Phosphorus in marine zooplankton

Water Research Pergamon Press 1973. Vol. 7, pp. 93-110. Printed in Great Britain

PHOSPHORUS IN MARINE ZOOPLANKTON

E. D. S. CORNER

Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth, U.K.

Abstract--In the euphotic zone, phosphorus compounds dissolved in sea water are utilized by growing plants, many of which are subsequently eaten by herbivorous zooplankton and the dietary phosphorus invested partly in growth and egg production, partly released in insoluble form as faecal pellets and partly metabolized. The fraction metabolized is excreted back into the sea water mainly as inorganic phosphate, which is again available as a nutrient for the plants.

Quantitative aspects of this cyclic process are discussed with particular reference to the Calanoid copepods, animals of central importance to the marine food web in several sea areas. Topics include: (1) laboratory and field investigations of the assimilation of dietary phosphorus and the efficiency of this process; (2) the rates at which zooplankton release soluble forms of phosphorus in relation to species, body size, food availability and season; (3) the importance of zooplankton in regenerating phosphorus compounds that may be used by the plant population; (4) the daily rations of phosphorus captured by zooplankton; (5) the total amounts and chemical forms of phosphorus in the animals; (6) growth of zooplankton and the use of N: P ratios in animals, diets and excretion products in estimations of gross growth efficiency interms of phosphorus; (7) future studies.

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

STUDIES on the metabol ism of phosphorus in zoop lank ton have been mainly q u a n -

titative, and st imulated by the undoub ted importance of these animals in the mar ine phosphorus cycle. A simplified version of this is shown in FIG. 1.

Zooplonk~oil

Herbivores Eupho~ic .,,o e' and

O~ ca rl l ivores z o. e ~ , o ~ ~ . . . - - - - ~ ~ ~

~,6~ ~ 'i Defri ~us ~ / I o~d

x5 ' ~ ~ bocfer lo k~ "a 7 , feeders

< 1/T f I_ ;I Oeod II Foecol I O~sso,~e~ l" il °r~°~'s~s I I ~e,,e,s

t ~. >{ Bo~,er~o I

IT I "0~,ego'e, I _ _ . _ _ t . . . . . . . . ..~ . . . . . . . .

Benmic region LBenmic orgoilisms J

Fro. 1. Aspects of the marine phosphorus cycle.

93

94 E.D.S. CORNER

It is clear from FIG. 1 that one important feature of the cycle is the conversion of dissolved phosphorus compounds into particulate phosphorus either in the form of living organisms such as plants and animals, or as detritus. There is conflicting evidence concerning the extent to which zooplankton may take up dissolved inorganic phosphate directly from sea water (vide KOBAYASr-tt et al., 1972). However, it is un- likely that the animals play a significant part in this process compared with plants and bacteria: instead, they are thought to obtain most of their dietary phosphorus from particulate material such as phytoplankton, detritus and bacteria.

Nutrient regeneration, a process by which organic phosphorus in particulate form is converted back into soluble inorganic phosphorus, is another important feature of the cycle and one in which zooplankton are usually involved to a large extent, particularly in comparison with other organisms. Thus, the role of bacteria has recently been reviewed by JOHANNES (1968), who concludes that only a small proportion of nutrient regeneration in lakes and oceans is due directly to these organisms; at certain times they may even compete with plant cells for dissolved nutrients. The main contribution of bacteria is more likely to be that of supplying food for animals such as protozoans, which are extremely active in terms of phosphorus excretion. Moreover, Johannes considers that it is bacteria-feeders, and not bacteria themselves, that accomplish much of the non-autolytic regeneration of nutrients from dead material.

Another aspect of the phosphorus cycle that is now attracting attention, but which cannot be dealt with in detail in the present account, is the possibility that non-living particulate material present in the sea may not entirely consist of dead organisms, but could include particles resulting from physico-chemical processes. For example, RILEY (1970) suggests that thin films of particulate material are produced by adsorp- tion of dissolved organic substances on bubbles, or other sub-surface objects. This material may then be aggregated into particles large enough to be retained by filter- feeding animals. It would be interesting to know whether this material is of nutritive value, but so far tests have been made only with Artemia salina (BAYLOR and SUT- CLIFFE, 1963), which is not a species truly representative of the zooplankton. However, evidence was found that detritus produced in sea water by the action of rising bubbles was of a certain nutritive value to these animals, and there is obviously a need for studying the importance of this detritus as a food for zooplankton.

It must be emphasized that the marine phosphorus cycle is by no means confined to the euphotic zone of the sea, where conditions near the surface favour the growth of phytoplankton. Thus, animals near the surface and those at depths cannot be considered as isolated. For example, inter-linking can arise through a number of overlapping vertical migrations ('VINOGRADOV, 1962). A further example of this inter- linking is the utilization by animals at depth of undigested food materials released in the form of faecal pellets by animals feeding near the surface. The importance of this latter process as a feature of marine ecology has led to many investigations both in the field and the laboratory; and the problem of food assimilation by zooplankton pro- vides a suitable starting point for assessing the role of these animals in the marine phosphorus cycle.

