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Diatoms in Shrimp Ponds

 

Daniel Gruenberg (daniel@acquestra.com): Dear Shrimpers, I am compiling information for a report on fertilizers that encourage the establishment and growth of diatoms in shrimp ponds.  If any of you have any information on this topic, please forward it to me.  Once I get all the information analyzed I will post it to The Shrimp List!

 

Alberto Barriga (albarriga7@gmail.com): Don’t look for a miraculous product.  Just mimic the nutritional composition of the diatoms that you’re attempting to fertilize.  If you look into algae stechiometry [a calculation of relative quantities of reactants and products in chemical reactions], you will find out that it’s a >16:1 nitrogen:phosphorus ratio, but in my experience I’d rather go with a 20:1 N/P ratio + 20 silica, which is a diatom selective nutrient.  It would be N:P+Si —> 20:1:20.

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Daniel, I agree with Alberto.  This is the same technique we used in the early 1970s at the Ralston Purina shrimp-farming project in Florida, USA, to shift the ponds from green to brown.  Generally it seems to have worked consistently, ending up with a dominant brown diatom bloom.

 

Daniel Gruenberg (daniel@acquestra.com): Thanks Durwood and Alberto.  I just want to note that the color of the bloom does not necessarily indicate the type of phytoplankton.  Blooms can be brown due to dinoflagellates or diatoms.

 

To check for diatoms, I always use a coffee filter (Asian cloth type that has a metal wire handle) with the bottom snipped off and connected to a test tube.  If you see copepods, you have diatoms.

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Daniel, at the Purina project, we confirmed our diatom counts daily with microscopes, without necessarily identifying the species.  The ponds were built on power plant coal-fly ash and native lime rock/calcium carbonate soils, and I think some clay was added after the fact when we had high percolation rates.  If I remember correctly, the hotter it got, the more difficult it was to get the shift to diatoms.  Summer rain runoffs raised the nitrogen and the phosphate levels, but not the silica levels.  The area where we were on the gulf coast had a deficit of silica in the water.

 

Daniel Gruenberg (daniel@acquestra.com): Do you have any idea of what the silicon dioxide (SiO2) [a chemical compound that is an oxide of silicon] levels were?

 

Durwood Dugger (ddugger@biocepts.com, http://www.biocepts.com/BCI/Home.html): Daniel, honestly I don’t remember, but when I saw Alberto’s 16:1:16 ratios, they seemed more familiar to my long ago experience than his 20:1:20 ratios.  I suspect you are correct about local conditions and seasonality having a significant impact on the respective protocols.  I’m pretty sure there is a lot of information on this topic from the 1970s and 1980s.  A little digging in the Journal of the World Aquaculture Society will probably turn up some good info.

 

Juan Aguirre (jxaquirre58@yahoo.com): Alberto, what form of silicate do you recommend for ponds?

 

Alberto Barriga (albarriga7@gmail.com): Juan, it depends on your sources of silica.  Magnesium silicate and rice-husk charcoal are cheap and contain a lot of silica.

 

Giovanni Chasin (gchasin_valencia@yahoo.com.br): Juan, we use pentahydrated sodium metasilicate, even better, though, is heptahydrated sodium metasilicate.

 

Daniel Gruenberg (daniel@acquestra.com): It seems like nobody knows of any specific products for fertilizing diatom blooms in shrimp ponds.

 

I also should note that at metasilicate concentrations over 1 part per million (ppm), silica is not limiting and any additional silica would not have much benefit.

 

Ramon Macaraig (monmac52@yahoo.com): The formula for green algae we are using is 06H263O110N16Si0P.

 

Rios et al. (1999) listed the following algae formulas:

 

Other Autotrophs C106H171O53N19Si0P2.0

Diatoms: C106H176O59N16Si6P1.2

Heterotrophs: C106H175O64N16Si3P1.9

Detritus: C106H181O60N13Si8P1.1

Total (Planktonic+Benthic) POM (Particulate Organic Matter): C106H177O59N15Si6P1.2

 

Loading 1 ppm of metasilicate converts to 10 kilograms per hectare in a pond with a depth of 1 meter.  That’s expensive.  It’s necessary to have the seawater analyzed and to factor in the salinity to quantify how much metasilicate is necessary.

