Practice question

Aquatic primary producers, especially phytoplankton, are often highly digestible because they contain little cellulose, lignin, or long-lived support tissue. Consumers can therefore absorb a large fraction of ingested material, producing higher assimilation efficiency than herbivores feeding on woody or fibrous terrestrial plants. The mechanism is food quality, not simply oxygen supply or species diversity. Aquatic detritus and heavily defended algae can be poorly assimilated, so the pattern is strongest when comparing zooplankton–phytoplankton links with terrestrial herbivory. Decomposition rate therefore emerges from interactions among substrate chemistry, decomposer traits, temperature, water, oxygen, and nutrient balance rather than from a single universal control. At ecosystem scale, these reactions regulate soil fertility, atmospheric carbon exchange, detrital food webs, and the residence time of organic matter. Mass loss alone cannot identify mechanism, because leaching, fragmentation, respiration, assimilation, and stabilization can produce different fates for carbon and nutrients. The distinction between gross transformation and net nutrient release is important: simultaneous microbial uptake can conceal substantial biochemical turnover.