Over the last month or so I've been trying to look at local ponds from an NVC perspective, to get an idea of how useful this classification system would be for categorising pond types (see above thread). These are some initial thoughts, focussing on specific types of ponds that I've noticed that seem to fall outside the NVC Aquatic Communities classification scheme.
Probably the most difficult problem I've come across is generating an NVC 'score' for ponds which do not have any aquatic vegetation species at all(!) Some of my favourite ponds are small concrete reservoirs that farmers have built (presumably to provide local water storage for crops / animals). In many cases they seem to attract more wildlife than similarly-sized natural ponds - I'm guessing that one reason is because they are often built in areas which have few natural ponds or streams, so draw wildlife from a large radius. They're alive with dragonflies in the summer, and I can find amphibians in or around many almost all the year round (maybe partially because they have difficulty getting out!). In many cases these artificial ponds don't support any higher plants - however this is where their similarities end. Some (at one end of the spectrum) contain clear water with little life except mosquito larvae, whilst many others (at the opposite end of the spectrum) are thickly coated with a floating layer of green algae - from a few inches to a foot or so thick - and support an amazing density and diversity of species (in particular snails, immature newts, caddisfly, stonefly and dragonfly larvae). There are also a number between these two extremes. As far as I can guess much of the variation arises from differences in inflows to the ponds (nutrient enrichment, etc), although I'm sure that things like depth, accessibility and a host of other factors are also relevant.
Small pools in dense woodlands.
By these I mean ponds where the bottom is covered by a layer of leaves for much of the year - I'm guessing that food-chains in these ponds are fuelled predominantly by decomposition processes. I know several examples that support very healthy newt populations. Again the ponds are very sparsely vegetated, either because of the dense shade or moveable substrate, and don't look as if they would fit into an NVC-based classification system easily. However I'm reserving judgement on these until later in the season (incase there is a brief window of growth).
Elongated, ditch-like ponds.
Highly linear ponds (which in many cases link to adjacent waterbodies) also present difficulties, because the majority of plants (apart from rushes) line the sides. The boundary between pond-related and adjacent vegetation is indistinct, and highly dependent on water level. If you follow the pond/ditch for a while it is often bordered by completely different vegetation, which further complicates matters.
I haven't had time to look at short-lived pools in any detail. However many of the sites that I check for frogspawn early in the year are shallow temporary pools on heathland, where the heath vegetation (mainly grasses) persist throughout the life of the pond. Where temporary pools are relatively short-lived, or where the vegetation community is resistant to submergence, it seems likely that such pools will have little effect on vegetation and as a result would fall outside the aquatic NVC classification. Temporary pools are used in preference by some species - notably frogs / natterjack toads, but also many invertebrates - as they offer a number of advantages in addition to local availability (rapid warming; predator avoidance; etc).
Ponds with perturbed vegetation communities.
Another problem worth noting is the increasingly common trend of introducing exotic plant species into natural ponds, either deliberately and accidentally. Several of my local ponds have been deliberately 'improved', in one particular case transforming a sparsely vegetated, acidic heathland pond with a dense covering of water-lilies. Using an NVC-based system this would be recognised as a completely different pond type; however it is arguable how much the aquatic community has changed, as the location is fairly remote and rate of colonisation by (for example) new invertebrate species likely to be low. Canadian pondweed offends in a similar way (although more often accidentally).
In summary, I'd say that my initial reaction is that the NVC classification system is (at best) likely to be useful for categorising a subset of ponds which exhibit 'classical' vegetation types, but of limited use in classifying the full range of pond types encountered in the UK. Part of the difficulty in adopting a purely plant-based classification scheme arises from the 'island' nature of discrete ponds, where colonisation by species (particularly plants) is a chance event. In particular, ponds in which plants have difficulty establishing, or where atypical (artificially introduced) species dominate will always fall outside such a scheme.
The ditch classification system proposed by JNCC extends the scope slightly; however even from my limited exploration it is clear that a number of additional pond types exist outside these classification schemes.
Green Man Software Ltd