Final answer:
Small phytoplankton are favored at low nutrient concentrations because they absorb nutrients more efficiently due to a higher surface area to volume ratio, and they can remain afloat in light-rich surface waters which is essential for photosynthesis and survival.
Step-by-step explanation:
At low nutrient concentrations, small phytoplankton are favored because they have a higher surface area to volume ratio compared to larger phytoplankton, which allows them to absorb nutrients more efficiently. Nutrients like nitrogen and phosphorus are limiting factors in the growth of phytoplankton in aquatic environments. Small phytoplankton can remain suspended in the upper layers of water where light penetration is sufficient for photosynthesis, and they can make more efficient use of available light and nutrients than their larger counterparts. Moreover, when nutrient levels are low, smaller species are less likely to sink and thus remain in the productive surface waters, where they can continue to photosynthesize and reproduce.
The ability to stay afloat and access light is crucial for phytoplankton survival, as light intensity and nutrient availability can significantly vary within the water column. In environments with heavy algal blooms due to an overabundance of nutrients, the water becomes aphotic and oxygen levels can drop significantly, leading to detrimental effects on marine life, as seen in bodies of water like Lake Erie and the Gulf of Mexico.