De­vel­op­ment de­liv­er­ing un­de­sir­ables

Central Otago Mirror - - CONVERSATIONS - LAUREL TIERNEY

In the first in a se­ries on wa­ter is­sues, former Guardians of Lake Wanaka mem­ber Laurel Teir­ney re­calls the begin­ning of her wa­ter fo­cused ca­reer in the 1970s.

At age 12, Lake Kaniere fos­tered my love of lakes. and I’ve in­dulged my pas­sion for study­ing wa­ter­ways over 45 years.

Since be­ing a Guardian of Lake Wanaka a decade ago, I be­lieve our lovely lake is not get­ting the at­ten­tion it de­serves, es­pe­cially mon­i­tor­ing trends in wa­ter qual­ity. Re­gional coun­cil pol­icy of in­ten­sively sam­pling a sin­gle lake site once a decade was en­tirely in­ad­e­quate. Thank­fully, monthly mon­i­tor­ing started in 2010, but still at just a sin­gle site.

Since ten­ure re­view, farm­ing the val­ley floors has in­ten­si­fied, with nu­tri­ents from fer­tiliser and live­stock car­ried by streams and rivers into the lake. Last time I vis­ited the Rob Roy Glacier, aerial top­dress­ing planes were work­ing the Matuk­i­tuki Val­ley and on my re­turn to the car, cat­tle were freely graz­ing the banks and re­liev­ing them­selves in the river.

In Wanaka, de­vel­op­ment is de­liv­er­ing a range of un­de­sir­ables to the lake. The most ob­vi­ous is sed­i­ment from huge new sub­di­vi­sions and slips. In­tense rain­storms, now an ac­cepted part of cli­mate change, can de­liver tonnes of sed­i­ment, and what­ever it is carrying, into the lake.

This hap­pened via Stoney Creek in 2004 and Bul­lock Creek last year. And when I was told that most of a lawn­mower’s clients fer­tilise and ir­ri­gate their lawns it struck me that a ‘mo­saic of mini farm pad­docks’ is be­ing cre­ated in town. Stormwa­ter drains also carry ev­ery type of chem­i­cal, from road sur­faces and town fa­cil­i­ties, into the lake.

Why be so con­cerned? Be­cause of Lake Tu­tira. In 1973, I be­gan work­ing on a lake ma­nip­u­la­tion project at the pop­u­lar recre­ational lake, 50km north of Napier. Lakes re­act to a one-way in­put of nu­tri­ents in a rea­son­ably pre­dictable way.

There’s usu­ally an ex­ces­sive growth of in­tro­duced plants, such as Hy­drilla in Tu­tira and La­garosiphon. Mi­cro­scopic green al­gae flour­ish, as nu­tri­ent con­cen­tra­tions in­crease. Dy­ing blooms sink to the bot­tom and are de­com­posed, re­leas­ing nu­tri­ents back into the wa­ter.

Lake Tu­tira’s sit­u­a­tion was so ex­treme the deep, cold wa­ter lost all oxy­gen within three weeks. With plant nu­tri­ents trapped in the deeper wa­ter, blue-green al­gae that fix ni­tro­gen from the air flour­ish and cre­ate thick lake scum. This can kill small an­i­mals. In my case, ear in­fec­tions and ran­cid abra­sions were the legacy of div­ing dur­ing monthly sam­pling over four years.

The ma­jor cause of the in­cred­i­bly rapid 10 year de­te­ri­o­ra­tion in Lake Tu­tira was the ad­vent of aerial top­dress­ing. De­spite a huge ef­fort, our at­tempts to lift de­oxy­genated wa­ter to the sur­face to ab­sorb oxy­gen made lit­tle dif­fer­ence.

Last sum­mer, 43 years and many other fu­tile at­tempts later, Lake Tu­tira lost all oxy­gen, killing most lake in­hab­i­tants - a ter­ri­ble first for a lake of this size in NZ.

SUP­PLIED

Wanaka aquatic sci­en­tist Laurel Teir­ney.

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