Does Orthophosphate Uptake Supply Sufficient Phosphorus to Phytoplankton to Sustain Their Growth?

It is generally supposed that phytoplankton obtain their P predominantly from orthophosphate uptake. Yet, phytoplankton are often observed to be responsible for only a very small fraction of the total planktonic community uptake of orthophosphate. Is this uptake of orthophosphate by algae nonetheless sufficient to meet their metabolic needs? To answer this question, I have compared the specific doubling rates of C and of P through orthophosphate uptake in the phytoplankton community of Lake Memphremagog. Orthophosphate uptake during the growing season supplied only a very small portion of the P required to support algal growth. Presumably, the phytoplankton obtained their P from some source other than orthophosphate uptake. Similar patterns of orthophosphate uptake in other P-limited lakes suggest that this phenomenon may be widespread.