Alterazioni nel regime idrogeologico nel fiume Po: analisi delle implicazioni nella composizione e nelle trasformazioni dei carichi di N E P.

Il regime idrologico è il determinante principale della struttura e della funzionalità degli ecosistemi acquatici lotici (Ward & Stanford,1989; Richter et al. 1996; Bunn & Arthington, 2002; Freeman & Marcinek, 2006). Le alterazioni nel regime idrologico interessano su scala globale gli ecosistemi fluviali e sono riconosciute come impatti diretti o indiretti delle intense attività antropiche (Poff et al.). Il mio progetto di tesi è dedicato all’approfondimento dell’effetto di due forzanti idrologiche nel fiume Po: gli eventi di piena e le opere di bacinizzazione. Una prima parte del progetto riguarda l’analisi della distribuzione dei carichi dei nutrienti durante gli eventi di piena, attuata attraverso una campagna di monitoraggio intensiva effettuata dal 2003 al 2007 in stazione di chiusura di bacino (Pontelagoscuro, Fe). La frequenza di campionamento si è rilevata cruciale nel determinare i carichi transitanti e nell’evidenziare i rapporti tra portata e principali parametri idrochimici ed è stato pianificato per tutto il 2008 anche un monitoraggio bisettimanale nel tratto potamale del fiume Po (Viadana, Mn) per approfondire l’evoluzione dei carichi, appunto, con la portata. Questo si è reso necessario data la scarsa disponibilità di dati esistenti in materia: per il fiume Po, parametri come la Silice Reattiva Disciolta o la frazione particellata di Fosforo vengono scarsamente considerati nei monitoraggi pregressi, anche se risultano avere un ruolo importante nei processi di eutrofizzazione degli ecosistemi lotici e costieri (Turner et al., 2003; Owens & Walling, 2002; Ittekkot,2000). I risultati evidenziano un ruolo dominante degli eventi di piena nella distribuzione spaziale e temporale dei carichi di Fosforo. Gli intensi processi erosivi che si attuano durante i transitori idrologici di piena aumentano il carico particellato di Fosforo ed il contributo del trasporto durante questi eventi rappresenta in media il 33.6 ± 6.3 % del carico totale annuo di Fosforo che nel periodo di analisi (2003-2007) non supera mai il valore di 10000 t P/a. Il rilascio del carico in tempi brevi è associata al trasporto soprattutto della componente particellata sospesa nella massa d’acqua, che negli eventi di piena più consistenti è costituita per circa il 50% da materiale organico refrattario e per un ulteriore 30-35% dalle componenti inorganiche legate al calcio, che sono particolarmente insolubili. Una velocità così elevata del rilascio del carico si configura come una sorta di “flash loading”, in cui si riconoscono fenomeni di hysteresis (Bowes et al.,2005) nell’evoluzione delle concentrazioni di Fosforo Particellato rispetto alle portate. Una parte consistente del carico del fosforo potrebbe non avere effetti biologici importanti, almeno nel breve termine. Una prima evidenza della scarsa biodisponibilità del fosforo particellato si è avuta da alcuni esperimenti svolti con Ulva sp., un’alga verde a rapida crescita. Il carico di Azoto risulta invece essere significativamente correlato alla portata del fiume (r = 0.877) e non alle caratteristiche idrodinamiche del transitorio. Nella seconda parte del mio progetto di tesi mi sono occupata dello studio dei potenziali effetti della regolazione del deflusso operata dallo sbarramento di Isola Serafini (Pc) sulla formazione dei carichi a valle e sulla loro trasformazione da monte a valle dello sbarramento. Sono stati caratterizzati i comparti acquatici e bentonici di stazioni a monte, nel bacino e a valle dello sbarramento per poter tracciare l’evoluzione dei carichi e poterne confrontare i parametri metabolici. E’ stato verificato come l’azione di rigurgito esercitato dallo sbarramento rallenti la velocità di corrente ed abbatta (sino all’80%) il carico particellato nel bacino. L’abbattimento del carico particellato nel bacino implica un accumulo di nutrienti e di materiale organico nei sedimenti. Nei sedimenti del bacino il contenuto di sostanza organica del sedimento è correlato in modo positivo con il consumo di ossigeno del sedimento e qui (3.2 ±0.6 mol O2 m-2 h-1) il tasso di rspirazione è quasi doppio rispetto alla stazione di monte (1.8 ±0.3 mol O2 m-2 h-1). Analogamente nel bacino risultano maggiori i tassi di rigenerazione di NH4+, NO3-, NO2-, PO43-, SiO2 e si riscontrano differenze nei flussi e tassi di rigenerazione delle altre specie chimiche indagate. Nel bacino, la ridotta velocità di corrente permette lo sviluppo di comunità fitoplanctoniche e durante le fioriture estive si verifica ,solo nel bacino, un abbattimento della silice reattiva pari a circa il 20% rispetto alle concentrazioni entranti. Meno rilevante è l’abbattimento di azoto e fosforo inorganici

