Macrofaunal impact on biogeochemical turnover in German Bight sediments

alexa.wrede [ at ]


Macrofaunal bioturbation is an important mechanism for the enhancement of remineralization and biogeochemical cycling in marine sediments. Reduction of bioturbation activity may accordingly have far-reaching negative implications for general ecosystem performance. This is especially the case for shallow shelf seas, such as the German Bight, which account for 50% of global benthic remineralization, although they cover only 7% of the total sea surface. Increasing anthropogenic activities (e.g. wind farm construction) in these shallow shelf seas have intensified the need for reliable quantifications and predictions of macrofaunal bioturbation activities and resulting biogeochemical processes. The aim of this thesis was,thus, to develop easily applicable concepts that allow for the quantification of sediment reworking and the prediction of bioirrigation. In order to simplify the quantification of sediment reworking, which can so far only be assessed experimentally, I compared two of the most commonly applied methods (sediment profile imaging (SPI) and standard slicing technique (ST)). In addition, the time-saving and easily applicable SPI method was tested for its suitability to assess sediment reworking from cylindrical multi-corer samples (Manuscript I). The results suggested that SPI is suitable and even more accurate than ST for the investigation of sediment reworking activity. This omits the previously necessary need for timeconsuming slicing or the complex transfer into rectangular aquaria. These findings will facilitate studies on spatiotemporal patterns of sediment reworking activity in the German Bight. Such studies are of special interest as the bioturbation potential (BPc), which was previously often applied to estimate the potential of communities to rework the sediment, does not correlate with actual sediment reworking rates (Manuscript II). Surprisingly BPc, which includes sediment reworking traits (i.e. mobility and reworking mode) but no specific bioirrigation traits, rather correlated with bioirrigation activity and nutrient fluxes of silicate, ammonium, nitrate, and nitrite (Manuscript II). To overcome ambiguity of BPc, I developed the irrigation potential (IPc), as an adaptation from BPc (Manuscript III). By incorporation of bioirrigation effect traits (i.e. burrow type, feeding type, injection pocket depth), IPc was specifically designed to predict bioirrigation and its influence on biogeochemical processes (Manuskript III). I could demonstrate that, in contrast to BPc, the modified index provides an accurate quantitative measure of macrofaunal bioirrigation for both single species and entire communities of various infaunal species, if the index is calculated with ash free dry body mass. IPc provided better estimations of phosphate, silicate, ammonium, nitrate and nitrite fluxes than BPc (Manuscript IV). The estimation of silicate, ammonium, nitrate, and nitrite fluxes may be further increased if IPcis calculated in wet body mass instead of ash free dry body mass. Wet body mass thereby serves as a proxy of the irrigated sediment volume. In general, IPc could become a valuable tool to support ecosystem management and future investigations on the effects of anthropogenic activities on biogeochemical turnover in shallow shelf seas. Findings of Manuscript IV however also demonstrated that IPc is a crucial but insufficient parameter for the modelling of sediment biogeochemical processes because these also dependent on environmental conditions (e.g. temperature, sediment organic matter content, permeability). A newly proposed temperature term (IcT) (Manuscript III) may provide a tool to identify spatiotemporal variations in macrofaunal bioirrigation activity. There is however a need to determine further how IPcor IcT relate to biogeochemical cycling under different environmental conditions as well as how the respective macrofaunal traits are affected by environmental parameters.

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Wrede, A. (2018): Macrofaunal impact on biogeochemical turnover in German Bight sediments , PhD thesis, University Bremen.

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