A Comprehensive Limnological Modelling of Lake Kinneret

A COMPREHENSIVE LIHHOLOGICAL MODELLI11G OF LAKL KINNERET

C. Serruya and S. Serruya Kinneret Limnological Laboratory P.O.B. 345, Tiberias, Israel

where:

INTRODUCTION

P

The combined effect of demographic development and industrial concentration in certain parts of the world led to considerable and rapid increase of the nutrient load of many lakes and deterioration of their quality. The mechanism involved is the intensification of the processes through which the photosynthetic organisms transform inorganic nutrients into organic material. The increment of matter synthesized at the primary level and injected in the food web causes deep modifications of the species composition of the conununity as well as of the chemical composition of the water. Conversely, the reduction of nutrient loads of severely disturbed water bodies showed that the process of eutrophication is reversible (9). However, although we know intuitively that global increase of nutrients leads to a global biomass increment, or vice versa in the process of lake correction, our understanding of the relationship nutrient load algal biomass is still very incomplete.

Vj 1 E kj

Concentration of chlorophyll; Lake segment volume;

G .

Bulk rate of transport of chlorophyll from segments k to j; ~et advective flow rate between segments k and j; Phytoplankton growth rates in j;

D .

Phytoplankton death rates in j.

Qkj PJ

PJ

The first two terms of the equations related to the transport of algae are easy to treat. ~ore serious problems arise with the expression which accounts for the effect of nutrient concentration on algal growth. The prerequisite of such an expression is an adequate hypothesis concerning the relationship between nutrient concentration and algal growth rate. The solution commonly adopted consists in assuming that the phytoplankton grows according to the "hyperbolic relationship": at high nutrient levels, the grm~th rate depends only on light and temperature, and at low nutrient levels, it becomes proportional to the concentration in the medium. The growth rate reduction factor due to low nutrient concentrations is expressed as follows:

In the past twenty years, numerous mathematical models have been published. Riley (1 ) expressed the grm~th of algae as a function of zooplankton grazing. Riley et al. (18) produced a more sophisticated model which included nutrient equations. More refined models were produced by Steele (26'27) and Davidson and Clymer (5). Vollenweider (2B) developed an empirical relationship between the external nutrient load of a lake and its trophic status. The recent tendency in modelling is to use models in order to account for eutrophication processes and to predict them. This is the line followed by Chen (4) and by Di Toro et al. (6,7). All these models are based on the law of mass conservation, which states that the mass of matter existing in a given volume of water must be accounted for by transport by dispersion or advection, or by production or removal within the considered volume. \o.'hen applied to the phytoplankton system, the law of mass conservation gives expression of the type (7): ph~toplankton

C N

K N m

Cp

+ C x K P + Cp N

(2)

m

~lhere

K Nand K P are the global half-saturation m m constants of the lake for total inorganic Nand P, and C and C are the concentrations of Nand P in N P the medium. As a matter of fact, the hyperbolic relationship was verified in experimental work carried out on a number of algal species for the various forms of inorganic N. Ketchum (13) shm.Jed that, for short periods of incubation, the uptake of ;lO3 by Nitzschia closterium ,,,as a hyperbolic function of the concentration of nitrate in the medium. Similar results have been found for other species and for ,,02, N03 and t:1i4 by Dugdale and Goering (B), Eppley and Coatsworth (10) and Eppley et al. (11). However, these experiments do not consider the interactions be t<-Ieen various species which, in natural

(1)

