Mathematical model for the ecosystem response of Lake Ladoga to phosphorus loading

The ecosystem response model described in this paper combines an ecosystem model and a three-dimensional circulation model of Lake Ladoga developed earlier by the authors. The ecosystem model describes the process of Lake Ladoga eutrophication, and its biological submodel describes changes in the phyto- and zooplankton. In the earlier model version, lake circulation was determined using a two-dimensional hydrodynamical model which was not completely adequate. The present model allows calculation of the distributions of phyto- and zooplankton and mineral phosphorus and nitrogen. One of its main advantages is that reliable computations of the ecosystem dynamics over an extended period of time are possible. The response of the ecosystem to different levels of phosphorus pollution loading and to weather conditions is studied.     

Seasonality and trends in the Secchi disk transparency of Lake Ladoga

Data on Secchi disk transparency in Lake Ladoga, the largest lake in Europe, collected between 1905 and 2003, were used to detect climatic (interannual) trends for lake regions with various depths. The seasonal variations in Secchi depth (D _s) during the ice-free period both for limnetic regions with large differences in bathymetric characteristics and for the whole lake were estimated by more than 7000 transparency measurements. The two-dimensional data sets have a spatial resolution of approximately 20 km and are geo-referenced by latitude and longitude in Lake Ladoga. Monthly mean spatial transparency distributions and their variances were calculated from May to October. The spatial distributions of the transparency for each month are discussed within the context of lake bathymetric patterns. The maximum values of Secchi depth (more than 4 m) occur during May and October in deep regions. Both the minimum mean value of water transparency and minimum horizontal gradients of D _s for the lake occur in August. The regions with significant interannual (climatic) decreasing trends of D _s have been identified. These areas increase in summertime and cover approximately half the lake area. In spring and autumn the areas decrease and occur in the southern near-shore regions. The mean downward climatic trend of water transparency in Lake Ladoga is 0.02 m/year.     

Distribution and summer diapause of Canthocamptus staphylinus (Jurine) (Copepoda: Harpacticoida) in Lake Ladoga

The spatial distribution of Canthocamptus staphylinus (Jurine) in Lake Ladoga was examined. C. staphylinus is one of the most numerous species of harpacticoids in the lake. It can dwell in almost all littoral biotopes. In profundal biotopes strongly exposed to anthropogenic influence (pollution and eutrophication) only this species of harpacticoid was found. A peculiarity of C. staphylinus biology is that in summer individuals are in the resting stage as cysts. In summer active individuals were found in littoral habitats at a water temperature of 20 °C, but their number was small in comparison with cold periods of the year. In the littoral zone the relative number of individuals in cysts was clearly related to water temperature. Twelve degrees temperature appears to be critical for development of littoral populations of C. staphylinus.     

Bottom sediments and biocoenoses of northern Ladoga and their changes under human impact

Silty and clayey sediments predominate in the bottom sediments of the deep basin of northern Ladoga. Chemical fractionation of organic constituents of the sediments indicate prevalence of chemically stable fractions in most samples. The sedimentary diatom assemblages contain large numbers of littoral taxa, but more than 90% of the total counts of frustules belong to planktonic forms. The macro- and meiozoobenthic communities are characterized by a limited number of species, and the biomasses are low as a result of low temperatures and the small amount of organic matter entering the sediments. There are several restricted areas, e.g. shallow bays, in which pollution by pulp and paper mill or sewage effluents has led into significant changes in the benthic conditions, including increased organic content and heavy metal concentrations of the sediments. There is no apparent change in radioisotope activity, and maximum values do not exceed the natural activity of Karelian granites. Benthic biocoenoses have been affected especially near the sources of anthropogenic pollution, where extensive lifeless areas, polysaprobic zones, and morphological deformities of invertebrates have been observed. Some definite changes have taken place even in the deep profundal benthic communities (e.g. decline of Pallasea quadrispinosa and Gammaracanthus loricatus, increase of Potamothrix hammoniensis and Limnodrilus spp.) and sedimentary diatom assemblages in the open lake (increase of Diatoma elongatum, Asterionella formosa and Aulacoseira italica) since the 1960's. Since the closure of two of the pulp mills and diversion of effluents at a third site, the situation has locally improved, especially at those sites where water exchange is effective.     

