77 T A N E (1967) 1 3 : 7 7 - 98

STUDIES ON T H E ZOOPLANKTON OF L A K E PUPUKE

B y J . D. Green *

I N T R O D U C T I O N

L a k e Pupuke , a l though c l ose to A u c k l a n d c i ty , was almost unknown b i o l og i ca l l y u n t i l 1964, when M. A . Barker , ( Tane , 1966) conducted a survey of i t s phys iochemica l nature i n re la t ion to the d i s t r ibut ion of the bottom fauna.

The lake i s a good prospect for systemat ic p lankton research, being the only body of fresh water i n the immediate v i c i n i t y of Auck l and exhib i t ing such important l imno l og i ca l phenomena as summer thermal s t ra t i f i ca t ion and winter homothermy. P rev i ous one - day t r ips to the lake by members of the Zoo logy Department had shown that because of i t s eutrophic nature the lake supported a moderately r i c h zoop lankton and a l so exhibi ted s easona l phytoplankton " b l o o m s " .

The purpose of the present study was to make a br ie f survey of the composi t ion and d i s t r ibut ion of the zooplankton i n the l a k e .

P L A N K T O N M E T H O D S

Fo r the purposes of th is study a plankton pump and f i l t e r were constructed, prov id ing a very convenient method for tak ing s m a l l quantit­ative samples of the p lankton . Apart from being compact and e a s i l y hand­led and operated, i t s accuracy i n procuring samples was exce l l en t when compared to methods used by other workers.

A t each sampl ing v i s i t the p lankton pump was used to make a ser ies of v e r t i c a l samples at a f ixed sampl ing point 200 metres from the southern shore. Samples were taken every 5 metres from the surface down to 28 metres. From each depth 5 l i t r es was f i l tered and the s i z e s of the catches so obtained were such that it was poss ib l e to count a l l the p lank-ters present; subsampl ing errors were thus avo ided . F o l l o w i n g the example of other workers, it was assumed that the plankton of s m a l l l akes wh i ch have regular shore l ines and bottom conf igurat ions, i s more or l e s s d i s ­tributed randomly hor i zonta l l y . T h u s , i n order that the r esu l t s obtained i n this study could be interpreted meaningful ly, it was assumed that the ecology, as observed from a s ing le sampl ing point near the middle , was representative of the whole l a k e .

T H E R M O M E T R Y

V e r t i c a l temperature recordings were made w i th a thermistor res is tance thermometer, capable of measuring accurate ly w i th in 0. 1 °C . ' D e p a r t m e n t o f Z o o l o g y , U n i v e r s i t y o f A u c k l a n d

78

Thermal s t ra t i f i ca t i on ex is ted i n L a k e Pupuke during the samp­l ing period from March to J u n e . The pos i t i on and range of the thermocl ine i s g iven i n Tab l e I.

T A B L E I

Date 6-4-66 13-5-66 1-6-66 15-6-66 18-8-66

Depth range Epi l imnion(m) 14 17. 5 23 26 12. 1°C

H O M O T H E R M A L

11. 8 °C

Depth range Thermocline(m) 2 2 1. 2 1. 5

12. 1°C H O M O T H E R M A L

11. 8 °C

Temp, range Thermocl ine( °C ) 4. 5 3. 4 1. 0 0. 7

12. 1°C H O M O T H E R M A L

11. 8 °C Lowes t Temp. Hypolimnion(°C) 11. 7 12. 1 12. 1 11. 3

12. 1°C H O M O T H E R M A L

11. 8 °C

Barker , (1964), a l so demonstrated the ex is tence of thermal strat­i f i ca t i on i n the l ake . In 1966, the conf igurat ional changes of the thermo­c l ine appeared to fo l low much the same course as they d id i n the previous study, although hypol imnet ic temperatures were s l i gh t l y lower i n 1966 than in 1964.

It was in teres t ing to observe whether the thermocl ine had any effect on the v e r t i ca l d i s t r ibut i on of the plankton, as C u s h i n g (1951) had c i ted a number of ins tances where members of the p lankton had been c on ­tained i n their d i s t r ibut i on by the presence of a thermocl ine . J o l l y and B a y l y showed that p lankton i n L a k e Hayes and in the Mayor I s land l akes , respect ive ly , was conta ined in the ep i l imnion, whi le J o l l y (1957) states that plankton i n New Z e a l a n d l akes occurs either above or below the thermocl ine.

Over the sampl ing per iod corre lat ions did appear to occur between plankton d i s t r ibut i on and the presence and pos i t i on of a thermocl ine i n Lake Pupuke . These corre lat ions are d i s cuss ed i n a later s e c t i o n .

B I O L O G I C A L R E S U L T S

a . Qua l i ta t i ve Compos i t i on of the P l ank ton The fo l l ow ing genera and spec i e s were found to be present i n

the p lankton . In the case of many of the rot i fers, because of d i f f i cu l t i e s i n ident i f i ca t ion , names are g iven mainly to indicate the type of an imal present, although there i s a good p o s s i b i l i t y that the names g iven are correct . C l a s s i f i c a t i o n fo l lows Pennak (1953) and Ward and Whipple.

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P H Y L U M A R T H R O P O D A C L A S S C R U S T A C E A Sub C l a s s

Order

Order

Sub C l a s s Order F a m .

P H Y L U M R O T A T O R I A C L A S S M O N O G O N O N T A

F a m .

F a m .

Order F a m .

F a m .

Order F a m .