488

environments, may influence the qualitative composition of the flora. Moreover, since the biomass:P or biomass:N ratio of the various species of algae is variable, these interactions may also influence the quantitative relationship between the nutrient supply to a lake and the resulting biomass. The analysis of the data obtained in Lake Kinneret is an illustration of the previous statement, and shows that we can hardly account for the succession of algae and resulting biomass values without introducing the concept of algal competition in our quantitative model. RELATIONSHIP BETWEEN PHOSPHORUS SUPPLY AND ALGAL SUCCESSION IN LAKE KINNERET General Background 1. Phosphorus Because of physico-chemical reasons (high concentrations of calcium and high pH), Lake Kinneret waters are very poor in dissolved inorganic P ( 23). So far, the lake sediments play the role of a phosphorus sink (concentrations of 0.12 - 0.42% on a carbonate free and dry weight basis in the sediments). The strong summer anoxia (up to 8-10 mg/l sulfides in deep layers) does not cause any significant release of phosphorus from the sediments, in contrast with the classical work of Mortimer (14). Conversely, the oxic period (January-February) is accompanied by a phosphorus supply from the sediments (19). This oxic release is caused by the oxidation of the organic phosphorus deposited at the end of the previous bloom. The decrease of pH observed at the turnover period in the whole water column as a result of oxidation of suI fides allows the concentration of orthophosphate to reach 5 ~g/l and even more. This is the only period of the year when measurable concentrations of P0 4 are found. 2. Other nutrients Although phosphorus, limited by chemical equilibria, is supposed to play a decisive role in Kinneret waters, other nutrients probably intervene in the succession of algae. The turnover recirculates the ammonia accumulated in summer in the hypolimnion, and allows its rapid nitrification. The mixed period is often accompanied by an increase of dissolved organic nitrogen (24). 3. Algae Although the algal population of the lake includes more than 250 species, it is heavily dominated by the dinoflagellate Peridi ni um cinctum fa westii , which blooms from January to May. At this period, Peridini um accounts for 60 to 99% of the total biomass, reaching absolute values of 300-400 g/m2 wet weight. It is worth noting that this alga is not grazed by zooplankton (12) but only by the fish rilapia, and this latter grazing does not exceed 10-15% of the Peridinium production (15). Therefore, in spite of the ponderal importance of Peridinium, the other families of algae have a more direct impact on the food chain; they also represent the natural competitors of Peridini um . This com-

petition generally ends with the victory of the However, in rainy ye ars, such as for example 1964, when the nutrient concentrations, and especially the nitrate, were hi ghe r tha n usual, Peridi niun did not develop, and was r e pl a ced by a bloom of :4icY'oeys ti s ( 21). The interesting point is that for higher nutrient loads, the YicY'ocystis bloom produced biomass values lower by one order of ma gnitude t han the us ual Periuiniwn bloom.

Peri dini um .

The Seasonal Succession of Algae In Fig. la, we have r epresented the total ph y toplankton biomass and the Peridinium biomass, and in Fig. lb, the fluctuations of the algae other than Peri dini um . Very regular sequenc es can be oLserved. The period January-February is characterized by a peak of diatoms (mainly t1elosiY'a sp.) and green algae (Pediastrum spp., CoelastY'um spp., Clos t eY'ium aci culare vaY'. sub pY'onum). The diatoms and gre en algae drop do,m during the Peri di nium maximum and show a second and hi gher pea k in June-July. In summer and fall, the diatoms disappear and are replaced by the association green and blue-green al g a~ A deviation from this regular pattern was observed in January-February 1970, when the diatom-green a lgae peak did not appear. Moreover, in 1973-74, a substantial increase of diatoms and blue-green algae was observed. The Mechanism of Algal Succession A major problem in the understanding of the nutrient concentration - algal growth relationship is the complexity of the process of uptake, that is, of transfer of nutrient through the cell membrane. We tried to avoid this difficulty by measuring the intracellular Nand P concentration of the phytoplankton (22). During the Pe ri dini um period, the N:P atomic ratios show the highest values (20:1 to 60:1). These values are much lower during th e peaks of diatoms and green algae (5:1 to 10:1). It is then obvious that the cellular P requirements are much smaller in Peri dinium than in the diatom - green algae association. We calculated by two independent ways the P level in Peri dini um cells at the end of the bloom (Table 1). In April-May, the PeY'i dini um cell contains no more than 0.9 to 1.5 • 10 12 mole P. Taking into account that the volume of a Peri dinium cell is about 1000 times that of a MicY'oey stis cell, we reach values of about 1.1015 mole P per cell unit, which is very close to the minimum cell level of P found by Bierman e t al. (3) for MicY'ocystis aeruginosa against 0.3 . 1014 for ChloY'ella. Recent research (1,16,25) demonstrated that alg a l growth could be expressed as a function of internal levels of phosphorus, and that th e division rate stops when a minimum cell level, specific for each species, is reached. In its turn, the intracellular P concentrations depend, among other factors, on the concentration of P in the medium. Taking into account our data and the experimental work previously mentioned, we can hypothesize that in Lake Kinneret, the diatoms and green algae have the highest P requirements, and highest minimum cell P concentration can develop only when P supply is at its highest. Conversely, blue-green algae and

