Estuarine, Coastal and Shelf Science 64 (2005) 241e248

www.elsevier.com/locate/ECSS

Settlement of marine periphytic algae in a tropical estuary

S. Nayar a,*, B.P.L. Goh b, L.M. Chou c

a South Australian Research and Development Institute e Aquatic Sciences, 2 Hamra Avenue, West Beach, Adelaide, SA 5024, Australiab Natural Sciences Academic Group, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637 616c Marine Biology Laboratory, Department of Biological Sciences, 14 Science Drive 4, National University of Singapore, Singapore 117 543

Received 6 July 2004; accepted 31 January 2005

Available online 7 April 2005

Abstract

This note describes settlement studies of marine periphytic algae on glass substrata in a tropical estuary in Singapore. The rates ofproduction in terms of 14C radiotracer uptake, biomass in terms of chlorophyll a, community structure and cell abundance weremeasured from the settled periphytic algae at various depths in the water column and compared with the prevailing hydrographical

conditions. Relatively higher periphytic algal settlement was observed at 1 m depth, even though it was not statistically differentfrom other depths. Diatoms such as Skeletonema costatum and Thalassiosira rotula dominated the assemblage, together with themarine cyanobacteria Synechococcus sp. The three settlement parameters viz., periphytic algal production, chlorophyll a and cellcounts showed significant differences between the days of settlement, with no significant differences observed for different depths.

The periphytic algal community in this study comprised 30 microalgal species, dominated by diatoms (78%), followed bycyanobacteria (19% e primarily Synechococcus sp.), green flagellates (1%), dinoflagellates (1%) and other forms accounting for theremaining 1% of the total cell counts. Correlation studies and principal component analysis (PCA) revealed significant influence of

silicate concentrations in the water column with the settlement of periphytic algae in this estuary. Though photoinhibited at thesurface, photosynthetically available radiation did not seem to influence the overall settlement of periphytic algae. Diatoms andSynechococcus in the periphytic algal community were influenced by water temperature, PAR, pH and dissolved oxygen as seen in

the PCA plots.� 2005 Elsevier Ltd. All rights reserved.

Keywords: periphytic algae; settlement; productivity; chlorophyll a; diatoms; photosynthetically available radiation; nutrients; Ponggol estuary

A consortium of algae, bacteria and micro-fauna,embedded in a polysaccharide matrix is called a biofilm.Biofilms are ubiquitous and form the base of local foodchains, governing the assimilation, retention and trans-formation of dissolved and particulate materials in anecosystem (Pusch et al., 1998). Periphytic algae are animportant component in biofilms and are the dominantprimary producers (Kairesalo, 1980; Robert et al., 1995).

* Corresponding author. SARDI-Aquatic Sciences, PO Box 120,

Henley beach, SA 5022, Australia.

E-mail address: nayar.sasi@saugov.sa.gov.au (S. Nayar).

0272-7714/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2005.01.016

In addition periphytic algae supply organic carbon tothe planktonic system during resuspension in the watercolumn (Delgado et al., 1991; de Jonge and vanBeusekom, 1992).

Periphytic algae are very sensitive to many of the land-derived substances, both organic and inorganic (Bring-mann and Kuhn, 1980; Guasch et al., 2003). Sinceperiphytic algae are an important ecological componentof aquatic ecosystems (Boston et al., 1991; McCormickand Stevenson, 1998) and have been reported to be goodindicators of aquatic pollution (Fjerdingstad, 1964), theyform an ideal candidate group for ecotoxicological tests.Since the periphytic algae form a biofilm, settlement

242 S. Nayar et al. / Estuarine, Coastal and Shelf Science 64 (2005) 241e248

studies are very critical as a prerequisite for anyecotoxicological or physiological experiments involvingthem. Though a lot of studies involving periphytic algaehave been carried out in freshwater systems such asstreams, lakes and rivers (for e.g., Guasch et al., 1997,2003; Carlton and Wetzel, 1988; Admiraal et al., 1999)there are very few published studies from marine systems(but see Mollander and Blanck, 1992; Mollander et al.,1992; Dahl and Blanck, 1996; Blanck and Dahl, 1998),especially from the tropics (Nayar et al., 2003).

The purpose of this study was to examine theperiphytic algal community, production and biomassfrom a tropical estuary, and to describe a methodologyfor effectively immobilizing periphytic algae on artificialsubstrates for ecotoxicological and physiological stud-ies. Since water quality has been reported by Guaschet al. (2003) to determine periphyton development andstructure, this note also examines some of the majorphysico-chemical parameters in relation to periphyticalgae.

