Primary Research Paper

Spatial variations in size, weight and condition factor of the females

of Acartia clausi (Copepoda: Calanoida) along a salinity gradient in two

contrasting estuaries of the Basque coast (Bay of Biscay)

Ibon Uriarte1 & Fernando Villate21Laboratory of Ecology, Department of Plant Biology and Ecology, Faculty of Pharmacy, University of theBasque Country, E-01006 Gasteiz, Spain2Laboratory of Ecology, Department of Plant Biology and Ecology, Faculty of Science and Technology,

University of the Basque Country, 64448008 Bilbao, Spain(*Author for Correspondence: E-mail: ibon.uriarte@ehu.es)

Received 21 February 2006; in revised form 19 May 2006; accepted 28 May 2006; published online 29 July 2006

Key words: Acartia clausi, body size, condition factor, estuaries, salinity, weight

Abstract

Variations in prosome length and width, dry weight and condition factor of female Acartia clausi copepodswere studied at three salinities (35, 34 and 33 psu) in the euhaline region of two estuaries (Bilbao andUrdaibai) of the Basque coast, with different level of anthropogenic impact. Effect of the environmentalvariables upon the morphology of A. clausi females on a small geographic scale is discussed. In general,biometric variables showed no significant differences between the two estuaries, but dry weight and conditionfactor were significantly higher in the estuary of Urdaibai at 35 and 34 psu, whilst at 33 psu they were higherin Bilbao. Body dimensions decreased significantly with decreasing salinity in both estuaries, however, nosimilar trends were observed for dry weight and condition factor. Temperature appeared the main variable toaccount size variations, but once eliminated seasonal effect of the temperature body size was related withoxygen concentration in the estuary of Bilbao and with salinity in Urdaibai. This study reveals that mor-phological characteristics of A. clausi not are only dependent on the temperature, but also, within a limitedgeographical zone, on local differences in environmental variables, mainly salinity and oxygen concentration.

Introduction

The body size of adult copepods depend on severalvariables, such as temperature, available food,salinity or predation (e.g. Durbin & Durbin, 1978;Klein Breteler & Gonzalez, 1982; Moraitou-Apostolopoulou et al., 1986; Warren et al., 1986;Gaudy et al., 1988; Gaudy & Verriopoulos, 2004).In a given geographical area, seasonal variations inthe size of individuals are well known, with max-imum lengths at minimum temperatures or duringalgal blooms (e.g. Deevey, 1960), but relativeimportance of both variables remains controver-sial. Thus, different works attribute the final size ofcopepods mainly to temperature (e.g. Durbin

et al., 1992; Christou & Verriopoulos, 1993), toavailable food (Vidal, 1980; Durbin et al., 1983;Klein Breteler & Gonzalez, 1988) or to synergicaleffect of both variables (Checkley, 1980; KleinBreteler &Gonzalez, 1982; Klein Breteler et al.,1982). In this sense, Viitasalo et al. (1995) pointedout the difference on the temporal scale; the sea-sonal variation in body size may depend on tem-perature whilst the food concentration woulddetermine the mean size in each season.

Estuaries are fluctuating environments wheresalinity is the main variable regulating distributionof the organisms. In such fluctuating systemscopepod viability requires both behavioural andphysiological adaptations (Fiksen & Giske, 1995),

Hydrobiologia (2006) 571:329–339 � Springer 2006DOI 10.1007/s10750-006-0258-1

such as adjustments in the osmoregulatorymechanisms, based ultimately upon differences inmetabolism (Kinne, 1964, 1971; Gonzalez &Bradley, 1994), affecting the relative growth ofthe body parts and the reproductive capability(Miliou, 1996). In this way, salinity plays a decisiverole in the final size of copepods (Moraitou-Apostolopoulou et al., 1986; Gaudy et al., 1988).Likewise, situations of high stress (such as, eutro-phication and pollution) can enhance the differ-ences in the metabolism (Rapport et al., 1985;Schindler, 1987). Thus, toxic substances mayproduce behavioural and physiological alterations(ingestion rates, velocity of swimming and respi-ration), increasing maintenance costs and reducingthe rate of growth, which produce smaller matureindividuals (Moore & Folt, 1993). So, differencesin the final size of copepods may reflect of thevariations in physiological variables due to fluc-tuations in environmental variables (Omori &Ikeda, 1984).

