© 2005 International Council for the Exploration of the Sea
Small-scale spatio-temporal variability in ichthyoplankton and zooplankton distribution in relation to a tidal-mixing front in the Irish Sea
a Department of Zoology, University College Dublin Belfield, Dublin 4, Ireland
b Department of Fisheries and Marine Biology, University of Bergen PO Box 7800, 5020 Bergen, Norway
*Correspondence to B. S. Danilowicz: College of Science and Technology, Georgia Southern University, PO Box 8044, Statesboro, GA 30460, USA; tel: +9126815111; fax: +9126810836. e-mail: bdanilowicz{at}georgiasouthern.edu.
This study examines the spatio-temporal dynamics of fish larvae and their prey at a tidal-mixing front in the central Irish Sea. The distribution of ichthyoplankton and zooplankton was analysed in relation to environmental variables (depth, surface temperature, surface salinity, and water column stratification) using Redundancy Analysis (RDA). Significant interannual variability in the formation and position of the tidal-mixing front coincided with large differences in the species abundances of both ichthyoplankton and zooplankton. During the summer, ichthyoplankton and zooplankton communities were structured by a combination of depth and hydrography, and the variability in species composition was directly related to the average value of the stratification parameter. Several ichthyoplankton species were consistently associated with frontal waters, while fewer species were concentrated in mixed water masses throughout the sampling period. The distribution of individual zooplankton species was also examined, and water mass affinities were shown to vary with developmental stage.
Keywords: fish larvae, Irish Sea, tidal-mixing front, zooplankton
Received 18 May 2004; accepted 27 April 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The spatio-temporal heterogeneity of plankton communities is frequently linked to the presence of localized oceanographic features (Govoni et al., 1989; Churchill et al., 1999; Rodríguez et al., 1999), and several investigations have concentrated on the effects of water column stratification on plankton distribution. Frontal structures have been shown to influence the overall abundance (Munk et al., 1995, 1999; Lochmann et al., 1997; Hays et al., 2001; Lough and Manning, 2001) and species composition (Valdéz and Moral, 1998; Flint et al., 2002; Munk et al., 2002) of ichthyoplankton and zooplankton assemblages. The variability in plankton distribution observed at frontal structures reflects interactions between hydrodynamic processes, spawning patterns, behavioural adaptations, and differential survival (Sánchez-Velasco and Shirasago, 1999; Gray and Miskiewicz, 2000; Carleton et al., 2001; Somarakis et al., 2002; Espinosa-Fuentes and Flores-Coto, 2004).
In the Irish Sea, a region of low tidal energy to the west of the Isle of Man results in the formation of thermally stratified waters during the early part of summer, which are separated from mixed waters by tidal-mixing fronts, the Western Irish Sea (WIS) fronts (Simpson and Hunter, 1974; Figure 1). The WIS front extends northwards alongside the west coast of the Isle of Man to form the Manx West Coast (MWC) front, which is identical in formation and structure to the WIS front (Fernandes, 1993) though the mixed water component is very small and very close to the coast in the MWC front.
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Although the Irish Sea represents an important spawning area for several commercial finfish (Fox et al., 1997), very few studies have examined the effect of seasonal stratification on plankton communities in the area. In a broad-scale study carried out in the northwestern Irish Sea, Dickey-Collas et al. (1996) described distinct inshore and offshore ichthyoplankton assemblages which varied temporally as the water column became stratified. Temporal fluctuations in zooplankton concentration have also been reported in the Irish Sea and have been linked to interannual differences in the extent and duration of stratification (Scrope-Howe and Jones, 1985).
