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Zooplankton of Wellington Harbour, New Zealand

The Plankton Calendar

The Plankton Calendar

Introduction

The plankton calendar (Text-fig. 7) 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 the five major faunal elements to this volume is illustrated and correlated with the mean of the surface water temperature readings for each month (Text-fig. 8). The monthly variation in the maximum number of larval decapod crustacean species is shown in Text-fig. 9.

Several 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.

An attempt was made to calculate the number of individuals of each species required to make up a given volume, but the very considerable size range of some species made this impracticable. For example the great size differences between solitary and aggregate forms of the Salpidae, between juveniles and adults of Pleurobrachia pileus, and between small and large medusae of Octophialucium funerarium and Phialella quadrata, made any such calculations misleading. However an approximate relationship between numerical abundance and volume with regard to Obelia geniculata, Octophialucium funerarium, Phialella quadrata, Pleurobrachia pileus, and Salpidae, may be obtained by correlating Text-figs. 7 and 8.

The important features of the plankton in each month are described below, and these are largely based on the data presented in Text-figs. 7, 8 and 9. All quantities are equated with a single 20 minute horizontal plankton tow, using a 36 gauge two-foot diameter cone net at depths between one fathom and six fathoms. The numerical equivalents of the expressions "rare", "few", "common", "very common", "abundant", and "very abundant" are given in the key to Text-fig. 7. Where these expressions have been used in this sense they have been italicized.

The Annual Cycle

January

Sea temperatures vary between 17°C. and 19°C. The plankton volume is generally about 100cc, or more when Thalia democratica is abundant. Obelia geniculata occupies less than 10% of the volume, while Salpidae, especially Thalia page 20
Text-fig. 7.—The Plankton Calendar. The Annual Cycle of Dominant Species in the Wellington Harbour Zooplankton based on their numerical abundance per 20-minute Horizontal Plankton Tow in 1961, 1962 and 1963.

Text-fig. 7.—The Plankton Calendar. The Annual Cycle of Dominant Species in the Wellington Harbour Zooplankton based on their numerical abundance per 20-minute Horizontal Plankton Tow in 1961, 1962 and 1963.

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Text-fig. 8.—Seasonal variation in the major faunal elements comprising the monthly mean settling volume of the Wellington Harbour plankton related to surface water temperature during 1961, 1962 and 1963. Based on 20-minute horizontal plankton tows.

Text-fig. 8.—Seasonal variation in the major faunal elements comprising the monthly mean settling volume of the Wellington Harbour plankton related to surface water temperature during 1961, 1962 and 1963. Based on 20-minute horizontal plankton tows.

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Text-fig. 9.—Annual variation in the monthly maximum numbers of larval decapod crustacean species in the Wellington Harbour plankton. Based on 1961, 1962 and 1963.

Text-fig. 9.—Annual variation in the monthly maximum numbers of larval decapod crustacean species in the Wellington Harbour plankton. Based on 1961, 1962 and 1963.

page 23 democratica, make up over 40% of the volume of most samples. Hydromedusae make up about 20% of the volume. Pleurobrachia pileus is rare and may be absent. About one-third of the volume comprises larval Crustacea, with 33 decapod species recorded for this month. Of these the larvae of Jaxea sp. and Petrolisthes novaezelandiae are usually the most common, but other species are strongly represented.

February

Sea surface temperatures vary between 17.5°C. and 20°C. but are consistently higher than in January. The volume of February samples is rarely more than 10cc, and all species are poorly represented. Pleurobrachia pileus is usually absent, and the only stomatopod larvae recorded are those of Squilla armata. Twenty decapod species are found in February, but these are represented by few individuals excepting the larvae of Petrolisthes elongatus, P. novaezelandiae, Jaxea sp. and Halicarcinus spp. which are all common.

