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Stand somewhere wet and peaty with your boots sinking in, and you are probably standing in one of two habitats. Bogs and fens are both peat-forming wetlands, both built from plant remains that never fully rotted away, and both far rarer than they once were. But they are fed by different water, that water carries different chemistry, and the plants growing around you will tell you which one you are in. Getting the distinction right changes how you read a site, and how you look after it.
Bogs are ombrotrophic, which simply means rain-fed. Everything the plant community receives arrives dissolved in precipitation or as dust. Fens are minerotrophic: their water has travelled through soil, rock or another water body first, picking up minerals on the way.
That single difference in where the water comes from drives almost everything else — acidity, nutrient supply, plant communities, and how the site responds to a dry summer or a new drainage ditch.
A bog sits on a mound of its own making. A raised bog is a lens of peat with a dome of water held above the surrounding groundwater level, the water table hovering close to the surface across the whole surface. Blanket bogs spread over flat or gently sloping ground, again fed by rainfall. Water movement is slow, mostly downward through the peat, and the site is effectively cut off from the landscape around it. Nothing flows through to bring nutrients in, and nothing flushes them out quickly either.
Fens occupy places where water gathers or passes: valley floors, springheads, lake margins, seepage slopes, old floodplain hollows. Many are fed by groundwater rising from a chalk or limestone aquifer, and those tend to be the most species-rich examples. Others are fed by surface water from streams or lakes. Either way there is through-flow, however gentle, and the water arrives carrying dissolved minerals.
Fen water tables often swing through the year, with summer drawdown and winter inundation, especially where surface water is the main supply. Bogs are more stable and more relentlessly wet. Peat accumulates in both because waterlogged, low-oxygen conditions slow decomposition, but the peat itself differs: fen peat tends to hold more remains of sedges, grasses and brown mosses, while bog peat is dominated by the remains of Sphagnum.
Bog mosses make a bog more acidic and keep it that way. Sphagnum exchanges hydrogen ions for cations in the water, so the system becomes poor in calcium and magnesium. Bog water is usually below pH 5 and often nearer pH 4, with very low nutrient levels, particularly phosphorus. The acidity and the waterlogging together suppress decomposition, which is why bog peat can build up so deep.
Fens are less acidic because base-rich water keeps arriving. Rich fens, fed by calcareous groundwater, can sit at neutral to mildly alkaline pH and may deposit marl — a pale grey, crumbly calcium carbonate sludge — in pools and channels. Poor fens are mildly acidic and much more modest in their plant diversity. So "fen" covers a wide chemical range, from a brown-moss carpet at a chalk spring to a tussocky, slightly acidic hollow.
Nutrient status follows the same pattern. Bogs are nutrient-poor, receiving only what the rain brings. Fens are richer, and the richest are those with a steady mineral supply. That richness is also their vulnerability: add nitrogen or phosphorus from fertiliser, drainage or polluted runoff and the balance tips towards a few vigorous grasses and nettles, at the expense of the specialist plants.
You can often name the habitat from a few metres away, before you get your pH meter out.
Sphagnum is the giveaway, but not every species of it. The hummock and lawn species that build bog are the ones to learn. Fens can hold Sphagnum too, usually in their poorer, more acidic corners, but they also support brown mosses that cannot tolerate the extremes of a bog.
Bogs and fens grade into one another, and that is part of the interest. A lagg fen forms around the edge of a raised bog, where water draining off the dome meets base-rich groundwater. "Poor fen" and "rich fen" describe where a site sits along the pH and mineral gradient.
Over time, fen can become bog. As peat accumulates, the surface lifts above the reach of groundwater, the plants lose their mineral supply, Sphagnum moves in and the site tips over into bog. Many of our remaining peatlands sit somewhere in that sequence, which is one reason the transitions are worth recording carefully when you survey them.
If the water arrives from somewhere else, you are in a fen. If the only water arriving is rain, you are on a bog.
Both are peatlands and both are protected, but the practical work differs. Restoring a bog usually means restoring the water table: blocking drains, removing planted trees, stopping burning and healing eroded peat faces. Keep people and machinery off the fragile surface where you can, and let the mosses do the work.
Restoring a fen may mean working at catchment scale, because the water quality and quantity come from elsewhere. A spring captured for a borehole, a road cutting a seepage line, or nutrient-rich runoff from farmland can all undo a fen from a distance. Fens often also need active management — light grazing or cutting — to stop scrub and rank grasses shading out the specialists. Bogs generally do best with less intervention and more water.
Wet peatland is not a safe place to wander without care. Keep to firm ground, use a pole to test depth, and never work alone on deep peat or near pools. If you are surveying for a management plan or a planning application, check the current statutory guidance and get advice from a qualified ecologist before making decisions on the ground.
Whatever you call the place, the water is the thing that matters. Protect the source, the flow and the chemistry, and the vegetation tends to follow.
Photo: Lauri Poldre / Pexels