P H Y L U M P R O T O Z O A C L A S S M A S T I G O P H O R A F a m .

C L A S S S A R C O D I N A

Copepoda Ca l ano idea Calamoecia lucasi-Cyclopoidea Macrocyclops albidus Paracyclops fimbriates Branchiopoda C ladoce ra Chydor idae Chydorus sphaericus Alona sp. Bosminidae Bosmina meridionalis Macrothric idae Ilyocryptus sordidus

P l o i m a Branch ion idae Keratella cochlearis K. quadrata Branchionus sp. Euchlanis sp. Platyais sp. Diplois sp. Asp lanchnidae

~~A~splanchna brightwellii F l o s c u l a r i a c e a F i l i n i i d a e Filinia sp.

Hexartha (Pedalia) mira

Per id in idae

Ceratium hirundinella (Muller) Ac t inopoda

C L A S S C I L I A T A

Sub C l a s s Actinophyrs sol

Order P e r i t r i cha F a m . Vo r t i c e l l i dae

Zoothamnion Smal l c lear , apparently free swimming mites were a l so found non - or ibat id and had few swimming ha i rs with large st i f f radiat ing ha i rs however. DIVISION C H L O R O P H Y T A Order Conjuga les

Sub order Desmidio ideae

They were The opisthosoma was covered

Closterium aciculare Staurastrum (2, p o s s i b l y 3 spec ies )

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b. Quant i tat ive Cons ide ra t i on of the Compos i t i on and D i s t r i bu t i on of the P l ank ton

From the resu l t s of the v e r t i ca l sampl ing procedures, the v e r t i ca l d is t r ibut ion and seasona l changes in the p lankton were s tud i ed . Studies were a l so made on the d iurna l changes in the v e r t i c a l d i s t r ibu t i on and on the l i t t o ra l p lankton .

1. Seasona l Va r i a t i ons i n the Re l a t i v e Abundance of the Dif ferent Spec ies :

F i gu r e s 1 and 2. (Zooplankton cons idered numer ica l l y , Phytop lankton on an 8 -po in t abundance s c a l e ; 0 to 7, 0 min. abundance, 7 max. abundance. )

Over the sampl ing per iod it was found that the zooplankton of Lake Pupuke exhib i ted the phenomenon of s easona l pe r i od i c i t y . From this study it was poss ib l e to obta in some idea of changes occurr ing i n the late summer, autumn (through the overturn) and winter.

The var ious spec i f i c components of the p lankton w i l l be con ­sidered f i rst , and second ly the i nd i v i dua l changes noted w i l l be d i s c u s s e d in cons iderat ion of the changes wh ich occurred in the to ta l p lankton pop­u la t ion .

P l ank ton i c Crustacea were at a l l t imes the most abundant members of the p lankton, except at one time during A p r i l when Ceratium appeared in great numbers. R o t t e r s dominated the l esse r groups.

*\Calamoecia ( F i g . l a ) was at a l l t imes the most dominant plankter present and as i t was larger than most others, th is dominance was both numerical and p h y s i c a l . (Un l ike Ceratium for example; a l though numer­i c a l l y great i t was p h y s i c a l l y s m a l l . ) Calamoecia exhib i ted three main pu lses , i n early A p r i l , m i d - M a y and m i d - J u n e , the latter be ing co inc iden ta l with a large phytoplankton " b l o o m " and occurr ing when the lake had reached a state of complete c i r c u l a t i o n . After th is l as t peak the numbers dropped away to reach the lowest l e v e l recorded, i n early J u l y ; they then s l ow ly increased t i l l at the end of the sampl ing period they reached numbers com­parable with those of m i d - M a r c h . Calamoecia i s a herbivorous ca l ano id , and it seemed poss ib l e that the peak i n June may have been a resul t of a great increase of an a l ga on wh i ch it fed. The subsequent drop i n numbers cou ld l i kew ise have been due to the a lga dying out at the end of the " b l o o m " .

(Bosmina ( F i g . l a ) was the second most dominant c rustacean, increasingNgradjial ly i n numbers over the sampl ing period and exh ib i t ing two sma l l inc reases , i n ear ly A p r i l and m i d - M a y w i th the larger peak In m i d - J u n e at the end of the overturn. T h i s a l so may have corresponded with an increase i n an a l ga l food organism. After th i s peak however there was a

81

dramatic increase i n numbers so that i n August Bosmina occurred i n almost the same numbers as Calamoecia.

pepods ' P i g . i a ) were present i n the l imnet i c region inssmal l numbers, mainly as young s tages , at a l l t imes . The numbers were a lmost constant except for a s l i ght increase in late A p r i l and a larger Increase at the end of the overturn i n June .

From co l l e c t i ons of the l i t t o ra l p lankton it was found that cyc lopo id copepods appeared there i n much greater numbers, and i n more mature stages than in the l imnet i c p lankton . During the peak per iods of cyc lopo ids the water was fa i r l y rough. It i s cons idered that these peaks could large ly be exp la ined by movements of the animal ( pass i v e l y or ac ­t ive ly ) into the l imnet ic region from the l i t t o ra l during t imes when the l i t t o ra l was disturbed by wave ac t i on . Consequent ly , In L a k e Pupuke, cyc lopo ids may be cons idered as members of the l i t t o r a l p lankton rather than as true l imnet ic s p e c i e s . (Al though some of the c y c l opo ids undoubtedly led a l imnet ic l i f e . )

N a u p l i i ( F i g . l a ) appeared to be of the ca lano id type and it seemed reasonable to assume that they belonged to Calamoecia. They d id not appear to show any increases corresponding to the highs in the pulses of the copepodite populat ion of Calamoecia, although any such increases may have been missed by occurr ing between sampl ing t imes . However there increase before the f i rst peak of Calamoecia and a smal ler increase after the second, whi le at the end of the overturn they increased w i th the other zooplankton members. From here, they increased s l ow l y to the end of the sampling per iod.

