Empirical large-scale evidence of algae growth control in rivers: Is total phosphorus control (still) a good management strategy?

Since the 1980s, the problem of algae blooms in rivers as a manifestation of eutrophication has been addressed by lowering nutrient inputs with a specific focus on limiting phosphorus. Despite the past achievements in phosphorus control, algae blooms are still frequently occurring, not least during the recent European drought years, with partly severe consequences for river ecology. This implies that additional parameters may have become important in controlling eutrophication. Regarding river management, this raises the question of whether a sole focus on further improvements in phosphorus control is still a good management strategy. Alternatively, an embedding in a multiple-stressor approach from a riverscape perspective might be necessary, particularly with regard to climate-related changes in temperature and precipitation.

Here, we analyze a Germany-wide dataset of chlorophyll a (Chl-a) concentrations over the period from 2000 to 2019 at 358 sites (33489 measurements in total) to address the following questions: (1) Are there geographical regions particularly threatened by algae blooms? (2) How sensitive are different river locations to elevated total phosphorus (TP) levels regarding algae development? (3) Can we explain spatial sensitivity differences by other in-stream parameters or catchment characteristics?

To understand when and where rivers are particularly effective in converting TP into algae biomass and thus prone to algae blooms, we use the measure of the degree of realized eutrophication, which is the ratio between the realized (i.e. the Chl-a measurement) and potential eutrophication (i.e. a theoretical upper Chl-a concentration at a given TP level if all TP is converted to biomass). Spatial differences in this degree of realized eutrophication are then analyzed together with in-stream parameters (e.g. water temperature) and catchment characteristics (e.g. topography, land use) using multivariate statistics.

We find algae blooms (> 30 µg Chl-a/l) across all analyzed German river basins and stream orders, making up 21 % of all measurements from March to November. They most frequently occur in large rivers (stream order 6 to 8) in catchments draining to the Baltic Sea and in the Elbe basin, constituting 58 % and 60 % of the measurements, respectively. For all stations, the median degree of realized eutrophication is only 1.3 %, whereas for single stations, it can go up to 20 %, revealing a large variability between sites. Results from a partial least squares regression analysis suggest that catchment characteristics like network length, seasonality of precipitation, lithology, and soil properties have predictive power, whereas in-stream parameters only play a secondary role.

While phosphorus is a critical prerequisite for algae growth, our results emphasize that its availability alone does not explain the development of algae blooms. For management, this means that a look beyond phosphorus control is necessary for preventing future river eutrophication.