Autotrofia ed eterotrofia in un sistema fluviale e lacustre eutrofico: bilanci di massa dei gas disciolti e dei nutrienti, ruolo della vegetazione macrofitica sommersa ed elofitica, del fitoplancton e dei tempi di residenza idraulica.

Il sistema fluviale del Mincio comprende segmenti a diverso grado di naturalità, morfometria e regime idrologico. Sono stati considerati in questo lavoro un segmento settentrionale caratterizzato da integrità ecologica e simile allo stato di riferimento, un segmento meridionale parzialmente bacinizzato a scorrimento lento ed un segmento centrale, assimilabile ad un lago per i tempi di ricambio lunghi e la velocità dell’acqua trascurabile. Obiettivi generali del lavoro sono stati: 1) confrontare le funzioni interne ai due segmenti fluviali in relazione al loro stato di conservazione e in particolare in relazione alle attività dei produttori primari dominanti (rispettivamente una macrofita radicata sommersa nel tratto nord ed il fitoplancton nel tratto sud); 2) evidenziare l’importanza degli scambi di gas disciolti (O2 e CO2) all’interfaccia acqua-atmosfera nel metabolismo complessivo di questi due sistemi; 3) quantificare l’importanza relativa del fitoplancton e di una prateria monospecifica di una pleustofita (Trapa natans) nell’evoluzione stagionale del chimismo e nei bilanci di massa di ossigeno, anidride carbonica e nutrienti in un sistema lacustre poco profondo. Le ipotesi generali associate a questi macrobiettivi sono: 1) la bacinizzazione può determinare la distrofia di sistemi fluviali eutrofici caratterizzati da un eccessivo arricchimento organico a livello dei sedimenti e dall’eccessiva proliferazione del fitoplancton; 2) l’idrodinamismo ha un ruolo fondamentale nel regolare i tenori dei gas e mantenere oscillazioni contenute nelle percentuali di saturazione; 3) le comunità a pleustofite favoriscono l’insorgenza dell’anossia con implicazioni sul metabolismo complessivo del sistema che le ospita. In estrema sintesi i risultati ottenuti permettono di affermare che: 1) Il sistema fluviale analizzato è una sorgente di CO2 verso l’atmosfera, in linea con quanto riportato dalla letteratura. I risultati di questo lavoro indicano però che i flussi sono fortemente regolati da aspetti idrologici e dalle comunità dei produttori primari dominanti e variano significativamente a seconda delle scale spaziali e temporali adottate nelle indagini. 2) La velocità dell’acqua nei segmenti fluviali eutrofici di alta pianura non permette la strutturazione di comunità fitoplanctoniche ed assume un’importante funzione regolatrice dei tassi di crescita, colonizzazione e sviluppo delle macrofite sommerse, garantendo buona ossigenazione e trasparenza. La disponibilità dei nutrienti simultaneamente determina un eccesso di proliferazione macrofitica con implicazioni a cascata per i segmenti a valle a causa del trasporto di materiale particellato. Nel segmento meridionale, la bacinizzazione si riflette in estremi valori di sovrasaturazione della CO2 e nella sottosaturazione dell’O2 in prossimità del fondo. Le densità di cellule microalgali sono inoltre tipiche di sistemi ipertrofici/distrofici. 3) Sorprendentemente, nonostante il carattere maggiormente distrofico del segmento fluviale meridionale, è il segmento a maggiore naturalità ad essere una maggiore sorgente di anidride carbonica per l’atmosfera. Questo a causa della maggiore velocità dell’acqua, che favorisce il rilascio del gas in eccesso, ma anche per i maggiori tassi di mineralizzazione del particellato. Nel segmento sud, invece, grandi quantità di CO2 disciolta vengono trasferiti a valle e tendono ad accumularsi in acqua (>400% di saturazione). 4) In un sistema eutrofico come il lago di Mezzo i bilanci dei gas indicano un metabolismo complessivo al netto autotrofo. In questo sistema viene ridimensionato il ruolo della pleustofita dominante rispetto al fitoplancton quale trappola di C, N e P. L’ombreggiamento della colonna d’acqua da parte di T. natans ha però implicazioni negative per i bilanci dell’ossigeno. 5) L’alterazione, indotta dalle attività antropiche, di aspetti idrologici quali i tempi di ricambio o la velocità dell’acqua inducono transizioni nelle comunità vegetali, nella zonazione dei processi e, in definitiva, nello stato trofico dei sistemi fluviali.The Mincio River is composed by adjacent reaches characterized by different levels of ecological integrity, morphology, and hydrological regime. This study considered a high plain reach (northern trait), similar to its reference status, a channelized low plain reach (southern trait), and a central reach, similar to a lake due to longer water retention-time (few days) and slow current velocity. The main objectives of this study were: 1) compare internal functions of two fluvial segments based on their conservation state and on the role of the dominant primary producers community (submerged macrophyte and phytoplankton in the northern and in the southern traits, respectively); 2) highlight the importance of dissolved gas (O2 e CO2) exchanges at the water-atmosphere interface on the whole metabolism of the two traits; 3) quantify the relative importance of phytoplankton community and of a pleustophyte stand (Trapa natans) on the seasonal evolution of the oxygen, carbon dioxide and nutrient mass balances in a shallow lake. The general hypotheses at the base of these objectives are: 1) the rectification and channelization could determine the dystrophy of eutrophic fluvial segments, characterized by nutrient and organic enrichment at sediment level and by microalgal blooms; 2) the hydrodynamism has a fundamental role as gas exchange regulator, and avoids excess super or undersaturation; 3) pleustophytic communities favor hypoxic events with implications for the whole ecosystem metabolism. In brief, the results obtained suggest that: 1) The Mincio River is a source of CO2 to the atmosphere, in accordance with the recent literature on river metabolism. Simultaneously, the results of this study show also that fluxes are strongly regulated by hydrological aspects and by the dominant communities of primary producers. Sharp variations in CO2 saturation, as those evidenced in the northern segment, suggest that the choice of adequate spatial and temporal scales, is of paramount importance. 2) The current velocity of the high plain river reach does not allow the structuring of phytoplankton communities and has an important role on regulating growth rates, colonization and development of submerged macrophytes, ensuring good water oxygenation and transparency. At the same time the high nutrients availability favoures the macrophytic development with cascade implications for downstream river reaches, mainly due to downwards particle transport. In the southern trait, the channelization results in extreme values of CO2 supersaturation and pronounced O2 decline at sediment level. Further, the microalgal biomass is typical of hypertrophic/dystrophic systems. 3) Surprisingly, the northern trait, characterized by high ecological integrity, is a major source of CO2 to the atmosphere. This is due to the higher current velocity which favours the release of the gas in excess, but also to the major mineralization rates of the particulate material. At the southern trait the CO2 excess is transferred downstream or accumulate into the water column (>400% of saturation). 4) In a eutrophic system, as the Lago di Mezzo, the gas mass balances demonstrate the net autotrophy of the whole system. In this shallow Lake, the dominant pleustophyte has a minor role as C, N and P trap compared to phytoplankton. The water column shading by T. natans has negative implications on oxygen balances and promotes anoxia. 6) The human modifications of hydrological aspects, as water retention time or current velocity, determines the shift of primary producer communities and of the metabolic processes. This modifies the slow fading of adjacent river reaches along longitudinal gradients, described in the River Continuum Concept. Human impacts induce local transitions of the trophic status in altered river reaches

