I'm trying to calculate secondary production of freshwater benthic communities. Some times (nearly in 50% of cases) I got negative numbers. Does it means that destruction in bigger that production?
I'm want to understand if I have done the mistake or not.
To explain the reason of negative numbers - in my opinion - the environmental background should be also taken in account. Eg. organic material input from outer sources.
Associating the negative coefficient with depth it may indicates that the production is highest near the surface water and decreases rapidly into the depth.
In streams export of animals / migration / predation are likely to affect the biomass of benthic animals present at a given river section. If you measure productivity from differences in biomass between sampling dates, these factors have to be taken into consideration.
Besides, the distribution of benthic invertebrates is notorously patchy, e.g. you may need a lot of samples, even if migration / predation etc are accounted for.
hope, this helps,
Irene
Have taken Habitat, watershed landuse into consideration? what kind of streams are you working on? Urban vs non urban, I can suggest a few papers if you like.
Dear Gitta,
I work on spring brooks. There are both kind of habitats urban and reserved ones. And negative numbers there are for different sites.
Dear Irene, I used temperature related P/B-ratios. But when I try to calculate the community production I got negative numbers. Because the formula for community prodation is Ptotal=P non-predators + Ppredators- Cpredators. Where P - production, C - food allowance of predator (near Cpredators =4,8*Ppredators ).
Dear Tatiana,
Two thoughts:
1) sometimes small invertebrates are underrepresented in the samples (mesh sie of net?)
2) some benthic invertebrates can be predators, but also use other feeding modes, for example Hydropsyche can feed in drifting particles, but also act as predator. How did you define predators? For Austria a list of macroinvertbrate taxa with feeding modes - as well as ecological preferences - does exist and it is available from the internet - for free. It is in german, though.
kind regards
Irene
Dear Irene,
1) I used net with the diameter of the mesh 0,5 mm, thought it possible that 2nd instar larvae of Chironomidae could went through it. Also it possible that meiobenthic organisms play bigger role in the diet of macroinvertebrate, but unfortunately we didn't collect and investigate them in this research.
2) I used Asterics database, where the percentage of feeding types is shown for bigger part of benthic animals. For animal whose were not in the data set I look for paper on their diet.
Given that you use P/B ratios to compute production, the negative values are caused by your way of handling predators, specifically the substraction of their consumption.
You wrote:
Ptotal=P non-predators + Ppredators- Cpredators. Where P - production, C - food allowance of predator (near Cpredators =4,8*Ppredators ).
I do not understand why you substract predator consumption.
If you want to estimate secondary production (and not the tertiary production by predators; this assumes clear and constant trophic levels which are not typical in streams during ontogenic development), then I would simply use Pnon-predators.or arguably the SUM of non-predator production and predator consumption (note that this asusmes that predator do not consume allochtonous preys).
If you want to get secondary+tertiary production, I would simply sum predator and non-predator production.
@Antoine: Nothing wrong with the formula
P non-predators provides the only basis for the non-detrius part of the food chain.
The part of P non-predators eaten by predators is no longer available, hence the
"- Cpredators" in the formula. However, part of the P non-predators consumed by predators is converted into P predators - so it has to be added to the formula.
One important aspect of measureing production in streams is to get an idea, how much fish (usually trout) can be obtained from it. The formula given by Tatina refers to the production in the stream available for fish.
@Irene: I reiterate that the formula does not work.
When you use B to estimate P (the P/B method), predator consumption is already accounted for (they ate some of the B of the non-predators). So the production available to fish is the sum of the production of predators and non-predators.
I am in agreement with Antoine on this one. When using P/B ratios to estimate secondary production, you would be double counting the consumption of predators in your current method.
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