Diego Copetti

Eutrophication management in surface waters using lanthanum modified bentonite: a review

This paper reviews the scientific knowledge on the use of a lanthanum modified bentonite (LMB) to manage eutrophication in surface water. The LMB has been applied in around 200 environments worldwide and it has undergone extensive testing at laboratory, mesocosm, and whole lake scales. The available data underline a high efficiency for phosphorus binding. This efficiency can be limited by the presence of humic substances and competing oxyanions. Lanthanum concentrations detected during a LMB application are generally below acute toxicological threshold of different organisms, except in low alkalinity waters. To date there are no indications for long-term negative effects on LMB treated ecosystems, but issues related to La accumulation, increase of suspended solids and drastic resources depletion still need to be explored, in particular for sediment dwelling organisms. Application of LMB in saline waters need a careful risk evaluation due to potential lanthanum release.

Coupling high-resolution measurements to a three-dimensional lake model to assess the spatial and temporal dynamics of the cyanobacterium Planktothrix rubescens in a medium-sized lake

In a medium-sized pre-alpine lake (North Italy) the cyanobacterium Planktothrix rubescens has strongly dominated the phytoplankton assemblage since 2000, similar to many pre-alpine lakes, despite improvements in water quality. The objective of this study was to determine the factors governing the spatial distribution of P. rubescens, including the major hydrodynamic processes and the influence of long-term reduction in nutrient concentrations during a period of climate warming. We used an intensive field campaign conducted from February 2010 to January 2011, to evaluate distributions of phytoplankton phyla, as well as P. rubescens, using spectrally resolved fluorescence measurements. These data provided highly spatially and temporally resolved phytoplankton population data suitable to calibrate and validate a coupled three-dimensional hydrodynamic (ELCOM) and ecological model (CAEDYM) of the lake ecosystem. The simulations revealed the fundamental role of physiological features of P. rubescens that led to observed vertical patterns of distribution, notably a deep chlorophyll maximum, and a strong influence of lake hydrodynamic processes, particularly during high-discharge inflows in summer stratification. The simulations are used to examine growth-limiting factors that help to explain the increased prevalence of P. rubescens during re-oligotrophication.

Using Impellers to Distribute Local Nutrient Loadings in a Stratified Lake: Lake Como, Italy

The feasibility of using a vertically downward pointing impeller to remove polluted water from a coastal margin was investigated with a pilot experiment in Lake Como, Italy. The vertical jet induced by the impeller entrained surface water downward to depth, where upon reaching neutral buoyancy it intruded horizontally into the lake proper. A three-dimensional numerical hydrodynamic model was validated with results from an intensive (eight-day) field study during which rhodamine WT dye was introduced in the lee of the impeller and then tracked as it was transported into the lake. One of the major sources of pollution to the lake is the Cosia River underflow that drains a catchment adjacent to the City of Como. The numerical model, once validated, was used to show that six impellers, with the correct thrust, could be used to reduce the pollutants adjacent to the City of Como by half within two weeks; this was achieved by injecting the surface water to a depth that would intensify the Cosia River intrusion.

Variability of microcystin cell quota in metapopulations of Planktothrix rubescens: Causes and implications for water management

In this study, we investigated the relationships between microcystin (MCs) concentrations and the biovolumes of Planktothrix rubescens (BPr) in 2 natural lakes (Pusiano and Garda) and 2 artificially dammed reservoirs (Occhito and Ledro) in Italy. In all the considered water bodies, P. rubescens was the dominant cyanobacterium. All the lakes were characterized by significant relationships between MCs and BPr, with limited variability in the MC quota (the content of MCs per unit of biovolume) within each water body compared with the variability between sites. The results were consistent with the development of specific MC-genotypes, with moderate seasonal and spatial changes in the proportion between toxic and non-toxic strains. The MC cell quota obtained in our work (ECQ, Environmental Cell Quota) were in the same range of values computed on the basis of analyses made on environmental samples dominated by P. rubescens or Planktothrix agardhii, and on isolates of the same two species (<1 to over 10 μg mm(-3)). Besides the usual ordinary least square regressions, models have been evaluated by using quantile regression, a method that allows estimating the conditional median or other quantiles of the response variable. We showed that the use of quantile regressions has different advantages, which included the computation of MC quota based on the whole range of available data, the robustness against outliers, and the ability to estimate models also in cases where there is no or only weak relationships. The highest ECQ values estimated from 95% quantile regressions in specific water bodies might be used to estimate the worst-case MC concentrations from algal abundances. Nevertheless, it was stressed that a realistic assessment of toxicity and potential adverse health effects necessarily should take into account the toxicity potential of the more abundant MC-congeners produced by specific cyanobacteria populations.

