364
K. Reise
Seagrasses and the Benthic Fauna
The seagrasses of the sedimentary shores tend to occur in dense meadows
(Den Hartog and Phillips, Chap. 9). These constitute an analogue to the kelp
forests of rocky coasts. Seagrass beds are highly productive, delivering leaf
material to the detritus pool of the entire coastal ecosystem, attract grazers
such as fish, ducks, geese and dugongs, calm down hydrodynamics, and stabilize and modify the sediment where they are rooted. At the lower shore, they
may provide a sheltered habitat for a rich community of algae, invertebrates
and fish. The composition and diversity of this associated community largely
depends on the quality of the surrounding habitats, and disjunct patches of
seagrass attract more visitors than large continuous stands.
The center of seagrass diversity resides in the subtidal zone of the tropical
Indo-West -Pacific. Plant size, structural complexity, and species richness of
the seagrasses tend to decline towards the intertidal and towards the temperate to arctic zones. Intertidal species may be found subtidally when the
larger subtidal species fail to occur. In the Baltic Sea, density and biomass of
the Zostera marina stands decrease with salinity (Baden and Bostrom,
Chap. 10). Seagrass meadows shift from shallow and sheltered sites on a rich
substrate under marine conditions to deeper and exposed sites on poor
substrates under the most brackish conditions. This niche displacement is
apparently caused by competition with fresh water plants in the brackish
waters. Seagrass beds occur usually separated from marshes and mangroves,
and do not constitute a successional step towards the terrestrial vegetation.
The dynamics of seagrass beds are usually slow, and many questions are
still unanswered. How much empty but suitable habitat is there available for
seagrasses at the sedimentary shores? Stands persisting over several decades
are known as well as spatially shifting beds (Den Hartog and Phillips,
Chap. 9). Except for the introduced seagrass species, Zostera japonica invading the North American Pacific coast and Halophila stipulacea the Mediterranean Sea, no recent expansions of seagrass beds have been recorded.
Instead, seagrass beds are in decline all over the world. Although climatic
events are often the proximate cause, as with exceptional cloudiness in the
northern Atlantic during the 1930s or a cyclone in northern Australia in 1992,
the ultimate causes for the decline or the lack of recovery often are anthropogenic. One such factor is deforestation with the resulting erosion and a subsequent increase in turbidity in the coastal waters. Dredging operations and
embankments, enhanced eutrophication and industrial waste, and the introduction of competitive algal species, are further factors contributing to the
decline of seagrasses.
An ecological comparison at the eastern Atlantic coast from cold to warm
temperate to subtropical climate reveals a number of interesting aspects
K. Reise
Seagrasses and the Benthic Fauna
The seagrasses of the sedimentary shores tend to occur in dense meadows
(Den Hartog and Phillips, Chap. 9). These constitute an analogue to the kelp
forests of rocky coasts. Seagrass beds are highly productive, delivering leaf
material to the detritus pool of the entire coastal ecosystem, attract grazers
such as fish, ducks, geese and dugongs, calm down hydrodynamics, and stabilize and modify the sediment where they are rooted. At the lower shore, they
may provide a sheltered habitat for a rich community of algae, invertebrates
and fish. The composition and diversity of this associated community largely
depends on the quality of the surrounding habitats, and disjunct patches of
seagrass attract more visitors than large continuous stands.
The center of seagrass diversity resides in the subtidal zone of the tropical
Indo-West -Pacific. Plant size, structural complexity, and species richness of
the seagrasses tend to decline towards the intertidal and towards the temperate to arctic zones. Intertidal species may be found subtidally when the
larger subtidal species fail to occur. In the Baltic Sea, density and biomass of
the Zostera marina stands decrease with salinity (Baden and Bostrom,
Chap. 10). Seagrass meadows shift from shallow and sheltered sites on a rich
substrate under marine conditions to deeper and exposed sites on poor
substrates under the most brackish conditions. This niche displacement is
apparently caused by competition with fresh water plants in the brackish
waters. Seagrass beds occur usually separated from marshes and mangroves,
and do not constitute a successional step towards the terrestrial vegetation.
The dynamics of seagrass beds are usually slow, and many questions are
still unanswered. How much empty but suitable habitat is there available for
seagrasses at the sedimentary shores? Stands persisting over several decades
are known as well as spatially shifting beds (Den Hartog and Phillips,
Chap. 9). Except for the introduced seagrass species, Zostera japonica invading the North American Pacific coast and Halophila stipulacea the Mediterranean Sea, no recent expansions of seagrass beds have been recorded.
Instead, seagrass beds are in decline all over the world. Although climatic
events are often the proximate cause, as with exceptional cloudiness in the
northern Atlantic during the 1930s or a cyclone in northern Australia in 1992,
the ultimate causes for the decline or the lack of recovery often are anthropogenic. One such factor is deforestation with the resulting erosion and a subsequent increase in turbidity in the coastal waters. Dredging operations and
embankments, enhanced eutrophication and industrial waste, and the introduction of competitive algal species, are further factors contributing to the
decline of seagrasses.
An ecological comparison at the eastern Atlantic coast from cold to warm
temperate to subtropical climate reveals a number of interesting aspects