Complex sentence: Phytoplankton and macroalgae present in the water mass produce oxygen by way of photosynthesis, and at the same time bacteria and all other organisms (zooplankton, algae, fish) consume the oxygen by respiration.
Split into simpler sentences: Phytoplankton and macroalgae present in the water mass produce oxygen by the way of the photosynthesis. At the same time bacteria and all other organisms (zooplankton, algae, fish) consume the oxygen by respiration.