Dry and wet atmospheric deposition of nitrogen and phosphorus in Singapore

Atmospheric nutrients have recently gained increased attention as significant additional sources of new nitrogen (N) and phosphorus (P) loading to aquatic ecosystems. The levels of nutrients (NO₃^--N, NH₄⁺-N, ON, DIP and OP) in aerosols and rainwater were measured and their respective deposition fluxes were estimated in the Marina catchment area, Singapore, from April 2007 to March 2008. During the study period, the mean atmospheric dry particulate fluxes (mgm^-2month^-1) were 21.64±13.56, 2.95±1.73, 24.21±9.59, 4.48±1.74 and 7.19±2.83 for NO₃^--N, NH₄⁺-N, ON, DIP and OP, respectively. In addition, the mean wet deposition fluxes (mgm^-2month^-1) were 59.60±45.29, 30.39±12.99, 39.03±20.70, 1.79±1.03 and 2.46±1.29 for NO₃^--N, NH₄⁺-N, ON, DIP and OP, respectively. Wet deposition made a larger contribution to total fluxes than dry deposition for N species especially NO₃^- and NH₄⁺, but P species showed the opposite trend. Both TN and TP showed different seasonal variations with peak wet deposition fluxes in Dec 2007 and Jan 2008 as compared to dry deposition fluxes which peaked during the inter-monsoon (IM) period. Other than the local emissions, air mass backward trajectory analysis indicated that the atmospheric nutrient concentration may be affected by transboundary transport of nutrients from regional sources during different monsoon seasons. Statistical correlation analysis showed that the deposition fluxes were associated with meteorological factors. In addition, the N:P ratio in annual total (dry particulate+wet) atmospheric deposition flux in this study was 10.5, implying that the aquatic ecosystems in Singapore, fed principally by atmospheric nutrients, may tend toward N-limitation.