daily and seasonal variability of co2 saturation and evasion in a free flowing and in a dammed river reach

The daily and seasonal evolution of O2 and CO2 saturation, water-atmosphere fluxes and budgets were measured in two fluvial reaches of the Mincio River (Italy). The northern reach is free flowing and is dominated by macrophytes while the southern reach is dammed, hypertrophic and phytoplankton dominated. We hypothesized short term regulation of gas saturation and fluxes by primary producers and the reversal of CO2 off-gassing in the southern reach. Results indicated that both reaches were always CO2 supersaturated. Higher CO2 evasion rates in the northern compared to the southern reach depended on reaeration coefficient, in turn depending on water velocity. In the northern reach dissolved inorganic carbon (DIC) production was one order of magnitude higher than oxygen consumption, likely due to a combination of anoxic heterotrophic activity in the hyporheic zone and carbonate dissolution. The activity of macrophytes influenced CO2 saturation on short time scales. A net summer abatement of DIC occurred in the southern reach, probably due to fixation by phytoplankton, which attenuated supersaturation but not reversed CO2 efflux. This study demonstrates how in small rivers CO2 evasion can undergo rapid and significant changes due to eutrophication, altered hydrology and shift in primary producer communities

Assessing macrophyte seasonal dynamics using dense time series of medium resolution satellite data

The improved spatial and temporal resolution of latest-generation Earth Observation missions, such as Landsat 8 and Sentinel-2, has increased the potential of remote sensing for mapping land surface phenology in inland water systems. The ability of a time series of medium-resolution satellite data to generate quantitative information on macrophyte phenology was examined, focusing on three temperate shallow lakes with connected wetlands in Italy, France, and Romania. Leaf area index (LAI) maps for floating and emergent macrophyte growth forms were derived from a semi-empirical regression model based on the best-performing spectral index, with an error level of 0.11 m2 m−2. Phenology metrics were computed from LAI time series using TIMESAT to analyze the seasonal dynamics of macrophyte spatial distribution patterns and species-dependent variability. Particular seasonal patterns seen in the autochthonous and allochthonous species across the three study areas related to local ecological and hydrological conditions. How characteristics of the satellite dataset (cloud cover threshold, temporal resolution, and missing acquisitions) influenced the phenology metrics obtained was also assessed. Our results indicate that, with a full-resolution time series (5-day revisit time), cloud cover introduced a bias in the phenology metrics of less than 2 days. Even when the temporal resolution was reduced to 15 days (like the Landsat revisit time) the timing of the start and the peak of macrophyte growth could still be mapped with an error of no more than 2–3 days.acceptedVersion© 2018. This is the authors’ accepted and refereed manuscript to the article. Locked until 11.7.2020 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0