Our study was designed to investigate the small-scale spatio-temporal variability in ichthyoplankton and zooplankton distribution in relation to the Manx West Coast front. The importance of conducting intensive, fine-scale studies, in addition to the more prevalent large-scale investigations, to fully assess the spatio-temporal heterogeneity of planktonic communities, has been discussed in the literature (Gallego et al., 1996; Gray, 1996; Sanvicente-Añorve et al., 2000; Ashjian et al., 2001; Hunt et al., 2002; Espinosa-Fuentes and Flores-Coto, 2004). Small-scale surveys are not confounded by time to the same extent as larger-scale studies, and generally allow for a greater number of sampling dates within shorter time intervals, thus enabling the temporal persistence of patterns to be tested. In a fine-scale study off the west coast of the Isle of Man, Nash and Geffen (2004) observed considerable interannual variation in the abundance of Calanus finmarchicus and Calanus helgolandicus between 1995 and 2001. They recorded very high abundance of C. finmarchicus in 1996 compared with 1997, and this coincided with strong year classes of cod, haddock, whiting, and plaice in the Irish Sea in 1996. Nicholas (1995) studied the structure of the plankton community off the west coast of the Isle of Man at three sites with varying hydrographic properties in 1993–1994, but found no consistent correlation between ichthyoplankton and zooplankton distribution and water column stability. In the current study, the area sampled by Nicholas (1995) was extended to include a larger number of stations in each hydrographic region. The principal objectives of the research were (i) to describe the spatio-temporal variability in the distribution of plankton off the west coast of the Isle of Man, and (ii) to assess the influence of the tidal-mixing front on the structure of ichthyoplankton and zooplankton communities.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Field investigations
In 1996–1997, a series of plankton surveys were carried out off the west coast of the Isle of Man at least once monthly between March and July from the RV "Roagan". The study area was covered by a grid of 18 stations located approximately 5 km apart (Figure 2). Stratified water was not present on every sampling date, and stations situated further offshore were omitted on several dates owing to logistical constraints which resulted in offshore, stratified waters being sampled only on a few occasions.
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Plankton samples were collected using a Gulf VII/PRO-NET high-speed plankton sampler with a 40-cm nose cone (Nash et al., 1998). The sampler was deployed for 20 min at 4–5 knots in a double oblique tow to within 2 m of the bottom. Maximum depth and volume filtered were recorded by the PRO-NET system. A mesh size of 275 µm was used on all sampling dates except June and July 1996 when a larger mesh size of 425 µm was used to minimize clogging (Table 1). All tows were conducted during daylight. Upon recovery, the net was rinsed and samples were fixed in 4% buffered formaldehyde. Fish larvae were removed from the samples in the laboratory and were identified following the keys of Russell (1976). The remaining zooplankton was identified to the lowest taxonomic level possible, and numbers of copepods at each developmental stage were also recorded.
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Hydrographic data were collected using a conductivity–temperature–depth (SeaBird SeaCat SBE-19, CTD) profiler. A stratification parameter (
) was derived from vertical profiles of density, 
(z), according to the following equation, after Simpson et al. (1979): |
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is the depth average density. Density profiles were calculated from temperature and salinity measurements averaged for 1-m intervals down through the water column. The value of represents a measure of the amount of energy required to mix the water column and thus increases with increasing stratification (mixed waters < 10 J m–3, frontal waters = 10–20 J m–3, stratified waters 
20 J m–3). The CTD profiler was deployed at a grid of stations similar to that used for the biological sampling (Figure 2). Physical data were collected within two days of the plankton samples. The number of stations surveyed varied with sampling date according to time and weather constraints. However, in general the CTD measurements were taken at more than 20 stations within the study area, and more offshore stations were surveyed in 1996 compared with 1997 (Figure 3).
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Data analysis
Multivariate ordination techniques using the CANOCO 4.0 software package (ter Braak and Smilauer, 1998) were applied to analyse the spatio-temporal variation in the distribution of zooplankton and ichthyoplankton. The seasonal progression of ichthyoplankton species over the sampling period was analysed by Detrended Correspondence Analysis (DCA) using the geometric mean abundance of each species (over all stations sampled) for each date. Detrending by segments was necessary to remove the arch effect (ter Braak and Smilauer, 1998).
Redundancy Analysis (RDA) was used to analyse the distribution of zooplankton and ichthyoplankton in relation to environmental variables for selected sampling dates (Table 1). Each date was analysed separately owing to the presence of strong seasonal heterogeneity in the data set. Ichthyoplankton samples from March and early April of 1996 and 1997 were not analysed with RDA because of low larval concentrations and species diversity. Zooplankton samples were analysed from the same sampling dates as fish larvae apart from late April 1997, when zooplankton data were not available.
Prior to analysis, species abundances were log(x + 1) transformed and those present at only one site were removed from the data matrix. A total of four environmental variables was included in preliminary analyses, three continuous variables (depth, sea temperature, and salinity (at 5 m)) and one categorical factor (stratification). The forward selection option in CANOCO was then used to identify environmental variables which significantly affected the distribution of fish larvae and zooplankton – environmental variables were considered if their sequential addition significantly improved the fit of species along the major RDA axis (p 
0.100, Monte Carlo permutation test). Partial effects of significant environmental factors were determined using the variance partitioning procedure. Having identified the principal environmental factors responsible for structuring the ichthyoplankton and zooplankton communities, the RDA species–environment correlations were examined to identify those species characteristic of mixed and frontal water masses throughout the sampling period when stratification was significant.
