Photosynthetic efficiency and seasonality of autotrophic picoplankton in Lago Maggiore after its recovery

1. Lago Maggiore is a deep subalpine lake that has regained its previous oligotrophic state during the last decade after going through a process of cultural eutrophication and subsequent restoration. Autotrophic picoplankton (APP) have been studied in Lago Maggiore since 1992, with the primary production of the fraction being measured in 1994 and 1995. In 1998, we began to study the population structure and photosynthetic characteristics of APP after the restoration of the lake. We also compared the seasonal dynamics of APP and nanoplankton biomass and production. In this paper, we show the trend of annual and interannual dynamics of APP from 1993 to 1998. 2. Since 1993 and 1994, APP have almost tripled in numbers (mean values: 1993=44 x 10(3) cell mL(-1) 1998=123 x 10(3) cell mL(-1) ) and in production. On average, APP produced 16 gC m(-2) year(-1) in 1994 and 41.2 gC m(-2) year(-1) in 1998, accounting for 10 and 20%, respectively, of total phototrophic production. 3. Although nanoplankton dominated in terms of biomass and production in the first 6 months of the year, APP were able to compete successfully with them in periods of P limitation. In September 1998, APP carbon fixation actually surpassed that of nanoplankton, reaching 13 mgC m(-3) h(-1) at 3 m. In a comparison of the daily chlorophyll-specific photosynthesis rates of nanoplankton and APP, the latter proved to be more efficient in fixing carbon, particularly in summer and autumn. 4. While the spatial distribution of APP abundance and production followed the isotherms of 18 and 20 degreesC, and was strictly related to water stratification and thermocline appearance, the APP efficiency [mgC (mg chl)(-1) Em(-2) ] had its maximum at the bottom of the euphotic zone, with irradiance in the range 0.5-18 muE m(-2) s(-1) . 5. The existence of a significant regression between APP abundance and carbon fixation (P < 0.001, r (2) =0.92, d.f.=17; APP(prod) =10.6 x 10(3) cell mL(-1) + 15) enabled us to infer APP production from cell numbers, and supports the view that picocyanobacteria production is closely dependent on its standing crop. 6. Lago Maggiore is a good example of how APP in a stabilised oligotrophic system can, under favourable conditions, increase in numbers and effectively exploit the few available resources. We can conclude that the APP population reacts to the reduction in P -levels, but only after several years of stable low nutrient conditions.