Hydrobiologia 200/201 : 427-436, 1990. R. D. Gulati, E. H. R. R. Lammens, M.-L. Meo'er & E. van Donk (eds), Biomanipulation - Tool for Water Management. © 1990 Kluwer Academic Publishers. Printed in Belgium.

427

Proposals for macrophyte restoration in eutrophic coastal lagoons

Francisco A. Comin, Margarita Men6ndez & Juan R. Lucena Department of Ecology, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain

Key words." Lagoons, macrophyte recovery, phytoplankton, nutrients, hydrology

Abstract

Based on the comparison of environmental requirements for Ruppia cirrhosa and Potamogeton pectinatus growth, macrophyte versus phytoplankton biomass and production features, and differences in hydrological and nutrient balances between Tancada lagoon (where macrophytes form dense beds) and Encafiizada lagoon (with no macrophytes at all), several proposals for macrophyte restoration are presented. The highest photosynthetic efficiency of R. cirrhosa takes place at high irradiance and it grows over a wide range of salinity. P. pectinatus is better adapted to lower light intensity and salinity than Ruppia. R. cirrhosa transplanted from Tancada to Encafiizada was successful in enclosures, where light availability increased (# = 0.013 cm-Z), but not in open waters where light extinction coefficient was 0.032 cm-~. Phytoplankton biomass (0.11-2.15 g C m -z) is much lower than macrophyte biomass (16-200 g C m -2) in Tancada lagoon. However, phytoplankton production (165 g C m -2 yr-~ in Tancada, 480 g C m - 2 yr- ~ in Encafiizada) is the same order of magnitude as macrophyte production (244-467 g C m - 2 yr- t). Turnover rates are 0.3-0.9 day- ~ for phytoplankton and 1.2-2.5 yr- 1 for macrophytes. Phytoplankton and inorganic particles are responsible for high turbidity of the water in Encaflizada lagoon. Phytoplankton blooms in Encafiizada lagoon are supported by high freshwater inflows from rice field drains from May to November. The Qs (seawater discharge)/Qv (Freshwater discharge) ratios are, respectively, 0.24 and 0.48, which denotes a higher seawater influence in Tancada than in Encafiizada lagoon. Decreasing freshwater inputs to Encaflizada lagoon both in May and November thus allowing greater inputs of sea water, is proposed as the most effective way to restore this eutrophic coastal lagoon.

The objective being to reduce nutrient loadings to the lagoon and phytoplankton in order to favour macrophyte re-colonization.

Introduction

Biomanipulation for restoration of aquatic eco- systems has focussed mainly on inland lakes. Because of the intensive occupation of the coasts in temperate latitudes all around the world, eutrophication of enclosed coastal waters has be- come more and more frequent. A need for water quality improvement has arisen from increasing

interest by groups concerned with fisheries, re- creation, and conservation.

The most characteristic feature of estuarine ('sensu lato') ecosystems is, perhaps, the mixing of continental and marine waters governed by physi- cal processes irregular in time and space (Knox, 1986). As a consequence productivity is promoted in these ecosystems, although it is also subject to irregular changes (Nixon, 1982).

428

The effects of eutrophication in coastal areas are similar to those in freshwater ecosystems (PaerI, 1988), namely increase of the nutrient con- centrations and plant populations with high nutrient uptake and growth rates.

In the Ebro Delta there is a demand to improve the water quality of coastal lagoons for fisheries, waterfowl, mad recreational purposes. Biomani- pulation should aim to control phytoplankton blooms in order to favour the development of rooted macrophytes which provide hiding and nesting areas for animals. Here, data on the inter- actions between phytoplankton and rooted ma- crophytes and about the requirements of macro- phytes for growth are presented. Further, a com- parison between two lagoons at the ecosystem

level is made and alternative proposals for using biomanipulation to restore eutrophic lagoons are discussed.

