This paper presents an analysis of 10 years of data of the nitrogen (N) to phosphorus (P) ratio in the Venice (Italy) lagoon.

The N/P ratio in the waters of the lagoon is shown to increase from about 40 to about 100 (by atoms) from 2000 to 2010.

The increase in the N/P ratio cannot be accounted by similar increases in the incoming freshwater nor incoming Gulf of Venice water.

It is hypothesized that the increase occurs because the sediments preferentially retain P over N.

The higher N/P does not account for the changes in the spatial distribution of the macroalgae over the period of study.

The results indicate the importance of sediments in regulating the ecosystems of shallow estuarine environments