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Minister approves moves on Rotorua lakes

Minister approves moves on Rotorua lakes

Environment Minister Marian Hobbs has approved a range of actions aimed at Rotorua's polluted lakes.

Releasing a Ministry for the Environment report to her on Lake Rotoiti and other Rotorua lakes Marian Hobbs said her ministry will contract an expert in managing degraded lakes to advise and assist Environment Bay of Plenty and will also consider ways of financially supporting a community programme of action.

Rotorua's lake problems will also be included in a programme of action being developed for water by an MfE working group under the government's sustainable development policy.

Marian Hobbs said she will meet representatives of Environment Bay of Plenty in Wellington soon to discuss the Rotorua lakes.

"There are no quick, cheap and easy solutions to the problems with Rotorua lakes but they are nationally important for tourism and recreation," Marian Hobbs said. "Once Environment Bay of Plenty has completed its investigations and has a proposed action plan for Lakes Rotorua-Rotoiti I will consider further government involvement."

Rotorua Lakes Background

There are 11 decent sized lakes in and around Rotorua. Lakes are extensive holes in the ground filled with water. Mostly the water comes from rain. The rain either gets to the lakes quickly through overland flow and streams, or slowly by seeping through the ground. On its journey to the lake, water picks up plant food (or nutrients) from the land.

Some of the lakes are interconnected and may overflow to each other.

Plants grow in the water in the lakes. In order to grow, the plants need only plant food and light for photosynthesis. When there is a regular amount of plant food, there is generally a resident population of water plants that grow there. When there is too much plant food, there can be virtual population explosions of fast-growing, mobile little plants called algae. The excess of algae is unpopular because it can smother out other life in the lake, and is considered to be water pollution.

There always have been algae explosions in the lakes near Rotorua, and there always will be. The trouble now is that they come more often; they stay much longer, and in some cases may become fixtures. Too much algae for too long smothers out everything else, and the lake may die.

Plant Food

The problem is too much algae. The cause of the problem is too much plant food that makes the algae grow and reproduce.

It would seem to obvious that if we know how much plant food is needed to grow algae, then we reduce the plant food to less than that, and the problem is fixed. Unfortunately this is not a case of what you see is what you get.

Firstly the science is at best approximate, and it always will be. There is a case for more and better science, but it will only ever give answers in generalities.

Secondly all plant foods are not equal. (the growth responses are not linear). Likely the algae risk from concentrated plant foods from septic tanks is greater than the same amount of plant food at lower concentration coming from rural sources. Conversely for some lakes the main source of new plant food is from rural sources.

Thirdly, and perhaps most importantly, the plant food comes in three different forms of availability to drive algae growth.

The immediate form of plant food is mineral dissolved in water. It is the smallest fraction of the plant food, and is the only form that the algae can eat today. In terms of immediacy it is equivalent to our food on the table at mealtime. It is immediately measurable and immediately available. It comes mainly from fresh inputs of contaminated water, but may come from being released from the other two less-available forms. Algae growth cant necessarily be reduced by controlling today’s inputs of plant food, but certainly the growth cant be reduced if today’s inputs are not controlled.

The intermediate form of available plant food is in short-term storage. If you like it is equivalent to our food stored in the cupboard. Some of it is packaged, but over the medium term quite a lot of it can be available if our table is depleted. In lakes this is food in living or recently dead plant material, or attached to fine soil particles, suspended throughout the water of the lake. This intermediate form tends to hang around for a while, and depending on conditions, may turn into the immediately available form, or even slip off into long term storage.

The least available form of plant food is in long-term storage. It has accumulated over the history of plant food inputs to the lake, and is now in the sediments at the bottom of the lake. If you like it is equivalent to us having dried food for an emergency stored in the basement. For the lakes around Rotorua, that basement now has much more stored food than it used to and some of it is spilling out onto the table. Changes to conditions like temperature, water flow etc has always caused some to spill out, and the algae population has always exploded to eat it up. These days there is so much plant food in the basement (sediment) that less change of conditions is needed to make it spill out, and the spilling may happen more often than not.