Description of sites studied

The two lagoons studied (Encafiizada and Tancada) are located in the Ebro Delta, NE Spain (Fig. 1). They are very shallow and surrounded by a narrow helophytic and halophylic plant belts. They receive drainage from irrigated rice-fields during April to November. Because of the freshwater inflows, conductivity decreases from 30 mS cm - ~ in early April to 1 mS c m - ~ in November in Encafiizada lagoon, while it changes

• / i 1 4 ',,

. ~ . / . ; - , ," - - t . . . . - " - - J

t ~ J " J $ s •

IO R I V E ~

; i i i i

I

, , , , , , ', , ',, ,;, - .

i ",~- "'.ENCAI~IZADA ~ ', t TANCA6"A-'.." " - -

i x " •

0 S K m I i

Fig. 1. Map of the Ebro Delta showing the location of the coastal lagoons and the sampling stations in Tancada and Encafiizada lagoons and channels. • water samples.

from 40 mS c m - I in late April to 2 mS c m - ~ in November in Tancada lagoon (Fig. 2). The bot- tom is very flat in both lagoons. Sediments are typical alluvial muds with a high proportion of silt, and sand is more abundant near the shore.

Some morphometric and hydrologic charac- teristics of the two lagoons are given in Table 1. Differences in qualitative and quantitative aspects of the biological populations have been described earlier (Comin, 1982). One important difference is

Table 1. Morphometric and hydrologic characteristics of Tancada and Encafiizada lagoons. Qv: freshwater dis- charges; Q~: seawater discharge; V: water volume of the lagoon.

Encafiizada Tancada

Surface, km 2 5.5 1.8 Volume, m 3 2.443.900 672.400 Average depth, cm 100 80 Mater level, cm 92-110 76-94 Turnover rate, month- I

Freshwater 2.9 2.5 Seawater 1.1 1.5

QF m3 (May-November) 51.000.000 10.500.000 Q~ m 3 (December-April) 12.300.000 5.000.000 Qv/V 20.8 15.7 Q.~/V 5 7.5 Q~/Qv 0.24 0.48

429

the presence and yearly development of extensive and dense, rooted macrophyte beds (mostly Ruppia cirrhosa (Petagna) Grande and Potamoge- ton pectinatus L. in Tancada lagoon (Men6ndez & Comin, in press) and the total lack of sub- merged macrophytes and the huge development of phytoplankton, including cyanobacteria in late summer, in Encafiizada lagoon (Comin, 1984), although submerged rooted macrophytes used to be diverse and extended over most of this lagoon until 1976 (Comin & Ferrer, 1979; Ferrer & Comin, 1979).

M a t e r i a l s and meth od s

Monthly estimations of water, macrophytes and phytoplankton variables were done during an annual period; for methods used see, Table 2.

Macrophyte biomass was determined monthly from triplicate samples collected with a core sampler (16-cm diameter) in two places of Tancada lagoon. Dry weights were measured after 24 h at 105 °C. Details of the methods are in Men6ndez & Comin (in press). Recommen- dations by Verhoeven (1980) were followed. Dry weight biomasses were converted to carbon using the carbon content of macrophytes determined

Table 2. Methods of analysis.

Parameter Methods of analysis Reference

Ammonia Indofenol. Grasshoff, 1983

Nitrate + nitrite Copper-cadmium reduction by autoanalyzer. Technicon industrial systems

Total nitrogen in water Digestion with basic persulfate solution, then nitrogen is Grasshoff, 1983 analized as nitrate + nitrite as above.

Single solution molybdenum blue by autoanalizer.

Digestion with persulphate solution, then determination of orthophosphate as above.

Wet digestion with nitric, perchloric and sulfuric acids then determination of phosphorus by vanadomol 1 bdo-phosphoric acid method.

Elemental analizer of C/N (Carlo Erba).

Orthophosphate

Total phosphorus in water

Total phosphorus in plant

Total nitrogen and car- bon in plant

Head, 1985

Grasshoff, 1983

Jackson, 1970

430

every month. Turnover times of 1.8-2.5 for Rupp& cirrhosa (Verhoeven, 1980) and 1.2 for Potamogeton pectinatus (Howard-Williams, 1978) were applied to obtain annual productions for both species.

Phytoplankton biomass and production were estimated from samples collected at 20-cm depth in two places in both lagoons. Chlorophyll-a and carbon (lac) assimilated were determined follow- ing Vollenweider (1969). Detailed methods and data can be found in Comin (1982, 1984). The ratio primary production/biomass (P/B) was cal- culated applying a conversion factor of 50 (car- bon/chlorophyll-a) (Margalef, 1983) to the chlo- rophyll-a concentration to obtain biomass as car- bon weight. Primary production is the assimilated (~4C method) carbon measured in the field.

