te ich ▁n ortheast ▁of ▁Sank t ▁Andreas berg , ▁which ▁is ▁the ▁only ▁p ond ▁not ▁gr out ed ▁with ▁grass ▁tur ves , ▁but ▁with ▁gran ite ▁sand ▁known ▁as ▁gran it gr us ▁and ▁with ▁a ▁dam ▁height ▁of ▁ 2 1   m ▁and ▁res er voir ▁volume ▁of ▁▁ 1 . 7   mill ion ▁m ³ ▁of ▁water ▁stands ▁head ▁and ▁shoulders ▁above ▁the ▁other ▁p onds ▁in ▁terms ▁of ▁size . ▁ ▁Flora ▁and ▁fa una ▁▁ ▁Although ▁these ▁water ▁features ▁have ▁been ▁artificial ly ▁created , ▁rare ▁fl ora ▁and ▁fa una ▁have ▁evol ved ▁in ▁and ▁around ▁many ▁of ▁the ▁Upper ▁Har z ▁p onds . ▁The ▁water ▁is ▁low ▁in ▁nut ri ents ▁and ▁rather ▁cool . ▁The ▁c ray fish , ▁which ▁has ▁died ▁out ▁in ▁most ▁European ▁water b od ies ▁due ▁to ▁c ray fish ▁pl ague , ▁has ▁been ▁able ▁to ▁surv ive ▁in ▁many ▁Upper ▁Har z ▁P onds ▁thanks ▁to ▁their ▁isolated ▁locations . ▁P ond ▁operators ▁and ▁fish ing ▁le ase hold ers ▁have ▁worked ▁successfully ▁to ▁increase ▁their ▁numbers . ▁▁ ▁The ▁operation ▁of ▁the ▁p onds ▁over ▁many ▁centuries ▁with ▁constantly ▁changing ▁levels ▁of ▁water ▁has ▁also ▁created ▁a ▁habitat ▁for ▁some ▁extremely ▁rare ▁plant ▁communities . ▁On ▁the ▁so ils ▁of ▁many ▁p onds ▁cor al ▁neck lace ▁( I le ce br um ▁vert ic ill atum ), ▁st rap wort ▁( Cor rig i ola ▁l itor alis ) ▁or ▁shore