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Seabird Observations at a Tidal Mixing Front in the Irish Sea
Estuarine, Coastal and Shelf Science (1998) 47, 153–164 Article No. ec980339
Seabird Observations at a Tidal Mixing Front in the Irish Sea R. Durazoa, N. M. Harrisonb and A. E. Hillc a
UABC-Facultad de Ciencias Marinas, Apdo. Postal 453 Ensenada, B.C., Me´xico Anglia Polytechnic University, East Road, Cambridge CB1 1PT, U.K. c University of Wales, School of Ocean Sciences, Menai Bridge, Gwynedd LL59 5EY, U.K. b
Received 2 October 1997 and accepted in revised form 22 January 1998 Seabird counts were carried out along six transects across a tidal mixing front in the western Irish Sea using, for the first time, a combination of conventional visual bird observation techniques and very high-resolution mapping of frontal structure using an undulating CTD. Four seabird species were clearly aggregated at a precisely defined frontal boundary. Flocks of seabirds were found to change location over short time periods (hours), maintaining their position relative to the surface physical parameters and the subsurface thermocline. Results confirmed considerable anecdotal and limited documentary evidence that suggest fronts are important feeding areas for seabirds. 1998 Academic Press Keywords: birds; fronts; CTD; Irish Sea
Introduction Seabirds are an important and visible component of the marine fauna. Despite their ability to search over large areas of ocean in order to feed (Duffy, 1983), they tend to congregate over small conspicuous geographic locations where local concentrations of prey occur. A clear example is the aggregation of seabirds near fishing boats where they gather to make extensive use of discarded fish and offal (Watson, 1981; Hudson & Furness, 1989). Surface-feeding seabirds also aggregate where other marine predators such as whales and diving birds make prey readily available close to the surface (Schneider et al., 1990; Grebmeier & Harrison, 1992). Seabird aggregations have also been found to occur at fronts that form a boundary between mixed and stratified waters (Schneider et al., 1987; Harrison et al., 1990; Russel et al., unpubl.). As a result of the spatial variation of tidal stirring, fronts occur during the summer over regions of the European continental shelf (ECS), and their positions are predictable (Simpson & Hunter, 1974; Pingree & Griffiths, 1978). On the ECS, the transition zone between stratified and tidally-mixed waters is typically about 5–10 km and the horizontal extent is about 1500 km (Hill et al., 1993). Typical cross-front gradients of surface temperature are about 1 C km 1 (Simpson, 1981). Slicks, ripples, surface convergences, frontal instabilities and eddies shedding from the frontal boundary are common features of the 0272–7714/98/020153+12 $30.00/0
highly active transition zone (Hill et al., 1993; Farmer et al., 1995). Frontal systems may be areas of greater phytoplankton concentration due to accumulation (convergence) or greater productivity (exchange of nutrients) (James, 1978; Simpson, 1981; Yoder et al., 1994). Shelf-sea fronts have long been recognized as areas of increased primary productivity (Savidge, 1976; Pingree, 1978; Kinder et al., 1983; Richardson et al., 1985; Le Fe`vre, 1986; Davenport & Rees, 1993; Olson et al., 1994). High productivity might be of both local origin, by the growth of phytoplankton induced by continuous replenishment of nutrients, or by the mixing of two different water masses, one stratified and less tidally-energetic and the other homogeneous, highly energetic (mixed) water. Fronts have been found to be areas of enhanced biological productivity and also areas with a high biomass at higher trophic levels (Fogg et al., 1985; Le Fe`vre, 1986). The importance of shelf-sea fronts as feeding areas for marine predators has been stressed by numerous authors [see Le Fe`vre (1986) for a review]. The occurrence of large numbers of seabirds near fronts in the South Atlantic Bight has been documented by Haney and McGillivary (1985), and in the Bering Sea by Kinder et al. (1983), Schneider et al. (1987, 1990), and Harrison et al. (1990). On the ECS, observations have been made of seabirds associated with shelfsea fronts in the Irish Sea (Fogg et al., 1985) as well 1998 Academic Press
154 R. Durazo et al.
F 1. The study area in the western Irish Sea. Insert shows where seabirds were counted and the front was mapped along the six transects. The zig-zag line denotes the front. Broken lines denote depth contours in metres.