 

I note, however, that our full-strength-seawater, biofloc systems are able to hold a diatom population at 15-20 transparencies without fertilizing with silicates.  We fertilize at six kilograms per hectare with diammonium phosphate (18-46-0, the soluble form with the highest P concentration available in the Philippines) to establish the transparency early in the growout cycle and to encourage chlorophytes.  The growth of a stable diatom population happened later and was a pleasant surprise.  The diatoms may be contributing to our average weight gains of 2.5-3.0 grams a week after the shrimp reach 20 grams!

 

Bob Rosenberry (bob@shrimpnews.com): I emailed Ramon to make sure that I understood some of the information in the above paragraph, and he forwarded the following additional information:

 

At our farm in the Southern Philippines, we classify the algae as diatoms, green algae and blue green algae (BGA) and manage the pond environment on based on the dominant species.  There are seasonal dinoflagelates, but they are easy to manage.

 

Our observations show that it is extremely rare at our temperatures, transparency and salinity levels for the BGAs and the diatoms to mix.  It is always ensembles of BGAs and green algae or diatoms and green algae that dominate at one time or other.

 

Daniel Gruenberg (daniel@acquestra.com): Ramon, sodium metasilicate is only 23% silica.  In order to increase silica by 1 ppm in a pond 1.5 meter deep, you would need 65 kg per hectare.   In a pond 1 meter deep, you would need 43.5 kg per hectare.

 

Also I would like to note that estuarine water contains relatively higher levels of silica, compared to open ocean water, so it’s not just simply related to salinity as some have mentioned.

 

With all the massive benefits that diatoms provide, I’m surprised there are so few products out there that encourage their establishment and growth.

 

Bob Rosenberry (bob@shrimpnews.com): After this report was first published on January 21, 2017, Daniel Gruenberg forwarded the following information: There are three forms of metasilicate: anhydrous, pentahydrate and nonahydrate.   Each of them has a different percentage of metasilicate:  anhydrous (23%), pentahydrate (13%) and nonahydrate (10%).

 

Patrick Wood (patrickjwood@yahoo.com): Rice husks are a good source for high-grade silicates.

 

Daniel Gruenberg (daniel@acquestra.com): Thanks for that information Patrick.  Again, my experience at farms in Asia is that silica is rarely a limiting factor in shrimp ponds, depending on the soil type and water quality.

 

Ramon Macaraig (monmac52@yahoo.com): Daniel, if your target is 1-ppm silicon, you are right at 65 kg/ha of sodium metasilicate.  I don’t know if there are commercial products of it that are cost effective for shrimp ponds.  I use technical-grade silica in my hatcheries for Chaetoceros culture, but I don’t use it in my ponds to stimulate diatom growth.

 

The silica in rice-hull charcoal is in oxide form, like in soil.  I have not done the experiments to find out at what acidity levels rice-hull charcoal acid can convert to soluble metasilicates.  The pond bottom silicates from the clay soils may be undergoing some chemical action, too.

 

We were surprised that diatoms appear spontaneously in our biofloc ponds and then stabilized about halfway through the growout cycle—but they have also crashed on us.  That’s why we’re converting brown ponds into green algae culture with phosphate fertilization, and then seeding with Nannochloropsis as soon as possible.

 

In clear water, we learned from our milkfish, tilapia and shrimp ponds that a diatom assemblage, created by making the system nitrogen rich with applications of urea, is the only antidote to a stable, but unfavorable, blue-green algae assemblage.  The brown diatoms are the first to respond to the urea and establish a brown canopy that smothers the benthic cyanobacteria.  It takes about 2-3 days.  Only then can we apply the phosphates to grow the stable green algae.  Applying phosphates perpetuates the BGAs.  The Aussies have used Phoslock to scrub the phosphates out of BGA-infested barramundi ponds.

 

Because diatoms have very short life cycles in our ponds, algae crashes and low-oxygen are major threats.  That’s why we have decided to go for a chlorophyte-dominated assemblage in our milkfish, tilapia and shrimp ponds.  We are happy that a stable diatom assemblage is being established in our full-strength-seawater, biofloc ponds.

 

Sources: 1. The Shrimp List (a mailing list for shrimp farmers).  Subject: Diatoms in Shrimp Ponds.  January 17, 2017.  2. Email to Shrimp News from Ramon Macaraig on January 20, 2017.  3. Email to Shrimp News from Daniel Gruenberg on January 21, 2017.  4. Bob Rosenberry, Shrimp News International, January 22, 2017.

 

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