Muck, mulm, sludge, sediment: there are lots of terms that refer to that slimy, cloudy, smelly mass that collects at the bottom of your pond (and many lakes - remember the squishy stuff that oozed between your toes as you waded?). It’s usually brown, although it can also sport a coat of green algae. The muck consists of decaying organic matter - fish waste, bits of leaves from your aquatic plants or that drifted into your pond from nearby trees, tiny bugs or larvae that didn’t quite make it, yard clippings and lots of other things. In small amounts, muck doesn’t pose much of a problem if your pond is well oxygenated and the chemistry is in good balance, but left unmanaged, it can build up to become a major threat to the ecosystem.
The stuff that makes up muck is in the process of decomposition, which is a natural process and generally a good thing. The problem begins to develop when there is inadequate circulation in that part of the pond or when the muck gets so thick that oxygenated water doesn’t pass through. As oxygen is depleted by the bacteria aiding decomposition, the good bacteria will die and be replaced by anaerobic bacteria which doesn’t require oxygen to live. This “bad” bacteria generates toxins as it consumes the waste, which not only creates an unpleasant “sewer” smell, but can injure and even kill your fish and plants. For this reason, you’ll want to manage your mulm. It may be worth doing a seasonal “muck out” of the bulk of the leaves and other detritus that have collected, and using a net to collect floating debris before it all sinks to the bottom. Maintaining good aeration and water circulation will prevent anaerobic bacteria from gaining a foothold and that’s always a good policy for pondkeeping.