Distribution and release of sedimentary phosphorus in Lake Ladoga

Sedimentary phosphorus fractions and phosphorus release from the sediments were studied in Lake Ladoga at altogether 46 sampling sites, representing the full range of sediment types encountered in the lake. Determination of P fractions and physico-chemical analyses were made of surface sediment cores (10–20 cm long, each sampled at 3–4 levels) and in the overlying water. The range of total phosphorus per dry weight of sediment was 0.2–3.3 mg g^–1, and that of inorganic P 0.1–2.5 mg g^–1. The levels of interstitial soluble phosphorus, range 2–613 µg 1^–1 for total P and 1–315 µg 1^–1 for inorganic P, were higher than those of dissolved P concentrations in the overlying water. Diffusive fluxes of phosphate from sediment to the overlying water were estimated using three independent methods. The estimated range was 4–914 µg P m^–2 d^–1; the mean value for the whole bottom area, 0.1 mg P m^–2 d^–1, is lower than previously published estimates. The estimated annual contribution of sedimentary inorganic P flux to Lake Ladoga water is equal to 620 tons of P per year, which amounts to more than 10% of the estimated external P load into the lake. 68% of the total diffusive flux emanates from deep water sediments, which are not exposed to seasonal variation of conditions. In deep lakes, such as Lake Ladoga, phosphorus release from the sediments is controlled primarily by diffusive mechanisms. Wave action and currents as well as bioturbation are probably of importance mainly in shallow near-shore areas. Phosphorus release by gas ebullition and macrophytes is considered negligible.     

Comparison of methods used in zooplankton sampling and counting in the joint Russian-Finnish evaluation of the trophic state of Lake Ladoga

The results of a joint Russian-Finnish investigation on zooplankton in Lake Ladoga are presented, and a comparison is made between two sampling techniques, tube sampler and plankton net, and between two counting methods. The precision of subsampling and sampling is further discussed on the basis of zooplankton data gathered in Lake Saimaa, Finland. The comparison clearly indicates that a tube sampler is required for reliable sampling of small-sized animals, while a plankton net saves time and is a more economical sampler for large, rare or active animals. The comparison between the results obtained by Finnish and Russian workers, using different counting procedures, shows that the main groups of crustacean zooplankton are similarly counted and identified in the two laboratories.     

Invertebrate communities associated with macrophytes in Lake Ladoga: effects of environmental factors

Littoral invertebrate communities (meio- and macrobenthos and zooplankton) were studied in seven types of macrophyte associations commonly encountered in Lake Ladoga: in reed (Phragmites) beds on sand, soft and hard bottoms, in associations with the prevalence of Potamogeton spp., Carex spp., and Equisetum fluviatile, and in diverse vegetation stands with e.g. Polygonum amphibium, Cicuta virosa, Typha latifolia and Eleocharis acicularis. Some of the studied habitats were affected by sewage pollution, others were in comparatively undisturbed areas. Statistically significant differences between invertebrate communities in the different macrophyte associations were found. In stepwise multiple linear regression analysis the following factors were identified as determinants of abundance of aquatic invertebrates in macrophyte associations: shoot density, plant dry weight biomass, periphyton biomass, periphyton chlorophyll a, periphyton primary production, and concentrations of Sr, Mg, Ca, P, Mn, Zn, Pb and Cu. Pollution was shown to have a minor effect on the composition of littoral invertebrate communities. It is not possible to determine one single principle factor responsible for the structure and density of invertebrates in macrophyte communities.     

