Studies on the Zooplankton of Pauatahanui Inlet

Abstract

Zooplankton abundance, and records of salinity' temperature and turbidity were determined from up to 9 stations on 17 occasions between September 1975 and October 1976 from Pauatahanui Inlet, Wellington, New Zealand, as part of a co-operative Pauatahanui Environmental Programme. Coincident samples were collected for water nutrient analysis and phytoplankton estimations. Inlet waters were cooler and less saline than adjacent coastal water during summer. The hydrology in the inlet was prone to rapid changes in salinity and occasional stratification in the entrance channel when there was a substantial amount of freshwater runoff into the inlet. Levels of suspended sediment were high and water transparency low. Nutrient levels were comparable with other estuarine systems. Changes in relative composition of the zooplankton were examined by settled volume of the major taxa. The plankton was dominated by copepods, medusae and polychaete worm larvae. The copepod assemblage was a depleted coastal group with the addition of Gladioferens pectinatus and the copepodites of an unknown canuellid harpacticoid copepod. The relative numerical proportions of copepods differed from previous studies in New Zealand waters. Acartia jilletti was dominant throughout the year apart from winter when the subdominant copepod Euterpina acutifrons exceeded it in abundance. The abundance of Acartia jilletti was high (max. 28,096 m-3) but extremely variable, and its success in Pauatahanui Inlet is attributed to tolerance of low salinities, few competing copepods, flexible feeding behaviour and adequate food supply. A weak positive correlation with salinity is at variance with previous New Zealand work on this species. Acartia jilletti had unusually high proportions of males, averaging about 80%. Variability in the sex ratio increased with distance up the inlet. Factors which were considered to be involved in the skewed sex ratio included high population density, unstable hydrology and reduced competition from other copepods. A relatively larger number of copepodites were found in the population at the head of the inlet. Two other members of the Acartiura subgenus occurring abundantly elsewhere in New Zealand waters were poorly represented. A decreasing order of salinity tolerance is proposed for the three members of the subgenus Acartiura found in Pauatahanui Inlet, with the tolerance of Acartia jilletti> Acartia ensifera> Acartia simplex. The centres of abundance of Acartia ensifera and Acartia simplex are believed to lie outside the inlet. Abundance of Euterpina acutifrons was maximal in late winter and spring. Minimum abundance was in early winter. This species achieved a level of abundance seldom reported elsewhere (max.2,819m-3). Densities decreased towards the head of the inlet and this was interpreted as freshwater dilution. There were weak positive correlations for all developmental stages with temperature and salinity. Male dominance in the population is thought to be linked primarily to hydrological instability, but several other factors may contribute. Female breeding activity was not limited by temperatures as low as 9.8°C. Male:female ratios of adults and copepodites stage 5 underwent parallel seasonal changes but adult males were dominant over adult females, while copepodite stage 5 females numerically dominated stage 5 males. Two separate male morphs were found in Pauatahanui Inlet and their relative abundance differs from that recorded in overseas studies. Reasons for this are discussed. Temperature and salinity are suggested as limits on the abundance of some less frequent copepods. The copepod assemblage and changes in its composition with time are examined. R-mode analyses (species associations) were carried out using a distance metric, the Czekanowski coefficient and also the Pearson correlation coefficient as measures of similarity. Q-mode analyses (station associations) were made using an index adapted from geochemistry' the Imbrie index of proportional similarity. One group of stations with high species diversity and abundance occurred in mid summer. A second, loosely associated group, with several subgroups, occurred in spring and summer with a depleted copepod population. A third, more loosely associated group of stations, mainly from autumn and winter, were more closely related to one another than to stations from other seasons in the inlet. Stations in this third group showed few faunistic similarities.