(chydorusC&lg. l b ) i s not normally reckoned on as be ing a true l imnetic c ras laoean, although It d id appear as such i n Pupuke (twice) during the sampl ing per iod; from March to A p r i l , and from J u l y to August when the sampl ing ended.

The numbers were very s m a l l compared to the other entomos-tracan populat ions, except for that of llyocryptus ( F i g . lb ) wh i ch was present during the latter part of the sampl ing period from ear ly J u l y to August .

Keratella cochleares ( F i g . lb ) was numer ica l ly greatest at the begini^ing of the sampl ing per iod, from when i t s numbers gradual ly became fewer t l t T m i d - J u n e when a s l ight increase occurred at the end of the over­turn. After th is increase K. cochlearis continued to decrease, so that at the end of sampl ing i t had disappeared complete ly .

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MARCH AfWL MA* JUNE JULY AUGUST

F I G . 1 a. Seasonal changes i n the numbers of the major entomostracan groups b. Seasonal changes i n the numbers of the l ess abundant p lankton —

minor entomostracans roti fers and mites

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Keratella quadrata ( F i g . lb ) exhib i ted two pu l s e s , the f i rs t wi th a peak in late March and the second with a peak in late June . From here K. quadrata dec l ined as did K. cochlearis and was not present i n raid-August . K. quadrata never reached as great numbers during i t s peaks as did K. cochlearis.

Asplanchnf/(Fig. lb ) was present i n s m a l l numbers in March and early AprilvJ>ukJr6m m i d - A p r i l to May it underwent a dramatic increase to reach a peak in ear ly May when it complete ly dominated the rot i feran fauna and was, after Calamoecia, the most abundant zooplankter present. There was an equal ly dramatic drop i n numbers over the rest of May and Asplan-chna eventual ly d isappeared by June . T h i s pulse was a fa i r l y t y p i c a l rotiferan one, and corresponded to the period when the lake was over­turning. At th i s time there was a decrease i n the numbers of phytoplankton and other zooplankton (on wh ich it may have fed), so that the increase i n Asplanchna would appear to be re lated to some phys i ochemica l change i n environment, such as a minor temperature change, to wh i ch rot i fers are part icular ly s e n s i t i v e .

Other rot i fers ( F i g . lb ) showed two pu l s es , one In late A p r i l the other in ear ly June , the peak in A p r i l being the larger. A smal ler increase a lso occurred i n m i d - J u n e . It i s poss ib l e that these peaks c ons i s t ed of different spec i es of ro t i f ers .

F i g . l b shows that a def inite s easona l s u c c e s s i o n cou ld be recognised i n the rot i feran populat ion . In i t i a l l y , Keratella cochlearis was dominant, but was fo l lowed by a smal ler peak of K. quadrata. T h e n i n late A p r i l some unident i f i ed rotifer(s) dominated and was c l o s e l y fo l lowed by a great increase i n numbers of Asplanchna. Then smal ler peaks of K. coch­learis, K. quadrata and an unident i f ied rotifer fo l lowed s u c c e s s i v e l y to the end of the sampl ing per iod.

Mites occured in the plankton during the samp l ing per iod from early A p r i l to August , exh ib i t ing peaks i n late A p r i l and J u n e . A s the mites were present during periods of rough water cond i t i ons , and as. they were found to be present i n l i t t o r a l co l l e c t i ons , it was cons idered that l i k e the cyc lopo id cope-pods they were members of the l i t t o ra l fauna wh ich had been dis lodged by rough weather.

Ceratium ( F i g . 2a) exhibi ted three pu lses of wh ich one (the second) was extremely marked. From March, the members dec l ined to a low in early A p r i l , after wh ich they increased to a peak i n late A p r i l de­c l i n ing again to reach a low l e v e l i n May and June . At the end of the over­turn i n late June , a smal ler peak was again es tab l i shed and at the end of the sampl ing per iod numbers were again d im in i sh ing .

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I , » I i I ' , ' MARCH APR! i MAY JUNE JULY AUGUST

1 — i . 1 1 I MARCH APRIL MAY JUNE JULY AUGUST

F I G . 2 a. Seasonal changes i n abundance of P lankton i c Pro tozoa and Phytop lankton b. Seasonal changes i n tota l zooplankton numbers

85

It must be pointed out however that the populat ion of Ceratium, l i k e that of the n a u p l i i , was subject to cons iderab le net error so that even i f counts were cons idered to be r e l a t i v e l y comparable, they ce r ta in ly d id not g ive a true ind i ca t i on of the abundance of these organisms, In re la t ion to Calamoecia for ins tance , an animal subject to l i t t l e net error.

The roti fers Branchionus and Euchlanis were present during March, whi le Filinia and Pedalia appeared in A p r i l . In May Branchionus, Euchlanis and Filinia had disappeared whi le i n ear ly June Euchlanis appeared. Towards the end of June a c o l o n i a l v o r t i c e l l i d , s im i l a r to Zoothamnion, appeared; as d id many Sarcodina s im i l a r to, or i n fact, Acttn-ophyrs sol or Actinosphaerium.