Water chemistry of Lake Ladoga and Russian-Finnish intercalibration of analyses

As part of the Russian-Finnish research studies on Lake Ladoga, joint expeditions were organized in 1992 and 1993. Water samples were collected for intercalibration of chemical analysis methods and to monitor the chemical quality of the lake water.In August of 1992 water samples were taken from northern Lake Ladoga for intercalibration of Russian and Finnish analysis methods. In August 1993 water samples were collected from 23 sampling stations in all parts of the lake; some of these were also used for intercalibration purposes.The oxygen, colour and COD_Mn results were at the same level in the intercalibration. In 1993, the P_tot results obtained were acceptable. In N_tot, Fe and Mn analysis there seemed to be systematic and random errors between some results.The Secchi depth ranged from 1.5 m to 3.3 m. The average concentrations for the total phosphorus ranged from 15 µg 1^–1 to 29 µg 1^–1. The total nitrogen values were from 620 µg 1^–1 to 690 µg 1^–1. The N:P ratio varied from 24 to 40. The concentration of phosphorus indicated mesotrophic or even eutrophic conditions in the lake. Phosphorus seemed to be the limiting nutrient to bacteria and algae.     

Protein indication method in monitoring of pulp mill effluent pollution in Lake Ladoga

The paper presents application of the protein indication method to studies of pollution of Lake Ladoga in the area affected by effluent discharge of the Pitkäranta pulp mill.Analysis of proteins gives new insight into the causes of water quality deterioration in stagnant zones near the mill discharge outlet at Pusunsaari Island. Anomalies in protein concentration coincide spatially with hydrochemical anomalies. Protein anomalies indicate the process of bacterial decomposition of wood fibre accumulated in the stagnant waters, which in turn is reflected in the chemical water quality parameters in the polluted zone.     

Land-use history of Riekkalansaari Island in the northern archipelago of Lake Ladoga,Karelian Republic,Russia

Agricultural history was investigated by means of pollen and charcoal analyses from the sediment of Lake Kirjavalampi in the Riekkalansaari Island, in the northern archipelago of Lake Ladoga, NW Russia (61°44N, 30°46E). Pollen and charcoal stratigraphies, and loss-on-ignition were analysed from a 0–294-cm profile cored from the deepest part of the small lake. The pollen profile was divided into six local pollen assemblage zones Kir 1–6 and dated by three radiocarbon samples. Lake Kirjavalampi was isolated from Lake Ladoga between 1460–1300 b.c., when the River Neva was formed as a new outlet for Lake Ladoga and the water level rapidly fell. The isolation is seen as a phase of rapid sedimentation in Kir 2 (237–173 cm). Spruce (Picea) starts to decline at 113 cm ca. a.d. 70, and the earliest cereal (Secale cereale) pollen was encountered at the 97-cm level, empirically dating the onset of cultivation to ca. a.d. 600. A marked intensification in agricultural activities occurs around a.d. 1200, and the indication of an open cultivated landscape is at its strongest during the time period 1700 to 1850.     

Ecologic and hygienic evaluation of Lake Ladoga as a source of drinking water

Eutrophication and pollution of Lake Ladoga cause epidemiologic and toxicologic risks for its use as a supply of drinking water. Increased levels of nutrients (N and P) and low molecular weight organic compounds enhance microbial activity and transformations of xenobiotic compounds in the lake, and promote the formation of chlorinated compounds in drinking water purification process. Experimental studies on the toxicity of water and sediment samples from Priozersk, Pitkärranta, Petrokrepost and Volkhov Bay areas have resulted in marked blood, immune system and genetic responses in laboratory rats. Severe toxicity is also evidenced by Daphnia biotests. Epidemiological studies have revealed elevated morbidity and mortality levels in the human populations in certain regions in the environs of Lake Ladoga, with respect to a group of diseases with a potential connection with water quality, i.e. diseases of digestive organs and genitourinary system as well as malignant neoplasms.     