as in the North Sea (Webb et al., 1984; Tasker et al., 1985). Considerable interest in the distribution of biomass at tidal mixing fronts is reflected in a number of studies (Le Fe`vre, 1986), but rarely have physical and biological investigations been conducted simultaneously. Previous attempts to correlate oceanographic and biological observations have been based on conventional, coarsely spaced CTD profiling (Kinder et al., 1983; Schneider et al., 1990; Davenport & Rees, 1993). Most of these studies have used mean hydrographic measurements and have been too coarse to detect short-term variability. A shelf-sea front may exhibit variations in the position of the transition boundary depending on the mixing regime over the seasons, or with tidal strength over a month (Simpson & Bowers, 1981), or as a result of changes in tidal stirring at the semi-diurnal period, over a matter of hours (Allen et al., 1980). Thus, while associations of marine life with fronts are numerous (Le Fe`vre, 1986), fine-resolution studies are required if the mechanisms driving these patterns are to be understood. Attempts to describe in detail the biological properties of fronts have been severely hampered by
the limitations of instruments in measuring complex and dynamically active environments. This study reports counts of seabirds feeding in the Irish Sea at a tidal mixing front; cross-frontal hydrographic structure was mapped with a continuous, rapid sampling towed undulating CTD. Aggregations of seabirds have been noted at the Irish Sea front (Fogg et al., 1985), however, this represents the first detailed description of the seabird aggregations relative to the frontal circulation. Irish Sea front This study was conducted across a thermal transition zone between the deep western Irish Sea (WIS) and the shallow eastern Irish Sea (Figure 1). The transition zone known as the western Irish Sea front (Simpson & Hunter, 1974) is a permanent feature of the summer. The dynamics of the eastern area are dominated by tidally-driven water movements, mainly the semi-diurnal tides, which generate currents of up to 1·2 m s 1. Here, the strong tidal currents assure that the water column remains vertically mixed throughout the year. In the WIS, tidal currents are weak (0·2 ms 1) which permits the development of
Seabird observations in the Irish Sea 155
a thermocline from early April–May until the end of October. A front extends from the southern tip of the Isle of Man to Dublin in Eire (Figure 1). The front is visible in infra-red images (Simpson, 1981) and instabilities and eddies shedding from the front are manifest in the surface (Pingree, 1978). The seasonal stability of the Irish Sea front is explained by recent oceanographic studies of the WIS (Hill et al., 1994). Lagrangian current measurements have indicated mean currents of the order of 0·05 m s 1, which flow as a cyclonic gyre around the centre of the WIS completing a revolution in about 15 days. The outstanding importance of the gyre on the ecology of the western Irish Sea has been summarized by Hill (1993) and Hill et al. (1994), who suggested that the cyclonic circulation is an effective physical retention mechanism permitting fish larvae to circulate through the region ensuring their return and recruitment into the adult population. Recruitment through seasonal stability may bring a year-to-year secure supply of food to higher trophic levels such as that of seabirds. The area of the WIS front is important for a number of fish stocks and supports commercial fisheries. Studies have shown that high productivity is associated with the front. Davenport and Rees (1993) found maximum concentrations of neuston at the front; Richardson et al. (1985) concluded that over the entire season of thermal stratification, the mean chlorophyll a concentration in surface waters was highest near the front as compared to adjacent waters. Methods Observations were made from the RRS Challenger between 16 and 18 July 1990. The WIS front was repeatedly traversed at a speed of 3·1–3·6 m s 1 (6–7 knots) along six transects, spaced 5 km apart (Figure 1), which had been chosen so as to cross the front at approximately 90 to its mean orientation, as observed using CTD casts. Seabird counts were carried out during daylight hours at 10-min intervals (about 2 km spatial resolution) and the number and species of seabirds on the water or feeding in a 300 arc from bow to beam were recorded (Tasker et al., 1984). The counts were made 300 m forward, whereas the undulating CTD was towed behind the ship. While fishing boats frequent the Irish Sea front, during the observations reported here there were no fishing vessels which could have biased the distribution of the surface feeding species. The ship position was obtained at 30 s intervals using a Decca navigation system. The cross-frontal variability of temperature and salinity was measured
