http://www.skepticalaquarist.com/phosphate-cycle

This is a link to a really good article on phosphates. To anyone like me who finds the whole subject hard to grasp sometimes it gives a good insight into the processes happening. Yes I know it is focused on freshwater but the principles are the same.
It might explain why people get varying readings for phosphate levels from one day\week to the next and why some people with zero readings have nuisance algae and others with readings have no algae!
The most interesting bit for me was towards the end with regards to cyanobacteria. It appears they can produce an enzyme called phosphatase in large quantities. This enzyme enables it to convert any phosphates bound up in organic matter, ie detritus, into a form it can use for its own needs. Phosphatase is relatively energy expensive so is only usually produced as a necessity during periods when phosphate is limited. Could explain why people who dose organic carbon sometimes get blooms of cyano, maybe the extremely low dissolved levels of phosphate encourage this phosphatase production.
Also may be a good reason to remove as much detritus as one can regularly to stop the cycle before it starts. Unlike the nitrogen cycle the phosphate cycle on a natural scale takes millions of years. The only way to help is export via your algae harvests and good husbandry.