Inconspicuous, industrious biodiversity

The United NATIONS celebrated the International Day for Biological Diversity on May 22, aimed at increasing understanding and awareness of biodiversity issues. Numerous articles on such issues were published in the international and local press, and predictably, cetaceans, turtles, butterflies and tropical rain forests were the main protagonists.

Important as these may be, however, they are not the only biodiversity worthy of note. An immense number of smaller, inconspicuous species exist, which also form part of global biodiversity and which play very important roles in the functioning of ecosystems, without which life as we know it on this planet would not be possible.

An example of this has recently been provided by a study on the fauna of detached, drifting plant material that accumulates on the shallow seabed in local bays. Everybody is familiar with the vast meadows of seagrass, mainly the Neptune Grass, Posidonia oceanica (Maltese: alka) that occur on mostly sandy bottoms at depths of a few metres down to about 40 m in the Maltese Islands as well as throughout most of the Mediterranean.

Most people are also aware that these meadows have a very high productivity. Productivity is the rate at which the plant converts inorganic carbon into organic material, that is, the rate at which the seagrass lays down new plant tissue that is potentially available as food for other organisms that feed on the plant. However, there are actually very few animals that feed directly on Posidonia, because of the high amount of indigestible fibre that the plant contains and because the plant produces chemicals that deter potential grazers. In reality, a couple of species of sea urchins and the fish Sarpa salpa (salema, M. xilpa) are practically the only animals that feed directly on Posidonia.

The large number of other species that live and feed within Posidonia meadows actually feed on the felt of very small plants and animals that cover the leaves of the seagrass or are predators on other animals, or use the seagrass as a substratum and feed on organic material that they collect from the surrounding water.

So what happens to the large amount of new plant tissue that is formed each year by the seagrass? As the seagrass grows, the leaves age and eventually die to be replaced by new leaves. The old leaves are ripped off the plant by water currents, especially during autumn and winter when the frequent storms cause a lot of water movement on the bottom.

Some of these leaves are transported by waves and end up on the shore where they are familiar as the banks of decaying 'weed' (wrack) deposited on local sandy shores and which are assiduously removed by beach cleaners during the summer months. However, large amounts of drifting leaves also end up as loose aggregations on the shallow sandy bottoms of local bays.

Here, these accumulations of seagrass-litter lie moving to and fro with the currents until they are either deposited on the shore by waves or are transported to deeper water by currents during stormy weather. It is estimated that 40-80 per cent of the organic material formed by the seagrass is exported from the meadows and ends up as leaf-litter and detritus. Therefore the masses of seagrass debris lying on the bottom potentially provide a very large amount of food for any animals that are able to feed on them.

A study carried out by Mark Dimech from the Department of Biology of the University of Malta, under the supervision of Dr Joseph A. Borg and this writer, aimed to characterise the assemblages associated with seagrass-litter present in shallow waters (six-eight metres) on sandy bottoms and to see if there are indeed any animals that use this food source.

This study was carried out at two bays, Qortin and Armier, on the northern coast of Malta. In these bays, the seagrass-litter accumulations consist mainly of: Posidonia leaves (more than 94 per cent) and in winter may cover up to 80 per cent of the otherwise bare sandy bottom with a layer 5-17 cm thick.

From these seagrass-litter accumulations 95 species of macro-invertebrates (animals other than fish larger that one mm in body size) were found and identified. At least five of these species have not been recorded previously from the Maltese Islands, showing not only that there is still much local biodiversity that we do not know about, but that even habitats that at first sight appear to be poor in life may harbour a large variety of species.

It was discovered that these animals were present in very high densities among the plant debris, reaching overall abundances of more than 5,990 individuals per square metre of seagrass-litter accumulation. Given that these accumulations may cover several hundred square metres of seabed within these bays, then the overall abundance of these species runs into the hundreds of millions of individuals in these two bays alone!

By far the most abundant animals of these seagrass-litter accumulations are crustaceans, which collectively make up some 97 per cent of the total abundance. Of the crustaceans, amphipods (small sideways-flattened shrimp-like animals) make up some 80 per cent of the individuals and isopods (marine relatives of the terrestrial woodlice) make up some 10 per cent.

Some of the species present are found in other habitats but the majority are exclusive to these seagrass-litter accumulations or else are only found in other habitats in very low abundances. These patches of seagrass-litter on the shallow sandy bottoms of local bays thus support unique populations of specialised species. But what do these species do?

Most of the amphipods and isopods present shred and consume pieces of decaying seagrass leaves to extract nutrients from the plant material as well as to digest the bacteria and fungi that coat the decaying leaves. In the process, they reduce the leaves to very fine particles that they excrete.

These fine particles fall to the bottom where they provide food for other organisms termed detritivores, since the deposited material is called 'detritus'. By reducing large pieces of leaves to fine particles, the crustaceans of the seagrass-litter accumulations speed up the process of decay tremendously since much more bacteria and fungi can live on the surface of small particles of plant debris than on large pieces of leaves.

Apart from providing a major source of detritus for hundreds of different species of detritivores, the animals of the seagrass-litter accumulations are fed upon directly by a number of other species, especially small fish, which of course are themselves the prey of larger fish. The seagrass-litter fauna thus channels organic material originally formed by the living Posidonia meadows and which is locked up in the relatively indigestible Posidonia leaves, to other marine food chains. This inconspicuous yet industrious biodiversity of seagrass-litter accumulations thus forms a vital link in the oceanic ecosystem that sustains all marine life, including the charismatic species most of us are more familiar with as well as those that we exploit commercially.

This article is based on a scientific paper by Mark Dimech, Dr Joseph A. Borg and Professor Patrick J. Schembri presented at the Mediterranean Seagrass Workshop 2006 held in Malta between May 29 and June 3, and will be published in the scientific journal Biologia Marina Mediterranea later this year.