5.1 Simulation of P. rubescens dynamics
The results addressed some of the knowledge gaps regarding the factors that govern its bloom in medium-sized and meso-eutrophic lake. The particular physiological traits, such as a cold-stenotherm, low growth rate, shade-requiring, low irradiances, buoyant and P-limiting adapted species (Walsby and Schanz, 2002; Dokulil and Teubner, 2000), gave it the ability to occupy and grow in the metalimnetic niche during summer, entrain into the surface mixed layer during autumn, survive during the winter turbulence and form the inoculum for the following growing season. The implementation of the three-dimensional ecological model revealed a clear vertical distribution of environmental drivers evidencing a niche separation between two depths and P. rubescens resulted to be influenced by lake hydrodynamics, particularly during high-discharge inflows. A simple seasonal succession was identified based on two more phytoplankton functional-groups which are in competition with P. rubescens for light and nutrients, each one in its optimal temperature range. The model lost performance in neshore and pelagic ar-eas, due to depth-specific calibration, the dilution processes by inflowing waters and the spatial interactions due to a different phytoplankton nutrient uptake associated with the stream-lake transition zone (Mackay et al., 2011).
The expected trophic status was assessed with a trophic index, evidencing that oligo-mesotrophy is the natural condition for Lake Pusiano, simulated under a natural external loading. The reference condition for the phosphorus loads and lake concentrations was confirmed through other available methods (paleolimnology, MEI indices, empirical equations). Even in its reference condition, the lake deep/hypolimnetic waters were simulated to remain anoxic during the thermal stratification, although the total anoxic volume and period are lower compared to the eutrophic simulated condition. This sug-gests the lake morphometry and physiographic features determine the oxygen depletion on the bottom and the lake monomictic pattern, more than the productivity. The general improvement of the trophic index has a potential implication for the lake water uses, i.e.
an increase of water trasparency and a reduction of P. rubescens blooming should result in a positive outcome for bathing or irrigation uses.
125 The interactions between P. rubescens eco-physiological traits and the lake ecological state are crucial factors in determining the effects of the climate forcing (e.g. tempera-ture warming) on its growth and may cause a different behaviour, depending on the lake trophic evolution. The Wavelet Analysis of 50-years daily output by simulated scenari-os highlighted a chaotic behaviour (no seasonal patterns) since the late 90s, when P. ru-bescens blooms started to occur after decades of phosphorus pollution from the catch-ment. A regular 12-month period (an autumn peak) was instead extracted during the simulation of a pristine condition, characterized by a low productivity.
After decades of lake eutrophication, the global warming did not increase the blooming events. In fact, its growth rate took benefit from a colder climate rather than the increas-ing temperatures, under the same eutrophic condition. Apparently, this outcome coun-teracts the recent findings about the positive relation between the establishment of P.
rubescens, a warmer climate and the longer-lasting stratification periods (i.e. more stable conditions for its growth), especially in deep subalpine lakes (Carey et al., 2012;
Posch et al., 2012). Yet, there are evidences that benefits from climate warming are re-stricted to the winter signal leading the on-set of thermal stratification, or even they are not directly demonstrated, while longer periods of summer stratification showed no ad-vantage for population development (Salmaso 2010, Dokulil and Teubner, 2012). Con-versely, a positive relation between P. rubescens and the increasing temperatures arose when the pressure from the catchment (e.g. phosphorus pollution) was switched off, as emerged by the Mann-Kendall statistics on daily output data. In other words, these out-comes suggest that the warming temperatures may have a different effect on P. ru-bescens dynamics, depending on the trophic evolution of a lake.
The present study did not solve, but could only confirm, the paradoxical behaviour of P.
rubescens (Padisák et al., 2010). Although the present study is based only on one case, the output gained separating the causal factors and comparing the effects support the formulation of some important hypothesis, to be tested in a further study on other lakes:
1. the large diffusion of the filamentous cyanobacterium was caused by the com-mon politics of lakes restorations, based on an unidirectional reduction of phos-phorus loads;
126 2. decades of pollution, catastrophic shifts, loss of biodiversity etc. weakened the natural fender of ecosystems, offering the gap for the instauration of invasive species;
3. the eco-physiological traits of P. rubescens became a key factor matching the first two conditions and determining its ubiquitous success;
4. climate change did not cause a direct effect on blooming mechanism in lakes that were already locally impacted;
5. lake ecosystems, that are in their original (or natural) condition, are stronger in contrasting P. rubescens instauration, being the ecological niches already occu-pied by several species, but they may be more vulnerable to global warming, fa-vouring the population growth.
5.2 Future perspectives
This study assessed an innovative methodological approach to allocate classic taxonom-ic determinations into ‘functional-groups’ in order to validate a three-dimensional mod-el with three ‘key’ groups based on cmod-ell enumerating. The use of a spectrally-specific submergible fluorimeter coupled to continuous simulations of a calibrated model can potentially supply a high-frequency monitoring for Lake Pusiano, in a next future. This device can also be employed for a real-time warning of the risk of P. rubescens blooms, or be used to manage the population growth predicting the effects of the regulation of the existing dam and extraordinary hydrologic events. The methodological approach was developed to integrate high-frequency measurement and models of the lake and its catchment is an exportable tool, regardless of the model options. The innovative use of an integrated lake-basin modelling tool enabled to separately simulate the effects on lake ecology of human pressures at acting at different spatial scales. Thanks to its archi-tecture, it can be used to predict the future effect of climate change and land use on lake thermal structure (as circulation, stratification or oscillation patterns), trophic evolution, sediment fluxes, cyanobacterial blooming etc. The Intergovernmental Panel on Climate Change scenarios (IPCC, 2007) can be integrated into the model as a meteorological forcing after they have been processed by a statistical downscaling.
127 In general, after the first effort to implement deterministic models for lakes, these tools are useful to predict changes within any quantitative variable, thus variation in biogeo-chemical cycles, consequences of a regime shift, the effects of different restoration plans or human pressure, physical processes and nutrients storage (e.g. carbon lake budget). A predictive study is already in progress by the author, based on scenarios simulating different practices to reduce the current phosphorus loading in Lake Pusiano, re-garding the goals of the European Water Framework Directive (2000/60/EU). The evolu-tion of lake ecological processes are distinguished under different management plan and the ef-ficiency is compared evaluating the simulated output by a cost/benefit analysis.
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