Ter­rawatch: Ro­man records show last­ing ef­fects of pol­lu­tion

The Guardian Australia - - Environment - Kate Rav­il­ious

All over the world lakes are in trou­ble. An ex­cess of nu­tri­ents – from fer­tilis­ers, de­ter­gents and sewage – is up­set­ting the bal­ance of life, lead­ing to al­gal blooms and bot­tom-wa­ter dead-zones. Many places are now try­ing to clean up their act, but how long does it take for a lake to re­cover?

Sed­i­ment cores drilled from a Swiss lake re­veal how long it took for the lake to bounce back af­ter the Ro­mans de­parted, and in­di­cate we might have to wait cen­turies for to­day’s pol­luted lakes to be­come prop­erly fresh again.

The Ro­man city of Aven­ticum, on the shores of Lake Murten in Switzer­land, grew rapidly from around AD30, sup­port­ing around 20,000 peo­ple dur­ing its hey­day in the first and sec­ond cen­turies. But now sed­i­ments from the lake bed re­veal that the land clear­ance and in­ten­sive agri­cul­ture as­so­ci­ated with the growth of Aven­ticum re­sulted in huge nu­tri­ent runoff into the lake, wip­ing out ecosys­tems and plung­ing the lake into cri­sis.

Dur­ing the third cen­tury the Ro­mans aban­doned Aven­ticum and the sed­i­ments show that life re­turned to the lake af­ter about 50 years. But the find­ings, which are pub­lished in Earth and Plan­e­tary Sci­ence Let­ters, re­veal that it took around 300 years for the lake to fully re­cover.

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