489

Peridinium have much lower requirements, and are able to develop even when the phosphorus levels are low. L~~guence

of Limno10gica1 Events and the Succes-

sion of Algae As mentioned above, the oxic period, when the lake is completely mixed, is the only period when orthophosphate is released from sediments and found in lake water. These favourable conditions allow the development of diatoms and green algae, which rapidly take up the orthophosphate. In late February, the waters become depleted of phosphorus, and the diatoms - green algae association drops down. Since the main diatom of this peak, NeLosira sp., is not eaten by zoop1ankton (Gophen, p.c.), its disappearance cannot be accounted for by grazing. In Fig. 2, we have represented the fluctuation of the diatoms and green algae in winter-spring 1974. The increase of orthophosphate is acco~panied by a peak of diatoms and green algae. The disappearance of orthophosphate leads to the rapid drop of these groups and a concomitant increase of the blue-green algae, and we note that the peak of blue-green algae corresponds to the peak of nitrate. In March, the blue-green algae decrease, and the Peridinium, more modest in its nutrient requirements, takes over. The Peridinium dominates in March-April and declines in May, but as early as March its high division rate is accompanied by an increase of the mortality rate (15). At this period, the lake is not thermally stratified, and part of the dying algae is sedimented. In April-May, the thermal barrier of the meta1imnion develops, and the bulk of the bloom decomposes in the epi1imnion. The simultaneous increase of temperature (25 0 C in May), and the excellent mixing caused by the westerly wind b1m.ing daily from 1200 to 2200 hours (20), accelerate the decomposition of the Peridinium bloom material and the release of phosphorus. This phosphorus is immediately taken up, and promotes the development of a second peak of diatoms and green algae. This explains the pulsatic pattern of P uptake rate (Table 2). From the data of C uptake rate (2) and the intracellular C:P ratios of the algae, we calculated the rate of P uptake and found that the highest ~alues are observed in January-February; they are followed by very low values in April-May and rise again in June-July, when the decomposition products of the bloom renew the sources of phosphorus. The observations in Lake Kinneret show that the succession of algae can be explained in terms of phosphorus availability, which depends mainly on physical events (turnover in winter, stratification and epi1imnion mixing in early summer), and of species specific internal P concentration. Can We Explain the Anomalies Observed in the Succession of Algae? 1. Absence of diatom - green algae peak in February 1970 \,e have connected the February peak of diatoms \.ith

the oxic release of phosphorus generated by the turnover. In winter 1970, the turnover occurred very late (in early February instead of late December), and the mixed period lasted less than one month. Consequently, the oxidation of the bottom sediments was poorer and of shorter duration than in other years. Fig. 3 shows the oxygen concentration of the deep water during the record period, together with the resulting winter association of diatoms and green algae. The year 1970, when the maximum oxygen concentration on the bottom only reached 4 mg/1 (compared to 9 mg/l in other years), was the only year when the winter peak of diatoms did not appear; in other words, the poor oxidation of the bottom organic matter did not release enough phosphorus for the diatoms to develop. 2. Increase of the non-dinoflagellate algae in 1973 and 1974 The outstanding limnologica1 event which accompanied the increase of non-dinoflage1late algae was the sharp drop of water level generated by the dry 1972-73 winter (Fig. 4). From mid-summer 1972, the \"ater level dropped below the average value, and the difference between the actual and average level was accentuated in 1973 and 1974. Since the solar radiation did not vary, the thermocline developed at a lower absolute altitude than in previous years. For example, in October 1973, the thermocline was at -233.6 m in comparison with -230.0 m in October 1972, causing a reduction of 30% of the volume of the hypo1imnion. Horeover, \.hen the \"ater level is lowered, the surface of the lake decreases less rapidly than its volume. The wind force, applied on a \.ater body of nearly unchanged surface but of smaller volume, generates better heat transfer resulting in a thicker and better-mixed epilimnion. In October 1973, the epi1imnion volume was 5,; higher than in the corresponding period of 1972. These purely mechanical modifications (lowering of water levels), without any increase of the watershed load (in fact, the external load of nutrient was 50% lower in 1973 than in 1972) (Fig. 5), tend to increase the nutrient concentrations in the lake for the following reasons: a) A given amount of organic matter decomposed in the hypolimnion, or nutrients released from the sediments, generate higher hypo1imnic concentrations when th e water level is lowered. b) The increase of hypo1imnic concentrations of certain elements, such as dissolv ed CO 2 , has a direct effect on the solubility of calcium phosphate. which is then released from th e sediments. c) The turnover process and oxidation of the organic matter on the bottom is much more effective in low level winters. d) At the turnover, these substances, redistributed in a smaller volume of water, produce higher concentrations in the whole water column. These reasons explain why, in 1973 and 1974, we observed increasing concentrations of phosphorus (total and dissolved) and of dissolved organic nitrogen

