r
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Lalws & Reservoirs: Research aud Management 1999 4: 15-22
Limnological comparison of a new reservoir with
one almost 40 years old which had been totally emptied
and refilled
A. M. Geraldes and M.
J.
Boavida*
Oepartamento de .Zoologin e Antropologia, Faculdarle de Ciencins da Universidadc de Lisbon, Campo Grandc C2 1749-016 Lis boa, Portugal
Abstract
In the present study, several limnological characteristics of a reservoir t-illed for the firs t time ar<' compared with th ose of another reservoir totally emptie d after being full for almost 40 years and then refilled. Three phases were considered during the s tudy, determined by the filling and emptying in the oldest reser voir: (i) Jilling phase. from August to October 1994; (ii) regular wate r-level phase, from Decembe r 1994 to April 1995; and (iii) e mptying phase. from May to .July 1995. Several form s of phosphorus, ph osphatase, chlorophyll a, and phytoplankton and zooplankton composition were de termined. T rophic states of the two reservoirs were compared. Major limnolog ical differences were not fo und between these two ar tificial lakes. In fac t, phys ical factors such as te mperature, chemical parameters s uch as the several for ms of phosphorus measured. and biologically derived variables such as chlorophyll a and phos phatase activity exhibited similar patterns and values in both reservoirs. as well as practically tlw same relationships among those.
Key words
emptying reservoir, new r eservoir, old r eservoir, phosphatase, phosph oru s, r e servoir (re)filling.
INTRODUCTION
Reser voirs arc s pecial lacustrine environme nts in which physical, chemical and biological features are strongly conditioned by s urface-level fluctuations caused by periodic, us ually seasonal, natural fi lling, and from almos t pe rennial, anthropogenic dewatering. These water movements are often dynamic. and therefore reser voirs are considered never to be in steady state (\.Yetzel1990). Although reser voir manageme nt meas ures are needed to minimize the effects of water-level flu ctuations, re markably few studies haw· been undertaken to unders tand the mechanisms by wh ich these s ing ular hydrological patterns may influence reser voir dynamics. 'l11e literature mainly discusses the effects of hydrology on plankto n co mmuniti es (Rey 1984. 1988; Schmid-Araya & Zufiiga 1992; Barone & !71ores 1994; Florcs & Bar onc 1994) an d on nutrient concentrations (fabre 1988: Barone & !7lores 1994). There are a few stu dies on fillin g processes in new reservoirs. focussing on zooplankton s
truc-*A uthor to w hom correspondence should be sent.
Email: zboavida@fc.ul.pt
Accepted for publication 20 November 1998.
ture (Pinel-Alloul & Methot 1984) and on both phytoplankton and zooplankton dynamics (Robarts et al. 1992) as well as on organic matter production and decomposition (Kimmel
et al. 1988) .
In Portugal, examples of complete e mpty ing and refilli ng of reser voirs are also scarce. However. there are studies on the effects of reservoir emptying and subsequent re filling on the structure and dynamics of zooplankton communities (Boavida & Crispim 1993: Crispim & Boavida H193) and on phosph orus dynamics (Marques & Boa vi cl a 1993. 1994) . All these s tudies for Portugal concern the same large reser voir and it was concluded that both zooplankt on communities and phos phor us dynamics were not much affected by the dis-turbing processe s of reservoir emptyi ng and refi lling .
The present study was carried ou t on two small moun tai n reser voirs so far not s tudied. Vale do Hossim I~ ese r vo ir
!G
3 km away) , both reservoirs are subject to the same climate. The aim of the present s tudy is to compare th e limnology of a recen lly inun dated reservoir with one almost 40 years old, which had bee n com pletely emptied (and remained empty fo r about 2 years) and was re fill ed at the s ame time as the new reservoir was receiving water fo r the fi rs t time. 1lw comparative limnology of the re se rvoirs was studied b y measuring th e concentrations of total phosph orus. total sol u ble phosphorus, and soluble reactive p h osphorus, phosphatase enzyme activity, and phosphomonoesters con-centrations in lake water, a nd chloroph yll a concentrations. Furthermore, the trophic state of the reservoirs was com-pare d by the dete rmination of Carlson 'sTrop hic State Index for lakt>s (Carlson 1977) and the constitution of both phy-toplankton and zooplankton communitie s was described for the two water-bodies.