The copepod calanoid Acartia clausi is awidespread species in neritic and coastal areas (e.g.Mauchline, 1998; Halsband & Hirche, 2001; Mir-alto et al., 2003; Chinnery & Williams, 2004). Thisspecies is a predominant element of mesozoo-plankton during most part of the year in the neriticwaters of the Basque coast, where it, mainly,occupies the outer part of the estuaries that drainsinto the Bay of Biscay (San Vicente et al., 1988;Villate, 1991a, b; Uriarte & Villate, 2005).

The aim of this paper is to compare the pro-some length and width, dry weight and conditionfactor of the adult females of A. clausi from theeuhaline region of the estuaries of Bilbao andUrdaibai in an attempt to evaluate the effect oforganic load and environmental variables on thefinal size of A. clausi.

Material and methods

Study area

The estuary of Bilbao (43� 23¢ N 3� W) is a shal-low (2–9 m) and highly stratified channel �15 kmlong that crosses urban and industrial settlementsand drains into a wide coastal embayment. Duringthe last century, a huge amount of untreatedwastewater from urban and industrial activities

has polluted waters and sediments of this estuary(Cearreta et al., 2000; Belzunce et al., 2001; Borja& Collins, 2004). Today, however, this system is inrecovering process (Borja & Collins, 2004). Incontrast, the estuary of Urdaibai (43� 22¢ N2� 43¢ W) is �13 km long, shallow (2.5 m meandepth), mixed and low perturbed mesotidal systemthat constitutes the central axis of the UrdaibaiBiosphere Reserve (Fig. 1).

Sampling

Ten sampling cruises were carried out monthlyfrom March to October 1997 (fortnightly in Mayand June) within the euhaline (salinity >30 psu)region of the estuaries of Bilbao and Urdaibai athigh tide. For the comparative purposes of thisstudy, zooplankton and water samples were al-ways obtained at fixed salinities around 35, 34, 33and 31 psu, respectively. Sampling was not per-formed at fixed stations because the location ofwater masses of a given salinity within both estu-aries is rather variable (Fig. 1) due to the effect oftides and river discharge. Vertical profiles ofsalinity and temperature were measured by aWTW LF 197 thermosalinometer (accuracy:±0.1) and of the dissolved oxygen saturation(DOS) by an YSI 55 oxymeter (accuracy: ±0.5).Water samples of the required salinity, for thedetermination of chlorophyll a (Chl a), dry weightof suspended particulated matter (seston) andparticulated organic matter (POM), were obtainedby means of a water pump below the halocline.Concurrent with the water sampling, zooplanktonsamples were collected at the same place and depthof water samples by towing a 200 lm net (mouthdiameter: 0.5 m). After retrieval, the catches werepreserved in 4% buffered formalin.

Seston determinations

Water samples for seston determinations werecarried to the laboratory in dark carboys and fil-tered through precombusted (450 �C, 24 h)Whatman GF/C glass-fibre filters within 3 h. Chl awas measured spectrophotometrically on 90%acetone extracts (Lorenzen, 1967). The seston wasmeasured by gravimetric method after 24 h dryingat 70 �C and POM, by difference, after 24 h cal-cination at 450 �C.

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Size determination in Acartia clausi

From each zooplankton sample, the prosomelength (PL) and width (PW) of �100 female adultA. clausi (without damages) were measured bymeans of an eyepiece micrometer under anOlympus inverted microscope at a magnificationof 100�. Female dry weight (DW) was obtainedby weighing 3 replicates of �30 individuals in a

M3P Sartorius electrobalance (±1 lg) after rins-ing with distillated water to remove salt and dryingat 60 �C for 24 h. In contrast to Kapiris et al.(1997), no effect of chemical preservation on thesize was observed. However, weights were cor-rected by a factor of 30% (Durbin & Durbin,1978; Landry, 1978), due to weight loss as conse-quence of preservation (Durbin & Durbin, 1978;Williams & Robins, 1982; Giguere et al., 1989;

