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Global Patterns of Bioturbation Intensity and Mixed Depth of Marine Soft Sediments

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Global Patterns of Bioturbation Intensity and Mixed Depth of Marine Soft Sediments Vol. 2: 207–218, 2008 AQUATIC BIOLOGY Printed June 2008 doi: 10.3354/ab00052 Aquat Biol Published online June 19, 2008 Contribution to the Theme Section ‘Bioturbation in aquatic environments: linking past and present’ OPENPEN ACCESSCCESS Global patterns of bioturbation intensity and mixed depth of marine soft sediments L. R. Teal1, 2,*, M. T. Bulling1, 3, E. R. Parker2, M. Solan1 1Oceanlab, University of Aberdeen, Main Street, Newburgh, Aberdeenshire AB41 6AA, UK 2CEFAS, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK 3University of York, Environmental Department, Heslington, York YO10 5DD, UK ABSTRACT: The importance of bioturbation in mediating biogeochemical processes in the upper centimetres of oceanic sediments provides a compelling reason for wanting to quantify in situ rates of bioturbation. Whilst several approaches can be used for estimating the rate and extent of bioturba- tion, most often it is characterized by calculating an intensity coefficient (Db) and/or a mixed layer depth (L). Using measures of Db (n = 447) and L (n = 784) collated largely from peer-reviewed litera- ture, we have assembled a global database and examined patterns of both L and Db. At the broadest level, this database reveals that there are considerable gaps in our knowledge of bioturbation for all major oceans other than the North Atlantic, and almost universally for the deep ocean. Similarly, there is an appreciable bias towards observations in the Northern Hemisphere, particularly along the coastal regions of North America and Europe. For the assembled dataset, we find large discrepancies in estimations of L and Db that reflect differences in boundary conditions and reaction properties of the methods used. Tracers with longer half-lives tend to give lower Db estimates and deeper mixing depths than tracers with shorter half-lives. Estimates of L based on sediment profile imaging are significantly lower than estimates based on tracer methods. Estimations of L, but not Db, differ between biogeographical realms at the global level and, at least for the Temperate Northern Atlantic realm, also at the regional level. There are significant effects of season irrespective of location, with higher activities (Db) observed during summer and deeper mixing depths (L) observed during autumn. Our evaluation demonstrates that we have reasonable estimates of bioturbation for only a limited set of conditions and regions of the world. For these data, and based on a conservative global mean (±SD) L of 5.75 ± 5.67 cm (n = 791), we calculate the global volume of bioturbated sediment to be >20 700 km3. Whilst it is clear that the role of benthic invertebrates in mediating global ecosystem processes is substantial, the level of uncertainty at the regional level is unacceptably high for much of the globe. KEY WORDS: Bioturbation · Sediment mixed depth · Bioturbation coefficient · Global analysis · Tracer · Sediment profile imaging Resale or republication not permitted without written consent of the publisher INTRODUCTION mixed layer, the recycling of nutrients, and the flux of materials from the sediment to the water column. The seabed is the most extensive habitat on the Accurate measurements of bioturbation rates are cru- planet, occupying >75% of the Earth’s surface. Most cial in determining how faunal-induced fluid and par- marine sediments are cohesive muds that are replete ticle redistribution affects the properties of the sedi- with complex biogeochemical cycles, which are critical ment profile. Given the vast area of marine sediments for the marine ecosystem. Benthic macrofauna biotur- across the globe and the importance of bioturbators bate and bioirrigate the upper centimetres of marine in mediating ecosystem processes, there are numerous sediments, significantly increasing the depth of the studies measuring bioturbation values worldwide. To- *Email: [email protected] © Inter-Research 2008 · www.int-res.com 208 Aquat Biol 2: 207–218, 2008 gether, these form a valuable repository of infor- al. 1997). Conversely, no apparent relationships between mation on global patterns of bioturbation intensity and L and ω or L and water depth were found. In fact, L the mixed depth of marine soft sediments (see e.g. exhibited a well-defined world-wide mean (±SD) of 9.8 Boudreau 1994, 1998, Middelburg et al. 1997). ± 4.5 cm (Boudreau 1994), which was later shown to re- Studies investigating the rate and extent of bioturba- sult from the feedback between the food dependence of tion have generally adopted modelling techniques that bioturbation and the decay of that resource (Trauth et allow the distribution and exchange rates of pore water al. 1997, Boudreau 1998, Smith & Rabouille 2002). In solutes, or sediment particles, across the sediment– this case, the independence of L would indicate that water interface to be quantified and compared. Biotur- considerable confidence can be placed in using an bation is characterized numerically by determining a average L in sediment modelling (Boudreau 