36 research outputs found
Spatial and temporal variability of a dinoflagellate-cyanobacterium community under a complex hydrodynamical influence:a case study at the entrance to the Gulf of Finland
Variability of nutrients and pelagic biological parameters (primary production and chlorophyll a [chl a] in flagellate and cyanobacterial size fractions, nitrogen fixation, phytoplankton species abundance) was followed for 12 d in July 1996 at an anchor station at the entrance to the Gulf of Finland, Simultaneously, meso-scale physical fields and plankton distribution were mapped over the surrounding 15 x 30 km area. The study period coincided with the intense blooming of a dinoflagellate Heterocapsa triquetra Ehrenberg and cyanobacterium Aphanizomenon flos-aquae (Linné) Ralfs community. A complex background of hydrodynamical processes was observed in the study area, including downwelling, formation and development of an anticyclonic eddy and jet currents. Our hypothesis was that the horizontal scale of patches decreases and the variation of biological parameters increases when moving from the overall community level (chl a) to the size class level and further to the species level. The horizontal distribution of chl a was closely related to the different water masses, but the distribution of the 2 dominant species differed and showed high variability even within water masses. The temporal variability of the pelagic biological parameters at the anchor station (estimated by the coefficient of variation) was between 25 and 95 % and it may be explained by horizontal patchiness. The results confirmed our hypothesis by showing that the coefficient of variation of summational parameters (total chl a, total primary production) was always lower than that of parameters specific to plankton size (chl a and primary production in 20 pm size classes), functional group (diazotrophs) or species. Phytoplankton in the size range equal to or greater than 20 μm exhibited particularly pronounced variability, while the smaller size fractions were less affected
Upwelling events, coastal offshore exchange, links to biogeochemical processes - Highlights from the Baltic Sea Sciences Congress at Rostock University, Germany, 19-22 March 2007
The Baltic Sea Science Congress was held at Rostock University, Germany, from 19 to 22 March 2007. In the session entitled"Upwelling events, coastal offshore exchange, links to biogeochemical processes" 20 presentations were given,including 7 talks and 13 posters related to the theme of the session.This paper summarises new findings of the upwelling-related studies reported in the session. It deals with investigationsbased on the use of in situ and remote sensing measurements as well as numerical modelling tools. The biogeochemicalimplications of upwelling are also discussed.Our knowledge of the fine structure and dynamic considerations of upwelling has increased in recent decades with the advent ofhigh-resolution modern measurement techniques and modelling studies. The forcing and the overall structure, duration and intensity ofupwelling events are understood quite well. However, the quantification of related transports and the contribution to the overall mixingof upwelling requires further research. Furthermore, our knowledge of the links between upwelling and biogeochemical processes is stillincomplete. Numerical modelling has advanced to the extent that horizontal resolutions of c. 0.5 nautical miles can now be applied,which allows the complete spectrum of meso-scale features to be described. Even the development of filaments can be describedrealistically in comparison with high-resolution satellite data.But the effect of upwelling at a basin scale and possible changes under changing climatic conditions remain open questions
Occurrence of cyanobacteria and cyanotoxin in the Southern Baltic Proper. Filamentous cyanobacteria versus single-celled picocyanobacteria
Simulation of nutrient transport from different depths during an upwelling event in the Gulf of Finland
Numerical simulation experiments with a high-resolution circulation model were
carried out to study nutrient transport from different depths to the surface 10-m
layer during an upwelling event along the northern coast of the Gulf of Finland
in July 1999. The initial nutrient distribution is based on field measurements
performed in the north-western part of the Gulf. Wind forcing covering the period
of the upwelling along the northern coast was turned through 180◦ to simulate
an upwelling along the southern coast. The simulation results showed that the
main phosphorus transport to the upper 10-m layer occurred from depths shallower
than 30 m for the upwelling events along both the northern and the southern coasts. Nitrogen transport to the upper 10-m layer was the largest from depths of
40–55 m for the upwelling along the northern and 40–65 m for the upwelling along
the southern coast. Simulated cumulative volume transports to the upper 10-m
layer from different depths showed that the contribution from deeper layers was
larger in the case of the upwelling along the southern coast. The reduction of wind
stress had a bigger influence on water transport from the deeper layers
Rapid changes in the winter near-bottom oxygen conditions in the Gulf of Finland due to variations of estuarine circulation
Observations of Stratified Turbulence Depending on the Different Wind, Strtification and Shear Scenarios.
Abstract not availableJRC.(SAI)-Space Application Institut
Flow, waves and water exchange in the Suur Strait, Gulf of Riga, in 2008
Wind, flow and wave measurements were performed in November–December in 2008 in the relatively narrow and shallow Suur Strait connecting the waters of the V¨ainameri and the Gulf of Riga. During the measurement period wind conditions were extremely variable, including a severe storm on 23 November. The flow speed along the strait varied between ±0.2 m s−1, except for the 0.4 m s−1 that occurred after the storm as a result of the sea level gradient. The mean and maximum significant wave heights were 0.53 m and 1.6 m respectively. Because of their longer fetch, southerly winds generated higher waves in the strait than winds from the north. All wave events caused by the stronger southerly winds induced sediment resuspension, whereas the current-induced shear velocity slightly exceeded the critical value for resuspension only when the current speed was 0.4 m s−1. A triplenested two-dimensional high resolution (100 m in the Suur Strait) circulation model and the SWAN wave model were used to simulate water exchange in 2008 and the wave-induced shear velocity field in the Suur Strait respectively. Circulation model simulations demonstrated that water exchange was highly variable, that cumulative transport followed an evident seasonal cycle, and that there was an gross annual outflow of 23 km3 from the Gulf of Riga. The horizontal distribution of waveinduced shear velocity during the strong southerly wind event indicated large shear velocities and substantial horizontal variability. The shear velocities were less than the critical value for resuspension in the deep area of the Suur Strait
Monitoring the effect of upwelling on the chlorophyll a distribution in the Gulf of Finland (Baltic Sea) using remote sensing and in situ data
The spatio-temporal variability of chlorophyl a (Chl a) caused by a sequence of
upwelling events in the Gulf of Finland in July–August 2006 was studied using
remote sensing data and field measurements. Spatial distributions of sea surface
temperature (SST) and Chl a concentration were examined using MODIS and
MERIS data respectively. The MERIS data were processed with an algorithm developed by the Free University of Berlin (FUB) for case 2 waters. Evaluation
of MERIS Chl a versus in situ Chl a showed good correlation (r2 = 0.67), but the
concentration was underestimated. The linear regression for a 2 h window was
applied to calibrate MERIS Chl a. The spatio-temporal variability exhibited the
clear influence of upwelling events and related filaments on Chl a distribution in
the western and central Gulf. The lowest Chl a concentrations were recorded in the
upwelled water, especially at the upwelling centres, and the highest concentrations
(13 mg m−3) were observed about two weeks after the upwelling peak along the
northern coast. The areas along the northern coast of upwelled water (4879 km2)
on the SST map, and increased Chl a (5526 km2) two weeks later, were roughly
coincident. The effect of upwelling events was weak in the eastern part of the Gulf,
where Chl a concentration was relatively consistent throughout this period