Viktor R. Tóth

Evaluating Sentinel-2 for Lakeshore Habitat Mapping Based on Airborne Hyperspectral Data

Monitoring of lakeshore ecosystems requires fine-scale information to account for the high biodiversity typically encountered in the land-water ecotone. Sentinel-2 is a satellite with high spatial and spectral resolution and improved revisiting frequency and is expected to have significant potential for habitat mapping and classification of complex lakeshore ecosystems. In this context, investigations of the capabilities of Sentinel-2 in regard to the spatial and spectral dimensions are needed to assess its potential and the quality of the expected output. This study presents the first simulation of the high spatial resolution (i.e., 10 m and 20 m) bands of Sentinel-2 for lakeshore mapping, based on the satellite’s Spectral Response Function and hyperspectral airborne data collected over Lake Balaton, Hungary in August 2010. A comparison of supervised classifications of the simulated products is presented and the information loss from spectral aggregation and spatial upscaling in the context of lakeshore vegetation classification is discussed. We conclude that Sentinel-2 imagery has a strong potential for monitoring fine-scale habitats, such as reed beds.

Ultraviolet Fluorescence LiDAR (UFL) as a Measurement Tool for Water Quality Parameters in Turbid Lake Conditions

Despite longstanding contributions to oceanography, similar use of fluorescence light detection and ranging (LiDAR) in lake settings is not routine. The potential for ship-mounted, multispectral Ultraviolet Fluorescence LiDAR (UFL) to provide rapid, high-resolution data in variably turbid and productive lake conditions are investigated here through a series of laboratory tank and field measurements carried out on Lake Balaton, Hungary. UFL data, calibrated empirically to a set of coinciding conventionally-analyzed samples, provide simultaneous estimates of three important parameters-chlorophyll a(chla), total suspended matter (TSM) and colored dissolved organic matter (CDOM). Successful UFL retrievals from both laboratory and field measurements were achieved for chla (0.01–378 mg∙m−3; R = 0.83–0.92), TSM (0.1–130 g∙m−3; R = 0.90–0.96) and CDOM (0.003–0.125 aCDOM(440); R = 0.80–0.97). Fluorescence emission at 685 nm is shown through tank measurements to display robust but distinct relationships with chla concentration for the two cultured algae species investigated (cyanobacteria, Cylindrospermopsis raciborskii, and chlorophyta, Scenedesmus armatus). The ratio between fluorescence emissions measured at 650 nm, related to the phycocyanin fluorescence maximum, to that at 685 nm is demonstrated to effectively distinguish these two species. Validation through both laboratory measurements and field measurements confirmed that site specific calibration is necessary. This study presents the first known assessment and application of ship-mounted fluorescence LiDAR in freshwater lake conditions and demonstrates the use of UFL in measuring important water quality parameters despite the more complicated hydro-optic conditions of inland waters.

Ground truth data on chlorophyll-a, chromophoric dissolved organic matter and suspended sediment concentrations in the upper water layer as obtained by LIF lidar at high spatial resolution

ABSTRACT This article is based on field measurements on the lake Balaton (Hungary) during the three days: 10, 11, and 12 September 2008. The expedition was performed with the aim to test recently developed ultraviolet (UV) fluorescent portable lidar UFL-8 in natural lake waters and to validate it by contact conventional measurements. We had opportunity to compare our results with the Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra spectroradiometer satellite images received at the satellite monitoring station of the Eötvös Loránd University (Budapest, Hungary) to make an attempt of lidar calibration of satellite medium-resolution bands data. Water quality parameters were surveyed with the help of UFL lidar in a time interval very close to the satellite overpass. High resolution maps of the chlorophyll-a, chromophoric dissolved organic matter and total suspended sediments spatial distributions were obtained.

Reed stands during different water level periods: physico-chemical properties of the sediment and growth of Phragmites australis of Lake Balaton

Water level fluctuations play a vital role in regulating macrophytes of shallow lakes. Morphology and growth dynamics of Phragmites australis, together with physico-chemical parameters of the sediment, were studied at stable (not degrading) and die-back (degrading) sites of Lake Balaton over an 8-year period that included low and average water levels. Lower water level increased plant density and green leaf number, positively affecting photosynthetically available leaf area. Nevertheless, rhizome carbohydrate content was not influenced by water level decrease. The physico-chemical parameters of the sediment did not vary greatly, although the nitrogen and phosphorus content and the midsummer redox potential of the sediment were higher at the low-water period. During the transition from average to low water levels, the sediment shifted from severely anoxic to poorly oxidised levels, with more favourable nutrient content, while the amount of ammonia and sulphides decreased, too. It was shown that lowering water levels could act on plants via increased redox potential of the sediment and could counteract the die-back of Phragmites, suggesting the effectiveness of water level decrease as a management practice to counter reed die-back.

