Knysna Seagrass Project
The extensive seagrass beds at Knysna, the largest and best developed in the entire country, may appear monotonous, and from a botanical point of view that may be true since they are mostly formed of the one species, Cape dwarf eelgrass, but they conceal a myriad of individuals and different species of tiny animals that form the food of the shoals of young fish that can be seen swimming through the beds at high tide. The beds not only feed but also shelter from their predators the young of many of the fish that are caught on line locally or out at sea after they have grown and migrated away from the estuary to breed. Without the beds, the local fish populations would undoubtedly decline. It is deeply ironic that those who dig up the seagrass to collect bait, and thereby impoverish or destroy considerable areas of beds, are in fact helping to eliminate the very fish that they are hoping to catch!
This invertebrate fauna, numbering well over 150 species, is seldom seen, in part because they are mostly less than 5 mm in size but also because many live hidden just below the surface of the sediment. It seems likely that the majority feed on relatives of the phytoplankton that themselves live on the surface of the seagrass leaves and on the sediment. Are the beds equally monotonous from the perspective of these fish-food animal species? Is one area of seagrass as good as any other for them?
The Knysna Seagrass Project, which has been studying the beds since 2009, has discovered that the answer is both ‘yes’ and ‘no’. ‘No’ in the sense that most of the animals are distributed very patchily, and the species found at one spot may be very different from those occurring only a few metres away. But ‘yes’ in the sense that the actual number of species per unit area is effectively constant, varying only in that the upstream half of the system supports fewer than the downstream half – relatively few can cope with the low and fluctuating salinity of the estuary proper. Otherwise, numbers of species per unit area vary little across large areas, essentially because the numbers of individuals taken by the young fish and others (maybe 95-99% of those born) keep the invertebrates so far below the level that their environment would support that none can affect the numbers of others via competition, etc. It is also worth stressing here that Knysna is a very special place for several of these species: for some it is apparently their only home in South Africa (and not surprisingly they bear the specific name knysnaensis), and for others it is the only place that they are known to occur in the whole of Africa!
The Project (under Prof Barnes) continues to try to produce understanding of the patterns of spatial distribution of these invertebrates within the Knysna estuarine bay and of the ways in which the presence or absence of seagrass plants affects these patterns. Hopefully, thereby it will be possible to provide to those with the duty of managing such systems the sort of information that will help in conservation of the beds, their fauna, and their role in the wider coastal ecosystem, not only at Knysna but elsewhere too.