Urmas Lips

On the estuarine transport reversal in deep layers of the Gulf of Finland

The Gulf of Finland is a 400-km long and 48–135-km wide tributary estuary of the Baltic Sea featuring the longitudinal two-layer estuarine flow modified by transverse circulation. Longitudinal volume transport in the deep layer is investigated by decomposing it into an averaged, slowly changing estuarine component (due to large-scale density gradients, river discharge and mean wind stress) and wind-driven fluctuating component. The derived expression relates the total deep-layer transport to the projection of wind stress fluctuation to a site-specific direction. The relationship is tested and calibrated by the results from numerical experiments carried out with the three-dimensional baroclinic circulation model. For the entrance to the Gulf of Finland, winds from northeast support standard estuarine circulation and winds from southwest work against the density-driven and riverine flow. The deep estuarine transport may be reversed if the southwesterly wind component exceeds the mean value by 4–5.5 m s−1. According to the data from hydrographic observations in the western Gulf of Finland, an event of advective halocline disappearance was documented in August 1998. Comparison of the deep-water transport estimates calculated from the wind data in 1998 with the observed salinity variations showed that the events of rapid decay of estuarine stratification were coherent with the estimated reversals of deep-layer volume transport, i.e. events of salt wedge export from the gulf.

Abiotic factors influencing cyanobacterial bloom development in the Gulf of Finland (Baltic Sea)

Blooms of cyanobacteria are a recurrent phenomenon in the Baltic Sea, including the Gulf of Finland. The spatial extension, duration, intensity and species composition of these blooms varies widely between years. Alg@line data collected regularly from ferries as well as weather service and marine monitoring data from 1997 to 2005 are analysed to determine the main abiotic factors influencing the intensity and species composition of cyanobacterial blooms in the Gulf of Finland. It is demonstrated that the development of the Nodularia spumigena Mertens bloom is highly dependent on weather conditions such as photosynthetically active radiation and water temperature. Nutrient conditions, especially the surplus of phosphorus (according to Redfield ratio) related to the pre-bloom upwelling events in the Gulf, affect the intensity of Aphanizomenon sp. (L.) Ralfs blooms. Differences in bloom timing and duration indicate that, if the preconditions (like nutrient ratio/concentration and weather conditions) for bloom formation are favourable, then the Aphanizomenon bloom starts earlier, the overall bloom period is longer and the Nodularia peak might appear in a wider time window.

Estuarine transport versus vertical movement and mixing of water masses in the Gulf of Finland (Baltic Sea)

Weekly mapping of vertical temperature and salinity fields was carried out across the Gulf of Finland in summer 2006 and spring 2007. Using successive cross-gulf vertical sections of salinity and wind data from the region the variations of estuarine and transverse circulation are described. Changes of deep layer phosphate-phosphorus concentrations are found to be related to the described variations in circulation patterns in a season with strong vertical stratification of the water column. Cumulative volume transport estimates were obtained using the results of a 3D baroclinic circulation model (HIROMB). We suggest that the north-easterly winds, which intensify the estuarine circulation and lead to the upwelling events along the southern coast of the Gulf of Finland, could have a major impact to the Gulfpsilas ecosystem by importing more saline and phosphorus rich waters. These events with many-fold more intense upward movement and mixing of deep waters (upward diapycnal transport) could contribute significantly to the ventilation of deep layers of the northern Baltic proper.

Ferrybox measurements: a tool to study meso-scale processes in the Gulf of Finland (Baltic Sea)

Ferrybox measurements are carried out in the Gulf of Finland (Baltic Sea) in a regular basis since 1997. Routines for data acquisition are developed enabling near real-time data delivery for operational models. Cross-gulf high-resolution temperature, salinity and chlorophyll a fluorescence profiles collected in 2007 are used to describe meso-scale variability of hydrophysical and -biological fields in the gulf. It is shown that higher values of chlorophyll a concentration are more often observed in the coastal areas and in the vicinity of a quasi-permanent salinity front in the central Gulf of Finland.

