Journal of Plankton Research Vol.23 no.9 pp.903-922, 2001
© Oxford University Press 2001
Biomass changes and bentho-pelagic transfers throughout the Benthic Boundary Layer in the English Channel
1 Laboratory Of Biological Oceanography, Department Of Bioengineering, Faculty Of Engineering, Soka University, 1–236 Tangi-Cho, Hachioji, Tokyo 192, Japan; 2 Station Marine, 28 Avenue Foch, Bp 80, Upres A 8013 Elico, 62930 Wimereux, France
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Abstract |
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 TOP Abstract INTRODUCTIONMETHODRESULTSDISCUSSIONREFERENCES |
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One hundred and eight suprabenthic hauls were taken from six sites in the English Channel with a modified Macer-GIROQ sledge that permitted the sampling of three kinds of organisms in the benthic boundary layer: mesozooplankton, macrozooplankton and suprabenthos. A complete annual cycle was sampled in the Bay of Saint-Brieuc but only spring and autumn samplings were taken from the other sites. Meso- and macrozooplankton biomasses were usually higher at every site during the daytime than at night; in contrast, suprabenthic biomasses were lower during the day than at night. However, at site 5, on pebble substrates, every faunistic group showed a higher biomass during the day than at night, while at site 2 the opposite occurred. Meso- and macrozooplankton biomasses were at their maximum during spring whereas the highest biomass of suprabenthos was observed from summer to autumn. Daytime exchanges were by mesozooplanktonic organisms and night-time exchanges were by suprabenthic species, especially amphipods, mysids and large decapods. Daily transfers showed the same pattern for every faunistic group, and transfers were higher in autumn than in spring, except at site 1 where it was similar during both seasons. Three groups of sites were identified from their annual transfers: sites 1 and 3, on the Brittany coasts, with lowest annual transfers; site 2, offshorePlymouth, with the highest transfer, and the three eastern sites (4, 5 and 6) showing similar annual transfers. The rate between macrobenthic production and annual transfers was high at coarse sand and pebble sites where the benthic macrofauna was endobenthic and sessile, suggesting a concentration of carbon in the bottom. On the other hand, this rate was low on medium sand substrates where the benthic macrofauna was vagile, suggesting that carbon remained concentrated in the benthic boundary layer where exchanges were most important by the migration of both pelagic and benthic organisms in this compartment.
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INTRODUCTION |
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TOPAbstractINTRODUCTIONMETHODRESULTSDISCUSSIONREFERENCES |
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There is growing interest in the rates and pathways of material cycling in the open ocean, and in particular in those processes which export material down from the upper mixed layer (Asknes and Wassman, 1993
; Lampitt et al., 1993; Lenz et al., 1993; Lane et al., 1994; Dam et al., 1995; Khripounoff et al., 1998; Bissett et al., 1999). Angel (Angel, 1989) highlighted that four types of biological processes could modify the downward flux of organic carbon in the ocean: (i) in situ production based on dissolved organic carbon, (ii) utilization by grazing including microbial degradation, (iii) active and passive processes modifying the particle-size spectrum and (iv) active vertical transfer of nutrients or organic matter. The bulk of papers describe the influence of mesozooplankton grazing and metabolism on downward vertical carbon fluxes (Noji, 1991; Dam et al., 1993; Lenz et al., 1993; Morales et al., 1993; Dam et al., 1995; Boyd and Newton, 1999), or the importance of marine snow (Shanks and Edmondson, 1990; Alldredge, 1998) or zooplanktonic faecal pellets (Lane et al., 1994) on carbon fluxes and recycling. Vinogradov (Vinogradov, 1962) and, later, Walsh et al. (Walsh et al., 1988) showed that the vertical flux of particles and its effects may be particularly important on a daily basis for some surface pelagic-feeding herbivores. Lane et al. (Lane et al., 1994) pointed out that the vertical carbon flux over the continental shelf may have complex spatial and temporal variability introduced by advection of distinctly different planktonic communities. They added that this advective transport by the grazing community can have a large impact on estimates of vertical carbon fluxes in continental shelf ecosystems because of coupling, in space and time, between pelagic processes, vertical fluxes and benthic deposition. However, our knowledge of carbon transfer from the sea floor to the surface is rather limited. It has been considered that the role of zooplankton in the active vertical transfer of nutrients or organic matter is negligible (Dugdale and Goering, 1967; Morel, 1989). Nevertheless, Ribera Maycas (Ribera Maycas, 1997) showed that organic matter transfer is explained by organism migration (biomass transfer) and by the matter transported by these organisms during their migration (biotransfer). Vallet et al. (Vallet et al., 1995) and Zouhiri and Dauvin (Zouhiri and Dauvin, 1996) used the data on daily biomass changes to estimate live organic matter transfers between the Benthic Boundary Layer (BBL), the benthos and the pelagic zone at a coarse sand site offshore Roscoff in the western part of the English Channel. These first results showed that the main vertical biomass transfers between these three compartments occurred at both sunset and sunrise, when migrations of different species were the most important. In this paper, live organic matter transfers are evaluated from daily biomass changes found at six different sites in the English Channel, and the annual carbon biomass flowing through the BBL is calculated in order to compare these values with secondary production of the pelagic and benthic zones of the English Channel.
