Mátyás Présing

Remote sensing of the water quality of shallow lakes: A mixture modelling approach to quantifying phytoplankton in water characterized by high‐suspended sediment

Remote sensing has the potential to provide truly synoptic views of water quality, in particular, the spatial distributions of phytoplankton. Whilst the spectral capabilities of satellites used in ocean colour work have improved significantly over recent years, the application of satellite remote sensing to lake water is constrained by the need for high spatial resolution image data and thus remains limited by spectral resolution capabilities. This becomes a significant problem when attempting to quantify chlorophyll a (Chl a) in waters characterized by high and heterogeneous suspended sediment concentrations (SSC). The SSC dominates the spectral reflectance, masking the spectral influence from other components in broad spectral band systems, making Chl a determination from remote sensing imagery difficult. This paper presents a linear mixture modelling approach to derive accurate estimates of Chl a from Landsat Thematic Mapper (TM) imagery. This approach was tested in Lake Balaton, Europes largest shallow lake characterized by high suspended sediment and, until recently, frequent eutrophic and hypereutrophic episodes. The last significant bloom occurred in September of 2000 and a Landsat TM image was acquired for 11th September, during which ground reference data of water quality was collected. The modelled image‐derived results of Chl a demonstrate an excellent correspondence (r2 = 0.95) between the ground‐based measurements of Chl a, and yield considerable detail of lake phytoplankton distributions. The September 2000 calibration was then successfully applied to a July 1994 Landsat TM image and validated with Chl a data collected coincidently within two days of the image. The comparability between water sample data and image results demonstrates that there is temporal stability and robustness in the approach and calibration described.

Characterizing the spectral reflectance of algae in lake waters with high suspended sediment concentrations

Remote sensing has the potential to provide truly synoptic views of water quality, the assessment of which is known to be affected by suspended sediment, phytoplankton biomass (chlorophyll a (Chl a) concentration) and dissolved organic carbon. These three components also control the spectral reflectance characteristics of waterbodies. This Letter uses in situ reflectance measurements of water in Lake Balaton on three different occasions, supplemented by controlled tank experiments, to characterize the influences of suspended sediment concentrations (SSC) and Chl a on spectral reflectance and simulated Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) response. The results confirm that Chl a cannot be estimated directly from Landsat reflectance data in waters characterized by heterogeneous SSC. However, principal component analysis (PCA) demonstrated that spectrally unique end‐members can be identified, indicating that a spectral linear mixture modelling approach combined with a multivariate regression analysis may be used to provide estimates of Chl a concentrations, which would be independent of SSC.

Size-selective filtration and taxon-specific digestion of plankton algae by silver carp (Hypophthalmichthys molitrix Val.)

A comparison of the composition of natural phytoplanktoncommunities with the gut content of silver carp (Hypophthalmichthys molitrix) showed that this fish speciescan nottake up algae smaller than 10 µm. The species compositionofalgal assemblages in the gut content of the silver carpusuallydiffers significantly from the composition of the >10 µmsizefraction of the natural phytoplankton assemblages. Thisdifferenceis mainly a result of the taxon selective digestion of silvercarp.In vitro experiments with digestive enzymes of thisspeciesresulted in a very fast (1–2 minutes) disintegration ofnon-mucilaginous cyanobacteria. The digestion of diatoms andcryptophytes was also effective, but mucilaginouscyanobacteria andgreen algae (Chlorococcales) proved practicallyindigestible.

Nitrogen uptake and fixation in the cyanobacterium Cylindrospermopsis raciborskii under different nitrogen conditions

Ammonium and nitrate uptake and N2-fixation of the heterocystous cyanoprokaryote Cylindrospermopsis raciborskii was examined in continuous cultures under different nitrogen concentrations and dilution rates using the 15N technique. It was found that at luxury phosphorus supply (5 mg PO4-P l−1) the biomass was similar in all cultures irrespective of the amount and portioning (continuous or pulsed) of available nitrogen forms. The added ammonium and nitrate was fully taken up by C. raciborskii and the remaining nitrogen demand was met by N2-fixation. Different ammonium concentrations (300, 750, 1500 and 3000 μg 15N l−1) added at the same dilution rate did not affect the growth of C. raciborskii. In the culture supplied with pulsed ammonium, N2-fixation was detected prior to ammonium addition only. After the ammonium pulse, the N2-fixation continued for a while then decreased and stopped. In addition, the inflowing ammonium was fully taken up by the organism. The rate of nitrogen fixation reached its original level after 8–24 hours, depending on the dilution rate. It can be suggested that the nitrogen fixation system stopped and was then activated again depending on the nitrogen content of the cells.