Kadagua

(a)

AbraHarbour

BILBAO

SESTAO

PORTUGALETE

SANTURTZI

LEIOA

ERANDIO

ZIERBENA

Galindo

Asua

Gobelas

Nerbioi-Ibaizabal

GETXO

SUKARRIETA

GERNIKA

KANALA

BUSTURIA

FORUA

(b)

Oka

Golako

TF

TF

TFTFMS

MS MS

MS

MS

MSMS

MS

MS = Marshes and supratidal areas

TF = Tidal flats

BARAKALDO

35 psu

Mape

34 psu

33 psu

35 psu

33 psu

MUNDAKA

31 psu

31 psu

34 psu

1km1km

Basque coast

Bay ofBiscay

N

Figure 1. Study area. Geographical locations of the estuaries of Bilbao (a) and Urdaibai (b). Arrows indicate the approximate extent

of salinity zones at high tide in each estuary.

331

Kapiris et al., 1997). Condition factor (CF), whichis a measure of the weight per unity of length (LeCren, 1951), was calculated as CF = aÆ(W/L3),where W = dry weight (lg), L = prosome length(mm) and a = 0, 1 (Durbin & Durbin, 1978).

Data treatment

The number of adult females of A. clausi obtainedin samples of 31 psu was scarce, and this preventedthe use of data from the 31 psu for statisticaltreatment. This way, the comparative study wasrestricted to data from 35, 34 and 33 psu. Para-metric tests were used for DW, CF and environ-mental variables because they showed normaldistribution. However, PL and PW showed non-normal distribution (even after logarithmic trans-formation) and non-parametric tests were used.A two-way ANOVA was used to test for spatialdifferences in environmental variables and bodysize data between estuaries. One-way ANOVA wasused to test for differences of the DW, CF andenvironmental variables between salinities withinestuaries and the Kruskal–Wallis H-test was usedfor the PL and PW. Multiple comparisons betweensalinities were carried out by the Tukey’s test (Zar,1999). Between-estuary differences for the totaldata set and between-estuary differences by salini-ties for the PL and PW were performed with the ztest, since>25measurements were obtained, whilstfor the DW, CF and environmental variables weremade by Student’s t, since there were <25measurements (Chalmers & Parker, 1989). Stepwiseregression analyses were used to identify between-variables relationships, and to determine the influ-ence of environmental variables on the sizevariables (PL, PW, DW and CF), respectively. Toeliminate temporal variability due to temperature,stepwise regression analysis was applied to biolog-ical variables (PL, PW, DW and CF) after sub-tracting the corresponding mean from each value.

Results

Variations of environmental variables

On average, salinity and temperature showed nosignificant differences between estuaries (Fig. 2).Chl a and seston were significantly higher in the

estuary of Bilbao only at 35 psu and the POM at35 and 34 psu. However, DOS was significantlyhigher at all salinities in the estuary of Urdaibai.On the other hand, DOS and Chl a showed dif-ferent trends of variation with salinity in eachestuary. Both variables increased significantly,mainly DOS, with salinity in the estuary of Bilbao,whilst they remained in similar values throughoutthe euhaline region of the estuary of Urdaibai(Table 1). The POM, however, increased signifi-cantly with decreasing salinity only in the estuaryof Bilbao.

10

8

6

2

4

0

** ** n.s.

35 34 33

15

10

5

0

20

25

30n.s.

35 34 33

** n.s.

33

31

29

27

35

37

39n.s. n.s. n.s.

35 34 33

Salinity (psu) Salinity (psu)

19

17

15

13

21

23

25n.s. n.s. n.s.

35 34 33

** *** ***

0

100

120

60

40

20

80

35 34 330

6

7

4

3

1

5

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** n.s. n.s.

