Hydrobiologia 506–509: 281–287, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

281

Phytoplankton changes in a shallow Mediterranean lake (Albufera ofValencia, Spain) after sewage diversion

Marıa-Jose Villena1 & Susana Romo1

1Area de Ecologıa, Facultad de Biologıa, Campus de Burjasot, 46100-Burjasot, Valencia, SpainE-mails: mjvillen@uv.es, Susana.Romo@uv.es

Key words: microalgae, flushing, phosphorus reduction, Mediterranean lake, restoration

Abstract

Phytoplankton from Lake Albufera, a shallow, polimictic, freshwater lake, was studied before and after sewagediversion. The lake is used as a reservoir for rice cultivation in the surrounding lake area. Due to antropogeniceutrophication in the 1960s, the lake turned from a mesotrophic to a hypertrophic, turbid state. In 1991, a restorationplan of the lake started which has reduced 30% of sewage effluents. Total phosphorus in the lake was reduced about31%, from 0.49 to 0.34 mg l−1, but nitrate did not vary. Chlorophyll-a mean annual values diminished to half,although hypertrophic levels remained (mean value from 318 µg l−1 to 180 µg l−1). The algal community structureindicates a response to P reduction in the direction of increasing equal contribution of algal groups. Cyanobacteriaare still the dominant group, but reduced their biomass by about 15%, due to filamentous cyanobacteria depletion(from 78% to 48% of total phytoplankton biovolume), in favour of chroococcal cyanobacteria (changed from 7to 23% in the 1990s). Other algal groups, especially diatoms and chlorophytes, also increased their contribu-tion in the phytoplankton after nutrient diversion, while euglenophytes, indicative of organic matter pollution inthe lake, decreased. There were pronounced changes in phytoplankton composition and a general trend towardpresence of smaller algal species. The dominant species in biovolume during the 1980s, Planktothrix agardhii,almost disappeared in the lake in the 1990s, and was replaced seasonally by the slender Pseudanabaena galeata.Disappearance of Planktothrix agardhii, allowed large Cladocera to control algae early in the year (dominant smalldiatoms and chlorophytes), during several clear water phases occurring in recent years that lasted up to five weeks.Even though some improvement of the lake water quality was observed, complementary restoration measures aresuggested, such as a reduction of phosphorus below 0.05 mg l−1, control of pesticides in the catchment area, andmanagement of benthivorous-planktivorous fish species, in order to re-establish phytoplankton composition andsubmerged plants as at the beginning of the 20th century.

Introduction

Understanding of the ecology of shallow lakes hasgreatly progressed in recent years, in part as a con-sequence of its importance to humans, due to differentuses (e.g., irrigation, recreation, etc.) and to theoreticalmodels that allowed scientists to test general patterns(Scheffer et al., 1993). After a period of global in-tensive eutrophication, in the last two decades therehas been an increasing interest for the restoration offreshwaters (see e.g. Kumagai & Warwick, 2003).Food-web manipulation has received special atten-tion since this is more feasible in shallow freshwaters

(Moss et al., 1996). A previous step, before biomanip-ulation or some other restoration measures, is a reduc-tion of external nutrient loading (Moss et al., 1996).The first step in the recovery of a eutrophic shallowlake is reduction of orthophosphate concentrations inthe lake below 0.01 mg l−1 (Sas, 1989). Accordingto the biestability model for shallow lakes (Schefferet al., 1993), phosphate reductions should lead to S-shaped responses of phytoplankton rather than a lineardecline. A linear regression between available phos-phorus and phytoplankton biomass should be expectedonly in cases of P limitation for algae (Sas, 1989). Fornorth temperate shallow freshwater lakes, reduction of

282

eutrophication symptoms and re-establishment of sub-merged macrophytes seem only to occur when totalphosphorus is lower than 0.1–0.05 mg l−1 (Jeppesenet al., 2000). For warmer shallow lakes in the Mediter-ranean region there is experimental evidence that thisthreshold should be even lower (Romo et al., 2003).

