Arduino Massimo Dattilo

The Spatial Distribution of Optical Properties in the Ultraviolet and Visible in an Aquatic Ecosystem

Abstract In aquatic ecosystems, the UV and visible radiation environment is strongly influenced by variation in the chemical and physical parameters of the ecosystems. In shallow lakes, highly heterogeneous water characteristics produce a wide variety of optical environments. Such ecosystems require analysis approaches that consider a potential variability. In this study, 77 stations were used to characterize the optical properties of a shallow lake (open water surface 54 km2). The vertical attenuation of solar radiation at 305, 313, 320 and 340 nm and at photosynthetically active radiation was measured during the seasonal cycle. Dissolved organic matter (DOM), turbidity, fluorescence, pH, temperature, conductance and dissolved oxygen were simultaneously measured. The spatial variation of the extinction spectra of the dissolved fraction at each sampling station was also measured and analyzed between 270 and 400 nm. The spatial heterogeneity of the lake was examined by determining the distributions of the attenuation coefficients and biooptical parameters at high spatial resolution and describing the distributions in a series of maps. The methodology permitted a quantitative description of the interaction between solar radiation and aquatic ecosystems as well as a spatial classification of the dominating processes within the lake. This included the determination of the role played by DOM loading and changing chemical properties within the lake optical environment.

Chemical and optical phototransformation of dissolved organic matter

Dissolved organic matter represents the main reservoir of organic carbon in most aquatic ecosystems. In the present study, we determined the optical changes and the quantum yields of transient species formation for chromophoric dissolved organic matter (CDOM) samples undergoing photodegradation. The results show that the triplet states (3)CDOM* are potentially key players in CDOM photodegradation and that such transformations are strongly influenced by small differences in CDOM sources and sinks. In contrast, ·OH radicals are very unlikely to play a key role in phototransformation. These results represent an important first step in combining optical and transient species analyses to understand photodegradation processes of dissolved organic matter.

Assessing the optical changes in dissolved organic matter in humic lakes by spectral slope distributions

The impact of photodegradation and mixing processes on the optical properties of dissolved organic matter (DOM) was examined using a distribution of absorption spectral slopes and fluorescence measurements in two Argentine lakes. By examining the variability of the absorption spectral slopes throughout the ultraviolet and visible wavelengths, it was possible to determine which wavelength intervals were most sensitive to dominant loss processes. For DOM photodegradation, results show that increases in the absorption spectral slope between 265 and 305 nm were highly sensitive to increased exposure to solar ultraviolet radiation. A slightly larger wavelength range (265-340 nm) was found to be influenced when both mixing and photodegradation processes were considered, in terms DOM residence time, DOM absorption and UV diffuse attenuation coefficients. This same interval of spectral slopes (265-340 nm) was found to highly correlate with changes in fluorescence emission/excitation in wavelengths that are typically associated with terrestrial humic-like DOM. The identification of specific wavelength intervals, rather than the use of standard wavelength intervals or ratios, improved our ability to identify the dominant dissolved organic matter (humic-like) and major loss mechanisms (photodegradation) in these lakes.

Basin-scale control on the phytoplankton biomass in Lake Victoria, Africa.

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria.

Extensive spatial analysis of the light environment in a subtropical shallow lake, Laguna Iberá, Argentina

AbstractVariations in the attenuation of photosynthetically available radiation were analysed using extensive spatial sampling in two seasons in a subtropical wetland lake. Simultaneously with the attenuation measurements, the principal absorption and scattering components of the water column were also measured. The elevated spatial resolution used in the study allowed the determination of spatially distinct optical water classes within the lake. Changes in dissolved organic matter, phytoplankton and tripton concentrations led to a wide variation in the vertical attenuation coefficients. These changes depended on local characteristics of the ecosystem and time of year. The spatial distribution of the attenuation coefficients was examined in relation to the hydrological and geomorphological characteristics of the littoral area of the lake. The impacts of two small rivers on the light environment and attenuation components are shown. Finally, the resulting model was used to examine the possible impacts of changes in light availability at the lake bottom in relation to recent changes in lake water level.

Chemical characterisation of a new estuarine pollutant (2,4-Dichloro-6-Nitrophenol) and assessment of the acute toxicity of its quinoid form for Artemia salina

It is known that the compound 2,4-dichloro-6-nitrophenol (2,4DC6NP) is formed upon nitration of 2,4-dichlorophenol, which in turn is a transformation intermediate of the herbicide dichlorprop. However, the chemical and spectroscopic characteristics of 2,4DC6NP, as well as its toxicity, are poorly known. This work shows that 2,4DC6NP behaves as a diprotic acid in aqueous solutions, with pKa values of 3.0 ± 0.9 and 4.9 ± 0.5. At pH < 3, 2,4DC6NP would undergo protonation. The absorption spectra suggest that anionic 2,4DC6NP, which prevails at pH > 5 would have an ortho-quinoid structure that is responsible for the absorption peak centred at 428 nm. Considering that 2,4DC6NP has been detected in the brackish lagoons of the Rhône delta (southern France), where its levels are comparable to those of the parent herbicide, it is necessary to examine the possible effects of 2,4DC6NP on the species living in that environment. For this reason, the acute toxicity of the anionic form of 2,4DC6NP was assessed for the brine shrimp Artemia salina, a zooplankton species that lives both in brackish and in saline aquatic environments. The toxicity test yielded a LC20 value of 8 ± 2 mg L−1 and a LC50 value of 18.7 ± 0.8 mg L−1. Such values are safely higher than the maximum detected concentration of 2,4DC6NP in the Rhône delta lagoons. Further studies should be concentrated on the long-term effects of 2,4DC6NP, and in particular on its potential genotoxicity.