490

(Figs. 6 & 7). These higher levels of nutrients enabled the diatoms and green algae to compete more successfully with Peri dinium, although the total amount of nutrient supply to the lake was lower than in rainy years. It follows that the 1973 bloom, and even more the 1974 one, were mixed blooms whe re Peridinium played a less dominant role than in the pas t (Fig. la).

do not produce the highest algal standing crops. Conversely, average external loads may be accompanied (1970) by very high standing crops if Peridinium takes over, and very low ext~rnal loads (1973) may produce twice as much biomass as the highest observed loads. As we have seen, the reasons for these peculiar features are:

We also note that in autumn 1973, an unusual peak of diatoms developed. Since, at this period, the temperature of the epilimnion is 27° C, this is clear evidence that high temperature, often invoked as a factor limiting the development of diatoms, has no part in the usual absence of diatoms in summer.

1. High external loads are mainly high nitrate loads. Even if the phosphorus load is then higher than in normal years, its available fraction does not increase as much as the available nitrogen. It appears that the major available phosphorus load is the organic phosphorus deposited on the lake floor, and its release is not monitored by the processes which determine the external load.

ThE LOAD-BIOMASS RELATIONSHIP IN LAKE KINNERET During the data record period, different limnological situations occurred and were accompanied by various algal reactions.

1. In 1964, unusual floods brought a nutrient load above the average; the nitrate load, generated by the flooding of the peat area of the Hula plain, was relatively higher than the phosphorus load. Concentrations of nitrates in the lake reached 1000 ~g/l. Peri dinium did not develop and was replaced by Microcysti s. The biomass of algae reached 86 g/m2. 2. In 1970, the flood and external nutrient load were slightly under average, and the turnover occurred later than usual. The Nand P concentrations in the lake ,~ere lower than usual, Peridinium developed early, and the winter diatom - green algae peak failed to appear. The algal biomass reached 341 g/m2. 3. In 1973, there was no flood, and the external N load was 50% lower than in normal years. Moreover, the turnover took place as usual but the water level was one meter below the usual level of this period. The Peri dinium bloom developed, but diatoms and green algae developed simultaneously. This tendency of a mixed bloom was accentuate~ in early 1974, when the water level was two meters below the normal level. The maximum biomass values were 197 g/m2 in 1973 and 152 g/m2 in 1974. Since the River Jordan and Lake Kinneret waters are limited in dissolved phosphorus for chemical reasons, the increase of the nutrient load during rainy years brings a relatively higher increment of available nitrogen than of available phosphorus. In rainy years, the external load is disequilibriated in favour of nitrogen, which allows the predominance of blue-green algae, especially Miarocystis. Conversely, in dry years, the nitrogen load is reduced. The internal phosphorus load generated from oxidation of organic phosphorus depends then on the timing and duration of the mixed period. If the turnover is early, the relative abundance of phosphorus enhances the development of diatoms; if the turnover is delayed, the low level of both Nand P favour the development of Peri dinium. An important point is that the high external loads

