Towards benthic ecosystem functioning maps: Quantifying bioturbation potential in the German part of the Baltic Sea
Introduction
Biogenic activities that cause sediment reworking, including burrow and mound construction, lateral ‘ploughing’ of the surface (e.g. by heart urchins), particle ingestion and egestion during foraging, food caching and prey excavation, wallowing and trampling, and the infilling of abandoned burrow structures, are known as bioturbation (Meysman et al., 2006). In a broad sense the effects of bioturbation by benthic macrofauna that helps to illustrate multiple transport mechanisms include changes of sediments diagenesis, bioirrigation, displacement of microorganisms and non-living particles (Volkenborn et al., 2007). These fundamental processes have implications for ecosystem related functions ranging from alteration of sediment biogeochemistry, organic matter regeneration and nutrient cycling to the provision and maintenance of habitats for other organisms (Mermillod-Blondin, 2011, Birchenough et al., 2012). The mixing of sediment particles and solutes through the activities of infauna is one of the most important factors controlling the fate of contaminants in sediments, because these activities enhance pore water oxygenation and keep contaminants in contact with active fauna (Konovalov et al., 2010). It is important to understand these ecosystem functions and underlying mechanisms to insure that they are safeguarded and benthic function are not adversely affected as required by the European Marine Strategy Framework Directive (MSFD). A better understanding of the functional role of benthic fauna in relevant ecosystem processes will lead to the evaluation of the ecological services of the sediments.
When bioturbation is studied from the biogeochemical angle, bioturbation rates are often estimated using a family of diagenetic bioturbation/non-local exchange models that are fitted to the in situ measured depth profiles of naturally-occurring, particle-reactive tracers, such as 210Pb, 234Th, or chlorophyll a (e.g. Boudreau, 1986, Soetaert et al., 1996). These methods are beneficial in that they may distinguish between local and non-local sediment mixing. Local mixing is characterized by random transports over very short distances as well as by an exponential decrease of the tracer with sediment depth and is quantified by a biodiffusion coefficient Db. Non-local sediment mixing is defined by the occurrence of subsurface maxima due to e.g. discrete burrowing events or feeding behavior (Boudreau, 1986). Subject to the chosen tracer these methods will reflect rather short term and discernible small-scale spatial heterogeneity in mixing, as indicated by large variations in replicate cores. The values of Db reported for the marginal sediments of the Kara Sea by Mulsow and Povinec (2002) ranged between 0.03 and 27.4 cm−2 yr−1 (based on single core collected per locations from 17 to 30 m water depth). Based on high sampling effort (with 4 cores at 6 locations few hundred meters apart collected at each of 6 stations) in the southern Baltic Sea Morys et al. (2016) suggested strong differences in modes of sediments reworking at replicate cores within one station; differences in Db estimated for replicate cores varied from factor 3 (at sandy stations) to factor 30 (at muddy Mecklenburg Bight). There was a factor of 20 between lowest injection flux (J = 0.04 μg cm−2 d−1) and highest estimate (J = 0.8 μg cm−2 d−1) for 24 cores collected in the Lübeck Bay. Based on mean chlorophyll profiles between-stations variability in local mixing (Db) showed a factor of 20 (with absolute values reported for Db ranging from 0.006 cm2 d−1 in Lübeck Bay to 0.4 cm2 d−1 in Stolteraa) and for stations characterized by non-local biomixing an injection flux differed by a factor of 6 (J ranged from 0.05 μg cm2 d−1 to 2.1 cm depth (Oder Bank) to subsurface maximum injection flux of 3.9 μg cm−2 d−1 in the Arkona Basin). According to Morys et al. (2016) non-local transport accounts for 33 to 50% of the investigated area in the west of the German part of the Baltic Sea and for 70–100% in the east, and there are first hints that the variability of DB, injection fluxes and amount of local versus non-local transports depend on the different compositions of the macrofauna population, the patchy distribution of the benthic organisms, and their adaptation to different salinities, as well as on food supply.