Phytop lankton abundance has been plotted qua l i t a t i v e l y i n F i g . 2a . T h i s figure refers mainly to Closterium and Staurastrum as most other phytoplankters were not reta ined in the f i l t e r . The large increase i n late May through to June occurred when the lake had taken up a state of com­plete c i r cu l a t i on . A t th i s time an obvious phytoplankton " b l o o m " was occurr ing . The water was a deep green co lour , the water surface g l i s tened and a " f i s h y " odour was most no t i ceab le . The " b l o o m " was ma in ly due to the great increase i n Closterium. There was a fa i r l y rapid decrease i n the phytoplankton, t i l l i n August there was so l i t t l e Closterium and Staur­astrum i n c o l l e c t i ons that they cou ld be detected only w i th d i f f i cu l t y .

T o t a l zoop lankton numbers are i n F i g . 2b. Welch (1948) states that i n temperate l akes of the second order (to which Pupuke i s s imi la r )

" t h e t o t a l annual production of p lankton often takes the form of a bimodal curve represent ing two maxima one i n spr ing and one i n a u t u m n "

" t h e large annual rythms (maxima and mimina) are the resu l t of con ­sp icuous increases and dec l ines i n ce r ta in , often few, p lankters accom­panied by those whose quanti tat ive changes have l i t t l e effect on the whole mass. "

In Lake Pupuke , because of the l i m i t s of the sampl ing period, i t was not poss ib l e to observe any spr ing inc rease . However the latter part of the summer minimum, an autumn increase and a winter l ow were observed.

From March to ear ly A p r i l ( F i g . 2b) the numbers were fa i r l y con ­stant. In late A p r i l there was a great increase i n numbers fo l lowed by a more gradual decrease to a low value at the end of May. There was then another (smaller) increase and dec l ine wh ich ended in ear ly J u l y . Then fol lowed a s low increase to the August populat ion l e v e l .

A s Calamoecia made up much of the p lankton numbers, the to ta l number of p lankters , apart from i t , have been plotted ( F i g . 2b) to obtain a

86

picture of the changes wh i ch occurred in the remainder of the populat ion .

Thus it can be seen that the f i rst peak cons i s t ed main ly of Calamoecia and Ceratium w i th rot i fers a l so p lay ing a part. However, the second peak was la rge ly due to Calamoecia a lone. T h i s peak co inc ided with the onset of the complete c i r cu l a t i on and the phytoplankton " b l o o m " , and as mentioned prev ious ly the herbivorous Calamoecia may have increased because of t h i s . Other p lankters p layed only a s m a l l part i n producing th is peak as a compar ison of the two diagrams in P i g . 2b w i l l show.

The f i rst populat ion peak occurred before any marked descent of the thermocline had occurred, and i t seemed that th i s increase was prob­ably due to ef fects other than those brought about by the d i s t r ibut ion of hypol imnet ic substances during the c i r cu la t i on per iod; un l ike the second peak. The subsequent s l i gh t increase after June was due main ly to the increase i n Bosmina; during th i s period the numbers of Calamoecia had remained constant at a f a i r l y low l e v e l .

While the s easona l changes descr ibed occurred i n 1966, it i s poss ib l e that i n the past they fo l lowed different courses , as they probably w i l l i n the future. However the resul ts show that over the part of the year studied, the zooplankton numbers i n L a k e Pupuke exh ib i t ed the main changes which have been found to be charac te r i s t i c of overseas l akes , as summarised in We lch 's statement prev ious ly .

2. V e r t i c a l D i s t r i bu t i on (F igures 3 to 5)

A s samples were taken as near to noon as po s s i b l e , these f igures represent the d i s t r ibut i on during the day on ly . D iurna l changes which occurred i n the v e r t i c a l d i s t r ibut i on are d i s cussed i n the next s e c t i on .

A t f irst the d i s c u s s i o n w i l l relate pr imar i ly to figure 5 wh ich shows the d i s t r ibu t i on of to ta l p lankton numbers over the s eason March to August . Secondly an out l ine of the general d is t r ibut ions manifested by the var ious spec i e s over the year w l U be g iven .

F i g . 3a shows that when the thermocline was deve loped, the majority of the plankton was present i n the ep i l imnion . On the 27th A p r i l and 13th May however, some of the plankton i s shown as extending below the thermocl ine to the lowest depth of sampl ing . It was shown that these plankters were introduced into the samples from these depths by a mal ­funct ioning of the sampl ing dev ice , and that at these dates no plankton was, i n rea l i t y , present below the thermocl ine.

Thus i t seemed reasonable to assume that the cutoff below the

87

thermocline was i n fact sharp, and i n rea l i t y no l i v i n g organisms (except mites, bacter ia , fungi, e tc . ) were present i n the hypol imnion when it ex­i s t ed . On 23rd March and 6th A p r i l when no samples were taken from below the thermocl ine, i t was assumed that the plankton was a l s o conta ined in the ep i l imnion .

What are the reasons for th i s cutoff? There are two poss ib l e exp lanat ions :

i ) Temperatures below the thermocl ine were too low for the p lankton to surv ive i n , or;

11) the bu i l d up or e l iminat ion of some chemica l substance(s)

as a consequence of the presence of a thermocl ine acted as a l im i t i ng factor .

