Meiofauna response to a dynamic river plume front

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Meiofauna response to a dynamic river plume front Article in Marine Biology · September 2000 DOI: 10.1007/s002270000353





4 authors: Roberto Danovaro

Cristina Gambi

Stazione Zoologica Anton Dohrn di Napoli

Università Politecnica delle Marche





Elena Manini

Mauro Fabiano

Italian National Research Council

Università degli Studi di Genova





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Marine Biology (2000) 137: 359±370

Ó Springer-Verlag 2000

R. Danovaro á C. Gambi á E. Manini á M. Fabiano

Meiofauna response to a dynamic river plume front

Received: 30 November 1999 / Accepted: 24 May 2000

Abstract The benthic response to a plume front was studied in two areas of the northern Adriatic (Mediterranean Sea) di€erently in¯uenced by the Po River freshwater input. Sediment samples were collected in June 1996 and February 1997 from 12 stations. The adopted sampling strategy was able to identify the front line in real time by satellite images and to locate sampling stations along an inner±outer plume gradient in order to cover the benthic area beneath the river plume, where enhanced biological production was expected, and open-sea sediments not directly in¯uenced by freshwater inputs. Meiofaunal parameters were compared to the physical conditions and to phytodetritus inputs, organic matter accumulation and bacterial secondary production. The sediments of the Adriatic Sea were characterised by high concentrations of phytopigments (0.6 to 13.9 lg g)1 for chlorophyll a and 1.2 to 17.7 lg g)1 for phaeopigments) and biopolymeric organic carbon (0.15 to 3.02 mg g)1). The plume system extended for a large sector of the northern Adriatic. In the northern area, a large and highly dynamic plume area was coupled with a sediment organic matter concentration signi®cantly higher than in open-sea sediments. In the southern sector, where the plume area and the front line did not change markedly during the year, plume±benthic coupling was evident only in the

sediments beneath the front, and corresponded to phaeopigment accumulation. Bacterial parameters and secondary production were high and signi®cantly higher in the frontal area than at open-sea stations. Meiofauna density (1342 to 8541 ind. 10 cm)2) did not change either by season or between areas and was signi®cantly correlated with phaeopigments and bacterial secondary production. Meiofauna displayed di€erent responses to plume inputs in the two sampling areas. In the northern sector, meiofauna density was coupled with organic matter distribution and displayed highest values beneath the plume. In the southern sector, the densities of copepods, turbellarians and kinorhynchs displayed highest values under the front in summer, and the same applied to total meiofauna density in winter. Juvenile decapods and copepod nauplii signi®cantly increased their densities in sediments beneath the front. Data presented in the present study suggest that plume inputs and frontal systems, enhancing phytodetritus accumulation and benthic bacterial response, might in¯uence density, composition and distribution of meiofaunal assemblages. As river plumes are highly variable systems a€ecting the trophic characteristics of the sediments underneath, their dynamics should be considered when analysing mesoscale spatial changes of meiofaunal assemblages.

Introduction Communicated by R. Cattaneo-Vietti, Genova R. Danovaro (&) á C. Gambi á E. Manini Sezione Biologia Marina, FacoltaÁ de Scienze UniversitaÁ de Ancona, via Brecce Bianche, 60131 Ancona, Italy Tel.:+39-71-2204654; Fax:+39-71-2204650 e-mail: [email protected] R. Danovaro Dipartimento di Zoologia, UniversitaÁ di Bari, Via Orabona 4, 70125 Bari, Italy M. Fabiano Dipartimento del Territorio e delle sue Risorse (DIPTERIS), via Benedetto XV, 61100 Genova, Italy

The coupling between pelagic and benthic systems is an issue attracting increased interest in the ®eld of marine ecology (Fleeger et al. 1989; Gooday and Turley 1990; Graf 1992; Danovaro et al. 1999). It is commonly accepted that benthic biomass, production and activity depend upon the amounts of organic matter reaching the sea ¯oor, after its production in the overlying water layers (Pfannkuche et al. 1983; Graf 1989). Among pelagic systems, fronts are hydrodynamic structures characterised by a vertical strati®cation of the water column that might in¯uence the benthic ecosystem. Fronts represent the interface between two water