Pearson's product moment correlation was used (with Bonferroni-adjusted probabilities for multiple comparisons) to test for significant correlations between zooplankton and ichthyoplankton concentrations and to compare zooplankton samples captured using different mesh types. The effect of mesh size on fish larval size and interannual variability in plankton concentrations was tested using analysis of variance (ANOVA). Data were tested for normality and homogeneity of variances prior to analysis and log(x + 1)-transformations were carried out as required.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Hydrography
There was considerable interannual variability in the pattern of stratification off the west coast of the Isle of Man (Figure 3). In 1996, stratification of the water column was already apparent when sampling began in early March. The position of the hydrographic front separating coastal, mixed waters, and the stratified water masses remained relatively stable throughout the year, generally moving closer to the coast as the summer progressed. In contrast, in 1997, frontal waters were not observed until late April. From April to May stratification was significantly less pronounced than observed in 1996. However, in June, a relatively similar pattern of stratification was found in both years. Stratification was mainly due to temperature differences down through the water column (Figure 4).
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Ichthyoplankton and zooplankton species encountered
In all, 16 607 fish larvae comprising 70 identifiable taxa (from 22 families) were recorded in the present study. The majority of larvae encountered represented common coastal fish species. Table 2 shows the numbers of fish larvae of each taxa found in 1996 and 1997 off the west coast of the Isle of Man. Sprattus sprattus (recorded as Clupeidae) and Limanda limanda larvae were very abundant within the study area and accounted for 15% and 13%, respectively, of the total number of fish recorded. Ichthyoplankton samples were also dominated by high concentrations of gobiids (26%), ammodytids (15%), gadoids (9%), and callionymids (7%).
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A total of 50 zooplankton taxa was recorded from the study area (Table 3). The dominant zooplankton group was the class Copepoda, which represented 23 of the observed taxa. The most abundant taxa encountered in the plankton samples (averaged over all stations and sampling dates) included several copepod species (Temora longicornis, Pseudocalanus elongatus, Paracalanus parvus, and Calanus spp.) as well as Oikopleura dioica and members of the Cladocera (Evadne nordmanni, Podon intermedius), euphausid larvae, and hydromedusae.
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Yolk sac larvae were rarely present in the samples, and the proportion of preflexion and postflexion larvae varied depending on the species and the time of year. Sprattus sprattus and Limanda limanda, the two most abundant fish larvae, ranged in size from 2 to 25 mm and 3 to 14 mm, respectively. There was no significant difference in the size distribution of these species collected using a 275-µm mesh (at three stations) compared with a 425-µm mesh (at 11 stations) on 05 June 1996 (Sprattus sprattus (log(x + 1)-transformed): ANOVA, F = 0.71, d.f. = 1/169, p = 0.399, r2 < 0.004; Limanda limanda: ANOVA, F = 3.44, d.f. = 1/76, p = 0.068, r2 = 0.043 (275 µm > 475 µm)). When the concentrations of each developmental stage for eight of the most abundant copepod species were compared for both mesh sizes, there was a strong positive correlation following log(x + 1) transformation (Pearson correlation = 0.813, p < 0.001, n = 56) indicating that both meshes collected similar proportions of naupliar and copepodite stages of copepods. Therefore, it was assumed that data collected using both mesh sizes could be compared directly, in particular, for assessing seasonal trends in ichthyoplankton and zooplankton abundance.
Temporal variability in ichthyoplankton and zooplankton
There was a strong seasonal trend in the distribution of ichthyoplankton shown in Figure 5 using DCA. A clear temporal gradient in species composition could be distinguished along the first ordination axis, which explained 51.8% of the variability in ichthyoplankton distributions. In March and April, plankton samples were dominated by Ammodytes marinus, Limanda limanda, Pholis gunnellus, and Cottidae. In May, high concentrations of Limanda limanda, Merlangius merlangius, Onos spp., and callionymids were recorded. By June to July, the callionymids continued to form a significant part of the ichthyoplankton, as did the Gobiidae. The summer-spawning ammodytids, Gymnammodytes semisquamatus and Hyperoplus lanceolatus, and the clupeoid, Sprattus sprattus, were also abundant at this time of the year. There was evidence of some interannual variability in the distribution of ichthyoplankton separated out by the second canonical axis, which explains a further 8.5% of the species composition. In particular, a relatively higher proportion of gadoids were recorded in the 1996 samples than in 1997. Seasonal and interannual differences in the dominant zooplankton taxa are shown in Table 4.
