| 1. |
- Eglite, Elvita, et al.
(author)
-
Nutrient turnover at the hypoxic boundary : flux measurements and model representation for the bottom water environment of the Gulf of Riga, Baltic Sea
- 2014
-
In: Oceanologia. - : Elsevier BV. - 0078-3234. ; 56:4, s. 711-735
-
Journal article (peer-reviewed)abstract
- Experimental studies of intact sediment cores from the Gulf of Riga, Baltic Sea, were conducted to estimate the response of sediment nutrient fluxes to various near-bottom water oxygen conditions. The experiment was performed in the laboratory using a batch-mode assay type system on the sediment cores held at 4 degrees C and oxygen concentrations maintained at 1, 2, 3, 4 and 5 mg l(-1). The results from the experiment were subsequently used to optimise the fit of the sediment denitrification sub-model of the Gulf of Riga basin. Sedimentwater fluxes of phosphate were low and directed out of the sediments under all treatments, demonstrating a general decreasing tendency with increasing near-bottom water oxygen concentration. The sediment-water fluxes of ammonium and nitrate + nitrite demonstrated opposing trends: ammonium fluxes decreased whereas nitrate + nitrite fluxes increased with rising near-bottom water oxygen concentration. The modelled fluxes agreed well with the measured ones, with correlation coefficients of 0.75, 0.63 and 0.88 for ammonium, nitrate + nitrite and phosphate fluxes respectively. The denitrification rate in sediments was simulated at oxygen concentrations from -2 to 10 mg l(-1). At oxygen concentrations < 2 mg l(-1) the modelled denitrification was sustained by nitrate transport from water overlying the sediments. With increasing oxygen concentrations the simulated denitrification switched from the process fuelled by nitrates originating from the overlying water (D-w) to one sustained by nitrates originating from the coupled sedimentary nitrification - denitrification (D-n). D-n reached its maximum at an oxygen concentration of 5 mg l(-1).
|
|
| 2. |
- Purmalis, Oskars, et al.
(author)
-
Ground-penetrating radar (GPR) screening in shallow endure and pape lagoon lakes
- 2016
-
In: Research for rural development 2016, vol. 1. - Jelgava : Latvia University of Agriculture. ; , s. 228-235
-
Conference paper (peer-reviewed)abstract
- Geophysical studies in mapping and screening applications are widely applied for archaeological, environmental, geological, hydrological and many other applications. Ground-penetrating radar (GPR) is one of methods from geophysical toolbox that is also called a ground-probing radar, subsurface radar surface-penetrating radar and 'georadar' or impulse radar it is a non-invasive and non-destructive technique. Pulsed electromagnetic signal is recording the reflected energy and scattering from subsurface objects. Studies were performed in former Littorina Sea lagoons that became lakes after the further Limnea Sea stage in the Baltic Sea established with comparatively lower absolute sea level that is close to present day situation. Characteriz ation of sediments as well as full sediment core description for comparison with GPRsignals were performed. Major results showthat GPR as non-destructive method in combination with geological coring followed by laboratory analysis of sediment properties can be successfully used to describe layering conditions, topography and depth of shallow lakes. Although there are some limitations regarding the electromagnetic (EM) noise and similar FM properties of analysed sediments, proper treatment of data gives complementary insight thus diminishing the necessity of dense coring network establishments in analysed areas of lakes. The aim of this screening study is to analyse potential advantages of GPR use for mapping sediments and topography of sandy bottom in shallow lagoon lakes as well as pinpoint problems during field and cameral works considering electromagnetic, geological and topographical disturbances.