Leaf reflectance can surrogate foliar economics better than physiological traits across macrophyte species

Abstract Background Macrophytes are key players in aquatic ecosystems diversity, but knowledge on variability of their functional traits, among and within species, is still limited. Remote sensing is a high-throughput, feasible option for characterizing plant traits at different scales, provided that reliable spectroscopy models are calibrated with congruous empirical data, but existing applications are biased towards terrestrial plants. We sampled leaves from six floating and emergent macrophyte species common in temperate areas, covering different phenological stages, seasons, and environmental conditions, and measured leaf reflectance (400–2500 nm) and leaf traits (dealing with photophysiology, pigments, and structure). We explored optimal spectral band combinations and established non-parametric reflectance-based models for selected traits, eventually showing how airborne hyperspectral data could capture spatial–temporal macrophyte variability. Results Our key finding is that structural—leaf dry matter content, leaf mass per area—and biochemical—chlorophyll-a content and chlorophylls to carotenoids ratio—traits can be surrogated by leaf reflectance with normalized error under 17% across macrophyte species. On the other hand, the performance of reflectance-based models for photophysiological traits substantively varies, depending on macrophyte species and target parameters. Conclusions Our main results show the link between leaf reflectance and leaf economics (structure and biochemistry) for aquatic plants, thus envisioning a crucial role for remote sensing in enhancing the level of detail of macrophyte functional diversity analysis to intra-site and intra-species scales. At the same time, we highlighted some difficulties in establishing a general link between reflectance and photosynthetic performance under high environmental heterogeneity, potentially opening further investigation directions

Aspects of Invasiveness of Ludwigia and Nelumbo in Shallow Temperate Fluvial Lakes

The relationship between invasive plant functional traits and their invasiveness is still the subject of scientific investigation, and the backgrounds of transition from non-native to invasive species in ecosystems are therefore poorly understood. Furthermore, our current knowledge on species invasiveness is heavily biased toward terrestrial species; we know much less about the influence of allochthonous plant traits on their invasiveness in aquatic ecosystems. In this paper, we present the results of a study on physiological and ecological traits of two introduced and three native macrophyte species in the Mantua lakes system (northern Italy). We compared their photophysiology, pigment content, leaf reflectance, and phenology in order to assess how the invasive Nelumbo nucifera and Ludwigia hexapetala perform compared to native species, Nuphar lutea, Nymphaea alba, and Trapa natans. We found L. hexapetala to have higher photosynthetic efficiency and to tolerate higher light intensities than N. nucifera and the native species especially at extreme weather conditions (prolonged exposure to high light and higher temperatures). Chlorophyll a and b, and carotenoids content of both allochthonous species were substantially higher than those of native plants, suggesting adaptive response to the ecosystem of Mantua lakes system. Higher variability of recorded data in invasive species was also observed. These observations suggest advanced photosynthetic efficiency of the invasive species, especially L. hexapetala, resulting in faster growth rates and higher productivity. This was supported by the evaluation of seasonal dynamics mapped from satellite remote sensing data. This study provides empirical evidence for the relationship between specific plant physiological traits and invasiveness of aquatic plant species, highlighting the importance of trait studies in predicting ecosystem-level impacts of invasive plant species

Investigating the Impact of Wildfires on Lake Water Quality Using Earth Observation Satellites

A study was carried out to investigate the effects of wildfires on lake water quality using a source dataset of 2024 lakes worldwide, covering different lake types and ecological settings. Satellite-derived datasets (Lakes_cci and Fire_cci) were used and a Source Pathway Receptor approach applied which was conceptually represented by fires (burned area) as a source, precipitation/drought representing transport dynamics, and lakes as the ultimate receptor. This identified 106 lakes worldwide that are likely prone to be impacted by wildfires via a terrestrial pathway. Satellite-derived chlorophyll-a (Chl-a) and turbidity variables were used as indicators to detect changes in lake water quality potentially induced by wildfires over a four-year period. The lakes with the largest catchment areas burned and characterized by regular annual fires were located in Africa. Evidence for a strong influence of wildfires was not found across the dataset examined, although clearer responses were seen for some individual lakes. However, among the hydro-morphological characteristics examined, lake depth was found to be significant in determining Chl-a concentration peaks which were higher in shallow and lower in deep lakes. Lake turbidity responses indicated a dependence on lake catchment and weather conditions. While wildfires are likely to contribute to the nutrient load of lakes as found in previous studies, it is possible that in many cases it is not a dominant pressure and that its manifestation as a signal in lake Chl-a or turbidity values depends to a large part on lake typology and catchment characteristics. Assessment of lake water quality changes six months after a fire showed that Chl-a concentrations either increased, decreased, or showed no changes in a similar number of lakes, indicating that a lake specific ecological and hydro-morphological context is important for understanding lake responses to wildfires