Net primary production and seasonal CO2 and CH4 fluxes in a Trapa natans L. meadow

The main hypothesis of this work is that Trapa natans L. and similar floating leaved macrophytes are only temporary sinks of atmospheric carbon dioxide and that they favour water hypoxia and large methane efflux from sediment to the atmosphere, due to their shading effect and scarce ability to transfer oxygen to submerged tissues. For this purpose, from April to August 2005, T. natans production, dissolved O2, CO2 and CH4 concentrations in the water column and CO2 and CH4 fluxes across the wateratmosphere interface were measured in an oxbow lake (Lanca di Po, Northern Italy) where a monospecific floating mat of water chestnut develops. Net primary production by T. natans was determined via biomass harvesting while gas fluxes were determined via short-term incubations of light and dark floating chambers. From July onwards, when the water surface of the oxbow lake was entirely colonized by the plant, the dense canopy resulted in a physical barrier for light and water reareation. As a consequence of sediment and plant respiration, persistent hypoxia and often anoxia, and CO2 and CH4 supersaturation occurred in the water column. Net primary production of T. natans, calculated at peak biomass, was 13.05 ± 0.32 mol CO2 m-2. The T. natans mat was a net sink for atmospheric CO2 from mid June to mid August, with an uptake peak measured at the beginning of July (229 mmol m-2 d-1); estimated net ecosystem metabolism was ≤10.09 ± 1.90 mol CO2 m-2. Contextually, during the vegetative period of T. natans, the oxbow lake was a net source of methane (9.52 ± 2.10 mol m-2), and the resulting CH4 to CO2 flux ratio across the water-atmosphere interface was ≥0.94. The large methane release was probably due to the persistent hypoxia and anoxia induced by the T. natans meadow, which uncoupled methane production from methane oxidation

Net primary production and seasonal CO2 and CH4 fluxes in a Trapa natans L. meadow

The main hypothesis of this work is that Trapa natans L. and similar floating leaved macrophytes are only temporary sinks of atmospheric carbon dioxide and that they favour water hypoxia and large methane efflux from sediment to the atmosphere, due to their shading effect and scarce ability to transfer oxygen to submerged tissues. For this purpose, from April to August 2005, T. natans production, dissolved O2, CO2 and CH4 concentrations in the water column and CO2 and CH4 fluxes across the wateratmosphere interface were measured in an oxbow lake (Lanca di Po, Northern Italy) where a monospecific floating mat of water chestnut develops. Net primary production by T. natans was determined via biomass harvesting while gas fluxes were determined via short-term incubations of light and dark floating chambers. From July onwards, when the water surface of the oxbow lake was entirely colonized by the plant, the dense canopy resulted in a physical barrier for light and water reareation. As a consequence of sediment and plant respiration, persistent hypoxia and often anoxia, and CO2 and CH4 supersaturation occurred in the water column. Net primary production of T. natans, calculated at peak biomass, was 13.05 ± 0.32 mol CO2 m-2. The T. natans mat was a net sink for atmospheric CO2 from mid June to mid August, with an uptake peak measured at the beginning of July (229 mmol m-2 d-1); estimated net ecosystem metabolism was ≤10.09 ± 1.90 mol CO2 m-2. Contextually, during the vegetative period of T. natans, the oxbow lake was a net source of methane (9.52 ± 2.10 mol m-2), and the resulting CH4 to CO2 flux ratio across the water-atmosphere interface was ≥0.94. The large methane release was probably due to the persistent hypoxia and anoxia induced by the T. natans meadow, which uncoupled methane production from methane oxidation

