Does the ability to control Phosphate constitute a new paradigm

in bloom management?

Talk presented by Adam Body at Australian Prawn and Barramundi Farmers Conference, Gold Coast, Queensland, August 2010

Further information on this subject: http://www.flickingfresh.com/Phos/A%20new%20tool%20for%20pond%20managers.html

Today I will be talking about algal blooms in outdoor ponds, why they are important, how we try to manage them and the influence of phosphate on these blooms. I will talk about the use of Phoslock, a phosphate management tool. Phoslock gives us the ability to easily and cheaply control phosphate in ponds. I will propose that this tool does indeed constitute a new paradigm in bloom management.

What is a bloom?

In outdoor pond aquaculture sunlight and pond fertilisation through the addition of feed results in the growth of algae and the proliferation of algal blooms .

The dynamics of algal blooms have been the subject of years of research. Is the control of the blooms top down by grazers such as rotifers and copepods? Or is control bottom up such as by the amounts of various nutrients such as phosphate in the water? Or, given the “feast and famine” nature of most of the rapidly reproducing organisms in our ponds, are blooms simply chaotic? As farmers we have to try to manage blooms. If we get it wrong it can be catastrophic for us. Heavy blooms can “crash”, causing stock losses due to depleted oxygen. Blooms can also become dominated by nuisance or even dangerously toxic algae.

It is because of my personal experience with a toxic bloom on my farm in 2009 that I am here talking to you today.

So I will give a brief background on that event. In May 2009 we lost a whole pond of barramundi due to the alga Prymnesium parvum.

At the time none of us had any idea whatsoever about Prymnesium. We had no idea what killed the fish. It was only through a combination of unlikely circumstances that we were able to identify Prymnesium as the cause.

Once it had been established that Prymnesium was the cause of the mortality our alarm grew as we found out more about how dangerous Prymnesium was. The DAC quickly organised a workshop with Gustaaf Hallegraeff an expert in HAB's from the University of Tasmania.

Gustaaf explained that Prymnesium can become toxic only under special circumstances. What we needed to do was maintain our N:P ratios. I walked out of the workshop thinking “how are we supposed to be able to do that?” This talk today will help you to see how we are now able to do exactly that.

Blooms and Water Quality

In outdoor pond aquaculture, when we are talking about blooms and water quality we are essentially talking about the same thing.

Dense blooms produce very high dissolved oxygen levels (up to 300 % saturation) and very high pH (over 9).

These extreme water quality fluctuations induce stress in our fish or prawns .

Heavy blooms and their impact on water quality also impact on pond ecology.

The influence of PO4 on blooms

PO4 is a primary plant nutrient.

It is generally accepted that there is an optimum molar ratio of N to P for algae and this is about 16:1. When building new blooms managers fertilise ponds to achieve roughly this ratio. Once the bloom is established, most managers have learned to manage blooms without giving much thought to this ratio, due in no small part to the fact that there has been no means to remove phosphate other than through water exchange.

What is bloom management and why is it important?

Pond managers try to manage their ponds to find the happy medium where a good diverse pond ecosystem can thrive and stress in the farmed animals is kept to a minimum.

A lot of pond management is about managing bloom density or transparency, but it is complicated by the types of organisms growing. Some of these organisms can a nuisance or can become very dangerous indeed, as the story of my own experience illustrates.

What tools are available to manage blooms?

To manage blooms we need access to management tools. These tools are of three types: those that measure the bloom, those that help us diagnose, and those that provide a means to alter the bloom.

Measurement tools are readily available for a variety of bloom related parameters including Secchi, oxygen, pH and water colour.

Diagnostic tools include those that measure ammonia, PO4 and so on. The phase contrast/dark field microscope is an especially important diagnostic tool on our farm.

There are fewer tools for altering the bloom. The most common of these are fertilisers and water exchange. But before talking about these I will just quickly mention pond stocking rate, because stocking rate has such a major impact on water quality and on bloom dynamics.

Stocking rate is driven by our ability to maintain a good pond environment, and also by the necessity we have to make some money out of our business.

The necessity to make a profit requires us to push the stocking rate boundaries. We have to stock higher (which translates to add more feed which equals fertiliser) than our natural comfort level and as a consequence provide more management inputs to maintain water quality and manage the bloom so we can bring the fish or prawns through to harvest. Although stocking rate is not strictly a management tool, it does dictate the level of management required for the pond.