Sas (1989) argued that nutrient loading reductionfirst affected phytoplankton biomass and later its struc-ture. In general, few works have studied phytoplank-ton changes in shallow lakes after nutrient reduction(Wojciechowski et al., 1988; Sas, 1989; Dokulil &Padisák, 1994; Stoyneva, 1998; Padisák & Reynolds,1998; Beklioglu et al., 1999; Jeppersen et al., 2003;Kangur et al., 2003; Köhler et al., 2000; Jeppesenet al., 2002), and there are no references for south tem-perate, tropical, or subtropical lakes. A general patternor model has not yet been generated. The present studyreports the main algal changes in a shallow Mediter-ranean lake (the Albufera of Valencia, Spain) aftersewage diversion, contributing with data from a southtemperate zone.

Material and methods

Study site

The Albufera of Valencia is a shallow, freshwater,polimictic lake located in the Natural Park of the Al-bufera (210 km2) in the Mediterranean Spanish coast(39◦ 20′ N, 0◦ 20′ W). It is the largest Spanish coastallake with a surface area of 23.2 km2, a mean depth of1 m, and a high water renewal time of about 10 timesper year. The lake has a belt of emergent vegetationand several small islands with reed. Since the 18thcentury, rice has been cultivated in the surroundingareas of the lake, and the lake has been used as a reser-voir for agricultural irrigation. The lake water levelis regulated by sluicegates situated at its three outletchannels which flow into the Mediterranean Sea. Thehydrological cycle markedly influences phytoplank-ton dynamics (Romo & Miracle, 1993). It has twoannual periods of water renewal during emptying ofricefields (January–March) and harvest (September–October), and periods of long or intermediate waterstability during the remaining months.

This shallow freshwater lake was in a submergedplants state at the first half of the 20th century(Arévalo, 1916; Pardo, 1942), and since the 1960seutrophication rapidly turned the Albufera lake intoa turbid, algal dominated state that eliminated sub-merged macrophytes and reduced general biodiversity

in the plankton and benthos (Blanco, 1974; Dafauce,1975). During 1980–1988 phytoplankton was over-whelmed by cyanophytes (Romo & Miracle, 1993,1994) and zooplankton almost wholely composed ofrotifers, with sporadic periods of cladoceran presence(Alfonso & Miracle, 1990; Oltra et al., 2001). Sincethe 1980s, the fish community is mainly composed ofmugilids (Blanco et al., 2003), commercially exploitedby local fishermen. Water physico and chemical vari-ables for the 1980s are reported elsewhere (Soria et al.,1987; Soria, 1997). In 1991, a restoration plan of thelake started by means of nutrient diversion, remov-ing industrial, agricultural, and urban sewage watersfrom the catchment area that has reduced about 77%phosphorus and 24% nitrogen loadings. Effluents fromricefields, mainly in the south lake area, are stilluntreated.

Methods

Monthly or bimonthly phytoplankton samples werecollected after nutrient diversion from November 1997to December 2000, at three sampling points of thelake, to allow comparison with previous algal stud-ies before nutrient diversion for 1980–1988 (Romo& Miracle, 1993). There was no spatial heterogen-eity in the phytoplankton (p > 0.05) and the aver-age of the sampling points is reported in this study.Similarly, algal counting method and biovolume es-timations were made according to Romo & Miracle(1993), counting error being less than ±6% (Lundet al., 1958). The greatest axial length or diameterfor phytoplankton (GALD) was estimated followingReynolds (1984). Algal diversity was measured ac-cording to Shannon & Weaver (1963). Water chem-istry was analysed using standard methods (APHA,1992). Statistical test of ANOVA or Mann–Whitney(non-parametric for variables without a normal distri-bution) were used to compare limnological variablesbefore and after nutrient diversion.

Results

After sewage diversion, some algal limnological vari-ables have changed significantly (Table 1). Orthophos-phate reduced below 0.01 mg l−1 and total phosphorusconcentration in the lake decreased by about 31%,while nitrate did no change (Table 1). Annual meanof chlorophyll-a concentrations diminished almost tohalf, from an annual mean of 318 µg l−1 to 180 µg l−1

283

(Table 1). The lowest concentrations were recordedduring unusual clear water phases ocurring in March1999 and in February 2000, with 1.6 µg l−1 and0.2 µg l−1, respectively. During theses phases, trans-parency reached the lake bottom and lasted up tofive weeks. Chlorophytes (Ankyra cf. judayi (G. M.Smith) Fott, Chlorella vulgaris Beijer, and Chlamydo-monas spp.) and diatoms (Cyclotella spp.) domin-ated the phytoplankton during the clear water phases,representing 65–85% of total algal biovolume.