During August the temperature from the surface to the lowest depth of sampl ing was s im i l a r to that of the hypo l imnion during May. Yet in August the plankton s t i l l thr ived and in fact a great increase i n the numbers of Bosmina had occurred . Thus it would seem that temperature was not the l im i t ing factor.

Therefore some chemica l substance must have been so depleted or increased i n concentrat ion i n the hypol imnion that i t l im i ted the v e r t i ­c a l d is t r ibut ion to the ep i l imn ion . Unfortunately, no chemica l readings were taken, but oxygen and su lph ide wh ich are common l im i t i ng factors , have been found to be depleted and increased respec t i ve l y i n the hypo-l imnia of New Zea land and overseas l a k e s .

After the overturn, on 29th June , the plankton was fa i r l y evenly d is t r ibuted , with a maximum at 25m. On the 6th J u l y the maximum had r i sen s l i gh t l y , but the ove ra l l d i s t r ibut ion was s t i l l uniform. However, during August , there was a concentrat ion into the upper waters (mainly the entomostraca); but the plankton was s t i l l obv ious ly present i n con­siderable numbers below the lowest depth of sampl ing . Why d id the p lank­ton come more to the surface at th is t ime? On the 27th June and 7th J u l y , because of the events of the overturn, strong ve r t i ca l currents would have existed wh ich would have tended to mix the populat ion evenly i n a ver t i ­c a l d irect ion to produce the observed d i s t r ibu t i on . After the per iod of ca lm weather of ear ly and m i d - A u g u s t , these currents would have d ied down, thus enabl ing the p lankters to r ise into the surface waters.

F i gures 3b to 5 show the v e r t i ca l d i s t r ibut ion of the i nd i v i dua l spec ies at se lec ted t imes during the sampl ing per iod. From these d i a ­grams and other observat ions, it was poss ib l e to make the fo l lowing general statements about the v e r t i ca l d i s t r ibut i on of the p lanktonic spec i es i n Lake Pupuke .

88

2 3 / 3 G/4 2 7 / 4 T 3 / 5 1 /6 1 5 / 6 2 9 / 6 6 / 7 1 6 / 8

= 2 0 0 = T r « r m o d i r t e p o s i t i o n TOTAL N U M B E R S O L .

: B ! A

1 6 / 4 / 6 6

c .

F I G . 3 a. V e r t i c a l d i s t r ibut ion of total plankton at the different sampl ing dates b - d . V e r t i c a l d is t r ibut ion on 23. 3. 66, 6. 4. 66, 27. 4. 66 r espec t i ve l y

89

1. Calamoecia had maximum dens i t i e s some distance above the thermocline, and even when the lake was homothermal, i t genera l ly preferred the surface waters above a depth of 20 m.

2. Bosmina was fa i r l y evenly d istr ibuted but wi th maximum numbers from the surface to 10 m.

3. N a u p l i i at f irst fo l lowed the thermocl ine down, after wh i ch they re­mained more i n the upper waters . Maximum dens i t i e s occurred at the same, or s l i gh t l y shal lower depths, than those where Calamoecia copepodites were at a maximum.

4. Cyclopoid copepods and llyocryptus were general ly found at depths below 10 m.

5. Chydorus was found i n the surface waters. 6. Ceratium and rot i fers preferred the upper waters, however maximum

dens i t i es occurred usua l l y at 5 m. or below rather than at the sur face. The lower l im i t s depended on the thermocl ine pos i t i on when the lake was s t ra t i f i ed .

7. Mi tes occurred i n , around, or below the thermocl ine. When no thermo­c l ine ex i s ted they s t i l l preferred the deeper water.

8. A l g a l d is t r ibut ion was re lated to ep i l imnion extent; their d i s t r ibut i on being contained by the presence of a thermocl ine wi th a s l i gh t tend­ency to maximum abundance i n the surface waters . When no thermo­c l ine was es tab l i shed they extended past the lowest depth of samp­l i n g .

The type of d i s t r ibut i on observed i n Pupuke would seem to be fa i r ly t y p i c a l of those found so far i n other New Zea l and l a k e s , ( Jo l l y ,

Symbols for v e r t i c a l d is t r ibut ion diagrams —

A a lgae

A s Asplanchna

B Bosmina

C a Calamoecia

C h Chydorus

C m Ceratium

C y cyc l opo ids

I llyocryptus

K c Keratella cochlearis

K q Keratella quadrata

M mites

N naup l i i

R roti fers

90

C y N YC A s R C m M A

M C m A

F I G . 4 a - c i . V e r t i c a l d i s t r ibut ion of p lankton on sampl ing dates 13. 5. 66, 1. 6. 66,

15. 6. 66 and 29 . 6. 66 respec t i ve ly

91

1951. B . T . Cunningham et. a l . 1952, I. A . E . B a y l y 1962. ), and indeed of l akes overseas . (Welch 1948, B i rge and Juday 1911. ) However the s ca r c i t y of information ava i lab le from other l akes i n New Zea l and enables only an incomplete comparison to be made between them and Lake Pupuke , and th is w i l l not be attempted here. Much further work must be done i n con ­junct ion wi th chemica l sampl ing before any re l i ab le c onc lus i ons may be made about the v e r t i ca l d i s t r ibut ion of p lankton i n New Zea land l a k e s .

3 . Studies on D iurna l Changes i n the V e r t i c a l D i s t r i bu t i on : (F igures 5c, 5d & 6)

A d iurnal sample was made on the 6th of J u l y . Samples were taken every 3 hours and thus f e l l into C u s h i n g ' s (1951) category of short time interva ls ( i . e. any in te rva l l e s s than 4 hours), which are cons idered best for studying d iurna l migrat ions.