In situ quantification of bioturbation can be achieved by many methods, requiring not only technology and resources not always available, and not feasible in some settings but also conventional expert knowledge on species identity and associated biological attributes such as traits. Where dedicated biogeochemical research programmes do not exist, a practical alternative is the adoption of a trait-based approach. When trying to categorize and understand ecosystem functions conducted by benthic communities, bioturbation intensity can be quantitatively estimated from benthic quantitative data using the metric of bioturbation potential BPc (Solan et al., 2004, Birchenough et al., 2012). BPc provides an estimate of relative bioturbation intensity integrated over time of the development of macrofauna community at its sampled state. It is a simple value of the possible bioturbation activity and does not include any realistic measurements. It means that it is a sum parameter without consideration of temporal variability, intra- and interspecific interactions, individual fitness and behaviour. Neither the realistic contribution of species/individuals nor the temporal and spatial patterns are reflected. Yet, due to the paucity of data concerning mechanistic attributes of bioturbation (transport rates, activity, mixing depth), and the fact that the majority of this data is focusing on single species, artificially manipulated assemblages or sediment geology, until now the use of BPc may indeed be the only available option to investigate bioturbation at the regional scale. This was underlined in the recent study by Queirós et al. (2015), dealing with quantification of variability of community level bioturbation and association with seasonal drivers in the Western Channel. By calculating BPc over time, or for different locations or scenarios, changes in the efficiency of the organism-sediment coupling can be monitored for compliance in support of management and policy objectives (Queirós et al., 2013). Our present study provides a regional contribution to this thematic. Particularly important for managers and ecologists, large scale representation of BPc, complementing more information on species’ ecological importance and providing a good test case to illustrate the uses of this metric, are lacking until now and are addressed in the present paper.
In order to create full-coverage spatial information of biological features spatial modelling techniques that link environmental parameters to biological information have become increasingly popular in recent years (Reiss et al., 2015). This work aims (1) to assess the seasonal and interannual variability of ecosystem functioning expressed by BPc in the German part of the Baltic Sea, (2) to identify key species contributing to bioturbation, and (3) to estimate, model and map its spatial differences in the study area.
Section snippets
Study area
The sedimentary habitats in the south-western Baltic Sea (Fig. 1, study area amounts to about 14.8*103 km2, average depth is 19 m) are mainly shaped by postglacial processes. Shallow areas along the shore and on top of the offshore glacial elevations are characterized by a mosaic of rocks, till, gravel and coarser sands. Substrate gets generally finer with increasing water depth. Organic-rich muddy sediments dominate in the basins and deeper part of trenches (Darr et al., 2014). Near-bottom
Validation of BPc against measured biodiffusion intensity (Db)
Based on the data collected at SECOS stations (Fig. 1) BPc was significantly correlated with Db (Pearson's product-moment correlation r = 0.46, p = 0.011, df = 25) and J (r = 0.52, p = 0.007, df = 24), however values of Pearson correlation coefficient between these estimates was at best moderate. No significant linear relation was found between BPc and derived first order ingestion rate r (applying the p < 0.05 significance threshold, df = 10). Interestingly, despite the very small sampling size, the only
Discussion and conclusions
Sediment destabilisation by sediment-reworking organisms is common in coastal aquatic environments, and observed changes in ecosystem properties are dependent on changes of functional attributes of inhabiting species community (Hale et al., 2014). Solan et al. (2004) have proposed the use of community bioturbation potential (BPc) as a quantitative, though relative, estimate of potential ecosystem functioning. The aim of this study was to describe the changes of this dimensionless index of
Acknowledgements
This work was supported by the German Federal Ministry for Education and Research (KÜNO Project SECOS, grant number 03F0666A). R.F. was partly funded by KÜNO-Project MOSSCO (03F0740B). Supercomputing power was provided by HLRN (North-German Supercomputing Alliance). We thank Martin Solan for his encouraging comments and willingness to help to improve the manuscript. We also thank two anonymous reviewers for their careful reading of our manuscript and their insightful remarks and suggestions.
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2021, Estuarine, Coastal and Shelf ScienceCitation Excerpt :In the context of global change (Worm et al., 2006), there is increasing interest in predicting ecosystem functioning and processes (Gogina et al., 2017) and many studies have focused on understanding the key role of organisms in benthic ecosystems (Gaston and Petchey, 2002; Queirós et al., 2015; Savage et al., 2007).