35 34 33

Salin

ity (

psu)

Oxy

gen

satu

ratio

n (%

)Se

ston

(m

g 1–1

)

POM

(m

g 1–1

)C

hlor

ophy

ll a

(µg

1–1)

Tem

pera

ture

(°C

)

Figure 2. Mean values of salinity, water temperature, dissolved

oxygen saturation (DOS), chlorophyll a concentration (Chl-a),

dry weight suspended particulated matter (seston) and partic-

ulate organic matter (POM) at each salinity site of the estuaries

of Bilbao (blank bars) and Urdaibai (filled bars). Bars show

standard deviation. Results of the Student’s t-test for the dif-

ferences between estuaries: ***p<0.001, **p<0.01,

*p<0.05, n.s. no significant.

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Morphological differences according to estuariesand salinity

The PL of the females ranged from 797.9 to970.2 lm in Bilbao and from 793.1 to 980.7 lm inUrdaibai and the PW from 251.6 to 305.4 lm inBilbao and from 254.8 to 311.6 lm in Urdaibai.The DW ranged between 5.98 and 12.05 lg in theestuary of Bilbao and between 6.3 and 12.8 lg inthe estuary of Urdaibai, and the CF values rangedfrom 0.9 to 1.5 and from 0.9 to 1.6 in Bilbao andUrdaibai, respectively. Seasonal variation of thebody size of the females of A. clausi showed asignificant (PL and PW: Kruskal–Wallis H-test,p<0.001; DW: one-way ANOVA, p<0.001)

trend of decreasing from March to August(Fig. 3). In the case of the CF no clear trend wasobserved.

In general, PL and PW of A. clausi showed nosignificant differences between estuaries, althoughthe PW was significantly higher in Bilbao at 34 psu(Fig. 4). The DW and CF showed significant dif-ferences between salinities but no between estuar-ies as a whole (Student’s t, p>0.05), due toinconsistent differences among salinities. As shownin Table 2, PL and PW decreased significantlywith decreasing salinity in both estuaries, but theDW and CF showed different trend of variationbetween estuaries. Thus, a weakly increase of bothvariables with decreasing salinity was observed in

Table 1. p-Values of the one-way ANOVA for differences within estuaries (p-values in bold indicate significant differences at

a = 0.05), and results of the test of Tukey for multiple comparisons between salinities (psu) of the environmental variables in the

estuaries of Bilbao and Urdaibai

Estuary of Bilbao Estuary of Urdaibai

p-Values Between-salinity differences p-Values Between-salinity differences

Salinity <0.0001 35>34>33 <0.0001 35>34>33

Temperature 0.9713 34 = 33 = 35 0.8540 34 = 33 = 35

DOS <0.0001 35>34>33 0.5404 33 = 34 = 35

Chl a 0.0406 35 = 34 = 33 0.1785 34 = 33 = 35

Seston 0.2697 33 = 34 = 35 0.1599 33 = 34 = 35

POM 0.0146 33 = 34 = 35 0.3976 33 = 34 = 35

Values decrease from the left located salinity to the right located salinity. >: significant differences between consecutive salinities,

=: no significant differences between consecutive salinities, underlined salinities: significant differences between them.

M A M J A O

1000

900

800

950

850

750

1050

285

255

300

270

240

315

M A M J A O0,8

1,0

1,2

1,4

1,6

6

8

4

10

12

14

J SJ S

PL (

µm)

DW

(µg

)C

F

PW (

µm)

Figure 3. Seasonal variation of the body size means of the females of Acartia clausi in the estuaries of Bilbao (open circles) and

Urdaibai (filled circles) during the period of study. Bars show standard deviation. (PL: prosome length; PW: prosome width; DW: dry

weigh and CF: condition factor). In August no data of DW and CF because females were very scarce to obtain reliable weights.

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the estuary of Bilbao, whilst in Urdaibai maximumvalues were recorded at 34 psu, but decreasingsignificantly at 33 psu.