ASSIMILATION OF PHOSPHORUS BY ZOOPLANKTON

Assimilation efficiency, i.e. the percentage of captured food digested and absorbed by the animals, has been measured using many species of zooplankton; but the first

Phosphorus in Marine Zooplankton 95

estimate, in terms of dietary phosphorus, was based on field observations made at Station L4 in the approaches to Plymouth Sound (HARVEY et al., 1935). Total plant production was estimated from the decrease in soluble phosphate in the sea water, which fell by 7 mg P m-3 over a period of 60 days. Thus, the average daily production of plant phosphorus was 0-11 mg m -3. On any one day during this 60-day period, the zooplankton present contained an average of 0.29 mg P, or roughly two and a half times that of the average daily plant production. Finding no evidence of diatom sinkage, Harvey et al. concluded that all the plant crop had been grazed by the zooplankton, each herbivore consuming 40 per cent of its body phosphorus daily. Further calculations (HARVEY, 1950), based on laboratory measurements of growth and respiration rates, showed that only 11-14 per cent of the body weight of the zooplankton was needed daily to replace respiratory losses and ensure growth. Thus, these findings indicated that about two-thirds of the daily ration of food captured by the animals must have been unassimilated: their assimilation efficiency was therefore 33 per cent.

The observations of HARVEY et al. (1935) led BEKLEMISHEV (1957, 1962) to put for- ward his theory of "superfluous feeding", according to which zooplankton release large quantities of undigested food as faecal pellets when the phytoplankton population in the sea rises above a certain critical level. Other observations by Harvey et al. lent additional support to this view. Thus, large numbers of faecal pellets were found in the sea at a time when the zooplankton were actively grazing and these pellets contained undigested plant cells. Moreover, the number of pellets produced was closely related to the level of the plant population.

Because the assimilation efficiency of zooplankton is a critical factor in the overall process by which plant food in the sea is converted into animal tissue, it has been the subject of numerous laboratory studies, several of which have dealt with the assimilation of dietary phosphorus. The first of these was made by MARSHALL and ORR (1955a), who cultured various diatoms and phytoflagellates in media containing radio-active phosphorus, fed the [32p]-labelled diets to the marine copepod Calanusfinmarchicus, over a period of 24 h, and measured the radioactivity in the animal body, eggs and faecal pellets. The sum of these three fractions was taken as the total quantity of 32p captured by the animals; and the amount in the body and eggs, when expressed as a percentage of the total quantity captured, was assumed to be the assimilation efficiency. It should be noted that at least one fraction of the captured phosphorus-- namely, the amount excreted in soluble form as an end-product of metabolism--was not included as part of the total quantity of phosphorus assimilated: to this extent therefore, the values obtained for assimilation efficiency were underestimated. Even so, they were much higher than expected. Thus, the data in TABLE 1 show that for diatoms and dinoflagellates the average assimilation efficiency over a wide range of food concentrations was never less than 60 per cent, and in some cases was higher than 90 per cent. True, certain species of flagellate were less well digested, 44-5 per cent being the value for Dicrateria inornata and 49.5 per cent for Chromulina pusilla, but these were exceptions: the other 7 species examined gave average values in the range 69.8-95"9 per cent.

A further important finding by MARSHALL and ORR (1955a) was that the percentage assimilation of labelled phosphorus changed very little with food concentration. Thus, in experiments with the diatom Skele tonema costatum (TABLE 2) a 20-fold increase

W.R. 7/I-2--'-O

96 E. D. S. CORNER

TABLE ], PERCENTAGE ASSLMILATION OF DIETARY PHOSPHORUS BY Ca[allu$ (Data from ~[ARSHALL and ORR, 1955a)

No. of species Assimilation Diet tested Cells m l - t efficiency ( ~ )

Diatoms 4 26--62,074 71.2-94'0 Dinoflagellates 6 167-7548 60.5-92'0 Flagellates 9 1335-833,000 44.5-95"9

in the level of plant food caused only a 10 per cent reduction in the proportion of captured food assimilated. It is important to note that the lowest concentration of Skeletonema cells used in these experiments was much higher than the rather low value of I0-100 cells 1-1 quoted by BEKLEMISHEV (1962) as representing the maximum above which "superfluous feeding" is presumed to take place.