MATERIALS AND METHODS
Both reservoirs are located at an altitude 1425 m a.s.l. in the mo untainous system of Serra da Estrela, Portugal (Fig . 1). on g ranitic bedrock, within the boundaries of a nal"ural park
*
N NE
0
c
SO SE
5
0(7"''""
Fig. 1. Locat 1on o f Rossi m and Lagoacho reservoirs in Portugal.
A. M. Geraldcs and M . .f . Boavida
(Parque Natu ral da Serra da Estrcla). iV!orphological a nd h ydrolog ical charact eristics of the re servoirs, as well as data on temperature and d issolved oxygen in the water column, are shown in Tablr 1. 'llw climate in Ser ra cl a Estrela is con-tine ntal, with warm. dry summe rs and long, cold winters. D espite the cold winters, snow is not always abundant and conse quently ice cover only seldom occurs in the dee pe st lakes and reservoirs. Du ring the p rese nt s tudy. winter was ·.abnormally warm and the re was no snow.
Rossim Reservoi r was built in 1956 to generate e lectrical power and supply water to surrounding villages. It was com-pletely em ptied fo r repairs in 1993, refilled in winter 1994, and partially emptied again in May 1995 for furth e r r epairs. During this pe riod, the water was c hannelled to Lagoach o through a tunnel connecting the two syste ms. However, in s pite of the tunnel , the re was no wate r exchange be twe en the two wate r-bodies beyond this water transference period. 'll1e direct influence of human activities on the impound-ments is g reater during s umme r when thP reservoirs a nd surrou ndings are used for recreation s uch as fishin g, boating, swimming and ca m ping . At that time of the year
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Natural Ga
~erra
Ua
E~trela
OVi scu
10 20 30
Limnological comparisons of a new and old reservoi r
it is also possible to find some domestic cattle (sheep and goat) gra%ing on the shores.
Lagoacho was built in 1993 and has the same uses as Rossim Reservoir. 11: was completely tilled for the first time in December 1994 . However, in May 1995 it was almost com-pletely emptied for repairs. Nevertheless, tlw drawdown was not so extensive as in tlw same period in Rossim I~ ese rvoir.
Since Lagoacho is of recent formation, was never used fo r recreation, and cattle g razing on thf' shores did not occur because of construction activities, human inOuence was con-s ide red negligible.
Vegetation is similar in the surroundings of both reser-voirs and is composed large ly of}zmiperus communis L. s sp.
nana, C.ytisus s p. , Erica sp. and Halimium alyssoides.
Phytoplankton in both reservoirs consisted mainly of species of Cosmarium , Botryoccocus, Dictyosphaerium, Pectodictyou, Scenedesmus, Ankistrodesmus, Dinobryon and Mallomonas.
Zooplan kton in both r ese rvoirs included th e rotifers
Po/yarthra dolichoptera, Conochi/us sp., Synchaeta sp.,
Keratella reticula la, Keratella serrulata, and Keratella valga. Tropocyclops prasinus, Eucyclops sp., Megacycloj;s viridis, A Iona costal a, A/on a quadrangularis, Chydorus spaericus and
Bosmina sp. were thf' most abundant c rustacean zoo-plankton species common to both reservoirs.
