RESIDENCE TIMES OF MANUKAU HARBOR, NEW-ZEALAND

Residence times of the three main channel systems of Manukau Harbour were calculated by methods based on the salinity measured in 10 tidal excursion segments: the freshwater replacement technique and a volume exchange model. The amount of freshwater present in the segments at high water was low (< 11% throughout the year), so salinities were high, with minimum values (winter) in each segment being in the range 30.1-34.0 (compared with a median value of 34.5 in the nearby coastal waters). The proportion of each segments' volume which was exchanged with an adjacent segment during a tidal cycle varied markedly across the harbour, from less-than-or-equal-to 5% in several shallow inner harbour segments to 65% in a deep segment adjacent to the entrance channel. For annual average conditions the freshwater replacement times of the channels were 11-18 days. Slightly longer residence times-12-26 days-were obtained from the volume exchange model. Calculated replacement times for two non-conservative tracers-inorganic phosphorus and nitrogen-were approximately half those for freshwater, as 5-12% of these species disappeared from the water column each day.