The study was carried out in May 2001 at PonggolMarina located along Ponggol estuary (Latitude:01 �25#27$ Ne01 �25#45$ N and Longitude: 103 �53#20$Ee103 �55#10$ E). Ponggol estuary is a mangrove-fringed estuary located on the Northeastern coast ofSingapore. The mouth of the estuary opens into the EastJohor Strait.

Based on previous research (Thielcke and Ratte,1994; Sreekumar, 1995; Vinyard, 1996; Nystroem et al.,

2000), the choice of the settlement substrate forperiphytic algae was glass. Multi-level rafts of PVCwere constructed with plastic clips attached to theframes of the raft (Fig. 1). The rafts were designed insuch a way that about 20 glass slides each could beplaced vertically for measurements of settlement at thesurface, 1 m, 2 m and 3 m depths. Glass slides of13.75 cm2 were cleaned with absolute alcohol prior toattachment onto two rafts. The rafts and glass slideswere submerged and anchored to a pontoon at themarina. The pontoon was located away from the zone ofboat traffic and was constantly flushed by the waters ofthe East Johor Strait. The glass slides for periphyticalgal production, chlorophyll a and cell abundance wereretrieved randomly each day from each depth for theduration of the experiment (5 days). For determinationof periphyton chlorophyll a and cell counts, theretrieved glass slides were transported to the laboratoryin 250 mL glass bottles containing 200 mL of filteredestuarine water maintained at 4 �C in the dark. Filteredestuarine water for transportation of glass slides andrinsing was prepared by filtering estuarine watercollected near the raft through a sterile Whatman�

47 mm diameter, 0.2-mm pore size membrane filter undervacuum.

Dissolved oxygen, photosynthetically available radi-ation (PAR), pH and salinity were measured in situ nextto the raft. Temperature and DO were measured witha YSI� 55 DO meter, photosynthetically available

Fig. 1. Construction details of the multi-level rafts used for periphytic algal settlement studies.

243S. Nayar et al. / Estuarine, Coastal and Shelf Science 64 (2005) 241e248

radiation (PAR) with a Licor LI 1000, pH with aWTW� pH330 pH meter and salinity with a WTW�

LF330 salinity meter. Water samples were also collectedfrom the four depths using an acid-rinsed Van Dornwater sampler. About 150 mL of the sample from eachdepth was filtered through a 0.2-mm pore size membranefilter and stored in the dark at 4 �C for analysis ofnutrients. Nutrients in the water samples viz., ammonia,nitrite, nitrate, phosphate and silicates were estimatedfollowing the standard colorimetric protocol of Parsonset al. (1984). A Shimadzu� RF 1601 was used for allspectrophotometric measurements.

For measurements of periphyton productivity, a setof glass slides retrieved from each depth was transferredinto paired 250 mL light and amber coloured bottlescontaining 200 mL of filtered estuarine water taken fromthe same depth. Each bottle was spiked with 5 mCi ofNaH14CO3 (ICN

� radiochemicals) and incubated in situfor 30 min at the corresponding depth at which the glassslides were collected. All incubations were carried out ona multi-level floating raft anchored to the mooringpontoon of the marina (Fig. 1). After incubation, thebottles were retrieved and transported immediately tothe laboratory at C4 �C (transit time approx 30 min) inthe dark. In the laboratory, periphyton settled on one ofthe surfaces of the glass slide was scraped off using a flatblade scalpel and resuspended in the filtered seawater inwhich it had been incubated. The slurry was furtherfiltered onto Whatman� 0.2-mm pore size membranefilters under vacuum, with repeated rinsing using filteredestuarine water to rinse off traces of unfixed radiotraceron the filter paper. Filters were then placed into 20-mLglass scintillation vials after which 1 mL of 0.5 Nhydrochloric acid was added to each vial, to removeinorganic carbon. Vials were left uncovered in a cleanfume hood for 24 h, after which 10 mL of scintillationcocktail Universol (ICN�) was dispensed into each vial,and capped tightly. A Wallac� 1414 liquid scintillationcounter, calibrated using Wallac� 14C unquenchedstandards, was used to assay the radioactivity of thefilters using the protocol of Parsons et al. (1984).Periphyton productivity was measured in duplicatesfor each day and each depth.

