THE DETERMINATION OF THE NET FLUXES FROM A MANGROVE ESTUARY SYSTEM

Determining the net exchange of constituents between a mangrove estuar y system and the adjacent ocean has been re-examined using an extensiv e dataset from the Sungai Merbok a short, tidally energetic estuary in Malaysia. Previous analysis of the data had indicated that the time-m ean sectionally averaged flow was not consistent with mass balance, ap parently preventing meaningful estimation of net nutrient fluxes from the mangrove system. In this case the problem was aggravated by the la ck of river gauge data and uncertainties introduced by the use of defl ected-vane current meters to make the flow measurements. In an alterna tive approach to the analysis, we have sought to put bounds on the net discharge and hence obtain limits for the nutrient output from the es tuary. Tide gauge measurements have been used in conjunction with the section flow data to determine the hypsometry of the mangrove system a nd hence yield an unbiased estimate of tidal transport Q(t). A salt ba lance condition, appropriate to a mixed estuary is then applied to per mit an estimate of Q(f), the freshwater discharge. Q(f) determined thi s way is found to be close to zero and certainly less than estimates f or the period (mean similar to 7 m(3) s(-1)) based on rainfall records and catchment area. The implication is that the combined effects of e vaporation and transpiration are removing a large proportion of the fr esh water entering the mangrove system from the rivers. The very low n et discharge indicates total nitrogen exchange is dominated by the cov ariance of Q(t) with the sectionally averaged concentration N-t. The c onsiderable variation in this latter term combined with the large ampl itude of Q(t) results in a high variability of the nitrate flux so tha t the estimate of the mean (0.5 g s(-1)) is subject to substantial sam pling uncertainty (SE = 12 g s(-1)). The application of the salt balan ce condition to flux studies in other estuarine systems is considered. Particular attention is drawn to the requirements of this approach to flux determination and especially the need for good timing control to allow the proper determination of the tidal diffusion flux of salt an d other components.