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METHOD |
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TOPAbstractINTRODUCTIONMETHODRESULTSDISCUSSIONREFERENCES |
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Study site
The English Channel represents a transition zone between the Atlantic Ocean to the west and the North Sea to the east. Because of the particular topography from west to east, this sea is affected by strong periodic tidal currents. It has been demonstrated that the English Channel, because of its strong tidal dynamic and its shallow depths (less than 200 m), is considered as a shallow well-mixed water (Pingree, 1980). Figure 1(A) aims to show the dynamics of the advection–dispersion process (Salomon et al., 1991). Iso-activity lines are rather straight in the eastern Channel and Dover Strait, but billows are visible in the Bay of Seine resulting from the combination of changes in the discharge function at La Hague and the presence of a tidally induced gyre in its western part (Breton and Salomon, 1995). The southern coast of England remains unaffected following a period of winds blowing from the west or the south, because of penetration of Atlantic waters toward the east (Salomon and Breton, 1993). Consequently, in areas of weak current, such as bays and estuaries, muddy fine sand predominates, whereas in the open sea, where currents are stronger, medium and coarse sand, pebbles and gravels predominate.
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Six sample sites were chosen as representative of the common hydrological and sedimentological conditions of this epicontinental sea (Figure 1B
, Table I): Three sites were in the western Channel. (i) Site 1, offshore Roscoff, was located within a thermohomogeneous area with low annual variations in salinity and consisted of bioclastic coarse sand. (ii) Site 2 was offshore Plymouth, where a seasonal thermocline was present from June to September separating surface and bottom waters at 25 m depth, and the sediment was of medium sand. (iii) Site 3 was in the Bay of Saint-Brieuc with low annual variations in salinity, absence of a seasonal thermocline, a moderate annual temperature variation and a coarse sand and pebbles substrate;
The other three sites were in the eastern Channel in shallow-mixed water throughout the year: site 4 in the Bay of Seine and both sites 5 and 6 in the Dover Strait. All sites were affected by the freshwater input of the Seine river, thus salinity remained lower than 35 throughout the year. Seasonal variations of temperature were well represented, with the lowest temperature in winter (5°C) and the highest at the end of the summer (20°C).
Sampling strategy
All hauls were collected with a new version of the Macer-GIROQ sledge (Dauvin and Lorgeré, 1989; Dauvin et al., 1995). This sledge consists of two parts. The head includes four 0.18 m2 boxes (width x height = 0.6 x 0.3 m) which are used to sample the water column in four layers: 0.10–0.40 m (net 1), 0.45–0.75 m (net 2), 0.80–1.10 m (net 3) and 1.15–1.45 m (net 4) above the bottom (BBL). Each box is linked to a WP2 zooplankton net (mesh size = 0.5 mm) and includes a Tsurimi-Seiki-Kosakusho (TSK) flow meter in the centre to measure the volume of the water filtered. The rear section supports the four nets and their collectors; a more detailed description can be found in Dauvin and Lorgeré (Dauvin and Lorgeré, 1989). The mean towing speed of each haul was 1.5 knots against the tide during the 15 min long haul.
