Final answer:
Sunlight is the essential abiotic factor to all aquatic ecosystems except ocean hydrothermal vents, as it enables photosynthesis which is vital for the majority of aquatic food webs. Other factors like temperature and dissolved oxygen also play critical roles in shaping these ecosystems.
Step-by-step explanation:
The abiotic factor that is essential to all aquatic ecosystems, with the exception of ocean hydrothermal vents, is sunlight. Sunlight is crucial for enabling photosynthesis in aquatic plants and phytoplankton, which form the basis of most aquatic food webs.
In the absence of sunlight, as is the case in hydrothermal vents, ecosystems rely on chemosynthesis for energy production, which is the process of converting chemicals in the vent fluids into usable energy. However, for the vast majority of aquatic systems, sunlight controls productivity through photosynthesis, making it a critical abiotic factor.
Other important abiotic factors in aquatic ecosystems include temperature, dissolved oxygen, and salinity, as these affect the physical and chemical properties of the water that in turn influence the life forms present.
In summary, while aquatic ecosystems share common abiotic factors with terrestrial environments, factors such as sunlight and dissolved oxygen are more variable within aquatic systems due to their distinct properties. In oceanic or freshwater environments that receive sunlight, these factors will influence the stratification, productivity, and composition of living organisms within the biome. Conversely, unique ecosystems such as hydrothermal vents do not depend on sunlight and have adapted to the dark, extreme conditions in which they exist.