Dear Maria Rousou ,

Fortnightly water quality analysis from the farm(s) would probably assist in understanding the increase/ decrease of levels. Again this depends on culture cycle duration and its  frequency. Having a sites within the farms, away from farm and a control site should help to correlate...

all the best

Dear Kumar, 

Thank you for your quick response!

Yes control site is really important in this case.

We have been carrying out the Environmental Monitoring (sediment, basic nutrients, chla, benthic macrofauna analysis) of five out of the six open-sea fish farms following National Legislation here in Cyprus. We have also set a control station 2km away from the farms to compare changes. We can use the same station to correlate TOC with the fish farm site.

The main difficulty is how to convert the TOC water analysis "snapshot data" to annual TOC emissions combining perhaps important factors such as the fish-farm production and fish feed as these two play a major role in the whole process.

Leave it to bureaucrats to specify equations that don't capture the real mass balance and environmental issues. 

For example, N isn't conserved.  Any SS (manure) going to the bottom can crate low ORP conditions and mineralize to ammonia.  This can then be oxidized to nitrite then utilized by anammox bacteria that live on ammonia + nitrite to N2 gas (lost of N) -- common in marine sediments.  Of course any nitrate can also be loss to pure denitrification in the sediments while suppressing H2S formation.  

Carbon will be very difficult or impossible to measure.   In most net pen operations, the flow through the net pen has to be high enough to prevent big O2 swings (especially with tidal current variations that go to zero flow), which means that the feed to flow ratio has to be in the low mg/l range.   From a kg of feed into the fish, you get about 0.2 kg of manure (dry) which can then be converted to soluble BOD on the bottom with a many months time scale.   But that soluble BOD will be oxidized in or near the bottom.   Microbiological ecologies evolve and change to use that food as fast as it comes in.  

The fraction of manure that is soluble BOD or soluble TOC is relatively small.  Remember, the potentially soluble and easily digested (mineralized)  components of the feed were digested and converted into CO2, H2O and fish biomass.   With the soluble and insoluble TOC from the net pens rapidly separating, what the local currents are and the amount of local turbulence and vertical mixing will determine your sampling results.  On top of that, with the low feed/flow inputs will put you in the noise of baseline natural TOC and BOD in ocean waters (background is higher in bays, etc.).   Sampling is probably a waste of money.

The soluble BOD from fish would be in 80% of the TOC is in the manure SS insoluble fraction), for many fish feed diets.  This is what I have seen in recycle systems where you eliminate most of the SS from the waste water then oxidize the BOD and ammonia (this is based upon having the solids mineralize for a variable time period of 1 to 7 days with the solubles generated going into the water column).   However, that data would be for BOD30+, not the standard BOD5.  With most of the manure containing more refractory polymers like cellulose and other low digestibility components along with much of the ash, most BOD5 testing isn't enough time for decomposition.  This is why the BOD/TOC (mentally correct that ratio for BOD being O2 and TOC being CO2 based measurements) ratios on the waste are relatively small.  

If the SS is going into the sediments, you probably have sediment traps.  The flux of the SS into the sediments and the mineralization of those solids will release the P into the sediments.  Being concentrated in the sediment will result in high local P concentration which will start to diffuse out of the sediments, meanwhile Ca and F from the seawater will diffuse in creating a chemical reaction to form phosphate rock (very insoluble precipitate) which becomes biologically unavailable to the ecology.  Another  loss, but this requires that the SS flux to the bottom be higher than the regulators probably want and will create a decrease in the ORP and decrease in the H2S boundary layer.   This is how some natural phosphate rock deposits were formed with high dead algae flux into the sediments, like is naturally occurring both in the Gulf off of Mississippi and off the coast in Santa Barbara area of California. 

For just a simple minded ball park number of both soluble and insoluble TOC and not caring about the fate or significance or impacts of the TOC (as carbon), a good guess would be about 0.2 * 0.5 (carbon/dry organic manure) * (1 - .25 (% ash in manure)) or about 7.5% of the feed.   Most of this TOC will be refractory and not show up as BOD5.  

There is a lot of information relevant to these microbiological processes and mass balances in the aquaculture literature like P and SS in trout raceways and N in recycle aquaculture systems.   

When the regulators have good estimates of the amounts emitted from the pens, how will they be wise enough to rationally use that information?  Unless your regulators are a lot smarter than those in the US, they will be clueless and really make arbitrary decisions based upon political considerations and activist group pseudo-science. 

Dear Dallas, 

Thank you so much for your in-depth answer.

Your concerns are exactly as ours.

To estimate such emissions scientific research projects on this subjects will need to be carried out, but they want it as simple, low-cost and accurate as possible... The bad thing is that if the aquaculture companies do not provide them with any data they will receive penalties. We will probable end -up giving them seawater raw data with a short report informing them on the difficulties to determine the TOC emissions.

As they are using simple minded equations for N and P, why not just give them a simple minded  equation for C.   You might add that most of this C will show up as TOC or COD not BOD5 and be in the noise.  

You probably have some natural COD levels and looking at the delta O2 levels have information on feed/flow ratios, you may be able to show that the delta TOC is < background and unmeasurable.

For any reasonable delta O2, you would be talking about a delta TOC of < 1 mg/l.  If your background is larger than that, don't bother measuring.