Research Note Note on Diatoms in Wellington Harbour in 1953–54.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 84, 1956-57, Page 197

Research Note Note on Diatoms in Wellington Harbour in 1953–54. By Brian E. Maxwell [Received by Editor, May 20, 1955.] Introduction Brewin (1952) has described seasonal changes in microplankton, including diatoms, in Otago Harbour, and Crawford (1947) has recorded a phytoplankton season in Cook Strait. The expeditions of the Terra Nova and the Dana in 1910 and 1928–29, respectively, have not yielded a complete account of the New Zealand diatoms. It might be expected that seasonal changes in the abundance of diatoms in Cook Strait would differ from those in Wellington Harbour owing to the turbulent tidal stream activity of the Strait waters in contrast to the protected waters of the almost landlocked harbour. This has not proven to be the case. Wellington and Otago Harbours, notwithstanding their difference in Latitude, show similarity in seasonal fluctuation in abundance of diatoms. The writer observed variations in the abundance of diatoms at the Queen's Wharf vicinity in Wellington Harbour during a twelvemonth period from July, 1953, to June, 1954, while engaged in a macro-zooplankton survey (Maxwell, 1955a) and therefore sets out the results of the observations in relation to the previously mentioned studies as the purpose of this note. These observations are not intended to constitute a complete diatom survey in the harbour for the diatom population was assessed only in respect to its most obvious members. Thus attention was directed to representatives of three genera of diatoms—i.e., those which were most noticeable, Coscinodiscus, Rhizosolenia, and Thalassiosira. Materials and methods pertaining to this study are described elsewhere (Maxwell, 1955a), and relevant hydrobiological and geographical data for the harbour region have been recorded (Maxwell, 1955b). Results General approximations of the monthly occurrence of diatoms are shown in the Table below. The rough estimates of the number of diatom cells per litre of sea Table Monthly Occurrence of Diatoms at the Queen's Wharf Vicinity, Wellington Harbour. Month and Year. Coscinodiscus Rhizosolenia Thalassiosira Jul., 1953 xx x Aug., 1953 xxx x x Sep., 1953 xxxx x Oct., 1953 x x Nov., 1953 x x Dec., 1953 x x Jan., 1954 x x Feb., 1954 x Mar., 1954 x Apr., 1954 x x xx May, 1954 x x June, 1954 xx x x x = very few; less than 1,000 cells/litre xx = moderate numbers; 1,000 to 10,000 cells/litre xxx = many; 10,000 to 30,000 cells/litre xxxx = extremely numerous; 30,000 to 50,000 cells/litre

Text-fig.—Seasonal fluctuation in abundance, of the monthly total diatom population at the vicinity of Queen's Wharf, Wellington Harbour, in 1953–54.

water probably indicate reliably the general character of the fluctuation in diatom population throughout the year. Coscinodiscus spp. were by far the most abundant constituent throughout the year, attaining a maximum in September and occurring in every month except March. Rhizosolenia spp. were either relatively low in number or absent and did not show a recognizable seasonal cycle. Thalassiosira spp. were on the whole absent during the summer months from November to March, and were low at other times excepting in the autumnal month of April, when they increased to moderate numbers. The relative proportions of the monthly total diatom population over the year are shown below in the Figure. The variation in abundance is manifest by the two maxima, the greater in spring (September) and the lesser in autumn (April). The former is mainly due to the Coscinodiscus element, while an increase in the numbers of Thalassiosira accounts for the latter. The Queen's Wharf vicinity is higher in the yield of diatoms during winter than during summer, when the yield is very low. Discussion Grawford (1947) reported a late summer (January-February) diatom maximum and a preceding lesser vernal (September) maximum in Cook Strait for the year 1944–45. This summer maximum contrasts markedly with the low summer diatom population in the harbour, although in both regions the vernal maximum occurs in September. At the Queen's Wharf vicinity the vernal diatom maximum immediately precedes, and the autumnal maximum coincides with, the total macro-zooplankton vernal and autumnal maximum, respectively (Maxwell, 1955a). The harbour and the Cook Strait regions correlate well in seasonal abundance of zoo-plankton, for Crawford has noted for the Strait that, although not profuse, zoo-plankton was present in the autumn; absent on the whole in winter; plentiful in spring following the diatom maximum; and that zooplankton was more plentiful than phytoplankton at the beginning of the summer. In Otago Harbour in 1944–45 a greater period of maximum abundance occurred in spring and a lesser maximum in autumn (Brewin, 1952). The former maximum was heralded by an increase in the number of Coscinodiscus. Thus it seems that, in general, the characteristics of the Wellington and Otago Harbour diatom populations are similar. September is evidently also the peak month of the vernal and greatest phytoplankton maximum in the coastal waters off New South Wales, Australia (Dakin and Colefax, 1940). Conclusions 1. The seasonal abundance of diatoms in Wellington Harbour is similar to that reported for other Australasian coastal waters excepting Cook Strait. 2 In Wellington Harbour the vernal diatom maximum occurs in advance of the vernal macro-zooplankton maximum, but the autumnal maxima of these groups occur simultaneously. 3. Wellington Harbour is indicated as being a rich field for future study of New Zealand Diatomales. Acknowledgment Grateful appreciation is acknowledged for the New Zealand University Research Fund Fellowship by means of which the writer could conduct his investigations of which this note forms a part. References Brewin, B. I., 1952. Seasonal Changes in the Micro-plankton in the Otago Harbour during the years 1944 and 1945. Trans. Proc. Roy. Soc. N.Z., 79: 614–627. Crawford, D. A., 1947. A phytoplankton Season in Cook Strait. N.Z. Sci. Cong., 173–175.

Dakin, W. J. and Colefax, A. N., 1940. The Plankton of the Australian Coastal Waters off New South Wales. Part I. Publ. Univ. Sydney, Dept. Zool. Monograph No. 1. Maxwell, B. E., 1955a. Observations on the Zooplankton in Wellington Harbour in 1953–54. Manuscript in preparation. Maxwell, B. E., 1955b. Hydrobiological Observations for Wellington Harbour. Trans. Roy. Soc. N.Z., in Press. Brian E. Maxwell, B.Sc., M.A., C/o Dept. Zoology, Victoria University College, P.O. Box 196, Wellington, New Zealand.