Stable Pond Blooms achieved through Phosphate and pH Control
Introduction
This article outlines the results of an investigation into the effects of limited phosphate (PO4) and reduced pH on saltwater pond bloom behaviour.
Background
The study was carried out on saltwater barramundi ponds located near Darwin in Australia. The production ponds received about 150 kg 50% protein feed per ha per day. This farm has had both acute and chronic problems with harmful algal blooms (HAB's) in particular Prymnesium parvum, Heterosigma spp, Gymnodinium spp, and Prorocentrum spp. These problems initiated the investigation into methods to manage algal blooms more effectively and so reduce risk of further fish losses.
Focus on Phosphate
This investigation concentrated on studying the impact of limited PO4 on blooms. The reason for this was technical developments in environmentally benign aquatic PO4 control had been made in Australia over the past few years. A product, Phoslock (www.phoslock.com) had been developed and proven in environmental applications as an effective PO4 removal tool. In addition, an Australian expert in harmful algal blooms, Dr Gustaaf Hallegraeff, had suggested that manipulation of N:P ratios may be one method to manage harmful blooms in ponds.
Methods for limiting P and reducing pH
Phoslock was applied to the ponds to remove PO4 . The active ingredient of Phoslock is Lanthanum which binds with PO4 forming insoluble rhabdophane, a stable mineral compound. The rate of application of Phoslock is 30 kilograms of product for each kilogram of PO4 that needs to be removed. Phoslock was able to consistently limit PO4 to less than 0.03 mg/l (Fig 1)
Molasses was used to reduce then to maintain pH. Molasses is a source of organic carbon that stimulates the heterotrophic bacteria in the pond. These bacteria then produce carbon dioxide which acts as a weak acid and causes pH to decline. These bacteria also consume oxygen so care must be exercised to prevent the oxygen demand overwhelming the aeration capacity of the pond. Judicious application of molasses was able to drop then maintain pH at less than 7.7, the pH value that had been targeted by previous studies. (Fig 2)
Effects of limiting P.
Limited PO4 had the effect of significantly reducing the abundance of some nuisance algae, including the very dangerous Prymnesium parvum. However, other genera such as Heterosigma and Gymnodium showed no such reduction when PO4 alone was limited. There was a some slowing down of the algal blooms as measured by the daily DO maxima, but this was erratic and unpredicatble..
Limited PO4 resulted in significant production of the enzyme alkaline phosphatase (APase) (Fig 3) APase has been shown in numerous studies to be produced by algae in PO4 limited conditions. This enzyme liberates PO4 from organic molecules so making the PO4 bioavailable. As a consequence of this enzymatic activity larger organic molecules are broken down thus speeding up the degradation of organic material in the pond. In essence, removing PO4 through application of Phoslock causes production of APase which liberates more PO4 .
Effects of combined limited PO4 and reduced pH.
Prolonged and stable reduced pH was achieved by application of molasses. When pH was maintained under 7.7 the blooms were relatively stable and predictable. (Fig 4) The dominant algae were nanochlorella types and other species including nanoflagellates and nanodiatoms (nano here means less than 5 um). Bloom stability and predictability are both very beneficial outcomes for pond management.
Limited PO4 assisted in the ability to cost effectively maintain a reduced pH. The assumption is that limited PO4 restricts microbial activities and this in turn restricts their rate of oxygen demand. This then means that the risk of dangerous DO depletion is reduced.
Although the quantity of APase produced in the low pH-low PO4 ponds was high, enzyme activity was slowed due to the low pH. Production of APase acts to liberate PO4 , which in an imposed low PO4 management regime must then be removed. APase is a potent enzyme that assists in speeding up the degradation of organic material thus helping minimise sludge accumulation. Regular PO4 removal thus has the positive effect of maintaining enzyme levels in the pond.
If pond pH was allowed to increase, the activity rate of APase increases releasing large amounts of PO4 very quickly. Data from this work showed that APase liberated PO4 2 – 3 times faster when pH was elevated. Elevated pH would allow the pond to revert back to the "natural" state of excess PO4, rapidly fluctuating DO and pH and unpredictable and potentially dangerous blooms.
Measurements of ammonia, nitrite and nitrate suggested very strongly that N turnover was very rapid indeed in the experimental ponds. Nitrite stayed surprisingly low and the assumption is that the reduced pH regime enhanced activity of Nitrobacter sp. allowing complete nitrification to nitrate followed by denitrification to N2. Regular molasses application as a response to rising pH enhances denitrification processes. The absence of N2- fixing cyanobacteria such as Oscillatoria spp in the ponds due to limited PO4 may also have assisted in the rapid removal of net N from the system.
Barnacles and calcareous tube worms are normally common on this salt water farm. However, in the experimental ponds they were absent. The assumption is that the low pH regime prevented or slowed development of calcareous fauna by restricting formation of their shells.
Mortalities of barramundi in the experimental ponds showed a downward trend as blooms were held in the low PO4, low pH regime (Figure 5). The assumption is that this is due to absence of nuisance algal species causing chronic problems in the fish.
A summary of the PO4, pH and pond parameter relationships is shown in Table 1.
Cost and benefit
The cost of inputs for limiting PO4 and reducing then maintaining pH must be balanced against the economic gains which include the reduction of risk. Quantities of Phoslock and molasses applied during this study, where feed inputs averaged 150 kg per ha per day, were about 100 kg per ha per month and 300 kg per ha per month respectively.
Summary
This study has shown that PO4 supply and pH are fundamental parameters impacting on bloom behaviour and bloom composition. By limiting PO4 and thereby promoting APase production and combining this with reduced pH (less than 7.7) a number of beneficial outcomes were achieved. These outcomes included stable, safe and predictable blooms and, for the experimental farm, a cost effective bloom management strategy.