with an undulating CTD (Searover) (Bauer et al., 1985). The descent and ascent of the instrument was computer controlled from the ship’s deck, and the instrument sampled from the surface down to 10 m above the sea-bed. Typical spatial resolution was 400 m in the horizontal and 1 m (after low-pass filtering) in the vertical. Flat seas throughout the measurements allowed visual detection of 100–200 m wide slicks and ripple bands at the time of crossings. Surface temperature was recorded continuously from a seawater source 3 m below the surface using a thermo-salinograph. Vertical casts carried out simultaneously with Searover and a conventional CTD indicated that the mean offset of Searover temperature readings was 0·1 C. Partial correlation was used to test for significant correlation between bird numbers and surface temperature, surface salinity, temperature and salinity gradients and visible sea-surface features. Water column stability was identified using a stratification parameter, the potential energy anomaly (PEA) defined as (Simpson, 1981):
where z is the vertical co-ordinate, g is gravity, h is depth and ñ is water density with mean ñ. The PEA represents the work which would be done to bring about complete vertical mixing. Hence, PEA is zero for a fully-mixed water column and becomes positive for stable (stratified) conditions. For the WIS, the transition boundary is known to occur close to the point where PEA drops from 10 joules m 3 to virtually zero (Simpson et al., 1977). In the present study, a depth of h=45 m, the maximum descent of the undulating instrument on the shallow edge of each transect, was taken to calculate the stratification parameter. A Kruskall-Wallis non-parametric analysis of variance was used to test for patterns in seabird distribution relative to water-column stability. Results Each of the transects illustrated in Figure 1 was run at least four times. A total of 11 runs were in full daylight over most of the transect, making bird counts possible. All 11 transect runs crossed the shallow stratification at the edge of the front, with only three of the 11 transects crossing into fully-mixed water (mixed top to bottom). The selected transects included here are those traversed at least twice with bird counts covering most of the section (Figures 2–6). On each figure, vertical arrows mark the bounds of the front at PEA=10 and 5, respectively.
156 R. Durazo et al.
10 Kittiwake 5 0 30 Other surface feeders 15
Number of birds
0 15 Razorbill
10 5 0 50
Guillemot 25 0 160 Manx shearwater
No observations
80
15
40
TS (°C)
PEA (J m–3)
0
20 12
0
0
B2
A2 14
20
Depth (m)
13 40
12 11
60
80
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0
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10
30
Distance run (km)
F 2. Top to bottom: number of birds at the surface for transect A2–B2 (run 5, 17 July 1990, 06.19–08.15h), potential energy anomaly (PEA) (thick line) and surface temperature (TS) (thin line), and the cross-frontal temperature structure. Contour intervals are 0·25 C. The broken line denotes periods over which no bird observations were carried out. Note that the horizontal axis for PEA graph has been displaced so as to cross the vertical axis at PEA=5. In the same graph, the symbols ` indicate the transition zone where PEA values drop from 10 to 5 J m 3.
Seabird observations in the Irish Sea 157 10 Kittiwake 5 0 30 Other surface feeders 15
Number of birds
0 15 Razorbill
10 5 0 50
Guillemot 25 0 160 Manx shearwater 80
50
16 TS (°C)
PEA (J m–3)
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25 13
0 0
B2
A2 14
Depth (m)
20
13 11
40
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80
100
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Distance run (km)
F 3. As in Figure 2 for transect A2–B2 (run 15, 18 July 1990, 05.20–07.04h). The inverted triangle indicates an area of surface convergence, where large quantities of scum and debris were observed.
