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