The six sites were sampled in spring and autumn, and at site 3 in addition over an annual cycle (Table I). At each date, at least two day hauls and two night hauls were sampled, the others occurred at sunset, sunrise. When the number of hauls was >6 (site 3) there was a similar number of day and night hauls except in February 1994 with more night hauls than day hauls. A total of 108 hauls and 432 net samples were taken and analysed. Organisms were fixed with 10% neutralized formaldehyde, rinsed and transferred to 70% ethanol about 1 week later. Organisms were then left in ethanol no longer than 3 months.This procedure with fixation with formaldehyde and conservation in ethanol reduce the loss weight of the preserved organisms (Vallet, 1997). Organisms were sorted under a dissection microscope, counted, identified to species level where possible, and classified into three groups: (i) mesozooplankton calanoid copepods and crustacean larvae (with a 0.5-mm-mesh-size net, only adult copepods and the biggest crustacean larvae were collected); (ii) macrozooplankton including euphausiids, chaetognaths, fish larvae, polychaetes and cephalopods; and (iii) suprabenthic organisms [as defined by (Brunel et al., 1978)], decapods, peracarids, pycnogonids and leptostraceans. The abundance of copepods, crustacean larvea, chaetognaths, euphausiids, some mysids and some amphipods, which were very numerous, was estimated using the following procedure: (i) measure of the biovolume of each sample after extracting individuals of the other species; (ii) count of abundant organisms in three replicates of 1 ml; and (iii) multiplication of the mean number per millimetre by the biovolume of the sample to have an estimation of the abundance of the dominant animals. The numbers of individuals were standardized to 100 m3. The individual preserved dry weight of each taxon at each site was measured after oven-drying at 80°C for 48 h and the ash-weight after further heating at 550°C for 2 h. Both weights were measured at a precision of 0.01 mg. The ash-free dry weight (AFDW) was obtained from the difference between these two values. The mean biomass of each species or taxon at each site in the four nets was estimated and expressed in mg of AFDW per 100 m3 (Vallet and Dauvin, 1998).
Statistical analysis
A Wilcoxon–Mann–Whitney U-test (Scherrer, 1984) was used to determine if there were significant differences between day and night mean biomass of each faunistic group at each site for each season or each month, and between mean spring and autumn biomasses at each site.
At site 3, a Kruskall–Wallis non-parametric test was used to test a possible change in the existence of average biomass of each faunistic group over the year. When the difference was significant, a non-parametric multiple comparison test [Student–Newman–Keuls: S.N.K. modified (Scherrer, 1984)] was used to determine which clusters of samples were significantly different from others. Each faunistic group—mesozooplankton (M), macrozooplankton (m) and suprabenthos (S)—was classified at each date both day and night according to their average biomass values, with 1 for higher values, 2 for medium values and 3 for lower values.
Estimation of live organic matter transfer
To estimate the transfer between the pelagic zone and the benthos, a method derived from Allen's method (Allen, 1950; Allen, 1971) used to estimate secondary production was used, calculated as follows:
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Ni is the positive or negative change in abundance of species i between two consecutive hauls.
We considered positive transfers to be when the biomasses in the BBL increased, and negative transfers when the biomasses in the BBL decreased. Positive values corresponded to transfers from benthos and/or pelagic zone to the BBL, and negative values to transfers in the opposite direction (Figure 2). The biomass values of the last haul during a series were deducted from the biomass values of the first haul of the same series in order to lock up exchanges in one series. Then the sum of positive and negative exchanges became equal to zero. In our approach, we considered that the unknown advective losses and gains in a site due to the tide were balanced (losses = gains), and that the losses of organisms through the predation were weak in comparison with the vertical transfer.
