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View Full Version : How is alkalinity affected with salinity flucutations??


jlinzmaier
10/10/2009, 07:06 PM
My question really isn't as simple as the title sounds. What I'm interested in is how much alk fluctuation will there be with salinity changes.

Let me provide an example:

For simplification lets assume two basic standards. When I refer to a raise in salinity let's assume that it's related to evaporation and "not" related to the addition of water of an elevated salinity. Also, when I refer to a decrease in salinity, let's assume that the decrease is related to adding 0 TDS RO/DI water.

For basic standards lets assume my base alkalinity is 8.0dkh and my base salinity is 35PPT.

Is there a way to calculate what my alk lvl would be if my salinity dropped 1PPT (related to the addition of RO/DI water)?? How about 2PPT's??

Same in reverse. If my salinity raises 1PPT (related to evaporation) how much would my alk level raise? How about 2PPT's??

I know that particularly SPS corals are very sensitive to alkalinity fluctuations when an aquarium is kept at a very low nutrient level (I won't use the overused and misrepresented reference of an "Ultra low nutrient system"). I'm referring to the low nutrient levels we are able to achieve in our aquariums which is what sort of environment these animals come from. Granted, there are a thousand and one differences between our tanks and the natural reefs, but many marine scientists have shown many similarities in how corals respond to elemental changes in the wild and in our living room. I believe Greenbean had once shown me a quote from an Eric Borneman about how nature has quite significant fluctuations in salinity and temperature thus providing the indication that the same fluctuations don't need to be to so significantly feared as most reefers do. A significant point in that concept is that the corals which are exposed to those conditions in a natural reef are adapted to those conditions; therefore, we couldn't take a tank which has been kept exactly at 35PPT and 80 degrees solid for several months then suddenly fluctuate the temp by 8 degrees and salinity by 2-3 PPT on a daily basis.

OK to the point:

If I have a tank with a very low nutrient level (similar to natural reefs) the standard response by corals to a significant alk fluctuation is stress, polyp retraction, and often tissue loss. I'm assuming that a flcutuation from 33PPT to 35PPT would create quite a change in total alk. If that assumption is true, why don't the corals in the wild exhibit the same negative reaction as they would in our tanks?? Is it as simple as the fact that they are "adapted" to the fluctuations?? If this is true then we should be able to "adapt" our own corals to the similar circumstances without ill effect (in theory). Obviously it would be silly to try to replicate this fluctuation, as I'm not aware that there is any great benefit, I'm just wondering if there is really as much necessity to limit salinity fluctuations on the basis that it will alter total alk levels. I guess that gets back to my primary question - How much does alk change in reference to a salinity change??

Jeremy

Randy Holmes-Farley
10/11/2009, 06:52 AM
Alkalinity provided by a salt mix is essentially directly proportional to the salinity. That is, proportional to the salt you use. Once in the tank and impacted by all sorts of things, they can move independently.

In a salt mix (or due to evaporation) 33 ppt to 35 ppt is only a change from 6.6 to 7 dkH or 10.4 to 11 dKH. :)

jlinzmaier
10/11/2009, 10:24 AM
Thanks Randy. That's just the answer I was looking for. In all reality, a salinity fluctuation doesn't create a huge alk fluctuation as long as there are no other variable factors affecting alk.

Thanks again.

Jeremy

Randy Holmes-Farley
10/11/2009, 10:48 AM
:thumbsup:

Happy Reefing. :)