3.1 Environmental gradient based on water quality
The averaged environmental parameters in the PCA analysis reflected a water quality trend from TM6 (SFB) to MM5 (TPC) (Figure 1, Table S1). The seven environmental parameters varied between sites. Nitrogen (nitrite and nitrate) and salinity have a greater contribution to the variation in TPC whilst ammonia and phosphorus to LCC. Overall, PC1 explained most of the variation observed (79.6 %) among sites and this was consistent with the geographic distribution, supporting the occurrence of a pollution gradient along Tolo Harbour (Figure 1). Based on the environmental data, sites were categorized into three relative pollution levels along the Tolo Harbour Channel (SFB as highly polluted, LCC as moderately polluted, and TPC as low polluted site).