Over the d iurna l sampl ing per iod, re lat ive l ight in tens i ty record­ings were made with a l ight meter, at the sur face . No apparatus was a v a i l ­able for making actua l measurements of l ight at the var ious sampl ing depths, but as l ight penetration was expected to be proport ional to l ight in tens i ty at the surface, the f igures obtained gave some i dea of changes in intens i ty which occurred i n the water. The o r i g ina l graph constructed from these resul ts i s not inc luded . L i gh t in tens i t y rose qu i ck l y at dawn between 0600 hrs . and 0800 hrs . to a fa i r l y constant day l e v e l , wi th a peak from 1100 hrs . to 1300 h r s . Intensity decreased rap id ly from 1600 h r s . to 1800 hrs . to the darkest hour, at 2100 hrs . The moon had r isen by midnight .

Welch states that the fo l l ow ing plankters may be expected to exhibi t v e r t i ca l d iurnal movements;

1. P l ank ton i c Crustacea 2. Cer ta in roti fers 3 . Diatoms and d ino f lage l la tes

Thus the spec i es se lec ted for study were Calamoecia lucasi, Bosmina meridionalis, Cy c l opo i d copepods, Keratella quadrata, K. cochlearis and Ceratium hirundinella.

Prev ious workers have found that i n a s ing le spec i es the different age groups and sexes may exhibi t different migrat ional cha rac t e r i s t i c s . To discover whether any such di f ferences occurred in the spec i e s s tud ied , d i s t inc t i on wi th in Calamoecia was made between —

1. The tota l populat ion 2. E g g bearing females 3. N a u p l i i

and w i th in the Bosmina populat ion between — 1. Those not bearing eggs 2. Egg bearing females

F I G . 5 a - b . V e r t i c a l d i s t r ibut i on of plankton 6. 7. 66 and 16. 8. 66 r espec t i ve l y c - d . D iurna l changes in v e r t i c a l d i s t r ibut ion

93

With juven i l e c yc l opo ids , roti fers and Ceratium, no such d i s t inc t i ons were poss i b l e .

From the resul ts f igures 5 and 6 have been const ructed . In the construct ion of the f igures, the ca tch at each depth has been converted to a percentage of the to ta l c a t c h .

Calamoecia ( total populat ion F i g . 5c) showed a v e r t i c a l migration which however d id not inc lude the whole popula t ion . At 0300 most i n d i v i d ­ua ls occurred at 20 m. T h i s would appear to be due to a s ink ing from the midnight depth which was 10 m. to 15 m. At 0600, when l ight increased , most of the populat ion migrated to an even lower l e v e l (25 m. ). However some d id migrate to the surface as seen i n the increase at 0 m. and 10 m . Thus some, but not a l l of the populat ion exhibi ted a dawn r i s e . B y 0900 the populat ion was concentrated i n the deeper waters, mainly at 25 m., as i t was at noon at 28 m., except that at th i s time a few had migrated upwards . B y 1500 more of the populat ion had gone deeper, and such a s i n k i n g d id not correspond to any increase in l ight in tens i t y at the sur face . A t 1800, when l ight intens i ty dropped, the populat ion segregated into two groups. One, wh ich rose qu i ck l y to the surface, gave a twi l ight r i s e , and the other rose more s l ow l y i f at a l l . Subsequent ly , the latter group rose to 20 m . at 2100 and reached a maximum l e v e l of 15 m. at midnight. The tw i l i ght m i ­grating group rose even c l ose r to the surface at 2100 when l ight was at a minimum, and at midnight some of the populat ion showed a midnight s i nk ing to 10 m.

Egg bearing females of Calamoecia ( F i g . 5d) showed a migrat ion s imi la r to that above, except that no dawn r i se appeared. The group at 5m. at 0900 i s a puz z l e . Al though it cou ld have cons i s t ed of a few ind i v i dua l s showing pos i t i ve he l io tropism to the l ight in tens i ty at 0900, other factors may have brought them to the sur face . A t noon the populat ion was concen­trated below 15 m. and from 1500 onwards they showed exact ly the changes descr ibed above, except that no midnight s ink ing occurred.

Thus , although some of the copepodite stages of Calamoecia d i d not migrate some (at least ) of the adult s tages underwent a d iurna l v e r t i c a l migrat ion.

N a u p l i i ( F i g . 6a) had a maximum day depth at noon. B y 1500 the populat ion had r i sen cons iderab ly so that maxima were present at 20 m. and 10 m. depth. B y 1800 nearly a l l of the populat ion appeared i n the sur­face waters, and th is may have been a response to the low l ight in tens i ty at the t ime. At 2100 the main body of the populat ion had sunk so that a maximum occurred at 5 m. If the naup l i i are to be considered as responding to an optimum low l ight in tens i ty , then when th is intens i ty had passed

94

2 0 % C L

C Y C L O P O I D C O P E P O D S

B o s m i n a - W I T H O U T E G G S

2 0 %

P I G . 6

a - d . D iurna l changes in v e r t i c a l d i s t r ibut ion

95

(2100) they would be expected to stop responding and to s ink , as was found to occur . The r i se at midnight cou ld therefore have been a response to the low l ight in tens i t i es provided by the moon wh ich has r i sen by then . The d is t r ibut ion observed at 0300, 0600 and 0900 are l e ss cons is tent with the above v iews , there was no d i s t inc t dawn r ise at 0600, rather there was a r i se at 0900.