Net primary production and seasonal CO2 and CH4 fluxes in a Trapa natans L. meadow

The main hypothesis of this work is that Trapa natans L. and similar floating leaved macrophytes are only temporary sinks of atmospheric carbon dioxide and that they favour water hypoxia and large methane efflux from sediment to the atmosphere, due to their shading effect and scarce ability to transfer oxygen to submerged tissues. For this purpose, from April to August 2005, T. natans production, dissolved O2, CO2 and CH4 concentrations in the water column and CO2 and CH4 fluxes across the wateratmosphere interface were measured in an oxbow lake (Lanca di Po, Northern Italy) where a monospecific floating mat of water chestnut develops. Net primary production by T. natans was determined via biomass harvesting while gas fluxes were determined via short-term incubations of light and dark floating chambers. From July onwards, when the water surface of the oxbow lake was entirely colonized by the plant, the dense canopy resulted in a physical barrier for light and water reareation. As a consequence of sediment and plant respiration, persistent hypoxia and often anoxia, and CO2 and CH4 supersaturation occurred in the water column. Net primary production of T. natans, calculated at peak biomass, was 13.05 ± 0.32 mol CO2 m-2. The T. natans mat was a net sink for atmospheric CO2 from mid June to mid August, with an uptake peak measured at the beginning of July (229 mmol m-2 d-1); estimated net ecosystem metabolism was ≤10.09 ± 1.90 mol CO2 m-2. Contextually, during the vegetative period of T. natans, the oxbow lake was a net source of methane (9.52 ± 2.10 mol m-2), and the resulting CH4 to CO2 flux ratio across the water-atmosphere interface was ≥0.94. The large methane release was probably due to the persistent hypoxia and anoxia induced by the T. natans meadow, which uncoupled methane production from methane oxidation

Nitrogen Removal in Vegetated and Unvegetated Drainage Ditches Impacted by Diffuse and Point Sources of Pollution

Secondary drainage canals have the potential to effectively mitigate excess nitrogen loads from diffuse and point sources. In vegetated (Phragmites australis and Typha latifolia) and in unvegetated canals subjected to diffuse and point pollution, nitrogen removal was evaluated by means of simple in–out mass balance and potential uptake by macrophytes was estimated from biomass data. Results suggest an elevated control of nitrogen in vegetated ditches receiving point source of pollution (average abatement of 50% of the total N load per linear km), whereas removal processes are much less effective in unvegetated ditches. The comparison between net abatement and plant uptake, highlights the presence of other unaccounted for processes responsible for a relevant percentage of total N removal. Overall, results from this study suggest the importance of actions aiming at the appropriate management of emergent vegetation, in order to improve its direct and indirect metabolic functions and maximize nitrogen removal in impacted watersheds

Nitrogen Budget in a Lowland Coastal Area Within the Po River Basin (Northern Italy): Multiple Evidences of Equilibrium Between Sources and Internal Sinks

Detailed studies on pollutants genesis, path and transformation are needed in agricultural catchments facing coastal areas. Here, loss of nutrients should be minimized in order to protect valuable aquatic ecosystems from eutrophication phenomena. A soil system N budget was calculated for a lowland coastal area, the Po di Volano basin (Po River Delta, Northern Italy), characterized by extremely flat topography and fine soil texture and bordering a network of lagoon ecosystems. Main features of this area are the scarce relevance of livestock farming, the intense agriculture, mainly sustained by chemical fertilizers, and the developed network of artificial canals with long water residence time. Average nitrogen input exceeds output terms by *60 kg N ha-1 year-1, a relatively small amount if compared to sub-basins of the same hydrological system. Analysis of dissolved inorganic nitrogen in groundwater suggests limited vertical loss and no accumulation of this element, while a nitrogen mass balance in surface waters indicates a net and significant removal within the watershed. Our data provide multiple evidences of efficient control of the nitrogen excess in this geographical area and we speculate that denitrification in soil and in the secondary drainage system performs this ecosystemic function. Additionally, the significant difference between nitrogen input and nitrogen output loads associated to the irrigation system, which is fed by the N-rich Po River, suggests that this basin metabolizes part of the nitrogen excess produced upstream. The traditionally absent livestock farming practices and consequent low use of manure as fertilizer pose the risk of excess soil mineralization and progressive loss of denitrification capacity in this area