Periphyton growing on one of the surfaces of anotherset of glass slides was scraped off using a clean flat bladescalpel and resuspended in 150 mL of filtered seawaterto be used for periphyton chlorophyll a. Resuspendedperiphyton for chlorophyll a measurements was filteredonto Whatman� 0.2-mm pore size membrane filtersunder vacuum. The acetone extraction fluorometricprotocol of Parsons et al. (1984) was followed, andfluorescence was read against a solvent and filter blankusing a Shimadzu� RF 1501 spectrofluorometer cali-brated with Sigma Aldrich chlorophyll a standards.Periphyton chlorophyll a concentrations were measuredin duplicates for each day and each depth.

For the analysis of cell counts and species compo-sition of periphytic algae, one of the surfaces of theglass slide was scraped using a clean flat blade scalpeland the material resuspended in 50 mL of filteredseawater. Two procedures were adopted for theanalysis. For larger cells (O20 mm), the resuspendedperiphyton samples were fixed with 1% formalin anddirectly enumerated under an Olympus� BX50 binoc-ular light microscope, and identified to species levelwherever possible. The smaller cells (!20 mm), 1 mLaliquot of the resuspended sample was filtered onto 0.2-mm pore size black Nuclepore filter and enumerated bythe autofluorescence technique using the same micro-scope (Porter and Feig, 1980). Total cell counts in termsof cell counts per square centimeter were estimated,accounting for the substrate area (i.e. 13.73 cm2),counting area under the microscope and the volumeof filtered estuarine water used to dilute the periphyticalgal material.

Results obtained for the settlement studies wereanalysed using the statistical package Minitab ver. 14and Statistica 98 (release 5.1). One-way analysis ofvariance (ANOVA) and Tukey’s post hoc pair-wisecomparisons were used to determine significant differ-ences between the days and depths of settlement for pe-riphytic algal production, periphytic algal chlorophyll aand periphytic algal cell counts. Simple correlationand Principal Component Analysis (PCA) were per-formed on the data to understand the relationshipbetween periphyton productivity, periphyton chloro-phyll a and periphyton cell counts to the prevailingphysico-chemical parameters at the estuary. The multi-variate statistical programme, MVSP Ver. 3.10 b (1985e1999, Kovach Computing Services, UK) was used tocarry out Principal Components Analysis (PCA). Thedata for PCA were standardised by a Log10 trans-formation to meet the basic requirement of the statisticaltest. The threshold level of statistical significance for allanalysis was p% 0.05.

Results of periphyton settlement, measured in termsof productivity, chlorophyll a and cell counts, showedan increase at all depths on all days (Fig. 2aec). Amongthe four depths, settlement was greatest on slides placedat 1 m depth. Among the days, increased settlement wasobserved from day 3 onwards. One-way ANOVAperformed on the three settlement parameters showedstatistically significant differences between the days ofsettlement (Table 1). In addition, Tukey’s post hoc testshowed no significant differences in settlement betweendays 1 and 4. Settlement on day 5 was significantlydifferent from the rest of the days with an exception toperiphyton productivity and periphyton cell countswhere no significant differences in settlement was ob-served between days 4 and 5 ( pO 0.05). No significantdifferences were observed in settlement at differentdepths ( pO 0.05).

244 S. Nayar et al. / Estuarine, Coastal and Shelf Science 64 (2005) 241e248

Water quality, current and light have been suggestedas factors in determining development of periphyticalgae (Strueder, 1999; Wellnitz and Rinne, 1999; Abeet al., 2000; Guasch et al., 2003). Even though PARmeasurements did not correlate with settlement ofperiphytic algae, it is believed that photo-inhibitioncaused the reduction in production, biomass andabundance of periphytic algae in surface waters (Doddset al., 1999). In studies carried out by other researchers,production and biomass maxima were observed at a 1-m

(a)

(b)

(c)

0

1

2

3

4

5

6

7

Day 1 Day 2 Day 3 Day 4 Day 5

Day 1 Day 2 Day 3 Day 4 Day 5

Day 1 Day 2 Day 3 Day 4 Day 5

Days of settlement

Perip

hyto

n pr

oduc

tion

( µgC

.cm

-2.h

-1)

Surface 1 m 2 m 3 m

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Days of settlement

Perip

hyto

n ch

loro

phyl

l a (µ

g.cm

-2)

Surface 1 m 2 m 3 m

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Days of settlement

Perip

hyto

n ce

ll co

unts

(L

og N

o.cm

-2)

Surface 1 m 2 m 3 m

Fig. 2. Settlement determined in terms of periphyton (a) productivity,

(b) chlorophyll a and (c) cell counts in Ponggol estuary. The error bars

for periphyton productivity and periphyton chlorophyll a represent

meanG standard deviation.

depth, followed by a decline with depth, in agreementwith the findings of this study (Strueder, 1999; Brandiniet al., 2001).

