A global analysis of oligotrophication trends in rivers and their ecological consequences
The goal of this project is to investigate the oligotrophication trends in rivers and estuaries, and the main ecological consequences with a focus on the possible shift from phytoplankton to macrophytes. The hypothesis that cultural oligotrophication is a general trend in Western rivers and estuaries will be tested through the creation and analysis of a global database, giving priority to data collection from U.S. rivers during the stage at SESYNC. The decrease in P in many rivers and estuaries during the last decades is well documented, but in most of cases the biotic variables responding to oligotrophication have not been sufficiently monitored. Thus, there are a scarce (but growing) number of cases in which chlorophyll (or phytoplankton) has been shown to decrease as a consequence of oligotrophication, and a few cases in which existing data allowed to show a regime shift from phytoplankton to macrophytes. Thus, an important question is why this type of regime shift is apparently exceptional in rivers whereas in shallow lakes it is common. The main reasons are likely that, besides the lack of data, factors such as river depth, pulsing flow regime or high suspended sediment prevent a general spread of submerged macrophytes. Then, the consequences of oligotrophication on fluvial ecosystems may be different as a function of river type. The hypothesis is that the regime shift to macrophytes may occur in mid-size rivers, but the response of primary producers (and the effects at whole-ecosystem level) is thought be different in streams, large rivers and estuaries.