Nitrogen uptake and the importance of internal nitrogen loading in Lake Balaton

1. The importance of various forms of nitrogen to the nitrogen supply of phytoplankton has been investigated in the mesotrophic eastern and eutrophic western basin of Lake Balaton.<br /> 2. Uptake rates of ammonium, urea, nitrate and carbon were measured simultaneously. The uptake rates were determined using N-15 and C-14 methodologies, and N-2-fixation was measured using the acetylene-reduction method. The light dependence of uptake was described with an exponential saturation equation and used to calculate surface-related (areal) daily uptake. <br /> 3. The contribution of ammonium, urea and nitrate to the daily nitrogen supply of phytoplankton varied between 11 and 80%, 17 and 73% and 1 and 15%, respectively. N- 2-fixation was negligible in the eastern basin and varied between 5 and 30% in the western region of the lake. The annual external nitrogen load was only 10% of that utilized by algae.<br /> 4. The predominant process supplying nitrogen to the phytoplankton in the lake is the rapid recycling of ammonium and urea in the water column, The importance of the internal nutrient loading is emphasized

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 alpha (Chl alpha) 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 alpha on spectral reflectance and simulated Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) response. The results confirm that Chl alpha 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 alpha concentrations, which would be independent of SSC

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, Europe's 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

Phytoplankton nitrogen demand and the significance of internal and external nitrogen sources in a large shallow lake (Lake Balaton, Hungary)

Since the middle of 1990s the trend of Lake Balaton towards an increasingly trophic status has been reversed, but N2-fixing cyanobacteria are occasionally dominant, endangering water quality in summer. The sources of nitrogen and its uptake by growing phytoplankton were therefore studied. Experiments were carried out on samples collected from the middle of the Eastern (Siófok) and Western (Keszthely) basins between February and October 2001. Ammonium, urea and nitrate uptake and ammonium regeneration were measured in the upper 5-cm layer of sediment using the <sup>15</sup>N-technique. Ammonium was determined by an improved microdiffusion assay. N2 fixation rates were measured by the acetylene-reduction method. Ammonium regeneration rates in the sediment were similar in the two basins. They were relatively low in winter (0.13 and 0.16 μg N cm<sup>−3</sup> day<sup>−1</sup> in the Eastern and Western basin, respectively), increased slowly in the spring (0.38 and 0.45 μg N cm<sup>−3</sup> day<sup>−1</sup>) and peaked in late summer (0.82 and 1.29 μg N cm−3 day−1, respectively). Ammonium uptake was predominant in spring in the Eastern basin and in summer in the Western basin, coincident with the cyanobacterial bloom. The amount of N2 fixed was less than one third of the internal load during summer when external N loading was insignificant. Potentially, the phytoplankton N demand could be supported entirely by the internal N load via ammonium regeneration in the water column and sediment. However, the quantity of N from ammonium regeneration in the upper layer of sediment combined with that from the water column would limit the standing phytoplankton crop in spring in both basins and in late summer in the Western basin, especially when the algal biomass increases suddenly