TABLE 2. PERCENTAGE ASSIMILATION OF DIETARY PHOSPHORUS AT DIFFERENT FOOD LEVELS

(Data from MARSHALL and ORR, 1955a) for Skeletonema costatum as the diet

Average assimilation Ceils ml- 1 efficiency ( ~ )

• 288,000 54"5 72,000 57.3 14,000 61 '5

Having obtained data for phosphorus assimilation by adult Calanus, MARSHALL and ORR (1956) again used [32P]-labelled diets in studies involving younger stages. They found that assimilation efficiency was again very high (see TABLE 3)" indeed, with two of the diets, naupliar stages were able to assimilate practically all the food they captured. The same technique has also been used in studies with a different species of copepod--Temora longicornis--and once again high assimilation efficiencies were found, averaging 77 per cent for a diet of S. costatum (BERNER, 1962).

TABLE 3. PERCENTAGE ASSIMILATION OF DIETARY PHOSPHORUS BY YOUNG STAGES OF Calanus

(Data from MARSHALL and ORR, 1956)

Average assimilation Diet Stage efficiency ( ~ )

Skeletonema costatum CI 68-9 (266,000 cells m l - t )

Ditylum brightwellii NVI 94.8 (57 cells ml- t) CII 80'8

Syracosphaera elongata NIII-NVI 98-I-99.7 (1250 cells ml- 1) CII 98.1


Several criticisms could be made of the tracer-isotope method. One, already mentioned, is that the quantity of phosphorus released in soluble form by the animals was not included in the calculations; another is that, unless there was a complete recovery of all faecal material, percentage assimilation would have been overestimated; a third is that the method measured only the assimilation of dietary compounds labelled with 32p. Nevertheless, there is now a growing body of evidence, from studies using several different methods, that supports the original findings of MARSHALL and ORg (1955a) and also shows that high values for assimilation efficiency apply to dietary constituents other than phosphorus (TABLE 4).

TABLE 4. PERCENTAGE ASSIMILATION OF DIETARY CONSrlTUENTS BY CALANOID COPEPODS

Species Food Method ~ Assimilation Reference

Cryptomonas sp. Tracer isotope 53-78 MARSHALL and 14C ORg (1955b)

] Skeletonema 1 ~C 60-75 / costatum

Calanus finmarchicus ~ Skeletonema Chemical 57.5-67.5 CORNER et al. (1967) [ costatum analysis |Skeletonema "Ratio method" 53.8-64.4 CORNER et al. (1967) \ costatum

C. helgolandicus Seston Chemical analysis 74-91 CORNER (1961) C. hyperboreus Thalassiosira "Ratio method" 70 CONOVER (1966a)

fluviatilis C. hyperboreus Seston "Ratio method" 67 CONOVER (1966b)

The methods used in these studies have been reviewed elsewhere ( C O R N E R and COWEr, 1968; CORNER and DAVIES, 1971), but it is worth considering briefly the "ratio" method of CONOVER (1966a) as this has the great advantage that the quantitative collection of faecal pellets is unnecessary. The method depends on the assumption that only the organic fraction of the food is sensibly affected by the digestion process: the inorganic fraction is released as faecal pellets. It is therefore only necessary to obtain the ash-free dry weight: dry weight ratios for samples of the food and the faeces to calculate the percentage assimilation of the organic fraction. Two further assumptions were made. The first was that the proportions of organic and inorganic material ingested by the animals were the same as those occurring in the natural food: the animal did not select the organic fraction in preference to the inorganic. The second was that no material, either organic or inorganic, was released from the gut in soluble form, but only as faecal pellets. None of the assumptions was tested: nevertheless, CONOVER (1966a,b) obtained values for the assimilation efficiency of Calanus hyperboreus that were in good agreement with those estimated by other techniques. He also found, in accordance with the data of MARSHALL and ORR (1955a), that the percentage of captured food assimilated was independent of the concentration of food available; and, further, that it did not vary with the amount of food ingested. This latter observation has now been confirmed by KblblALEVA (quoted by SUSHCHENYA, 1970) for A. salina feeding on Dunaliella sp. and for Calanus helgolandicus feeding on Biddulphia sinensis (CORNER et al., 1972: see FIG. 2).

98 E.D.S. COR.~Ert

T

0 E

Z

::L

I0

0

°

I I I 5 ] 0 15

#g N capl'ured (0nirnal)-f (day) "I

I 2 0

FIG. 2. Relationship between quantities of food captured and rejected as faecal pellets by Calanus feeding on Biddulphia. Y = 0-659X; standard error of line = 0.02; correlation

coefficient = 0.98.

