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.

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.

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.

Variability in photobleaching yields and their related impacts on optical conditions in subtropical lakes.

Changes in the concentration and spectral absorption of chromophoric dissolved organic matter (CDOM) may strongly condition the optical properties of tropical and subtropical water bodies. We examined the spatial distribution of CDOM-related absorption, spectral slope and vertical attenuation of solar radiation in two shallow lakes in the Esteros del Iberá wetland system. In situ measurements were made to examine spatial variations in photobleaching yields in natural lake conditions. The results showed that "fresh" allochthonous CDOM is more susceptible to phototransformations than either "aged" allochthonous organic matter or autochthonous sources, if the distances from sources are considered as proxies for residence time. Based on measured changes in absorption spectral slope in relation to solar ultraviolet irradiance, a model was developed which used CDOM as a non-conservative tracer of water masses. Spatial changes in CDOM absorption within the lake were then used to compare photo related transformations to those associated with conservative mixing.

Satellite-based indices in the analysis of land cover for municipalities in the province of Siena, Italy.

Territorial indicators based on the satellite measured reflected or emitted energy can provide valuable information on the spatial evolution of a territory. Information regarding land use, biomass coverage and radiant temperature were obtained from remotely obtained measurements for each municipality in the Province of Siena. Remotely sensed data were combined into different indices to compare characteristics of land cover between territories and create an information base for continued study. A classification was performed on a municipal level and land use classes were grouped together and a general index of use pressure was created. A vegetation index was used to compare biomass densities. A radiant temperature index was calculated using measure thermal infrared emissions. The results of these analyses allowed researchers to examine the intra and inter-municipal spatial heterogeneity within the Province. A comparison between the results of each index was made and the problems related to the use of such indices were examined.

Estimate of the effects of ultraviolet radiation on the mortality of Artemia franciscana in naupliar and adult stages.

The impact of different doses of artificial ultraviolet (UV) radiation on the growth stages of a marine zooplankton was investigated using laboratory microcosms. Mortality percentages of naupliar and adult samples of Artemia franciscana were recorded in relation to different UV doses (single exposure: 75, 150, 300, 600, 1,200, 2,400, 3,900, 7,800 J m(-2)) at specific observation times after exposure (24, 48, 72, 96 and 120 h). The relationship between mortality percentage and UV dose showed significant differences in relation to the zooplankton growth stage. The elevated susceptibility of the naupliar samples to UV radiation is described through a mortality model based on a logistic equation. The data analysis shows that the slope of mortality versus dose remains the same for the two growth stages while the lethal dose in the naupliar stage was 3.3 smaller than that determined for the adult stage. The slope of the UV mortality rate versus post-incubation time was found to be significantly different (P<0.05) at low UV doses for the two life stages examined, i.e. naupliar and adult. The lower value of LD(50) in naupliar stages compared to that for adults confirms that in the early growth stage this marine zooplankton is more susceptible to UV radiation.

Morphological anomalies in pollen tubes of Actinidia deliciosa (kiwi) exposed to 50 Hz magnetic field.

The role of the pollen grain, with respect to the reproductive process of higher plants, is to deliver the spermatic cells to the embryo sac for egg fertilisation. Delivery occurs through the pollen tube, a self produced organ that is generated when the pollen grain reaches the stigma surface. The effect of magnetic fields on pollen tube growth was reported in a recent publication by Germanà et al. Pollen tube growth is an interesting candidate for the detailed study of the effects of electromagnetic fields on cytoplasmic structures and organelles. In this research Actinidia deliciosa (kiwifruit) pollen grains were germinated in the presence of an alternating magnetic field (50 Hz). Our results, although of preliminary nature, show that pollen tube growth is affected by magnetic fields. The analysis of the observed anomalies in the pollen tube appear to be the result of changes in the ionic charges within the pollen tube cytoplasm.

Analysis of extinction in ultraviolet and visible spectra of water bodies of the Paraguay and Brazil wetlands.

The extinction spectra in ultraviolet and visible radiation were analyzed using filtered and unfiltered water samples obtained in 11 open water bodies in the Neembucù (Paraguay) and Pantanal (Brazil) wetlands. The role of dissolved and suspended matter in the total extinction was analyzed between 260 nm and 700 nm. The chromophoric dissolved organic matter (CDOM) was the major component in extinction of considered ultraviolet radiation (260-400 nm). The differences in CDOM concentrations explained the main pattern of extinction of the ultraviolet radiation in the samples. Nevertheless, differences between the studied water bodies were found also to depend on the rate of photodegradation and photobleaching. The methodology developed in the present study was to distinguish "humic optic waters" according to quantity and quality of dissolved and suspended matter present. In the "humic optic water", the penetration of 10% of incident UV radiation and the photoactive layer are estimated. The influence of particulate matter increases in the total extinction of the wavelengths higher than 400 nm. The integral of the extinction curve of suspended matter in the visible wavelengths (400-700 nm) was found to relate with the total suspended solids and chlorophyll concentrations.