masses with di€erent physical and chemical properties, e.g. temperature, salinity and/or nutrient concentration (Jeverson et al. 1979). Their most relevant e€ects are the con®nement of the phytoplankton and the local enhancement of primary production (Riegman et al. 1990; Heilmann et al. 1994). The resulting production of large amounts of fresh organic detritus and the absence of water column strati®cation lead to increased organic matter vertical ¯uxes to bottom sediments (Frontier 1986). Among the di€erent frontal structures (that include also shelf-break, upwelling, divergence and convergence fronts), river plumes have the peculiarity of carrying to the sea large amounts of nutrients derived from natural or anthropogenic sources. Such nutrient inputs increase primary productivity at the mouth of the river and at the frontal area where lower salinity plume waters meet the nutrient-poor, high-salinity open-sea waters (Lalli and Parsons 1993). As a consequence, it may be expected that benthic activity and secondary production are enhanced in the sediments in¯uenced by the plume. In this regard, pioneer studies relating pelagic fronts with macrobenthos have been carried out in the North Sea (e.g. Skagerrak±Kattegat front; Josefson and Conley 1997). However, the benthic response to pelagic inputs is seldom easy to assess, and di€erent living components might react di€erently to the organic matter inputs. The scant information available deals only with macrofauna and suggests that front-related organic matter inputs might signi®cantly increase subsurface-feeding mollusc densities, and depress surface deposit-feeding molluscs and most crustaceans, whereas other components are simply not responsive (Josefson and Conley 1997). Due to their life cycles and high turnover rates, meiofauna are expected to respond rapidly to changes in food availability (Danovaro 1996), in particular, to the organic loading produced by river discharge (Albertelli et al. 1999). In this regard, meiofauna density and diversity in the Rocas Atoll have provided indirect evidence of the presence of a topographically controlled front (Netto et al. 1999). However, literature dealing with meiobenthic response to plume fronts is, to our knowledge, non-existent. The present study was designed to examine the e€ects of a dynamic river plume on the sediment below it. The Po River plume in the Adriatic Sea has important largescale e€ects, as the Po River represents the major freshwater input for the entire Mediterranean basin, accounting for 30 to 50% of the total river inputs (Frascari et al. 1988). The Adriatic Sea generally exhibits a decreasing trend of nutrient concentrations from north to south, due to the nutrient input by rivers in the northern area. The dominant cyclonic circulation determines a southward nutrient ¯ow along the western coast of the basin that becomes relatively less important in the middle and southern Adriatic. The northwestern coastal waters are strongly in¯uenced by the freshwater out¯ow of the Po River (OrlieÁ et al. 1992), which dominates the annual mean total ¯ow of all the rivers of the

northern and middle Adriatic (Artegiani et al. 1997). Freshwater inputs from the Po show a strong seasonal variability, averaging about 1500 m3 s)1. In summer, the front is parallel to the western shoreline, whereas, during winter, Bora winds combined with increased river out¯ow spread the Po waters over the entire interior of the North Adriatic, extending the front across the Adriatic. Po waters spread southward along the western coast down to Ancona (OrlieÁ et al. 1992) where they display a surface salinity ranging from 31.6 (coastal area in¯uenced by the plume) to 37.5& (open-sea area). The high organic loading, mostly due to Po River discharge, is coupled with increased primary production as far as 150 km south of the Po delta (Zoppini et al. 1995). During the last decades, eutrophication of the Adriatic Sea has increased notably, due to nutrient enrichment combined with anthropogenic activities (including tourism and ®sheries; Vollenweider et al. 1992). In the present study we investigated the e€ects of the plume on the accumulation of sedimentary organic matter (as biopolymeric carbon), the distribution of phytopigments (as tracers of primary production), and bacterial biomass and production. The direct or indirect river plume in¯uence on meiofauna assemblages was tested in di€erent seasons by comparing the di€erent meiofaunal responses (in terms of density and community structure) in open-sea areas (as a control) and beneath the river plume and its front dynamics.

Materials and methods Sampling strategy Sediment samples were taken during two cruises on board the R.V. ``Urania'' in June to July 1996 and February to March 1997. These periods, selected as representative of opposite conditions (summer vs winter), also re¯ected opposite freshwater inputs (highest in late spring-summer and lowest in winter). Two areas, the northern and middle Adriatic Sea (hereafter northern and southern area), were sampled in each period in order to explore di€erences in the e€ects of the Po River plume at an increasing distance from the delta. Samples were taken across the frontal area from the inner to the outer sector. Instead of selecting ®xed stations, we assumed a dynamic benthic response, and we identi®ed six stations per sampling area (northern and southern) and period (Fig. 1). These stations were selected from two main grids of 30 ´ 40 nautical miles containing 48 to 60 stations (in the two sampling periods, respectively), where CTD downcasts were carried out to de®ne the thermohaline conditions. In each area and during each sampling period, three main ecological sectors (i.e. beneath the plume, under the front and in the open sea; see Table 1) were covered with two stations each. As front position and dimensions were highly dynamic, ATSR (microwave radiometer for cloudy skies) and SAR (Synthetic Aperture Radar) measurements were taken to monitor plume extension and displacement with time. The front position and displacement were also followed using satellite images and measurements of water temperature and salinity with a SARAGO system (to detect vertical and horizontal changes in the physical characteristics of the water column). The zigzag temperature and salinity pro®les obtained using SARAGO were utilised for identifying the plume front (de®ned as the area where the sharpest salinity gradient was observed). All these measurements were carried out synoptically 1 to 3 d before benthic sampling. In the northern