The periphytic algal community in this studycomprised 30 microalgal species, 78% of which werediatoms, 19% cyanobacteria (primarily Synechococcussp.), 1% green flagellates, 1% dinoflagellates andmiscellaneous forms accounted for the remaining 1%of the total cell counts (Table 2). Diatoms weredominated by Skeletonema costatum, Thalassiosiracondensata and Thalassiosira rotula. Dominance ofdiatoms in the periphytic algal community is wellknown (e.g., Greenwood et al., 1999; Havens et al.,1999; Kostel et al., 1999; Muller, 1999; Baffico, 2001;Brandini et al., 2001). The occurrence of meroplank-tonic forms such as S. costatum and Thalassiosira sp. inthe plankton community and in epibenthic and peri-phytic algal assemblage has been reported before(Blanck and Dahl, 1996; Rautio et al., 2000). Eventhough, Synechococcus sp. is known to be abundant inthe coastal waters of the South China Sea (Agawin et al.,2003), there is no published literature reporting itsoccurrence in the periphytic or benthic algal communi-ties. In the present study, the relative dominance ofS. costatum was observed to alternate with Synechococ-cus sp. (Fig. 3). Similar observations on the alternationof dominance between diatoms and cyanobacteria inperiphytic algal community have been reported byBlanck (1985). Periphytic algal assemblage was domi-nated by Synechococcus sp. at all depths on the first dayof settlement, while S. costatum dominated the assem-blage on day 2. By the third day, the surface water glassslides registered a relatively higher abundance ofT. rotula, while the glass slides placed at 1, 2 and 3 mdepths recorded a dominance of Synechococcus sp. Onthe fourth day of settlement, the abundance ofS. costatum was higher on glass slides at all depths,

Table 1

One-way analysis of variance (ANOVA) for periphyton settlement

studies in Ponggol estuary

Parameter Effect df

Effect

f p

Value

Tukey’s

pair-wise

comparisons

Periphyton

productivity

Depths 3 1.09 0.382 1OSO2O3

Days 4 4.40 0.015

Periphyton

chlorophyll a

Depths 3 0.49 0.693 1OSO2O3

Days 4 13.32 !0.001 5O4O3O2O1

Periphyton

cell counts

Depths 3 1.75 0.197 1OSO2O3

Days 4 6.49 0.003

Results of Tukey’s pair-wise comparison are arranged in the ascending

order of their means and lines are drawn over treatment groups that

are not significantly different from each other ( pO 0.05).

Abbreviations: S, 1, 2 and 3 represent surface, 1 m, 2 m and 3 m

depths.

245S. Nayar et al. / Estuarine, Coastal and Shelf Science 64 (2005) 241e248

with an exception to those at 3 m depth, whereSynechococcus sp. dominated the assemblage. On theother hand, the distribution of T. rotula was consistentthrough depth for all the glass slides. However, on thefifth day of settlement, S. costatum dominated theperiphytic algal assemblage on the glass slides placed atall depths except surface where there was a near equaldominance of both S. costatum and Synechococcus sp.

Summary of the hydrobiological parameters moni-tored at various depths during the settlement study didnot indicate much variation (Table 3). Nutrient concen-trations were high, especially of silicates. Correlationmatrices in Table 4 showed that silicates (positive) andtemperature (negative) correlated significantly to peri-phytic algal productivity, chlorophyll a and cell counts( p! 0.05). A significant negative correlation was alsoobserved between periphytic algal productivity andsalinity and between periphytic algal cell counts andnitrite. Periphyton productivity, chlorophyll a and cell

Table 2

Species composition of periphytic algal assemblage from Ponggol

estuary, Singapore

Surface 1 m 2 m 3 m

Synechococcus sp ! ! ! !

Actinophychus undarlatus !Anabaena sp. !

Biddulphia aurita !

Coscinodiscus radiatus !

Coscinodiscus sp. ! !Cyclotella sp. !

Cyclotella striata !

Ditylium brightwelli !Favella ehrenbergii ! ! ! !

Fragillaria oceanica ! !

Goniodoma sphaericum !

Hyalodiscus stelliger ! ! !Merismopedia elegans ! ! ! !

Navicula sp. ! ! !

Nitzchia closterium !