Extraction methods for phycocyanin determination in freshwater filamentous cyanobacteria and their application in a shallow lake

Phycocyanin (PC) is one of the water-soluble accessory pigments of cyanobacteria species, and its concentration in aquatic systems is used to estimate the presence and relative abundance of blue-green algae. PC concentration and the PC/Chl-a ratio of four N2-fixing filamentous cyanobacteria strains (Cylindrospermopsis raciborskii, Anabaena spiroides, Aphanizomenon flos-aquae and Aphanizomenon issatschenkoi) common to Lake Balaton (Hungary) were determined using repeated freezing and thawing. A strong linear correlation was found between the extracted PC and Chl-a concentrations for all strains at high Chl-a concentrations (almost stable PC/Chl-a ratio in the range of 20−100 µg l−1 Chl-a). Extraction of PC and Chl-a from samples with low biomass of cyanobacteria (less than 20 µg l−1 Chl-a) proved to be unreliable using the standard protocol of freeze–thaw cycles (coefficients of variation exceeding 10–15%). In order to find an extraction method that is robust in fresh waters characterized by low algae biomass (e.g. Lake Balaton), the effectiveness of four extraction methods (repeated freeze–thaw method and homogenization with mortar and pestle, Ultrasonic, and Polytron homogenizer) were compared using C. raciborskii. It was found that the efficiency of extraction of phycocyanin was highest when a single freeze–thaw cycle was followed by sonication (25% additional yield compared with using the freeze–thaw method alone). Applying this combined method to surface water samples of Lake Balaton, a strong correlation was found between PC concentration and cyanobacterial biomass (R 2 = 0.9436), whilst the repeated freezing–thawing method found no detectable PC content. Here we show that the combined sonication/freeze–thaw method could be suitable for measuring filamentous cyanobacteria PC content, even at low concentrations; as well as for the estimation of cyanobacterial contribution to total biomass in fresh waters.

Some special morphological features of Cylindrospermopsis raciborskii in batch and continuous cultures

Morphological characteristics of Cylindrospermopsis raciborskii were investigated in batch culture under P-starvation and in continuous cultures at two growth rates with different N forms. The species displayed an enormous morphological variability under controlled condition. In the P-starved batch culture very short and extreme morphology trichomes with conical or drop-shape heterocytes were formed. In continuous culture extremely long, twisted filaments were found near the maximum growth rate. Rarely, C. raciborskii formed intercalary heterocytes.

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.

Diversity patterns of trait-based phytoplankton functional groups in two basins of a large, shallow lake (Lake Balaton, Hungary) with different trophic state

The application of functional approaches in understanding phytoplankton community-level responses to changes in the environment has become increasingly widespread in recent years. Eutrophication is known to cause profound modifications in ecosystem processes; however, the underlying relationships between environmental drivers and phytoplankton diversity and functioning are complex and largely unknown. Therefore, in the present study, we investigated and compared the temporal diversity patterns of phytoplankton functional groups in the mesotrophic eastern and eutrophic western basin of the shallow Lake Balaton situated in Hungary. Diversity data were derived from taxonomic composition and biomass data corresponding to the years 2005–2006 and 2008–2009. With the use of cluster analysis, phytoplankton species were classified into eight distinct groups representing different combinations of functionally relevant traits including greatest axial linear dimension; surface-to-volume ratio; photosynthetic pigment composition; N2 fixation; phagotrophic potential; growth form/complexity (unicell, filamentous, colony-, or coenobium-forming); and motility. Our results have revealed that there is a significant inverse relationship between the functional group diversity used in our study and trophic state (total phytoplankton biomass) as opposed to species diversity, where no correlation was observed. In addition, group evenness showed an even stronger negative correlation with trophic state, while species evenness yielded only a weak relationship. The observed variability in functional group diversity suggests that such an approach could provide an efficient means of revealing structural changes in phytoplankton communities, establishing new hypotheses and highlighting fundamental points in ecosystem functioning.

