The activities of benthic fauna have profound impact on physical, chemical and biological conditions in aquatic sediments. Sediment reworking and water irrigation (or ventilation) displaces both particles and porewater within the sediment.[22] By volume, the amount of water moved is much larger than that of particles, and burrows act as channels for the direct communication between subsurface porewaters and overlying water. The enhanced solute exchange, therefore, redistributes dissolved reactants and products of microbial reactions within sediments.[23,24]
Irrigation (ventilation)
Most infaunal animals actively irrigate their burrows with oxygen-rich overlying water. The renewal of burrow water serves important transport functions for the animals, such as supply of oxygen, removal of toxic metabolites and providing suspended food items. However, burrow irrigation may also affect the distribution of meiofauna and microorganisms, as well as the associated biogeochemical processes within the sediment.[4,5,25]
The irrigation of Nereis and Arenicola is intermittent with active and quiescent periods in a more or less rhythmic fashion. N. virens is active for about 20% of the time with 5–10 min irrigation periods followed by 20–30 min inactivity[26] (Fig. 6). N. diversicolor is considerably more active, particularly when it is suspension-feeding.[9] It also has active irrigation periods of 5–10 min duration, but only interrupted by very short periods of inactivity in a very regular pattern. During the inactive periods it ingests the ripe mucus net and spins a new one. Even when there are no food particles in the overlying water and the suspension-feeding activity of N. diversicolor ceases, this worm remains active for about 50% of the time. The irrigation activity of Arenicola marina proceeds in cycles of 40 to 60 min duration (Fig. 6). These very regular cycles are probably under the control of a pacemaker situated in the nervous system[27,28] and are linked to the feeding and defecation cycles.
The total volume of water pumped by populations of these worms is impressive (Table 1). Particularly, the suspension-feeding N. diversicolor is capable of pumping large volumes of water into the sediment. Natural populations (1000–3000 m-2) of this species may pass a volume corresponding to a water column of 3 to 9 m through their burrows each day.[29] Consequently, their burrows are continuously flushed with overlying water resulting in highly oxic and oxidized conditions in burrow water and surrounding sediment (Fig. 7).
Reworking
Macrofaunal reworking affects the stability and composition of coastal marine sediments. Thus, organic matter deposited in the sediment is usually redistributed, i.e. from surface to subsurface layers and vice versa. However, various burrow-dwelling animals disturb the sediment structure differently depending on their specific life habit and feeding type.[22,30]
The sedentary A. marina is a typical head-down 'conveyor-belt' feeder, which stays more or less permanently in its burrow while eating subsurface sediment and defecating at the surface. This is done in a cyclical pattern in phase with the irrigation cycles. Defecation is evident in Fig. 6 as short bursts of negative water flow caused by the backward movement of the worm. The amount of sediment displaced by populations of A. marina is quite considerable with the potential of complete turnover of up to a 40 cm thick sediment layer per year (Table 2). Since lugworms only ingest particles of less than 2 mm, all larger particles are left behind just below the feeding depth of the worms. Thus, a 'graded bedding'[22] with a distinct layer of gravel and bivalve shells at a depth of about 40 cm can usually be observed in areas with dense populations of A. marina. Another visual evidence of the intense reworking by lugworms is the characteristic troughs and piles at the sediment surface indicating the position of feeding funnels and the tail shafts, respectively (Fig. 5).
Free-living polychaetes, like Nereis spp., are actively moving around at the sediment surface or within their burrow systems in search for food. Occasionally they abandon burrows, either willingly or forced by inter- and intraspecific fights, and move some distance before digging a new 'home'. The amount of particles moved by these feeding and digging activities of Nereis spp. is quantitatively small (i.e. 2%) compared with A. marina (Table 2). However, the selective feeding of Nereis spp. on fresh plants, animals and microorganisms at the sediment surface associated with suspension feeding, subsurface defecation and mucus secretions along burrow walls may redistribute significant amounts of reactive organic matter within the sediment.[9]
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