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Zooplankton of Wellington Harbour, New Zealand

Summary

Summary

(1)The present series of Wellington Harbour plankton collections commenced on 2 January 1961, under the direction of Professor L. R. Richardson assisted by members of the staff and research students from the Department of Zoology, Victoria University of Wellington. By January, 1964, the collection consisted of 250 plankton samples.
(2)This collection has been used to form the basis of an analysis of the seasonal changes in the abundance of the more important macro-zooplankton organisms over the three years.
(3)It was not possible to identify all the taxa to the specific level as the systematic status of many New Zealand marine organisms has not yet been determined.
(4)Plankton samples were obtained from two stations in Wellington Harbour with a two-foot diameter cone net. These samples were analysed in the laboratory. Unidentified larval Crustacea were reared further in the laboratory. Decapod crustacean larvae found in the plankton were in some cases identified by comparison with larvae hatched from the eggs of ovigerous females obtained from beach collections.
(5)The plankton fauna discussed in this study can be divided into two sections.
(a) Permanent Plankton: The species in this section are present in their adult form all the year round and do not fluctuate significantly during the year. These include Isopoda, Amphipoda, Copepoda and Chaeto-gnatha.
(b) Temporary Plankton: This section includes the larger number of species. Their planktonic existence is a phase in their life history and is seasonal. Of the species in this section larval Crustacea have been made the subject of special study. Many species have been identified and their seasonal abundance discussed.
(6)The plankton calendar has been compiled from seasonal variation in the numerical occurrence of the described species in Wellington Harbour plankton samples from January 1961 to August 1963. Seasonal trends in the monthly mean volume of plankton, and the relative contributions of five major faunal elements to this volume is illustrated and correlated with the mean of the surface water temperature readings for each month. The monthly variation in the maximum number of larval decapod crustacean species is shown.
(7)Some of the species discussed were occasionally present in quite small numbers, but by virtue of their large size they were found to make an appreciable contribution to the volume of plankton. Such species were Thalia democratica, Ihlea magalhanica, Octophialucium funerarium, Phialella quadrata, and Pleurobrachia pileus. When present, these species together with the smaller but very abundant medusae of Obelia geniculata cause major volumetric increases.
(8)The important features of the plankton in each month are described, and these are based on the data presented in Text-figs. 7, 8 and 9. The first evidence of the spring increase in the zooplankton is in mid-July, and this continues through August to reach a maximum in September, which is sustained through to late January. There is a sharp fall-off in February, which recovers to reach a very minor autumn maximum in March and early April.page 29
(9)The seasonal cycle shown by the majority of species is independent of the seasonal trends in the total volume of plankton. The volume of plankton is greatest in winter, and is largely dependent on the abundance of Obelia geniculata medusae and Pleurobrachia pileus which tend to have winter temperature optima. These differ from the majority of species which seem to have late spring and summer temperature optima. Few other species are recorded in winter.
(10)Surface water temperature probably influences numerically dominant species, thereby controlling many of the fluctuations in the volume of plankton. A critical temperature seems to be between 15°C. and 16°C.
(11)Although usually conforming to the described monthly pattern, the plankton is characterised by short-period fluctuations which are non-seasonal. In several cases plankton samples were neither quantitatively nor qualitatively representative of the month in which they were taken. Also, the larvae of species occurring seasonally were occasionally identified in samples taken outside the expected seasonal boundaries. During the spring and summer months monospecific swarms of crustacean larvae were produced by large numbers of adults liberating their larvae simultaneously, and the larvae were not dispersed by tidal action. Such irregular variations as these cannot be interpreted without continuous plankton recording over an uninterrupted period of time correlated with meteorological and hydrological data.
(12)Skerman (1958) considers that the short-period temperature fluctuations in the harbour environment may be outside the eurythermal capacity of the shelf forms beyond the harbour entrance—a factor which may determine the composition of the harbour population. Laboratory experiments have shown shelf forms to have a wide eurythermal capacity. A more probable limiting factor would be an insufficient period of sustained optimum temperatures to induce species to breed or to complete their larval metamorphosis. Wilson (1951) showed that the nature of the water itself and substratum specificity are important factors for the completion of larval metamorphosis and the establishment of a species in a given environment.
(13)Dakin and Colefax (1933, 1940) have outlined a calendar for the more frequently occurring organisms in the plankton of the coastal waters of New South Wales, Australia. These are the only published seasonal analyses of a Southern Hemisphere plankton community available for comparison with the zooplankton of Wellington Harbour. The Wellington Harbour zooplankton follows essentially the same seasonal pattern described for New South Wales.