A Systematic and Ecological Survey of the Summer Macroplankton, Southern New Zealand

Abstract

Three months of plankton sampling, January to March, 1951,has been possible by the presence of H.M.N.Z.S. LACHLAN, a frigate on surveying duties, on the New Zealand coast. Samples include two series of stations from Cook Strait to Dunedin, a month’s sampling concentrated in Foveaux Strait, interspersed with series of stations eastwards of the Straits proper and between the Straits and Dunedin, and during February and March, a number of stations, including those from special cruises, chiefly from Dunedin southward to Nugget Point. As well, three sample s obtained by LACHLAN in November, 1951, between New Zealand and Campbell Island have been utilized for confirmation of sub-Antarctic influence on southern New Zealand. Eighty six samples have been subjected to qualitative and quantitative analyses; the volume of individual tows has been determined by the displacement method (large and unrepresentative specimens removed), and individual species sorted and counted, or where catches ore large, numbers of n species estimated from an enumerated fraction of the sample. In view of the rigid control exercised in length of tow, depth of net, speed of tow and type of net used, the quantitative procedures adopted are regarded as justified, and giving a fair measure of the population during the period of sampling and comparable results from the several localities and conditions sampled. Unavoidable restrictions from survey routine prevented extensive stations being kept, or the maintenance of regularly kept series of stations. Instead, to obtain a reasonably accurate estimate of numbers and types of organisms present, numerous short (three-minute) tows were made under all conditions of weather and sea, and time of day, and usually covering an area in some concentration. Later in the season (Appendix I and II) these were supplemented by oblique tows from up to 100 ms. to the surface. From these the average planktonic conditions of an area, during a reasonably defined period, were obtained. Oceanographic investigations to date, indicate that New Zealand waters are influenced by three water-masses arriving from three directions; cool to cold sub-Antarctic water of the West Wind Drift strongly influences at least eastern South Island waters; warm water, originating in the South Equatorial Current and travelling via the East Australian Current, extends in n broad belt across the Tasman Sea onto the West Const of New Zealand, particularly the North Island; and a southward deflection of Tropical waters reaches the east const of the North Island and largely contributes to the East Cape Current. The net result of these influences in New Zealand is the existence of a warm, high-salinity water north, and a lower salinity, cold water to the south, the two meeting in the sub-Tropical Convergence, which during the time of this survey was located (eastwards of the South Island), between Kaikoura and Banks Peninsula, but nearer the latter. Consideration of the overall distribution of species shows the presence of two clearly defined faunal groups, corresponding to the warm and cold-waters described and separated by the sub-Tropical Convergence. The southern of these groups shows strong influence of sub-Antarctic species in most groups, but especially Amphipoda and Copepoda, while the northern shows strong sub-Tropical to Tropical influences, particularly in Copepoda, to n lesser extent in Salpidae and most markedly by the absence of many southern species. Distinction can be drawn too, between oceanic-, coastal-, and slope-water faunas (the two last constituting the neritic fauna), although little oceanographic data have accumulated to show how well founded are these distinctions. Species, regarded on present knowledge as being representative of the warm and cold water, and neritic and oceanic faunas, have been selected with a view to their use as indicating the presence of these water types. Cold-water species are Cyllopus magellanicus, Parathemisto gaudichaudi, Sagitta lyra (gazellae-type) as well as Eucalanus acus and Clausocalanus laticeps and other Copepoda. The various species of Sapphirina, Pleuromamma abdominalis and possibly Thalia democratica are thought to be representative of warm water, while Nyctiphanes australis, Tenagomysis spp.,Calanoides carinatus, Neocalanus tonsus, Oikopleura dioica and Ihlea magalhanica appear typically in coastal water in the Southern Area of this survey. It is realised that only further sampling will show whether these species will retain their position ns indicators at all times for New Zealand waters. A total of 80 species have been identified during the survey (including 24 by specialists -- Amphipoda, Polychaeta, Pteropoda and Ostracoda) covering all groups except Coelenterata and all species encountered during analyses. Species include 37 Copepods, 2 Mysids, 5 Euphausiids, 16 Amphipods, 2 Copelata, 2 specie s of Pyrosoma, 1 Doliolid, 5 Salps, 2 Ostracods, 2 Chaetognaths and 4 of each Pterorpoda and Polychaeta. The systematics of Calanus tonsus Brady are discussed and it is concluded that this species belongs in the genus Neocalanus; it has therefore been transferred, and is called Neocalanus tonsus (Brady). Additional material for. The Mysid Tenagomysis tenuipes (known previously from a single male) has provided information on larval history and development, and on the range of variation to be expected in the adults. The previously unknown “Old Nurse” stage of the Doliolid, Dolioletta valdiviae, Neumann, is believed to have been taken along with gonozooids ancl phorozooids of this species. Two species are regarded as new, namely, Erythrocypris sp and Tomopteris sp., and are being more fully studied by the specialists on Ostracoda and Folychaeta respectively. The ranges of a number of sub-Antarctic species have been extended northward to New Zealand, e.g., Eucalanus acus, Clausocalanus laticeps among Copepoda; Cyrlopus spp., Hyperoche medusarum among Amphipoda; the Mysid, Tenagomysis tenuipes, and others. As well new records for the Australasian and South Pacific regions have been made, the most striking of which are the appearances of Sapphirina pyrosomatis (Copepoda), Nematoscelis megalops (?) (Euphausiidae), and Dolioletta valdiviae (Doliolida). Occurrences of many species in the Southern Area have been plotted on charts according to relative densities of specimens and these indicate a more or less clear-cut delineation between oceanic and coastal faunas, and as well indicate the presence of a third, the slope-water fauna. These distinctions have been. indicated more objectively and precisely by plotting occurrences of species (relative densities shown) against the salinity and temperature of the water at the stations, temperatures being shown in the ordinates, salinities in the abscissae. The groups of occurrences thus obtained show clearly and precisely the limits of range of salinity and temperature from which specimens have been taken, and also show any habitat “preferences”; from the groupings too, possible indicator species for any particular type of water may be readily selected. This has been found to apply equally well to the neritic and oceanic faunas. Diurnal variation in numbers of specimens at the surface occurs in all species analysed, and such variation, supported by evidence derived from comparisons of oblique and surface hauls, indicates that this is the result of vertical migration. A number of deviations from the usual pattern of “main rise” before midnight and a “pre-dawn rise” have been noted for several species, ranging through to reversed patterns (i.e. a daylight rise) for Parathemisto gracilipes and Cyllopus magellanicus. Differential migration has been detected between adults and Juveniles of Parathemisto spp.and Metridia lucens. The average catch of total plankton in ccs. per tow per month shows that there is a decline in volume taken from January to February, followed by an increase in March and April. This is regarded as indicating a portion of a seasonal planktonic cyole. The abundance of individual species as shown by counts of specimens are compared with the volumetric changes, and these show that a few common species contribute towards the volumetric changes, with the majority of cycles behaving independently of that illustrated by total volumes.