A swimmer can sense the turning of the lake. There''s a moment in the season when the water changes. It isn''t something you can see, it''s something you can feel. In spring, the winter ice melts, and the warm and cold of the lake intermingle, flowing together. In summer, as the lake grows warm, a green froth of algae caps the surface of the water, and when it cools again in autumn, the green disappears. The air thins. The leaves flash red and gold. And the water ''turns'' .

You come to know the consistent cool of spring and the stagnant warmth at the top of a summer lake. When the water clears in the autumn, you can feel it: the lake feels cleaner on your arms, less like velvet and more like cut glass. And then winter comes, sharper than ever. Swimming year-round means greeting the lake''s changes. There is an English expression for the lake''s changes: the ''breaking of the meres''. It describes the point in late summer when shallow lakes - meres - turn a turbid blue-green, algae breaking atop the surface like yeast froths on beer. The Germans also have a word for the green of summer: umkippen . It describes the point when the water has turned to slick green, fizzling with iridescent algae.

But the breaking of the meres and umkippen capture only that single moment of algal rupture, the death of the lake from too much algae and too little oxygen. We tend to notice the obvious thing - the emerging sheen of an algal bloom - and reduce a word''s meaning to that tiny moment, that fleck of green on the surface. The lake''s turning - ''lake stratification'' and ''overturn'' - runs deeper, taking in an entire year''s worth of changes in the water. Turning is perpetual. It points to the wider transformations in the water, as layers below billow and rearrange themselves beneath the surface. Even in winter, the lake is alive beneath the ice. I long for the ice. The sharp cut of freezing water on my feet.

The immeasurable black of the lake at its coldest. Swimming then means cold, and pain, and elation. When I was twenty-eight, almost as if by accident, I was sent to Berlin on a five-month research placement. I moved into a second-floor Altbau flat, one of the crumbling, enormous apartments that looks straight out of a Stasi spy drama. And from this old place with an old cellar that had once been used for escape tunnels, I set out into a world of pine and silken water, of craggy cobbles and peeling paint. Berlin resembled the other places I''ve called home - Canada, Britain - but only in glimpses: in the way the skeletal pines would edge the lake, in how the old stones would grow thick with moss. Pain, brightness, loss and renewal were layered in the landscape: in the lush shade of Tiergarten, which in my grandfather''s days was barren, razed and desperately carved up for allotments, and in the crooked edges of concrete that had slowly been dismantled as kids my age grew up. I was three when the Berlin Wall came down.

I don''t remember it, but I came to know it in my own way. The footprint of the Wall was turned into a hiking trail. Pavements stopped you in your tracks, Stolpersteine, brass stumbling stones marking the lost. Roads radiated out like a dial, a stretched palm pressed on to the city. I thought the roads here had to be so wide, if only to hold the ghosts. Half a year later, as I was retreating from the deep end of depression, I surfaced with the bizarre notion that the solution to my problems lay in swimming. I felt furious that I had succumbed to the dark vacancy of my moods, as though it were my fault. My heart was broken.

Above all, I thought that swimming might help me find some new place in the world in a year in which I''d changed address five times. A place in a city that wasn''t mine, and that held layered in its streets a century of change and grief, ghosts in the landscape. Naively, perhaps, I believed that if I could find that place in the middle of the lake where every feeling slipped away, I might undo the hurt. I''d moved again - this time into a stark white room with ceilings thrice my height - but spent only a few moments unpacking and settling in before turning my eyes to the map. The city at its centre, cut through by a fan of broad avenues and the rivers, the sudden countryside at its edges. Hundreds of spots of blue multiplied exponentially as the city lines crept into the surrounding land. These lakes and rivers - their intricate weave of water laid on to the flat North German Plain by retreating glaciers in the last ice age - had worked a tiny hook into my heart, and I could do nothing for it but swim. Perhaps it was a drastic response.

In depression, I had become someone I hadn''t wanted to be, emptied and hardened. I felt that I had to respond to it in kind, as if lake water might blast away my sadness and fear. So I decided to swim for a year, in the hope of finding some reserve of joy and courage in myself. It was a means of greeting the ghosts - mine and others - as they appeared around unknown corners. I knew there was no untouched landscape here: there is hurt that cannot be undone. I wanted to find a way to negotiate it, to live with it. Of all the lakes near to the city, I planned to swim in fifty-two, a whole year''s worth, stretching my swims out through each season. Prone to rules, I kept the parameters simple: no cars, no wetsuits.

I could take friends from time to time. My daily life would continue as normal. I was in the final year of my doctorate, finalising and refining a dissertation in environmental history. I was living an ocean away from family and home. There was pressure to hold things together. Swimming would be a way of staying with my fears, a way of staying in place. Above all, I sought to find some balance in it. The summer in Berlin began coolly and then arrived fully formed, hot.

Bright swathes of sunlight stretched over the cobbles, and the sky painted itself cornflower blue. The temperature rose and the air thickened only slightly as a hot, dry June settled on the city. I looked at the map, traced my fingers over the lakes I knew - Krumme Lanke, Weißer See, Liepnitzsee, Bötzsee, Mühlenbecker See - and decided, as if by habit, to start at the beginning. Krumme Lanke, my first German lake.