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The average concentration of ichthyoplankton over all the sampling dates was significantly higher in 1996 than in 1997 (Figure 6, ANOVA, F = 14.18, d.f. = 1/179, p < 0.001, r2 = 0.076) mainly due to significantly higher larval concentrations in 1996 in April (ANOVA, F = 31.69, d.f. = 1/22, p < 0.001, r2 = 0.590) and May (ANOVA, F = 130.10, d.f. = 1/32, p < 0.001, r2 = 0.803). Average zooplankton concentrations were also significantly higher in 1996 than in 1997 over all the sampling dates (Figure 6; ANOVA, F = 26.48, d.f. = 1/132, p < 0.001, r2 = 0.169) and, in particular, in May (ANOVA, F = 56.27, d.f. = 1/31, p < 0.001, r2 = 0.645) and June (ANOVA, F = 11.37, d.f. = 1/28, p = 0.002, r2 = 0.289). There was a strong correlation between total zooplankton and ichthyoplankton concentrations throughout the sampling period in 1996 (Pearson correlation = 0.480, p < 0.001, n = 71), with the main peak in abundance in May and July, and in 1997 (Pearson correlation = 0.395, p = 0.002, n = 60) where the peak was significantly lower in June 1997.
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Spatial variability in ichthyoplankton and zooplankton
There was generally very good agreement between environmental variables found to be significant in structuring ichthyoplankton and zooplankton communities based on the forward selection procedure in the RDA (Table 5). Depth was significantly correlated to the distribution of ichthyoplankton and zooplankton on all sampling dates analysed, while stratification was significant on all dates except April and May 1997 for ichthyoplankton and early June 1996 and May 1997 for zooplankton. The total percentage of variance in species composition explained by environmental variables was compared for ichthyoplankton and zooplankton taxa in relation to sampling date (Figure 7). Although the variance explained by the environmental variables is consistently higher for the fish larvae than for zooplankton, the relative amount of explained variance follows a similar pattern in relation to sampling date in both years. The most notable difference occurred in early June 1996, when stratification was significantly correlated with the distribution of ichthyoplankton but not with other zooplankton species. Figure 8 shows the total variability explained by the environmental variables in relation to the average value for the stratification parameter calculated for each date across all sampling stations. As stratification becomes more pronounced within the grid, the percentage of explained variance in species composition increases for both ichthyoplankton (% variance explained = 43.07(log(average stratification)) + 3.81, r2 = 0.804) and zooplankton ((% variance explained = 50.40(log(average stratification)) – 3.87, r2 = 0.895).
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The species–environment correlations between individual ichthyoplankton and zooplankton species (present on two or more sampling dates) and stratification were examined to identify those species that were associated with mixed and frontal waters throughout the sampling period when stratification was significant (five dates of 13 for ichthyoplankton and four dates of nine for zooplankton). Ichthyoplankton and zooplankton species were then assembled into three groups: "mixed", "frontal", and "variable" (Tables 6, 7). The "variable" group comprised those species that did not show consistent correlations with either the mixed or the frontal waters.
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There were relatively few species which were positively associated with the mixed waters throughout the sampling period. These included Symphodus melops, an inshore, shallow-water species with benthic eggs, and Platichthys flesus, a common pleuronectid in brackish waters and estuaries. The gobiid, Lebetus scorpioides and the pipefish, Syngnathidae, were also placed in the "mixed" group, but were present in low concentrations in the samples.
The majority of larvae characteristic of the frontal waters were pelagic spawners. Sprattus sprattus and Limanda limanda, which have a very widespread distribution within the Irish Sea, were included in the "frontal" group. Although several coastal and deeper water species of gadoid larvae were present in this group, Gadus morhua and Onos spp. were the only species that were abundant in the samples. Other fish taxa that dominated this group included triglids, callionymids, and Ammodytes marinus, an offshore ammodytid species with benthic eggs.