361 Fig. 1 Sampling area and location of the sampling stations. The position of the plume front in June 1996 and February 1997 is illustrated as detected by SARAGO thermohaline pro®les coupled with satellite images

sampling area, where the frontal pattern was more dynamic, coastal sampling stations were located 7 to 10 nautical miles landward of the front; two ``frontal stations'' were positioned close to each other and on both sides of the front, one on the internal and one on the external side, and the last two stations were located in the open sea, about 7 to 10 nautical miles seaward of the front. This station spacing was selected to ensure that the front line could not reach the open-sea stations within a period of 4 weeks. In the southern area, the location of the front did not change greatly with time (Fig. 1). Sediment sampling All sediment samples (for biochemical, bacterial and meiofaunal analyses) were taken using a multiple corer (model Midi, four core tubes with 5.7 and 9.5 cm diameter). Six to seven undisturbed replicate cores were selected from a total of 10 to 20 deployments. Three cores were analysed for meiofauna. The top 6 cm were sectioned into di€erent sediment layers (0 to 1, 1 to 3 and 3 to 6 cm) and preserved in bu€ered 4% formalin solution using 0.45 lm pre®ltered arti®cial seawater containing MgCl2 (80 g l)1) and stained with about 200 ll of Rose Bengal (1&). For organic matter analysis (i.e. chlorophyll a, phaeopigments, carbohydrates, lipids and proteins), the surface sediment (0 to 1 cm) from two or three additional cores was frozen at )20 °C. For bacterial analyses, subsamples (1 ml) were incubated as described below for secondary production estimates or added to 10 ml of 0.2 lm ®ltered seawater with pre®ltered formaldehyde (0.2%) for bacterial counting. Environmental parameters Temperature at the water±sediment interface (down to 1 cm depth into the sediments) was measured with an immersion thermometer; surface salinity was determined by CTD (SeaBird, SBE); grain size was determined with a dry sieve technique. Vertical pro®les of redox potential (Eh) were determined immediately after collection using an Eh-meter (HI-8424) down to 6 cm depth, only values relative to the top 5 mm are reported here. Redox potential discontinuity depth (RPD), estimated at the depth at which sediment colour turns from brown to black and expressed in centimetres, is reported here as an integrated measure, complementary to Eh pro®les, for identifying oxygenation conditions of the sediment core. Chlorophyll a and phaeopigment analyses (n = 3) were carried out according to Lorenzen and Je€rey (1980). Pigments were extracted from about 1 to 2 g of sediment with 90% acetone. After

chlorophyll a determination, samples were acidi®ed with 0.1 N HCl to determine phaeopigment concentration. Carbohydrates were analysed according to Gerchakov and Hatcher (1972). Lipids were extracted from sediment subsamples by direct elution with chloroform and methanol. Analyses were carried out using the methods of Bligh and Dyer (1959) and Marsh and Weinstein (1966). Absorbance was measured at 375 nm. Concentrations were reported as tripalmitine equivalents. Proteins were analysed according to Hartree (1972). The three biochemical classes (lipids, proteins and carbohydrates), converted to carbon equivalents, were summed up and de®ned as the biopolymeric carbon concentration (BPC; sensu Fabiano and Danovaro 1994), which is reported here as a synthetic measure of the sediment organic carbon potentially available to benthic heterotrophs. Data were normalised to dry weight after desiccation (60 °C, 24 h). Water content was determined simultaneously as the di€erence between wet and dry sediment weight. For comparison, data were also normalised to sediment volume (ml)1), using water content and assuming a speci®c sediment density of 2.4 g cm)3. Bacterial analysis (density and biomass) was carried out as described by Danovaro et al. (1994). Portions of the subsamples were stained with Acridine Orange and ®ltered onto black, 0.2 lm Nuclepore ®lters, which were analysed by epi¯uorescence microscopy (Zeiss Axiolab, HBO 50 W). Data were normalised to dry weight after desiccation (60 °C, 24 h). Bacterial production was measured with the 3H-leucine incorporation method, following the procedure for sediments suggested by van Duyl and Kop (1994) with a few adaptations. The radioactivity was counted on a liquid scintillation counter (Packard, Tri-Carb 2100 TR). Data on bacterial production have been summarised from Fabiano et al. (1997). Meiofaunal analysis For meiofaunal analysis, sediment samples were passed through 1000 and 20 lm mesh sieves to retain the smallest meiofaunal organisms. The fraction remaining on the 20 lm sieve was centrifuged three times with Ludox HS 40 (density 1.15 g cm)3) as described by Heip et al. (1985). All meiobenthic animals were counted and classi®ed taxanomically under a stereomicroscope 40 or 80´ (Zeiss, Stemi 2000); soft body meiofauna were mounted on slides and identi®ed, if possible, with a microscope at 400 to 1000´ (Zeiss, Axiolab HBO). Statistical analysis The relationship between meiofaunal and sediment parameters was tested initially using a Spearman rank correlation analysis. Meio-

February 1997 Northern area 16 plume 11 plume 18 front 13 front 20 open sea 15 open sea Southern area 5 plume 1 plume 6 front 2 front 8 open sea 4 open sea