Laboratory experiments were done to know the optimal salinity, temperature and light conditions for R. cirrhosa growth. Net productivity of Ruppia was measured using the oxygen technique (Vollenweider, 1969). Ruppia leaves were en- closed in light and dark bottles and the dissolved oxygen in the water measured by the Winkler method (Grasshofet al., 1983). Short-term (2-h) experiments were done (three replicates) for dif- ferent combinations of light (20-2500/~E m -2 s-~), temperature (10-40 °C) and conductivity (9.12-52. I mS cm - 1) at several stages of Ruppia development.

Water discharges to the lagoons were estimated from monthly flows of water integrated for the particular morphometry of the different channels. Nutrient discharges were estimated as the pro- ducts of the water discharges and the analyzed nutrient concentrations (Table 2).

Results

Growth of R. cirrhosa and P. pectinatus L. under different light and salinity conditions

The water variables most significant for the growth of both species in the Ebro Delta lagoons are salinity and light. The other water characteris- tics mentioned in Table 3 are between the range of both species requirements. R. cirrhosa does not occur in water where the salinity is below 1.5%o C1- and it grows well in waters in salinity up to 35%0 CI- (Verhoeven, 1979). The best growth of P. pectinatus takes place at salinities below 3%0 Ct- (Van Wijk, 1988, Van Wijk et al., 1988).

Laboratory experiments showed that R. cir- rhosa net productivity is optimum at light inten- sities about 700 #E m -2 s - l, salinities of 20 parts per mil and a temperature of 25 °C (Fig. 2). Unfortunately, there are no experimental data about photosynthetic rates of P. pectinatus. How- ever, germination of P. pectinatus is not affected

Table 3. Environmental requirements of Ruppia cirrhosa and Potamogeton pectinatus as defined by selected variables. Minimum and maximum values (between brackets) and average values or ranges are shown.

Ruppia cirrhosa Potamogeton pectinatus

Salinity, %0 CI- (1.5) 2-35 (60) (0.3) 3 (9) Temperature, °C (0) 10-30 (38) (10) 15-25 (37) Light,/~E m - 2 s - ~ (200) 700 (2000) - Extinction coefficient, cm- ~ 0.0025-0.0095 0.0106-0.025 pH (7.4) 7 (10.4) (7) 8.5 (9.8) Alkalinity, meq 1- ~ 1-4 1.4-10.4 Maximum depth, cm 150 500 Sediment Mud (Sand) (Clay) Clay-sand (Sand) Hydrodynamism Optimum not in shallow dynamic

zones but deeper waters. Resistant to water level changes and

even short dry periods.

1. Verhoeven (1980), and authors non published data. 2. Van Wijk (1988); Van Wijk et al. (1988); Howard Williams (1978); Purohit & Singh (1987).

6E4

5E4 u u~

4E4

3E4 g

2E4 0 u

IEI

F M A M J J A S O N D J F M A

Fig. 2. Seasonal changes of conductivity in Tancada and Encafiizada lagoons. O Encafiizada • Tancada.

by light (Purohit & Singh, 1987) and the early growth of the plant is mainly controlled by tem- perature (Spencer, 1986). Moreover, Spencer also stated that irradiance only affected early growth of P. pectinatus at temperatures between 23 and 30 °C.

An other point on the relative importance of light on both species growth comes from their architecture and life cycle. R. cirrhosa growth takes place exclusively from rhizomes while P. pectinatus grows from both rhizomes and tu- bers. R. cirrhosa grows In'st during April-May horizontally by developing the rhizomes in a net- work from which new leaves arise. Then, during June-July vertical growth takes place by develop- ing new branched leaves from the shoots (see Verhoeven, 1979 for details). In contrast, P. pecti- natus grows predominantly vertically, both below and above ground parts, particularly so in turbid waters where it reaches the water surface in an earlier stage and the foliage is concentrated in the surface water layer (Van Wijk et al., 1988). These authors also remark the adaption of P. pectinatus populations from brackish waters, which usually are turbid, to produce long shoots even when they were cultured in sparce clear water. These all are evidences of higher requirements of light for R. cirrhosa than for P. pectinatus growth and more resistance of the latter under turbid conditions of the water.