Competition for spectral irradiance between epilimnetic optically active dissolved and suspended matter and phytoplankton in the metalimnion. Consequences for limnology and chemistry

In deep lakes, water column stratification isolates the surface water from the deeper bottom layers, creating a three dimensional differentiation of the chemical, physical, biological and optical characteristics of the waters. Chromophoric dissolved organic matter (CDOM) and total suspended solids (TSS) play an important role in the attenuation of ultraviolet and photosynthetically active radiation. In the present analysis of spectral irradiance, we show that the wavelength composition of the metalimnetic visible irradiance was influenced by epilimnetic spatial distribution of CDOM. We found a low occurrence of blue-green photons in the metalimnion where epilimnetic concentrations of CDOM are high. In this field study, the spatial variation of the spectral irradiance in the metalimnion correlates with the observed metalimnetic concentrations of chlorophyll a as well as chlorophyll a : chlorophyll b/c ratios. Dissolved oxygen, pH, and nutrients trends suggest that chlorophyll a concentrations were representative of the phytoplankton biomass and primary production. Thus, metalimnetic changes of spectral irradiance may have a direct impact on primary production and an indirect effect on the spatial trends of pH, dissolved oxygen, and inorganic nutrients in the metalimnion.

Spatial dynamics of chromophoric dissolved organic matter in nearshore waters of Lake Victoria.

The underwater light conditions in the African Great Lakes depend on the complex dynamics of ecological and hydrological forces, and are strongly influenced by local environmental conditions and global biogeochemical cycles. Changes in the optical conditions in these lakes have direct impacts on ecosystem productivity, carbon dynamics and nutrient availability. A central role in the underwater light climate is played by dissolved organic matter which is present in all aquatic ecosystems. The chromophoric fraction of these compounds can mediate ecosystem change through its influence on the attenuation of ultraviolet and PAR radiation, microbial carbon cycling and radiative transfer. In the African Great Lakes, little information is available regarding the dynamics of dissolved organic matter and those sources and sinks which control its presence in the water column. We present an extensive spatial analysis of three major bays on the Kenyan and Ugandan shores of Lake Victoria. We use these data to examine the dynamics of chromophoric dissolved organic matter in different bays and we develop a model to estimate its flow from these bays to the Lake, considering both conservative mixing and photodegradation processes. While some bays release chromophoric dissolved organic matter practically unmodified into the Lake, increased residence time and exposure to solar ultraviolet radiation create conditions where chromophores are lost before entering the open lake.

The microalgae Tetraselmis suecica in mesocosms under different light regimes

Pigment profiles and pheopigment accumulation of the microalga Tetraselmis suecica were studied under different spectral irradiances. Irradiance conditions typical of coastal waters with (‘clear’) and without (‘coloured’) chromophoric dissolved organic matter (C-DOM) were simulated in mesocosm cultures. The lag, growth and stationary phases were found to follow a logistic model, as light limitation controlled growth where maximum carrying capacity and incremental growth were sensitive to irradiance conditions, both being higher in mesocosms with full irradiance conditions (clear sea conditions without attenuation due to C-DOM). Through daily measurements of chlorophylls, carotenoids and pheopigments, it was found that the highest concentrations of pigments occurred in the growth phase: in particular in the clear sea conditions. Lower concentrations of lutein were measured in the coloured mesocosms showing their protective function against photo-oxidation. Pheopigment concentrations increased linearly throughout the experiment, being highest in the mesocosm with clear sea conditions. In general, clear sea conditions, including ultraviolet irradiance, showed the higher production of primary and secondary pigments, demonstrating the high tolerability of T. suecica to a range of solar irradiance conditions.

The optical qualities of shallow wetland lined bays in Lake Victoria

In tropical lakes, the characteristics and dimension of the coastal wetlands can have a strong influence on the quality of the inshore waters. The interaction between littoral wetlands and the open water environment is complex and requires an understanding of the material and energy exchanges between these compartments. In the present analysis, we examine the impact of wetlands on the optical properties (underwater light environment) of two adjacent bays on the Ugandan side of Lake Victoria. We use both irradiance profiles within the water column as well as measurements of the dissolved and particulate fractions of the water column. By introducing a new term to identify that part of the solar spectrum most affected by wetland released dissolved organic matter, it is possible to determine the area of each bay that is influenced by the wetland. Depending on the period of analysis, wetland released chromophoric dissolved organic matter can play a dominating role in the attenuation of UV and visible radiation in the underwater environment.