By use of what may loosely be called the "growth efficiency" method, BUTLER et al. (1970) estimated the percentage of captured phosphorus assimilated by Calanus spp. during a spring diatom increase in the Clyde, when plant food was well in excess of the concentration quoted by BEKLEMISREV (1962) as inducing superfluous feeding. Adult animals belonging to the first generation of the year were collected each day during the diatom bloom and analysed for body phosphorus and body nitrogen so that the daily increase in these quantities could be calculated. Laboratory experiments were then carried out to measure the daily amounts of phosphorus and nitrogen excreted in soluble form as end-products of metabolism. Use was made of the two equations:

DNQN = TN + WN (1)

DpQp --- Tp q- Wp (2)

where DN and Dp are the percentages of captured nitrogen and phosphorus digested, QN and Qp are the daily rations captured, TN and Tp are the daily amounts metabolized, and WN and Wp are the daily quantities invested in growth. The ratio N:P (by weight) was determined for the soluble excretory products, the plant food, the faecal pellets and the animal growth; and, by using these various ratios together with equations (1) and (2), values for DN and Dp were calculated as 62-4 and 77.0 per cent respectively. This value of 77 per cent for the percentage of captured phosphorus digested is very close to those found in the laboratory experiments of MARSHALL and ORR (1955a) but conflicts with the poor levels of assimilation (25-33 per cent) deduced by BEKLEMISHEV (1962): indeed, the data as a whole imply that when food is plentiful-- and during the period of this study the average concentration of plant cells was over 5 million l - l - - a faster turnover of nutrients, particularly phosphorus, had more

P hospho rus in Mar ine Zoop l ank ton 99

meaning than superfluous feeding. Thus, it is clear from the data in TABLE 5 that the quantity of phosphorus excreted in soluble form was alone greater than the combined amounts lost as faecal material and invested in growth.

TABLE 5. PERCENTAGE DAILY RATION LOST AS FAECAL PELLETS AND RELEASED IN SOLUBLE FORM BY Calanus

FEEDING ON A SPRING DIATOM INCREASE (Data f rom BUTLER et al., 1970)

Daily rat ion P N

As faecal pellets 23.0 37.5 As soluble excret ion 59.8 35.7 Available for g rowth 17"2 26-8

P H O S P H O R U S E X C R E T I O N B Y Z O O P L A N K T O N

The quantities of soluble phosphorus excreted by zooplankton are usually estimated as the increase in the phosphorus content of sea water containing the animals. The experiments are normally of short duration in order to reduce the effects of bacteria; but the number of animals needed to produce a large enough change in phosphorus content for reliable measurement is greater than the population density found in nature.

From the ecological standpoint it is necessary to know how phosphorus excretion by animals varies with food level, and attempts have been made to estimate the quantities excreted by animals when actively feeding. This type of experiment is complicated by two factors: firstly, it is necessary to know to what extent the algal food may change the levels of solubl e phosphorus compounds in sea water; secondly there is the question of whether those released by the animal represent end-products of metabolism, or soluble undigested material released from the gut together with faecal pellets. The first of these difficulties can usually be overcome by the use of adequate controls. The second possibility has not yet been thoroughly investigated for zooplankton in general, although BurgER et al. (1970) found no significant difference in the levels of phosphorus compounds excreted by Calanus with food in the gut and by animals with the guts empty.

The variation in phosphorus excretion with season has been studied by BUTLER et al. (1970) using Calanus captured in the Clyde Sea-area and has been found to resemble the seasonal variation in available food (see FIG. 3). It could be argued that conditions in the sea favouring an increased supply of plant food induce the animals to increase the metabolic rate: in other words, both high levels of food and high levels of phosphorus excretion have some common cause. However, the results obtained in several laboratory investigations indicate that, compared with unfed animals, actively feeding Calanus release significantly greater quantities of soluble phosphorus compounds. For example, HARGRAVE and GEEN (1968) found that the copepod Acartia tonsa excreted about 40 per cent more phosphorus when fed, and even greater increases were found in experiments with other species. Likewise, CORNER et al. (1972) have shown that C. helgolandicus feeding on high concentrations of B. sinensis excrete 34 per cent more phosphorus than starved controls.

100 E . D . S . C O R N E R

0-4

g 3"

o ~ 0-2

12_

7 &, "~ 1.5

T m o .E_ c o

Z 0"5

Old, (~lenerati0n of Cf' .V's

Old generation of

New qenerofion of o I. and V's

Mixed ~1 First gener Firsf ~ner(3fian of (2

O'7

0"5

I

"0

12

o x

"- v

0.1 [ t.)

FIG. 3. Changes in nitrogen and phosphorus excretion (as tzg animal-~ day -t) by Calanus during a spring diatom increase. O ©, females; • • , males; y y , stage V. Stippled areas show chlorophyll a in 5 I. of sea water (Elcra 66~ ) in a i0 ml acetone extract.

Hatched areas show diatom counts 1-~. Horizontal scale expanded during April.

