l38
G.c. CaMe
Herbivores, particularly those feeding on rhizomes below the sediment
surface, also make holes to reach their food. This is described for the Greater
Snow Geese (Anser caerulescens atlanticus) by Dionne (1985): in search of
rhizomes of Scirpus americanus geese dig many thousands of small holes in
the substrate causing on average 10 em lowering of the lower tidal marsh of
the St. Lawrence Estuary. Reise (1985; Fig. 7.3) illustrates pits dug by ducks
(Anas penelope) and geese (Branta bernicla) in search of rhizomes of intertidal seagrass Zostera noltii in K6nigshaven, Sylt. Dugongs (Dugong dugong) ,
herbivorous sea-mammals, are reported to produce unvegetated feeding trails
in seagrass fields (Nakaoka and Aioi 1999) or even to change seagrass beds
into bare sand flats (Preen 1995).
These examples may illustrate how important other organisms - in particular large herbivores and predators - are in sediment reworking next to
deposit feeders. Their under-water hole digging may produce large clouds of
suspended sediment in the water column, thereby also causing lateral transport of sediment. Holes made in the vegetation by, e. g., dugongs (Preen 1995),
or in the mats of tube-building amphipods the Gray whales use as food,
enhance erosion (Johnson and Nelson 1984). They may influence sediment
dynamics on a scale similar to that of deposit feeders.
6.4 Seasonal Variation
"Change is the norm within the estuarine system, whether related to individual tides, to lunar cycles of tides, to seasonal or annual variations, or even
to controls with longer periodicities;' wrote McManus (1998) and "some of
the greatest dangers to present -day estuarine science lie in the need to
respond to outside pressures: to provide rapid answers for administrators
under pressure ... and to ensure rapid publication".
Bioturbation, like most biological processes, shows variations on different
time scales from tidal, diurnal, seasonal to interannual. Quantification of sediment reworking therefore needs to be based on at least one year of measurements to give meaningful results. Seasonal variation in bioturbation is best
studied in temperate areas; little is known about seasonal variation in tropical
areas. A large seasonal variation in sediment reworking in temperate areas is
apparent from studies of Arenicola marina (Cadee 1976; Riisgard and Banta
1998) and burrowing callianassid shrimps such as Callianassa subterranea in
the North Sea (Rowden et al.1998) and the intertidal C.filholi in New Zealand
(Berkenbusch and Rowden 1999). Year-to-year variation may also be considerable and could be related to comparable variations in density in the
polychaete Heteromastus filiformis on tidal flats (Cadee 1979). Hole-digging
activity of predators such as birds shows clear seasonal variations (Cadee
G.c. CaMe
Herbivores, particularly those feeding on rhizomes below the sediment
surface, also make holes to reach their food. This is described for the Greater
Snow Geese (Anser caerulescens atlanticus) by Dionne (1985): in search of
rhizomes of Scirpus americanus geese dig many thousands of small holes in
the substrate causing on average 10 em lowering of the lower tidal marsh of
the St. Lawrence Estuary. Reise (1985; Fig. 7.3) illustrates pits dug by ducks
(Anas penelope) and geese (Branta bernicla) in search of rhizomes of intertidal seagrass Zostera noltii in K6nigshaven, Sylt. Dugongs (Dugong dugong) ,
herbivorous sea-mammals, are reported to produce unvegetated feeding trails
in seagrass fields (Nakaoka and Aioi 1999) or even to change seagrass beds
into bare sand flats (Preen 1995).
These examples may illustrate how important other organisms - in particular large herbivores and predators - are in sediment reworking next to
deposit feeders. Their under-water hole digging may produce large clouds of
suspended sediment in the water column, thereby also causing lateral transport of sediment. Holes made in the vegetation by, e. g., dugongs (Preen 1995),
or in the mats of tube-building amphipods the Gray whales use as food,
enhance erosion (Johnson and Nelson 1984). They may influence sediment
dynamics on a scale similar to that of deposit feeders.
6.4 Seasonal Variation
"Change is the norm within the estuarine system, whether related to individual tides, to lunar cycles of tides, to seasonal or annual variations, or even
to controls with longer periodicities;' wrote McManus (1998) and "some of
the greatest dangers to present -day estuarine science lie in the need to
respond to outside pressures: to provide rapid answers for administrators
under pressure ... and to ensure rapid publication".
Bioturbation, like most biological processes, shows variations on different
time scales from tidal, diurnal, seasonal to interannual. Quantification of sediment reworking therefore needs to be based on at least one year of measurements to give meaningful results. Seasonal variation in bioturbation is best
studied in temperate areas; little is known about seasonal variation in tropical
areas. A large seasonal variation in sediment reworking in temperate areas is
apparent from studies of Arenicola marina (Cadee 1976; Riisgard and Banta
1998) and burrowing callianassid shrimps such as Callianassa subterranea in
the North Sea (Rowden et al.1998) and the intertidal C.filholi in New Zealand
(Berkenbusch and Rowden 1999). Year-to-year variation may also be considerable and could be related to comparable variations in density in the
polychaete Heteromastus filiformis on tidal flats (Cadee 1979). Hole-digging
activity of predators such as birds shows clear seasonal variations (Cadee