June 1996 Northern area 1 plume 17 plume 12 front 13 front 8 open sea 24 open sea Southern area 25 plume 43 plume 33 front 40 front 30 open sea 48 open sea


44°40¢50¢¢ 44°34¢34¢¢ 44°35¢11¢¢ 44°30¢35¢¢ 44°31¢17¢¢ 44°26¢28¢¢

43°46¢58¢¢ 43°45¢04¢¢ 43°48¢10¢¢ 43°46¢42¢¢ 43°53¢51¢¢ 43°51¢41¢¢

13°14¢39¢¢ 13°18¢06¢¢ 13°17¢10¢¢ 13°20¢44¢¢ 13°23¢07¢¢ 13°26¢20¢¢

43°54¢30¢¢ 43°44¢32¢¢ 43°55¢43¢¢ 43°54¢31¢¢ 44°05¢30¢¢ 43°55¢25¢¢

13°02¢26¢¢ 13°17¢55¢¢ 13°13¢52¢¢ 13°21¢13¢¢ 13°16¢30¢¢ 13°31¢48¢¢

12°54¢30¢¢ 12°55¢19¢¢ 13°05¢50¢¢ 13°03¢21¢¢ 13°13¢32¢¢ 13°11¢16¢¢

44°38¢74¢¢ 44°27¢47¢¢ 44°33¢14¢¢ 44°33¢30¢¢ 44°38¢22¢¢ 44°28¢08¢¢

12°35¢09¢¢ 12°35¢28¢¢ 12°47¢58¢¢ 12°52¢22¢¢ 13°04¢57¢¢ 13°04¢53¢¢

Longitude Latitude (E) (N)

15.0 14.9 18.5 19.4 49.8 50.0

36.9 42.3 42.0 40.2 47.8 48.9

27.0 13.5 36.0 47.0 57.0 66.0

28.6 27.7 35.6 40.1 40.0 41.5

9.5 10.0 10.5 10.0 11.0 11.8

10.8 10.5 10.9 10.9 11.5 11.4

18.0 19.0 13.0 18.5 15.0 18.5

15.5 15.0 12.5 12.5 12.0 12.0

Depth Bottom (m) temp. (°C)

33.9 32.0 36.0 36.3 36.2 37.3

34.5 33.4 32.8 31.6 34.9 37.0

35.2 35.3 36.5 36.2 37.5 36.4

34.2 33.5 34.8 35.7 37.2 37.5

2.5 4.0 6.0 3.5 6.0 5.0

4.5 4.5 6.0 nd 4.0 4.0

3.5 1.5 4.0 3.5 4.5 3.5

8.0 5.0 3.5 2.5 7.0 6.0

42.7 72.1 88.1 94.9 86.0 89.7

38.2 56.5 3.0 5.9 79.0 57.7

8.8 1.2 69.4 51.0 87.8 91.6

Grain size (% sand)

104.0 30.2 90.0 32.3 110.0 4.4 )11.0 5.2 nd 4.6 44.0 1.7

40.0 20.0 110.0 nd nd 120.0

180.0 35.0 69.0 74.0 65.0 97.0

77.5 )10.0 42.9 91.0 60.1 57.8

Salinity RPD Eh (&) (cm) (mV)

2.1 1.8 2.3 3.4 1.1 1.3

13.9 1.4 0.9 1.0 0.6 0.7

2.5 7.9 2.5 2.2 1.2 1.0

7.9 4.8 3.2 1.5 1.3 2.6

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

Phaeop. (lg g)1)

Biopolymeric carbon (lg C g)1)

5.4 8.6 7.4 8.3 3.5 2.8

‹ ‹ ‹ ‹ ‹ ‹

0.5 1.1 0.6 1.5 0.1 1.3

1800.7 1487.0 2357.7 2310.5 584.5 795.0

‹ ‹ ‹ ‹ ‹ ‹

611.8 327.0 732.2 624.5 98.2 252.0

0.5 9.9 ‹ 2.2 923.9 0.7 5.7 ‹ 1.5 479.4 0.0 10.4 ‹ 1.6 1359.1 0.7 10.5 ‹ 0.6 1397.5 0.1 7.2 ‹ 0.7 1544.4 0.5 9.3 ‹ 2.4 1331.1

‹ ‹ ‹ ‹ ‹ ‹

210.8 128.3 392.6 389.0 451.7 314.2

2.5 17.7 ‹ 2.0 1435.6 ‹ 253.8 0.7 5.9 ‹ 1.9 421.6 ‹ 111.3 0.2 2.0 ‹ 0.3 249.1 ‹ 47.2 0.2 1.8 ‹ 0.3 148.6 ‹ 31.6 0.1 1.2 ‹ 0.2 209.4 ‹ 35.2 0.1 2.2 ‹ 0.2 196.3 ‹ 40.4

