Fig. 3.23 shows average concentrations of nitrate and phosphate in surface waters (0-10 m) of the Black Sea. The data are presented with reference to a quantile (5-class) system, such that for each parameter some 20% of the results should fall into each category. For each map, the darker the colour the greater the concentration, with class boundaries as follows:

Nitrate	Phosphate
Figure 3.23    Mean concentrations of nitrate and phosphate in surface waters of the Black Sea, 200-2005 (See note below Fig. 3.21.)

A clear pattern of nitrate enrichment occurs (despite many of the results falling below the analytical limit of detection), with high concentrations along the western edge of the sea. Levels around Odessa are also nitrate-enriched, with moderate levels of contanination along much of the east Turkish coast. A small number of sites offshore of the Turkish/Georgian border show high levels of enrichment, which are likely to be the result of either a lack of summer results (when nitrate levels are lowest) or poor analytical quality control.