An Estimate on External Fluxes of Phosphorus and its Environmental Signi®cance in Xiamen Western Sea HUASHENG HONG*, SHAOLING SHANG and BANGQIN HUANG The Research Laboratory of SEDC of Marine Ecological Environment/Environmental Science Research Centre, Xiamen University, Xiamen 361005, People's Republic of China Atmospheric deposition, benthic release, freshwater runo and sewage discharge inputs of phosphorus to the Xiamen Western Sea were estimated. It was found that benthic release was probably most signi®cant, having a ¯ux rate of the order of 1 mmol P m 2 d 1 . The Xiamen Western Sea is P limited, and the results of this study suggest that pulse inputs of P may play an important role in the trig- gering of red tides. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: phosphorus; ¯uxes; model; Xiamen Sea; China; Red tide. An increasing body of evidence suggests that red tides may be induced in coastal waters by escalating eutro- phication. It is therefore important to understand how an ecosystem responds to the increasing input of nutri- ents. Xiamen Western Sea is a semi-enclosed bay with an area of 50 km 2 that is subject to many environmental stresses, caused by the rapid economic development of Xiamen (Fig. 1). Phosphorus has been identi®ed as a limiting nutrient in this sea. Furthermore, nutrient en- richment has been implicated in the frequent occurrence of red tide in the inner bay (Zhang, 1988; Chen et al., 1990; 1993a,b). Despite a number of studies in the last ten years, focusing on P in Xiamen Western Sea, the mechanism of P cycling in this water body remains poorly understood. Such information is obviously useful for the long-term management of Xiamen Western Sea. Shang (1995) developed a dynamic box model in an attempt to better understand the dynamics of P in Xi- amen Western Sea. This workerÕs model considered variables such as: the species of P (viz. dissolved inor- ganic phosphorus, i.e., DIP, dissolved organic phos- phorus, i.e., DOP and particulate phosphorus, i.e., PP) and phytoplankton biomass. The following three pro- cesses were also considered in the model (Fig. 2): (a) transport between the segments due to water movement and eddy diusion; (b) internal cycling by biological assimilation, regeneration and transformation; and (c) external input from atmospheric deposition, benthic release, freshwater runo and sewage discharge. In this paper, we estimate: external input to the water body, from various pathways, i.e., atmospheric deposi- tion, benthic release, estuary transport and sewage dis- charge, and the factors controlling them. Furthermore, the kinetics of phosphate uptake for dierent size phy- toplankton was studied and related to the eects of nutrient status in ambient water body. External Input Fluxes of P Atmospheric deposition Atmospheric deposition was considered unimportant, until the last decade, because the air±sea exchange mechanism was poorly understood and experimental methods were not developed. Chen (1989) considered atmospheric input of P to the open ocean to be less signi®cant than that of N. Duce et al. (1991) calculated, however, that the global atmospheric ¯ux of P was greater than that contributed from freshwater runo. The situation is therefore still poorly understood, in particular, in coastal regions where the land-ocean in- teraction is important. Dry deposition can be de®ned as the process by which chemicals are transferred from the atmosphere to the ocean in gaseous or solid phases. Wet deposition is de- ®ned as chemical transfer by rainfall. Both forms of deposition are dicult to measure directly. Indirect ap- proaches (usually by measuring the concentration of chemicals in the collected aerosol particles) are, there- fore, often used to estimate air±sea ¯uxes. In this study, aerosol samples were collected (by the Third Institute of Oceanography, State Oceanic Administration, China) during four seasons, i.e., August 1993, November 1993, January 1994 and May 1994. The P content of the aerosol (C pa ) was measured using a method developed for particulate phosphorus in seawater (Shang and Hong, 1997). Dry deposition ¯ux (Fp), wet deposition PII: S0025-326X(99)00080-6 Marine Pollution Bulletin Vol. 39, Nos. 1±12, pp. 200±204, 1999 Ó 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/99 $ - see front matter *Corresponding author. 200