Browsing by Subject "sediment"

As water flow encounters an object on the sea floor, its hydrodynamics change. Accelerated currents and vortices develop around the object with changing intensity as a function of distance from its proximity. This leads to erosion and aggradation of sediment, known as scour. Studies focusing on formation processes of scour often involve locating visible scour sites by sonar scanning the geomorphology of the seafloor. However, the effects of scour on macroinfauna and small-scale sediment characteristics are not visible in sonar images. In this Master’s thesis, scour at a shipwreck of a timber-built historic sailing ship, the Joskär shipwreck, was first identified by scanning the study area with side-scan sonar, and by measuring water depth contours around the shipwreck by scuba diving. Sediment samples were then taken inside the area assumed to be under the most pressure from scour. Samples from three separate distances on two transects drawn outwards from the hull of the shipwreck were collected and analysed for sediment grain size, organic content, and species assemblages of macroinfauna. In addition, macrofauna were analysed for individual lengths, number of individuals, diversity index, and functional groups. All samples were collected with a core tube sampler operated by a scuba diver. The methods used in this Master’s thesis widen the concept of scour past the sole physical processes observable with sonar to a more holistic level that considers the quality of biological, geological, and chemical characteristics of the benthic environment. The results of the present Master’s thesis show that the quality of the sediment near Joskär shipwreck varies within a relatively small scale. Organic content of the sediment was the most potent descriptor of scour at the study site, exhibiting a consistent decreasing trend as distance to the shipwreck increased on both sampled transects. Sediment grain size became finer as distance to the shipwreck increased. However, compared to grain size, based on visual observations of the sediment samples, shell debris content of the sediment could possibly act as a better measure of presence of scour. The variability of characteristics of macroinfaunal communities as a function of distance from Joskär shipwreck was not a viable tool to describe the presence of scour, as no consistent trends of the variables were observed. As no control site was included in the study design, the characteristics of the benthic environment inside the scour around Joskär shipwreck could not be compared to the seafloor unaffected by scour. Further research could reveal possible variation between these distinct habitats, and that way produce valuable indicators of scour. The hypothesis in the present thesis was that macroinfaunal assemblages and sediment characteristics would exhibit variation between the sampling sites as a function of distance from the shipwreck. The observed trends of sediment characteristics validated a part of the hypothesis, showcasing the utility of sediment characteristics in describing scour at Joskär shipwreck. However, a part of the hypothesis was rejected, as no consistent trends of macroinfaunal features were present.

The ambition of the agricultural environmental programme is to reduce nutrient load, because greater part of the diffuse loading of phosphorus is caused by agriculture. A eutrophic influence of the phosphorus in water systems tends to be limited inter alia by constructed wetlands. Their main task is to allow sedimentation of eroded soil into the bottom of the wetlands. There is ambiguity on the findings of the functionality and the importance of the prevention of water loading among scientific research in Finland. The aim of this study is to examine by utilizing soil analyses what happens to the basin water eroded phosphorus in the wetland sediment and wether the sedimentary constituent of the soil be suitable for a substratum of plant production. Comparing the samples of basin soil and wetland sediments revealed that the eroded constituent of the basin soil got assorted on wetlands. The samples collected from the wetlands contained 48 % more clay than the samples collected from the basin soil. The growth of the clay concentration increased the reactive area of the sediment. In consequence, it contained 45 % more hydroxides of aluminium and iron in the samples of the sediment than the samples of the basin soil. Because of the hydroxides, the phosphorus sorption capacity was 52 % higher than in the samples of the basin soil. However, the degree of phosphorus saturation was equal in the sediment and basin soil, because the oxidized sediment contained 50 % more phosphorus extracted from hydroxides of aluminium and iron. At the time of sampling the sediment was in reduced state and the amount of its water extracable phosphorus was significantly higher compared to the oxidized sediment. Correspondingly, when the sediment became oxidized the sorption capacity for phosphorus increased significantly, therefore the phosphorus was desorbed from reduced sediment to the wetlands water. This was also proven in a pot experiment, where rye-grass that grew in the sediment suffered from a severe shortage of phosphorus. In contrast, rye-grass grown in the basin soil didn’t suffer from the deficiency of phosphorus at the same fertilization levels. After threefold extra fertilization of phosphorus, the dry matter yield, concentration of phosphorus and uptake of phosphorus on the second yield grown in sediment were equal to the results of the first yield grown in basin soil. According to the results of the pot experiment, the sediment in reduced state is weakly suitable for the substratum of plant production, because the sorption capacity of phosphorus is high. Instead, sediment suits well to be utilized in the areas wherein the soil includes plenty of easy soluble phosphorus, such as for the material of subgrade for the corral of livestock, because the sediment reduces the load of phosphorus directed to the environment.

Human-induced nutrient enrichment has led to eutrophication, which is globally a severe environmental problem in aquatic ecosystems. Eutrophication has a variety of deteriorating effects on marine ecosystems in the form of e.g., cyanobacterial blooms, bottom water hypoxia and anoxia, as well as increased fish and benthos mortality. The Baltic Sea is especially prone to eutrophication due to the combined effects of restricted water exchange and extensive nutrient loads. Nutrient enrichment reinforces primary production which further enhances organic matter remineralisation in the sediment – water interface, leading to oxygen depletion in the bottom waters. Decreased oxygen concentrations on the seafloor can lead to the release of phosphorus bound to reducible iron oxides. The so-called ‘vicious circle’ of internal loading is formed through the further enhanced nutrient release from the sediments into the water column due to the reduced bottom water conditions resulting from increased supply of organic matter into the system. However, the processes controlling phosphorus transport from land to sea through the ‘coastal filter’ remain poorly understood. In this study, sediments from Paimionlahti estuary were examined for phosphorus content and bulk elemental composition. Sedimentary phosphorus contents were determined through chemical extractions. The extracted fractions of phosphorus (P) include Fe oxide bound P (Fe-P), authigenic apatite P (Ca-P I), detrital apatite P (Ca-P II), and organic P (org-P). The fraction of Fe-P dominated in the upper sediment layers in most sites, whereas more unreactive fractions associated with P burial remained constant through sediment depth. The generally unreactive forms of P illustrated increasing trends towards open sea areas, partly explained by changes in the overall sediment composition as well as by potential differences in environmental conditions among sampling sites. The highest amounts of Fe-P were recorded in sites with the highest sediment accumulation. The results demonstrate that P from rivers is transformed and processed in the coastal zone, delaying its transport to the open sea.