March

Sea surface temperatures in this month are between 16°C. and 17°C. Plankton volume is about 15cc, with greater numbers of Thalia democratica being mainly responsible for this small increase. Larval Crustacea have increased with first stage larvae of Squilla armata, Munida spp., Paguridae and some brachyuran species recorded. Twenty-six larval decapod species are present with Petrolisthes novaezelandiae and Paguridae the most common.

April

Sea surface temperatures are between 14°C. and 16°C. but may fall to 12°C. by the end of the month. Plankton volume is about 50cc. Large numbers of Obelia geniculata medusae and to a lesser extent Pleurobrachia pileus are together responsible for this increase. Phialella quadrata and Thalia democratica are recorded, but usually in small numbers. Fish eggs are abundant, but fish larvae are still rare. Twenty-two larval decapod species were found in April, with Petrolisthes novaezelandiae and Paguridae still the most common of these.

May

Sea surface temperatures are between 12°C. and 14°C. The volume of May samples is extremely variable. Catches of up to 500cc may be taken towards the end of the month with vast numbers of Obelia geniculata medusae and Pleurobrachia pileus comprising over 90% of this volume. Thalia democratica occurs only rarely, usually towards the beginning of the month. Decapod larvae are poorly represented and only 15 species are found. Pagurid larvae are the most common of these.

June

Sea surface temperatures fall from about 12°C. at the beginning of the month to 10°C. at the end of June. The volume of June samples is generally more than 130cc, and occasional tows may produce more than a litre of plankton mainly comprising a great abundance of Obelia geniculata medusae and Pleurobrachia pileus. Copepoda are now abundant and occasionally make a large contribution to the volume of plankton. June is the poorest month for larval Crustacea. The most common crustacean larvae are those of Squilla armata. Only nine decapod species occur and all of these are very poorly represented.

July

Sea surface temperatures range between 10°C. and 11°C. but slightly lower temperatures are occasionally recorded. Vast numbers of Obelia geniculata medusae and Pleurobrachia pileus still persist and may comprise over 90% of the volume page 24 of one litre or more in some samples. The majority of samples produce a volume of between 50cc and 100cc of plankton. Phialella quadrata was very common in July samples of 1963, but was absent in July of 1961 and 1962. The first evidence of the spring increase in the zooplankton appears in mid-July. Fish eggs are very abundant and fish larvae are now common. As in June Copepoda are abundant. Of the decapods, 18 species are found in July, but these are represented mainly by a few first stage larvae occurring towards the end of the month.

August

Sea surface temperatures are the lowest for the year in early August and readings below 9.5°C. are relatively common. However towards the end of August there is a rise to approximately 11°C. The volume of plankton is usually greater than 100cc, and is comprised mainly of Obelia geniculata medusae and Pleurobrachia pileus which occur in abundance. Octophialucium funerarium and Phialella quadrata are present but their abundance is variable. Fish eggs are very abundant and fish larvae very common. Copepoda are very common. The spring increase of larval Crustacea makes up about one-third of the volume of plankton. Twenty-eight species of larval decapod can be recognised in August samples, the most common larva being of Callianassa filholi.

September

Sea surface temperatures between 11°C. and 13°C. are recorded in this month. The average volume of plankton is about l00cc, and over one-third of this volume is made up of Copepoda and crustacean larvae. Obelia geniculata medusae are still very abundant and Pleurobrachia pileus is common, but these species do not occur in the vast numbers characterising the winter months. Large numbers of fish eggs and larvae are usually present. In September larval Crustacea reach a maximum which is sustained through to late January. Stomatopod larvae are common and cirripede nauplii and metanauplii occur in abundance. Larval decapod Crustacea are very numerous and 44 species occur. Of these Callianassa filholi dominates all samples and may be very abundant. Other species are well represented. Jasus edwardsi Phyllosoma larvae occur in September only.