Although annual chlorophyll-a decreased after nu-trient diversion, the mean annual algal biovolumeremained almost the same, with maximum valuesin spring, while total phytoplankton abundance al-most doubled after nutrient reduction (Table 1). Algalnumbers increased due to unicellular and small colon-ies mainly of chroococcal cyanobacteria, especiallyChroococcus dispersus (Keiss.) Lemm. (mean cellsize 4.1 µm, single cells being the dominant morpho-type in the lake), Chr. minutus (Kütz.) Näg (meancell size 6.9 µm), and Aphanocapsa incerta (Lemm.)Cronb. Kom. (mean size colony 14 µm) (Fig. 1). Thehighest period of algal growth corresponded with thelowest water renewal in the lake in April–September(Fig. 1). Cyanobacteria reduced by about 15% inthe lake (Table 1), due to filamentous cyanobacteriadepletion (from 78% to 48% of total phytoplanktonbiovolume), in favour of chroococcal cyanobacteria(changed from 7 to 23% in the 1990s). Other groupsalso contributed greater in the phytoplankton, espe-cially diatoms (changed from 9 to 20% of the totalbiovolume) and chlorophytes (from 4 to 7%), whileonly euglenophytes were reduced (from 1.4 to 0.4%)(Table 1).

Seasonally, algal composition and structurechanged between the two compared periods (Fig. 1).Before nutrient diversion, Planktothrix agardhii(Gom.) Anag. & Kom., dominated phytoplank-ton abundance early in the year (January–May,50–80% total algal abundance), while other non-heterocystous cyanobacteria, such as Geitlerinema sp.,Pseudanabaena galeata Böcher, and Planktolyngbyalimnetica Kom.-Legn. et Cronb., dominated duringsummer and autumn (40–70% of total algal abund-ance) (Fig. 1). In total biovolume, however, Plankto-thrix agardhii was the dominant algae over all seasons(Fig. 1). After nutrient diversion, P. agardhii almostdisappeared from the lake, annual mean biomass shif-ted from 86 ± 81 mg l−1 in the 1980s to 1.1 ±1.3 mg l−1 in the 1990s, and was replaced season-ally by the slender Pseudanabaena galeata in winter

and spring, and by Planktolyngbya contorta (Lemm.)Anag. & Kom., Pl. limnetica, Chroococcus dispersus,Chr. minutus, Aphanocapsa incerta, and Jaaginemacf. metaphyticum Kom. during summer and earlyautumm (Fig. 1). In abundance, however, Chroococ-cus dispersus predominated during summer and earlyautumn (with 36–50% of total algal numbers), but rep-resented less than 18% of total biovolume. Diatoms(mostly Cyclotella spp. and some Nitzschia species)peaked mainly during periods of lake water renewal, inJanuary–March (35% of total biovolume) and Septem-ber (27% of total biovolume). Shannon-diversity andspecies richness did not change between study peri-ods (Table 1), but mean size of phytoplankton spe-cies showed a significant decrease and phytoplanktonbiomass of the edible size classes increased (GALD<50 µm, Table 1).

Discussion

After nutrient diversion in 1991, planktonic chloro-phyll-a was reduced to half, although hypertrophicvalues persist. According to Cullen & Forsberg(1988), three main responses after external nutrientloading reduction can be expected. The Albufera lakeshowed a response of Type 2, in which the lowerlevels of phosphorus and chlorophyll-a were insuf-ficient to change lake trophic category, although thesystem became less eutrophic. Our results agree withSas (1989) who argued that a response in chlorophyll-a is only expected when orthophosphate in the lakeis below 0.01 mg l−1. In correspondence with sewagediversion, total phosphorus in the Albufera lake alsodiminished about 31%, but mean values still highto reduce eutrophic chlorophyll values or re-establishsubmerged macrophytes for warmer shallow Medi-terranean lakes (TP < 0.05 mg l−1, Romo et al.,2003), which have lower P values than that recommen-ded for northern temperate shallow lakes (Jeppesenet al., 2000). For the Albufera this implies a moresevere removal of sewage effluents and the use of com-plementary restoration measures, to counterbalanceinternal nutrient loading (Moss et al., 1996). Sewagediversion in the lake mostly reduced P-levels, but fer-tilization in the rice fields prevented any reduction innitrate.

