The seasonality of meiofauna abundance was examined at an intertidal mud flat located in the oligo-mesohaline reach of the Gironde Estuary, France. Samples.
Estuarine, Coastal and Shelf Science (1996) 43, 549–563
Seasonal Variability of Meiofaunal Abundance in the Oligo-mesohaline Area of the Gironde Estuary, France
P. J. P. Santos, J. Castel and L. P. Souza-Santos Laboratoire d’Océanographie Biologique, Université Bordeaux I, 33120 Arcachon, France Received 27 January 1995 and accepted in revised form 4 August 1995
Keywords: meiobenthos; seasonal variations; estuaries; French coast The seasonality of meiofauna abundance was examined at an intertidal mud flat located in the oligo-mesohaline reach of the Gironde Estuary, France. Samples were collected weekly for 1 year at two stations located at the lower and the upper level of the mud flat (Stations L and H, respectively). Clear seasonal variations were evident for the meiofauna at both stations. Multivariate analysis revealed that meiofaunal composition, as well as biomass, was mainly regulated by temperature and salinity. Meiofaunal abundances were dominated by harpacticoid copepods at Station H and by nematodes at Station L, with meiofaunal biomass dominated by harpacticoid copepods at both stations, an unusual situation in estuarine muddy sediments. The meiofaunal community response to salinity occurs both as a chronic effect and as an anticipation to estuarine water salinity changes, thus suggesting that the animals response to this factor in oligo-mesohaline areas can also be an acute response to local heavy rains which can influence the meiofauna due to both variations of interstitial salinity and by erosion. Microphytobenthic resources, as estimated by chlorophyll a and pheopigment concentrations, had no strong influence on either meiofauna composition or biomass. Furthermore, evidence is given that microphytobenthic food supply is not a limiting factor to the benthic fauna in the oligo-mesohaline area of the Gironde Estuary. ? 1996 Academic Press Limited
Introduction Although meiofauna has been the subject of studies in several European estuaries (see references in Smol et al., 1994), and some studies have been done on the correlation between seasonal variations of meiofauna and trophic resources (see references in Bodiou et al., 1990), only few, intermittent data are available regarding meiofauna and microphytobenthos for French estuaries. Estuaries are amongst the most productive of non-cultivated ecosystems although the reduced and fluctuating salinities result in a harsh environment. In contrast to macrobenthic species, which decrease in numbers from the mouth to head of estuaries, many meiofaunal species have a high tolerance for brackish waters. This results in a relative increase of meiofauna in brackish waters where sometimes the biomass relation between 0272–7714/96/050549+15 $25.00/0
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the two faunal groups can become 1:1 (Giere, 1993) or even higher [up to 90% of total living biomass measured by ATP concentration (Sikora et al., 1977)]. However, when compared to the lower and middle reaches of estuaries, the invertebrate communities of upper oligo-mesohaline regions have received little attention (West & Ambrose, 1992). One of the main features of the Gironde Estuary is the high turbidity of the water, with suspended particulate matter concentrations (SPM) which can exceed 1 g l "1 in the oligo-mesohaline area. Due to the high turbidity, phytoplankton blooms are restricted to areas and periods of low SPM concentrations, occurring generally only during summer in the lower estuary (Irigoien, 1994). Particulate organic carbon (POC) and bacterial numbers are also low (1–1·5% of SPM and 105 cells ml "1, respectively) in the oligo-mesohaline area (Fontugne & Jouanneau, 1987; Prieur et al., 1987), and the readily degradable carbon represents only a small fraction (10–15%) of POC (H. Etcheber, pers. comm.), indicating that the SPM is of poor nutritional value. This suggests that the trophic resource utilized by meiofauna may be the autochthonous benthic production. This study focuses attention on meiofauna, microphytobenthos and the associated physico-chemical parameters in the oligo-mesohaline zone of the Gironde Estuary, and gives an estimation of meiobenthic biomass and food requirement which is compared to microphytobenthic resources. Materials and methods The study area is located in the middle Gironde Estuary (45)16*N, 0)47*W) 55 km seaward from Bordeaux. Tidal amplitude varies between 2·5 and 5·0 m and current velocities can reach 2 m s "1. Freshwater discharge varies seasonally, usually reaching a maximum in January–February (mean: 1500 m3 s "1) and a minimum in August– September (mean: 250 m3 s "1). Two intertidal stations were sampled at the lower (Station L) and upper (Station H) level of the mud flat. Between April 1992 and April 1993, both stations were surveyed weekly, during low water, resulting in 47 sampling data sets. Salinity was measured in the channel using a hand refractometer, and sediment temperature was measured with an electronic thermometer at 3 cm depth. Published data were used for insolation (Bulletin Climatologique de la Gironde) and for the tidal coefficient, a specific French unit showing a logarithmic relationship with tidal amplitude (Annuaire des Marées, Port Autonome de Bordeaux). Four replicate cores (inner diameter 1·4 cm) were taken at each station for meiofauna (to a depth of 5 cm), microphytobenthic pigments, particulate organic carbon (three replicates to a depth of 1 cm) and the sand content of the sediment (two replicates to a depth of 1 cm). Meiofauna samples were preserved in 10% formalin and extracted from the sediment using the Ludox technique (modified from de Jonge & Bouwman, 1977). Animals retained on a 63-ìm sieve were stained with Rose Bengal. Meiofaunal biomass was obtained by weighing 10–200 individuals of the major taxa (rinsed in distilled water and dried at 60 )C for 24 h) on a Mettler ME 22 microbalance (sensitivity 0·1 ìg). Copepods were identified to species level. Magnesium hydroxycarbonate (1%) was added to the microphytobenthic pigment samples which were stored frozen at "20 )C. Pigments were extracted from lyophilized sediment samples in 90% acetone. Chlorophyll a and pheopigment concentrations were determined spectrophotometrically using the equations of Lorenzen (1967). Particulate
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organic carbon was determined using a CHS-LECO analyser (values were expressed as the percentage of carbon to sediment weight). The sand content was determined as the percentage (weight/weight) of the sedimentary fraction greater than 63 ìm. Conversion of meiofauna biomass from dry weight to carbon content was made using a factor of 0·4 (Feller & Warwick, 1988). Microphytobenthos carbon was estimated using a C:chlorophyll a ratio of 40 (de Jonge, 1980; Gould & Gallagher, 1990). Estimations of meiobenthic and microphytobenthic production are given based on published P/B ratios. Meiobenthic food requirements were calculated using the estimated production and published values for efficiency of food conversion and assimilation. A similar approach was used by Witte and Zijlstra (1984) and Escaravage et al. (1989). In order to investigate general patterns in the meiofauna community a detrended correspondence analysis (DCA) was used on the density data [ln(x+1) transformed] of the higher meiofaunal taxa, for both stations using the DECORANA computer program (Hill, 1979). Since the fauna may respond to changes in environmental factors with a lag (as occurs in predator–prey interactions), cross-correlation analysis was used to identify lag phases between environmental factors and axis scores variation. After this step, multiple correlation analysis (CA) was used to correlate the axis scores (resulting from the ordinations) with the lag-moved physicochemical and microphytobenthic parameters. Stepwise multiple regression analysis (MRA) was used to find the most important factors explaining meiofauna biomass variability (P