The large amounts of plant food in storage in the sediments means that removing the day-to-day inputs will not remove the risk of algal growth from the most polluted lakes. Even with no more external inputs of plant food, there is likely enough there to feed algae for years.

What to do and what not to do?

It is possible to use weedicides to kill off most of the algae (and most other things) but they breed up again while you watch.

If we decide that killing algae is a waste of effort, there are three basic strategies – manage the cause, manage the symptoms, or change the rules.

Manage the cause

The cause is too much plant food that is available to algae; if you like, on the table. Some of the lakes have a level of plant food that they can cope with in most circumstances. For these the objective is to manage the inputs of plant food at a level such that they continue to cope.

In the most polluted of the lakes it is unlikely that the problem can be managed with the present rate of inflow of plant food. For some lakes most of the excess of plant food is directly or indirectly from farms. For some lakes it is mainly from urban run-off, including sewage (septic tanks). For some lakes (e.g. Rotoiti) it may be inflow from other lakes (Lake Rotorua) that the other lake can cope with but they can’t. It is not certain that fixing the rate of plant food going into the lakes will solve the problem. It may be that the spilling out of the stored food will be sufficient to feed the algae. Conversely the problem of the excess algae will not be managed if the present rate of inflow of plant food continues.

It seems likely that it will be necessary to cap the plant food input to those lakes that are coping, and reduce the inputs to those that are not coping. This means obvious things like reticulated sewerage with compulsory connection and no more building without it around polluted lakes like Rotoiti (it doesn’t have it now despite the level of development), and less obvious things like constraints or reductions on agricultural inputs. Even these activities will not give guaranteed improvements, and certainly not immediately, because of the large amounts of plant foods already in storage in the sediments.

Remember also that this is not and never will be exact science.

Manage the Symptoms

Algae smother other life for oxygen and light. There can be some relief from strand –type algae by harvesting them with special machines and removing them from the lake. Unfortunately the algae of concern here are so tiny as to be not harvestable. Another technique is to introduce extra oxygen to make up for that used up by the algae. Oxygen can be introduced via water fountains, air pumps, or even by changing internal patterns of flow and mixing in the lake. The introduction of new sources of clean water can also help. Certainly these are worth considering in some areas whilst there is longer term attack on the causes. Furthermore, if oxygen levels are kept high, there is less release of plant food from the stored sources in the water column and lake sediment.

Again the science is not exact, and some cautious adaptive management approach would be necessary.

Change the Rules

The first rule that may be changed is the balance between plant food in the sediment and plant food released to be immediately available for algae growth. As above, oxygen keeps that balance away from release from the sediments. More direct impact may come from adding to the lake clay-like materials that absorb plant food and sink to the bottom.

The second possible rule change is the natural flow of waters. It is possible to divert away sources like inflowing waters (even natural) with high amounts of plant foods. This may be particularly pertinent in Rotoiti which receives plant food in excess from Rotorua, and from thermal springs.

Such changes are likely expensive, and need to be weighed against the risk both of failure in this inexact area, and secondary impacts elsewhere that might eventuate.

Timing for Decisions

The timing for decisions-making is soon. The EBOP-led investigations will produce plans for consideration by around March 2004. A reasoned investigation and proposal takes this reasonable amount of time.

Unfortunately the past warm season was accompanied by more algal growth than expected especially in Rotoiti. We can not predict if the next warm season is going to be better, the same or worse. If it is worse the impacts could be undesirable and the longevity of those impacts is unknown.

It would seem timely to make some strong decisions within the next year, and to prepare for either rapid response or ameliorative positions even faster than that.

(As important but less urgent there are at least two other issues, which would be prominent if our attention was not focused on algae. The lakes are carrying too much suspended soil from run-off, and there is significant alien invasion of waterweeds.)

© Scoop Media

 
 
 
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