Relationships between body size and environmentalvariables

From the stepwise regression analyses (Table 3),temperature appeared the predominant variable to

account for the PL, PW and DW variability in theestuaries of Bilbao and Urdaibai, and in bothestuaries taken together. Among the other vari-ables, only DOS contributed to some extent to thevariability, which was the case only for CF in theestuary of Bilbao. Once the temporal variabilitywas eliminated (Table 4), DOS was the main var-iable to account for the PL and PW variability inthe estuary of Bilbao, salinity in the estuary ofUrdaibai and again salinity in both estuaries takentogether. However, for the PW in the estuary ofUrdaibai the positive effect of DOS was second-arily added. Likewise, for the other two biologicalvariables, only the CF variability in the estuary ofBilbao was explained by the salinity. However, inall cases regression model accounts for less than50% of the variability.

Discussion

Environmental variables

The analysis of salinity data set showed thatsalinity might be responsible for differences inbody size variables within each estuary but not fordifferences between estuaries. DOS, however,showed high differences between estuaries, as wellas along the euhaline region of the estuary ofBilbao, denoting the effect of organic inputs to thissystem. Poor conditions of oxygen concentrationcharacterize organically enriched water massessuch as the estuary of Bilbao, where the valuesmeasured at 33 psu indicate a situation of frequenthypoxia, as consequence of the high biologicaloxygen demand by heterotrophic bacterial activity

Salinity (psu)

35 34 33

950

1000

900

850

800

n.s. n.s. n.s.

35 34 33

320

300

280

260

n.s. * n.s.

Salinity (psu)35 34 33

11

13

9

7

5

** * *

35 34 33

1.4

1.2

1.0

0.8

1.6

1.8n.s. ** *

PL (

µm)

PW (

µm)

CF

DW

(µg

)

Figure 4. Mean values of the prosome length (PL), prosome

width (PW), dry weigh (DW) and condition factor (CF) of the

females of Acartia clausi at each salinity of the estuaries of Bil-

bao (blank bars) and Urdaibai (filled bars). Bars show standard

deviation. Results of the z-test (PL and PW) and of the Student’s

t-test (DW and CF) for the differences between estuaries:

***p<0.001, **p<0.01, *p<0.05, n.s. no significant.

Table 2. p-Values of the Kruskal–Wallis H-test (PL and PW) and of the one-way ANOVA (DW and CF) for differences within

estuaries (p-Values in bold indicate significant differences at a = 0.05), and results of the test of Tukey for multiple comparisons

between salinities (psu) of prosome length (PL), prosome width (PW), dry weight (DW) and condition factor (CF) of the females of

Acartia clausi in the estuaries of Bilbao and Urdaibai

Estuary of Bilbao Estuary of Urdaibai

p-Values Between-salinity differences p-Values Between-salinity differences

PL <0.0001 35>34>33 <0.0001 35>34 = 33

PW <0.0001 35 = 34>33 <0.0001 35>34 = 33

DW 0.7951 33 = 34 = 35 0.0491 34 = 35>33

CF 0.5461 33 = 34 = 35 0.0253 34 = 35 = 33

Values decrease from the left located salinity to the right located salinity. >: significant differences between consecutive salinities,

= : no significant differences between consecutive salinities, underlined salinities: significant differences between them.

334

(Iriarte et al., 1998), which could perturb thephysiological and behavioural functions oforganisms (Diaz & Rosenberg, 1995; Wu, 2002).

Potential food variables, such as Chl a, sestonand POM, showed that the between-estuary dif-ferences were more evident at high salinities, whilethose of oxygen availability were more evident atlow salinity. The inverse distribution of phyto-plankton and organic materials with salinity inBilbao may to some extent explain it, denotingthat phytoplankton and autotrophic processes aremore important at higher salinities, while detritus

and microbial activities responsible for oxygendepletion are more important at lower salinities(Orive et al., 2004). In Bilbao, at lower salinitylight seems to limit phytoplankton development,whilst at higher salinity conditions improve due toa decrease in turbidity and high concentrations ofnutrients (Agirre, 2000). In contrast, in the euha-line region of the estuary of Urdaibai phyto-plankton and seston organic compoundconcentrations, both increased with decreasingsalinity. In fact, a peak of phytoplankton biomassis usually recorded around 30 (Ruiz et al., 1998)