2. The Peridinium cell is able to produce more organic matter from a given amount of Nand P than other algae. Moreover, the low grazing pressure on Peridi ni um causes the accumulation of organic matter in the water. Conversely, the mixed blooms observed in 1973 and 1974 were accompanied by an increase of the herbivorous zooplankton groups, mainly the Cladocera (Fig. 8). It seems that a right management for this lake should consist in encouraging the mixed blooms. By doing so, we could reach two important aims: a) decrease the overall standing crops; and b) orientate the matter flow towards the grazing pathway instead of the present detritus pathway. The Nutrient Reduction Factor in Limnological Models We have shmm that in Lake Kinneret, the development of algae and the resulting values of the standing crops do not depend simply on the external loads and concentrations of nutrients, but also on the algal association which predominates in a given nutrient situation. Even when conSidering only two major nutrients, nitrogen and phosphorus, the large variety of nutrient situations leads to flexible and continuously changing biological responses, which are not properly expressed by a global Michael-Menton expression. A limnological model of Lake Kinneret, for example, should be able to simulate the response of diatoms - green algae to the winter load of phosphorus, and their replacement by Peridinium or by Miaroaystis when the oxidation of the lake bottom material is slowed down by the onset of stratification. It should also simulate the second peak of diatom - green algae at the end of the winter-spring bloom by taking the decomposing Peridinium as the nutrient input of this bloom. Such an approach requires: 1. Measuring parameters ,·,hich are rarely determined

routinely, such as the chemical composition of the plankton and the release of phosphorus from sed iments. 2. Analyzing limnological data with the purpose of clarifying the succession of algae and their relationship to the chemical and physical chain of events taking place in th e lake. 3. Determining experimentally the P and N uptake

491

rate of the main competitor species of algae, as well as their P and N minimum cell levels, their division rate at different nutrient levels, etc.

(11) Eppley, R.W., J.N. Rogers and J.J. McCarthy, "Half-Saturation Constants for Uptake of Nitrate and Ammonium by Marine Phytoplankton," LIMNOL. OCEANOGR., Vol. 14 (1969), 912-920.

In Lake Kinneret, points 1 and 2 (above) have been carried out routinely as part of the master plan which directed the research of the Laboratory. Point 3 is being carried out on five algal species for nitrogen and phosphorus, as a special project undertaken as a preliminary step of the lake modelling. This approach joins that of Bierman et al. (3), who already demonstrated that it is feasible to introduce the concept of algal competition and succession in models.

(12) Gophen, H., "Zoo plankton in Lake I:inneret," In: LAKE KINNERET DATA RECORD, (T. Berman, ed.), Israel Scient. Res. Conf., 1973, 61-67. (13) Ketchum, B.H., "The Absorption of Phosphate and Nitrate by Illuminated Cultures of Nitzschia closterium," AM. J. BOT., Vo1. 26 (1939), 399-407. (14) Mortimer, C.!:., "The Exchange of Dissolved Substances Between Hud and Water in Lakes," J. ECOL., Vol. 29 (1941), 280-329.

REFERENCES (1) Azad, H.S. and J.S. Borchardt, "Variations in Phosphorus Uptake by Algae," ENVIRON. SCI. TECHNOL., Vol. 4 (1974), 737-743.

(15) Pollingher, U. and C. Serruya, "The Division Rate of PeriJiniwn and the Development of the Illoom in Lake Kinneret, Israel," J. PHYCOL. (1975), in press.

(2) Berman, T. and U. Pollingher, "Annual and Seasonal Variations of Phytoplankton, Chlorophyll and Photosynthesis in Lake Kinneret," LHlNOL. OCEANOGR., Vol. 19 (1974), 31-54.

(16) Rhee, G., "Competition Bet\~een an Alga and an Aquatic Bacterium for Phosphate," LI~OL. OCEANOGR., Vol. 17 (1972), 505-514.

(3) Bierman, V.J., F.H. Verhoff, T.L. Poulson and

!-I.W. Tenney, ":'Iulti-nutrient Dynamic ~lodels of Algal Grovlth and Species Competi tion in Eutrophic Lakes," PROC. SYHP. HODELLING THE EUTROPHICATION PROCESS, Workshop, Utah State Univ., Logan, Utah, Sept. 5-7,1973,89-109.