Assessing the impact of wildfires on water quality using satellite remote sensing: the Lake Baikal case study

Lakes have been observed as sentinels of climate change. In the last decades, global warming and increasing aridity has led to an increase in both the number and severity of wildfires. This has a negative impact on lake catchments by reducing forest cover and triggering cascading effects in freshwater ecosystems. In this work we used satellite remote sensing to analyse potential fire effects on lake water quality of Lake Baikal (Russia), considering the role of runoff and sediment transport, a less studied pathway compared to fire emissions transport. The main objectives of this study were to analyse time series and investigate relationships among fires (i.e., burned area), meteo-climatic parameters and water quality variables (chlorophyll-a, turbidity) for the period 2003-2020. Because Lake Baikal is oligotrophic, we expected detectable changes in water quality variables at selected areas near the three mains tributaries (Upper Angara, Barguzin, Selenga) due to river transport of fire-derived burned material and nutrients. Time series analysis showed seasonal (from April to June) and inter-annual fire occurrence, precipitation patterns (high intensity in summer) and no significant temporal changes for water quality variables during the studied periods. The most severe wildfires occurred in 2003 with the highest burned area detected (36,767 km2). The three lake sub-basins investigated have shown to respond differently according to their morphology, land cover types and meteo-climatic conditions, indicating their importance in determining the response of water variables to the impact of fires. Overall, our finding suggests that Lake Baikal shows resilience in the medium-long term to potential effects of fires and climate change in the region

Remote sensing of macrophyte morphological traits: Implications for the management of shallow lakes

Macrophytes are important elements of freshwater ecosystems, fulfilling a pivotal role in biogeochemical cycles. The synoptic capabilities provided by remote sensing make it a powerful tool for monitoring aquatic vegetation characteristics and the functional status of shallow lake systems in which they occur. The latest generation of airborne and spaceborne imaging sensors can be effectively exploited for mapping morphologically – and physiologically – relevant vegetation features based on their canopy spectral response. The objectives of this study were to calibrate semi-empirical models for mapping macrophyte morphological traits (i.e., fractional cover, leaf area index and above-water biomass) from hyperspectral data, and to investigate the capabilities of remote sensing in supporting macrophyte monitoring and management. We calibrated spectral models using in situ reflectance and morphological trait measures and applied them to airborne hyperspectral imaging data, acquired over two shallow European water bodies (Lake Hídvégi, in Hungary, and Mantua lakes system, in Italy) in two key phenological phases. Maps of morphological traits were produced covering a broad range of aquatic plant types (submerged, floating, and emergent), common to temperate and continental regions, with an error level of 5.4% for fractional cover, 0.10 m2 m-2 for leaf area index, and 0.06 kg m-2 for above-water biomass. Based on these maps, we discuss how remote sensing could support monitoring strategies and shallow lake management with reference to our two case studies: i.e., by providing insight into spatial and species-wise variability, by assessing nutrient uptake by aquatic plants, and by identifying hotspot areas where invasive species could become a threat to ecosystem functioning and service provision

Climate change and ecological assessment in Europe under the WFD – Hitting moving targets with shifting baselines?

The Water Framework Directive (WFD) sets the fundamental structure for assessing the status of water bodies in the European Union. Its implementation is currently entering its fourth six-year cycle assisted by a total of 38 guidance documents. The principal objective is to ensure good status for surface and ground waters. The functioning of the WFD is based on detecting the impact of human pressures on biological, physico-chemical, or hydromorphological parameters, and reducing these causal pressures through a program of measures to achieve good status. Climate change can exert a significant influence on ecological status by directly altering parameters monitored, pressure interactions, or influencing the effectiveness of programs of measures. Aquatic systems respond holistically to climate change with different pressures having additive, synergistic, or antagonistic interactions. The challenge is how to adapt the framework to manage aquatic systems in the context of climate change while maintaining focus on implementing measures to tackle key pressures. This paper examines potential approaches, including reassignment of waterbody type, quantifying the portion of Ecological Quality Ratio (EQR) driven by climate change, and creating an assessment module of climatic pressures and ecological responses. The overall purpose is to stimulate discussion and explore ways to incorporate climate change into the WFD structure.publishedVersio