Bioturbation in estuarine sediments: modelling macrofauna-mediated oxygen dynamics
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The aim of this thesis was to provide a comprehensive examination of the roles of macrofauna in estuarine biogeochemistry by adopting a multidisciplinary field and laboratory-based approach and integrating species distribution modelling with biomass-dependent ecosystem function relationships. Eventually, an ecosystem management tool to provide insights into the ecological consequences of anthropogenic disturbance in estuaries was developed using the Western Scheldt as a model system. To establish a baseline understanding of the spatio-temporal patterns of macrobenthos (activities) and ecosystem functioning in the Scheldt estuary, a seasonal survey was carried out during four consecutive seasons in 2015 and 2016, sampling three habitats with different hydrodynamic regimes (low-dynamic intertidal, high-dynamic intertidal, subtidal) in each of the three main salinity zones (polyhaline, mesohaline, oligohaline). Fluxes of dissolved oxygen, nitrate, nitrite, ammonium and phosphate were measured, as well as environmental properties of the sediment and water, macrofaunal activity (bioturbation and bio-irrigation rates) and macrofauna trait composition with respect to sediment reworking. Luminophores were applied on top of the sediment to measure sediment particle mixing modes and rates. The decline in the concentration of the added inert tracer sodium bromide in the water column was used to estimate pore water exchange rate as a proxy for bio-irrigation. Data and insights obtained from this field survey are presented in Chapter 2 and Chapter 3. Chapter 2 assesses the spatial and temporal variability of these benthic ecosystem processes (i.e. particle mixing and bio-irrigation) in the Scheldt estuary. Luminophore profiles showed that biodiffusion was the dominant particle reworking mode. Rates of both particle reworking and bio-irrigation were highly variable among seasons and habitats, and different species were found to contribute unevenly to both processes, and between habitats and seasons. Habitat structuring effects on populations, density-dependent interactions with the habitat, and temperature-driven variability in macrobenthos activity and living position in the sediment are suggested to explain the observed spatial and temporal differences. In Chapter 3, the relative contributions of macrofauna and the environment to the spatial and temporal variability in benthic biogeochemistry were investigated. Using distance-based redundancy analysis, we found total density, bio-irrigation and temperature to be the main contributors to biogeochemical variability, but this model explained only 23 % of the total variation. Variation partitioning and analyses of subsets of the data in the different seasons, showed that processes linked to the biota were lowest and overruled by environmental steering in the coldest period of the year, while processes related the burrowing behaviour of macrofauna, or densities and biomasses of specific sediment reworking functional groups, predominantly determined biogeochemical variability from June onwards. Macrofaunal contribution to biogeochemistry was highest in low-dynamic intertidal habitats that are densely inhabited by biodiffusing and bio-irrigating fauna. We conclude that the impacts of macrofaunal and environmental factors on the biogeochemical fluxes in the Scheldt estuary vary along the estuarine gradients and with season and are thus highly context-dependent. This spatial and temporal variability should therefore be considered to extrapolate biogeochemical fluxes to entire ecosystems. The impact of two bioturbating benthic invertebrates, Limecola balthica and Hediste diversicolor, on sediment community oxygen uptake in the Scheldt Estuary was examined in Chapter 4. Both H. diversicolor and L. balthica irrigate the sediment, the former by ventilating their burrows and the latter by siphoning water. Laboratory-controlled microcosms containing defaunated sediment amended with artificially composed faunal densities of different body sizes were used to test how species identity, habitat, and population density influence O2 uptake in different habitats (muddy and sandy sediments) in monoculture. Both L. balthica and H. diversicolor facilitated O2 fluxes between the sediment and the overlaying water, and a major portion of the variance in sediment metabolism and bio-irrigation could be explained by the per capita body size and density, or by the total biomass of the inhabiting bioturbators. H. diversicolor showed a more pronounced relationship between biomass/density and faunal-mediated O2 consumption than L. balthica. Respiration was significant in predicting faunal-mediated O2 uptake among different species (H. diversicolor and L. balthica) across different habitats (sandy and muddy sediments); whilst the relationship between bio-irrigation and faunal-mediated O2 uptake was significant for H. diversicolor in both sediment types and and for L. balthica in sandy sediments only, with a lower predictive power of bio-irrigation compared to respiration as the predictor. Analysis of covariance demonstrated significant habitat effects in biomass-dependent bio-irrigation, which might be attributed to different physical constraints (e.g. O2 availability) in sandy and muddy sediments. In summary, we demonstrated that the faunal-mediated O2 consumption by macrobenthos is density- and biomass-dependent, but the extent to which the variance of faunal-mediated O2 uptake can be explained is also conditioned by the interplay between the abiotic environment and the biological traits of the species, which is in support of our hypothesis that biological traits (e.g. bio-irrigation) and sediment physico-chemical properties significantly affect