Another factor influencing the rate of phosphorus excretion is body weight. JOHANNES (1964a) drew attention to earlier reports that microzooplankton frequently account for a substantial fraction, sometimes exceeding 50 per cent, of the total animal biomass in certain coastal areas. He expressed excretion in terms of the time taken by an animal to release an amount of dissolved phosphorus equal to its total body phosphorus content (Body Equivalent Excretion Time, or BEET) and found that this value decreased with decreasing body size. For example, whereas a lamelli- branch weighing 12 g had a BEET of 438 days, that of a ciliate weighing only 0-4 × 10-3/zg had a BEET of only 14 min. Further work (JoHaNNES, 1964b) confirmed that certain marine protozoa excreted phosphorus at a very rapid rate and that although these animals were present only as a minor fraction of the marine faunal biomass they could still be responsible for a major quantity of the dissolved phosphorus excretion. It is in the benthic region that nutrient regeneration by protozoa is of greatest

Phosphorus in Marine Zooplankton I01

significance; but certain protozoans, such as heterotrophic flagellates, are numerous in the euphotic zone.

Many studies have been made of oxygen uptake by zooplankton and the data in terms of body weight are such that heavier animals respire at rates lower than those of lighter animals of the same species. Although comparatively few studies have been made on phosphorus excretion, there is some evidence that the same relationship applies. For example, HARGRAVE and GEEN (1968) found that phosphorus excretion by the young naupliar stages of A. tonsa was some 40 per cent higher than that of the adult animals in terms of body weight.

Quantitative data for phosphorus excretion by various species of zooplankton are summarized in TABLE 6 and show that the turnover of phosphorus by these animals can sometimes be remarkably high.

TABLE 6. LEVELS OF SOLUBLE PHOSPHORUS RELEASED BY ZOOPLANKTON

Phosphorus excretion daily Species Season [p.g(mg body wt)- t] % Body P Reference

Acartia spp. Spring 11 130 HARRIS (1959) Acartia spp. Spring 2"4 -- MARTIN (1968) Acartia spp. Autumn 6"6 -- MARTIN (1968)

Acartia nauplii 1 1"4 -- J t Acartia CII-IV 1"3 - - Acartia CV-VI 1"0 - - HARGRAVE and Pseudocalanus minutus, Summer 1.5 -- ~ GEEN (1968)

Oithona similis, and 0"9 - - Temora longicornis 1 "3 - -

Mixed 3'7 - - / POMEROY et al. (1963) 9"4

Sagitta hispida ? 2"4 40 BEERS (1964) Calanusfinraarchicus Spring 2.2 23 '3 BtrrLER et ak (1969)

Autumn 0-6 8-5

REGENERATION OF SOLUBLE PHOSPHORUS COMPOUNDS

Several estimates have been made of the extent to which soluble compounds released by zooplankton account for the nutrient requirements of the phytoplankton in lakes, estuaries and oceanic waters; and it has occasionally been found that, in terms of phosphorus, the animals supply more than 100 per cent of the phosphorus removed by the phytoplankton (see TABLE 7). Obviously, a process in which the animal population loses more phosphorus daily than can be added to the plant population will be one of limited duration unless the animals make use of supplementary sources of dietary phosphorus, such as detritus.

Analysis of the relationship between zooplankton and phytoplankton in terms of nutrient regeneration is complicated by the fact that, in some studies, the zooplankton were captured in nets too coarse to retain the microzooplankton, such as protozoans which, as mentioned earlier, are considered to make a substantial contribution to the quantity of phosphorus released by the animal population. In addition, some of the phosphorus excreted by the animals may be in the form of organic phosphorus

102 E.D.S. CORNER

TABLE 7. PERCENTAGE OF PHYTOPLANKTON PHOSPHORUS REQUIREMEN-t" SUPPLIED BY ZOOPLANKTON EXCRETION

Sea area Percentage Reference

Gulf Stream off Georgia 107 Continental Shelf off Georgia 31 Doboy Sound, Georgia 8.5 Narragansett Bay (Spring) 16-9

(Autumn) 200 Bras d'Or Lake, Nova Scotia 150

(Summer) Morrison's Pond, Nova Scotia 25

(Summer)

) POMEROY et al. (1963)

MARTIN (1968)

HARGRAVE a n d GEEN (1968)

compounds which cannot be used by the plants. Thus, POMEROY et al. (1963) estimated that nearly half the phosphorus released by mixed zooplankton collected from the Gulf Stream and Doboy Sound, Georgia, was in the "organic" form; and HARGRAVE and GrEy (1968) obtained even higher values (67-74 per cent) in their experiments using various copepods from Bras d'Or Lake and Morrison's Pond, Nova Scotia. BUTLER et al. (1969, 1970) showed that the levels of "organic" phosphorus released by Calanus spp. varied considerably with season: thus, in winter, nearly all the excreted phosphorus was in the inorganic form, whereas, during the spring diatom increase, the quantity of "organic" phosphorus released rose to about 70 per cent of the total.

The compounds comprising the "organic" phosphorus fraction have never been identified chemically: the amount is simply expressed as the difference between the quantities of total phosphorus compounds and inorganic phosphate excreted. Whether this "organic" phosphorus represents an end-product of metabolism excreted from the tissue of the animal, or whether it consists of soluble "organic" phosphorus compounds that are released from the gut, has never been thoroughly investigated. Obviously, the chemical forms of these compounds and their method of release by the animal are aspects of the marine phosphorus cycle that deserve further study.

