Aarhus University Seal / Aarhus Universitets segl

No. 323: Relationships in the marine environment - Significance of sediment changes

Høgslund, S., Carstensen, J., Krause-Jensen, D. & Hansen, J.L.S. 2019. Sammenhænge i det marine miljø - Betydning af sedimentændringer. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 74 s. - Videnskabelig rapport nr. 323. http://dce2.au.dk/pub/SR323.pdf

Summary

This report has been prepared for the Danish Environmental Protection Agency and is part of the project “Relationships in the marine environment - stress factors other than nutrients". The report describes how changes in sediment pools of C, N and P affect central biological quality elements in Danish coastal waters. The report is based on data collected in research projects and the national monitoring programmes on model calculations and reviews of scientific literature.

A review of published measurements of sediment characteristics and monitoring data showed that there is insufficient data to quantify changes in sediment characteristics in individual water areas in Danish coastal waters. Analysis of monitoring data from the two periods, 1999-2003 and 2017-2018, showed a significant decrease in the content of organic matter and total nitrogen at the station at the South Funen Archipelago. Apart from this, there were no significant changes in sediment characteristics in depth-integrated pools or in the individual sediment depths.

The sediment receives nutrients bound in the organic matter that settles on the seabed. Here, the organic material is converted and some nutrients are released and returned to the water column. The dynamics of this "internal load" with nutrients thus depend largely on organic matter supply to the sediment and turnover of the organic substance. Modelling of sediment-biochemistry in the Bay of Aarhus showed that a reduction in the supply of organic matter to the sediment of 38% over 10 years leads to a decrease in the internal N load of 40% in summer and autumn. This decrease may reduce the growth of phytoplankton in the water column, as the water column’s primary production is particularly dependent on internal (recycled) nutrients during this time of year.

The primary source of N from the sediment is ammonium, and it is the changes in the release of ammonium that drive the quantitatively most important changes in the internal N load. Ammonium release is to a great extent determined by the turnover of the labile and refractory organic matter pools in the sediment. Therefore, the largest impact on the internal load takes place during the first four years following changes in the sediment pools.

The exchangeable pools of organic matter also determine the phosphate release from the sediment that decreases as they are turned over. At the same time, the changed redox conditions in the sediment cause the pool of iron bound phosphorus in the sediment to rise, and if this pool is reduced in connection with oxygen depletion, it will give rise to periodically increased release of phosphate to the bottom water.

Sediment oxygen uptake and hydrogen sulphide depth change immediately and in line with the change of organic matter content in the sediment. A decrease in organic matter content results in reduced oxygen uptake and a deeper position of the hydrogen sulphide front.

Changes in the sediment pools of organic matter change the redox conditions in the sediment, and model calculations for the Bay of Aarhus showed that it takes decades (> 40 years) before the conditions that are controlled by the redox chemistry are stabilised at new levels. This applies to both oxygen uptake, which is important to the oxygen conditions in the bottom water, and hydrogen sulphide content in the sediment.

Chemical sediment parameters such as organic matter content, oxygen conditions and hydrogen sulphide greatly impact the bottom fauna and eelgrass.

The hydrogen sulphide content of the sediment impacts the bottom fauna through direct toxic effects and indirect effects on habitat quality. Analysis of data from the national monitoring programme and measurements made in connection with an extensive research project demonstrate how changes in the depth of the hydrogen sulphide front directly affect the DKI index describing the bottom fauna’s ecological status. The impact can be documented both in coastal sediments and in open waters. The relationship is best described by a saturation function, such that changes in the hydrogen sulphide depth have relatively little impact on the bottom fauna diversity, as long as the change takes place deep within the sediment. For example, you might expect a change in DKI of 10-20% through a change in the position of the hydrogen sulphide front from 5 cm to 3 cm depth in open waters, while the effect is increased to about 50% if the position is changed from 3 cm to 1 cm depth.

Modelling of the sediment-biochemistry of the Bay of Aarhus confirms that the content of hydrogen sulphide sediment is affected both by current and former eutrophication conditions, and this parameter may be a more sensitive indicator of the possible eutrophication-related load on the bottom fauna than e.g. frequency of oxygen depletion.

Sediment changes are generally driven by changes in the organic matter supply, changes in deposition and resuspension conditions and changes in seabed flora and fauna. Since seabed flora and fauna are also affected by sediment conditions, there is also an important interaction between sediment characteristics, fauna activities and vegetation.

Bottom fauna oxidises sediment and has an important function in substance exchange between the more or less oxidised/reduced zones in the sediment. In this way, fauna contributes to keeping hydrogen sulphide away from the sediment surface and pumping oxygen deep into the sediment.

The mutual interaction between the content of hydrogen sulphide and seabed fauna activity means that stress factors that e.g. affect the bottom fauna’s ability to ventilate the sediment indirectly affect the sediment hydrogen sulphide and, thus, suitability as a habitat for both the seabed fauna and eelgrass.

Scientific literature documents a non-linear correlation between seagrass depth limit and sediment characteristics. When certain critical thresholds are exceeded, there is a negative effect from organic rich sediments on the eelgrass depth limit. This is, among other things, due to impaired anchoring possibilities for eel grass. Effects of sediment characteristics on seagrass distribution are supported in several modelling studies in Danish fjords, and modelling of sediment-biochemistry emphasises that a reduction in the supply of organic matter to the sediment only slowly spreads to the total sediment pool of organic matter, which  has an impact on the physical characteristics of the sediment and the anchoring possibilities of the eelgrass.

Changes in external nutrient supply affect the supply of organic matter into the sediment and lead to changes in the sediment’s C, N and P pools. These changes directly affect the biological quality elements linked to the seabed fauna, including the DKI index, and the growth conditions for eelgrass. Changes in sediment pools also affect the internal load and, thus, the availability of nutrients and growth conditions for the water column phytoplankton.