202
w. Ritzrau . H. Fohrmann
cluded (Fig. 5f, Table 2). The largest class of aggregates (ws = 80 m day-I) contain
about six times more of the initial mass the medium size aggregates (ws = 50 m
day-I; Table 2). In the rougher hydrodynamic regime 2, hydrodynamic sorting
did not occur, the whole BBL was dominated by the largest aggregate class. One
interesting feature was the distinct c-shaped vertical distribution pattern of the
relative mass fractions below 15 cm above the sediment (Fig. 6d-f). This shape
occurred only in the stronger hydrodynamic regime 2 and was more pronounced if aggregate interaction was included. The vertical distribution patterns suggest, by linking particle distribution and interaction with the current
regime, hydrodynamic sorting of different aggregate size classes can be simulated.
11
Relevance of Model Results to Field Studies
Only little information on the size distribution of aggregates is available with
high resolution in the benthic boundary layer. Using an in situ particle camera at
I-2m above the seafloor in a tidal inlet near Texel revealed that aggregate dynamics were strongly related to the tidal cycle (Chen et al. 1994). For the bottom
water the aggregate size distribution over a tidal cycle was dominated by aggregate sizes between 93xlO- 6 and 379xlO- 6 m (93-373 flm). In this study, the
mean current velocity in the inlet was more than 1 ms- I , applying the same
methods in the Elbe estuary, variations of mean particle sizes between 150x 10- 6
and 350x 10- 6 m were observed (Chen and Eisma 1995). The authors relate size
distribution of aggregates to various parameters describing the composition of
aggregates (e.g. organic content). Prediction of small-scale variations in the distribution of aggregate sizes due to hydrodynamic sorting has not been attempted. Our simulated size distribution patterns support the idea of hydrodynamic
sorting in the BBL. Even though hydrodynamic sorting of aggregates could not
be proved by the observed distribution of bulk measurements, this process was
proposed to explain elevated microbial activities in the BBL (Ritzrau 1996). The
numerical simulations presented here suggest that hydrodynamic sorting occurs
even under a very simple approach of parameterisation of aggregate interaction.
12
Vertical distribution of aggregate size classes and benthic organisms
The distribution of aggregate size classes has major implication for feeding
strategies of suspension-feeding benthic animals. If the aggregate formation due
to turbulent aggregation and differential settling dominates the aggregate dynamics close to the seafloor, it is very likely that small-sized classes of aggregates
« 10- 6 m) may not be present and hence not be available to benthic organisms;
but also the opposite may occur. Close to the seafloor, aggregates are subjected
to the strongest shear, so that aggregate disintegration may produce small fragments which then become available for suspension feeders. Further on, the size
w. Ritzrau . H. Fohrmann
cluded (Fig. 5f, Table 2). The largest class of aggregates (ws = 80 m day-I) contain
about six times more of the initial mass the medium size aggregates (ws = 50 m
day-I; Table 2). In the rougher hydrodynamic regime 2, hydrodynamic sorting
did not occur, the whole BBL was dominated by the largest aggregate class. One
interesting feature was the distinct c-shaped vertical distribution pattern of the
relative mass fractions below 15 cm above the sediment (Fig. 6d-f). This shape
occurred only in the stronger hydrodynamic regime 2 and was more pronounced if aggregate interaction was included. The vertical distribution patterns suggest, by linking particle distribution and interaction with the current
regime, hydrodynamic sorting of different aggregate size classes can be simulated.
11
Relevance of Model Results to Field Studies
Only little information on the size distribution of aggregates is available with
high resolution in the benthic boundary layer. Using an in situ particle camera at
I-2m above the seafloor in a tidal inlet near Texel revealed that aggregate dynamics were strongly related to the tidal cycle (Chen et al. 1994). For the bottom
water the aggregate size distribution over a tidal cycle was dominated by aggregate sizes between 93xlO- 6 and 379xlO- 6 m (93-373 flm). In this study, the
mean current velocity in the inlet was more than 1 ms- I , applying the same
methods in the Elbe estuary, variations of mean particle sizes between 150x 10- 6
and 350x 10- 6 m were observed (Chen and Eisma 1995). The authors relate size
distribution of aggregates to various parameters describing the composition of
aggregates (e.g. organic content). Prediction of small-scale variations in the distribution of aggregate sizes due to hydrodynamic sorting has not been attempted. Our simulated size distribution patterns support the idea of hydrodynamic
sorting in the BBL. Even though hydrodynamic sorting of aggregates could not
be proved by the observed distribution of bulk measurements, this process was
proposed to explain elevated microbial activities in the BBL (Ritzrau 1996). The
numerical simulations presented here suggest that hydrodynamic sorting occurs
even under a very simple approach of parameterisation of aggregate interaction.
12
Vertical distribution of aggregate size classes and benthic organisms
The distribution of aggregate size classes has major implication for feeding
strategies of suspension-feeding benthic animals. If the aggregate formation due
to turbulent aggregation and differential settling dominates the aggregate dynamics close to the seafloor, it is very likely that small-sized classes of aggregates
« 10- 6 m) may not be present and hence not be available to benthic organisms;
but also the opposite may occur. Close to the seafloor, aggregates are subjected
to the strongest shear, so that aggregate disintegration may produce small fragments which then become available for suspension feeders. Further on, the size