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A classmate designed an ecosystem, as shown in the figure. Lettuce is planted in water, and algae, nitrifying bacteria, Daphnia, carp and saprophytic bacteria live in it.
A, in the ecosystem, the energy flow is characterized by one-way flow and gradual decline, so to maintain the normal operation of the ecosystem, the device must be placed in a place suitable for lighting, and A is correct;

B, plant transpiration will lose a lot of water, so to maintain the normal operation of the ecosystem, the substance that must be supplemented regularly is water, and B is correct;

C, in this ecosystem, nitrifying bacteria (chemoautotrophs), lettuce and algae (photosynthetic autotrophs) all belong to producers, and C is correct;

D, the shorter the food chain, the more energy the highest trophic level gets, so the food chain where carp get the most energy is algae → carp, D error.

Therefore, choose: d.