Restoration of Lake Geneva: Expected versus observed responses of phytoplankton to decreases in phosphorus

Abstract

Long‐term phytoplankton responses in Lake Geneva to a decline in phosphorus (P) loading are examined in terms of summer (July–September) biomass and community structure. With the rapid development of human activity on its banks and within its catchment area in the 1960s, this large subalpine hydrosystem shifted from oligotrophy to eutrophy within approximately one decade. Measures to reduce P loading were initiated successfully in the mid‐1970s, when total P concentrations in the winter overturn altered from 90 μg/L in 1980 to 40 μg/L in 1998. Until the 1990s, algal descriptors improved as expected (biomass decline, reappearance of diatom species, increased contribution of nanoplankton). Then, paradoxically, and in contrast to the reappearance of oligotrophic species, summer algal biomass began to increase. Pre‐summer (period prior to the beginning of the clear water phase) dissolved inorganic phosphorus concentrations and summer phytoplankton composition presented similar interannual trends. However, the succession of phytoplankton structure during the reoligotrophication phase differed greatly from that during the eutrophication period, and a recent abnormal upward trend in algal densities is mainly the result of the development of large species that formerly were only common from late September until November. This community change, mainly triggered by filamentous (Mougeotia gracillima, Tribonema) or motile forms (Dinobryon sociale, Cryptophycea), seems to have been induced by the earlier and greater deepening of the P‐depleted layer. In addition to milder summers, this massive development of larger forms seems to be favoured by four of their biological features: tolerance to warm temperatures, tolerance to low‐light intensity (might exploit deeper layers where P is not yet limiting), shapes not only providing a large surface to volume ratio or motility (adaptation to low‐nutrient concentrations), but increasing resistance to zooplankton grazing. This paradoxical trend, perhaps reinforced by the decline on roach Rutilus rutilus abundance (an opportunistic planktivore), is likely to remain until the P‐depleted zone is extended below the layers that can be frequently resupplied in nutrients by hydrodynamic processes.

Central European water quality indices applied to long-term data from peri-alpine lakes: test and possible improvements

“…Müller) Bergh blooms in Fig. 1 Ecological Quality Ratios (EQRs) derived from the German national method (white circles) and the Austrian national method (black circles) using 3 years running averages for Lake Geneva (A), Lake Annecy (B) and Lake Bourget (C) summer (Anneville et al, 2002b). BI means scatters less than BI WFD and shows a slight but significant increase from 1974 to 2006 (Fig.…”

“…This is a real problem since the indices are expected to be used to monitor the quality within single lakes. Such mismatches can be due to the delay phenomenon that is part of the re-oligotrophication process in lakes (Jørgensen & de Bernardi, 1998) and occurs in terms of phytoplankton abundance in Lake Geneva (Anneville et al, 2002b) and Bourget . Similar observations were made in Lago Maggiore where water quality improvement was retarded until TP concentration fel-l below a threshold concentration (de Bernardi et al, 1988;Sas, 1989).…”

Application of remote sensing for the optimization of in-situ sampling for monitoring of phytoplankton abundance in a large lake

“…The excessive nutrient loading has strongly impacted the lake by increasing the algae concentration. The water transparency decreased, summer algal blooms became more frequent and certain fish species diminished drastically (Anneville et al, 2002). After the implementation of phosphorus reduction plans in the early 1970's, the phosphorus concentration in Lake Geneva has decreased by almost 80% (BFS, 2013; www.cipel.org), going along with an improvement of the water quality and a slight reduction of the algal biomass towards the 1990's (Rimet, 2013).…”

“…After the implementation of phosphorus reduction plans in the early 1970's, the phosphorus concentration in Lake Geneva has decreased by almost 80% (BFS, 2013; www.cipel.org), going along with an improvement of the water quality and a slight reduction of the algal biomass towards the 1990's (Rimet, 2013). Thereafter, despite the ongoing decrease of phosphorus in Lake Geneva, phytoplankton biomass started to increase again (Anneville et al, 2002), reaching new record peaks in (Rimet, 2013Tapolczai et al, 2015).…”

Middle and late Holocene climate change and human impact inferred from diatoms, algae and aquatic macrophyte pollen in sediments from Lake Montcortès (NE Iberian Peninsula)

“…The vectors corresponding to these taxa are virtually orthogonal to those of both Cyclotella species, suggesting a strong inverse relationship. The dominance of C. comta, a species of the Cyclotella radiosa complex that is common in meso-eutrophic waters (Anneville et al 2002;Krammer and Lange-Bertalot 2004;Wunsam et al 1995), is interrupted by short episodes of decline, when Fragilariales become more important. Throughout the whole record, periods dominated by C. comta may indicate stability phases, with higher lake levels favouring plankton development.…”