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From estimated positive vertical daily fluxes of living matter (mg AFDW 100 m–3) at each site, annual transfers between BBL, benthos and pelagic zone, were estimated in g C m–2. The conversion from AFDW to dry weight (DW) was obtained by a conversion factor, DW = 1.25 x AFDW [proposed by (Le Borgne, 1975
)], and from DW to carbon weight according to conversions given by Raymont et al. (Raymont et al., 1966): carbon weight = 0.35 x DW. Estimations were made for each faunistic group at each site as follows: (i) sum of positive transfers for n hauls in mg AFDW 100 m–3; (ii) division of this sum by the number of hauls to obtain living matter transfer in mg AFDW 100 m–3 h–1; (iii) multiplication by 24 to estimate the transfer per day (mg AFDW 100 m–3 d–1); (iv) division by 100 to obtain mg AFDW m–3 d–1; (v) division by 0.3 to obtain mg AFDW m–2 d–1, [following the formulae given by (Dauvin et al., 1994) and summarized below]:
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): g DW m–2 yr–1; and (viii) multiplication by 0.35 (Raymont et al., 1966): g C m–2 yr–1;
The different stages resulted in the following equation:
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For all sites, an average transfer was estimated from observations of spring and autumn, and during an annual cycle at site 3, where an average transfer was estimated from six transfer values obtained at six different dates along the year (Table I).
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RESULTS |
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TOPAbstractINTRODUCTIONMETHODRESULTSDISCUSSIONREFERENCES |
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Daily changes
Annual cycle (Site 3)
Significant diel differences occurred for macrozooplankton and suprabenthos at each date, while for mesozooplankton significant differences appeared only in June and November with day biomass < night biomass and vice versa (P <0.05) (Table II). Macrozooplanktonic and suprabenthic biomasses were higher at night than at day, except in February (Table II).
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The results of Kruskall–Wallis non-parametric test showed a significant difference between mesozooplankton, macrozooplankton and suprabenthos biomasses between the day and night (Table III
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February was characterized by higher macrozooplanktonic biomass in the day and higher suprabenthic biomass at night. The chaetognath Sagitta elegans Verrill dominated the macrozooplankton (D > N), and the decapod Pandalina brevirostris Rathke dominated the suprabenthos (N > D) which formed at night about 70% of the suprabenthic biomass and 60% of the total biomass (Table IV
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April and June were characterized by higher mesozooplanktonic biomass at both day and night (Tables II, III
). The copepod Parapontella brevicornis Lubbock was dominant (Table IV) and represented about 35% of the mesozooplanktonic biomass in June and 21% of the total biomass.
Later, August and October were characterized by higher mesozooplankton biomass during the daytime and higher suprabenthic biomass at night (Tables II, III). Crustacean larvae dominated mesozooplanktonic biomass, while the mysid Anchialina agilis (Sars) and the amphipod Apherusa bispinosa (Bate) dominated the suprabenthic biomass (Table IV).
In November, the BBL fauna was characterized by higher meso- and macrozooplanktonic biomass during the day and higher suprabenthic biomass at night (Tables II, III). The chaetognath Sagitta setosa Müller became the dominant species of the macrozooplankton, and the mysids A. agilis, Siriella clausii Sars, like the amphipod A. bispinosa remained dominant (Table IV).
Usually mesozooplanktonic organisms left the BBL some hours before and/or after sunset and returned some hours before and/or after sunrise, while the other two groups remained in the BBL at night (Figure 3). The biomass transfer was low in February, moderate in April, October and November and reached a maximum in June and August. The macrozooplankton and the suprabenthos dominated in February, whilst the mesozooplankton dominated during spring in April and June; during the summer and the autumn, the suprabenthos was the dominant group in biomass and transfer (Table IV, Figure 3).
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Spring and autumn hauls
Site 1 was characterized by higher meso- and macrozooplankton biomasses at daytime over each season (Table V
). At night, in spring, it was characterized by higher mesozooplanktonic biomass while no significant difference between the three groups appeared in autumn (Table V). The copepod P. brevicornis dominated the mesozooplankton (D = N) at spring and the euphausiid Nyctiphanes couchii (Bell), the macrozooplankton (D > N) at both seasons (Table VI).
