Long-term comparison of dissolved nitrogen species in tropical estuarine and coastal water systems

Abstract

Long-term observation of aquatic habitat is invaluable as it reveals trends that are not evident from shorter-term measurements. In this study, we assessed measurements of dissolved nitrogen species over a 12-year period (2004–2015) at Port Klang estuary and Port Dickson coastal water. Total suspended solids (TSS) was also measured, and increased at 6–9 mg L−1 yr−1. In contrast, total dissolved nitrogen (TDN) did not increase but was higher at Port Klang (8–320 μM) relative to Port Dickson (2–30 μM). Chl a concentration was also higher at Port Klang (5.18 ± 7.79 μg L−1) than at Port Dickson (2.27 ± 1.39 μg L−1). Among the dissolved nitrogen species, we found that ammonium (NH4) was predominant at Port Klang where dissolved inorganic nitrogen (DIN) contributed 71 ± 3% of TDN. At Port Dickson, dissolved organic nitrogen (DON) predominated, contributing 92 ± 7% of TDN. When we calculated the nitrogen loading at Port Klang, dissolved nitrogen load increased over time (1.13 ± 0.29 kg d−1 for TDN, 1.11 ± 0.29 kg d−1 for DIN, and 0.15 ± 0.06 kg d−1 for DON). We showed that the nitrogen loading increase at Port Klang was not due to climatic forcing but was most probably due to urbanization occurring upstream that inevitably increased river discharge rates (4.04 ± 0.12 m3 s−1 yr−1).