Common Structures and Properties of Seagrass Beds Fringing the Coasts of the World 197
In the intertidal zone of western Europe, the narrow-leaved Zostera noltii is
generally dominant. This species is usually a biennial. It survives the winter in
a green condition, but in less dense beds than in the summer. In many areas,
this species is accompanied by a narrow-leaved morph of Z. marina. This
latter species develops particularly in small depressions on sandy or muddy
flats, where a thin film of water prevents the total desiccation of the plants
during low tide. In contrast, Z. noltii inhabits mainly the inconspicuous
elevations on these flats.
Around the low-water mark and deeper in the sublittoral zone, the beds of
the perennial morph of Z. marina are the most conspicuous feature. In many
areas, beds comprised of annual and perennial seagrasses are separated by
miles depending on the slope of the tidal flats.
In the Mediterranean, where tidal movements are more or less restricted to
the northern part of the Adriatic Sea, Z. noltii forms the upper belt where it is
permanently submerged under very sheltered conditions. It borders on beds of
Cymodocea nodosa that border in this turn on beds of Posidonia oceanica.
However, it appears that Posidonia beds extend over large areas, down to at
least 40 m, and that areas unsuitable for this species are often occupied by C.
nodosa.A Posidonia bed bordered on its upper limit as well as on its lower limit
by C. nodosa has been described by Bay (1984) from Corsica. Z. marina is very
restricted in its occurrence in the Mediterranean, being mainly confined to the
northern Adriatic and northern Aegean Sea and to the Black Sea.
In the Baltic Sea with no tides, and a salinity gradient to fresh-water, beds
of Z. marina occur fully submerged, forming mosaics with brackish-water
plants, such as Ruppia cirrhosa and Potamogeton pectinatus.
On the North American Atlantic coast, the situation is simpler, as only Z.
marina occurs with annual and perennial morphs. In the Gulf of California
(Mexico), where high water temperatures exceed the lethal point by June or
early July each year, all Z. marina is annual (Phillips et al. 1983a). Along the
North American Pacific coast, originally only Z. marina occurred, but during
the last century, the mainly intertidal species Z. japonica has been introduced
(Harrison and Bigley 1982). This species has more or less the same ecological
niche as Z. noltii in Western Europe. In more recent times, Phillips and WyllieEcheverria (1990) have discovered the sublittoral Z. asiatica in North
America, but still very little is known about the ecological demands of this
species.
On the western side of the North Pacific, Z. marina is the common species
in the sublittoral, while Z. japonica occupies the intertidal zone. In East Asia
there may be large gaps between the communities of these two species, but Z.
japonica extends much further to the south, viz. to Vietnam, where its stands
may border on growths of tropical species. Of the other three Zostera species
in this region, e. g. Z. asiatica, Z. caulescens and Z. caespitosa, very little is
known.
In the intertidal zone of western Europe, the narrow-leaved Zostera noltii is
generally dominant. This species is usually a biennial. It survives the winter in
a green condition, but in less dense beds than in the summer. In many areas,
this species is accompanied by a narrow-leaved morph of Z. marina. This
latter species develops particularly in small depressions on sandy or muddy
flats, where a thin film of water prevents the total desiccation of the plants
during low tide. In contrast, Z. noltii inhabits mainly the inconspicuous
elevations on these flats.
Around the low-water mark and deeper in the sublittoral zone, the beds of
the perennial morph of Z. marina are the most conspicuous feature. In many
areas, beds comprised of annual and perennial seagrasses are separated by
miles depending on the slope of the tidal flats.
In the Mediterranean, where tidal movements are more or less restricted to
the northern part of the Adriatic Sea, Z. noltii forms the upper belt where it is
permanently submerged under very sheltered conditions. It borders on beds of
Cymodocea nodosa that border in this turn on beds of Posidonia oceanica.
However, it appears that Posidonia beds extend over large areas, down to at
least 40 m, and that areas unsuitable for this species are often occupied by C.
nodosa.A Posidonia bed bordered on its upper limit as well as on its lower limit
by C. nodosa has been described by Bay (1984) from Corsica. Z. marina is very
restricted in its occurrence in the Mediterranean, being mainly confined to the
northern Adriatic and northern Aegean Sea and to the Black Sea.
In the Baltic Sea with no tides, and a salinity gradient to fresh-water, beds
of Z. marina occur fully submerged, forming mosaics with brackish-water
plants, such as Ruppia cirrhosa and Potamogeton pectinatus.
On the North American Atlantic coast, the situation is simpler, as only Z.
marina occurs with annual and perennial morphs. In the Gulf of California
(Mexico), where high water temperatures exceed the lethal point by June or
early July each year, all Z. marina is annual (Phillips et al. 1983a). Along the
North American Pacific coast, originally only Z. marina occurred, but during
the last century, the mainly intertidal species Z. japonica has been introduced
(Harrison and Bigley 1982). This species has more or less the same ecological
niche as Z. noltii in Western Europe. In more recent times, Phillips and WyllieEcheverria (1990) have discovered the sublittoral Z. asiatica in North
America, but still very little is known about the ecological demands of this
species.
On the western side of the North Pacific, Z. marina is the common species
in the sublittoral, while Z. japonica occupies the intertidal zone. In East Asia
there may be large gaps between the communities of these two species, but Z.
japonica extends much further to the south, viz. to Vietnam, where its stands
may border on growths of tropical species. Of the other three Zostera species
in this region, e. g. Z. asiatica, Z. caulescens and Z. caespitosa, very little is
known.