It is known that some "organic phosphorus" is released by phytoplankton as well as zooplankton (KUENZLER, 1970). On the other hand, CHu (1946) and HARVEY (1953) showed that organic phosphate esters could provide a source of phosphorus for plant growth; and KUENZLER and PERRAS (1965) have shown that, under phosphate deficient conditions, the enzyme alkaline phosphatase is produced in the cells and enables them to use the phosphate fraction of these esters. However, no-one has yet examined the organic phosphorus compounds released by zooplankton in terms of their nutritive value to plants, although this type of study would probably greatly increase our understanding of the zooplankton--phytoplankton relationship in the sea.

The rapid regeneration of nutrients by zooplankton helps to maintain the levels of soluble phosphorus compounds in the euphotic zone beyond the period of the phytoplankton bloom. However, this is not necessarily true for all sea areas: for


instance, in a region of upwelling off the coast of Peru excretion by the anchovy population is the major source of nutrients for phytoplankton growth (Wma'LEDGE and PACKARD, 1971).

DAILY RATIONS IN TERMS OF PHOSPHORUS

The daily rations of food needed by zooplankton have been frequently measured, mainly because these data are essential to a proper understanding of the grazing pressure exerted by a herbivore population in the sea. Several studies have been made of the ways in which these animals capture plant cells (see review by GAULD, 1966) and two different feeding mechanisms are apparently involved. One of these, used to capture very large algal cells, is an active seizing of the food, which is then broken up into smaller fragments before being taken into the gut: the other is a filtering mech- anism by means of which small algal cells are swept out of suspension in sea water and propelled as a food bolus into the mouth of the animal.

When the animals act as filter-feeders, they do not ingest plant cells at a constant rate. Thus, MtrLLIN (1963), studying grazing by C. hyperboreus, showed that the number of plant cells ingested (or ration) increased at first with cell concentration, but, having reached a maximum, continuously decreased as progressively higher levels of food were used. Similar results have been obtained by HAQ (1967). Further studies (PARSONS et al., 1969; ADAMS and STEELE, 1966) have shown that when the concentration of plant food falls below a certain critical value, the animals stop grazing.

° f 4 0

-z

22o

i- R r T I I 3

t

I I I I r I I 6 9 12

F o o d l e v e l a s c e l l s x t 0 "3 L -~

FIG. 4. Daily rations as percentage body phosphorus captured by Calanus feeding on Bid- dulphia.

So far, few studies of this kind have been made using zooplankton feeding on very large food particles which are actively seized. However, PAFFENaOFER (1971) has shown that C. helgolandicus, when grazing larger algal cells, captures larger daily rations; and this observation is supported by the results of further studies (CORNER et al., 1972) in which the extremely large cells of B. sinensis were used as a food for this animal. The data obtained are summarized in FIG. 4, and show that the daily ration captured does not fall when very high food concentrations are used: moreover,

104 E.D.S. CoR.~Eg

the animals still continue to capture cells at very low food levels. Indeed, the curve shown in FIG. 4 is similar to that given by IvLEv (1945) for feeding by planktivorous fish. Thus, the rate of increase of food consumed, dR, with an increase in the concentration of food available, dp, is proportional to the difference between the maximum ration, R . . . . and the actual ration, R. Thus:

dR = K (R,,a~ --R) or Rma~ = R (I -- e-kP).

dp

It was found in these experiments using B. sinensis that the maximum ration captured daily by the animals was equivalent to nearly half the body content of phosphorus: but in an earlier study (BUTLER et al., 1970), in which the animals were feeding on much smaller algal cells, the corresponding value was less than 20 per cent. The high ration of phosphorus taken daily by animals feeding on B. sinensis is similar to the value of 50 per cent proposed by CUSHING and VUdETId (1963) for Calanus spp. feeding on a diatom bloom in the North Sea. There were two possible reasons for this high ration. One was that these large algal cells are poorly disgested. Another was that the cells are damaged outside the animal's body during capture. These two possibilities have important implications in terms of the ultimate fate of the plant population. Thus, the first envisages the loss of plant production as faecal pellets that sink below the euphotic zone and are eventually used as a food by animals at depth: whereas the second view stresses the "spillage" of soluble materials from broken plant cells as a source of nutrients supporting the further growth of phytoplankton used as a food by herbivores near the surface. The extent to which each of these two processes occurred was sufficiently important to merit quantitative investigation (CoRNE~ et al., 1972). It was found, using the large cells ofB. sinensis as a food, that the proportion of the ration digested was independent of the size of ration consumed (see FIG. 2), confirming earlier observations by MARSHALL and ORR (1955a) and CONOVER (1966a,b). However, in contrast to these earlier results, the digestion ofB. sinensis was relatively poor (about 40 per cent in terms of dietary phosphorus). Evidence was next sought for damage to the algal cells outside the animal's body during capture; but none was found. True, considerable quantities of soluble phosphorus compounds were detected in the sea water containing the grazing animals, but further experiments showed that this material could all be accounted for as an excretion product of the animals.