Nitzchia lanceolata ! ! ! !Nitzchia philippinarum !

Nitzchia pungens ! !

Nitzchia seriata !

Nitzchia sigma !Nitzchia sp ! ! ! !

Oscillatoria sp. !

Pleurosigma fasciola ! ! !Rhizosolenia sp. ! !

Skeletonema costatum ! ! ! !

Stephanophyxis sp. !

Thalassiosira condensata ! !Thalassiosira rotula ! ! ! !

Thalassiosira sp. ! ! !

Unidentified dinoflagellates !

Green flagellates ! !Miscellaneous ! ! ! !

The symbol ‘!’ indicates the presence of the species.

counts were positively and significantly correlated witheach other.

The scree plot of the data set for PrincipalComponent Analysis revealed the largest variance forthe first two PCA axes. Based on this, all representationsand conclusions of the analysis account for the first twoPCA axes only. The scatter plot of the PCA loadingscores (Fig. 4) for periphytic algal production, chloro-phyll a, total cell counts, cell counts of diatom andSynechococcus in the periphyton community withphysico-chemical parameters revealed two distinctclusters. Periphytic algal production, chlorophyll a andtotal cell counts ‘clustered’ with silicates and separatelyfrom other physico-chemical parameters on one axis,while, cell counts of diatoms and Synechococcus

Surface

0%

20%

40%

60%

80%

100%

Synechococcus sp Actinophychus undarlatus Cyclotella striata

Favella ehrenbergii Fragillaria oceanica Green flagellatesMerismopedia elegans Miscellaneous Navicula sp.

Skeletonema costatum Thalassiosira condensata Thalassiosira rotula

1 m

0%

20%

40%

60%

80%

100%

2 m

0%

20%

40%

60%

80%

100%

Perc

enta

ge o

f tot

al p

erip

hytic

alg

al c

ell c

ount

s

3 m

0%

20%

40%

60%

80%

100%

1 2 3 4 5Days of settlement

Fig. 3. Percentage composition of dominant periphytic algal species

to total cell counts at four settlement depths over the 5 days of

settlement.

246 S. Nayar et al. / Estuarine, Coastal and Shelf Science 64 (2005) 241e248

Table 3

Summary of the hydrobiological conditions in Ponggol estuary during settlement studies

Parameters Depth Surface 1 m 2 m 3 m

DO (mg L�1) Range 6.3e8.9 6.6e10.5 4.9e8.5 3.5e6.3

MeanG SE 7.9G 0.5 8.2G 0.8 6.4G 0.8 4.9G 0.6

Temp. ( �C) Range 29.0e31.7 29.3e31.6 29.5e31.3 29.5e31.2MeanG SE 31.0G 0.6 30.8G 0.4 30.7G 0.3 30.6G 0.3

PAR (mmol cm�2 s�1) Range 1676.4e1850.0 1583.4e1654.0 1453.0e1494.8 1262.0e1397.4

MeanG SE 1730.2G 34.3 1613.4G 14.6 1476.6G 9.8 1359.8G 28.6

pH Range 7.8e8.3 8.0e8.4 8.0e8.4 8.0e8.4

MeanG SE 8.0G 0.1 8.1G 0.1 8.2G 0.1 8.1G 0.1

Salinity (!103) Range 22.9e26.5 24.9e26.9 26.6e27.4 26.7e27.6

MeanG SE 24.9G 0.9 26.0G 0.4 27.0G 0.2 27.3G 0.2

NH3-N (mmol) Range 1.0e16.1 1.6e10.9 4.1e9.4 2.5e18.0

MeanG SE 7.5G 2.8 6.1G 1.9 6.8G 1.0 9.5G 2.9

NO2-N (mmol) Range 0.5e1.1 0.5e1.1 0.6e1.2 0.6e1.4

MeanG SE 0.8G 0.1 0.8G 0.1 0.8G 0.1 1.0G 0.1

NO3-N (mmol) Range 1.2e7.7 1.2e8.4 1.9e6.6 1.7e5.9

MeanG SE 4.6G 1.2 4.8G 1.5 4.1G 1.0 4.1G 0.8

PO4-P (mmol) Range 0.7e2.0 1.0e2.2 1.2e2.9 1.4e3.6

MeanG SE 1.3G 0.3 1.4G 0.2 2.0G 0.4 2.6G 0.4

SiO3-Si (mmol) Range 37.6e302.1 35.0e167.1 28.6e195.4 41.5e231.6

MeanG SE 135.4G 56.8 95.5G 31.3 114.7G 39.9 132.8G 45.1

Periphyton production

(mg cm�2 h�1)