Total phosphorus reference condition for subalpine lakes: A comparison among traditional methods and a new process-based watershed approach

Different methods for estimating the total phosphorus (TP) reference conditions of lakes have rarely been compared. This work tests the uncertainty and accuracy of the most frequently used approaches (Morpho-edaphic index -MEI-, export coefficient, diatoms and pigment-inferred TP models) for 35 subalpine lakes. Furthermore, we propose a new process-based watershed approach that was tested on a subalpine environment and consists of combining a space for time substitution with a space for space substitution. The possible presence of uncontaminated or less contaminated environments inside or next to the watershed can be exploited by training a hydrological transport watershed model according to the uncontaminated conditions and then applying the calibration to the entire watershed, which reconstructs a natural or semi-natural TP load scenario. We found that the root mean square error (RMSE) for the MEI is 4xa0μgxa0L(-1). However, its application is limited for lakes that present with an alkalinity ≤1xa0meqxa0L(-1). For lakes with a higher alkalinity, we observed a loss of predictive capability that results from the lower solubility of phosphorus under conditions of high calcium content. The export coefficient model was applied with a mean export coefficient and presents similar prediction capabilities as the MEI. The chlorophyll-inferred TP model shows a higher uncertainty (RMSExa0=xa08xa0μgxa0L(-1)); however, it produced fewer underestimations and overestimations. With regards to the diatom-inferred TP model, we are only able to evaluate an uncertainty of 5xa0μgxa0L(-1) at the European level. Finally, the proposed process-based watershed approach adequately predicted the reference condition of the selected lake and had an uncertainty lower than the other methods (2xa0μgxa0L(-1)). We conclude by revealing the potential and limitations of this approach in the field of ecological lake modelling more and more attracted by TP pristine load inputs in studies on the effects of climate change and eutrophication of lakes.

Impact of Global and Local Pressures on the Ecology of a Medium-Sized Pre-Alpine Lake

Abstract Both global and local anthropogenic pressures influence the process of lake eutrophication. The evaluation of their relative contribution requires long-term observation and articulated methodologies. An integrated lake and catchment modeling was performed on a medium-sized phosphorus-limited lake (Lake Pusiano, North Italy). Pristine conditions were simulated coupling model outcomes to paleolimnological analysis, while a specific statistical technique (Spectral Singular Analysis) was used to flatten the air temperature increasing trend between 1960 and 2010. Four different scenarios were simulated to characterize different degrees of local and global pressures on the lake ecological responses.

Impacts of Climate Change on Water Quality

In this chapter we present the result of two model exercises aiming at simulating the impact of climate change onto two classes of surface aquifers: lakes and rivers. Section 10.1 focuses on the impact of global warming on the thermal structure of two Italian South alpine lakes: Lake Como and Pusiano. Long term hydrodynamic simulations (1953–2050) were performed using the hydrodynamic model DYRESM (Dynamic Reservoir Simulation Model). DYRESM simulations were forced with downscaled regional climate scenarios undertaken within CIRCE. Our model simulations projected a yearly average temperature increase of 0.04°C year−1 for the period 1970–2000 and 0.03°C year−1 for the period 2001–2050 (A1b IPCC scenario). These results are in line with those detected in long term research studies carried out world-wide. This temperature increase is first responsible for a general increase of the water column stability and for a reduction of the mass transfer between deep and surface waters with direct implications on the oxygen and nutrient cycles. The magnitude of the temperature increase is also sufficient to impact on the growth of phytoplankton populations and it is likely one of the concurrent causes promoting the massive cyanobacteria blooms, recently detected in the two Italian case studies and in different lake environments in Europe. Section 10.2 approaches the problem of establishing a methodology to estimate the average yearly nutrient (phosphorus and nitrogen) river loads under present climate conditions and under the forcing of climate change. The case study is the Po River the largest hydrological basin in Italy and the third tributary of the Mediterranean semi-enclosed basin. The methodology developed in this study is based on a hierarchy of different numerical models which allowed to feed the MONERIS model (MOdeling Nutrient Emissions into River System) with the necessary meteorological and hydrological forcing. MONERIS was previously calibrated (1990–1995) and validated (1996–2000) under past conditions and then run under current conditions to define a control experiment (CE). Current nutrient loads have been estimated in 170,000 and 8,000 t year−1 respectively for nitrogen and phosphorus. Approximately 70% of the nitrogen load is from diffuse sources while 65% of the phosphorus load originates from point sources. Nutrient loads projections at 2100 (under different IPCC scenarios) allowed to estimate that both nitrogen and phosphorus loads are strictly dependent on the resident population which is responsible of a 61 and 41% increase respectively for nitrogen and phosphorus. Projected nutrient load variations were found to be negligible when holding the resident population constant. Finally the phosphorus load is markedly influenced by the efficiency of the waste water treatment plants (WWTPs).