|
|
| 3. |
- Aigars, Juris
(author)
-
The role of sediments in the biogeochemical cycles of nutrients in the Gulf of Riga
- 2001
-
Doctoral thesis (other academic/artistic)abstract
- The sediment biogeochemistry of C, N, P and biologic silica (BSi) plays an important role in cycling of these elements in aquatic ecosystems. In shallow coastal and estuarine systems, water-sediment nutrient dynamics can influence biological processes (e.g., primary production) in overlying waters.The objectives of this study are:characterize C, N, P and BSi geochemistry of the surface sediments in the Gulf of Riga,estimate the spatial distribution, including net flux and sink, for organic C, N and P in the Gulf of Riga,examine if there are any appropriate bulk, regional and/or vertical relationships between the nutrients,examine if nutrient biogeochemistry in surface sediments vary as a response to naturally occurring processes i.e., intensity of sedimentation and bioturbation, oxygen deficiency, variation of temperatureSpatial and vertical distribution reveals that organic C and N ratio is almost independent of sediment characteristics and location, and exhibit a stable value down the sediment core. Over 90 % of carbon and nitrogen in sediments is organic. In contrast, organic P constitutes less than 50 % of the total phosphorus pool. The organic C:N:P ratio in sediments indicate that N and P are decomposed preferentially to C, whereas P is decomposed preferentially to N. The stable C:N ratio in sediments indicates that preferential N decomposition occurs in water column and/or immediately upon settling at sediment surface. Distribution pattern of BSi suggests that accumulation rate of sediments controls BSi concentration. Although nutrient loading from drainage area increased drastically over the past 100 years, vertical profiles of C, N and BSi show no variation in element concentrations except in the top few centimeters. A corresponding increase of benthic biomass most likely results in low elemental accumulation in these sediments.Although Gulf of Riga is one of the most productive areas in the Baltic Sea, seasonal alterations of C and N levels were limited. However, during spring and autumn algae blooms total C and N concentrations increased in the study area. Limited data suggests that burrowing amphipods directly impact the C and N concentrations in the top 2-3 cm of sediments. The vertical distribution of P is more dependent on oxygen concentration, which is largely controlled by bioturbation and sedimentation rates. Moreover, the results suggest that bioturbation is largely responsible for temporal accumulation of inorganic P in surface sediments.The low unidirectional fluxes of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in early spring is consistent with low water temperature and poor nutritional quality of experimentally added material. The high water temperature and better nutritional quality of material added in summer, comparatively to winter, resulted in large DIN and DIP fluxes. Moreover, flux experiments under low oxygen conditions and pulse input of large quantities of settling seston suggest that sediment surface might experience lack of oxygen despite availability of oxygen in the overlying water column.
|
|
| 4. |
- Andrén, Elinor, et al.
(author)
-
Defining Reference Conditions for Coastal Areas in the Baltic Sea
- 2007
-
Reports (other academic/artistic)abstract
- The overall aim of DEFINE is to provide a methodology to define reference conditions for nutrient concentrations in the coastal zone of the Baltic Sea. This will aid the national authorities that surround the Baltic basin in implementing the EU's Water Framework Directive (WFD) by providing decision-makers with a methodology to assess reference conditions and the degree of past and present departure from this state, such that appropriate policy and management measures can be taken at national and European levels. DEFINE adopts a palaeoecological approach grounded on diatom-based transfer functions, which can then be applied to define background total nitrogen (TN) concentrations in estuaries and coastal areas over the entire Baltic Sea. All transfer functions and necessary supporting documentation will be publicly available as a coherent management tool and accessible via the MOLTEN/DEFINE web page (http://craticula.ncl.ac.uk/Molten/jsp/).
|
|
| 5. |
- Conley, Daniel, et al.
(author)
-
Hypoxia is increasing in the coastal zone of the Baltic Sea
- 2011
-
In: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 45:16, s. 6777-6783
-
Journal article (peer-reviewed)abstract
- Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955-2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.
|
|
| 6. |
- Conley, Daniel, et al.
(author)
-
Past, present and future state of the biogeochemical Si cycle in the Baltic Sea
- 2008
-
In: Journal of Marine Systems. - : Elsevier BV. - 0924-7963 .- 1879-1573. ; 73:3-4, s. 338-346
-
Journal article (peer-reviewed)abstract
- The Baltic Sea is one of many aquatic ecosystems that show long-term declines in dissolved silicate (DSi) concentrations due to anthropogenic alteration of the biogeochemical Si cycle. Reductions in DSi in aquatic ecosystems have been coupled to hydrological regulation reducing inputs, but also with eutrophication, although the relative significance of both processes remains unknown for the observed reductions in DSi concentrations. Here we combine present and historical data on water column DSi concentrations, together with estimates of present river DSi loads to the Baltic, the load prior to damming together with estimates of the long-term accumulation of BSi in sediments. In addition, a model has been used to evaluate the past, present and future state of the biogeochemical Si cycle in the Baltic Sea. The present day DSi load to the Baltic Sea is 855 ktons y(-1). Hydrological regulation and eutrophication of inland waters can account for a reduction of 420 ktons y(-1) less riverine DSi entering the Baltic Sea today. Using published data on basin-wide accumulation rates we estimate that 1074 ktons y(-1) of biogenic silica (BSi) is accumulating in the sediments, which is 36% higher than earlier estimates from the literature (791 ktons y(-1)). The difference is largely due to the high reported sedimentation rates in the Bothnian Sea and the Bothnian Bay. Using river DSi loads and estimated BSi accumulation, our model was not able to estimate water column DSi concentrations as burial estimates exceeded DSi inputs. The model was then used to estimate the BSi burial from measured DSi concentrations and DSj load. The model estimate for the total burial of BSi in all three basins was 620 ktons y(-1), 74% less than estimated from sedimentation rates and sediment BSi concentrations. The model predicted 20% less BSi accumulation in the Baltic Proper and 10% less in the Bothnian Bay than estimated, but with significantly less BSi accumulation in the Bothnian Sea by a factor of 3. The model suggests there is an overestimation of basin-wide sedimentation rates in the Bothnian Bay and the Bothnian Sea. In the Baltic Proper, modelling shows that historical DSi concentrations were 2.6 times higher at the turn of the last century (ca. 1900) than at present. Although the DSi decrease has leveled out and at present there are only restricted areas of the Baltic Sea with limiting DSi concentrations, further declines in DSi concentrations will lead to widespread DSi limitation of diatoms with severe implications for the food web.