Four species were associated with the front: three subsurface-feeding species, Manx shearwaters (Puffinus puffinus), guillemots (Uria aalge), razorbills (Alca torda) and one surface-feeding species, the
kittiwake (Rissa tridactyla). A fifth group is depicted in the figures to represent the assembly of all other surface-feeding species; included in this group are fulmar (Fulmarus glacialis), European storm-petrel
158 R. Durazo et al. 10 Kittiwake 5 0 30 Other surface feeders 15
Number of birds
0 15 Razorbill
10 5 0 50
Guillemot 25 0 160 Manx shearwater 80 No observations
16
50
TS (°C)
PEA (J m–3)
0
25 13
0
0
B2
A2 15
14
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20
Depth (m)
13 40 12 60
80
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Distance run (km)
F 4. As in Figure 2 for transect A2–B2 (run 17, 18 July 1990, 09.46–11.52h).
(Hydrobates pelagicus), northern gannet (Morus bassanus), herring gull (Larus argentatus) and lesser black-backed gull (Larus fuscus).
The hydrographic profiles in Figures 2–6 show a broad frontal zone of 10 km or more over which horizontal gradients in temperature and water column
Seabird observations in the Irish Sea 159 10 Kittiwake 5 0 30 Other surface feeders 15
Number of birds
0 15 Razorbill
10 5 0 50
Guillemot 25 0 160 Manx shearwater 80
40
17 TS (°C)
PEA (J m–3)
0
20 14
0
0
B3
A3 15
14
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Depth (m)
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40 12 60
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Distance run (km)
F 5. As in Figure 2 for transect B3–A3 (run 8, 17 July 1990, 13.25–15.18h).
stability (PEA) were evident. The transects illustrated in Figures 2, 3–4 crossed into fully-mixed water at the shallow end (surface temperature <14 C and PEA <5). Over this broad zone, the shallow stratification
was pushed to the surface by the strong tidally-driven flow below. The surface expression of the front varied on the crossings illustrated, with some transects showing a relatively abrupt transition from shallow
160 R. Durazo et al. 10 Kittiwake 5 0 30 Other surface feeders 15
Number of birds
0 15 Razorbill
10 5 0 50
Guillemot 25 0 160 Manx shearwater 80
40
17 TS (°C)
PEA (J m–3)
0
20 0
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14 B3
A3 15
14 13
Depth (m)
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Distance run (km)
F 6. As in Figure 2 for transect B3–A3 (run 18, 18 July 1990, 12.18–14.12h).
stratification to mixed water, and a marked drop in surface temperature (Figure 3). On other transects (Figure 2), the transition was gradual. On all 11 transects there were high numbers of birds aggregated over areas where the thermocline
rose close to the surface. Very large peaks in seabird numbers were found at the surface expression of the front, which was characterized by a marked change in surface temperature (TS) on four out of 11 transects (e.g. Figures 3 and 4).
Seabird observations in the Irish Sea 161 T 1. Occurrence of bird peaks (>20 birds) over maximum ÄT at surface, to one side of ÄT toward the mixed side, or seawards over stratified water
Peak at ÄT Peak to mixed side Peak to stratified side
Manx shearwater
Guillemot
Razorbill
5 0 3
2 2 2
— 2 —
Manx shearwaters, guillemots, razorbills and kittiwakes all contributed to the numbers in the largest aggregations, but overall the seabird species differed in their distribution along the transects. The differences between the three subsurface-feeding species can be described by counting the aggregations of each species across three regimes: fully stratified water, the boundary defined as the maximum ÄTS along each transect, and over fully-mixed waters (Table 1). An aggregation was defined as 20 or more birds on a 10 min transect section. Peak numbers of shearwaters, often hundreds of birds, appeared to congregate along maximum temperature gradients at the surface. Smaller peaks occurred over the shallow stratification immediately adjacent to the area of maximum ÄTS. The largest aggregations of guillemots were also at the maximum ÄTS (Figures 3 and 4), but aggregations were found in stratified water and mixed water as well. Guillemots were the most widely dispersed species of those observed, and the only species to occur at all in the deeply stratified water at a distance from the front. Razorbills appeared in large aggregations only on the mixed side of the front. Temperature gradients at the surface appear to be a good predictor of seabird aggregations, indicating the extent to which the thermocline has broken down along the transect. These temperature gradients are not very large, but are a marker of flow gradients which shape the circulation at the frontal boundary. To test the relationship between the seabirds and stability, the 10-min bird observations were categorized into: PEA <5; PEA 5–10; PEA 10–25; and PEA >25. Razorbills were significantly more abundant in mixed water with a PEA of <5 (Kruskall-Wallis ANOVA, P <0·05). For guillemots and Manx shearwaters there were no significant differences (guillemot; ns, P=0·07, Manx shearwater; ns, P=0·53). The results reflect the fact that there is not a simple decrease in stability across the frontal boundary. The PEA values vary substantially over relatively short distances along the transects (Figures 4–6).