The cyc l opo id copepods ( F i g . 6b) were a l l juven i l e s tages and were not ident i f i ab l e . A s d i s cuss ed above, these are to be cons idered as the young of l i t t o ra l c y c l opo i d s . T h i s be ing so , then two spec i es would be present, Macrocyclops albidus and Paracyclops fimbriatus. F i g . 6b shows two migrating groups, one 10 m. below the other, and th is i s cons is tent with the v iew that the l imnet ic c y c l opo id populat ion i s composed of the young of the above s p e c i e s . The upper group showed the better sequence, and the gaps at noon and 1800 in the lower group poss ib l y occurred because plankters were present below the lowest depth of sampl ing . The migration of the upper group w i l l be descr ibed , it being assumed that the lower group migrated s im i l a r l y , but at greater depths; ( i . e. the optimum l ight in tens i ty was l e s s ) . From 0300 the copepods dropped to the greatest day depth at noon and exhib i ted no dawn r i s e . After noon they rose from the day depth to reach the maximum day l e v e l at 2100 when they were present at the sur face . At midnight a s l i gh t s i n k i n g occurred. Thus these copepods showed a c lear d iurna l migrat ion of 15 m. extent.

The Bosmina populat ion ( F i g . 6 c & d) was sp l i t into two groups for study, those wi th and those without eggs. Those wi th eggs w i l l be c on ­s idered f irst ( F i g . 6d). Dashed l ines represent the l i n e s the path a t y p i c a l plankter may have taken.

At 0300 the main body of the populat ion occurred at 15 m. A t 0600 a marked dawn r ise occurred, but by 0900 the populat ion had once more returned to deeper waters th is time to 20 m. The noon d i s t r ibut i on was unusual with a s l i gh t maximum at the surface, but general ly appears to be uniform. T h i s d is t r ibut ion i s incons is tent wi th the rest of migrat ion pattern, and with the migration patterns of other parts of the Bosmina populat ion and may be erroneously i nd i ca t ed . Dur ing the afternoon the populat ion rose to a maximum l eve l at 2100. A t midnight s i n k i n g occurred and th i s app­arently continued t i l l the l e ve l of 0300 was reached.

A n explanat ion for th i s pattern of migration would be that egg bearing Bosmina females show a pos i t i ve he l iotropism to a low l ight i n ­tens i ty and a negative he l io t rop ism to high l ight i n t ens i t i e s . C u s h i n g (1951) states that the midnight s ink ing phenomenon may be due to e i ther a true s i n k i n g of animals or to a reduct ion i n the upwards swimming response because of the disappearance of the low l i ght in tens i ty at midnight. He

96

states that i f a moon i s up (as occurred i n the example under cons iderat ion ) then the plankters might be expected to stay at the sur face. T h i s d id not occur i n the present c a s e . The depth reached after the dawn r i se would be expected to be the result of an ac t i ve downswimming away from l i gh t . A s the depth reached at 0300 after s i nk ing s ince 2100 was cons iderab ly l e s s , the an imals were thus not a c t i v e l y swimming to this depth; and i t may be conc luded therefore that they remained inac t i ve and rea l l y sank. The depth reached between 2100 and 0300 was too great to be expla ined by random movement, but too s m a l l to be exp la ined by act ive downward swimming.

The rest of the Bosmina populat ion ( P i g . 6c) appeared to be com­posed of two groups, each exh ib i t ing a different migratory pattern. These are shown by dashed l i n es " a " and " b " the pattern undergone by " b " would appear to be the same as that descr ibed for females w i th eggs, and this group may have cons i s t ed of females which had lost their eggs either by natural shedding or during the f i l t e r ing operat ion. The second group represented by l ine " a " had a b a s i c a l l y s imi la r pattern, but ins tead of descending after 0600 they cont inued to ascend t i l l 0900 after wh i ch de ­scent occurred. They d id not have as great a day depth as group " b " . Therefore in the tw i l i ght migrat ion they reached the surface ear l i e r than group " b " ( i . e. at 1800) wh i ch reached the surface at 2100 h r s . Both groups then underwent a midnight s i n k i n g . Such a pattern cou ld be exp la ined on the grounds of react ion to l ight i f group " a " had a higher optimum l ight in tens i ty than group " b " .

Keratella showed irregular migration but it appeared to occur at maximum depths during the day whi le coming to the surface at night . Keratella cochlearis showed a migrat ion pattern i d en t i ca l with that of K. quadrata.

Ceratium a l so showed maximum depth during the day whi le r i s i n g s l i gh t l y at night . T h i s smal ler range was to be expected because of i t s rather l im i ted powers of locomot ion.

The pattern of migrat ions of the spec i es studied were i n agree­ment with that out l ined by Cush ing (1950) as being the general picture obtained from the resu l ts of workers us ing short time in t e r va l s . T h i s pattern c ons i s t s of

1. Ascent towards the surface from the day depth, the animals not necessa r i l y reaching the surface but merely r i s ing to a higher l e v e l .

2. Departure from the surface at or before midnight. 3 . Return to the surface just before dawn. 4. A sharp descent to the day depth when the sunl ight

starts to penetrate the water.

97

5. The var iab le day depth.

The resu l ts obtained a l s o seem to be cons is tent w i th the f o l l ow ing conc lus i ons summarised by C u s h i n g .

