204
C. den Hartog and R.C. Phillips
habitat and varies seasonally with changes in growth rate, with life spans of
leaves ranging from 11-24 days in Halophila ovalis, over about 50 days in
Zostera marina, and up to 125 days in Posidonia australis (West and Larkum
1979; Hillman 1985; Borowitzka and Lethbridge 1989). These rates are also
affected by grazing, and differences in susceptibility to grazing (Hootsmans
and Vermaat 1985). Life spans of leaves range from 1-4 months. Ott (1980)
demonstrates that a plant of Posidonia oceanica produces about 10 leaves per
year, but the leaves produced in autumn may persist up to two to five times
longer than those produced in spring. West and Larkum (1979) found that the
turnover rate of leaves of P. australis is about 2.5 times greater in summer than
in winter. There are also significant differences in the leaf turnover rates at
different sites in the same area.
The percentage of flowering stalks and the number of eelgrass seeds
produced are directly related to the degree of stress in the habitat, e. g. more
seeds were produced per square metre in the Gulf of California and in the
southerly ends of the area of distribution along the Pacific coast of North
America and in Alaska than in the mid-range locations in Washington State
(Phillips et al. 1983a). Also, more seeds were produced at anyone site in the
intertidal zone than in the sublittoral. The same relationships appear to be
valid for the Atlantic coasts of North America (Phillips et al. 1983a) and
Europe (Jacobs and Pierson 1981).
9.S Seagrass Dynamics
Very little attention has been paid to successional concepts (Molinier and
Picard 1951,1952). It has become evident that seagrass beds do not function
as a prelude to terrestrial ecosystems. Seagrass beds are almost always separated spatially from marshes, and, in the tropics, the same is true for seagrass
beds and mangrove stands.
The seagrasses, however, show year-to-year patterns in their development.
In the intertidal zone of the Wadden Sea, most of the Zostera marina is
annual. Whether this annual behaviour is genetically or environmentally
determined has not yet been established. Probably both options are possible.
In normal winters, Z. marina does not survive, but, after mild winters, survivors are often found. The accompanying Z. noltii rhizomes survive; the
above-ground biomass is considerably reduced in winter. These seagrass
beds are heavily grazed by Brent geese in autumn. According to Jacobs et al.
(1981), the geese and ducks (widgeon, mallard and pintail) consume almost
the total above-ground biomass as well as part of the underground biomass,
i.e. approximately half the annual production of the seagrass. As a consequence of ice scour in winter, sediment movement under the influence of
C. den Hartog and R.C. Phillips
habitat and varies seasonally with changes in growth rate, with life spans of
leaves ranging from 11-24 days in Halophila ovalis, over about 50 days in
Zostera marina, and up to 125 days in Posidonia australis (West and Larkum
1979; Hillman 1985; Borowitzka and Lethbridge 1989). These rates are also
affected by grazing, and differences in susceptibility to grazing (Hootsmans
and Vermaat 1985). Life spans of leaves range from 1-4 months. Ott (1980)
demonstrates that a plant of Posidonia oceanica produces about 10 leaves per
year, but the leaves produced in autumn may persist up to two to five times
longer than those produced in spring. West and Larkum (1979) found that the
turnover rate of leaves of P. australis is about 2.5 times greater in summer than
in winter. There are also significant differences in the leaf turnover rates at
different sites in the same area.
The percentage of flowering stalks and the number of eelgrass seeds
produced are directly related to the degree of stress in the habitat, e. g. more
seeds were produced per square metre in the Gulf of California and in the
southerly ends of the area of distribution along the Pacific coast of North
America and in Alaska than in the mid-range locations in Washington State
(Phillips et al. 1983a). Also, more seeds were produced at anyone site in the
intertidal zone than in the sublittoral. The same relationships appear to be
valid for the Atlantic coasts of North America (Phillips et al. 1983a) and
Europe (Jacobs and Pierson 1981).
9.S Seagrass Dynamics
Very little attention has been paid to successional concepts (Molinier and
Picard 1951,1952). It has become evident that seagrass beds do not function
as a prelude to terrestrial ecosystems. Seagrass beds are almost always separated spatially from marshes, and, in the tropics, the same is true for seagrass
beds and mangrove stands.
The seagrasses, however, show year-to-year patterns in their development.
In the intertidal zone of the Wadden Sea, most of the Zostera marina is
annual. Whether this annual behaviour is genetically or environmentally
determined has not yet been established. Probably both options are possible.
In normal winters, Z. marina does not survive, but, after mild winters, survivors are often found. The accompanying Z. noltii rhizomes survive; the
above-ground biomass is considerably reduced in winter. These seagrass
beds are heavily grazed by Brent geese in autumn. According to Jacobs et al.
(1981), the geese and ducks (widgeon, mallard and pintail) consume almost
the total above-ground biomass as well as part of the underground biomass,
i.e. approximately half the annual production of the seagrass. As a consequence of ice scour in winter, sediment movement under the influence of