Redox-specialized bacterioplankton metacommunity in a temperate estuary.

This study explored the spatiotemporal dynamics of the bacterioplankton community composition in the Gulf of Finland (easternmost sub-basin of the Baltic Sea) based on phylogenetic analysis of 16S rDNA sequences acquired from community samples via pyrosequencing. Investigations of bacterioplankton in hydrographically complex systems provide good insight into the strategies by which microbes deal with spatiotemporal hydrographic gradients, as demonstrated by our research. Many ribotypes were closely affiliated with sequences isolated from environments with similar steep physiochemical gradients and/or seasonal changes, including seasonally anoxic estuaries. Hence, one of the main conclusions of this study is that marine ecosystems where oxygen and salinity gradients co-occur can be considered a habitat for a cosmopolitan metacommunity consisting of specialized groups occupying niches universal to such environments throughout the world. These niches revolve around functional capabilities to utilize different electron receptors and donors (including trace metal and single carbon compounds). On the other hand, temporal shifts in the bacterioplankton community composition at the surface layer were mainly connected to the seasonal succession of phytoplankton and the inflow of freshwater species. We also conclude that many relatively abundant populations are indigenous and well-established in the area.

Use of ferrybox measurements for the Baltic Sea environment assessment

The spatially heterogeneous character of aquatic life and rapid changes in pelagic communities make it difficult to assess the status of the marine environment using traditional monitoring methods. In order to distinguish between human induced and natural changes in the ecosystem the environmental parameters and the factors affecting them need to be monitored at a wide range of temporal and spatial scales. The capacity of any single monitoring method or strategy should not be overestimated. Instead, different methods complement each other. In the Baltic Sea high frequency recordings of phytoplankton biomass and related environmental parameters (temperature, salinity, nutrients, chlorophyll a) in the near surface layer (4-5m) have been conducted with unattended water sampling onboard several commercial ferries over the last 14 years. The collected data have been used for delivery of almost on-line information for environmental authorities and public, for developing HELCOM indicator fact-sheets and indexes/maps characterising inter-annual and short-term changes of productivity and phytoplankton biomass in different sea areas, and for forecasts of algae blooms. The combination of high frequency automated sampling onboard merchant ships with satellite imagery, traditional sampling and meteorological information has increased the understanding of ecological processes in the Baltic Sea.

Sub-regional observing and forecast system for the NE Baltic: Needs and first results

Following the operational observing system development under BOOS and application of local stand-alone coastal circulation models, acquisition of HIROMB forecasts started regularly in 2005. The forecasts, working on-line with the operational observations (automatic coastal stations and recently, also FerryBox), proved very useful in numerous cases, especially during storm surges and broadcasting relevant warnings well in advance of critical floodings. For the intermediate and downstream services that require resolving of detailed patterns of mesoscale and coastal circulation, installation and application of 0.5-mile resolution HIROMB started in 2007. The system is integrated with the Baltic-wide HIROMB and is driven by sub-regional HIRLAM weather data with enhanced resolution, forecasted and delivered by the Estonian Meteorological and Hydrological Institute.

Near-Bottom Hypoxia Impacts Dynamics of Bacterioplankton Assemblage throughout Water Column of the Gulf of Finland (Baltic Sea)