" T h e s e different parts of the migrat ion have been l inked together in a s ing le and cont inuous process . Ascent i n the evening and descent i n the morning become the cont inuous change i n day depth with the changing penetration of l i ght ; the midnight s ink ing is assumed to be due to a pass ive state i n f u l l darkness ; the dawn r i se i s a return by the animals to the mean optimum l ight in tens i ty for the populat ion. The v e r t i c a l migration of p lanktonic Crustacea i s mediated by the change i n l ight pen­etration during the day. An ima l s aggregate i n an optimum band of l ight i n ­tens i ty . They have the capac i t y of moving phototact ica l ly , and i n f u l l darkness they move probably at min imal speeds . Migrat ions between per iods of f u l l darkness may be operated by phototax is , photok ines is or by a com­binat ion of these mechanisms. "

D ISCUSSION & S U M M A R Y

L a k e Pupuke was shown to be a fa i r ly t y p i c a l second order l ake , and the moderately dense p lankton populat ion which i t supported ind ica ted i t s eutrophic nature.

T h i s plankton populat ion was in a broad sense s imi la r to those found i n other l akes in New Zea l and , be ing composed mainly of micro-crustaceans and rot i fers. J o l l y (1952) makes the fo l l ow ing remarks " B r a d y (1906) and Henry (1924) note that few spec i es of Entomostraca occur in New Zea land l a k e s . In comparison wi th l akes such as Mendota — 12 spec i es (Birge , 1897), N i p i s s i n g - 12 (Langford, 1938), Luce rne — 9 (Worthington, 1931), Lough Derg - 67 (Southern and Gardner, 1926) , Lake Hayes shows a charac te r i s t i c l ack of d i v e r s i t y " F i v e genera of ento­mostraca were recorded from the p lankton of Lake Hayes , and i n Pupuke s t i l l fewer were found; (3 spec i es — Calamoecia, Bosmina and " C y c l o p s " ) . The number of spec ies found by B a y l y i n L a k e Aroarotamahine was f ive , and it i s interest ing to note that spec i es of Daphnia wh ich were present i n the l akes studied by J o l l y and B a y l y were absent from L a k e P u p u k e . However, only part of the year ly c y c l e was observed and other spec i es not noted over the sampl ing per iod may have been present i n the sp r ing and summer months.

The plankton in L a k e Pupuke was found to exhib i t the phenomenon of seasona l per iod ic i ty ; however complete disappearance was conf ined mainly to the l e ss abundant groups whi le the dominant spec i es were present a l l through the sampl ing periods even though their numbers d id f luctuate markedly .

98

The plankton was not found to be evenly d istr ibuted throughout the l a k e ; v e r t i ca l di f ferences i n the p lankton d is tr ibut ion were marked, e spec i a l l y when thermal s t ra t i f i ca t i on was deve loped. In winter, however, the d i s t r ibut ion was general ly more even. Observat ions over a 24-hour pariod showed that the v e r t i c a l d i s t r ibut ion patterns changed d i u r n a l l y . P e w studies of th is k ind have been made in New Zea l and , the only two publ ished accounts being those of J o l l y (1952) and B a y l y (1062). Calamoecia had not been s tud ied i n th is respect before, and it was interes t ing to note that some stages of i ts l i fe c y c l e , at l eas t , underwent a fa i r ly t y p i c a l c y c l e of d iurna l migrat ion. The observat ions on the roti fers and Ceratium a l so appeared to be the f irst of their k ind from New Zea l and . Bosmina, which was a l so s tudied by B a y l y , was found to exhib i t a migration as i t d id i n the Mayor Is land l akes . Other spec i e s of Bosmina have been known to show diurnal v e r t i c a l migrat ions; and some of these have been documented (Cush ing , 1951) .

Pour of the spec i es s tud ied , namely Bosmina, Asplanchna, Keratella and Ceratium have been found to exhibit seasona l changes of body form in the Northern Hemisphere l a k e s . Welch suggests that th is may be because of marked temperature di f ferences before and after the autumn overturn. A s th is was the period s tud ied , any such changes would have been noted. However, such was not the case ; the spec i es noted above d id not show any seasona l changes i n body form i n L a k e Pupuke .

R E F E R E N C E S

Barker , M. A . 1966. Tane 12: 93 - 95.

B a y l y , I. A . E . 1962. " E c o l o g i c a l s tudies on New Zea land Lacus t r i n e Z o o p l a n k t o n " . Aust. J. Mar. Freshw. Res., 13 (1): 143 - 197.

Cunningham, B . I., 1953. " A survey of the Western C o a s t a l Dune l ake s of Moar, N . T . , Tor r i e , the North I s land , New Z e a l a n d " . Aust. J. Mar. A . W., & Parr , P . J . Freshw. Res. 4 (1): 343 - 386.

C u s h i n g , D. H . 1951 " T h e V e r t i c a l Migrat ion of P l ank ton i c C r u s t a c e a " . Biol. Rev. 26: 158 - 192.

F i s h , G . R. 1963. " L i m n o l o g i c a l Cond i t i ons and Growth of Trout i n Three L a k e s near R o t o r u a " . Proc. N. Z. Ecol. Soc. 10.

J o l l y , V . H . 1952. " A pre l iminary study of the L imnology of Lake H a y e s " . Aust. J. Mar. Freshw. Res. 3 (1): 7 4 - 9 1 .

Welch, P . S. 1952. " L i m n o l o g y " Second E d . McGraw H i l l .