0.1 0.9 0.2 0.2 0.0 0.3

0.8 14.0 ‹ 1.5 3017.8 ‹ 800.8 0.3 16.5 ‹ 2.3 2917.3 ‹ 823.7 0.0 9.1 ‹ 0.8 1786.1 ‹ 378.9 0.2 5.0 ‹ 0.8 1046.8 ‹ 237.0 0.2 3.1 ‹ 0.4 470.8 ‹ 122.0 0.0 2.0 ‹ 0.1 377.7 ‹ 81.3

Chl a (lg g)1)

12.0 11.8 20.1 22.5 12.5 14.5

8.9 2.4 2.3 1.9 2.7 3.1

22.8 10.6 28.8 17.6 8.9 20.2

36.5 25.0 30.6 15.8 16.1 6.6

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

1.8 4.5 1.4 4.6 4.6 3.0

0.8 0.6 0.2 0.2 1.0 0.4

8.3 2.9 9.6 1.0 1.6 3.4

7.4 5.5 1.9 3.2 1.5 0.6

24.0 23.5 40.2 44.9 25.1 29.0

17.8 4.9 4.6 3.7 5.3 6.3

179.7 95.8 251.8 140.8 68.6 150.0

301.6 205.1 267.0 147.6 154.7 57.4

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

3.6 9.0 10.0 9.2 9.1 5.9

16 5.6 0.4 0.4 2.0 0.8

65.6 26.4 83.1 7.8 16.3 25.4

63.5 46.5 15.8 32.8 15.8 5.0

Bacterial Bacterial density biomass (cells ´ 108 g)1) (lg C g)1)

0.26 0.14 0.32 0.41 0.17 0.24

0.49 0.27 0.09 0.08 0.12 0.10

0.62 0.63 0.60 0.81 0.20 0.26

1.51 0.43 0.73 0.22 0.05 0.07

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

‹ ‹ ‹ ‹ ‹ ‹

0.25 0.05 0.03 0.21 0.06 0.05

0.13 0.23 0.01 0.00 0.02 0.02

0.20 0.05 0.15 0.34 0.06 0.05

0.80 0.14 0.20 0.09 0.02 0.01

Bacterial production (lg C g)1 h)1)

Table 1 Station location, environmental parameters, phytopigments, organic matter concentration and bacterial parameters (mean ‹ SD) collected in June 1996 and February 1997 in the northern and southern areas (RPD redox potential discontinuity depth; Eh redox potential; nd not determined)


363 faunal parameters were tested for di€erences between open-sea stations and stations beneath the plume or beneath the plume front, using two-way analysis of variance (ANOVA) and post hoc Tukey tests. Data normality was tested before analysis, and when normality conditions were not matched data were log-transformed.

Results Environmental parameters Data on temperature, salinity, RPD depth, redox potential and grain size are reported in Table 1. Temperature varied with season and ranged from 12.0 to 19.0 °C in June and from 9.5 to 11.8 °C in February. The RPD depth showed a strong variability within season and sampling area, ranging from 1.5 to 8.0 cm in June and from 2.5 to 6.0 cm in February. Redox potential analysis indicated the presence of suboxic conditions (generally 0.625 mm accounting for 51 to 91.6%) at all stations except those

near the coast, which were characterised by muddy sediments (fraction Front±Open sea Open sea < Plume±Front Season p-level p-level p-level p-level Temperature ns Salinity 0.001 RPD ns Eh ns Grain size 0.001 Chlorophyll a 0.001 Phaeopigment 0.001 Biopolymeric 0.001 carbon Bacterial 0.020 density 0.001 Bacterial secondary production Nematodes 0.010 Copepods 0.020 Nauplii 0.050 Polychaetes ns Ostracods ns Kinorhynchs ns Others ns (meiofauna) Total meiofauna0.010 a