Establishment of the Baikalian endemic amphipod Gmelinoides fasciatus Stebb. in Lake Ladoga

Gmelinoides fasciatus Stebb., a small amphipod from the Lake Baikal basin, was discovered in July 1988 in Lake Ladoga, the largest European lake. G. fasciatus likely invaded Lake Ladoga as a consequence of its intentional introduction, aimed at enhancing fish production, in some Karelian Isthmus lakes close to Lake Ladoga's western shore in the early 1970's. Benthos studies conducted in 1989 and 1990 revealed that G. fasciatus was well established in littoral communities along the western and northern shores of Lake Ladoga. G. fasciatus was the dominant species in these littoral communities and contributed over 70% of the macroinvertebrate biomass. The species was abundant in different macrophyte beds and stony littoral areas, both in heavily polluted and undisturbed sites. The maximum abundance and biomass of G. fasciatus was about 54,000 ind m^–2 and 160 g (wet wt.) m^–2. Negative impacts of G. fasciatus on native species have been observed. Studies are needed to identify the effects of G. fasciatus on the functioning of littoral communities and to predict and control the spread of this amphipod.     

Mathematical modelling as a tool for management in eutrophication control of shallow lakes

The eutrophication model Delwaq-Bloom-Switch is developed to be a functional tool for water management. Therefore it includes nutrients, algal biomass and composition as well as water transparency. A module describing the interaction between water and bottom gives the model the flexibility to deal with measures, such as a decrease of the external phosphorus loading and flushing with water differing in composition from the lake water. This paper focuses on the functional aspects of the model, the results of an application on Lake Veluwe, The Netherlands, and the implications for water management.With one set of coefficients DBS reproduces the most important characteristics of Lake Veluwe for a period of two years before measures (reduction of the external loading and flushing during the winter months) and eight years after the measures. The phosphorus concentration decreased and became growth limiting for algae instead of nitrogen and light. Both in measurements and modelling results, the algal composition changed from blue-green algae dominance to green algae and diatom dominance. Lake Veluwe had a relatively short transient phase after reduction of external loading, because high nitrate concentrations in the flushing water inhibited a long period with high phosphorus releases from the bottom.Model calculations were carried out to investigate the effects of fish stock management and optimization of flushing. Both measures are promising.     

Combined use of spectral brightness and polarization characteristics of upward radiation in remote sensing of inland waterbodies

In freshwater bodies, the presence of considerable amounts of optically active components with a high changeability of qualities makes the interpretation of remote sensing measurements difficult, when these are based only on the upward radiation brightness characteristics. A higher information content and reliability of measurements in such cases may be ensured by means of a combined use of both the brightness and degree of polarization of the upwelling radiation. An experimental test of this approach was made in Lake Onega. The effective wavelength, obtained from the spectral distribution of the brightness coefficient, was considered a brightness characteristic. Water quality indices were determined with the help of multiple regression equations. The obtained correlation coefficient for relative water transparency is R=0.78 ± 0.06 and the regression level average quadratic error is S _z = ± 0.32 m.     

3D-numerical modelling of cohesive suspended sediment in the Western Scheldt estuary (The Netherlands)

A cohesive sediment transport model considering the effects of flocculation, deposition and erosion is used in an attempt to simulate the suspended sediment distribution in a mesotidal estuary. The numerical model solves the three-dimensional (3D) advection-diffusion equation using a two-time level scheme, and a semi-implicit finite difference approach. The transport model is coupled to a 3D-barotropic hydrodynamic model for the simulation of the major tidal components reproducing the non-linear effects. An application of these models in the Western Scheldt estuary is described. The results of the different tests show that the adopted approach provides a useful basis for a good understanding of the physical processes involved in sediment transport and for the study of practical problems. The sensitivity of the model to key parameters controlling the simulation of bed sediment/water exchanges, shows the importance of a good definition of bottom sediment characteristics and the importance of further development of a consolidation algorithm.     

