The most common approach to mitigation of lake eutrophication is reduction of phosphorus emissions, in particular by changing farm management. This reduction can be combined with landscaping retention structures upstream of the lake, the analyses of which the paper is based on. The management of these structures currently focuses on maximising the quantity of phosphorus trapped, regardless of lake dynamics. This paper adapts a dynamical model of lake phosphorus to examine the effects of these phosphorus retention structures. We highlight two effects: first, a structure that traps some of the phosphorus load before it reaches the lake reduces the amount of phosphorus in lake water. Second, some retention structures slow down lake phosphorus dynamics in a way that may perversely slow lake restoration. We propose a cleaning strategy that maximises the chances of restoring a lake to an oligotrophic condition. We demonstrate our model with a real-world case study.