1994), al- mixed layer depth (L), over which sediment mixing though we recognize that the factors influencing L are most frequently occurs, and/or a bioturbation coeffi- likely to be numerous and interact with one another. cient (Db) (for review, see Meysman et al. 2003). Db is This contribution builds on that of Boudreau (1994, defined as the rate at which the variance of particle 1998) by extending the database of L and Db. Our location changes over time, where the variance is a objectives were to: (1) examine the relationship be- measure of the spread of particles in a tracer profile tween measured estimates of bioturbation and the and is proportional to the velocity of the diffusing method of determination, (2) determine the influence particle (Crank 1975), thus providing a convenient of season, water depth and location on Db and L, (3) descriptor of the intensity of bioturbation. Both L and identify areas of high and low Db and L, (4) highlight Db can be determined by measuring the vertical distri- regions or benthic habitats that have been poorly bution of a tracer through the sediment profile. Tracers researched, and (5) refine the global estimate of biotur- commonly used for bioturbation experiments include bated sediment. In so doing, we wish to summarise the naturally occurring, particle reactive radionuclides and present knowledge of bioturbation at the global level artificial tracers such as glass beads, fluorescent sedi- and, after taking into account biases in the data, rec- ment particles (luminophores), metal-doped sediment ommend areas of future research. and isotopically labelled algae (Mahaut & Graf 1987, Wheatcroft et al. 1994, Blair et al. 1996, Gerino et al. 1998, Sandnes et al. 2000, Berg et al. 2001, Green et al. METHODS 2002, Forster et al. 2003). Incorporation of the tracer into the sediment by faunal activity results in a vertical Global database. Global data on L and Db were col- tracer profile that is either: (1) smooth and exponential lated from peer-reviewed literature. Data were re- in decline, in which case faunal-mediated particle trieved from the ‘ISI Web of Knowledge’ using the ‘Sci- movements approximate diffusional mixing, or (2) more ence Citation Index Expanded’ and ‘Social Sciences complex, resulting from discrete burrowing events, in Citation Index’ databases. A ‘general search’ using the which case particles are displaced by the fauna in a search term bioturbation in the titles and key words of series of ‘non-local’ movements. Mathematical models all document types, in all languages, was performed appropriate to the form of the tracer profile are for the publication years 1970 to 2006. All published applied, and a Db coefficient can be estimated (for an sources were manually searched for values of L and/or overview, see Meysman et al. 2003, 2008, this Theme Db, and, where available, additional information was Section). L can also be determined from tracer profiles, gathered on the geographical position of the study (lat- or, when using alternative methods such as sediment itude, longitude), water depth, sedimentation rates (ω), profile imaging (SPI; Rhoads & Cande 1971), can be type of tracer used (hereafter method) and month of measured directly from the sediment profile. year the measurement was taken. Additional Db and L Despite methodological differences (Gerino et al. 1998), values were added from the publications cited in some analyses of published Db and L values have been Boudreau (1994), and further estimates of L were used to determine empirical relationships between obtained from the sediment profile imaging literature. parameters used in diagenetic models and water depth Much of the latter includes monitoring studies that (Boudreau 1994, 1998, Middelburg et al. 1997). Such document localized benthic impacts. As our focus was empirical relationships can enable predictions to be to determine representative estimates of bioturbation, made about biogeochemical rates and processes in only reference sites furthest away from any anthro- oceanic sediments where data are still limited or absent. pogenic impact (as defined by the study authors), In a rudimentary analysis of >200 datapoints, for exam- or sites acknowledged as having no discernable ple, Boudreau (1994) found a significant correlation anthropogenic impact, were used. In addition, we between Db and sedimentation rate (ω), both of which determined additional values for L from previously decrease with increasing water depth (Middelburg et unpublished SPI surveys (n = 31; for sources, see Teal et al.: Bioturbation intensity and mixed depth of marine soft sediments 209 ‘Acknowledgements’). In SPI images, the depth of L is formation reported in each study using Google Earth delineated using the vertical colour transition (from (http://earth.google.com/). To account for regional dif- brown to olive green/black) that occurs within the ferences in environmental conditions, Bailey’s oceanic sediment profile (Fenchel 1969, Lyle 1983). This col- regions (Bailey 1998) and Spalding’s coastal regions oration is dictated by the redox state (ferrous or ferric) (Spalding et al. 2007) were combined to ensure com- of the dominant electron acceptor iron (Lovley & plete global coverage.
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