431

Biomass and production of phytoplankton and ma- crophytes

Phytoplankton biomass and macrophytes between the two lagoons remarkably differed (Table4). Phytoplankton biomass is always higher in Encaflizada than in Tancada. Maximum biomass is between 2.5 and 6 times higher. Phyto- plankton primary production rate in the lagoons also differed although not so strikingly as does biomass. Macrophyte biomass in Tancada lagoon was 16 to 34 times higher than phyto- plankton biomass at any time (Table 5). How- ever, macrophyte production was only 2.8 (in the case of R. cirrhosa) and 1.4 (in the case of P. pectinatus) times greater than phytoplankton production in Tancada lagoon. Annual produc- tion of phytoplankton in Encafiizada was similar to Ruppia annual production and 2 times higher than Potamogeton annual production in Tancada lagoon. Phytoplankton turnover rates were much higher than macrophyte turnover rates. R. cir- rhosa showed a higher P/B ration than P. pectina- tus, which would also confer to the former higher requirements of energy (e.g. nutrients, light) per biomass and time unit to support its high produc- tion.

Encahizada and Tancada lagoon

The two lagoons had contrasting characteristics at the ecosystem integration level. First of all, the

Table 4. Characteristics of the phytoplankton in Tancada and Encafiizada lagoons during two periods of the year, (freshwater period: April to December, seawater period: De- cember to April).

Max. Primary P/B biomass production

mgCm-2 mgCm-2 day-' Enca~izada

Freshwater period 4.3 148.3 0.08 0.55 Seawater period 6.2 332.0 0.11 0.87

Tancada Freshwater period 1.9 119.4 0.08 0.95 Seawater period 1.0 45.5 0.41 1.37

432

Table 5. Differences between phytoplankton and macrophytes in Tancada lagoon.

Biomass Production P/B

C:N:P g C m -2 g C m - 2 y r -~

Phytoplankton 100: 16-23:1 0.113-2.154 164,87 0.08-1.37 day- M-D 906:38:1

Ruppia cirrhosa J-A 731:29:1 16.76-217.15 466.87 1.8-2.5 yr- 1 M-D 1216:31:1

Potamogeton pecthmtus J-A 838 : 20: I 17.65-203,82 244.58 1,2 yr '

M-D: May to December. J-A: January to April.

water in Encafiizada was much more turbid than in Tancada lagoon (Fig. 3). Consequently the light extinction coefficients were much higher in the former than in the latter lagoon (Table 6). In Depth Encafiizada Taneada

cm cm- ' em- ' fact, only 1.6% of the surface light (2800 #E m - 2 s - ~ in July) reached 70 cm depth in Encafiizada, lO 0.047 0 while 51% does so in Tancada. The fact that 20 0.042 0.0024 Tancada lagoon is shallower that Encafiizada 30 0.039 0.0059

40 0.047 0.0066 (Table 1) is not important p e r se because quite a 50 0.057 0.0072 large area in Encafiizada lagoon is as shallow as 60 0.054 0.0071 Tancada and the water there is also very turbid. 70 0.059 0.0095

The ratio between freshwater discharge and 8o 0.079 water volume is 28 for Encafiizada and t5.6 for loo 0.072 Tancada (Table 1). Most of the water discharge to the lagoons takes place in the early phase of the rice growing period (May) and, particularly, during harvest when rice fields are drained (Oc- tober-November).

~Em s

0 500 ~000 t500 2000 2500 0 a i . ~ . . . .

1 0 . 20- 30. 40- 50- 60. 70: ~o 90

100

30O0 i

Fig. 3. Depth profiles of light intensity (#E m-2 s " ) in Tancada and Encafiizada lagoons. • Encafiizada • Tan-

cada,

Table& Light extinction coefficients in Encafiizada and Tancada lagoons in July 1988.

In addition to water volume, Encafiizada lagoon also receives higher concentrations of DIN than Tancada lagoon, but the other nutrient concentrations did not differ from those in the water flowing into Tancada lagoon. Nutrient dis- charges estimated per unit area are similar for both lagoons but for DIN, which is four times higher for Encafiizada, about half of the dis- charges of nutrients to the lagoons take place in May (early rice growth stages) and November (emptying rice-fields period).

The TN : TP ratio increase from the inflowing waters to the lagoon waters, which means that phosphorus is removed from the water column (its concentration decreases) more readily than nitrogen and almost in the same magnitude in both lagoons (Table 7). However, the changes in the ratios DIN : DIP from the inflowing waters to the lagoon waters are different. It increases in

433

Table 7. Comparative data on nitrogen and phosphorus (total and dissolved inorganic) between Encafiizada and Tancada lagoons.