Spatial and temporal changes were evident in the pattern of bird aggregation. Changes took place on very short time-scales apparently in response to changes in thermocline depth and surface temperature gradients. Figures 2–4 illustrate the temporal evolution during three runs of transect A2–B2, and show a typical frontal structure, with the surface expression of the front at around 20 km and sharp changes in surface temperature (TS) in the vicinity of the front. While similarities in the hydrographic structure were evident, short-term changes were seen in the thermocline depth. The first two of these runs (Figures 2 and 3) were carried out approximately 24 h apart. The two were similar, with the thermocline reaching the surface at about 12 km. The third run (Figure 4) was traversed 5–6 h after the run in Figure 3, and shows the thermocline to have been displaced downwards. This phenomenon may reflect the influence of internal waves, as is suggested by the shape of the thermocline. Figures 5 and 6 show two runs along transect A3–B3, with differences in surface temperature and stability. Surface temperature was generally 0·5 C or more warmer on the run in Figure 5 than on that in Figure 6. On both runs a very shallow, strong thermocline can be seen, buckling toward the surface; fullymixed water was never reached. Figure 5 shows noisy fluctuations in the surface temperature and stability, and aggregations of birds distributed across the wide frontal boundary. Where there was a more regular drop in PEA and surface temperature (Figure 6), there was a different pattern, with many of the same bird species present, but only Manx shearwaters forming a very large aggregation (at 25 km). The same transect A3–B3 was crossed 10 h earlier (not shown) and revealed a surface–temperature distribution similar to that in Figure 6, suggesting a predictable shift between the conditions illustrated in Figures 5 and 6. The distributions of the most abundant birds were compared to patterns in surface temperature and salinity (Table 2). All species except for fulmar were significantly associated with temperature gradients. All species were found over more saline surface waters (e.g. away from the strongest stratification). However, changes in salinity were very small in the cross-frontal direction, and no patterns were found in seabird distribution relative to salinity gradients. There was no significant correlation with time of day. Slicks are a good indication of property changes at the surface, such as where internal waves come to the surface, or the surface expression of fronts. For all crossings, the null hypothesis of a random distribution relative to these small-scale structures was tested. The results presented in Table 2 indicate that birds were more often associated with streaky waters than
162 R. Durazo et al. T 2. Results of partial correlation for five seabirds
Manx shearwater Guillemot Razorbill Kittiwake Fulmar
Surface temperature
ÄT
Surface salinity
ÄS
Streaky water
Time of day
<0·01 ns <0·01 0·059 ns
<0·01 <0·01 <0·01 0·03 ns
<0·01 <0·01 <0·01 <0·01 <0·01
ns ns ns ns ns
0·05 0·02 ns <0·01 ns
ns ns ns ns 0·022
ns, non-significant. ÄT and ÄS are cross-frontal temperature and salinity gradients, respectively.