Remediation of internal phosphorus loads with modified clays, influence of fluvial suspended particulate matter and response of the benthic macroinvertebrate community

Clay-based phosphorus (P) sorbents have been increasingly used as geoengineering materials for the management sediment-derived internal P loading in eutrophic lakes. However, the long-term behavior of these sorbents has remained elusive along with their response to burial under suspended particulate matter (SPM), and their effect on macroinvertebrate communities occupying dynamic regions at the sediment-water interface of shallow and turbid lakes. In this study, field mesocosm experiments were undertaken in Lake Chaohu, China, to study the effects of the application of lanthanum-modified bentonite (LMB) and thermally-modified calcium-rich attapulgite (TCAP) on sediment internal P loading and to assess their influence on macroinvertebrate community structure. A complementary laboratory core incubation study was also undertaken to investigate the effects of SPM deposition on LMB and TCAP performance. In the field, both LMB and TCAP effectively intercepted P released from sediment for up to five months. A P fractionation analysis indicated that LMB and TCAP application results in a substantial increase in inert P fractions in sediment. Laboratory studies indicated that deposition of SPM may increase in mobile P both in the upper sediment and across the new post-SPM deposition sediment-water interface. Importantly, a comparison of sediment chemical extractions and estimated P fluxes suggests that chemically-defined forms of P in the sediment may be used as a proxy to estimate the net sediment P flux. Significantly, the surficial application of either LMB or TCAP did not cause negative effects on macroinvertebrate communities. This study indicates that to sustain a low P flux across the sediment-water interface in shallow, turbid lakes, repeat dosing of geoengineering materials, temporally aligned to the deposition of fluvial SPM, may be required.

Restoring lakes through external phosphorus load reduction: the case of Lake Pusiano (Southern Alps)

Abstract We estimated the external phosphorus load (EPL) between 1960 and 2015 in Lake Pusiano, a mid-sized subalpine lake that reached its maximum trophic state in the mid-1980s. Using historical data we also estimated the internal phosphorus load (IPL) between 1972 and 2015. EPL reached its maximum in the mid-1980s with phosphorus (P) values ~21 t yr−1 compared to the current value close to 6 t yr−1. IPL was one order of magnitude less and ranged between 0.25 t yr−1 (2015) and 3.6 (1985) t yr−1. The strong reduction of the P load determined a marked decrease of both P (from 200 to 23 μg L−1) and chlorophyll a (from 18 to 8 μg L−1) in-lake concentrations. The process of eutrophication and subsequent recovery, however, showed hysteresis between P load and in-lake P concentrations. In recent years, in particular, the P concentrations at winter overturn seem to be independent from the P load, related to modifications in the hydrological management of the lake that favored P flush-out in October–November, when the EPL is maximal. This process led to a marked decrease of the total P concentrations at winter overturn (23 μg L−1), which recently resulted in lower-than-target concentrations (30 μg L−1) established by the Lombardy Region. The lake, nevertheless, suffers from the presence of the toxic cyanobacterium Planktothrix rubescens, and eradicating this species and further improving water quality will likely require an additional abatement of the EPL.

Endogenous origin of foams in lakes: a long-term analysis for Lake Maggiore (northern Italy)

The formation of foams on lakes is a complex phenomenon whose origin is often hardly identifiable. Recently (2007, 2008, and 2010) foam episodes started to occur in Lake Maggiore, northern Italy. The present work aimed to verify the hypothesis of an endogenous-natural origin of these foams, driven by trophic or climatic changes. To this purpose, a long-term (2000–2013) analysis of phytoplankton biovolumes, meteorological, and hydrological data has been performed together with the chemical characterization of foams. Foams resulted of endogenous origin and likely related to phytoplankton biomass degradation. Data analysis highlighted atypical warm temperature and residual lake stratification in winter in two of the three years of foam events, coupled with exceptional Bacyllariophyceae blooms in spring. Tabellaria flocculosa mostly contributed in terms of biomass in 2007 and 2008, but not in 2010; thus overall algal biomass seemed a better predictor of the risk of foam formation. Foam events occurred from July to December, driven by atypically windy conditions, and congruently with the time needed to degrade biomass into surface-active compounds. A co-occurrence of different factors resulted essential to generate foams, and climate changes likely contribute to enhance their occurrence in Lake Maggiore.