Lagoon water Nutrient discharge Maximum discharge

ItM Range ~M Range May November mg m - 2 month - ~ mg m - 2 month - i

Encafiizada

TN 95 30-270 69 40-97 TP 2,69 1,28-3.48 3.7 1.48-8.99 DIN 21.1 8-41 47 1-76 DIP 1.56 0,4-4.65 2.21 0.6-4,41 T N : T P 37:1 15:1-93:1 31:1 27:1-34:1 D I N : D I P 16:1 9:1-28:1 37:1 10:1-77:1

T{tltcad(1

TN 94 36-230 66 38-80 TP 2.47 1.02-4A7 3.76 1.20-9.18 DIN 12.93 1-31 12.5 3.5-25 DIP 0.52 0.1-1.78 1.16 0.17-3.42 T N : T P 25:1 32:1-160:1 42:1 13:1-67:1 D I N : D I P 45:1 108:1-5:1 31:1 4:1-148:1

1274 896 (10%) 2963 (33%) 152 - (-) 395 (37%) 909 608 (9%) 3642 (56%)

o/ 91 18 (3%) 298 (47/.o)

767 1182 (22%) 647 (12%) 968 - ( - ) 185 (27%) 122 214 (26%) 205 (24%) 66 112 ( 1 6 ~ ) 390 (55%)

Tancada and decreases in Encafiizada, perhaps because denitrification proceeds actively in this highly nitrogen loaded lagoon but not in Tancada. Moreover, phosphorus could be taken up pre- ferentially by the macrophytes in Tancada lagoon as their C : N : P ratios (Table 5) indicate phos- phorus limitation according to Atkinson & Smith (1983). The crucial point is that water discharge from rice-fields contains a high proportion of par- ticulate and dissolved matter (For6s, 1989). Then, it can be processed in the lagoons depending on their trophic status, slowly and accumulating in Encafiizada and faster, oxydized and exported in Tancada.

Discussion

Proposals for biomanipulation

Restoration of macrophytes in Encafiizada lagoon should be the main purpose of biomanipu- lation because in Tancada lagoon fisheries and waterfowl are important resources in comparison

with Encafiizada lagoon, where both resources decreased in relation to the eutrophication pro- cess (Demestre et al., 1977; Comin et al. unpub- lished data).

The first approach, at the population inte- gration level, is an attempt to directly introduce again macrophytes in Encafiizada lagoon (Fig. 1). Transplants of R. cirrhosa from Tancada to Encafiizada lagoon were successfull only if enclosed in microcosms where turbidity de- creased due to particle sedimentation and the ab- sence of grazing by fishes (Table 8, Fig. 4). More- over, data about environmental requirements of R. cirrhosa and P. pectinatus'indicate that the for- mer is more adequate to be replanted if turbidity can be decreased about ten times (Tables 3 and 6) because it can tolerate the annual salinity changes in that lagoon. P. pectinatus would be more suited than R. cirrhosa to be replanted because it can tolerate turbid waters. However, P. pectinatus would not be able to maintain itself if salinity keeps fluctuations as it does by now (Fig. 2, Table 3). Then, this proposal could be experimen- tally implemented in artificially isolated areas

434

Table 8. Comparative results of growth of Ruppia cirrhosa replanted in Encafiizada within microcosm and in open water.

Light Time Percent Condition extinction preserved plant of the plants coefficient alive alive cm - I year

Within microcosms 0.013 1 25

In open water 0.032 0.08 10

Health growth slight epiphytism flowers, fruits

Weak consistency highly epipjyt- ized no flowers, no fruits

DIRECT REPLANTING

IN ( <

OPEN WATER

[ PHYTOPLANKTON TURBIOI,T Y

iT,; DIRECT REPLANTING IN

I ENCLOSE WATER

MACROPHYTE ~,/_~ BENTI C ALGAE 1 GROWTH GROWTH

Fig. 4. A diagrammatic representation of the interactions between populations in Encafiizada lagoon after macrophyte replanting.

where freshwater inputs and turbulance could be controlled. This is expensive both in time and people but could be temporarily successful and provide complementary information about ma- crophyte survival relative to a low cost.

