mml:mi> </mml:math> and phosphorus concentrations: River length and upstream lakes control re

Eutrophication, i.e., the enhanced primary production above the natural level due to nutrient enrichment, remains a serious problem in river ecosystems despite substantial reduction of the limiting nutrient phosphorus (P). This study investigates to which extent P can predict phytoplankton patterns and which factors beyond P are relevant on a spatial scale. We analyzed a comprehensive dataset of chlorophyll a (Chl-a) and total phosphorus (TP) concentrations from 329 German river monitoring sites spanning 2000-2019. Our approach involved: (1) examining spatial Chl-a patterns, particularly exceeding critical levels (>30μg/l), (2) quantifying the role of TP introducing the degree of realized eutrophication (α_realized), the ratio of observed to potential Chl-a (i.e., the maximum Chl-a for the given TP); and (3) employing statistics with multiple predictors to identify catchment and stream network predictors of median α_realized at all stations. Results revealed critical Chl-a levels across diverse river sizes, with the large northeastern lowland rivers exhibiting the highest concentrations, contrasting with lower concentrations in the large southern and western rivers. We found α_realized to be highly variable in space and consistently below 100%, indicating that P alone does not limit eutrophication. Our analysis pointed to distinct control patterns emerging for river length. In long rivers (>408 km), α_realized predictably increased with length, except for the Rhine river, which showed a decrease. Conversely, in shorter rivers, the presence of upstream lakes and cumulative lake residence times was a dominant control of α_realized though predictability was lower. We conclude that in longer rivers, phytoplankton growth predominates over loss, mainly controlled by residence time. The Rhine's deviation is probably attributed to concentrated TP point sources and elevated loss rates though grazing by invasive mussels. Short rivers can reach high α_realized through higher residence times caused by upstream lakes embedded in the river network. Our study implies that P reduction is unavoidable for eutrophication management in coupled river network-lake ecosystems and may become more important due to prolonged residence times in a changing climate.