Water exchange is the traditional tool used to maintain a stable bloom and good water quality. Water exchange is simple and cheap and does the job of diluting the bloom. In most cases water exchange is all a manager needs to maintain a good bloom.

If the pond manager has access to a bountiful supply of good water that he can exchange at will, then he may have little need of other tools. An exception is if nuisance or harmful algae develop in the ponds.

Fertilisation is the other traditional tool. N and P can be added at will. So,recalling our Darwin toxic algae workshop and Gustaaf's reflections on preventing toxicity, my first action when returning to the farm after that workshop was to measure phosphate. This was about 0.6 ppm.

In order to achieve the 16:1 molar ratio required to make any future Prymnesium blooms lose their desire to create poison in my pond, I would need to raise my ammonia to around 1.8 mg/l.

Such a course of action may make the Prymnesium very content and prevent them manufacturing toxins, but it would also allow them to proliferate very intensively in my pond – Catch 22 springs to mind.

Phoslock is the tool that is at the center of this talk. Phoslock binds with free phosphate and enables us to accurately bring phosphate into the range that we desire.

Can blooms be managed?

Blooms are traditionally managed through water exchange.

Maintaining the bloom at the 30 – 40 cm transparency that is considered optimal and preventing or minimising growth of nuisance or dangerous algae can however be difficult.

Blooms constantly change. And they can change very quickly.

Bloom management on a particular farm usually comes down to the experience and “art” of the manager.

The manager will know each pond as an individual. Can this art be taken out and replaced with science? On our farm our ability to control phosphate is bringing us closer to this goal.

PO4 control and bloom management.

Phosphate control is now an integral part of our bloom management. The hard edge of this relates directly to toxic algae and Prymnesium. Prymnesium is now present in our system and in our area and we have no choice but to manage it.

Diligence and the ability to control phosphate have provided us with the means to proceed with our farm business with the knowledge that we can deal with Prymnesium

As far as general bloom management we have introduced a pond protocol that directs we now keep phosphate down to less than 0.05 mg/L. We have found that as long as phosphate is kept down at these levels, then our afternoon DO max rarely exceeds 10 ppm and our night time DO's remain constantly in the safe zone. Low phosphate slows the whole system down.

We are also saving electricity (almost 25%) as the aeration is running for less hours each night.

The blooms are much more stable with very little of the boom and bust that we used to have to deal with and this means the risk of bloom crash and associated water quality problems and fish “down time” is markedly reduced.

How do we manage phosphate?

We measure phosphate and if it is high we bring it down slowly, and we calculate carefully so that we leave a small amount of free phosphate in the system. Measurement is quick and simple. We use the Palintest System.

During the time we are bringing phosphate down we have to measure 2 or 3 times a week, and it can take 1 or 2 weeks to get the system stable with phosphate in the range we want. We apply Phoslock such that each application does not exceed 50 kg per ha, ensuring that the removal of phosphate is not too rapid.

Once the pond has stabilised, we monitor DO max and min using our Datacall system, and if max DO is over 10 ppm then we will check phosphate and apply Phoslock as required.

How much Phoslock is needed?

Theoretically 30 kg of Phoslock will remove 1 kg of phosphate . In practice this holds true when phosphate is over 0.1 ppm. However as phosphate drops under 0.1 ppm it gets harder to remove the phosphate

This is to be expected. But the fact is that in order to see real results we have to get the phosphate down under 0.05. So in order that we do not blow our budget we have to get a little bit smart. We know that we are competing with the algae for the phosphate, and as we have gained experience, we have learned that phosphate concentrations change throughout the day, and also from day to day. So we target the times when phosphate is peaking, and apply Phoslock at those times. This way we get the most out of the Phoslock.

Has the paradigm changed?

I think it has, simply because we now have the means to manage phosphate in a cheap and effective way. Phoslock is another tool, a vital one when it comes to managing toxic algae because it can be life or death for the pond.

In normal bloom management Phoslock is a handy tool that will be of use to some pond managers with particular problems. It may for example be useful for helping to control off flavor or persistent oscillatoria or consistently thick blooms. It may also be useful for cutting power costs or for complying with environmental regulations. Each farm manager will need to assess his own situation and firstly, measure phosphate to see if it is potentially part of the problem. But what the farm manager now knows is that he has available to him the tool to manage phosphate if he wants to. He did not have this tool in the past.