Common Structures and Properties of Seagrass Beds Fringing the Coasts of the World 207
the Gulf of Carpentaria in March, 1985. Of the 183 km 2 of seagrass beds,
151 km 2 were completely destroyed. In April 1986, it became clear that the
beds that had survived the storm had disappeared too. As a result of scouring,
the remaining shallow beds had been eroded away, and the remaining deeper
beds had disappeared as a result of deposition of fine silt that smothered the
seagrasses. This disaster had a purely natural cause. However, Preen et al.
(1995) described the effects of another cyclone, "Fran", on the seagrass beds of
Hervey Bay in 1992. In this case, the cyclone was followed by extremely large
river discharges, which caused a considerable decrease in salinity. The sediment discharged by the floods in its turn caused a low-light environment and
deposition of material on the bottom. In a period of 6 weeks, more than
1000 km 2 of seagrass beds were lost, along with the accompanying organisms.
The authors mentioned that there has been a slight recovery of seagrass in the
deep water but, in the shallow environments, no recovery has been observed.
In this case, the damage by the floods can be ascribed without hesitation to a
century of human mismanagement of the adjacent terrestrial environment:
deforestation with the unavoidable result of erosion, degradation of the soil,
and a highly decreased water-holding capacity of the bottom.
Small-scale disappearances of seagrass beds are repeatedly observed. Their
cause is not always clear. Usually, they are the result of one or other type of
pollution, dredging, or construction works such as dams, ports, etc. Due to the
continually increasing pollution, the green alga Enteromorpha radiata has
increased in the last decades. This alga grows loosely anchored in the mud
and can form large blankets under sheltered conditions, which may float and
become deposited in other places. If these blankets become deposited on a
seagrass bed, this may lead to the complete destruction of the bed owing to
suffocation, as has been observed in Langstone Harbour, Hampshire, England
(Den Hartog 1994). Although the event took place in 1991, the seagrass has
not returned (Den Hartog, personal observation, summer 1999).
Other impacts to seagrass beds are adventive algae, which accidentally
arrive in other areas and upset the existing balance between the available
native species of the system. In Western Europe, the brown alga Sargassum
muticum gradually replaces Zostera beds in the lower part of the intertidal
zone on gravelly or mixed substrata (Den Hartog 1997). In the Mediterranean,
the green alga Caulerpa taxifolia is disastrous to the beds of Posidonia
oceanica (Meinesz and Hesse 1991).
In the past decades, we have seen the disappearance of many large and
small seagrass beds. Unfortunately, we have to admit that we do not know of
any newly formed seagrass bed by native species in their own area of distribution. This is an alarming observation. For this reason, it is urgently necessary that the study of the dynamics of seagrass beds be promoted, from the
very beginnings through maturity to senility. This knowledge is necessary to
understand this very valuable ecosystem. Without this knowledge, it will not
the Gulf of Carpentaria in March, 1985. Of the 183 km 2 of seagrass beds,
151 km 2 were completely destroyed. In April 1986, it became clear that the
beds that had survived the storm had disappeared too. As a result of scouring,
the remaining shallow beds had been eroded away, and the remaining deeper
beds had disappeared as a result of deposition of fine silt that smothered the
seagrasses. This disaster had a purely natural cause. However, Preen et al.
(1995) described the effects of another cyclone, "Fran", on the seagrass beds of
Hervey Bay in 1992. In this case, the cyclone was followed by extremely large
river discharges, which caused a considerable decrease in salinity. The sediment discharged by the floods in its turn caused a low-light environment and
deposition of material on the bottom. In a period of 6 weeks, more than
1000 km 2 of seagrass beds were lost, along with the accompanying organisms.
The authors mentioned that there has been a slight recovery of seagrass in the
deep water but, in the shallow environments, no recovery has been observed.
In this case, the damage by the floods can be ascribed without hesitation to a
century of human mismanagement of the adjacent terrestrial environment:
deforestation with the unavoidable result of erosion, degradation of the soil,
and a highly decreased water-holding capacity of the bottom.
Small-scale disappearances of seagrass beds are repeatedly observed. Their
cause is not always clear. Usually, they are the result of one or other type of
pollution, dredging, or construction works such as dams, ports, etc. Due to the
continually increasing pollution, the green alga Enteromorpha radiata has
increased in the last decades. This alga grows loosely anchored in the mud
and can form large blankets under sheltered conditions, which may float and
become deposited in other places. If these blankets become deposited on a
seagrass bed, this may lead to the complete destruction of the bed owing to
suffocation, as has been observed in Langstone Harbour, Hampshire, England
(Den Hartog 1994). Although the event took place in 1991, the seagrass has
not returned (Den Hartog, personal observation, summer 1999).
Other impacts to seagrass beds are adventive algae, which accidentally
arrive in other areas and upset the existing balance between the available
native species of the system. In Western Europe, the brown alga Sargassum
muticum gradually replaces Zostera beds in the lower part of the intertidal
zone on gravelly or mixed substrata (Den Hartog 1997). In the Mediterranean,
the green alga Caulerpa taxifolia is disastrous to the beds of Posidonia
oceanica (Meinesz and Hesse 1991).
In the past decades, we have seen the disappearance of many large and
small seagrass beds. Unfortunately, we have to admit that we do not know of
any newly formed seagrass bed by native species in their own area of distribution. This is an alarming observation. For this reason, it is urgently necessary that the study of the dynamics of seagrass beds be promoted, from the
very beginnings through maturity to senility. This knowledge is necessary to
understand this very valuable ecosystem. Without this knowledge, it will not