Range 0.11e3.41 0.14e6.46 0.03e1.32 0.08e1.12

MeanG SE 1.09G 0.67 2.03G 1.29 0.55G 0.25 0.55G 0.25

Periphyton chlorophyll

a (mg cm�2)

Range 0.12e2.43 0.03e2.71 0.17e2.13 0.12e2.11MeanG SE 0.75G 0.48 1.32G 0.58 0.70G 0.40 0.70G 0.42

Periphyton cell

counts (no cm�2)

Range 9352e42,794 9614e86,509 1846e32,138 167e31,011

MeanG SE 22,464G 7679 38,834G 15,238 17,360G 6932 11,854G 6921

Table 4

Statistically significant correlation matrices of periphyton productivity,

periphyton chlorophyll a and periphyton cell counts to physico-

chemical parameters in Ponggol estuary

Periphyton

productivity

Periphyton

chlorophyll a

Periphyton

cell counts

Periphyton

productivity

e

Periphyton

chlorophyll a

*[R2Z 0.596] e

( pZ 0.000)

Periphyton

cell counts

*[R2Z 0.854] *[R2Z 0.682] e

( pZ 0.000) ( pZ 0.000)

Dissolved

oxygen

NS NS NS

Temperature �*[R2Z 0.373] �*[R2Z 0.411] �*[R2Z 0.209]

( pZ 0.004) ( pZ 0.002) ( pZ 0.043)

PAR NS NS NS

pH NS NS NS

Salinity �*[R2Z 0.303] NS NS

( pZ 0.012)

NH3-N NS NS NS

NO2-N NS NS �*[R2Z 0.213]

( pZ 0.041)

NO3-N NS NS NS

PO4-P NS NS NS

SiO3-Si *[R2Z 0.253] *[R2Z 0.464] *[R2Z 0.289]

( pZ 0.024) ( pZ 0.001) ( pZ 0.014)

NS: Not significant; *Significantly positively correlated; �*Significantly

negatively correlated.

clustered with water temperature, PAR, pH anddissolved oxygen on the other axis. This distinct‘clustering,’ indicated by a dotted line, together withthe high loading scores is indicative of the majorinfluence of silicates on the settlement of periphyticalgae on the whole and the influence of watertemperature, PAR, pH and dissolved oxygen specificallyon diatom and Synechococcus in the periphytoncommunity. This is further supported by the significantpositive correlation of periphyton productivity, chloro-phyll a and cell counts with silicate in this study,identical to the conclusions of Sommer (1996). Adominant species in the present study, Synechococcussp. have been reported to occur in greater abundance inthe coastal waters and river estuaries of the SouthChina Sea which are rich in inorganic nutrients (Agawinet al., 2003).

In conclusion, diatoms dominated the periphyticalgal community in Ponggol estuary, with silicatesplaying a role in the settlement of periphytic algae.High light irradiance in the surface waters is likely tohave photoinhibited the periphytic algae in these watersaccounting for the lower production rates, biomassand abundance. Diatoms and Synechococcus in the

247S. Nayar et al. / Estuarine, Coastal and Shelf Science 64 (2005) 241e248

Axis

2

Axis1

Periphyton productionPeriphyton chlorophyll a

Periphyton cell countsDO

Temp.

pH

ytinilaS

NH3

NO2

NO3

PO4

SiO3PAR

Diatoms

Synechococcus

-0.09

-0.17

-0.26

-0.35

-0.43

0.09

0.17

0.26

0.35

0.43

-0.0

9

-0.1

7

-0.2

6

-0.3

5

-0.4

3

0.09

0.17

0.26

0.35

0.43

Fig. 4. Scatter plot of PCA loading scores for periphytic algal parameters and the physico-chemical parameters monitored from Ponggol estuary. The

‘clouds’ grouped under the discontinuous line represent strongly correlated parameters.

periphytic algal community were influenced by watertemperature, PAR, pH and dissolved oxygen.

Acknowledgements

Our sincere gratitude to Dr Ruth O’Riordan, De-partment of Biological Sciences, National University ofSingapore for her critical comments on the manuscript.This study was supported by the Singapore Institute ofBiology research trust fund, RTF 30/2001. Thanks aredue to Mr Abdul Latiff for his help during the field workand Ponggol Marina Pte Ltd for boat berthing facilitiesand the use of a pontoon for the settlement experiments.

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