Table 1. Morphometnc, physrcal, and chem1cal charactenst rcs of
Rossim and Lago acho reservoirs. Values other than morphomet ric and on water temperature and d1ssolved oxygen w ere obtamed 1n
June 1994
Ross1m Lagoacho
Reservoir Reservoi r
Area (ha) 37 .0 23.7
Total capacity (m3) 3 567 440 1 052 000
M ax. depth (m ) 17.46 29.00
A lkali nity (meq/L) 0 .03 0
os
Cl (mg/L) 1.90 1.60
SO} (mg/L) 0 95 0 83
N-N0 1 (j.Jg/L) 0 00 0 00
N-NH I (j.Jg/L) 29.0 38.0
Ca2' (mg/L) 0.6 1 0.86
Mg2 ' (mg/L) 0 16 018
Na (mg/L) 1 41 1.24
K (mg/L) 0 . 14 0 .22
TN (mg/L) 0. 12 0 22
Si (mg/L) 1 15 1 27
pH 6 .28 6 42
Conductivi ty (J.JS/cm at 18"C) 11.4 11 7
Water temperature range ("C) 2 0-21.5 3.0-22.0
D1ssolved 0 2 range (mg/L) 8.5- 13.0 8 0-1 2 5
17
Samples were taken monthly in both reservoirs from August 1994 to Aprill995. From May to June. samples were taken weekly. A final sample was taken in July 1995. Water samples were collecled close to th e shore from the upper 30-40 cm stratum directly into acid-rinsed polyethylene bottles a nd were transported to the laboratory in a cold con-tainer (about8°C) within 24 h. Seasonal temperature profiles were recorded. \Vater temperature and dissolved oxyge n were mea sured by meter (YSI 5075; Yell ow Spri ngs Ins trument Co., OH, USA). I~ eadily available phosphorus was estimated as soluble reactive phosphorus (SRP) . Solu ble react ivf' phosphoru s concentrations were estimated s pectro-photometrically by the method of MuqJhy and Riley (1962). Total phosphoru s (TP) was determined after acid hydroly-s ihydroly-s with pe rhydroly-sulfatc for fiO m in under high tempera turf' and pressure (Ame rican Publi c Health Association 1989) . 'll1C' same procedure was carried out to dete rmine total solubk phosphorus (TSP) after filte ring the water (0.45 f.lm me m-brane filter). For all SRP, TP and TSP determinations, five replicates were run. Phosphomonoesters (PME) concen-tration and acid phosphatase (APA) activity of lake water were determined as described in Boavida and Heath (1988) . Acid phosphatasf' and PI\IJ E cleterminations were pe rformed on three replicates. Chlorophyll a values were obtained from 150 to 300 mL of water filtered through a Whatman GF/ A filter no more than 2 h after collection. Concentrations were determined spectrophotometrically after overnigh t extrac-tion in 90% acetone, according to American Public Health Association (1989) . All conce ntrations were expressed in !J.g/ L exceptAPA activities which were expressed in e nzyme units (e.u.) . One e.u. was defin ed as the amount of enzyme which hydrolyses 1 f.l rnoi/L of substrate (pN PP, Sigma. Sigma-Aldrich Co. , MO, USA) in 4 h. A Bauch and Lomb Spectronic 710 dig-ital spectrophoto mete r (Bauch & lomb GMB H. Munich, c;ermany) was used to measu rf' absorbance.
18
componen ts ( r SI (TP) and TSI (Chi) . respectively) of Carlson's Trophic State Index (Carlson 1977).
Four seasonal samples were obtained in order to deter-mine phytoplanklon and zooplankton composition. Sam ples were taken and analyzed according to Am erican Public Health Association (1989).
RESULTS
During the study period ne ither reservo ir stratified. Homeo thermy was observed for the whole period . with water temperature ranging from 2.0 to 21.soc in Rossim Reservoir and from 3.0 to 22.ooc in Lagoacho Reservoir (Table 1).
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Fig. 2. (= ). to tal phosphorus; (·-), total solu ble phosphorus and
( ) soluble reactive phosphorus concentration in (a) Ross1m and
(b) Lagoacho reservo1rs
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:h ;;! .., ~ ~Fig. 3. Changes m aod phosphatase act1v1ty 1n (- ) Ross1m and
(- --) Lagoacho reservo1rs.
A. M. Geraldes and M. J. Boavicla
Trends in TP, TSP and SRP concen trations were similar in both r eservoirs (Fig. 2) . However, during the first phase the highest concentrations were obtained in Lagoach o Reservoir. In Rossim Reservoir, TP concentntions ranged between 33.00 and 43.65 11.g/ L d uring this period, whil<" in Lagoacho Reservoir the variation was between 35.4 1 and 55.37 fl.g / L. To tal phosphor us and TSP concentr ations de creased until the end of the (re) fi lling phase in both syste ms (Fig. 2). Soluble reactive phosphorus ranged between 2.34 fl.g/ L (18 August 1994) and 12.23 fl.g/ L (29 Octobe r 1994) in Rossim Reservoi r, and betw·een 4.02 fl.g/ L (18 Aug u st 1994) and 14.54 fl.g/ L (29 October 1994) in Lagoacho. According to th ese low and high limits for both TP and SRP, available phosphorus in Rossim Rese rvoir varied between 7 and 28% ofTP. and be tween 11 and 26% in Lagoach o Reservoir. 'l11e concentrations of all measured phosphorus fractions decreased to m inimu m values during the regular water phase (Fig. 2).