Influence of an insecticide, K-Othrine, on the reproduction and mortality of the pond snail (Lymnaea stagnalis L.)

The toxicity of K-Othrine, an insecticide containing a synthetic pyrethroid derivative (deltamethrin) as an active ingredient, was studied on the pond snail, Lymnaea stagnalis L. This insecticide is used for the control of mosquitoes around Lake Balaton, Hungary.The adult snails were kept in a variety of concentrations (1, 10, and 100 μg/L) of K-Othrine solution (1st, 4th, and 7th weeks) as well as in clean tap water (2–3rd and 5–6th weeks) over a 7-week period. The pesticide treated snails deposited significantly more eggs than those in the control, but the hatched snails showed a survival rate significantly lower than the control values.The snails that were hatched in the insecticide solution and survived were kept in clean water (2nd generation, group I). By the end of the experiment (18th week) the survival rate of the snails was 30% lower than that of the snails hatched and kept in clean water (2nd generation; control, group IV). The number of deposited eggs was 45% less than in the control.The snails hatched in insecticide-solution and treated with K-Othrine similarly to their parents from the 1st till the 7th week (2nd generation, group II) showed a survival rate 35% lower than that of the control. The snails reached maturity with a significant delay, and 73–94% fewer eggs were deposited than in the control.The survival rate of the snails hatched in clean water and kept in K-Othrine solution similarly to group II (2nd generation, group III) was 15–20% lower, and the number of deposited eggs were 73–85% fewer than in the control.The data on survival and reproduction support the hypothesis that the considerable toxic aftereffect of K-Othrine is of a cumulative nature.

Relative nitrogen deficiency without occurrence of nitrogen fixing blue-green algae in a hypertrophic reservoir

The N:P ratios (TN:TP, NO3-N:TP and DIN:SRP) in theshallow,hypertrophic Marcali reservoir varied about 1, far below thephysiological optimum for the planktonic algae. Moreover,N2-fixing cyanobacteria were absent in the vegetationperiod.One possible explanation for this phenomenon could be thefiltration of the stocked silver carp (Hypophthalmichthysmolitrix Val.), a phytoplankton feeder, and bighead carp(Aristichthys nobilis Rich.), an omnivorous plankton feederpopulation (600 kg ha−1) which strongly reduces the largesized algae.Enclosure and uptake experiments were carried out to confirmtherelative N deficiency in the reservoir and, by excluding thefish,to check whether the reservoir water is suitable for thedevelopment of N2-fixing cyanobacteria, or theplanktivorousfish are responsible for the lack of blue-greens. Theenclosureswere free of fish. In one of them the NO3-N enrichmentresulted in a rapid growth of diatoms and cryptophytes,confirmingthe relative N limitation of the phytoplankton biomass. Theresultsof 15N uptake experiments showed a high preference ofalgaefor ammonium. Its actual uptake velocity (19.5 µg l−1h-1) was much higher than that for nitrate (1.0 µgl-1 h−1). But the relatively high value of thehalf-saturation constant (32 µg l−1) in the ammoniumuptake experiment indicated only a moderate affinity of algaeevento the preferred nitrogen source. The other enclosure wasinoculated with N2-fixing cyanobacteria. During the 29daysexperiment they did not grow in the enclosure and weregraduallyeliminated from the water column. The results of enclosureexperiments suggested that factors such as the relatively highabundance of inorganic N (30–50 µg l−1 ammoniumand90–140 µg l−1 nitrate, respectively) could beresponsiblefor the absence of N2-fixing cyanobacteria in thereservoir,rather than the size selective filtration of planktivorousfish.