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Site 2 was characterized by higher mesozooplanktonic biomass in the daytime than at night in spring, while in autumn higher suprabenthic biomass characterized this site over both day and night (Table V
). The copepod Calanus helgolandicus Claus dominated (N > D) in spring and formed about 60% of the mesozooplanktonic biomass and 34% of the total biomass (Table VI). Even if it showed the highest daytime biomass in autumn, suprabenthic species became dominant, especially Schistomysis ornata (Sars), with higher biomasses at night (Table VI).
Site 3 was characterized by higher mesozooplanktonic biomass over both day and night in spring and autumn by higher mesozooplanktonic biomass at daytime and higher suprabenthic biomass at night (Table V). In spring, the copepod P. brevicornis dominated mesozooplankton biomass while in autumn, crustacean larvae dominated mesozooplanktonic biomass and the mysid A. agilis and the amphipod A. bispinosa dominated the suprabenthic biomass (Table VI).
Site 4 was characterized by higher mesozooplanktonic biomass at day and by higher suprabenthic biomass at night, for each season (Table V). In spring, the copepod Temora longicornis Müller was dominant (D > N) and formed at day about 84% of the mesozooplanktonic biomass and 74% of the total biomass (Table VI). In autumn, the amphipod A. bispinosa became dominant (N > D) and represented about 42% of the total biomass (Table VI).
Site 5 was characterized by higher suprabenthic biomass at both day and night, and for each season (Table V). The large decapod Pandalus montagui Leach dominated and formed about 40% (spring) and 38% (autumn) of the total biomass (Table VI).
Site 6 was characterized by higher meso- (spring) and macrozooplanktonic (autumn) biomass at daytime, and higher suprabenthic biomass at night (spring and autumn) (Table V). At each season, the mysid Gastrosaccus spinifer (Goës) was dominant and represented about 31% (spring) and 33% (autumn) of the total biomass (Table VI).
At each site and at each season, the most important biomass exchange occurred at sunset and sunrise(Figure 4). As in site 3 during the annual cycle, usually mesozooplanktonic organisms left the BBL at sunset to come back at sunrise, while the two other groups remained in the BBL at night (Figure 4). In spring, meso- and macrozooplankton dominated while in autumn, suprabenthos was the dominant group at each site, except at site 1 where macrozooplankton remained the dominant group (Figure 4).
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Seasonal changes
Annual cycle (Site 3)
The organic carbon transfer was provided essentially by mesozooplankton and suprabenthos in the BBL (Table VII). During spring, carbon exchanges came from the water column by mesozooplankton dominance. From August to February, carbon transfer was essentially by suprabenthic individuals (A. bispinosa). The carbon transfer increased from a minimum in February to a maximum in August, decreased by half in October and remained at that latter value in November (Table VII). In June, the high carbon transfer observed was by the mesozooplanktonic organisms (P. brevicornis), and in August was by A. bispinosa and A. agilis. However, in August mesozooplankton already provided about 30% of the carbon transfer while in October and November it represented less than 10% (Table VII). At the same time, the proportion of carbon transfer produced by suprabenthic organisms increased by an increase of their biomass, especially those of A. bispinosa.
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On average, the mean carbon transfer was essentially due to the suprabenthos (about 50%), then the mesozooplankton (36%) and macrozooplankton (14%). It rose about 24 mg m–2 d–1, corresponding to 9 g m–2 yr–1 (Table VII
).
Spring and autumn hauls
Carbon transfer showed no significant differences between both seasons at site 1 (Table VIII). At site 2, carbon transfer was about ten times lower in spring than in autumn (Table VIII). This high difference was essentially due to a high increase of suprabenthic carbon transfer between spring and autumn because of high recruitments of the mysid S. ornata. In spring the abundance of this species was around seven individuals per 100 m3 and its population consisted essentially of males and females. On the other hand, in autumn its abundance reached 2087 individuals per 100 m3 and its population was represented by 83% juveniles and 17% males and females, the sex ratio male to female being equal to 1. At site 3, as at site 1, no significant differences occurred between spring and autumn carbon transfers (Table VIII). In spring, meso- and macrozooplankton showed the highest proportions of carbon transfer. In autumn, the carbon transfer of suprabenthos was higher (Table VIII). The estimation of annual transfer with only two data (22.6 mg C m–2 d–1; Table VIII) was similar to that estimated from the annual cycle (24.3 mg C m–2 d–1; Table VII), the difference reaching only 8%.
