Phytoplankton productivity increased in Lake Geneva despite phosphorus loading reduction

Abstract

Long-term (1972-2005) data from Lake Geneva were analyzed to assess whether primary productivity (PP) has declined over years in response to phosphorus (P) reduction measures implemented in the 1970s, and to determine which factors were responsible for the response observed. The results revealed increases in PP and chlorophyll a, and a concomitant decrease in water transparency, which contrasts sharply with other large lakes undergoing nutrient loading controls. Our data revealed long-term increases in incident light, water temperature, water column stability (WCS), N:P ratios and Baccilariophyceae. Nitrogen, Daphnia and WCS, rather than P, were the major factors driving PP during the first part of the study (1972-1988). When P became the main nutrient driving PP (after 1988), the latter still increased, partly as a result of long-term increases in light and WCS, two factors not often considered when exploring climate influence on phytoplankton. Daphnia abundance decreased over time, and its coupling with phytoplankton changed from positive during the period of P abundance (1972-1988) to negative during the period of P scarcity (1989-2005), suggesting a change in the influence of Daphnia on phytoplankton and in food web structure. These data support the view that increases in climate-related factors may render the restoration of lakes more difficult, and showed that neglecting nitrogen, light and WCS, may hamper our understanding of lake responses to re-oligotrophication or lead to inappropriate management decisions, particularly in the context of global warming.

Quantitative PCR Enumeration of Total/Toxic Planktothrix rubescens and Total Cyanobacteria in Preserved DNA Isolated from Lake Sediments

“…were detected at 1-, 4-, and 10-cm depths (for dating, see Table S2 in the supplemental material) but not at the 24-cm core depth, while other cyanobacteria were detected down to the 24-cm layer. Interestingly, total cyanobacterial qPCR counts increased gradually from 1 to 10 cm, consistent with the period of strong eutrophication observed in the late 1980s (53), and decreased again at the 24-cm core depth (consistent with a period of pre-eutrophication according to data from the long-term monitoring of deep perialpine lakes).…”

“…Lake Bourget and Lake Geneva were indeed affected by eutrophication in the second half of the 20th century, and these systems are now recovering toward an oligotrophic status, while Lake Annecy has not been impacted by such strong eutrophication, and its oligotrophy has been preserved over the past decade (42). Lake Geneva underwent strong eutrophication in the early 1960s, but following the implementation of phosphorus reduction measures in the 1970s, the phosphorus concentration decreased from ϳ90 g liter Ϫ1 in the mid1970s to ϳ30 g liter Ϫ1 in 2005 in Lake Geneva (53) and from 120 to 26 g liter Ϫ1 between 1980 and 2001 in Lake Bourget (24).…”

Planctomycetes in Lakes: Poor or Strong Competitors for Phosphorus?

“…S1 in the supplemental material). This response was consistent with the very small P amounts in initial samples and with the TN:TP ratio being largely higher than 16 (by mass), the threshold beyond which P limitation of phytoplankton often occurs (39,40). Bacterioplankton-phytoplankton interactions are very complex, and understanding them requires sophisticated approaches (41).…”

Dynamics of auto- and heterotrophic picoplankton and associated viruses in Lake Geneva

“…The abundance of planktonic communities positively correlated with P tot and negatively correlated with NO 3 . P is known as an important component in the dynamics of planktonic communities in lakes (Wetzel, 2000), but the significance of N in regulating the plankton dynamics especially in Lake Geneva has only been recently reported (Tadonléké et al, 2009). In this regard, the capacity of picocyanobacteria to use N sources like NH 4 /NO 3 or switch over from P or N subject to the nutrient availability could be considered as an important factor in determining the plankton dynamics in this lake.…”