Pond Boss Forums GENERAL QUESTIONS Water chemistry nutrient overload??

There is a 60 acre coal mine/strip pit lake that has a max depth of 72ft that used to be extremely clear with 10ft+ visibility. For the last 3 years there has been between 600-1200lbs of fish food added, per day, for at least 3 months a year by a guy who was allowed to operate a fish farm in floating cages. This year visibility is reduced to <2ft. Question #1, will roughly 81,000lbs+/- of fish food per year for 3 years have an affect on a lake that size with that type of water volume? Assuming it does, how long, if ever, after the fish farming operation is stopped would it take for the nutrient levels to drop off enough to reduce the algae blooms? We're going to have some nutrient tests done in the next week or 2.

Thanks,
Mike

So, we're talkin' a quarter million pounds of food??

The maximum depth is isn't the most important thing. The average depth is what matters.

Let's say the average depth is 25 feet. That's 1,500 acre-feet of water.

My pond is about 15 acre-feet at the very, very most. So let's say your coal mine pit is 100 times the water volume of my pond.

That would mean that the same question would be..."would 800 pounds of feed per year affect the water quality of Bruce's pond"?

The answer is "Yes, it would".

Does water flow out of this system in any way? Or is this purely a groundwater situation? I don't know much about coal mine/strip pit lakes, but I'd be concerned.

I guess another way to put it is "If 600 deer ran into your pit and died every year would it affect the water quality?" Or how about 8 full grown elephants? Four bulls and four cows.

Keep in mind that when an aquaculturist tells you that his fish are converting feed at 2:1, that's a dry weight to wet weight conversion. In reality, a fish is defacating 8 times as much waste as they are creating new muscle mass. A dry weight to dry weight conversion is something like 8:1.

I understand what you guys are saying. However I see the question a little different than that. The problem with that line of thought (I learned this the hard way) is it does not factor in the water quality on day 1. Here is an example. The water could on day 1 be short only a little in Phosphorus (P) resulting in the high visibility (no visible bloom). If the entire pond is short only a little on P , say the equivalent of that produced by 10,000 lbs of fish food , then at that point the bloom is going to go from low to high as the sole limiting factor is removed.

This is another one of those "It depends" answers as are so common in water quality questions. Stated another way - depending on water quality anything that removes the limiting factor or adds a missing ingredient can cause a big change in fertility and other things. That includes the addition of fish food even at much lower rates.

That's extremely well stated.

Thanks for the input. I don't know much about fish farming but that did sound like an aweful lot of food but when you start relating to the weight of 4 elephants it REALLY sounds like a lot. I believe the owners are going to ask the fish farmer to remove the operation after this year. Bruce-yes the lake does have a single culvert type overflow. My understanding is that the water doesn't really fluctuate more than a couple feet throughout the year. We haven't had rain in a couple of weeks, the temps have been 90+ and the water was only a foot below the pipe. I would guess that the ave water depth is somewhere between 25-35ft. After we get the results I'll try to post them and see what you guys think.
Thanks again,
Mike

I have received the results from the Tubidity/Nutrient tests today and I was hoping you guys could give me your opinions as to what may be going on.

Two water quality tests were taken. One about 80yds out from the fish cages and the other at the other end of the lake.
The results reported for both sites were identical;
Alkalinity 41
PO4 0.03
Ortho 0.02
Nitrate 0
Hardness 324.9

I also took a sample from a nearby lake that was similar in size that did not have a fish farm operation but would have similar runoff. The thought was to use this as a baseline and we thought we would be able to pinpoint the fish operation as the cause for the declining water clarity. Here are the results for our "control"

Alkalinity 47.9
Total PO4 0.01
Ortho P 0.01
Nitrate 0.0
Hardness 290.7

Does anyone notice any significant differences between the results or any reason why the water might have gone from "gin clear" to clarity of 2ft or less. Any outher tests that I should consider??
-Mike

Assuming your tests are accurate, the orthoP levels in your lake are high enough compared to nitrate to cause an algae bloom (phytoplankton). I find it hard to believe the nitrate is 0.0 in both lakes considering the info you provided earlier. Total PO4 levels in your lake compared to other lake indicate a plankton bloom. To verify density of planktonic algae you need a quantitative analysis of phytoplankton (not a qualitative analysis) or a chlorophyll a test for both lakes.

Ditto Bill. All that feed and no nitrates? Hard to believe.

I've never claimed to know much of anything but I was also expecting some nitrates as well. Every test for both sites within the lake reported identical numbers across the board. Is that normal for everything to be so uniform especially if one sample was taken so close to a source like the fish cages? Would sample depth have made a difference? I was only able to go down an arms length out of the boat. For the Ortho Phos and Phosphate tests what values would be considered significant and what would you define as "normal"? Also, the Ph values were around 8.8 if that helps with anything and the turbidity tests were very low. Thanks,
Mike

Q - Is that normal for everything to be so uniform especially if one sample was taken so close to a source like the fish cages? It all depends. If there is good mixing due to current, wind or mechanical aeration then water chemistry can be fairly uniform in many of the areas. Unmixed areas will show higher and lower concentrations.

Results. I assume the testing laboratory was certified and knew what they were doing regarding the tests and proper collection and storage of samples.
Nitrate is vulnerable to biological activity thus the test should be started promptly after collection. If the sample is to be stored it should be keep just above freezing with or without preservatives such as sulfuric acid.

Phosphate. These sample should not be collected in plastic due to P can absorb on to the walls of the bottle. If samples are not analyzed immediately then storage should be slightly above freezing or preserved with mercuric chloride to stabilize the sample. If total P only is to be tested no special storage is necessary.

I consider your P values lower than expected for your situation. Total P and ortho P values can fluxuate quite a bit due to production and condition of plants which absorb ortho P (reactive). Ortho P of cage operation is twice that of control which indicates more reactive P regardless of the amount of error in sample collection or analysis. I assume that the P values are in mg/L. It is best to analyze P values with accuracy in micrograms/liter (parts per billion).

I consider your values for P and N in error. Typically total nitrogen is 12 to 35 times more abundant the total P. Total P typically ranges over the trophic range from lows of 10ug/L to 100ug/L or 0.01 - 0.1 mg/L. There are instances where N can be limiting and quite low while ortho P is still available. In these cases certain types of algae will bloom.

pH values will not have much bearing on what you are trying to do which I assume is determine the impact of cage operation on water quality.

Making evaluations or judgements from a one time sampling is very risky because the condtions are constantly changing and often the changes can be quite dramatic depending on the prevailing conditions.

Many an incorrect management decision has been made on faulty data.

Not sayin' we know for absolute sure, but you need to keep your mind open and listen to Bill.

Bill...Thanks for the lesson. Here is the chain of events. I took the samples on the afternoon of saturday the 11th. They were placed in a cooler and then stored in the refridgerator until monday. They were then shipped priority overnight to the lab and then analyzed on the 17th. The kit consisted of 2-16oz plastic sampling bottles. Based off of that would you completely disregard the Nitrat tests? I believe I've seen some simple field kits for Nitrates and Phos. Are the kits fairly reliable?

Thanks again,
Mike

Those figures are in line or even lower than what is seen in ultra-oligitrophic lakes like Crater Lake. Something is definitely amiss here.