The Stangrove Pond Survey - 2006

Stangrove Pond Slide Show

The Duckweed Problem

A symptom rather than the main disease but, nevertheless, a major factor in the decline of the aquatic biodiversity of the site has been the excessive growth of lesser duckweed. Although a big problem across the south-east this year it was no great problem here during 2006 (see Complications) but, apparently, was so dense throughout 2005 as to prevent light penetrating to any reasonable depth. Light is the great ecological power source: without it no green plant can survive, and in this situation its exclusion is helping to denude the pond, which as previously stated is already prone to oxygen deficiency because of the way it is constructed, of vital oxygenating plants.

In moderation a healthy population of duckweed is not a problem, indeed it is wholly beneficial, removing various toxins and pollutants from the system, it only generally becomes a nuisance in a mesotrophic/eutrophic (enriched) environment. There was no problem with excessive cover during my last association with the site and for several years after. The best chance of restoring the status quo, should the problem recur, lies in first identifying any change (increase) in the amount of nutrients i.e. phosphates, nitrates, etc, that are entering the water either directly or via the groundwater system. Has there, for instance, been any change in the management regime of the surrounding grassland: additional applications of fertilizer or other treatment in the vicinity perhaps? Could some source of enrichment have been created or released into the groundwater during the construction of the Edenbridge relief road or other nearby building projects? Faecal input from the numerous ducks inhabiting the pond and an indeterminate amount of input from uneaten food, in the form of bread and grain, both of which seem to have greatly increased since my last involvement, will have an adverse impact. Too many dead leaves left rotting in the pond could also well be responsible.

It should be mentioned that all the ponds of my acquaintance, that have had a problem with excessive duckweed, have been of the Stangrove ilk, i.e. excessively shaded by trees, with a high input of leaves, and, in most cases, a number of water fowl that were being fed supplementary, unnatural, food. Whatever the cause it is imperative that it be identified and, if at all feasible, eliminated, as it is otherwise only possible to contain the problem without ever arriving at a truly realistic solution.

Methods of containment fall into four basic categories: manual, mechanical, biological and chemical, listed here in order of preference.

Once the origins of the problem have been found and resolved the excessive duckweed, may, with plenty of hard work, be simply raked off the surface with the assistance of hand rakes and an easily manufactured floating boom (a length of wooden baton with a rope attached at either end works well). Unless the root cause has been identified, however, there is little point in using this method of control. As if only a few pieces of duckweed are left (it is constantly being imported on the feet and feathers of waterfowl and a myriad submerged seeds and turions will inevitably be left behind anyway) it will very rapidly spread back across the whole surface.

Mechanical methods of control are not really worthy of consideration on this scale of operation. Suffice it to say that various plant and machinery is available (as seen in use, on TV, this summer, clearing The Lee Navigation Canal, North London and areas of Dockland) but this is best suited to large expanses of open water such as reservoirs.

Ducks and some other waterfowl, such as moorhen, eat duckweed but not, quite obviously, at a fast enough rate to provide any reasonable kind of control in this instance, and any benefit will probably be outweighed by other problems when their numbers are high enough to make any difference.

Ignoring various bacterial products, which are at best a stop gap solution. The only valid biological agents that I would consider, in this situation, are fish (a bale or two of barley straw may work for the blanket algae, which is also a symptom of the same problem, but not on this order of plant). And then only very reluctantly, if all else failed, as they would hugely devalue the pond as a habitat both for invertebrates, of all kinds, and for newts of both species (great crested are particularly vulnerable) resident in Stangrove Pond, as they will predate on both their eggs and tadpoles. They would also be fairly obvious in their presence and would doubtless attract fishermen and vandals, with the attendant problems of discarded line and an increase in litter.

Many of our native fish, classified as cyprinids (carp family), will eat duckweed but it must be remembered that anything that feeds on this will, in all probability, eat the oxygenating plants which we are trying to benefit in the first place. Premier amongst the weed eaters is the exotic grass carp, which I have seen used to great effect in a friend's pond, of similar size and layout to this one. The pond in question, however, was already well stocked with other species of fish, which had failed to control it. It must be said that there did not appear to be any dramatic impact on the other vegetation present, post introduction.

Chemical control is the least desirable of all options and one that I would not wish to be involved with in any way. Fluridone and diquat will kill duckweed and I am told that terbutryne will kill floating vascular plants and algae as well. All of these agents will, to a greater or lesser degree, also kill off any submerged plants including the valuable oxygenators. If you do decide to go down this route, against my advice, you will need to seek expert opinion in this field, as I am not sufficiently knowledgeable to make any recommendations. Perhaps East Surrey Water Company have had some experience of the problem?

The only real solution then, in my view, is to identify the source of the problem and eliminate it, after which it may still be necessary to clear any new growth from the surface, a couple of times, by hand, until a tolerable balance is achieved.

Occasionally this kind of infestation will spontaneously disappear for no apparent reason. In its early years Bough Beech Reservoir was afflicted by a massive growth of Canadian pondweed all around its shallower margins, and then, in one season, it vanished, with no trace to be found to this day. The reason remains a mystery.