Many thanks for the quick replies. I am grateful that people take time to try to answer my curiosity. I had read the other thread, but it does not address the question.
I have few interests outside farming and gardening, but I am deeply curious about all sorts of farming, and that is what you are doing, raising, breeding, and some are selling, livestock. It does not matter if you only have half a dozen shrimp in one tank – you are looking after your livestock just the same as me with 100 pigs in England, 400 cattle and 1500 sheep in Australia, 2000 hens in Scotland, or a dozen goats in Portugal. These were not the only enterprises on the farms, but still, you are a farmer – a shrimp farmer.
There are things to be learned from all branches of farming that can be applied to other branches. The reason I am so keen to follow this through is because of my use of mulches for plants. We have hot and dry summers here and mulches help to ameliorate the ground temperatures around plants. I am increasingly using cut comfrey leaves for this purpose and am working towards providing two comfrey plants for each of my almost 500 olive trees. Whilst I found out about the use of dead leaves by chance, I have spent a lot of time in the last 10 days or so (almost constant rain so no outside work possible) trying to find why shrimp farmers use fallen dead dry leaves (DDL) and not harvested dried leaves (HDL).
If there is a compelling reason to use DDL, would that reason also apply to my mulching of the olive trees? You might think that this forum is a strange place to be pursuing the question, but in turn I think you might be able to teach the world of mulchers something. Alternatively, the answer might be that there is no reason not to use HDL instead of DDL.
I also did some research on periphyton, taking the view that whilst some animals may consume the leaves, many, including the very young, were grazing the periphyton. I noted that periphyton production was often restrained by limitations in the supply of Nitrogen(N) and Phosphorus(P). Suggesting that HDL would be a better source of nutrients for the periphyton. Furthermore, that in streams where flood waters have removed the periphyton it recovers much more quickly in water with better nutrient levels. Also that, as posted by plantbrain, I have not seen anything to suggest that one species of tree is better than another at providing the periphyton for the shrimp. Nor, indeed, that DDL are needed to produce periphyton. On two of my farms the house has been situated within 50 yeards of a river. I have also been an angler for about 60 years. I know that summer storms can cause immense numbers of growing leaves to be knocked off trees and into the river.
Leading on from the periphyton research I considered the role of mycorrhiza underneath the comfrey mulches. In much the same way as the periphyton use the leaves, some mycorrhiza can supply available P (a very immobile mineral and often unavailable) to plants from leaf litter on the surface, rather than take it from the soil. The leaf litter is normally naturally fallen leaves.
As for possible contaminants of the water when using HDL, I cannot find any evidence to show that the levels of Carbohydrates, N, P, or K in leaves would cause any problems in your tanks. In my OP I posted that I was familiar with the use of chemical analysis of leaves. N levels could be up to 2%, P unlikely to be more than about 0.2% and K a maximum of 1% even on heavy clay soils that release K. Total carbohydrates are somewhat higher in fresh leaves, but the weight, and it would have to be water soluble carbohydrates to have any effect, is miniscule. Chlorophyll is the green pigment in the cyanobacteria in periphyton, also the chloroplasts of algae and plants. It is not soluble in water so cannot be a problem. I weighed some Loquat leaves last week, because they are the heaviest I have. I chose the biggest leaves I could find and they averaged out at 4g per leaf, including the short leaf stalk, for a freshly harvested leaf. Dried they will weigh about half that. At the levels being discussed for inclusion in a tank, there is no possibility of leaves providing excess amounts of nutrients to a tank. Not all the NPK is available, nor even soluble in water, when the leaves are introduced. But some of the periphyton organisms can use these insoluble nutrients. The slow breakdown of leaves in the tank, is surely a good reason to use them, not a detrimental factor.
Perhaps I should not have used the phrase “everything returned to the parent plant”. I was thinking useable plant nutrients, not everything held in the leaf tissue. I believe we all accept that some tannins are not taken back into the roots. Similarly, heavy metals are not either. Consequently, leaves from any plant used in phytoremediation, at whatever stage of the leaves’ life or death, will contain those heavy metals accumulated in the growing leaf. Nobody would knowingly take the risk of using leaves from a plant that had been used as a phytoremedialist.
I still cannot find anything that says HDL should not be used, but I still do not know which is best for shrimp farmers and mulch users – HDL or DDL. Maybe there is no difference.
I appreciate that forum users might not want to continue responding, and I again thank those who have done so.