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At site 4, no differences appeared between spring and autumn carbon exchanges. However in spring mesozooplankton was dominant while in autumn, the main carbon transfer was due to the suprabenthos (Table VIII
). At site 5, carbon exchange was higher during the spring than the autumn, and at every season, the contribution of the suprabenthos for this transfer was the highest (Table VIII). At site 6, contrary to site 5, autumnal carbon transfer was higher than that of spring. However, as at site 5, the suprabenthos showed the highest contribution for all seasons (Table VIII). Spatial changes
Table IX gives average biomasses and annual transfers for each faunistic group at each site and the total annual biomass and transfer at each site. It shows that suprabenthos was the most important compartment in the total annual carbon transfer, except at site 1 when macrozooplankton was dominant (51%). At site 4, mesozooplankton formed about 40% of the total annual carbon exchange.
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Moreover, from total annual biomasses and carbon transfers, three groups of sites can be distinguished: the Brittany coast sites 1 and 3 showing the lowest total annual biomasses and an annual transfer lower than 10 g C m–2 yr–1; site 2 offshore Plymouth having the highest total annual biomass and carbon transfer compare to the other five sites, and the three eastern sites with similar annual carbon transfers (from 26 to 32 g C m–2) and total annual biomasses (from 443 to 586 mg AFDW 100 m–3) (Table IX
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DISCUSSION |
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TOPAbstractINTRODUCTIONMETHODRESULTSDISCUSSIONREFERENCES |
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Annual changes
At site 3, zooplanktonic biomass (WP2 net with 0.5-mm-mesh size) was maximum in June (Vallet, 1997) while those of BBL fauna reached their peak in August (Table X), when crustacean larvae and the amphipod A. bispinosa showed their highest biomasses (Table IV). During spring (April and June), zooplanktonic biomass was about twice as high as the biomass sampled in the BBL, when the primary production reached its maximum (Table X). From summer (August) to winter (February), both zooplanktonic and BBL fauna biomasses were similar. Zouhiri and Dauvin (Zouhiri and Dauvin, 1996) showed that at site 1 BBL fauna biomass was higher in July (698 mg AFDW 100 m–3) than in November (154 mg AFDW 100 m–3), so the July value at site 1 was twice the August value at site 3. Nevertheless, autumnal values were similar. Wildish et al. (Wildish et al., 1992) estimated summer biomasses of the BBL fauna in the Browns Bank area (66°00'W–42°75'N) in the Northwest Atlantic of 2355 mg AFDW 100 m–3, which was greater than those observed at sites 1 and 3 in the English Channel. The Bay of Saint-Brieuc appeared to be a poor environment with low biomasses, related to low trophic inputs (Anonymous, 1996). Moreover, Table X shows that the maximum biomass of the BBL fauna occurred after the chlorophyll maximum, when primary production was high. In Arctic regions, Noji (Noji, 1991) underlined that there is a ‘delay’ between the maximum stock of the macrozooplankton and the phytoplankton bloom; macrozooplankton reached its maximum after the phytoplankton bloom. In the North Sea, a successional phase between minimum macrozooplankton biomass in March, prior to the spring bloom, and maximum biomass soon after the spring bloom were also observed (Dam et al., 1993).
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Seasonal changes
In the eastern part of the English Channel the continental inputs of nutrients from the Seine river stimulated the phytoplanktonic biomass during the spring as well as autumn, especially at site 4 (Table XI). The high spring primary production at site 4 coincided with the highest BBL mesozooplanktonic biomass occurring in the English Channel (Tables V, XI). This high primary production was essentially by important inputs of nutrients by the Seine River (site 4). However, the mean annual biomass of BBL fauna remained very low compared to that of the zooplankton, which was 25 times higher than that of BBL fauna (Table XI). Moreover, BBL biomasses at sites 4 and 5 decreased at the same time as the biomass of chlorophyll a (Table XI). It can be suggested that primary production had a direct effect on the BBL biomass over the seasons. It has been highlighted (Colebrook, 1979; Morales et al., 1993; Dam et al., 1993) that a large proportion of phytoplankton biomass is not utilized by herbivores and senescing phytoplankton stocks may culminate in sedimentation pulses involving an increase of the macrozooplankton biomass in the deeper layer. On the other hand, at site 6, the decrease of the biomass of chlorophyll a went with an increase of BBL biomass. We can emphasize that in the seaward areas, sedimentation pulses of senescing phytoplankton stocks occurred later than in the landward areas, thus a gap was observed between phytoplankton bloom and increased BBL biomass.