During the emptying phase, concentrations of th ese parameters increased again. In Rossim Reservoir, TP con-centrations rose to 61.30 fl.g/L and SRP values to 16.30 fl.g/ L. During this phase. a large concentration of suspended par ticles was observed . 'Il1is cou ld have resulted from increased water turbulence during e mptying, and could have caused the return of complex phosphorus to the wate r, thus co ntributing to the observed hig h TP concentration . In Lagoacho RPser voir, the concentration of all forms of phosphorus s lightly increased. The biggest increase was o bser ved at the en d of the e mptying pro cess. Phosphomonoesters were below detection levels in both systems in all periods. In both Rossim and Lagoach o, sig-nilica nt correlations were found between TP and TSP (r
=
0.56;P
< 0.05) and (r=
0.94;P
< 0.05) , respectively. In Lagoacho Reservoir, a sign ificant correlation was found betwee n TP and SRP (r = 0.61; P < 0.05). Variables selected by multiple stepwise linear regression analysis arc shown in Table 2. Because both TSP a nd SRP are fractions of TP, these cor relations have no par ticular ecological interest.During the (re)fi lling phase in Rossim Reservoir, APA activi ty incre ase d sligh tly, ranging from 3.80 X 10 ~ to 4.56 x 10-2 e.u .. T he same pal! ern was observed in Lagoacho
Reser voi r, wherP APA values varied between 5.47 X 10 ~ and 7. 19 x 10 2
e.u. (Fig. 3) .
During t hr regular wa ter-level phase, APA activity decreased to the m inimum values recorded: 1.52 X 10 ~ e.u. in Rossim Reservoir a nd 1.82 x 10 ~ e.u. in Lagoacho Reser voir. During the em ptying pha se APA activit ies increased again in both systems. However, in Rossim Reservoir the increase was hig her and activity rose to 3. 75 x 10 1 e.u .. wh ile in Lagoacho Reservoir the highest
.t
Limnological comparisons of a new and o ld reservoir 19
Tab le 2. Variables selected by m ultiple stepwise linear regression analysis 1n Rossim and in Lag oacho reservoirs
r2 F p
Ross1m Reservoir
log TSP = 0.674 log TP + 0.174 0.69 8 .974 < 0 .05
log SRP = 0.451 log APA + 0.292 0 .7 1 10.1 55 < 0 .01
log TP = 0.428 log APA + 0.509 log TSP + 0.320 0.74 10.433 < 0 .0 1
log APA = 0.629 1og TP + 0 .76 1 log SRP - 0 .340 0 .87 12.360 < 001
Lagoacho Reservoir
log TSP = 1.105 log TP + 0 .687 log D0 - 0.943 0.99 338.873 < 0.01
log TP = 0.891 log TSP - 0.602 log DO + 0.846 0 .99 318.77 5 < 0 01
TSP. total soluble phosphorus; TP, total phosphorus; SRP, soluble reactive phosphorus; APA, acid phosphatase; DO, dissolved oxygen .
emptying period APA activity slightly decreased again. ln Rossim Rese rvoir, significant correlations were fo und between APA and TP (r = 0.73; P < 0.01), between APA and SRP (r = 0.66; P < 0.05) and between APA an d water temperatu re (r
=
0.63; P < 0.05). The latter has no ecological meaning. The positive correlation between enzyme activity and SRP is different to expected (significant, though nega-tive) when planktonic organisms pr oduce phosphatase in response to the scarcity of available ph osphorus in the water; this probably means that phosphatases are produced in a constitutive way in Rossim Reservoir, i. e. indepe ndently of the phosphorus concentration in the reservoir's water. A correlation between APA and TP is often found; for a correct interpretation of this relationsh ip, however, other COITel-ations with ecological meaning should have been found. Multiple stepwise linear regression analyses are presented in Table 2. In I..agoacho Reservoir, no significant correlations were found betwee n APA and othe r parame ters.During the (re) filling phase, chlorophyll a concentrations varie d between 5.57 and 12.47 J.l.g/L in I ~oss im Reservoir, and be tween 15.15 and 16.93 J.l.g/L in Lagoacho Reservoir (Fig. 4). During the fill ed phase, chlorophyll a concentrations we re low in both reservoirs. During the e m ptying phase chlorophyll a concentrations s ignificantly inc reased in
17 16
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Fig. 4. Chlo rophyll a concentrations 1n (- ) Ross1m and (- - - )
Lagoacho reservoirs.