In the Albufera lake, changes in the algal com-munity structure indicates a response to P reductionin the direction of increasing equal contribution ofalgal groups, similar to that described in some north

284

Fig

ure

1.M

onth

lym

eans

ofab

unda

nce

(ind

ml−

1)

and

biov

olum

e(m

m3

l−1)

ofth

edo

min

ant

phyt

opla

nkto

nsp

ecie

sin

the

lake

ofth

eA

lbuf

era

(Val

enci

a,Sp

ain)

befo

re(1

980–

1988

)an

daf

ter

(199

7–20

00)

nutr

ient

dive

rsio

n.D

otte

dlin

ere

pres

ents

tota

lphy

topl

ankt

onnu

mbe

rsor

biov

olum

e.

285

Table 1. Limnological and biological variables before (1980–1988) and after (1997–2000) sewagediversion in the lake Albufera of Valencia (Spain). Annual means and standard deviations

Before sewage After sewage

diversion diversion

P Significance

Conductivity (µS cm−1) 1747 ± 270 1878 ± 267 0.480 n.s

Ta ( ◦C) 18.9 ± 6.8 19.6 ± 2.1 0.928 n.s

pH 8.7 ± 0.3 9.0 ± 0.5 0.014 ↑Secchi disc (m) 0.21 ± 0.06 0.27 ± 0.16 0.565 n.s

Nitrate (mg l−1 N) 0.78 ± 0.43 0.97 ± 0.88 0.875 n.s

Ammonium (mg l−1 N) 0.99 ± 1.64 0.84 ± 0.67 0.008 ↓Orthophosphate (mg l−1 P) 0.17 ± 0.22 <0.01 0.004 ↓Total phosphorus (mg l−1P) 0.49 ± 0.20a 0.34 ± 0.16 0.166 n.s

Chlorophyll-a (µg l−1) 318 ± 83 180 ± 53 0.000 �Phytoplankton biovolume (mm3 l−1) 134 ± 80 140 ± 66 0.051 n.s

Phytoplankton abundance (ind ml−1) × 105 3.1 ± 3.0 6.8 ± 3.7 0.000 �Diversity (bits ind−1) 2.38 ± 0.29 2.44 ± 0.41 0.505 n.s

GALD biovolume <50 mm (mm3 l−1) 17 ± 10 65 ± 23 0.000 �GALD biovolume >50 mm (mm3 l−1) 99 ± 78 74 ± 50 0.489 n.s

Algal group biovolume (mm3 l−1)

Cyanobacteria 114 ± 138 106 ± 58 0.315 n.s

Filamentous cyanobacteria 108 ± 138 74 ± 51 0.000 �Chroococcal cyanobacteria 6.8 ± 15.5 32 ± 21 0.000 �

Chlorophyta 2.7 ± 2.4 7.2 ± 4.6 0.000 �Bacillarophyta 5.5 ± 5.9 23 ± 16 0.000 �Cryptophyta 0.56 ± 0.59 1.3 ± 1.0 0.015 �Dinophyta 0.16 ± 0.22 0.73 ± 0.7 0.000 �Euglenophyta 0.75 ± 1.77 0.21 ± 0.19 0.018 �

aTotal phosphorus data before nutrient diversion correspond to 1988. n.s. = not significant. Arrowsindicate the direction of change. For details see Material and methods.

temperate lakes (Köhler et al., 2000; Jeppesen et al.,2002). In addition, euglenophytes which indicate or-ganic matter polution in the lake (Romo & Miracle,1994) steeply decreased. According to Sas (1989)P reduction will first affect algal biomass and laterphytoplankton community structure. Our results sup-port the opposite response, such as was also observedby Padisák & Reynolds (1998) in another shallow lake.In Lake Albufera, algal composition had pronouncedchanges but algal biovolume remained unchanged andabundance even increased. There was a significanttrend of reducing algal and cyanophyte size, similarto that found in some shallow lakes under oligo-trophication (Wojciechowski et al., 1988; Stoyneva,1998; Köhler et al., 2000). Smaller dominant spe-cies had less chlorophyll content (Reynolds, 1984),which could explain discordance between chlorophylland algal biovolume trends. According to Berger &Sweers (1988) and Nicklisch et al. (1991), the large