(17) Riley, G.A., "Factors Controlling Phytoplankton Populations on Georges Bank," J. liAR. RES., Vo1. 6 (1946), 54-73. (18) Riley, G.A., H. Stommel and D.F. Ilumpus, "Quantitative Ecology of the Plankton of the Western North Atlantic," BULL. BINGHAt'1 OCEANOCR. COLL., Vol. 12 (1949), 1-169.

(4) Chen, C.W., "Concepts and Utilities of Ecologic Models," J. SANIT. ENG. DIV. NI. SOC. CIVIL ENG., Vol. 96 (1970), 1085-1097. (5) Davidson, R.S. and A.B. Clymer, "The Desirability and Applicability of Simulating Ecosystems," ANN. N.Y. ACAD. SC!., Vo1. 128 (1966), 790-794.

(19) Serruya, C., "Nitrogen and Phosphorus Balances and Load-Biomass Relationship in LaLe Kinneret (Israel) ," VERB. INT. VER. LI~OL., Vo1. 19 (1975), in press. (20) Serruya, S., "Wind, \,ater Temperature and Motions in Lake Kinneret: General Pattern," VERB. INT. VER. LIMiIOL., Vo1. 19 (1975), in press.

(6) Di Toro, D.M., D.J. O'Connor and R.V. Thomann,

"A Dynamic Model of the Phytoplankton Population in the Sacramento-San Joaquin Delta," In: NONEQUILIBRIUM SYSTEHS IN NATURAL \,ATER CHEI1ISTRY, ADVfu~CES IN CHD1ISTRY SERIES 106, Amer. Chem. Soc., Wash., D.C., 1971, 131-180. (7) Di Toro, D.M., D.J. O'Connor, J.L. Mancini and R.V. Thomann, "Preliminary Phytoplankton-Zooplankton-Nutrient Model of Western Lake Erie," SYSTEHS ANALYSIS fu'lD SHlULATION IN ECOLOGY, Vol. 3 (1973), Academic Press (8) Dugdale, R.C. and J.J. Goering, "Uptake of New and Regenerated Forms of Nitrogen in Primary Productivity," LI'fNOL. OCEA.."lOGR., Vo1. 12 (1967), 196-206. (9) Edmondson, W.T., "The Present Condition of Lake Was hing ton," VERll. INT. VER. LIMNOL., Vol. 18 (1972), 284-291.

(21) Serruya, C. and U. Pollingher, "An Attempt at Forecasting the Peridiniwn Bloom in Lake Kinneret (Lake Tiberias)," HITT. INT. VER. LHlNOL., Vo1. 19 (1971), 277-291. (22) Serruya, C. and T. Berman, "Phosphorus, Nitrogen and the Growth of Algae in Lake Kinneret," J. PI~COL., Vol. 11 (1974), in press. (23) Serruya, C., M. Edelstein, U. Pollingher and S. Serruya, "Lake Kinneret Sediments: Nutrient Composition of the Pore Water and ;·!ud-Water Exchanges," LIHNOL. OCEANOGR., Vo1. 19 (1974), 489-508. (24) Serruya, C., u. Pol1ingher and :1. Gophen, "The Nand P Distribution in Lake Kinneret (Israel) with Emphasis on Dissolved Organic Nitrogen," OIKOS, (1975), in press.

(10) EppIey, R. \,. and J. 1. Coatsworth, "Uptake of Nitrate by Ditylum brightweHi - Kinetics and Mechanisms," J. PHYCOL., Vol. 4 (1968), 151156.

492

g.m_ o

(25) Soeder, C.J., H. Muller, H.D. Payer and H. Schuller, "Mineral Nutrition of Planktonic Algae; Some Considerations, Some Experiments," INT. ASS. THEOR. APPL. LIMNOL., Vol. 1 9 (1971), 39-58. (26) Steele, J.H., "A Study of Production in the

Gulf of Mexico," J. MAR. RES., Vol. 22 (19 64), 211-222. (27) Steele, J.H., " Notes of Some Theoretical Prob-

lems in Production Ecology," MEM. INST. IDROBIOL., Vol. 18, suppl. (1965), 383-398. (28) Vollenweider, R.A., "Scientific Fundamentals

of the Eutrophication of Lakes and Flowing Waters with Particular Reference to Nitrogen and Phosphorus as Factors in Eutrophication," OECD REP., Paris, 1970, 159 p.

