Ecosystem‐level effects of re‐oligotrophication and N:P imbalances in rivers and estuaries on a global scale

• Published in: Global change biology 2023-03, Vol.29 (5), p.1248-1266
• Main Authors: Ibáñez, Carles, Caiola, Nuno, Barquín, José, Belmar, Oscar, Benito‐Granell, Xavier, Casals, Frederic, Fennessy, Siobhan, Hughes, Jocelyne, Palmer, Margaret, Peñuelas, Josep, Romero, Estela, Sardans, Jordi, Williams, Michael
• Format: Article
• Language: English
• Subjects: Aquatic plants; Biomass; cascading effects; chlorophyll; Dominance; Ecological effects; Ecosystem; ecosystem shift; Ecosystems; Environmental changes; Estuaries; Estuarine dynamics; Eutrophication; Income; Lakes; Macrophytes; Metabolism; Nitrogen; oligotrophication; Phosphorus; Phosphorus - metabolism; Physical factors; Phytoplankton; Phytoplankton - metabolism; Plankton; primary producers; Primary production; Review; Rivers; running waters; stoichiometry; Trends; Turbidity
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.16520

Trends and ecological consequences of phosphorus (P) decline and increasing nitrogen (N) to phosphorus (N:P) ratios in rivers and estuaries are reviewed and discussed. Results suggest that re‐oligotrophication is a dominant trend in rivers and estuaries of high‐income countries in the last two–three decades, while in low‐income countries widespread eutrophication occurs. The decline in P is well documented in hundreds of rivers of United States and the European Union, but the biotic response of rivers and estuaries besides phytoplankton decline such as trends in phytoplankton composition, changes in primary production, ecosystem shifts, cascading effects, changes in ecosystem metabolism, etc., have not been sufficiently monitored and investigated, neither the effects of N:P imbalance. N:P imbalance has significant ecological effects that need to be further investigated. There is a growing number of cases in which phytoplankton biomass have been shown to decrease due to re‐oligotrophication, but the potential regime shift from phytoplankton to macrophyte dominance described in shallow lakes has been documented only in a few rivers and estuaries yet. The main reasons why regime shifts are rarely described in rivers and estuaries are, from one hand the scarcity of data on macrophyte cover trends, and from the other hand physical factors such as peak flows or high turbidity that could prevent a general spread of submerged macrophytes as observed in shallow lakes. Moreover, re‐oligotrophication effects on rivers may be different compared to lakes (e.g., lower dominance of macrophytes) or estuaries (e.g., limitation of primary production by N instead of P) or may be dependent on river/estuary type. We conclude that river and estuary re‐oligotrophication effects are complex, diverse and still little known, and in some cases are equivalent to those described in shallow lakes, but the regime shift is more likely to occur in mid to high‐order rivers and shallow estuaries. The re‐oligotrophication of rivers and estuaries is a widespread process in high‐income countries that was preceded by the re‐oligotrophication in lakes and anticipates a similar process in middle‐income countries. This process is very often accompanied by an imbalance of the N:P ratio which tends to increase due to a stronger decrease in P in regard to N. The ecological effects of P decrease and N:P increase are pervasive and trigger cascading effects that change the species composition, productiv