xiv
Foreword
So what, taxonomically, are seagrasses? This
question is tackled in Chapters 1 and 2. However,
as will be seen in those pages the answer is not so
easy as we would like. Seagrasses are certainly the
only flowering plants that are found in the sea and
they belong to the Monocotyledons. They belong
to at least two families, the Potomagetonaceae and
the Hydrocharitaceae, and maybe more. However, in
contrast to freshwater and land plants, there are relatively few genera (ca. 12) and these are very diverse,
indicating that seagrasses have had a multi-pronged
origin. Furthermore, when it is realized that many
of these genera have species, which grow in estuaries and coastal lagoons alongside other hydrophytes,
the real difficulty in defining seagrasses becomes apparent. Should one include the Ruppiaceae in the
list of seagrasses and, if one does, why exclude certain members of the Zanichelliaceae? These arguments are set out in Chapter 1 and 2; yet surprisingly questions remain. Suffice it to say that up to the
present time a pragmatic assessment has been made
by the seagrass community that seagrasses are those
flowering plants that occur in estuaries or on continental sea margins – and Ruppia and Zanichella
species are only allowed in under sufferance, as
these can complete their life cycles outside marine
waters.
And this discussion begs the question as to the
definition of a seagrass? This again is a contentious
question. Apart from the systematic questions posed
in Chapters 1 and 2, there is the question of what morphological characteristics do seagrasses possess in
common. As discussed in Chapter 3, it can be seen
that most of the individual characteristics of seagrasses are possessed by other hydrophytes. Not even
the ability to live in increased salinity is a unique
feature of seagrasses. Certainly they all possess the
ability to pollinate underwater and most reproduce
underwater. Most are adapted to the wave-prone environment of the sea, with tough linear leaves and
an underground rhizome system, but the widely distributed genus Halophila is an exception here. The
characteristics most often cited as unique to seagrasses are as follows:
• Live in an estuarine or marine environment, and
nowhere else.
• Pollinate underwater with specialized pollen.
• Produce seeds underwater which can be dispersed by both biotic and abiotic agents .
• Have specialized leaves with a very reduced cuticle and an epidermis which lacks stomata and
is the main photosynthetic tissue.
• Have a rhizome or underground stem which is
important in anchoring.
• Have roots that can live in an anoxic environment and are dependent on oxygen transport
from the leaves and rhizome but are also important in the nutrient transfer processes.
The importance of seagrasses as it emerged over
the last quarter of the 20
th Century is in their key role
on the shallow seaward margins of our continents.
Seagrasses are now considered one of the important
‘ecosystem engineers’ (see Chapter 6) given their
ability to change significantly numerous aspects of
their environment. Their importance here is summarized in a set of axioms, which have become familiar
to planners and conservationists, now often referred
to as ‘ecosystem services’ (Costanza et al., 1997):
• Seagrasses are important primary producers,
that is they convert sunlight and carbon dioxide efficiently into organic form;
• Seagrasses supply organic food to a variety of
dependent food webs;
• Seagrasses stabilize the seabed in which they
grow;
• Seagrasses structure the seabed on which they
grow into a complex environment which provides places for many organisms to exist;
• Seagrasses act as the nursery ground for many
commercially-caught species.
Are these axioms correct or are they “feel good” sayings that have been used to encourage the workers
in the field. One of the recommendations to authors
of chapters of this book was to challenge the current
paradigms, and therefore the reader will find this reflected in many chapters. For example, the axiom that
“seagrass ecosystems are driven by detritus” is challenged in Chapter 7 (Mateo et al.) where it is shown
that production from epiphytic algae and benthic algae may well exceed that from the detrital production
system of seagrasses.