October

Sea surface temperatures are between 13°C. and 15°C. Average tows produce between 50cc and l00cc of plankton. Obelia geniculata is very abundant and Pleurobrachia pileus is common, but together they comprise only about 50% of the volume. Fish eggs and larvae are still very common, but these do not occur in the abundance characterising the previous three months. In comprising about 45% of the volume, larval Crustacea are more abundant than in any other month of the year. Squilla armata larvae are very common with early stage larvae dominant. Forty-six larval decapod species are recorded with Callianassa filholi the most abundant species.

November

Sea surface temperature recordings vary between 15°C. and 16.5°C. Between l00cc and 200cc of plankton is produced by an average tow in this month. The bulk of this volume is made up with Obelia geniculata, Pleurobrachia pileus, Octophialucium funerarium, and Phialella quadrata. Crustacean larvae are still very abundant, but these comprise less than one-third of the volume of plankton. Decapod Crustacea are represented by the larvae of up to 45 species, with Petrolisthes novaezelandiae, Callianassa filholi and Halicarcinus spp. the most abundant.

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December

Sea surface temperatures vary between 15°C. and 17.5°C. but most of the recordings are around 17°C. The volume of plankton in this month is similar to November, and varies between l00cc and 150cc. The abundance of Obelia geniculata medusae falls off rapidly towards the end of the month, and by December Pleurobrachia pileus is usually rare. Ihlea magalhanica occurs rarely, but Thalia is still absent. Larval Crustacea are abundant but comprise less than one-third of the volume of plankton. Forty species of decapod larvae have been isolated in December samples. Petrolisthes novaezelandiae, Callianassa filholi and Jaxea sp. are dominant among the Decapoda. Up to 5,000 larvae of Jaxea sp. are recorded in some samples. In December brachyuran zoeas occur in peak abundance for the year.

Irregular Variations

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. The following volumes were recorded for successive samples from Station One in June, 1961 and July, 1961:

June 1 270cc
June 13 120cc
June 15 190cc
June 22 120cc
July 5 20cc
July 12 180cc
July 20 270cc
July 26 200cc

The sample taken on 5 July produced only a small fraction of the volume normal for these months, and Obelia geniculata medusae and Pleurobrachia pileus which usually dominate the June and July plankton were very scarce. The larvae of several brachyuran species present in the majority of the above samples were absent on 5 July.

A similar example occurred on 25 January 1962, when a volume of 10cc was recorded compared with 230cc, 180cc and 230cc recorded on 3, 10 and 15 January respectively. Neither the volume nor the faunal composition of the sample taken on 5 January compared with other January samples. Brachyuran larvae representing 15 species dominated this sample, while Copepoda, porcellanid larvae, and Jaxea sp. were rare, and Salpidae, Octophialucium funerarium and Phialella quadrata were absent.

The larvae of species occurring seasonally were occasionally identified in samples taken outside the expected seasonal boundaries. A single stage one larva of Petrocheles spinosus was recorded on 2 May 1962. This species was absent from all samples between December 1961 and 2 May 1962, and did not reappear in the plankton until July 1962. Several stage one larvae of Jaxea sp. were recorded on 24 May 1962. Only late stage larvae were present in February 1962, and stage one larvae of this species were not subsequently recorded until September 1962. In such out of season occurrences as these larvae did not appear in sufficient numbers to suggest a secondary period of liberation.

During the spring and summer months occasional plankton samples were dominated by many hundreds of early stage larvae of any one of several crustacean species. These monospecific swarms were probably produced by large numbers of adults liberating their larvae simultaneously, and the larvae were not dispersed by tidal action. This is supported by the observation that species occurring sparsely in the plankton for several weeks may be suddenly represented by an abundance page 26 of first stage larvae in a single plankton sample, with subsequent samples yielding very few.

Identified decapod Crustacea forming such larval swarms are as follows: Hemigrapsus edwardsi, Petrolisthes novaezelandiae, Callianassa filholi, Jaxea sp. Of these, Callianassa filholi and Jaxea sp. swarm quite frequently in the months of their greatest abundance.