Planktothrix agardhii is outcompeted by slender fila-mentous cyanobacteria, under conditions of reducedP supply and higher underwater light. In the Albufera,this species was described as sensitive to SRP shortage(Romo & Miracle, 1993), and during the 1990s hasbeen replaced by Ps. galeata. A similar P. agardhiidecline was also observed in some shallow restoredlakes (Scheffer et al., 1997; Köhler et al., 2000). Incontrast with northern lakes, in the Albufera, smallcolonies of chroococcal cyanobacteria replaced fila-mentous cyanobacteria mainly during summer. Thereis experimental evidence that TP levels lower than0.3 mg l−1, water quiescence, and low zooplanktongrazing replaced filamentous cyanobacteria in favourof chroococcal cyanobacteria in a shallow Mediter-ranean lake (Romo et al., 2003). These conditionsoccur during summer in the Albufera, with maximumfish populations (Blanco et al., 2003). Cyanobacteriapredominance in the lake is in accordance with this

286

group temperature dependence (Reynolds, 1992) andits more relevant presence on food-webs of warmershallow lakes (Romo et al., 2003). Since the beginningof the 20th century cyanobacteria were well repres-ented in the Albufera, mainly during summer-autumnand co-existing with chlorophytes and tycoplanktonicdiatoms in other seasons (Pardo, 1942).

According to Reynolds (1992), filamentous non-heterocystous cyanobacteria will dominante eutrophicpolimictic shallow lakes, but alternatively with diat-oms at higher flushing rates. Seasonally, the mainchange between the study periods accounted earlierin the year, between January and March. Disappear-ance of Planktothrix agardhii early in the year allowedcentric diatoms and other small, fast-growing algaeto dominate during these months of water renewal.Cyclotella spp. may/can come from the paddy fieldsduring this period (Romo, 1997). Edible algae facilit-ated subsequent clear water phases produced by largeCladocera. In temperate lakes, the clear water phasesoften take place between April and August (Schef-fer et al., 2001), but in the Albufera these were inFebruary–March, due probably to algal biomass re-duction by flushing and change in composition, mildwinter water temperatures for cladoceran to overcrop(winter mean 12 ◦C), and decreasing fish predation onzooplankton by maximum planktivorous and benthi-vorous catches by commercial fishermen. Responseto nutrient loading reduction also depends on waterretention time (Jeppesen et al., 2003). Some studiessupport the idea that phytoplankton biomass or cy-anobacteria depletion could be accelerated by flushing(Olli, 1989; Bailey-Watts et al., 1990; Jagtman et al.,1992; Köhler et al., 2000), even for large lakes suchas the Albufera (e.g. lake Worderwijd 2600 ha, Zm= 1.5 m, Meijer & Hosper, 1997). But this is a spe-cific measure that requires changes in the Albuferahydrological cycle and land use (ricefields) and is onlyeffective in the long-term in combination with com-plementary restoration measures, such as a reductionof lake-P below 0.05 mg l−1, control of nitrogen andpesticides in the catchment area, and managementof fish communities, in order to re-establish phyto-plankton composition and submerged plants as at thebeginning of the 20th century.

Acknowledgements

We wish to thank Juan Miguel Benavent, MaríaSahuquillo, and the guards from the Albufera Nature

Park for their valued assistance during all years’samplings. We specially thank Carmen Ferriol for de-termining chlorophyll-a during clear water phases andDr Erik Jeppesen for valuable discussion of the res-ults. We are also grateful to the Consellería de MedioAmbiente de la Comunidad Valenciana and the Ofi-cina Técnica Devesa-Albufera for the facilities givenand supply of the physical and chemical data in the1990s. This work was funded during 1997–2000 by aEuropean Community project (SWALE EnvironmentProject ENV4-CT97-0420).

References

APHA-AWWA-WEF, 1992. Standard Methods for the Examinationof Water and Wastewater, 18th edition. American Public HealthAssociation. Washington D.C.

Alfonso, M. T. & M. R. Miracle, 1990. Distribución espacial delas comunidades zooplanctónicas de la Albufera de Valencia.Scientia gerundensis, 16/2: 11–25.

