Temp and Salinity relationship....Is it simple or deeper???

SoAg Reefer · Aug 25, 2020

So has anyone else noticed the correlation between temp and salinity? Maybe it’s well known and I’m late to the party. Higher temp equals higher salinity and lower temp, salinity drops again. I understand the evaporation rate with higher temps and water then is replaced with fresh dropping salinity and a higher rate of evaporation also lowers temp. So I am assuming that as water warms the salinity increases because the fresh water is fully diluted by this point and evaporation is occurring. Temp keeps rising, salinity goes with it As water evaporates then comes back down when fresh water is added via ato which usually also drops temp a bit. Is it that simple?

Thats my assumption. However, I have not had a large temp swing like I did this weekend in some time and salinity stayed somewhat steady as temp varied from 77.5-79 ish. Wasn’t until the temp hit 81+ this weekend that I noticed a very high salinity level in conjunction with the high temp. All other parameters remained steady, just those two were off this weekend. Issue fixed now but my curiosity is there nonetheless.....Is there something more that I’m missing?

chipmunkofdoom2 · Aug 25, 2020

Your tank is 34 gallons? In order for 34 gallons of water at 34ppt salinity to go to 37ppt, your tank would have to evaporate 3 gallons per day. That seems unlikely.

I would suspect this is all measurement error. You're measuring salinity with a conductivity probe, and conductivity changes with temperature temperatures. On top of that, it's notoriously difficult to temperature-correct the Apex salinity probe properly.

SoAg Reefer · Aug 25, 2020

That is correct. I use the salinity module in the Apex for directionality. I know its off by 1.5 or so ppt versus a refractometer but since Im not with the tank at night or on weekends it helps me identify things. If my salinity drops too much it could mean my skimmer is overflowing into the 5 gallon bucket in which it drains and my ato is feeding fresh ro like crazy so i turn off the skimmer remotely just in case. easier way to do that? yes, but this is my office tank.

When I got in yesterday morning I fully expected to find the fan turned off due to a power outage which caused the temps to rise or an overflowing skimmer bucket and the ato was not working. I found neither. Fan was on, skimmer overflow was normal. Everyone in the tank was happy as could be, in fact if i didnt have the apex measuring everything i would know anything was out of whack this weekend. Temp jumped and salinity followed. Im not worried about 81.5F even when running 77.5-78.5 normally. Thats an easy fix and I know why it jumped. What surprised was that the salinity followed the temp. Just seemed odd. Checked salinity yesterday morning and it was 1.026 which is only a little higher than the 1.024-1.025 i usually run. Alk, Mag, Cal, po4, no3, potassium, iodine....everything i test for was fine. I ran extra trident tests all weekend thinking theyd be out of whack...they werent. The rest i tested yesterday before i fixed the issue and all were fine.

Hopefully youre right and its a measurement thing but its just odd that it followed temp so closely. Appreciate the input for sure

Randy Holmes-Farley · Aug 26, 2020

SoAg Reefer said:
So has anyone else noticed the correlation between temp and salinity? Maybe it’s well known and I’m late to the party. Higher temp equals higher salinity and lower temp, salinity drops again. I understand the evaporation rate with higher temps and water then is replaced with fresh dropping salinity and a higher rate of evaporation also lowers temp. So I am assuming that as water warms the salinity increases because the fresh water is fully diluted by this point and evaporation is occurring. Temp keeps rising, salinity goes with it As water evaporates then comes back down when fresh water is added via ato which usually also drops temp a bit. Is it that simple?

Thats my assumption. However, I have not had a large temp swing like I did this weekend in some time and salinity stayed somewhat steady as temp varied from 77.5-79 ish. Wasn’t until the temp hit 81+ this weekend that I noticed a very high salinity level in conjunction with the high temp. All other parameters remained steady, just those two were off this weekend. Issue fixed now but my curiosity is there nonetheless.....Is there something more that I’m missing?

To expand on Chipmunks post...

There is not any significant relationship between temperature and salinity.

There is a relationship between the output of many devices used to measure something that correlates with salinity and temperature.

In this case, you are using conductivity. It is highly temperature dependent. Conductivity measures the ability of ions to flow through the seawater, and at higher temperatures ions travel faster at higher temperatures. Conductivity meters also measure temperature and make a correction back to 25 deg C. They may or may not do this correctly, and some devices allow you to change how they do it, potentially making it incorrect, or turning the correction off entirely (bad).

You might check into the setting of your conductivity meter's temperature correction, but I do not know how one does that with that device.

Other things may also impact conductivity, such as electrical interference, bubbles on the probe, etc.