The interplay of nutrients, dissolved inorganic carbon and algae in determining macrophyte occurrences in rivers

Nitrogen and phosphorous concentrations are widely considered to drive macrophyte assemblages in rivers. However, Dissolved Inorganic Carbon (DIC) - available for plants as CO₂ and HCO₃^- - is also of major relevance. Based on literature, we present a conceptual model on the interaction between algae, macrophytes, DIC, pH, light, N, P and the surface water and sedimental compartment. Analysing two separate datasets (i) on river physico-chemistry and chlorophyll-a, and (ii) on river physico-chemistry and macrophytes we quantify three connections within this concept: (1) the correlation of chlorophyll-a versus pH, (2) the correlation of TP versus chlorophyll-a and (3) the occurrence of HCO₃-users and CO₂-only-users among macrophytes along the DIC gradient. Chlorophyll-a correlated positively with pH (R-squared = 77%, p < .001) due to increased carbon dioxide uptake of phytoplankton. Surface water TP did not linearly correlate with chlorophyll-a concentrations. Obligate and optionally submerged macrophyte species that utilise HCO₃^- were separated from CO₂-only-users by HCO₃^- concentrations, with an area under the curve (AUC) of 68% and 70% (both p < .001) between groups. Obligate and optionally submerged macrophyte assemblages only composed of HCO₃-users and those exclusively composed of CO₂-only-users showed an even stronger separation based on the HCO₃^- concentration, with both an AUC of 82% and 78% (both p < .001). Our results underline that DIC can greatly affect riverine macrophytes. However, absolute concentrations of HCO₃^- are less relevant, while the connection to pH is more important, reflecting CO₂ concentrations. River monitoring and management should consider the interaction between nutrients DIC, surface water and sedimental compartment as important factors affecting macrophyte occurrence, rather than solely focussing on surface water nutrients.