Arévalo, C. 1916. Indroducción al estudio de los Cladóceros delplancton de la Albufera de Valencia. Trab. Lab. Hdrob. Esp. 1.Anales del Instituto General y Técnico de Valencia, vol. I.

Bailey-Watts, A. E., A. Kirika, L. May & D. H. Jones, 1990.Changes in phytoplankton over various time scales in a shallow,eutrophic lake: the Loch Leven experience with special referenceto the influence of flushing rate. Freshwat. Biol. 23: 85–111.

Beklioglu, M., L. Carvalho & B. Moss, 1999. Rapid recovery of ashallow hypertrophic lake following sewage effluent diversion:lack of chemical resilience. Hydrobiologia 412: 5–15.

Berger, C. & H. E. Sweers, 1988. The Ijsselmeer and its phytoplank-ton with special attention to the suitability of the lake as a habitatfor Oscillatoria agardhii Gom. J. Plankton Res. 10: 579–599.

Blanco, C. 1974. Estudio de la contaminación de la Albufera deValencia y de los efectos de dicha contaminación sobre la faunay flora del lago. PhD Thesis, Universidad de Valencia (Spain),193 pp.

Blanco, S., S. Romo, M. J. Villena & S. Martínez. 2003. Fish com-munities and food web interactions in six shallow Mediterraneanlakes. Hydrobiologia 506–509: 473–480.

Cullen, P. & C. Forsberg, 1988. Experiences with reducing pointsources of phosphorus to lakes. Hydrobiologia 170: 321–336.

Dafauce, C. 1975. La Abufera de Valencia. Un estudio piloto.Monografías ICONA 4: 1–127.

Dokulil, M. T. & J. Padisák, 1994. Long-term compositionalresponse of phytoplankton in a shallow, turbid environment,Neusiedlersee (Austria/Hungary). Hydrobiologia 276: 125–137.

Jagtman, E., D. T. Van Der Molen & S. Vermij, 1992. The influ-ence of flushing on nutrient dynamics, composition and densitiesof algae and transparency in Veluwemeer, The Netherlands.Hydrobiologia 233: 187–196.

Jeppesen, E., J. P. Jensen & M. Sondergaard, 2002. Response ofphytoplankton, zooplankton, and fish to re-oligotrophication: An11 year study of 23 Danish lakes. Aquat. Ecosyst. Health Manag.5: 31–43.

Jeppesen, E., J. P. Jensen, M. Sondergaard, T. Lauridsen & F.Landkildehus, 2000. Trophic structures, species richness andbiodiversity in Danish lakes: changes along a phosphorus gradi-ent. Freshwat. Biol. 45: 201–213.

287

Jeppesen, E., M. Sondergaard, N. Mazzeo, M. Meerhoff, C. C.Branco, V. Huszar & F. Scasso, 2003. Lake restoration and bio-manipulation in temperate lakes: relevance for subtropical andtropical lakes. In Reddy, V. (ed.), Tropical Eutrophic Lakes:Their Restoration and Management (in press).

Kangur, K., T. Möls, A. Milius & R. Laugaste, 2003. Phytoplanktonresponse to decreasing nutrient level in Lake Peipsi (Estonia) in1992–2001. Hydrobiologia 506–509: 265–272.

Köhler, J., H. Behrendt & S. Hoeg, 2000. Long-term responseof phytoplankton to reduced nutrient load in the flushed LakeMüggelsee (Spree system, Germany). Arch. Hydrobiol. 148:209–229.

Kumagai, M. & F. V. Warwick, 2003. Freshwater Management.Global Versus Local Perspectives. Springer-Verlag, Berlin.

Lund, J. W. G., C. Kipling & E. D. Le Cren, 1958. The inverted mi-croscope method of estimating algal numbers and the statisticalbasis of estimations by counting. Hydrobiologia 11: 143–170.

Meijer, M. L. & H. Hosper, 1997. Effects of biomanipulationin the large and shallow lake Wolderwijd, the Netherlands.Hydrobiologia 342/343: 335–349.

Moss, B., J. Madgewick & G. Phillips, 1996. A guide to the restor-ation of nutrient-enriched shallow lakes. Environmental Agency,Broads Authority, Manchester, 180 pp.