expected by chance, which suggests that birds were using these clues at the surface to position and feed. Discussion The results show persistent seabird aggregations at a well-defined frontal boundary, and prompt several questions on how seabirds locate fronts, and how they benefit from frontal circulations. The data suggest that seabird flocks form and disperse over very short time-scales (hours), and that different species either prefer to use different parts of the circulation or have different capacities to locate the best feeding areas. In the case of the surface-feeding species, their distributions may reflect the concentrations of prey in fine-scale convergences at the surface. Diving species may instead benefit most from areas with strong vertical flow, making prey available closer to the surface or in a predictable location. The significant relationship between low stability and razorbill numbers suggests that strong vertical flow may bring prey near the surface. The fact that there is no such significant relationship between stability and guillemots or Manx shearwaters suggests they have a complex response to the circulation along the frontal boundary. The Irish Sea front is characterized by a complex circulation, with eddies and internal waves being two ways through which seabird prey could be concentrated in the surface waters. The changes in the distribution of birds through time may provide some clues as to how birds locate frontal boundaries. The correlation of most species with visible streaking at the surface suggests this may be an important clue for locating fronts. In weak winds it is common to observe the surface manifestation of internal waves as streak bands of different colour or roughness, oriented parallel to the frontal boundary. The rapid sampling instrument was able to detect vertical displacements of the thermocline from one pass to the next, and internal waves are
clearly visible in figures. Other features of the surface include along-front lines of scum and debris. Convergence zones were noticeable as lines of scum and floating jellyfish. Figure 3 presents the most striking example of convergence: at distance run 13 where the thermocline reaches the surface (), large quantities of floating material and a large number of surface-feeding birds were observed. Another reason for convergences may be the presence of eddy instabilities (Farmer et al., 1995). These are shed from the front and may trap high concentrations of prey in surface convergences. Eddies in the WIS front are common features during the summer (Pingree, 1978; Simpson, 1981) and a sequence of daily satellite images presented by Pingree (1978) suggest that these structures last for about 3–5 days. However, at the time of the observations it was not possible to obtain clear satellite images of the study area to further explore the importance of eddy features to seabird aggregations. The occurrence of big flocks of seabirds at the front and to the mixed side of the front is consistent with work of Schneider and his colleagues (1990) in the Bering Sea. Schneider (1990) attributed this pattern to the increased concentrations of chlorophyll a in the mixed region compared to stratified waters. In the WIS front, large concentrations of chlorophyll a (Savidge, 1976; Richardson et al., 1985) and high productivity (Beardall et al., 1982) have been frequently reported for the transition region. Similarly, larger biomass measurements have been reported for areas closer to the front and landwards (Nichols et al., 1990). The high productivity is reflected in the very large amount of biological activity observed: persistent large aggregations of birds and basking sharks. The exact location of the birds must relate to the circulation concentrating their prey at the frontal boundary. Changes in surface temperature, thermocline shape and depth as well as variations in the strength of the
Seabird observations in the Irish Sea 163
frontal circulation may be more or less predictable over a tidal cycle. Similar behaviour has been depicted in an intensive study of frontal variability in the WIS during June by Allen et al. (1980). The predictability of the WIS front in time and space may explain the high numbers of birds; earlier studies have suggested ephemeral fronts are less important for birds than strong predictable features (Schneider et al., 1987). It is likely that the importance of the WIS front to seabird populations is due to its geographical and temporal persistence and the magnitude or strength of cross-frontal gradients. The WIS front develops during spring each year, reaches maximum cross-frontal gradients by mid July to early August, and its position remains more or less fixed every year (Simpson & Hunter, 1974; Allen et al., 1980; Richardson et al., 1985; Hill, 1993). Strong parallel and cross-frontal flows are predicted with strong across-the-boundary gradients of physical parameters. Persistence and seasonal strengthening make for predictable feeding sites for breeding seabirds and may determine breeding success. Persistence and