|
|
| 7. |
- Müller-Karulis, Bärbel, et al.
(author)
-
Modeling the long-term dynamics of nutrients and phytoplankton in the Gulf of Riga
- 2011
-
In: Journal of Marine Systems. - : Elsevier BV. - 0924-7963 .- 1879-1573. ; 87:3-4, s. 161-176
-
Journal article (peer-reviewed)abstract
- The long term dynamics of nitrogen, phosphorus, and phytoplankton in the Gulf of Riga were simulated with a biogeochemical box model that resolved seasonal cycles. The model was calibrated using a numerical optimization procedure that adjusted 37 parameters to maximize the model data fit for field observations from 1973 to 2000 and validated with an independent dataset covering 2001-2007. Both the long-term increase and subsequent decline in winter nitrogen concentrations, as well as the continuous increase in winter phosphate levels were well reproduced by the model, which also gave reasonable representations of the seasonal dynamics of nutrients and phytoplankton. Starting from the mid-1990s, the model simulated an increase in cyanobacteria growth sustained by internal phosphorus loading. While nitrogen was efficiently removed by denitrification from the Gulf of Riga, comparatively slow export to the Baltic Proper was the main removal pathway of phosphorus. Modeled residence times were 5.4 years for nitrogen and 38 years for phosphorus. Scenario simulations indicated that the Gulf of Riga responds to phosphorus load reductions with a gradual decrease in primary production and cyanobacteria growth, while the effect of nitrogen load reductions is largely offset by nitrogen fixation.
|
|
| 8. |
- Olli, Kalle, et al.
(author)
-
Diatom stratigraphy and long-term dissolved silica concentrations in the Baltic Sea
- 2008
-
In: JOURNAL OF MARINE SYSTEMS. - : Elsevier BV. - 0924-7963. ; 73:3-4, s. 284-299
-
Journal article (peer-reviewed)abstract
- In many parts of the world coastal waters with anthropogenic eutrophication have experienced a gradual depletion of dissolved silica (DSi) stocks. This could put pressure on spring bloom diatom populations, e.g. by limiting the intensity of blooms or by,causing shifts in species composition. In addition, eutrophication driven enhanced diatom growth is responsible for the redistribution of DSi from the water phase to the sediments, and changes in the growth conditions may be reflected in the sediment diatom stratigraphy. To test for changes in diatom communities we have analyzed four sediment cores from the Baltic Sea covering approximately the last 100 years. The sediment cores originate from the western Gulf of Finland, the Kattegat, the Baltic Proper and the Gulf of Riga. Three out of the four cores reveal only minor changes in composition of diatom assemblages, while the Gulf of Riga core contains major changes, occurring after the second World War. This area is set apart from the other Baltic Sea basins by a high frequency of low after spring bloom DSi concentrations (<2 mu mol L-1) during a relatively well defined time period from 1991-1998. In 1991 to 1993 a rapid decline of DSi spring concentrations and winter stocks (down to 5 mu mol L-1) in the Gulf was preceded by exceptionally intense diatom spring blooms dominated by the heavily silicified species Thalassiosira baltica (1991-1992; up to 5.5 mg ww L-1). T baltica has been the principal spring bloom diatom in the Gulf of Riga since records began in 1975. DSi consumption and biomass yield experiments with cultured T baltica suggest that intense blooms can potentially exhaust the DSi stock of the water column and exceed the annual Si dissolution in the Gulf of Riga. The phytoplankton time series reveals another exceptional T baltica bloom period in 1981-1983 (up to 8 mg L-1), which, however, took place before the regular DSi measurements. These periods may be reflected in the conspicuous accumulation of T baltica frustules in the sediment core corresponding to ca. 1975-1995.
|
|
