A 2-week multidisciplinary study of the physical, chemical, and biological mechanisms controlling the initiation of the late summer blooms of the diazotrophic cyanobacteria, Aphanizomenon jlos-aqua2 Ralfs and Nodularia spumigena Mertens, in the Baltic Sea was carried out in a frontal region at the entrance to the Gulf of Finland in July 1993. The front is formed by inflowing sa'itier waters of the northern Baltic proper and outflowing fresher waters from the gulf, and its position and shape are largely controlled by wind conditions. In general, the waters of the northern Baltic proper are less stratified than the outflowing lesssaline waters. At the time of the study, the two major water masses differed in terms of phytoplankton community structure, both at species level and at the level of functional groups. Wind-induced vertical mixing was instrumental in bringing nutrient pulses to the upper mixed layer in the less-stratified, high-saline water mass. Nutrient pulses were followed by enhancement of primary productivity and assimilation number (primary productivity/Chl a) in cyanobacterial (> 20 pm) and flagellate (< 20 vrn) size fractions. It is proposed that mesoscale blooms of A. flos-aquae benefit from the nutrient-pulsing events. Calm weather and solar heating, as reflected by rising temperatures in the upper mixed layer and overriding of water masses in the frontal region, resulted in substantial shallowing of the upper mixed layer, which initiated the bloom of N.spumifena.
The development of a deep chlorophyll maximum (DCM) at a depth of 30-35 m was followed during a 15-d case study in July 1998 at the entrance to the Gulf of Finland. The study consisted of three 18-24-h periods of biological (chlorophyll a, phytoplankton, primary production), chemical (nitrate, phosphate) and physical (CTD, turbulence, vertical particle size distribution) measurements at an anchor station and six mesoscale towed CTD/ fluorometer mappings over the surrounding area. Exceptionally cold and windy weather led to a red tide of the dinoflagellate Heterocapsa triquetra instead of the cyanobacterial bloom that frequently occurs in late summer.Comparison of the estimated amount of nitrogen required for an H. triquetra bloom biomass with external loading affirmed that the bloom had been formed on the basis of the nitrate pool below the thermocline. The development of the bloom, therefore, led to the extremely deep nitracline. The DCM formed by H. triquetra developed at the top of the nitracline at an illumination of Ͻ0.1% of the sea surface illumination. A temperature-salinity analysis showed that the DCM was not caused by intrusions from inshore regions. It was concluded that the DCM was formed as a result of changing migratory behavior of H. triquetra after an upwelling event that fertilized the upper layer with phosphorus.
Abstract. The reversal of estuarine circulation caused by southwesterly wind forcing may lead to vanishing of stratification and subsequently to oxygenation of deep layers during the winter in the Gulf of Finland. Six conductivity, temperature, depth (CTD)+oxygen transects (130 km long, 10 stations) were conducted along the thalweg from the western boundary to the central gulf (21 December 2011-8 May 2012. Two bottom-mounted ADCP were installed, one near the western border and the second in the central gulf. A CTD with a dissolved oxygen sensor was deployed close to the western ADCP. Periods of typical estuarine circulation were characterized by strong stratification, high salinity, hypoxic conditions and inflow to the gulf in the near-bottom layer. Two circulation reversals were observed: one in DecemberJanuary and one in February. The first reversal event was well developed; it caused the disappearance of the stratification and an increase in the oxygen concentration from hypoxic values to 270 µmol L −1 (to 6 mL L −1 ) throughout the water column along the thalweg and lasted approximately 1.5 months. Shifts from estuarine circulation to reversed circulation and vice versa were both associated with strong longitudinal (east-west) gulf currents (up to 40 cm s −1 ) in the deep layer. The change from oxygenated to hypoxic conditions in the western near-entrance area of the gulf occurred very rapidly, within less than a day, due to the intrusion of the hypoxic salt wedge from the NE Baltic Proper. In the eastern part of the gulf, good oxygen conditions caused by reversals remained for a few months.
[1] A high resolution numerical study is undertaken to simulate an upwelling event along the northern coast of the Gulf of Finland, 21-29 July 1999, which was documented well by in situ and remote measurements. The simulated sequence of SST maps shows a reasonably good resemblance to that of satellite infrared imagery, including both mesoscale coherent structures (filaments or squirts) and the whole process of post-upwelling relaxation of the temperature field. Upwelling along the northern coast of the Gulf is accompanied by downwelling along the southern coast so that two longshore baroclinic jets and related fronts are developed simultaneously. When the strong westerly winds producing the upwelling/downwelling weaken, the longshore jets become unstable and produce transverse jets, cold/warm water squirts. Using pseudo-random simulated fields of temperature and velocity of currents, the apparent lateral diffusivity due to squirts is directly estimated at 500 m 2 s À1 . The model is also applied to estimate nutrient transport. Simulated phosphate concentration in the surface layer at the cold side of upwelling front is found to be about 0.3 mmol m À3 which is consistent with observations. The total content of phosphorus and nitrogen in the upper 10 m layer of the Gulf introduced by the upwelling event is estimated to be 387 and 36 tons, respectively. It follows, that the upwelling event transports nutrients into the upper layer with clear excess of phosphorus (N:P = 36:387 = 0.093) compared to the Redfield ratio of 7.2. Therefore phosphorus input caused by upwelling during summer most likely promotes nitrogen-fixing cyanobacteria blooms.Citation: Zhurbas, V., J. Laanemets, and E. Vahtera (2008), Modeling of the mesoscale structure of coupled upwelling/downwelling events and the related input of nutrients to the upper mixed layer in the Gulf
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