Table 3. Results of the stepwise forward multiple regression analysis for prosome length (PL), prosome width (PW), dry weight (DW)

and condition factor (CF) of the females of Acartia clausi in the estuaries of Bilbao and Urdaibai, and in both estuaries taken together

(Tª: temperature, DOS: dissolved oxygen saturation, n.s.: no significant)

Variables Regression R2 p

Estuary of Bilbao PL 1200.19) 18.91 Tª 0.91 <0.0001

PW 381.19) 6.21 Tª 0.82 <0.0001

DW 16.04) 0.45 Tª 0.59 <0.0001

CF 1.36) 0.0018 DOS 0.08 0.0444

Estuary of Urdaibai PL 1151.81) 15.74 Tª 0.58 <0.0001

PW 375.91) 5.81 Tª 0.68 <0.0001

DW 17.63) 0.51 Tª 0.60 <0.0001

CF – – n.s.

Both estuaries PL 1187.45) 18.08 Tª 0.81 <0.0001

PW 379.99) 6.11 Tª 0.78 <0.0001

DW 16.63) 0.47 Tª 0.59 <0.0001

CF – – n.s.

Table 4. Results of the stepwise forward multiple regression analysis for prosome length (PL), prosome width (PW), dry weight (DW)

and condition factor (CF) of the females of Acartia clausi in the estuaries of Bilbao and Urdaibai, and in both estuaries taken together,

once temporal variability is eliminated (DOS: dissolved oxygen saturation, Sal: salinity, n.s.: no significant)

Variables Regression R2 p

Estuary of Bilbao PL )15.78 + 0.21 DOS 0.27 0.0001

PW )4.83 + 0.07 DOS 0.19 0.0020

DW – – n.s.

CF 1.15) 0.03 Sal 0.10 0.0301

Estuary of Urdaibai PL )200.73 + 5.85 Sal 0.42 0.0028

PW )100.22 + 2.22 Sal + 0.24 DOS 0.49 0.0047

DW – – n.s.

CF – – n.s.

Both estuaries PL )174.19 + 5.10 Sal 0.26 <0.0001

PW )56.47 + 1.65 Sal 0.20 <0.0001

DW – – n.s.

CF – – n.s.

335

and dilution and nutrient limitation seem to beresponsible of the decrease of phytoplankton bio-mass seawards (Franco, 1994).

Morphological variations

On seasonal scale negative effect of temperature oncopepod body size has widely been remarked(among others Ambler, 1985; Durbin et al., 1992;Escribano & McLaren; 1992; Christou & Verrio-poulos, 1993; Norrbin, 1994; Viitasalo et al., 1995;Gaudy & Verriopoulos, 2004). Although, limita-tion on food supply would also affect the final sizeof copepods negatively (Vidal, 1980; Klein Breteleret al., 1982; Klein Breteler & Gonzalez, 1988;Viitasalo et al., 1995), in our case no relationshipbetween size and available food was observed, inagreement with other works (Le Borgne et al.,1985; Pagano & Saint-Jean, 1989; Hopcroft &Roff, 1990). The CF standardizes the comparisonsof animals from different environments (Durbin &Durbin, 1978), and varies according to the quan-tity of available food (Durbin & Durbin, 1978;Durbin et al., 1983; Christou & Verriopoulos,1993). However, our data showed no relationshipbetween CF or body size and available food. Overall study period, Chl a concentration was>0.47 lg l)1, which is considered the lowest levelfor development of Acartia genus (Landry, 1978;Durbin et al., 1983). Moreover, Acartia can alsoingest heterotrophic micro- and nano-plankton(e.g. Wiadnyana & Rassoulzadegan, 1989; White& Roman, 1992; Gasparini & Castel, 1997; Roll-wagen Bollens & Penry, 2003). Therefore, ourresults suggest that our estuaries are food-richenvironments that do not limit the development ofA. clausi by food availability.

