Impact of polychaetes (Nereis spp. and Arenicola marina) on carbon biogeochemistry in coastal marine sediments†
Institute of Biology, Odense University, SDU, DK-5230 Odense M, Denmark
Known effects of bioturbation by common polychaetes (Nereis spp. and Arenicola marina) in Northern European coastal waters on sediment carbon diagenesis is summarized and assessed. The physical impact of irrigation and reworking activity of the involved polychaete species is evaluated and related to their basic biology. Based on past and present experimental work, it is concluded that effects of bioturbation on carbon diagenesis from manipulated laboratory experiments cannot be directly extrapolated to in situ conditions. The 45–260% flux (e.g., CO2 release) enhancement found in the laboratory is much higher than usually observed in the field (10–25%). Thus, the faunal induced enhancement of microbial carbon oxidation in natural sediments instead causes a reduction of the organic matter inventory rather than an increased release of CO2 across the sediment/water interface. The relative decrease in organic inventory (Gb/Gu) is inversely related to the relative increase in microbial capacity for organic matter decay (kb/ku). The equilibrium is controlled by the balance between organic input (deposition of organic matter at the sediment surface) and the intensity of bioturbation. Introduction of oxygen to subsurface sediment and removal of metabolites are considered the two most important underlying mechanisms for the stimulation of carbon oxidation by burrowing fauna. Introduction of oxygen to deep sediment layers of low microbial activity, either by downward irrigation transport of overlying oxic water or by upward reworking transport of sediment to the oxic water column will increase carbon oxidation of anaerobically refractory organic matter. It appears that the irrigation effect is larger than and to a higher degree dependent on animal density than the reworking effect. Enhancement of anaerobic carbon oxidation by removal of metabolites (reduced diffusion scale) may cause a significant increase in total sediment metabolism. This is caused by three possible mechanisms: (i) combined mineralization and biological uptake; (ii) combined mineralization and abiogenic precipitation; and (iii) alleviation of metabolite inhibition. Finally, some suggestions for future work on bioturbation effects are presented, including: (i) experimental verification of metabolite inhibition in bioturbated sediments; (ii) mapping and quantification of the role of metals as electron acceptors in bioturbated sediments; and (iii) identification of microbial community composition by the use of new molecular biological techniques. These three topics are not intended to cover all unresolved aspects of bioturbation, but should rather be considered a list of obvious gaps in our knowledge and present new and appealing approaches.
Geochemical Transactions 2001, 2:92.Open Access Article.
It has long been recognized that activities of macrobenthic organisms have significant effects on sediment–water solute exchange and diagenetic reactions within sediments. Much experimental and modelling work done over the years on the effects of reworking and irrigation by burrowing animals such as polychaetes, crustaceans and bivalves [1-5] have not only enhanced our understanding of bioturbation effects, but also provided important knowledge on general mechanisms controlling diagenetic processes in sediments. It is now well established that the distribution of a porewater solute is determined by the balance between transport and reaction processes, and that irrigating infauna may change this balance dramatically by enhancing transport conditions. This has been modelled by a variety of excellent models such as the cylinder model of Aller and the non-local exchange model of Emerson. Although these two models basically are equivalent, the construction of the cylinder model allows the incorporation of additional realistic complexity (e.g. semipermeable burrow linings and periodic irrigation).
Nevertheless, the real complexity created from a multitude of behavioural activities by the wide variety of burrow-dwelling animals found in marine sediments is extremely difficult to describe mathematically. Activities, such as filter-or suspension-feeding and specific locations of faecal pellet deposition, may have serious impact on diagenetic reactions and solute distributions within sediments. While models may improve our general knowledge on bioturbation effects, the particular activity of any specific assembly of infauna can only be determined by examining both the impact of each individual species as well as of the entire community.
This paper attempts to review the current knowledge on the impact of common intertidal polychaetes, Nereis spp. and Arenicola marina, on fluxes and reaction rates involving carbon in coastal sediments. The behaviour and life habits of these animals are linked to measured fluxes and porewater profiles of CO2 as well as carbon reaction rates within sediments. The results are also used for the construction of simple (conceptual) models describing the impact of these species on carbon diagenesis in sediments. Finally, some guidelines are given for future work still needed to provide a more complete understanding of sediment carbon biogeochemistry and microbiology associated with Nereis spp. and A. marina.