Over the past century the spread of hypoxia in the Baltic Sea has been drastic, reaching its ‘arm’ into the easternmost sub-basin, the Gulf of Finland. The hydrographic and climatological properties of the gulf offer a broad suite of discrete niches for microbial communities. The current study explores spatiotemporal dynamics of bacterioplankton community in the Gulf of Finland using massively parallel sequencing of 16S rRNA fragments obtained by amplifying community DNA from spring to autumn period. The presence of redoxcline and drastic seasonal changes make spatiotemporal dynamics of bacterioplankton community composition (BCC) and abundances in such estuary remarkably complex. To the best of our knowledge, this is the first study that analyses spatiotemporal dynamics of BCC in relation to phytoplankton bloom throughout the water column (and redoxcline), not only at the surface layer. We conclude that capability to survive (or benefit from) shifts between oxic and hypoxic conditions is vital adaptation for bacteria to thrive in such environments. Our results contribute to the understanding of emerging patterns in BCCs that occupy hydrographically similar estuaries dispersed all over the world, and we suggest the presence of a global redox- and salinity-driven metacommunity. These results have important implications for understanding long-term ecological and biogeochemical impacts of hypoxia expansion in the Baltic Sea (and similar ecosystems), as well as global biogeography of bacteria specialized inhabiting similar ecosystems.

The Importance of Mesodinium rubrum at Post-Spring Bloom Nutrient and Phytoplankton Dynamics in the Vertically Stratified Baltic Sea

The inter-annual dynamics of the photosynthetic ciliate Mesodinium rubrum in the central Gulf of Finland in spring-summer continuum during five years were followed. The analysis was mainly based on high-resolution measurements and sampling in the surface layer along the ferry route Tallinn-Helsinki. The main purpose was to analyze the dynamics of M. rubrum biomass, its contribution to the photosynthetic plankton biomass, and the influence of water temperature and variations of inorganic nutrients in the surface and sub-surface layer on its dynamics. The analysis revealed that the outcome of the M. rubrum bloom in spring was largely related to the surface layer water temperature – in the years of earlier warming, the higher biomass of this species was formed. The photosynthetic ciliate was an important primary producer in all studied years during the late phase or post-spring bloom period in the Gulf of Finland. The maximum proportion of M. rubrum in the photosynthetic plankton community was estimated up to 88% in May and up to 91% in June. We relate the observed post-spring bloom decrease of phosphate concentrations in the surface layer to the dominance and growth of M. rubrum. We suggest that this link can be explained by the vertical migration behaviour of M. rubrum and phosphate utilization in the surface layer coupled with inorganic nitrogen assimilation in the sub-surface layer. Thus, the dynamics of M. rubrum could strongly influence the amount of post-spring bloom excess PO43- in the euphotic layer and the depth of nitracline in the Gulf of Finland.

High-resolution monitoring of environmental state variables in the surface layer of the Gulf of Finland (during a dynamic spring bloom in March-May 2010)

The flow-through system (Ferrybox) installed onboard a ferry cruising between Tallinn and Helsinki in the Gulf of Finland (Baltic Sea) measures temperature, salinity, chlorophyll a fluorescence, turbidity and since January 2010 also pCO2. In March-May 2010, the water sampling was conducted on a weekly basis at 17 locations along the ferry route to measure nutrient concentrations (NO2+NO3 and PO4), chlorophyll a content and phytoplankton species composition and biomass. Our aim was to show that the Ferrybox technology can be successfully applied to follow the rapid changes of state variables during a very dynamic season of the year — phytoplankton spring bloom. High variability of environmental parameters has been observed both in space and time in the Gulf of Finland in spring 2010. It is suggested that both the general circulation in the surface layer and mesoscale hydrodynamic processes are influencing the bloom evolution and spatio-temporal variability. The observed coincidence of Chl a peaks with the periods of relatively fast temperature increase indirectly shows the importance of positive buoyancy fluxes (vertical stratification) for phytoplankton growth in spring. Spatio-temporal distribution of pCO2 was in a good accordance with the Chl a dynamics confirming that the pCO2 measurements can be used for the estimates of phytoplankton productivity. Our data confirm that the regular late evening fluorescence measurements can be successfully applied to determine the Chl a content in the surface waters in spring. It is concluded that autonomous high-resolution in-situ monitoring in combination with adaptive water sampling and remote sensing could give a full enough data set to assess the environmental state of the Gulf of Finland during this highly dynamic season of the year.