ns 0.001 ns ns 0.001 0.030 0.030 0.010

ns ns ns ns

Front values signi®cantly higher than plume and open-sea values

June > February p-level

ns 0.001 ns ns

0.020 ns ns ns 0.040 ns ns 0.000

0.020 ns ns ns 0.000 ns ns 0.000







0.010 0.010 0.030 ns ns ns ns

ns 0.001 0.000 0.020 0.030 ns 0.000

ns 0.001 0.020 0.020 0.020 ns 0.020





decreasing from the coast to the open sea. In the southern area, during summer, BPC reached highest values at the frontal stations, whereas in winter, its concentrations increased from the coast to the open sea (Table 1). Phytopigments and BPC concentrations were signi®cantly correlated (Table 3), and both parameters consistently displayed signi®cantly higher values in sediments under the river plume (Table 2). Bacterial parameters In both areas, bacterial abundance, biomass and production ¯uctuated over time with a clear seasonality (Table 1), characterised by signi®cantly higher values in June than in February (Table 2). No signi®cant di€erences between sampling areas were observed (ANOVA, ns). In June, bacterial abundance ranged from 6.6 ‹ 0.6 to 36.5 ‹ 7.4 ´ 108 cells g)1, whereas in February it varied from 1.9 ‹ 0.2 to 22.5 ‹ 4.6 ´ 108 cells g)1. Similar temporal and spatial patterns were observed for bacterial biomass (from 57.4 ‹ 5.0 to 301.6 ‹ 63.5 lg C g)1 and from 3.7 ‹ 0.4 to 44.9 ‹ 9.2 lg C g)1, in June and February, respectively). Bacterial production on average was signi®cantly higher in summer than in winter (Table 2), ranging from 0.05 ‹ 0.02 to 1.51 ‹ 0.80 lg C g)1 h)1 in June and from 0.08 ‹ 0.00 to 0.49 ‹ 0.13 lg C g)1 h)1 in February. In the northern area, bacterial abundance signi®cantly decreased from the plume to the open-sea sediments (Table 2), but in the southern area, in both sampling periods, bacteria displayed highest density and production in sediments beneath the plume front (Table 2). Meiofauna abundance and community structure Total meiofauna density (in the top 6 cm of the sediment) did not show any seasonal variability (ANOVA, Table 3 Results of the Spearman rank correlation analysis between sedimentary, bacterial and meiofaunal parameters: temperature (Temp), chlorophyll a (Chl a), phaeopigments (Phaeop), biopolymeric carbon (BPC), bacterial density (TBN), biomass (BBM) and secondary production (BSP), meiofaunal density (Meio) nematode Temp Chl a Temp Chl a Phaeop BPC TBN BBM BSP Meio Nem Cop Naupl Pol Ostr Kin Juv Dec

Phaeop BPC

1.000 0.166 1.000 0.044 0.751 1.000 0.455 0.467 0.759 1.000 0.386 0.263 0.558 0.771 0.554 0.205 0.309 0.736 0.464 0.518 0.598 0.821 0.093 0.717 0.742 0.517 0.062 0.704 0.732 0.474 0.369 0.167 0.211 0.504 0.498 0.125 0.165 0.546 0.472 0.244 )0.091 0.042 0.223 0.215 0.142 0.405 0.078 0.592 0.666 0.560 0.138 )0.166 )0.137 0.235

ns). Average meiofaunal density was 3139.5 ‹ 904.4 ind. 10 cm)2 in June and 3156.8 ‹ 1659.6 ind. 10 cm)2 in February. Furthermore, no di€erences were observed comparing sampling areas: average meiofauna density was 2984.9 ‹ 1541.7 ind. 10 cm)2 and 3311.3 ‹ 1080.9 ind. 10 cm)2 in the northern and southern area, respectively (ANOVA, ns; Fig. 2a). Meiofauna density decreased with depth in the sediment, and more than 98% of the total density was concentrated in the top 6 cm of the sediments, whereby 30 to 60% of the total density was con®ned to the top 1 cm. In the northern area, in both sampling periods, meiofauna density increased from the coast to the open sea, ranging from 1341.9 ‹ 540.3 to 5748.7 ‹ 1368.6 ind. 10 cm)2 in June and from 1559.4 ‹ 486.0 to 8541.0 ‹ 4111.6 ind. 10 cm)2 in February. In the southern area, however, no clear trend in meiofaunal density was observed in June (range: from 2855.8 ‹ 1031.3 to 3787.9 ‹ 1304.1 ind. 10 cm)2), whereas in February highest densities were observed at frontal stations (7863.6 ‹ 2957.2 and 3495.7 ‹ 1218.8 ind. 10 cm)2, respectively). Total meiofauna density was signi®cantly correlated with phytopigments (both chlorophyll a and phaeopigments, n = 23, p < 0.01, Table 3), BPC concentrations (n = 23, p < 0.01), and bacterial density and secondary production (n = 23, both p < 0.05). Spatial and temporal changes of the main meiofaunal taxa are reported in Fig. 2b to h. Nematodes, the dominant taxon, ranged from 67 to 96% of the total meiofaunal density in June and February, respectively (Fig. 3a, b). In the northern area their absolute abundance decreased from the coast to the open sea in both periods (peaks at coastal stations: 5270.0 ‹ 1182.0 ind. 10 cm)2 in June and 7750.3 ‹ 3628.9 ind. 10 cm)2 in February). Conversely, in the southern area, in winter, nematodes showed a clear response to the river plume (peaks in frontal stations: 7569.2 ‹ 2800.9 and 3223.4 ‹ 1105.8 ind. 10 cm)2). Harpacticoid copepods density (Nem), copepod density (Cop) and their nauplii (Naupl), polychaete (Pol), ostracod (Ostr), kinorhynch (Kin) and juvenile decapod density (Juv Dec). Bold values signify: Roman type, p < 0.05, r = 0.380; italic type, p < 0.01, r = 0.486






1.000 0.741 0.592 0.388 0.365 0.432 0.418 )0.015 0.337 0.329 0.212

1.000 0.720 0.167 0.114 0.558 0.539 0.115 0.458 0.345 0.514

1.000 0.445 0.424 0.335 0.333 0.088 0.327 0.293 0.118

1.000 0.992 1.000 0.324 0.215 0.184 0.079 0.261 0.225 0.096 0.035 0.618 0.540 )0.050 )0.151


