Reports water temperature monitoring carried out as part of an investigation into the possibilities of the introduced pacific oyster Crassostrea gigas breeding in British waters. Temperature recordings (data loggers) were obtained for one site (Oyster Hut, just W. of west end of Narrows) approximately 15 times per day from March 1993 to February >94, and from two other sites for shorter periods; Ferrybridge from November >93 to February >94 and Morkham=s Lake from mid July to mid August 1993.

From March to August, Fleet water temperatures (at the Oyster Hut) were higher (by approx. 5EC) than those of mid channel (Channel Light Vessel), similar in September, then lower (dropping rapidly by 5-10EC) than mid channel from October to February. Fleet water temperatures were within the maximum-minimum range of Weymouth air temperatures throughout the year. Peak Fleet water temperature of 17-18EC occurred in June-August, and minimum water temperature of 6EC in February.

Other workers (Whittaker, 1978 and data from Abbotsbury Oyster Farm) indicate maximum water temperatures in other years of 26-28EC, and minima in January/February of below freezing, when the water surface has frozen. No evidence for vertical stratification has been found, and it is unlikely in such shallow open water, nor in the deeper Narrows-Ferrybridge area where tidal currents are strong.

Depending on the stage in the tidal cycle, highest and lowest temperatures were recorded during both night and day. In summer, when Fleet water temperatures are warmer than sea temperatures, warmer (during the day) or colder (during the night) water from the western Fleet flowed past the temperature sensor at the Oyster Hut. In winter, when sea temperatures are generally warmer than Fleet temperatures, the reverse is true. Temperature swings are greater further west in the Fleet (away from the buffering effects of seawater exchanged by the tide).

AEven on neap tides, there was usually some evidence of a second diurnal cycle at Morkham=s Lake... this suggests that the statement by Robinson et al (1988, 668 [should be 1983, 668]) that >the semidiurnal tide propagates into the west Fleet only weakly ... at spring tides, and not at all ... at neaps= may need some clarification. Robinson et al (1983, 659) recognise a >fortnightly fluctuation of water level, related to the spring-neap cycle, which is strongest towards the western end=, and state that at neap tide the water >remains in West Fleet ... and for several days is virtually isolated from the tide=. The short record available from Morkham=s Lake does not indicate an obvious fortnightly temperature cycle and ... even at neaps a semidiurnal variation is apparent@.

The paper concludes that in 1993-94, temperature was not a limiting factor to spawning and settlement of Crassostrea gigas, but despite this, successful spawning of this species had not been observed to date.

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