Another area where science has made great
progress is seagrass genetic structure. This is reflected in work on seagrasses and Chapter 2 (Waycott
et al.) surveys the current status of seagrasses from
this stand point. The result is a fresh approach to
the taxonomy and systematizing of seagrasses. The
Foreword
So what, taxonomically, are seagrasses? This
question is tackled in Chapters 1 and 2. However,
as will be seen in those pages the answer is not so
easy as we would like. Seagrasses are certainly the
only flowering plants that are found in the sea and
they belong to the Monocotyledons. They belong
to at least two families, the Potomagetonaceae and
the Hydrocharitaceae, and maybe more. However, in
contrast to freshwater and land plants, there are relatively few genera (ca. 12) and these are very diverse,
indicating that seagrasses have had a multi-pronged
origin. Furthermore, when it is realized that many
of these genera have species, which grow in estuaries and coastal lagoons alongside other hydrophytes,
the real difficulty in defining seagrasses becomes apparent. Should one include the Ruppiaceae in the
list of seagrasses and, if one does, why exclude certain members of the Zanichelliaceae? These arguments are set out in Chapter 1 and 2; yet surprisingly questions remain. Suffice it to say that up to the
present time a pragmatic assessment has been made
by the seagrass community that seagrasses are those
flowering plants that occur in estuaries or on continental sea margins – and Ruppia and Zanichella
species are only allowed in under sufferance, as
these can complete their life cycles outside marine
waters.
And this discussion begs the question as to the
definition of a seagrass? This again is a contentious
question. Apart from the systematic questions posed
in Chapters 1 and 2, there is the question of what morphological characteristics do seagrasses possess in
common. As discussed in Chapter 3, it can be seen
that most of the individual characteristics of seagrasses are possessed by other hydrophytes. Not even
the ability to live in increased salinity is a unique
feature of seagrasses. Certainly they all possess the
ability to pollinate underwater and most reproduce
underwater. Most are adapted to the wave-prone environment of the sea, with tough linear leaves and
an underground rhizome system, but the widely distributed genus Halophila is an exception here. The
characteristics most often cited as unique to seagrasses are as follows:
• Live in an estuarine or marine environment, and
nowhere else.
• Pollinate underwater with specialized pollen.
• Produce seeds underwater which can be dispersed by both biotic and abiotic agents .
• Have specialized leaves with a very reduced cuticle and an epidermis which lacks stomata and
is the main photosynthetic tissue.
• Have a rhizome or underground stem which is
important in anchoring.
• Have roots that can live in an anoxic environment and are dependent on oxygen transport
from the leaves and rhizome but are also important in the nutrient transfer processes.
The importance of seagrasses as it emerged over
the last quarter of the 20
th Century is in their key role
on the shallow seaward margins of our continents.
Seagrasses are now considered one of the important
‘ecosystem engineers’ (see Chapter 6) given their
ability to change significantly numerous aspects of
their environment. Their importance here is summarized in a set of axioms, which have become familiar
to planners and conservationists, now often referred
to as ‘ecosystem services’ (Costanza et al., 1997):
• Seagrasses are important primary producers,
that is they convert sunlight and carbon dioxide efficiently into organic form;
• Seagrasses supply organic food to a variety of
dependent food webs;
• Seagrasses stabilize the seabed in which they
grow;
• Seagrasses structure the seabed on which they
grow into a complex environment which provides places for many organisms to exist;
• Seagrasses act as the nursery ground for many
commercially-caught species.
Are these axioms correct or are they “feel good” sayings that have been used to encourage the workers
in the field. One of the recommendations to authors
of chapters of this book was to challenge the current
paradigms, and therefore the reader will find this reflected in many chapters. For example, the axiom that
“seagrass ecosystems are driven by detritus” is challenged in Chapter 7 (Mateo et al.) where it is shown
that production from epiphytic algae and benthic algae may well exceed that from the detrital production
system of seagrasses.
Another area where science has made great
progress is seagrass genetic structure. This is reflected in work on seagrasses and Chapter 2 (Waycott
et al.) surveys the current status of seagrasses from
this stand point. The result is a fresh approach to
the taxonomy and systematizing of seagrasses. The