Factors Controlling Phytoplankton Blooms in a Temperate Estuary: Nutrient Limitation and Physical Forcing

A combination of enclosure nutrient enrichment experiments and historical data analysis was used to identify the factors controlling seasonal dynamics and competition of the phytoplankton community in the Curonian lagoon (Southeast Baltic Sea). Experiments using different nutrient (N, P and Si) manipulations were performed in 10-l enclosures for 48 h. Changes in chlorophyll a concentrations, inorganic nutrient concentrations, and plankton cell density were monitored. Results revealed that phytoplankton development in the lagoon is strongly affected by ambient physical factors (wind, temperature). Nutrient limitation, however, also plays an important role in seasonal succession mechanisms showing quite distinct seasonal development patterns. Based on the data, available phytoplankton seasonal succession in the Curonian lagoon could be described as composed by three phases corresponding to different domination and regulatory mechanisms.     

Temporal and spatial patterns of crustacean zooplankton dynamics in a transitional lagoon ecosystem

Patterns and mechanisms of plankton crustacean seasonal succession in the eutrophic freshwater Curonian lagoon (south-eastern Baltic Sea) were analysed on the basis of four-year (1995, 1996, 1998 and 1999) field sampling results. The seasonal crustacean zooplankton succession in the lagoon appears to be the consistent six-stage sequence of four distinct species complexes. Each stage is characterised by its individual species composition and quantitative characteristics. The uniform and periodic pattern of the limnetic zooplankton crustacean successional stages in the lagoon indicates that the seasonal succession of the limnetic zooplankton is not disturbed by unpredictable environmental fluctuations, such as brackish water inflows. Seasonal zooplankton succession is also comparatively uniform at a spatial scale. Not more than two adjacent successional stages were found across the northern part of the lagoon during each of 11 seasonal surveys. Comparison between monthly water residence time and dominant plankton crustacean species life cycle duration points to a more transitory plankton community in spring while in the summer it is not much influenced by lagoon hydrodynamics. Consequently, the Curonian lagoon crustacean community quite closely follows the Plankton Ecology Group (PEG)-described freshwater lake seasonal succession in summer and turns into a lentic-like system in spring and autumn.     

Patterns of benthic oxygen uptake in a hypertrophic lagoon: spatial variability and controlling factors

Water temperature, organic matter quality and quantity and macrofauna activity generally regulate the seasonal evolution of benthic oxygen uptake in coastal areas. We hypothesize that highly productive lagoons can represent an exception in this respect, due to alternating sequences of phytoplankton bloom, dystrophy and collapse events, coupled with water anoxia and azoic sediments. In order to verify this assumption, total oxygen uptake (TOU) and diffusive oxygen uptake (DOU) were determined during the ice-free period of 2009 in the sediments of a hypertrophic basin (the Curonian Lagoon, Baltic Sea). Seasonal measurements were carried out via sediment incubation and microprofiling in littoral and pelagic areas. TOU increased from spring to summer, but it remained elevated also in autumn likely due to accumulation of labile organic matter after algal blooms. TOU and DOU closely agreed in pelagic areas, while at littoral sites TOU exceeded DOU, suggesting temporal or local importance of bioturbating organisms. Water chlorophyll a and oxygen saturation were likely the most important driving factors for benthic respiration. Very limited oxygen penetration (<1?mm) over a 6-month period possibly enhances nitrogen removal via denitrification and reactive phosphorus efflux.     

Hydraulic regime-based zonation scheme of the Curonian Lagoon

The aim of this study was to delineate the spatial zonation of the Curonian Lagoon based on the hydraulic regime and the sediment characteristics. A finite element hydrodynamic model has been applied to the Curonian Lagoon to simulate the circulation patterns for three years. With the help of a transport diffusion model the salinity distribution and the residence times of the Curonian Lagoon have been investigated when forced by river runoff and by wind. The finite element method permitted to follow the details of bathymetry and morphology of the lagoon, describing the areas of special interest with higher resolution. The hydrodynamic model has been validated using in situ water level and salinity measurements. A statistical GIS analysis of the bottom sediment characteristics and the modeled residence times and salinity distribution led to a synthetic hydraulic regime-based zonation scheme. The derived classification scheme is of crucial value for understanding the renewal capacity and biota distribution patterns in the lagoon. Guest editors: A. Razinkovas, Z. R. Gasiūnaitė, J. M. Zaldivar & P. Viaroli European Lagoons and their Watersheds: Function and Biodiversity     

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, NO₃, NH₄, PO₄, 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. Guest editors: A. Razinkovas, Z. R. Gasiūnaitė, J. M. Zaldivar & P. Viaroli European Lagoons and their Watersheds: Function and Biodiversity