Nicklisch A., B. Roloff, & A. Ratsch, 1991. Competition experi-ments with two planktonic blue-green algae (Oscillatoriaceae).Verh. int. Ver. Limnol. 24: 889–892.

Olli, E. V., 1989. Simulated impacts of flow regulation on blue-green algae in a short retention time lake. Arch. Hydrobiol. 33:181–189.

Oltra, R., M. T. Alfonso, M. Sahuquillo & M. R. Miracle, 2001.Increase of rotifer diversity after sewage diversion in the hy-pertrophic lagoon, Albufera of Valencia, Spain. Hydrobiologia446/447: 213–220.

Padisák, J. & C. S. Reynolds, 1998. Selection of phytoplanktonassociations in Lake Balaton, Hungary, in response to eu-trophication and restoration measures, with special reference tocyanoprokaryotes. Hydrobiologia 384: 41–53.

Pardo, L., 1942. La Albufera de Valencia. Biología de las aguas con-tinentales II. Instituto Forestal de Investigaciones y Experiencias,Madrid, 263 pp.

Reynolds, C. S., 1984. The Ecology of Freshwater Phytoplankton.Cambridge University Press, Cambridge.

Reynolds, C. S., 1992. Eutrophication and the management of

planktonic algae: what Vollenweider couldn’t tell us. In Sutcliffe,D. W. & J. G. Jones (eds), Eutrophication: Research andApplication to Water Supply. Freshwater Biological Association,Ambleside: 4–29.

Romo, S., 1997. Importance of allochthonous phytoplankton in acoastal freshwater lake. Verh. int. Ver. Limnol. 26: 610–614.

Romo, S. & M. R. Miracle, 1993. Long-term periodicity of Plankto-thrix agardhii, Pseudanabaena galeata and Geitlerinema sp. ina shallow hypertrophic lagoon, the Albufera of Valencia (Spain).Arch. Hydrobiol. 126: 469–486.

Romo, S. & M. R. Miracle, 1994. Population dynamics and ecologyof subdominant phytoplankton species in a shallow hypertrophiclake (Albufera of Valencia, Spain). Hydrobiologia 273: 37–56.

Romo, S., M. R. Miracle, M. J. Villena, J. Rueda, C. Ferriol &E. Vicente, 2003. Mesocosm experiments on shallow lake foodwebs in a Mediterranean climate. Freshwat. Biol.. In press.

Sas, H. 1989. Lake restoration by reduction of nutrient load-ing: expectations, experiences, extrapolations. Academia VerlagRicharz, Sankt Augustin, 497 pp.

Scheffer, M., S. Carpenter, J. A. Foley, C. Folke & B. Walker, 2001.Catastrophic shifts in ecosystems. Nature 413: 591–596.

Scheffer, M., S. H. Hosper, M. L. Meijer, B. Moss & E. Jeppesen,1993. Alternative equilibria in shallow lakes. Trends Ecol. Evol.8: 275–279.

Scheffer, M., S. Rinaldi, A. Gragnani, L. R. Mur & E. H. van Nes,1997. On the dominance of filamentous cyanobacteria in shallow,turbid lakes. Ecology 78: 272–282.

Shannon, C. E. & W. Weaver, 1963. The Mathematical Theory ofCommunication. University of Illinois Press, Urbana, Illinois,117 pp.

Soria, J. M., 1997. Estudio limnológico de los ecosistemas acuáti-cos del ‘Parc Natural de l’Albufera’ de Valencia. PhD Thesis,Universidad de Valencia, Valencia, 289 pp.

Soria, J. M., M. R. Miracle & E. Vicente, 1987. Aporte de nutrientesy eutrofización de la Albufera de Valencia. Limnetica 3: 227–242.

Stoyneva, M. P., 1998. Development of the phytoplankton of theshallow Srebarna Lake (north-eastern Bulgaria) across a trophicgradient. Hydrobiologia 369/370: 259–267.

Wojciechowski, I., W. Wojciechowska, K. Czernas, J. Galek & K.Religa, 1988. Changes in phytoplankton over a ten-year periodin a lake undergoing de-eutrophication due to surrounding peatbogs. Arch. Hydrobiol. 78: 373–387.