PHOSPHORUS LEVELS IN ZOOPLANKTON

The total phosphorus content of zooplankton is normally low (see TABLE 8), often accounting for less than 1 per cent of the dry weight. It varies considerably with species. For example, it accounts for 0-55-I.16 per cent of the dry weight of copepods (wet weight:dry weight ratio -~- 6: 1) but only 0.14 per cent in the case of "watery" forms such as siphonophores (wet weight:dry weight ratio ~ 25:1). It also varies with season. Thus, BtrrLER et al. (1969) found that Calanus spp. contained about 50 per cent more phosphorus during a spring diatom increase than in winter.

Although little is known about the detailed composition of zooplankton in terms of phosphorus, CO~OVER et al. (1961) have interpreted the results of studies using 32p as showing that at least two "pools" of phosphorus exist in C. finmarchicus. One "pool" consists of labile compounds with a half-life of only a few hours and accounts


TABLE 8. LEVELS OF PHOSPHORUS IN ZOOPLANKTON (Data from BEERS, 1966 for yearly average found

in Sargasso Sea)

Phosphorus as Species dry body wt

Copepods 0-79 Euphausiids-mysids 1.48 Other crustacea 1.26 Chaetognaths 0'63 Polychaetes 0-99 Siphonophores 0-14 Hydromedusae 0-17 Pteropods 0"30

for 6 per cent of the total phosphorus content. The other "pool" , containing the remainder of the phosphorus, has a half-life of 13 days. It would be interesting to know how these so-called "pools" are distributed throughout the tissues of the animal, and the chemical forms of phosphorus contained in them. However, as yet the only analysis of the various phosphorus fractions in Calanus is that shown in TABLE 9. The data (HEAD and KILVINGTON: unpublished observations) are compared with those obtained for the diatom Rhizosolenia styliformis used as a food, and it is clear that the levels of phosphorus in the various fractions were fairly similar for the animals and the diet. Thus, the largest amount of phosphorus in both was present as the acid-soluble fraction and the smallest as phosphoprotein. Phospholipid phosphorus in the plants accounted for 12.6 per cent of the total and in the animals 17.3 per cent; the phosphorus present in nucleic acids accounted for 19.5 per cent of the total in the planks and 20.9 per cent of that in the animals. These data were obtained by the fractionation method described by SCHNEIDER (1945). It would be interesting to apply this method to the analysis of both feeding and starving Calanus in order to see whether the relative sizes of these phosphorus fractions change during starvation and how the labile and stable "pools" of phosphorus reported to be present in the animals (CoNOVER et aL, 1961) are distributed among them.

TABLE 9. DISTRIBUTION OF PHOSPHORUS BETWEEN VARIOUS FRACTIONS IN Calanus AND Rhizosolenia

Calanus R h izo so len ia Percentage total P Percentage total P

Range Mean Range Mean

Acid-soluble P 51.6-53.3 52-8 56.0-59.3 57.5 Phospholipid P 16.3-18.1 17-3 10.8-16.3 12.6 Nucleic acid P 20.6-21.2 20-9 18.1-22.2 19"5 Phosphoprotein P 8.2-9.6 9'0 4-3-14"0 10-6

106 E.D.S. CORNER

Certain phosphorus compounds found in marine animals may be of special interest. One of these is the phosphonic acid analogue of taurine, 2-aminoethylphosphonic acid (2-AEP):

O

HO - - P - - CH2 - - CH2 - - NH2 (2-AEP) I

OH

This compound, which is present in the free state and also as a major component of the phospholipids of certain ciliates, coelenterates and molluscs (QuIN, 1965) has now been isolated from hydrolysates of the planktonic amphipod Amomyx nugax, as well as those of several species of phytoplankton (KITTREDGE et al., 1969). The C--P bond is very stable chemically and substances containing it may be metabolically inert. Accordingly, the level of 2-AEP accumulated in the animals may serve to indicate the total quantity of phytoplankton assimilated in a given period.

Another group of phosphorus compounds of special interest is the nucleic acids. Thus, SUTCLIFFE (1965) has found a correlation between concentrations of RNA and growth rates in various invertebrates such as A. salina, and there is a possibility that the rate of growth of zooplankton can be calculated from RNA measurements. As the rate of production of zooplankton in the sea is difficult to measure, the possibility of using RNA measurements as a means of predicting this is of considerable interest. However, it will be necessary to know how RNA levels vary with species and stage of development, as well as with external factors such as food supply, before the value of the method can be properly assessed.