Between-estuaries environmental differencesseem not to affect the body size of A. clausi,denoting its high capacity to develop in eutrophiedand polluted systems (Moraitou-Apostolopoulou& Verriopoulos, 1978; Arfi et al., 1981; Siokou-Frangou & Papathanassiou, 1991; Uriarte &Villate, 2005). The weight was significantly higherin Urdaibai at 35 and 34 psu but in Bilbao at33 psu. This could be interpretated as a higherindividual metabolic cost in Bilbao at high salini-ties, since for similar sizes the female weight waslower. However, the lower weight in Urdaibai at33 psu must be taken with caution since at this

salinity high physical deterioration of individualswas observed. Rapid mixture of bottom watersfrom 30 to 35 psu in less than 3 km (Ruiz, 1995)may have a negative effect on neritic speciesinward estuaries.

Although in the case of macrozoobenthos andmeiobenthos of confinement environments, therole of salinity as major factor in driving bioticfeatures is questioned, focusing on the role ofhydrodynamic patterns (Guelorget et al., 1994;Lefebvre et al., 1997), in our study, significantdecrease of the prosome length and width in bothestuaries at decreasing salinity corroborates thecrucial role of salinity on the size of copepods inestuaries and coastal systems (Moraitou-Aposto-lopoulou et al., 1986; Shanmungan et al., 1986;Gaudy et al., 1988; Pagano & Saint-Jean, 1989).This relationship with salinity is in agreement withthe observed for the zooplankton community(Uriarte & Villate, 2004) and the population ofA. clausi (Uriarte & Villate, 2005) in the sameestuaries, but not for the egg production (Uriarteet al., 2005). Divergences of the optimal salinityenhance the respiration rate of copepods (Gaudyet al., 2000), due to necessities of an energy sup-plement for osmoregulation and maintenance(Kinne, 1964; Moore & Folt, 1993) and conse-quently result in smaller individuals (Gaudy et al.,1988; Milliou, 1996). Differences in body sizecould reflect ecophysiological variations (Milliou,1996) as a result either of adaptations to environ-mental differences in the systems (Gaudy et al.,2000) or to genetically separated populations(Deevey, 1960; Vianello, 1968; Caudill & Bucklin,2004). Thus, smaller sizes at low salinities may be aphysiological and ecological advantage, since thenecessary extra energy for the osmorregulation isbalanced by the energetic saving as consequence oflow growth rate and low oxygen consumption(Miliou, 1996). Our results demonstrate that in anestuarine context, relatively small variation insalinity (35–33 psu) produces significant changesin the body size of copepods. However, this effectseems to be restricted to neritic species that pene-trate into estuaries, since the estuarine speciesA. tonsa showed no relationship between salinityand length in a larger salinity variation (18–30 psu) (Ambler, 1985), confirming the mostmarine character of A. clausi (Chinnery & Wil-liams, 2004; Uriarte & Villate, 2005). Likewise,

336

environmental deterioration inward the estuary ofBilbao enhanced this effect, as is shown in thestepwise regression analyses where DOS appearedthe main factor to account for the morphologicalvariability. Thus, the depletion in available oxygensuggests an increase of the energetic costs (Moore& Folt, 1993) that might negatively influence thefinal size of copepods. On a small geographic scaleenvironmental variables may be responsible forsignificant variations of the morphology of thespecies (Gaudy & Verriopoulos, 2004). On thecontrary, in this study, the spatial variation trendof the size dimensions along salinity gradientcontrasts with that of the weight, which increasedweakly in both estuaries. However in Urdaibai sizedecreased significantly at 33 psu probably due tothe physical deterioration observed at this salinitypreviously discussed.

In summary, our results show that the mor-phological characteristics of A. clausi are not onlyrelated to the season, influenced by temperature,but, in a given geographical zone, they are alsorelated to local differences in environmental vari-ables, such as salinity and oxygen concentration.Future studies should focus on the biologicalmechanisms that produce equilibrium between sizereduction and biomass stability under stress con-ditions, such as low salinities, pollution or eutro-phication

Acknowledgements

Financial support for this research was providedby the University of the Basque Country UPV118-310-EA018/96 and by a grant to I. Uriarte fromthe Department of Education, Universities andResearch of the Basque Government. Thanks tothe team of the ecology laboratory, and especiallyto U. Cotano for help in carrying out this research.

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