Using density and body-size data for a well-documented food web, the Ythan estuary, Aberdeenshire, we show that the shape of the constraint space is broadly similar to that proposed by Brown, and the slope (regression coefficient) of the plot of the central tendency is not significantly different from global or community data sets. However, the steepness of the plot is very sensitive to the degree of taxonomic resolution of species entities, particularly those of small body-size, although the overall shape of the constraint space does not change. When more fully resolved, the regression slope differs markedly from those of most freshwater and terrestrial studies. In addition, the upper bound of the constraint space is, contrary to theoretical expectations, insensitive to gross changes in the system’s productivity. The dramatic increase in productivity that has led to an increase in abundance of invertebrates in the Ythan does not visibly affect the upper bound of the constraint space. Our results show that the traditional approach to comparing systems by the regression slopes of central trends is probably meaningless unless data sets are resolved to a similar degree of taxonomic resolution, and that detecting productivity effects on the location of the upper bound will be difficult.