Burrow- Sediment Relations 



The sediment color along present and past burrows was also useful in 

 semi-quantifying the role of A^. incisa in 1) aeration of sediment and 2) in 

 increasing the effective surface area of the sediment-water interface into the 

 benthos. There is always some oxygen diffusion across any sediment-water 

 interface, assuming the overlying water is oxygenated. Wherever burrows 

 penetrate the sediment and are irrigated with oxygenated water, a halo of light 

 brown or yellow oxygenated sediment soon develops around the burrow. The 

 transition of yellow-brown to black, 2-5 cm deep, approximates the limit of 

 oxygen penetration. Thus an increase in "aerobic" sediment is described by the 

 expression: 



halo ~ burrow-halo system ' burrow 



Tliis volume was estimated here by measuring the light brown oxidized zone 

 visible against the thin, sediment-filled glass aquaria using planimetry. This 

 subsurface oxygenation persists for some time after a worm abandons the 

 burrow, since efferent oxygenated irrigation water typically continues to 

 course through old burrows. The rate of increase in sediment aeration 

 following introduction of a single worm in a thin aquarium at 18°C is 

 summarized in Figure 20-8. The 2-5 mm thick "aerobic" halo is continuous 

 with and as thick as the aerobic zone at the sediment-water interface (see 

 Figure 20-3). The dotted line in Figure 20-8 represents the depth of aerobic 

 sediment in an aquarium without any worm present, that is, the aerobic zone 

 at the sediment surface. Any increase above this level represents that resulting 

 as a consequence of burrow irrigation activities. By comparing the aeration 

 rates depicted in Figure 20-8, it is apparent that the slope of the curve increases 

 with water temperature. This rise in sediment aeration eventually levels off in 

 time as an equilibrium develops between oxygenation of new burrow sediment 

 and chemical reduction of oxygenated sediment along old abandoned burrows. 

 Figure 20-8 summarizes the relationship between temperature and rate of 

 Nephtys sediment aeration observed in laboratory in situ thin aquaria. It is 

 apparent that the extent of sediment oxygenation is positively correlated with 

 temperature, i.e. more oxygen is delivered to deeper layers during warmer 

 seasons. Hence, even thougli oxygen demand by the benthos is at its maximum 

 during warm periods, the actual sediment aeration ihiou^ Nephtys burrowing 

 and irrigation can be even greater. 



DISCUSSION 



A^. incisa burrows through sediment using adaptations previously described 

 for other Nephtys and Nereis species (Schafer, 1962) and for Arenicola marina 

 (Wells, 1952). This specialized locomotion called "bolting" refers to the head 



313 