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In the western part of the English Channel, during the spring, site 2 showed a seasonal thermocline favourable to high phyto- and zooplanktonic biomasses (Table XI
). However, during spring, BBL biomass at this site was lower than those observed at sites 4 and 5; nevertheless, their zooplanktonic biomasses sampled were respectively six and 16 times lower than those of site 2 (Table XI). In fact, at site 2, large copepods like C. helgolandicus were present in high abundances, giving rise to high zooplanktonic biomass, while at both sites 4 and 5, the composition of the zooplankton was dominated by the small copepod, T. longicornis. Site 1 showed the lowest phytoplanktonic biomass while its zooplanktonic biomass was higher than those of other sites, except for site 2. Moreover, in spring, its BBL biomass (340 mg AFDW 100 m–3) was similar to those of both western sites 2 (376 mg AFDW 100 m–3) and 3 (462 mg AFDW 100 m–3), and in autumn (165 mg AFDW 100 m–3) at site 3 (141 mg AFDW 100 m–3) (Table XI). During spring, the suprabenthic biomass increased from the western part to the eastern part of the Channel: on average, it varied from 58 mg AFDW 100 m–3 in the western part to 201 mg AFDW 100 m–3 in the east. This increase was by the high biomass of mysids (Gastrosaccus spp.) and also to large decapods (P. montagui and P. brevirostris) which occurred only in the eastern part. In autumn, the highest biomass occurred at site 2 (1800 mg AFDW 100 m–3) and the lowest at site 1 (13 mg AFDW 100 m–3). However, if we exclude site 2, the same gradient was also observed in autumn.
In the English Channel, the BBL fauna was affected by seasonal biomass changes similar to those of benthic (Dauvin, 1988) and zooplankton communities (Colebrook, 1986), according to the recruitment period and the growth of individuals. Moreover, it was highlighted that seasonal changes of the BBL fauna were also linked to the availability of the phytoplanktonic biomass and to the zooplankton in the water column as well as to sedimenting organic matter to the BBL fauna (Vallet and Dauvin, 1999).
Biomass exchanges and live organic matter transfers
Many species carry out ontogenic, seasonal or diel vertical migrations (Brunel et al., 1978; Macquart-Moulin, 1984; Angel, 1989; Vallet et al., 1995; Zouhiri and Dauvin, 1996; Vallet, 1997; Ribera Maycas, 1997). One of Longhurst and Harrison's conclusions (Longhurst and Harrison, 1988) was that these vertical migrators inevitably ‘pump’ material to deeper water layers by ingesting material near the surface and egesting at least a part of the material in deeper-water layers. Consequently, they can qualitatively influence sedimentation by discriminant feeding, selective utilization of ingested particles, and processes associated with the biological and physiological properties of faecal material (Noji, 1991). This transport of matter ingested by BBL organisms during their vertical migrations merits consideration, in particular transport linked to migrations of BBL fauna which can ensure contacts between the pelagic zone and the benthos. The great importance of pelago-benthic fluxes of interaction among the BBL, the benthos and the pelagic zone has been shown (Sainte-Marie and Brunel, 1985). This study in the English Channel pointed out that biomass exchanges between compartments occurred mostly around sunset and sunrise at every site and every season, and for every faunistic group (meso- or macrozooplankton or suprabenthos), except in February at site 3 where it was during the night that exchanges were higher (Figure 3a) as in other studies (Vallet et al., 1995; Zouhiri and Dauvin, 1996). Usually biomass exchanges were limited during the day and at night (Zouhiri and Dauvin, 1996). Lane et al. (Lane et al., 1994) also highlighted that the maximum descent of the zooplanktonic biomass on the shelf of the Middle Atlantic Bight occurred early in the morning just after the sunrise. However, Ribera Maycas (Ribera Maycas, 1997) underlined the daily balance of transfer of matter between the sea bottom, the water column and the surface, illustrating the relative importance of nocturnal transfers compared to daytime transfers. On the other hand, in the English Channel, it was observed that biomass exchanges could be different according to the taxa throughout the day: mesozooplankton presented higher biomass exchanges during the daytime than at night, while biomass exchanges of macrozooplankton and suprabenthos were higher at night-time than during the day. However these daytime and night-time exchanges often remained lower than those observed at both dawn and dusk. It is also possible that the tidal currents can have effects on the swimming activity and the horizontal transport of organisms during their pelagic phase (Dauvin and Zouhiri, 1996).