Naupl Pol



Juv Dec

1.000 0.897 0.300 0.560 0.597 0.816

1.000 0.171 0.494 0.486 0.705

1.000 0.224 0.629

1.000 0.353


1.000 0.237 0.304 0.248


Fig. 2 Meiofaunal density in the northern Adriatic in¯uenced by the Po River plume during two sampling periods (June 1996 and February 1997). Reported are: a total meiofaunal density; b nematodes; c copepods; d nauplii; e polychaetes; f kinorhynchs; g ostracods; h juvenile decapods. Data are expressed as ind. 10 cm)2 (‹SE )

were the second most important taxon, accounting for 2 to 18% of the total meiofaunal density. Copepod density ranged from 24.6 ‹ 11.6 to 329.1 ‹ 127.3 ind. 10 cm)2 and was signi®cantly correlated with BPC, and bacterial density and biomass (p < 0.05, r = 0.380; p < 0.01, r = 0.487, respectively; Table 3). They were

followed by polychaetes, turbellarians, ostracods and kinorhynchs, but their contributions to the total density were modest (from 0.0 to 10.5%). Juveniles of decapods, accounted on average for only 0.5 to 1% of the total meiofaunal density, but this value is extremely high when compared to the composition of meiofaunal assemblages in other coastal systems (Higgins and Thiel 1988). The category ``others'' included bivalves, tanaidaceans, cumaceans, priapulids, gnathostomulids, amphipods and cnidarians, but their overall contribution ranged between 0.6 and 7.8% of the total meiofauna.

366 Fig. 3 Meiofaunal composition during the two sampling periods (expressed as percentage)

Discussion Organic matter composition in relation to the front structure and dynamics The sediments of the Adriatic Sea were characterised by high concentrations of labile and fresh organic matter. Sedimentary phaeopigment concentrations, used as tracers of primary production inputs to the sea ¯oor, were very high, suggesting a strong coupling with water column processes. In addition, the high sedimentary chlorophyll a concentrations suggested the presence of high microphytobenthos densities (reaching about 9000 cells ml)1, Totti 1999), typical of a eutrophic area characterised by strong nutrient inputs from the Po River. Although the northern and southern areas displayed di€erent hydrographical features, no signi®cant di€erences were observed comparing the average values of most benthic parameters. Generally, phytopigment concentration and bacterial biomass were barely higher in the northern area, whereas BPC concentrations were slightly higher in the southern sector. However, the two investigated areas responded di€erently to the presence of the river plume. In the northern area, the plume was

much more extended than in the southern area, and the freshwater input determined a permanent salinity gradient. As a consequence, chlorophyll a and BPC concentrations strongly decreased from the sediments beneath the plume towards the open sea. In the southern area, where the position of the front line did not change greatly with time, remaining more-or-less parallel to the coast (see Fig. 1), an enhanced organic matter input was evident along the front (i.e. along the border between lower-salinity waters and open-sea waters). Here we observed that phytodetritus (expressed as phaeopigments) and BPC concentrations accumulated at frontal stations. These data con®rm the expectation of enhanced primary production and stronger pelagic±benthic coupling in plume-front areas (Smith and DeMaster 1996). Bacterial secondary production (BSP) in the sediments of the northern Adriatic was comparable to values reported from highly productive systems (Boto et al. 1989; Alongi 1990; van Duyl and Kop 1990). According to estimates of sediment oxygen consumption carried out in the Adriatic (Moodley et al. 1998), bacterial density and production were signi®cantly higher in summer than in winter (Table 2) and were signi®cantly enhanced in sediments beneath the front when compared to open-sea stations. BSP was also signi®cantly corre-


lated with phytodetritus inputs (Fig. 4a) and with BPC distribution (Fig. 4b), suggesting the presence of an evident bacterial response to plume-associated inputs of organic matter in the sediments of the Adriatic Sea. Abundance, composition and variability of meiofaunal assemblages In this study meiofauna showed very high densities, comparable to those previously reported for the Adriatic Sea (Tahey et al. 1996; Moodley et al. 1998) and even higher than those observed from rich/eutrophic coastal sediments (Feder and Paul 1980; Ansari and Ingole 1983; Elmgren et al. 1984; Radziejewska 1984; Colangelo and Ceccherelli 1994). Meiofaunal community structure, dominated by nematodes (about 86% of total meiofaunal density) and followed by harpacticoid copepods (about 7%) and polychaetes (about 2%), was typical of most coastal systems (including the Adriatic: Tahey et al. 1996; Moodley et al. 1998). Total meiofaunal densities in the Adriatic Sea did not change markedly between either seasons or sampling areas: as nematodes accounted for the largest component of meiofaunal density, this is primarily due to the consistency in nematode abundance. The lack of seasonal changes in nematode density is somewhat surprising, as biopolymeric organic carbon and bacterial