The second approach, at the community inte- gration level, is an attempt to decrease phyto- plankton populations in Encafiizada lagoon by interaction with other populations (Fig. 5). Re- ducing fish populations has been successfully used for light and macrophyte improvements in inland small lakes (Gulati, 1989; Meijer etaI., 1989). The operativeness of such biomanipulation is doubtful in large lagoons because of the diffi- culties to control fish movements between the lagoons and to close systems. However, setting up barriers for sediment disturbing fishes (e.g. carp) to enter the lagoon would decrease sediment resuspension favouring light transmittance

through the water column. Moreover, seasonal changes of salinity linked to water turnover do not permit zooplankters to develop dense and per- manent populations in these lagoons. In fact, zooplankton biomass is about ten times higher in Tancada than in Encafiizada lagoon but the populations of the different species develop only for short periods of time (Men6ndez & Comin, 1986). Then, it is not likely that zooplankton grazing plays a key role, even when increased ten times, in the control of phytoplankton, particular- ly when filamentous cyanobacteria are abundant (Comin, 1982).

The third approach deals with manipulation at the ecosystem integration level. Eutrophication of the coastal lagoons in the Ebro Delta is caused by the inflow of agricultural irrigation water. Re- ducing the nutrient discharge, particularly phos- phorus inputs, if P is the limiting nutrient, is one

435

mACROPHYTE $ J

PHYTOPCANKTONI , I "I .... I. I'ILIGHT ..... l CONSUMPTION~ I //'-"~ PH Y TOPLANK TON ~-'~ ~ - I I i IAVAILABIL!IY ] SEDItIENT l j

DISTURBANCE t PHYTOBE~THOS J GROWTH I

Fig. 5. A diagrammatic representation of the interactions between different components of the community in Encafiizada lagoon after biomanipulation to increase phytoplankton consumption.

of the most frequent and recommended actions to recover eutrophicated inland aquatic ecosystems (Vollenweider, 1968; Likens, 1972; Golterman, 1975). However, reducing nutrient discharge may not be enough in shallow lakes as nutrient release from the sediments can reactivate the phytoplank- ton growth. Removal of sediment has been also necessary to be successful in these environments (Moss et al., 1986). In the Ebro lagoons, decrease of nutrient discharge can easily be accomplished by diverting water. This is much cheaper than industrial water treatment. In addition, lowering freshwater discharge would also imply decreasing the inflows of particulate and organic materials (Prat et al., 1988). In any case, if the N : P ratios in Encafiizada lagoon change towards Tancada values phytoplankton production would be more likely limited by phosphorus rather than by nitro- gen (Smith, 1979), which could be a starting point to avoid cyanobacteria proliferation (Smith, 1983). It is possible that for phytoptankton pro- duction there is a shift from P-limitation during freshwater periods to N-limitation during sea- water periods in the coastal lagoons of the Ebro Delta (Comin & Valiela, m press). Besides, inter- nal biogeochemical processes in Encaflizada lagoon are likely to be more important than loadings in determining the N : P ratios, as in many other estuaries (Howarth, 1988). Then attempts to control N/P ratios of the inflowing waters to coastal lagoons may not be effective.

Reducing freshwater inputs to Encafiizada lagoon will also decrease the water depth (Fig. 6). Increasing salinity and available light in the water column (which will promote R. cirrhosa growth are among the positive effects. On the negative side, increase in the sediment-water exchanges

FRESHWATER ISEAWATER l 0,sc,A 0E | ,,PoTs , (~AY & 0CTOB~R) 1 (MAY ~ NOVE[IBER)

* i Y SEOIMENT , /

EXCHANGE - J " J AVAILABLE

J, LIGHT I >-IMACROPHYTES~ I

Fig. 6. Interactions among the water characteristics and the biota promoted by biomanipulation at the ecosystem inte-

gration level in a shallow eutrophic coastal lagoon.

may result in increased phytoplankton. The balance of positive and negative effects is not predictable. However, increasing lagoon-sea- water exchanges, naturally after decreased fresh- water inputs or artificially, would be the best com- plementary action to decreasing freshwater inputs in order to maintain water level and reduce nutrient concentrations in the lagoon because of the relatively low nutrient concentrations in sea water. Nevertheless, management of coastal lagoons should aim to restore degraded commu- nities without exporting the problems to closed ecosystems.

Acknowledgements

This work was supported by CICYT-Spain (PAC84-16-C02-02), Diputaci6 of Tarragona and EEC (EV4V-O132-E, TT).

436

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