During the annual cycle, biomass exchanges varied from a minimum in February to a maximum from June (corresponding to a high biomass of mesozooplankton) to August (corresponding to a high suprabenthic biomass), decreased in October and remained stable until November (Figure 3) confirming previous results for site 3 (Vallet and Dauvin, 1999).
The estimation of the secondary production of the BBL fauna is rather limited, except for some suprabenthic species especially mysids (Sorbe, 1991; San Vincente and Sorbe, 1990; San Vincente and Sorbe, 1993; Elizalde, 1994); no data were available for a whole community. However, it is interesting to compare estimations of annual organic matter transfers obtained for the BBL with secondary production of macrobenthos, obtained in the English Channel, e.g. in the Bay of Morlaix (Dauvin, 1991), or estimated from the productivity of macrobenthos on medium sands (P/B = 1.5), coarse sands (P/B = 1.0) and pebbles (P/B = 0.9) (Dauvin, 1988; Dauvin, 1991; Davoult, 1989). Rates between macrobenthic secondary production (PB) and annual transfers (TA) strongly varied from one site to another (Table XII). They were high on coarse sand and pebble substrates of the English Channel (site 3 and 5), but appeared very low on medium sand bottoms (sites 2, 4 and 6). It is on medium sand substrates that macrobenthic biomasses are the lowest; but on those substrates, suprabenthic mysids presented their highest biomasses, and correlated with this the PB/TA ratio reached its minimum. For example, on site 6, the annual TA was more than 150 times higher than the annual benthic production. Moreover, on medium sand substrates (sites 2, 4, 6) where macrobenthic biomasses were represented by vagile species, the annual TA was higher than PB while on coarse sand and pebble substrates (sites 3 and 5) where macrobenthic biomass were represented by sessile organisms, PB increased (Table XII). It can be suggested that where the benthic macrofauna was vagile, carbon remained concentrated in the BBL where exchanges were the most important by the migration of both pelagic and benthic organisms in this compartment; on the other hand, where the benthic macrofauna was endobenthic or sessile, carbon was concentrated in the bottom where exchanges between pelagic zone and benthos remained lower and much less important than on medium sand substrates. Contrary to sites 3 and 5, site 1, on coarse sand substrates, showed similar carbon production between the three compartments.
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It would be interesting for future studies to focus on (i) estimating the consumption of the particular organic matter by demersal organisms as well as the rate of respiration and nitrogen excretion to estimate a total energetic budget of such a compartment, (ii) estimating the role of the tide in the horizontal transfer, and (iii) quantifying the part of demersal organisms in the food of upper trophic compartments, especially demersal ichthyofauna to estimate the BBL macrofauna/ichthyofauna coupling.
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Acknowledgments |
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This study was carried out as a part of the ‘Programme National d'Océanographie Côtière (PNOC), Chantier Manche (CNRS-INSU and IFREMER)‘. The authors thank the crews of the NO ‘Le Suroît’, ‘Le Noroît’, ‘Côte de Normandie’, ‘Côte d'Aquitaine’ and ‘Pluteus II’, J.C. Lorgeré and C. Conq for their technical support during the cruises, and K. Ghertsos for help with English.
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Received on February 12, 2000
; accepted on March 27, 2001
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