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Zooplankton of Wellington Harbour, New Zealand

Discussion

Discussion

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. When the temperature falls consistently below this level Obelia geniculata medusae and Pleurobrachia pileus occur in great abundance and cause a sharp rise in the volume of plankton. As the temperature rises above 16°C. in early summer these two species become rare or absent. The converse probably applies to the Salpidae, especially Thalia democratica which makes a large contribution to the volume of summer and autumn samples. Salpidae do not occur until early December when the surface water temperatures are consistently above 15°C. or 16°C. but in late May when the temperature again falls below 15°C. Salpidae are absent.

Minor variances from the described monthly pattern were noted during the research period. For example in 1963, Jaxea sp. and Callianassa filholi larvae appeared in the plankton two or three weeks earlier than expected, and Pleurobrachia pileus became absent at the end of November, which was over a month earlier than in 1961 and 1962. Also, in 1961 cirripede nauplii were present in September only, but in 1962 and 1963 their occurrence was quite considerably protracted, with fewer larvae occurring over the three months of August, September and October.

It is not possible to interpret the causes of early or delayed larval liberations, shortened or protracted liberation periods, or the many irregular variations characterising the plankton samples without continuous plankton recording over an uninterrupted period of time correlated with meteorological and hydrological data.

Skerman (1958) gives evidence of diurnal temperature variations in the magnitude of 4.5°C. in summer. He writes that thermograph records showed 4.0°C. differences in temperature within the uppermost six feet of water on several occasions. However Maxwell (1956) found that differences between bottom and surface temperatures in Wellington Harbour were at the most 2.0°C.

Skerman's data and the data obtained during this current research programme suggests that marine animals within the harbour environment are influenced by sudden short-period variations in temperature. During laboratory rearing experiments, adult Crustacea collected from the littoral and sub-littoral margins of Wellington Harbour, and juvenile Crustacea reared from planktonic larvae, survived abrupt temperature changes of plus or minus 16°C. without obvious detriment. This showed that many species possess a wide eurythermal capacity.

Skerman contends 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. From laboratory experiments involving living plankton, the fact that the larvae of adult Crustacea known to occur only beyond the harbour entrance (Sergestes sp., Pandalus spp., Munida spp., and possibly Petalomera sp.) survived abrupt temperature changes over a wide range suggests that short-period temperature variations are not the limiting factor determining the harbour population. A more probable limitation would be an insufficiently long period of sustained optimum temperatures to induce some species to breed, or to complete their larval metamorphosis.

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Wilson (Plymouth) showed that the nature of the water itself had a very marked influence on the development of larvae (polychaetes and echinoderms), and it is possible, therefore, that the oceanic larvae which come into the harbour environment are unable to develop in this water mass. Many species living on or in different types of substratum are unable to metamorphose unless the required substratum is found (Wilson, 1951). Larvae of parents living within the harbour environs will therefore be the only species likely to complete their larval metamorphosis and to become established. All others will be transitory.

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 plankton of Wellington Harbour.

The annual range of sea surface temperatures for New South Wales is from 23°C. in February to 15°C. in August. The temperatures are therefore generally higher than in the Wellington Harbour where slightly lower summer maxima (20°C.) and much lower winter minima (9°C.) are recorded.

In the New South Wales plankton the abundance of Thalia in all months except June, July and August is probably a result of the relatively high sea temperatures. Thalia occurs only in late summer and autumn in the cooler Wellington waters.

Obelia geniculata and Pleurobrachia pileus which dominate the Wellington Harbour winter plankton are not discussed by Dakin and Colefax (1933, 1940). Ctenophores are recorded in the late summer and autumn samples from New South Wales, but these are at no stage dominant, and it is not known whether or not these ctenophores include P. pileus. The winter "bloom" of Obelia geniculata and Pleurobrachia pileus in the Wellington plankton suggests that these species favour lower temperatures than occur in New South Wales waters. It is probable that the distribution of Obelia geniculata follows the southern circumpolar, cold-temperate distribution of Macrocystis pyrifera outlined by Knox (1960).

The zooplankton of the Wellington Harbour follows essentially the same seasonal pattern described for that of New South Wales. However, the zooplankton of New South Wales developed in late spring or in early summer (November or December) and tended to sustain itself at a high level during the summer and into late autumn. The peaks are in November-December and in March (Dakin and Colefax, 1933). In the Wellington Harbour 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. This late summer decrease and autumn recovery of the zooplankton is not as marked in New South Wales (Dakin and Colefax, 1940).

From a consideration of the evidence presented in this study, it is clear that the majority of zooplanktonic species occur in a seasonal cycle. This cycle is independent of the seasonal trends in the total volume of the plankton. Total volume is largely dependent on the abundance of Obelia geniculata and Pleurobrachia pileus which tend to have temperature optima differing from the majority of species in the plankton.

As the major components of the Wellington Harbour plankton are known to occur elsewhere in New Zealand waters, it is possible that the plankton of other coastal or harbour areas of New Zealand shows a similar seasonal pattern to that described for Wellington Harbour. There is a need for further work in this field which is largely still untouched, and studies of this type from other New Zealand localities would prove especially valuable.

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