Morphological and photosynthetic acclimation of Potamogeton perfoliatus to different environments in Lake Balaton

Comparative significance and synchronicity of morphological and photosynthetic adjustments of Potamogeton perfoliatus to shore-specific environments were examined on plants growing at the maximum depth of colonisation of the northern and southern shores of Lake Balaton. The shore-specific environments did not affect photophysiological parameters: the photosynthesis of plants on both shores was high, coupled with low respiration and compensation irradiances. In contrast, morphological and habitual differences between the shores were significant: plants of the shady, northern shore had lighter, but larger leaves, and longer internodes concentrated in the apex of the plants. Thus, photophysiological variability of Potamogeton does not follow its morphological differentiation.

A comparison of airborne hyperspectral-based classifications of emergent wetland vegetation at Lake Balaton, Hungary

ABSTRACT Earth observation has rapidly evolved into a state-of-the-art technology providing new capabilities and a wide variety of sensors; nevertheless, it is still a challenge for practitioners external to a specialized community of experts to select the appropriate sensor, define the imaging mode requirements, and select the optimal classifier or retrieval method for the task at hand. Especially in wetland mapping, studies have relied largely on vegetation indices and hyperspectral data to capture vegetation attributes. In this study, we investigate the capabilities of a concurrently acquired very high spatial resolution airborne hyperspectral and lidar data set at the peak of aquatic vegetation growth in a nature reserve at Lake Balaton, Hungary. The aim was to examine to what degree the different remote-sensing information sources (i.e. visible and near-infrared hyperspectral, vegetation indices and lidar) are contributing to an accurate aquatic vegetation map. The results indicate that de-noised hyperspectral information in the visible and very near-infrared bands (400–1000 nm) is performing most accurately. Inclusion of lidar information, hyperspectral infrared bands (1000–2500 nm), or extracted vegetation indices does not improve the classification accuracy. Experimental results with algorithmic comparisons show that in most cases, the Support Vector Machine classifier provides a better accuracy than the Maximum Likelihood.

Morphological and Genetic Variability of Myriophyllum spicatum in Different Shallow Water Bodies of Hungary

Deteriorating environments in shallow waters place significant pressure on aquatic plants. To estimate the morphological and genetic diversity of submerged macrophytes at 5 sites characterized by varying degrees of human impact, Myriophyllum spicatum L. was studied. Sediment nitrogen concentrations were found to be significantly correlated with the morphological properties of M. spicatum: the site with high sediment nitrogen concentration had 5 to 10 times larger, but rather uniform (low morphological variability) plants, while the site with the least nitrogen in the sediment had smaller M. spicatum plants that were 4 times more variable than the larger plants. Nevertheless, genetic variability of the studied plants showed no response to nutrient loading in the shallow lakes. The adverse environmental conditions therefore seem not to have affected the genetic material of the sampled plants, thus the large morphological variability observed in association with the nitrogen poor sites was predominantly a result of plastic reactions of M. spicatum to environmental conditions.

Contribution of Violaxanthin Cycle to the Stress Tolerance of Semiarid Grassland Species

The de-epoxidation process of violaxanthin to zeaxanthin induced by light was recognized a long time ago (Ref. 1), but it is only a recent achievement that the role of violaxanthin cycle components have been revealed as being decisive in heat dissipation of excess excitation energy in light harvesting complexes with respect to capacity for photochemical utilization (Ref. 2).

The effects of interspecific interactions between bloom forming cyanobacteria and Scenedesmus quadricauda (chlorophyta) on their photophysiology

Eutrophication and enhanced external nutrient loading of lakes and seas are most clearly reflected by increased cyanobacterial blooms, which are often toxic. Freshwater cyanobacteria produce a number of bioactive secondary metabolites, some of which have allelopathic properties, significantly influencing the biological processes of other algae, thereby affecting species composition and succession of the phytoplankton. The goal of this work was to investigate the influence of bloom-forming cyanobacterial exudates on the photophysiology of the green alga Scenedesmus quadricauda by chlorophyll fluorescence analysis. We were able to prove the effect of algal cell-free filtrates on the performance of S. quadricauda and demonstrate for the first time that the freshwater picocyanobacterium Cyanobium gracile has strong negative impact on the coexisting green alga. Neither the cyanotoxin (MYC, CYN and ATX) producing, nor the non-toxic strains showed any systematic effect on the production of S. quadricauda. Various strains of the cyanobacterium Cylindrospermopsis raciborskii inhibited the performance of the green alga independently of their origin. Our results urge further studies for a better understanding of the factors affecting the release of allelopathic compounds and the mechanisms of their effects on target organisms.

Harmonic analysis of Lake Balaton phytoplankton blooms using 9 years of MERIS-derived chlorophyll-A

This study is the first application of harmonic analysis to remotely sensed chlorophyll-a concentration in lakes. Freshwater phytoplankton phenology can track the development of algal blooms in lakes. The dynamics of algal blooms in Lake Balaton is analyzed over 9 years by harmonic analysis of MERIS-derived chlorophyll-a estimates. An analysis of the significance of the individual harmonic terms at each significant frequency provides the amplitudes and phases. Four lake basins are analyzed and phenological indicators are presented. The basins show distinct algal bloom phenologies.