Drivers of Cyanobacterial Blooms in a Hypertrophic Lagoon

The Curonian Lagoon is Europe's largest lagoon and one of the most seriously impacted by harmful blooms of cyanobacteria. Intensive studies over the past 20 years have allowed us to identify the major drivers determining the composition and spatial extent of hyperblooms in this system. We summarize and discuss the main outcomes of these studies and provide an updated, conceptual scheme of the multiple interactions between climatic and hydrologic factors, and their influence on internal and external processes that promote cyanobacterial blooms. Retrospective analysis of remote sensed images demonstrated the variability of blooms in terms of timing, extension and intensity, suggesting that they occur only under specific circumstances. Monthly analysis of nutrient loads and stoichiometry from the principal tributary (Nemunas River) revealed large interannual differences in the delivery of key elements, but summer months were always characterized by a strong dissolved inorganic N (and Si) limitation, that depresses diatoms and favors the dominance of cyanobacteria. Cyanobacteria blooms occurred during high water temperatures, long water residence time and low-wind conditions. The blooms induce transient (night-time) hypoxia, which stimulates the release of iron-bound P, producing a positive feedback for blooms of N-fixing cyanobacteria. Consumer-mediated nutrient recycling by dreissenid mussels, chironomid larvae, cyprinids and large bird colonies, may also affect P availability, but their role as drivers of cyanobacteria blooms is understudied

2D finite element ecological model for the Curonian lagoon

The results of application of 2D finite element model SHYFEM to the Curonian lagoon (Baltic Sea) are considered. SHYFEM consist of a physical processes module and an eutrophication module EUTRO adapted for the SHYFEM code from well known modelling system WASP. The SHYFEM/EUTRO model calibration results were compared with the performance of various biogeochemical models analysed in other studies (153 studies published from 1990 to 2002). The performance of all corresponding state variables-dissolved oxygen, NO(3), NH(4), PO(4), phyto- and zooplankton-was slightly lower than median model performance which could be considered satisfactory given the initial state of model formulation and calibration. Model underestimates phytoplankton autumn blooms, especially for the southern part of the lagoon, where fine sediments dominate and water residence time is high. It can be concluded that, in order to increase model performance, the eutrophication module should be improved to account for the dominance of different phytoplankton groups as well as for the exchanges between the sediments and the water column. The amount and quality of the data available for the model setup and calibration are unsatisfactory and should be improved for the development of the next enhanced model version

Linking Carbon-Nitrogen-Phosphorus Cycle and Foodweb Models of an Estuarine Lagoon Ecosystem

In this study, an NPZD model and a trophic network model that contains organism groups on the higher trophic levels were developed and linked using the “bottom-up control” approach. Such a linkage of models provides the possibility to use the advantages of both models; reproducing of the erratic behaviour of nutrients and plankton as realistic as possible, while still taking the more complex organisms in the trophic network, which respond to external forcing in a larger time scale. The models developed in this study were applied to the Curonian Lagoon that is an important estuarine ecosystem for Lithuania. The tests and simulations have proven that the results of the NPZD model were accurate enough for representing the nutrient and phytoplankton dynamics in the Curonian Lagoon as well as spatial differences which are of ecological interest. Linkage with trophic network model demonstrated NPZD model results to be consistent with the Curonian Lagoons ecosystem. The modelling results showed that primary production is relatively high in the Curonian Lagoon and is unlikely to be controlled by the organisms that are on the higher trophic levels of the food web. Analysis of the NPZD model scenarios with different nutrients inputs revealed that phosphorus is the main limiting nutrient for primary production in the Curonian Lagoon. However, different combinations of nitrogen and phosphorus inputs control the relative abundance of different phytoplankton groups. Investigation of reaction of ecosystem to water temperature increase showed that the temperature increase finally leads to decrease of available phytoplankton to upper levels of the food web.DOI: 10.15181/csat.v3i1.109

Linking Carbon-Nitrogen-Phosphorus Cycle and Foodweb Models of an Estuarine Lagoon Ecosystem

In this study, an NPZD model and a trophic network model that contains organism groups on the higher trophic levels were developed and linked using the “bottom-up control” approach. Such a linkage of models provides the possibility to use the advantages of both models; reproducing of the erratic behaviour of nutrients and plankton as realistic as possible, while still taking the more complex organisms in the trophic network, which respond to external forcing in a larger time scale. The models developed in this study were applied to the Curonian Lagoon that is an important estuarine ecosystem for Lithuania. The tests and simulations have proven that the results of the NPZD model were accurate enough for representing the nutrient and phytoplankton dynamics in the Curonian Lagoon as well as spatial differences which are of ecological interest. Linkage with trophic network model demonstrated NPZD model results to be consistent with the Curonian Lagoons ecosystem. The modelling results showed that primary production is relatively high in the Curonian Lagoon and is unlikely to be controlled by the organisms that are on the higher trophic levels of the food web. Analysis of the NPZD model scenarios with different nutrients inputs revealed that phosphorus is the main limiting nutrient for primary production in the Curonian Lagoon. However, different combinations of nitrogen and phosphorus inputs control the relative abundance of different phytoplankton groups. Investigation of reaction of ecosystem to water temperature increase showed that the temperature increase finally leads to decrease of available phytoplankton to upper levels of the food web.DOI: 10.15181/csat.v3i1.1093</p

Study of ice cover impact on hydrodynamic processes in the Curonian Lagoon through numerical modeling

Summary In this study, we present an analysis of the hydrodynamic processes under ice cover in the largest lagoon in Europe – the Curonian Lagoon. By applying a finite element numerical modelling system (SHYFEM) and remote sensing ice cover data, the residual circulation, water fluxes through specific areas of the lagoon, saltwater intrusions, and water residence time (WRT) were investigated. The results, taken over an 11 year period, show that ice cover affects the circulation patterns in the lagoon by forming and shifting different gyre systems. Different circulation patterns are observed throughout all the meteorological seasons of the year. Since ice decreases circulation, water fluxes also decrease, especially in a cross-section in the middle of the lagoon, where the ice-cover suppressed wind-stress has a higher impact on the water movement rather than it has in the north. The presence of ice cover also decreases the salinity of the water in the northern part of the lagoon. In general, the salinity in the water column-averaged over different periods is vertically uniform, however, a slight increase of salt concentration can be observed at the bottom layers in the Klaipėda Strait, where the difference of >1 PSU between bottom and top layers shows up on average 130 hours per year. The ice cover also decreases the saltwater intrusions into the lagoon by nearly 14 days per year. The increase of WRT is most prominent after long ice cover periods, away from the river inlets, especially in the southern part of the lagoon, where without the help of the wind action, water takes a longer time to renew than in the northern part