GROWTH OF ZOOPLANKTON IN TERMS OF PHOSPHORUS

Of the food captured by zooplankton only a fraction will be invested in growth or egg production, the remainder being lost as undigested material and moults as well as through metabolic activities concerned with maintenance. It has already been mentioned (p. 97) that the assimilation of phosphorus is in general an efficient process: but also (p. 101) that the metabolic losses represented by the amounts of phosphorus excreted in soluble form are often high. In recent years, there has been considerable success in culturing species of zooplankton in the laboratory (ZILLIOUX and WILSON, 1966; HEINLE, 1966; MULLIN and BROOKS, 1967, 1970) and several factors affecting growth rate have been studied using these cultures. For example, growth is faster at higher temperatures and varies with both quality and quantity of diet. Growth rates for copepods reared in the laboratory have been measured as increases in body weight or body carbon, but so far no data have been obtained in terms of body phosphorus,

Egg production has also been studied and recent findings with copepods (PAFFEN- H6FER, 1969) are that the number of eggs laid is very high, being approximately two thousand per female. CORNER and DAVIES (1971) have calculated, as a first approxi- mation, that three times the nitrogen content of a female C. helgolandicus is used to form eggs and it is possible that egg production accounts for an even higher factor in terms of body phosphorus. Thus, CONOVER et al. (1961) showed that up to 70 per


cent of the phosphorus assimilated by Calanus was accounted for, within 1 week, as eggs laid.

Gross growth efficiency, i.e. the proportion of the daily ration captured invested in growth by the animal, is an important variable in the study of the phosphorus cycle in the sea; but few attempts have been made to measure this in terms of phosphorus. KETCm_rM (1962) pointed out that the ratio N: P in phytoplankton (average value 7.3 in terms of weight) was substantially less than that in zooplankton (average value 10.9); therefore, animals living on phytoplankton and retaining an excess of nitrogen, must show an N:P ratio in their excretion products of less than 6"1:1. Ketchum used values for the ratio N:P in the phytoplankton, zooplankton and their excretion products obtained from studies in Long Island Sound and calculated that the gross growth efficiency in terms of phosphorus was nearly 50 per cent. Ketchum's estimate covered the whole period of growth of the animals and he assumed that all the captured food was assimilated. BtrrLER et al. (1969) found that, when allowance was made for the proportion of captured food lost as faecal material, gross growth efficiency in terms of phosphorus was only 28"3 per cent. This value was calculated as applying throughout the whole period of growth, but excluding egg production. In a later study (BUTLER et al., 1970), gross growth efficiency was calculated over a much more re- stricted part of the life span, a 2-week period during the spring diatom increase in the Clyde when a mixture of stage V, male and female C.finmarchicus increased their body content of phosphorus. It was found, using these mature animals, that only 17.2 per cent of the daily quantity of phosphorus captured was invested in growth. Data by PETn'A (1967) and PAVLOVA (1967) have shown that gross growth efficiencies in terms of calories are markedly higher for young copepods than for older animals: it seems likely, from the work of BUTLER et al. (1969, 1970) that the same holds true for dietary phosphorus.

CONCLUSIONS

This brief account of the importance of zooplankton in the marine phosphorus cycle has been mainly concerned with studies using copepods: admittedly, there are many other groups of animals present, but the numerical importance of copepods is overwhelming (HARDY, 1956). It is well known that this one group of animals includes many hundreds of species. However, studies so far have been concerned with only a few of these and it is essential that more work be done to provide a wider conspectus.

Most of the studies made so far have been concerned with animals feeding on algal diets. However, it is necessary to know more about the nutrition and metabolism of carnivorous zooplankton; and there is also a need for further work on the nutritive value of detritus.

Much work has been done on the percentage assimilation of dietary phosphorus by zooplankton and the values obtained are generally high. However, there is a need for further assimilation studies using particularly large algal cells, as well as microzooplankton, as the diets; and more information is needed concerning the assimilation of different phosphorus fractions and individual phosphorus compounds present in the food.

There have been several investigations of the levels of soluble phosphorus compounds excreted by zooplankton. However, the precise way in which these substances, particularly "organic" phosphorus compounds, are released by the animals deserves

108 E .D .S . CORNER

more detailed investigation: as does the chemical nature of these "organic" phosphorus compounds and their possible use as nutrients by phytoplankton.

The successful culturing of several species of zooplankton in the laboratory will doubtless pave the way for further studies of the factors affecting the gross growth efficiency of these animals in terms of phosphorus; and the data obtained could be useful in the formulation of mathematical models related to the production of zooplankton in the sea. However, there is also a need for work of a more biochemical nature, particularly the use of artificial diets in detailed investigations of zooplankton nutrition and the development of suitable methods for studying phosphorus metabolism at the tissue and cellular level.

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Phosphorus in marine zooplankton