Rossim Reservoir, whi le in Lagoacho they remained similar to those observed during the regular water-level phase. In Rossim Reservo ir a significant correlation was foun d between chlorophyll a and TP (r = 0.66; P < 0.05) meaning that most phosphorus was in the algal biomass. ln Lagoacho Reservoir, no s ignificant correlation was found between chlorophyll
a
and oth er variables.Principal components analysis and cluste r analysis applied to data from l~ossim Reservoir indicated the existence of two groups, on e of samples obtained during the end of the refi ll-ing phase and durll-ing the regular water-leve l ph ase, and one
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Fig. 5. Pnncipal componen ts analysis and cluster analysis of data
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Fig. 6. Pnnc1pal components analysis and cluster analySIS o f data
from Lagoacho Reservo1r.
of s amples obtained at the beginning of the refilling phase and during the e mptying phase (Fig. 5) . ln contras t. for Lagoacho Reservoir data. a clear definition between samples was not obtained in PCA analysis . Howeve r, cluster analysis separated samples obtained in the filling/ regular wate r-level phase from those of the emptying ph as<> (Fig. 6) .
According to Carlson's Trophic State Index , both reser-voirs were class ified as eutroph ic until 12 January 1995. ln Ross im Reservoi r, TS I (T P) varie d be tween 53 and 58, and TSI (Ch i) be tween 47 and 55. In Lagoacho Reservoir, TSI ( rP) ranged b etween 52 and 62, and TST (Chi) between 57 and 58. From 13 f'e bruary 1995 to 23 March 1995 these systems were considered as mesotrophic (Rossim Reservoir: TSI (TP), 42-44: TSl (Chi) , 28-45; Lagoacho Reservoir: TSI (TP), 22- 55; TST (Chi), 29- 35) . During the e mptying ph ase, Rossim Reservoir became eut rophic again; TSI ( I'P) ranged be tween 51 and 63 and T SI (Chl) be tween 47 a nd 50, while Lagoacho Reservoir remained mesotrophic; TSI (rP) ranged between 35 and 48 and TSI (Chi) between 29 and 45.
DISCUSSION
The re lative ly high concentrations of TP, TSP and SRP recorde d duri ng the (re)filling phase in both reser voirs mig ht have been caused by the leaching of nutrients. particularly phosphorus, (ru m nooded te rrestrial vegetation and soil. In addition , in l{ossim Reser voir, the r esuspension
A. M. Geraldes and M . .J. Goavida
of sed iments as the water rose cou ld have contributed to observed concentrations (Fabre 1988) . T he nutrient release in Lagoacho Rese rvoir might have been higher tha n in Rossim as a consequence of the existence of larger amounts of te r restrial vegetation on this previously non-inundated reservoir. Similar results were obtained for other reser voirs during the filling phase by Pinel-Alloul and l\llethot. (1984), Kimmcl et al. (1988), Robarts et al. (1992), who referred to this phenomenon as a 'trophic upsurge'. A significant increase of primar y production as a result of high nu trient conce ntrations during th e 'trophic upsurge' was also obser ved by tl1ese authors. 1l1t' increase in c hlorophyll a
du ring the (re) fill ing phast' in both reservoirs cou ld refl ect the raising primar y production ascribed to this phenomenon. According to Pine i-Alloul a nd Meth ot (1984) , the 'trophic upsurge' is followed by a decline, refer red tu as a 'tr ophic depression', and a retur n to a stabilized trophic state; this would b e the result of th e exhaustion of the available nutrien ts from the flood ed land and the gradual fl ushing of the reservoirs towards a st eady lake-type system (Pinel-Alloul & Methot 1984; Kimmel eta/. 1988; Robarts et al.
1992) . This phenomenon may have occurred in the reser-voirs studied. T h e decreas<> in phosphorus concentrations during the regular water phase could be re lated to an increase in sedim entation rates as well as to the exhau stion of the available nu trients by phytuplanklon (Bostrom et al.
1988; Wetzel 1993). T hus, th e diffe rences in the s tudied reser voirs could be explaine d by different sedime ntation rates, which in turn are explained by intrins ic moq)hological and hyd rological characteristics. In Rossim Reservoir. nutri-e nts m ig ht havnutri-e bnutri-enutri-en usnutri-e d mornutri-e quickly as a consnutri-equnutri-encnutri-e of hig he r phytoplankto n biomass derived from recolon-ization from re sting forms, commonly produced in situations of stress s uch as emptying.
vida
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1111
mts 1ted oirs 84). d to :ant ie nt 1ls o dl
a
le et
1011.
