Dear Eveline, top down control refers to the grazing of plankton by higher organisms in the food chain such as zooplankton and fish. Bottom up control refers to the food that phytoplankton consumes and its concentration in the environment, in other words the nutrients that phytoplankton needs for growth. (I.e nitrae, phospate, iron, silica etc). Both bottom up and top down can control the assembleges of phytoplankton and in a way it acts as a self balancing system. Some people refer to virus infection also as a bottom up control as they kill many phytoplankton species and blooms, but this can also be named: side-in control. I hope this helps. Jozef.
Yes a lucid explanation given by Jozef, I wish to further add with example
let us say that (i) the abundance of piscivorous fish increases then (ii) this will result in increased predation pressure on planktivorous fish so the abundance of planktivorous fish will decrease (iii) this will reduce the predation pressure on plankton so plankton abundance will increase. This is all top down effect
On the other hand (i) nutrient concentration in water increases. (ii) This helps in phytoplankton growth so phytoplankton concentration will increase, (iii) there will be more food available to Zooplankton so zooplankton density will increase (iv) more food available for planktivorous fish so fish abundance will increase, and hence piscivorous organisms. However, it may be noted that in eutrophic system or in case of algal bloom top down effect is normally not translated to higher trophic levels.
Here is a journal article describing the process in tidepools from Marine Ecology Progress Series.
http://www.int-res.com/articles/meps/145/m145p161.pdf
I am very thankful to all of you for the details.
Regards
Eveline
Are upstream and downstream mechanisms to avoid a single species dominates the rest
hi
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