faunal-mediated O2 consumption. The specific example of using macrobenthos for ecology conservation and estuarine ecosystem management is explored in Chapter 5, with H. diversicolor as the model species, since this species was found to be the dominant contributor to biogeochemical fluxes in the Scheldt estuary (Chapter 2-3). We quantified the contribution of H. diversicolor across a range of biomasses in sediment metabolism and extrapolated the spatial variability of the faunal-mediated O2 consumption based on its biomass and distribution within its natural habitats in the Western Scheldt. Biomass-scaling of faunal-mediated O2 uptake by H. diversicolor was quantified from laboratory-controlled microcosms containing defaunated sediment with artificially composed faunal densities and body sizes in its habitats (polyhaline sandy sediment, polyhaline muddy sediment and mesohaline muddy sediment) along the estuarine gradients of the Western Scheldt; and the spatial variability of the fauna-mediated O2 consumption was extrapolated at the landscape scale by combining spatial mapping of H. diversicolor developed from multi-quantile regression modelling. Furthermore, the years 1955 and 2010 were compared to investigate changes in H. diversicolor-mediated O2 consumption in relation to the anthropogenic modifications of the estuary between both years. By tuning the quantile of the responses (upper quantile and full quantile), two species distribution scenarios were developed to describe organisms’ responses to different environmental constraints. Biomass of H. diversicolor was a highly significant (R2 = 0.9) determinant of fauna-mediated O2 uptake (F1,26 = 119.6, p < 0.001) that was unaffected by habitat type (F2, 26 = 3.12, p = 0.06). Therefore, the governing function of biomass scaling can be used to scale the biomass-dependent fauna-mediated O2 uptake to each of three different habitat types. In the intertidal region, maximal fauna-mediated O2 uptake estimated from the upper quantile regression (tau=0.95) was 8772 and 6201 mol d-1 in 1955 and 2010, respectively, corresponding to respective total biomasses of 4407.12 kg and 2915.03 kg H. diversicolor for the entire Western Scheldt. The full quantile regression model revealed a decline in total biomass of H. diversicolor to 34% of its 1955 level by 2010 (from 3044.71 kg to 1040.03 kg) in the intertidal region of the Western Scheldt, which corresponded with a reduction in the faunal-mediated O2 consumption by H. diversicolor in the intertidal area to 39.7% of the 1955 value by 2010, i.e. a decline from 5401.31 to 2145.93 mol d-1. Overall, variabilities in maximum current velocity induced by dredging activities most likely affected the spatial distribution and biomass of H. diversicolor. In Chapter 6, the key findings from data chapters Chapter 2-4 are combined to constitute a synthetic discussion about benthos contribution to ecosystem functioning across space and time, along with implications of the modelling methodology applied in Chapter 5 for the use of macrobenthos for estuarine ecosystem management. The approach adopted in this thesis, which combines small-scale experiments (Chapter 2-4) with broad-scale modelling (Chapter 5), could be used to predict faunal-mediated O2 consumption at an estuarine landscape level. The integration of species distribution and biomass-dependent ecosystem functioning models allows quantification and upscaling of variation in oxygen dynamics induced by bioturbators at high spatial resolution over a large temporal scale. Additionally, reflections of this study and an outlook for future research are presented. This thesis bridges small-scale experimental studies with the broad-scale mapping needs of society and managers, developing a predictive framework that can inform policy makers and conservation practitioners for sustainable management of estuaries. This approach is expected to have direct applications for society through the identification of risks to ecosystem functioning and through the generation of advice on species management in view of the provisioning of ecosystem services. It is clear from the work presented in this thesis that macrobenthic bioturbators play an important role in regulating oxygen dynamics in the Scheldt estuary, therefore conservation of specific species (e.g. H. diversicolor and L. balthica) should be a key priority. These commonly occurring species should be properly monitored and preserved to ensure that their population biomass remains sufficiently high to secure the delivery of ecosystem services. Human activities such as dredging can yield important direct and indirect negative impacts on habitats for macrobenthos, so the natural habitats of key bioturbators should be protected from human impacts through environmental legislation and management (e.g. European Habitats Directive and the EU Marine Strategy Framework Directive). In a wider context, species and ecosystem conservation and management need to integrate the vulnerability of key species to climate change (e.g. temperature and ocean acidification) and the predicted changes to the provisioning of ecosystem services. Additionally, the potential habitat loss caused by sea level rise and coastal squeeze are major challenges for coastal management in the Scheldt estuary. There is no place for intertidal habitats to migrate along with sea level rise since the estuary is heavily confined by dikes. On the one hand, sea level rise may pump saltwater further upstream; on the other hand, the changes in rainfall may affect freshwater intrusion. Even though more research is needed, the results in this study can act as a baseline for future studies, and the proposed predictive framework is expected to be more broadly applicable to quantify the bioturbation impact of other key species on landscape evolution and ecosystem functionality.