Several fish larval species varied in their affinity to specific water masses throughout the sampling period and were placed in the "variable" group (Table 6). Many members of this group were present in low numbers within the study area (e.g. Centrolabrus exoletus, Scophthalmus maximus, Microstomus kitt, Lebetus guilleti). Some flatfish species (Glyptocephalus cynoglossus, Phrynorhombus norvegicus) showed a consistent positive correlation with stratification on dates when larvae were most abundant. However, an opposite relationship was observed outside the peak spawning season when low numbers of larvae were present. The family Gobiidae, which represents a diverse group of both inshore and offshore species, was also present in this group.
Table 7 shows zooplankton species associated with "mixed" and "frontal" waters throughout the sampling period. Species which have inconsistent distributional tendencies are also listed. It is clear that there is a great deal of ontogenetic variation particularly for copepod species, with adults, nauplii, and various copepodite stages of a given species often varying in their distributional tendencies. Very few taxa were positively correlated with the mixed water masses apart from the copepods Centropages hamatus (c V) and Parapontella brevicornis (c V and adults). In contrast, several developmental stages of the most abundant copepod species (including Calanus finmarchicus, Acartia clausi, Oithona similis) were concentrated within the tidal-mixing front on all dates when stratification was significant. When the distributional tendencies of members of the "variable" group were examined, several of those species were positively associated with the mixed waters in the earlier part of summer, but as the tidal-mixing front became firmly established in late June and July, these species concentrated within the frontal waters.
There was no evidence of significant frontal accumulation of total ichthyoplankton or zooplankton at the MWC front. Although average concentrations of fish larvae were higher at the front than in mixed waters on all sampling dates, these differences were not statistically significant. Furthermore, total ichthyoplankton and zooplankton distributions were not significantly correlated with each other spatially on individual sampling dates apart from in early April 1997 (Pearson correlation = 0.524, p = 0.026, n = 18).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Our study dealt with two contrasting years in terms of the formation and location of the Manx West Coast front. The significantly higher ichthyoplankton and zooplankton concentrations recorded in 1996 than in 1997 may be related to the early onset of stratification and generally more stable frontal structure within the study grid in that year. Similar results were obtained from a large-scale study in the Irish Sea, where a decrease in overall concentrations of zooplankton followed the breakdown of stratification by severe gales (Scrope-Howe and Jones, 1985). In the present study, a higher proportion of gadoid larvae was recorded when the front was stable in 1996 than in 1997. Munk et al. (1999) observed a similar pattern at a shelf-break front in the northeastern North Sea where a weakening of the frontal structure resulted in a decline in gadoid larvae.
The increased zooplankton abundances in 1996 should provide more favourable feeding conditions for fish larvae in that year than in 1997, ultimately leading to increased larval survival. Several recent studies have demonstrated a relationship between interannual differences in prey availability and recruitment of fish larvae in the Irish Sea. Nash and Geffen (2004) recorded a maximum abundance of the copepod species Calanus finmarchicus in 1996 than in the period 1997–2001 which, coinciding with a high abundance of young gadoids (cod, haddock, and whiting) in the water column and a large settlement of plaice in that year. Hindcast interannual variability in Calanus egg production appeared to have a significant effect on cod recruitment in the Irish Sea (Brander et al., 2001). Therefore, interannual variability in ichthyoplankton concentrations recorded in the current study may be due to a combination of the favourable hydrographic conditions and increased food availability in 1996 compared with 1997. Such fluctuations in fish larval concentrations could have major implications for recruitment to adult populations if large-scale disruption of stratification occurred.
Apart from affecting overall abundance, the onset of water column stratification was also shown to influence the species composition of ichthyoplankton and zooplankton communities significantly. Plankton communities were structured predominantly by depth and hydrography. Many species were found associated with the tidal-mixing front, while a smaller group was positively correlated with the mixed waters throughout the sampling period. Several investigations conducted at the shelf-slope front in the western Mediterranean have observed similar results. Although the species composition of ichthyoplankton assemblages was primarily determined by depth (Sabatés and Masó, 1990), variations in the position of the front were correlated with large changes in the distribution and abundance of fish larvae (Sabatés and Masó, 1992; Sabatés and Olivar, 1996).