Fig . 1. a ) Fl uctua t ions of t o t al Pv rr hophyta hiomass .

a l ~al

hiomass and

h ) Fluc t ua t ions of t he a l gae othe r than

Py r rhophy t a : bl ue- green a l gae (ho r izon t al l i nes ) . dia t oms (whi t e area) a nd gr ee n algae (shadowed a r ea) . ~o t e the r e gul a r peaki ng of diatoms a nd gr een algae . TA.LjLE 1

Alal)unt of P ill Pe J'iv.i11i W'/ cells . Computa tions hdve heen made in ,\pril I'hen qq:-: of the algal biomass is r('presented bv Pf;ridini~4l'I .

la . The V:l.!ues of total P deteITIlned hv che rnctl'y l pne bluc TJcthod \.'crc assUMl.cl t o be o r r.:lnic int r accllul.u P . The cell nunt-cr and tot.1.1 P \,c r e determined on the same sarr.plc . j

20

E

Dep t h

Oate

_ _ _ _ _1':'1_,_ _

TO (<11 P

Ct'll number

tlc le P

72

2fi82

0 . 80 . 10- 12

761

0 . 96 .10- 12

·, ··

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35

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g

lb . The perCf'ntahc of P in Pcri"':'ini:...:", uas r elated to t he corrcs pondlnr, IlUl':'lber of cells ca l culated from the ave ra ge cel l d ry \~eight of !'"ri(~iniur .

30 _

'

!...20

•

~

10

1

*

1oI 0 J· 1oI

• ___ po_- p

3

1

9

7

"

Fig . 2 . lieekl v fl u ctua tions of the green algae . bl ue- g r een a l gae and dia t oms with t he concen tr a ti ons of ort hopi1osrha t e an d nitr a t es . The peak of dia t oms is indica t ed hy a ve rti ca l full l i ne a nd th e peak of bl ue- gr een algae by a do tt ed line.

f r om Pollinghcr and £em<\n . 1973.

TABLE 2 Ra te of P up take bv t he al~al popula ti on in 1972.

_ _"-19,,,7::.2::.-_ _ Jan

Results in f'J.'I./m2/dav .

_ - 'F.!:eb"---_-"'!2.
:·· "...!

~·L
;

8 P uptake:

85

75

42

13

26

16

;

------. ----r------.---- ----- --- --~ --.- ---~

Linnological even t s:

,

o

Dccor.posi tion of t:lf' t-loon

Fig . 3 . Mon t hl y f l uc t ua t io ns of the oxy~e n concent ra t ions in the wa t e r ove rlying t he mud (40 m dep t h) . The ver t ical li nes show the peaks of diatoms (full line) and ~ r een al~ae (dot t ed l ine) ,

493

Oroonic N Avorogo 510. A 01969 . 1970 0191 1

IIIJ

1972

~ 1973

m 1974

Fi;'.. 4. F1llctll.ltion of th e lak e l ev(·ls for the pe riod 1 969 - 1 9 74 . .1O' m' JORDAN

r ~

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7 . Fluctuation of concentrations of organic N

f .i f,.

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_

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"ARTICULATE

(monthl y 3veragcs) at tll(' c e ntral s tation A (411 m depth) for the period 19(·9-· 1974 .

o 'OOj 50

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6

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,

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100

fir,. 5. Fluctuation of tile Jordan lZiver yield and of tbe external nutrient load .

Tolol P AverQ90 510. A

o .... •

191'0

brill::: 01971

IV

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VI

II

III

IV

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x

XI

XI I

%

100

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x, fig.

. XII

G. Fluctuation of concentratien s o f total P (r lon thl y avera ~" es) at tiI e· central station A (40 m depth) for t it e l'l'riotl 19(,9-l n 74 .

VII

~"

fi ~ .

494

CO PEPO DA

D '.cLADO CERA

S . J(e lative contriuution of the :,erbivorous Cladocera and th e preda t or Copepoda during the period 1 9~9- 1974 .