Competitive Bioturbators in the Wadden Sea and Ariake Sound
163
During the stable period for the population on the Tomioka Bay sand flat,
the higher densities of newly recruited shrimps at the seaward stations
(Tamaki and Ingole 1993; Tamaki et al.1997) and its reflection in adult shrimp
densities (Fig. 7.7a) could primarily be due to the above-mentioned positive
intraspecific relationship. Intraspecific competition for space could also
regulate the population density. In fact, the burrow space exclusively occupied
by each shrimp is a good predictor of the carrying capacity for the population
on the sand flat (Tamaki et al. 1997; Tamaki and Veno 1998). For the population decline at the two seaward stations in recent years, one possibility is the
sudden increase in the abundance of the stingray, Dasyatis akajei, from 1995,
the fish voraciously feeding on ghost shrimps and disturbing the substrate to
a considerable depth (up to 20 em), especially in the lower two-thirds of the
sand flat (A. Tamaki, unpublished data).
7.6 Effects of the Ghost Shrimp Expansion and Decline
7.6.1 Effects on Sediment Properties
Following the expansion of N. harmandi over the Tomioka Bay sand flat,
sediment properties were remarkably altered by the shrimp bioturbation on
the landscape scale (Tamaki and Suzukawa 1991; Tamaki 1994; Fig. 7.6).
Destabilisation occurred because mounds deposited on the sand flat surface
were easily dispersed by tidal currents and waves causing increased
erodability. An estimated deposition rate of 2.6-12.7 mm/day would result in
a mixing of the entire sediment column in 47-235 days. The silt-day content
at the surface sediment has been reduced due to the ghost shrimps 'blowing
off' the fine particles resuspended into the water column, thereby reducing
the cohesiveness of the sediment. Increased oxygenation of the entire sediment column has occurred due to the overlying water introduced into the
burrows, such that the brown layer that was originally close to the surface is
now extended to the substrate base. Finally, larger shell material, such as
empty shells of U. moniliferum, was buried deeper by continual sediment
deposition by ghost shrimps. Thus, the substrate properties of the entire
dissipative sand flat were altered by the ghost shrimp to approximate those of
exposed sandy beaches. Since 1995, the substrate properties have not been
examined in detail, but are likely to have shifted due to the lowered ghost
shrimp densities (Fig. 7.7a) and increased disturbance by stingrays.
163
During the stable period for the population on the Tomioka Bay sand flat,
the higher densities of newly recruited shrimps at the seaward stations
(Tamaki and Ingole 1993; Tamaki et al.1997) and its reflection in adult shrimp
densities (Fig. 7.7a) could primarily be due to the above-mentioned positive
intraspecific relationship. Intraspecific competition for space could also
regulate the population density. In fact, the burrow space exclusively occupied
by each shrimp is a good predictor of the carrying capacity for the population
on the sand flat (Tamaki et al. 1997; Tamaki and Veno 1998). For the population decline at the two seaward stations in recent years, one possibility is the
sudden increase in the abundance of the stingray, Dasyatis akajei, from 1995,
the fish voraciously feeding on ghost shrimps and disturbing the substrate to
a considerable depth (up to 20 em), especially in the lower two-thirds of the
sand flat (A. Tamaki, unpublished data).
7.6 Effects of the Ghost Shrimp Expansion and Decline
7.6.1 Effects on Sediment Properties
Following the expansion of N. harmandi over the Tomioka Bay sand flat,
sediment properties were remarkably altered by the shrimp bioturbation on
the landscape scale (Tamaki and Suzukawa 1991; Tamaki 1994; Fig. 7.6).
Destabilisation occurred because mounds deposited on the sand flat surface
were easily dispersed by tidal currents and waves causing increased
erodability. An estimated deposition rate of 2.6-12.7 mm/day would result in
a mixing of the entire sediment column in 47-235 days. The silt-day content
at the surface sediment has been reduced due to the ghost shrimps 'blowing
off' the fine particles resuspended into the water column, thereby reducing
the cohesiveness of the sediment. Increased oxygenation of the entire sediment column has occurred due to the overlying water introduced into the
burrows, such that the brown layer that was originally close to the surface is
now extended to the substrate base. Finally, larger shell material, such as
empty shells of U. moniliferum, was buried deeper by continual sediment
deposition by ghost shrimps. Thus, the substrate properties of the entire
dissipative sand flat were altered by the ghost shrimp to approximate those of
exposed sandy beaches. Since 1995, the substrate properties have not been
examined in detail, but are likely to have shifted due to the lowered ghost
shrimp densities (Fig. 7.7a) and increased disturbance by stingrays.