In the present study, the patterns of ichthyoplankton distribution in relation to the tidal-mixing front are most likely caused by a combination of physical and biological processes, the relative importance of which would be largely dependent on the locomotory capabilities of the fish larvae involved (Bradbury and Snelgrove, 2001; Epifanio and Garvine, 2001; Hare et al., 2001). The distribution of early stage, preflexion fish larvae is predominantly related to adult spawning patterns and hydrodynamic processes (Doyle and Ryan, 1989; Grioche and Koubbi, 1997). Fish larvae of shallow-water, inshore species were found concentrated in the mixed waters while those of offshore species were positively associated with the tidal-mixing front reflecting the spawning habits and distribution of the adults. Preflexion larvae may be retained in the frontal waters by the advective processes commonly associated with tidal-mixing fronts (Scrope-Howe and Jones, 1985; Munk et al., 1999; Lough and Manning, 2001). Swimming abilities of fish larvae improve markedly with age (Champalbert and Koutsikopoulos, 1995; Fisher et al., 2000), and later stage, postflexion larvae may play a more active role in their distribution on the small scale of the current study. Fish larvae control their horizontal dispersal through the selective use of various currents by altering their vertical position in the water column (Churchill et al., 1999; Jager, 1999; Armsworth, 2001; Forward and Tankersley, 2001). Off the west coast of the Isle of Man, postflexion fish larvae may utilize horizontal currents to maintain their position in more favourable conditions. However, a more detailed analysis of the hydrodynamics at the MWC front would be necessary to understand fully the influence of passive physical processes and active behavioural adaptations on the distribution of fish larvae. Information on the vertical distribution of ichthyoplankton down through the water column would also provide further insight into factors affecting their distribution. Differential mortality, attributable to predation or starvation, is also likely to play a role in the distribution of later life history stages of ichthyoplankton.
The distribution of zooplankton should also reflect interactions between spawning patterns, hydrodynamics, active vertical transport, and differential mortality (Scrope-Howe and Jones, 1985; Sullivan, 1993; Gallagher et al., 2004; Iguchi, 2004). The significance of these processes varies with developmental stage, as described for fish larvae. Although similar environmental factors were significant in structuring both ichthyoplankton and zooplankton communities, the variance in species distribution explained by environmental variables was consistently higher for fish larvae than for zooplankton. This would indicate that the distribution of zooplankton is less closely correlated with the physical environment. Detailed studies on the hydrodynamics of the region in relation to the distribution (horizontal and vertical) of fish larvae and zooplankton would be required to explain this pattern. However, it could reflect the ability of fish larvae to position themselves in particular water masses optimal for their growth and survival. Differential survival of larvae in water masses with different levels of food availability could also result in a closer linkage between ichthyoplankton and the physical environment.
Spawning strategies in adult fish are believed to be adapted to result in the co-occurrence of fish larvae and their prey thereby enhancing the survival and development of larvae in accordance with the "match–mismatch" hypothesis (Cushing, 1969). In the current study, high concentrations of fish larvae were positively correlated with high prey densities temporally, with peak abundances of both groups observed at the same time in both years.
Much of the research on temporal and spatial variability in plankton communities has been carried out on a large scale, and there has been a tendency to over-average the data (Cowen et al., 1993), thereby overlooking much of the small-scale variability in the distribution of plankton. Our study allowed us to identify changes in species–environment associations through time at a tidal-mixing front. The influence of environmental variables on species composition showed a similar pattern for both zooplankton and ichthyoplankton throughout the sampling period, and the definition of plankton communities was directly related to the strength of stratification during summer. The effect of the Manx West Coast front and the inherent dynamism within the plankton community would have been underestimated if this investigation had been conducted on a large spatial and temporal scale. Therefore, the development of stratification, and the strength of the stratification parameter, may need to be sampled and modelled at small spatial scales to understand observed patterns at the more typically surveyed large spatial scales.
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Acknowledgements |
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The authors are grateful for assistance in the field from the skipper and crew of the RV "Roagan", Graham Hughes, Cheryl Corkill, and numerous other shipboard assistants. The authors also acknowledge the zooplankton identifications undertaken by the Arctic Agency, Poland. This research was supported by an Enterprise Ireland Research Grant to B. S. Danilowicz, Enterprise Ireland/British Council Grants to B. S. Danilowicz and R. D. M. Nash, and an Enterprise Ireland post-graduate fellowship to O. Lee.
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Footnotes |
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1 Current address: Department of Zoology, Trinity College, Dublin 2, Ireland.
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References |
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