)hie )hie
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::1 by a se. cl in :1 be ·ess. that mge e of ·ond tere.
Limnological comparisons of a new and old reservoir
morphometry a nd the fact that Lagoacho Reser voir (recip-ie nt) was also being empt(recip-ied could have 's moothed' tlw procf'ss; the water co ming from Ro ssim Ht>servoi r was rapidly removed and did not cause a s ignifican t increase in turbulence.
The increase in chlorophyll a conce ntration s during the e mpty ing phase in Rossim Reservoir could have been related to the occurrence of a bloom of sma ll Cosmarium sp. in this period (Geraldt's, unpubl. data).
Phosphomonocslers were not found, e ith e r because they did not exis t in the lake or because they were not detected d ue to rapid hydrolysis as they were released into the water (Boavida 1991). Acid phosphatase activity in Lagoacho Reser voir was higher than in Rossim Reser voir. Part of Uw
APA activity de tected in Lag oacho Reservoi r could have been of terrestrial origin. resulting from the first noocling of soil Oansson et al. 1981) . The lowest APA activity was observed in both r<'servoirs during the regular water-level phase. This mig ht have been because this phase occurred between winter and spring when phytoplankton biomass was low (Berman 1970; Boavida & Heath 1988; We tzcl 1993). In fac t, Boavida and Heath (1988) noted that phos phatase activity and ph ytoplankton biomass were corre lated. However, in these reservoirs. no significant correlations were found. 'This mig ht be attributed to the large dis turba nce that occurred in both systems. The increase of APA activity in Rossim Reser voir during the e mptyi ng phase could be explained by the bloom of s mall Cosmari11111. According to .Jansson et al. (198 1), O lsson (1983) and Rai and .)acobsen ( 1993), a major part of sestonic phosphatases is associated with s mall algae (<10 !-L111) and probably with bacteria. In co ntras t. the e mptying process mig ht have caused high plankton mortali ty; ge ne rally, phos ph a tase ac tivity is highest whe n high rates of plankton decomposition occur (Hale rnejko & Chrost 1984; Boavida & Marques 1995). In Lagoacho Reservoir, the small increase in APA activity during this period could be explained by the lowest biomass of Cosmarium recorde d.
Reduction of APA activity at the e nd of tl1e e mptying phas e in both reservoirs might be related to progressive inactiv-ation of the enzyme. Phosphatases fou nd in natural e nviron-me nts persis t fo r about 2 mon ths with no appreciable loss of activity (e.g. Boavicla 1991). If no more enzymes were pro-duced to replace that inactivated, phosphatase' activity of lake water would ce rtainly decrease. The positive correlation and th e functional relation ship in Rossim Reservoir, or the ir absence in Lagoacho Reservoir, between APA and SRP, might suggest that APA is constitutively produced in both syste ms.
T h<:> effects of e mptying/ refilling hydrological patterns see med not to be importan t. No importa nt diffe rence was
21
found between the new n •ser voir and the one that has bee n filled for almost40 years before being emptied and dried for about 2 years. 'l11e conclusion is that for struct1Jral/func-tio nal characteristics brought about by ageing to dominate more than 40 years are needed. It seem s that the strongest innuence in these reservoirs was that oi climate and soil (in th is case the same inlluence . g iven th e vici nity of the wate r-bodies) which lead to similar limnological characteris tics.
ACKNOWLEDGEMENTS
We would like to thank B. Paulo and E Geraldcs for th eir as istanc<' in the fi eld. AMG was supported by M.S. grant PRAXJS XXl/ BM/ 1299/ 94 . .Ju nta Nacionale de I nvesl i ga~ · ao
Cie ntilica e Tecnol6gica, Portugal. Ch em ical a nalyses other than chlorophyll, phosphoru s and phosphatase determina-tions we re kindly pe rforme d at the Istituto ltaliano di Idrobiologia, Pallan za, Ital y. Pertinent comments and suggestions by an anonymou s referee are appreciat ed. This paper is a contribution from Centro de Biolog ia Ambie nl al. llnivers idade de Lis boa. Por tugal.
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