Numerical study on amelioration of water quality in Lakes Shinji and Nakaumi: a coastal brackish lagoon system

Aiming at estimating the effect of an amelioration scheme for water quality in a highly eutrophicated and strongly enclosed estuary, especially placing emphasis on the alleviation of a oxygen-depleted water body, a three-dimensional (3-D) time-dependent coupled physical and ecological model was applied to Lakes Shinji and Nakaumi, a typical coastal brackish lagoon system in Japan. First, the model was run to reproduce the seasonal variation in water temperature, salinity, dissolved oxygen, and other ecological constituents during the period from April 1998 to March 1999. The daily change regimes of the observed sea surface winds, tidal elevation at the open boundary, river discharges, and meteorological parameters were incorporated into the forcing variables of the ecosystem model. Next, as a numerical experiment, a scenario simulation was carried out under four premises to alleviate the oxygen-depleted water body. The model results agreed fairly well with the field measurements obtained through the regular environmental monitoring in the lagoon system. It was found that an oxygen-depleted water body appears in the eastern part of Lake Nakaumi in early May and develops to cover the whole lagoon in midsummer to leave it almost anoxic. The scenario simulation revealed that each amelioration scheme has a considerable effect in alleviating this hypoxic water body. It was found from the model results and the additional flux analyses that the oxygen concentration will improve as a result of (1) change in the flow field by cutting the dikes built up during lake reclamation and (2) decrease in benthic oxygen consumption by filling up deep holes once created by dredging activities. An erratum to this article is available at .     

Zooplankton characteristics in monitoring of Lake Ladoga

Water lettuce (Pistia stratiotes L. (Araceae)) is alien to Africa and a declared weed in South Africa. In large perennial rivers it is effectively controlled by its biological control agent, Neohydronomus affinis (Coleoptera: Curculionidae). On those shallow isolated water bodies which are regularly subjected to alternate wet and dry regimes that become infested with water lettuce, chemical control is necessary to prevent further spread of the weed and to facilitate access to water. This paper discusses the short-term chemical control and the long-term biological control of water lettuce. The need for further research is outlined.     

Lakes Hula and Agmon: destruction and creation of wetland ecosystems in northern Israel

A portion of the former Lake Hula wetland (northern Israel) was re-flooded in spring 1994 and the physical, chemical and biological developments within the resulting new lake and wetland complex (Agmon) was followed closely by a multi-disciplinary scientific team. The first three years of study relating to Lake Agmon are reported in this issue of Wetlands Ecology & Management. We provide in this paper a general background on the Lake Hula Draining Project in the 1950s and the recent re-flooding and creation of the Agmon wetland.     

Density structure of the fine surface water layer of large lakes and remote measurement of the temperature

Remote sensing of surface temperatures in large lakes differs in some critical respects from that in marine conditions. Firstly, the atmospheric structure over large lakes has some continental features, which makes the calculation of its influence more complicated. Secondly, the vertical temperature structure in the fine surface layer of a lake is more complex than in marine waters. A prominent feature of the thermal structure of spring heating of large lakes (e.g. in Lake Ladoga from May to July) is the existence of a thermal front, which is manifested on the lake surface as a fine strip with steep horizontal temperature gradient. The existence of a thermal front makes it possible to calibrate infra-red space survey data without using in situ surface temperature measurements. The calibration of arbitrary heat brightness values for +4 °C can be obtained by plotting a frequency distribution histogram of the heat brightness values obtained for the IR-image pixels. While the thermal front is in existence, the heat brightness frequency distribution appears bimodal, and the brightness value at the minimum between the peaks is assigned the temperature of +4 °C. Laboratory and field experiments have enabled us to separate two types of near-surface density structures. Under conditions of density instability in the fine surface layer, microconvection limits the magnitude of vertical temperature difference in this layer to a constant range of 0.6–0.8 °C. However, when a stable density structure prevails during calm conditions, much greater vertical temperature differences may exist.