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

Abstract

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 × 103 cell mL–1 1998=123 × 103 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 °C, and was strictly related to water stratification and thermocline appearance, the APP efficiency [mgC (mg chl)–1Em–2] had its maximum at the bottom of the euphotic zone, with irradiance in the range 0.5–18 μE m–2 s–1. 5. The existence of a significant regression between APP abundance and carbon fixation (P < 0.001, r2=0.92, d.f.=17; APPprod=10.6 × 103 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.

Flow-cytometric mapping provides novel insights into the seasonal and vertical distributions of freshwater autotrophic picoplankton

“…Using flow cytometry, we could efficiently and precisely enumerate 4 subpopulations among the APP, several of which had been lumped together in previous studies where epifluorescence microscopy was used to count APP (e.g. Søndergaard 1991, Weisse & Kenter 1991, Callieri & Piscia 2002. The refined analysis allows us to draw safer conclusions as to the potential causes for the observed seasonal variation among freshwater APP populations.…”

“…Whilst the overall seasonal pattern of APP abundance in oligo-mesotrophic temperate lakes appears to be reasonably well described, there remains considerable uncertainty as to the taxonomic identity of the various populations and the extent to which different environmental and ecological factors are responsible for the observed seasonal trends (Weisse 1993, Callieri & Piscia 2002. Experimentation has been used in previous studies in attempts to identify potential causal factors (e.g.…”

“…Weisse 1988, Wehr 1991. In lieu of direct manipulation, researchers have also used regression techniques and/or have undertaken time-series studies to identify which measured variables are likely to causally influence the seasonal abundance of autotrophic picoplankton (Gaedke & Weisse 1998, Callieri & Piscia 2002.…”

Picocyanobacterial community structure and space-time dynamics in the subalpine Lake Maggiore (N. Italy)

“…The samples were kept in the dark and immediately processed (~1 hour) after sampling. The depth of 3 m was selected because Photosynthetically Active Radiation (PAR) is always higher than 10% of surface PAR and also because this is the typical depth of phytoplankton peak abundance in Lake Maggiore (Callieri, and Piscia 2002). The depth of 20 m was chosen because it is always deeper than the depth of mixing (Z m ) and has around 0.1% of surface PAR.…”

“…The space-time dynamics of freshwater Pcy along environmental gradients have been extensively studied by means of microscopic and flow cytometric investigations (Callieri, and Piscia 2002;Crosbie et al 2003a;Sarmento et al 2008). In temperate lakes, the biomass maxima generally conform to a typical bimodal pattern, with a spring or early summer peak and a second peak in late summer or autumn (Stockner et al 2000).…”