Browsing by Subject "SHALLOW"

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  • Haavisto, Noora; Tuomi, Laura; Roiha, Petra; Siiria, Simo-Matti; Alenius, Pekka; Purokoski, Tero (2018)
    We made an assessment of the hydrography in the Bothnian Sea based on data collected by the Argo floats during the first 6 years of operation in the Bothnian Sea (2012-2017). We evaluated the added value of Argo data related to the pre-existing monitoring data. The optimal usage and profiling frequency of Argo floats was also evaluated and the horizontal and vertical coverage of the profiles were assessed. For now we lose 4 m of data from the surface due to sensor design and some meters from the bottom because of the low resolution of available bathymetry data that is used to avoid bottom collisions. Mean monthly temperature and salinity close to surface and below halocline from the float data were within the boundaries given in literature, although some variation was lost due to scarcity of winter profiles. The temporal coverage of the Argo data is much better than that of ship monitoring, but some spatial variability is lost since the floats are confined in the over 100 m deep area of the Bothnian Sea. The possibility to adjust the float profiling frequency according to weather forecasts was successfully demonstrated and found a feasible way to get measurements from storms and other short term phenomena unreachable with research vessels. First 6 years of operation have shown that Argo floats can be successfully operated in the challenging conditions of the Bothnian Sea and they are shown to be an excellent addition to the monitoring network there. With multiple floats spread in the basin we can increase our general knowledge of the hydrographic conditions and occasionally get interesting data related to intrusions and mixing during high wind events and other synoptic scale events.
  • Plikk, Anna; Engels, Stefan; Luoto, Tomi P.; Nazarova, Larisa; Salonen, J. Sakari; Helmens, Karin F. (2019)
  • Niemistö, Juha; Silvonen, Soila; Horppila, Jukka (2020)
    Effects of hypolimnetic aeration (pumping of epilimnetic water into the hypolimnion) on the quantity of settling material in eutrophied Lake Vesijarvi, Finland were studied by comparing spatially comprehensive gross sedimentation rates as dry and organic matter prior to aeration activity and during two aerated years. Possible changes in the organic matter (as loss on ignition, LOI), carbon (C) and nitrogen (N) contents and changes in the C/N ratio of the settling material and surface sediment were quantified. Thermal stratification broke up earlier due to aeration and was followed by sedimentation peaks. The absolute amount of dry and organic matter as well as C and N settling to the lake bottom were significantly higher in the aerated years. Increased sedimentation rates were especially pronounced in the deep zones indicating enhanced sediment focusing. Increased sedimentation of C and N reflected higher primary production during the aerated years, which most likely was associated with increased temperature and turbulence and the subsequent regeneration and recycling of nutrients in the water body. Aeration seemed to slightly enhance degradation, but contrary to its ultimate aim, it failed to decrease the phosphorus content of the water column and deposits of organic material in the deep zones of the lake.
  • Roiha, Petra; Siiriä, Simo-Matti; Haavisto, Noora; Alenius, Pekka; Westerlund, Antti; Purokoski, Tero (2018)
    Argo floats have been used in the environmental monitoring of the very shallow Bothnian Sea, a sub-basin of the Baltic Sea, for 5 years as part of the Finnish Euro-Argo programme. The Bothnian Sea is so far considered to be an environmentally healthy part of the Baltic Sea because the deep waters of the basin are well-ventilated by inflowing oxygen-rich saltier and heavier surface layer waters of the Baltic Sea proper. Thus the deep water flow is of interest in the Bothnian Sea. In this study, we used Argo float data from six different long-term missions, from 111 to 512 days, to analyze the deep-water flow in the Bothnian Sea where no continuous monitoring of currents exist. We estimated mainly the flow below the expected halocline from the paths of the floats. We analyzed the movements statistically and estimated the error caused by the surface drift of the floats during their stay at the surface by using 3D hydrodynamic model results as reference data. Our results show a northward flowing resultant current in the deep trench of the Bothnian Sea. There seemed to be very little exchange between coastal zone and open-sea waters in deeper layers. The drifting speed of the floats in the deep layers of Bothnian Sea generally was around 2 cm/s but instantaneous speeds of up to 30 cm/s in the middle-layer (50 dbars) were observed. In the Bothnian Sea deep, the deep trench on the Finnish side of the Bothnian Sea, the vast majority of the observations showed deep currents from south to north, with the same average speed of around 2 cm/s but the instantaneous maximum was smaller at 13 cm/s. Our study indicates that the routine Argo float observations can be used to get information on the deep currents in the basin in addition to hydrographic observations.
  • Rask, Martti; Malinen, Tommi; Olin, Mikko; Peltonen, Heikki; Ruuhijärvi, Jukka; Vesala, Sami; Hietala, Jaana (2020)
    Fish community responses to long-term mass removal of planktivorous fish from eutrophicated Lake Tuusulanjarvi were examined during 1996-2018 by gill net test fishing, hydroacoustics with simultaneous trawling, virtual population analysis (VPA) and fish growth measurements. The gillnet catches of white bream and roach increased first while bream and bleak decreased but recovered in the early 2000s. Perch and pikeperch increased but ruffe decreased and remained low. According to hydroacoustics, pelagic fish biomass decreased. Smelt was the most abundant species at the beginning and the end of the monitoring period but bream dominated the fish biomass in most years. VPA calculations indicated a five- and threefold increase in the biomass of bream and white bream, respectively, during 2005-2011. Significant increases appeared in the growth of perch, bream and roach. Overall, the responses in the fish community structure and fish abundance to the food web management were slight and mostly masked by changes in reproduction and growth of fish and annual variability in environmental conditions such as temperature and water turbidity. Gillnet test fishing and echo sounding complemented each other well in monitoring the effects of management fishing. Where bream is one of the dominant species VPA is also recommended.