• Global patterns of bioturbation intensity and mixed depth of marine soft sediments

      Teal, L. R.; Bulling, Mark T.; Parker, E. R.; Solan, Martin (2013-06-11)
      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 (D b ) and/or a mixed layer depth (L). Using measures of D b (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 D b . 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 D b that reflect differences in boundary conditions and reaction properties of the methods used. Tracers with longer half-lives tend to give lower D b 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 D b , 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 (D b ) 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 km 3 . 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.
    • Indirect effects of non-lethal predation on bivalve activity and sediment reworking

      Maire, O.; Merchant, J. N.; Bulling, Mark T.; Teal, L. R.; Grémare, A.; Duchêne, J. C.; Solan, Martin (2013-05-24)
      Deposit-feeders are the dominant bioturbators of aquatic sediments, where they profoundly impact biogeochemical processes, but they are also vulnerable to both lethal and non-lethal predation by a large variety of predators. In this study, we performed a series of experiments to test the effects of predation avoidance on the feeding activity and sediment reworking intensity of the deposit-feeding bivalve Macoma balthica. Feeding activity at the sediment–water interface and sediment reworking intensity (vertical displacements of inert particle tracers) were monitored using image analysis techniques for treatments including and excluding the predatory shrimp, Crangon crangon. Detection of C. crangon by M. balthica resulted in an immediate retraction of the feeding siphon and a reduction in feeding activity. M. balthica also buried deeper into the sediment in the presence of C. crangon. This predator avoidance behaviour indirectly affected sediment reworking modes and rates, increasing the thickness of the bioturbated sediment layer as well as the non-local transport of sediment particles at depth. Conversely, feeding activity and sediment reworking processes remained unaffected when C. crangon was present, but isolated from the sediment, suggesting that predator perception in M. balthica is tactile (i.e. induced by direct encounter) rather than being chemosensory. Collectively, these results demonstrate that predatory avoidance behaviour by benthic infauna can significantly impact benthic bioturbation and the incorporation of organic matter into the benthic food web.