Fig. 4 Relationships between benthic bacterial secondary production (BSP) and a phaeopigments or b biopolymeric carbon. Slopes of the linear regressions are also given

parameters displayed values signi®cantly higher in June than in February, but is consistent with the lack of seasonal changes in phytopigment concentrations. On the contrary, copepods, nauplii, polychaetes, ostracods and other less represented taxa showed clear di€erences between seasons, being signi®cantly higher in June than in February (Table 2). Meiofaunal parameters did not change between the northern and southern areas (ANOVA, ns) and did not show any clear relationship with sediment texture. In fact, the northern area displayed coarser sediments directly in¯uenced by Po River out¯ow, whereas the southern area displayed a lower median grain size, but such di€erences were not re¯ected by changes in meiofaunal density or composition. The interactions between organic matter content/origin and meiofaunal assemblages were complex, as di€erent taxa apparently displayed di€erent driving variables. Nematode, copepod and polychaete densities were, on average, slightly higher in the southern area, in correspondence with greater food availability (expressed as BPC concentrations). Conversely, minor taxa, such as kinorhynchs, turbellarians and ``other'' organisms, showed only slightly higher densities in the northern area, in correspondence with higher phytodetritus concentrations and greater bacterial biomass. In both seasons meiofauna were signi®cantly correlated with phytopigments and BSP (Table 3), con®rming


the in¯uence of potential food sources on meiofaunal abundance and distribution (Montagna et al. 1983; Danovaro 1996). However, the slope coecients of these relationships were always higher in February than in June (Fig. 5a, b). Though correlation analyses do not allow us to infer on cause±e€ect relationships, these results, supported by ANOVA results, suggest that meiofauna might vary their response with season, depending upon the availability of microphytobenthos and benthic bacteria. In particular these data point out a much stronger meiofaunal response to bacterial production and phytopigment accumulation in February (during more limited food availability conditions) than in June. Meiofauna response to the river plume front: seasonal versus spatial variability The present study was designed to test meiofaunal coupling with pelagic processes (i.e. enhanced primary production) in relationship to the presence and variability of a large river plume. Considering that meiofaunal assemblages in the northern Adriatic are characterised by rather limited temporal changes and homogenous distribution (Moodley et al. 1998), the adopted sampling

Fig. 5 Relationships between meiofaunal abundance and a phaeopigment concentration or b bacterial secondary production

strategy appeared to be the most appropriate to better focus on meiofaunal response to locally and temporarily enhanced production of organic matter. Meiofaunal density and composition are assumed to be strongly in¯uenced by di€erent parameters, including physical variables and biological interactions, such as the quantity and quality of available food (Soyer 1985; Coull 1988; Soltwedel 1997; Moodley et al. 1998). In this regard, a plume front is an optimal case study for investigating the meiofaunal response to changing pelagic processes, as frontal systems a€ect all above-mentioned parameters (Frontier 1986; Netto et al. 1999). In general, total meiofauna, nematode and polychaete densities re¯ected the distribution of phaeopigments (Table 3). As frontal systems enhance primary production and pelagic±benthic coupling, it is possible to conclude that river plumes, supplying the sediments with phytodetritus and inorganic nutrients, might in¯uence the development and distribution of meiofaunal assemblages living in the sediments beneath a low-salinity plume. Similar conclusions were drawn by Netto et al. (1999), who observed increased nematode diversity and meiofaunal densities in sediments in¯uenced by a topographically controlled front causing debris and organism accumulation in the sediments.


As plume characteristics (in terms of plume dimension and front position) were di€erent in the two sampling areas (being more variable in the northern area), di€erent meiofaunal responses might be expected. Meiofaunal response to the organic matter accumulation associated with the river plume was evident in the northern area, as meiofaunal density in plume and frontal sediments were always signi®cantly higher than in open-sea sediments (Table 2). In particular, harpacticoid copepod density increased notably in sediments under the plume front, apparently responding to the increased bacterial density. In the southern area, the e€ects of enhanced production associated with the plume were generally more evident at frontal stations, and the di€erent meiofaunal components varied their responses during the two sampling periods. In winter, total meiofauna density displayed signi®cantly higher values at frontal stations: the taxa responsible for this response were nematodes and copepods (ANOVA, p < 0.05 for both taxa), whereas other minor taxa did not display any signi®cant response. Conversely, in summer, total meiofaunal density did not display any ``front e€ect'', but most minor taxa (including nauplii, kinorhynchs, turbellarians and juvenile decapods: ANOVA, all taxa p < 0.05) did. The presence of signi®cantly higher nauplius and juvenile decapod densities in frontal sediments suggests that these might be important sites for post-recruitment processes. Acknowledgements This work was carried out within the framework of the Programme PRISMA2: WP4 ``Biogeochemical cycles'' and partially supported by 60% funding, University of Bari. The authors are indebted to the crew of the R.V. ``Urania'', Dr. G. Catalano (responsible for the cruise) and to M. Armeni, A. Covazzi, A. Dell'Anno, A. Pusceddu, C. Totti and L. Vezzulli for kind assistance in di€erent phases of this research.

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