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Assessment of the Present Nutrient Variations at the Elbe Estuary Outflow: Seasonal and Interannual variations

Abstract

Estuaries are highly dynamic systems, which not only provide important ecosystem, cultural and economic services, but also represent unique ecosystems with complex transport processes. They have been shaped particularly strongly by human settlement and marine resource use, leading to an increase in nutrient inputs and to associated negative effects such as hypoxia or eutrophication. Due to extensive restoration and conservation efforts, estuaries and coastal waters in the United States and Europe have had remarkable improvements in recent decades, but there is still a continuing concern about nutrient eutrophication in estuarine and coastal ocean waters of the world, because hypoxia from anthropogenic sources is still a major occurrence. In addition, new challenges such as climate change, population growth and intensive agriculture will likely continue and intensify the anthropogenic impacts on estuaries and coastal waters. Therefore, it is necessary to provide an assessment of the current state of estuarine ecosystems, in order to validate the success and effectiveness of restoration and conservation strategies. In Germany, the Elbe estuary has experienced major changes due to a period of high pollution in the 1980s, from which it is still recovering. Therefore, the current state of the nutrient outflow from this estuary to the German Bight will be investigated in this thesis, based on data sets from the FerryBox station in Cuxhaven. In particular, the current inter-annual and seasonal nutrient variations of nitrate (NO3-), nitrite (NO2-), phosphate (PO43-) and silicate (Si(OH)4-) in the Elbe estuary outflow were evaluated, from Systea Micromac nutrient analyser data, for 2014-2018. In addition, the effect of light availability on biological nutrient uptake was evaluated. Also, dissolved nutrient ratios were examined, and an inverse correlation between the seasonal nitrogen and phosphorus concentrations was identified. Since the 1990s, the nutrient concentrations in the Elbe have decreased. For the recent years 2014-2018, it was determined that the concentrations are stable, and no further decrease in nutrient concentrations has been observed. The nitrate and silicate inputs are still very high, and every year up to 100 kilotons of silicate and over 100 kilotons of nitrate are discharged through the Elbe into the German Bight. The lowest nutrient concentrations were measured in summer and spring, so it can be concluded that primary production helps for nutrient uptake in the estuary, but also that during winter and fall, light is most likely a growth-limiting factor. Nitrate concentrations show a different seasonal pattern than phosphate and are highest in spring. Overall, every year between 2014 and 2018, nitrate and phosphate concentrations show an inverse seasonal fluctuation. Also, nitrate concentrations are linked to discharge patterns, and the seasonal peak in nitrate matches the seasonal peak in river discharge.
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