short-term strengthening of cross-frontal gradients may not only determine the position of prey and feeding birds, but may also promote interaction between predators. Attraction to the strongest part of the circulation may be by other seabirds (Schneider et al., 1990) or by large predators below the surface (Grebmeier & Harrison, 1992). Species interactions will influence the distributions of the most abundant species in the area of the front. The very large aggregations of Manx shearwaters could disrupt feeding by other species, or alternatively surface-feeding species may benefit from the active feeding by diving species. Schneider et al. (1990) found auks were making food available at the surface for kittiwakes; dead and disoriented euphausiids were floating up from a subsurface feeding frenzy and accumulating in fine-scale surface convergences. The main aim of this work was to determine whether there was a predictable association of seabirds with the tidal-mixing front in the WIS. Results showed that peak numbers occurred close to the transition boundary and that flocks of birds moved rapidly according to the changing conditions of the physical environment. Thus, the extensive shelf-sea fronts in the ECS (Pingree & Griffiths, 1978; Hill et al., 1993) must play a key role in seabird ecology. Time spent at foraging may be directly linked to the presence of frontal areas at a suitable distance from the breeding site (Wanless & Harris, 1992). Systematic counts in the North Sea (Dunnet et al. 1990) have found a large number of seabirds during the breeding season close to the shore along the eastern coast of Scotland, a region where a persistent
along-coast tidal front is formed during the summer season. Offshore, peak seabird densities also appear to coincide with the eastern extension of the Flamborough front (Webb et al., 1984; Tasker et al., 1985), and may explain the seabird colony at Flamborough Head and the southern boundary of breeding for many species. It is possible that similar patterns in seabird distribution may be found along the ECS whenever a shelf-sea front develops during the summer. Seabird breeding success depends largely on an abundant food source. Food may not be available when rough weather results in turbulent seas and prey concentrations become dispersed, reducing a seabird’s chance of success in a breeding season (Dunn, 1973; Wanless & Harris, 1992). Observations have shown that shelf-sea fronts are eroded from the surface to the depth of the mixed layer by the passing of storms, but that conditions prevailing prior to the storm are rapidly restored afterwards (Allen et al., 1980; Wang et al., 1990; Hill, 1993). Thus, there may be much greater success in seabird colonies which have access to predictable, persistent fronts. Colony growth and success may depend on the capacity of individual birds to identify the physical clues which indicate the position of fronts, where prey may be easily available. Acknowledgements We would like to thank to officers and crew of the RRS Challenger for their valuable help during the observations. N. Mathers and D. Boon maintained the Searover in working order. R.D. acknowledges support through scholarships from Conacyt and UABC in Me´xico and from the Overseas Research Studentship Committee in Great Britain. The authors would also like to thank M. Whitehouse and J. Reid for their help reviewing the manuscript. Comments from two anonymous reviewers greatly improved the manuscript. This work was supported by NERC under research grant GST/02/272. The seabird survey was made possible through support from the Joint Nature Conservation Committee and grants from the Departments of Transport, Energy and the Environment (Northern Ireland), BP, Shell, Esso, Hydrocarbons GB and Chevron. References Allen, C. M., Simpson, J. H. & Carson, R. M. 1980 The structure and variability of shelf sea fronts as observed by an undulating CTD system. Oceanologica Acta 3, 59–68. Bauer, J., Fischer, J., Leach, H. & Woods, J. D. 1985 SEA ROVER data report I—North Atlantic summer 1981—NOA’81. Ber. Inst. fu¨r Meereskunde, Kiel 143, 155 pp.
164 R. Durazo et al. Beardall, J., Foster, P., Voltolina, D. & Savidge, G. 1982 Observations on the surface water characteristics in the western Irish Sea: July 1977. Estuarine, Coastal and Shelf Science 14, 589–598. Davenport, J. & Rees, E. I. S. 1993 Observations on neuston and floating weed patches in the Irish Sea. Estuarine, Coastal and Shelf Science 36, 395–411. Duffy, D. C. 1983 The foraging ecology of Peruvian seabirds. Auk 100, 800–810. Dunn, E. K. 1973 Changes in fishing ability of terns associated with windspeed and sea surface conditions. Nature 244, 520–521. Dunnet, G. M., Furness, R. W., Tasker, M. L. & Becker, P. H. 1990 Seabird ecology in the North Sea. Netherlands Journal of Sea Research 26, 387–425. Farmer, D. M., D’Asaro, E. A., Trevorrow, M. V. & Dairiki, G. T. 1995 Three-dimensional structure in a tidal convergence front. Continental Shelf Research 15, 1649–1673. Fogg, G. E., Egan, B., Hoy, S., Lochte, K., Scrope-Howe, S. & Turley, C. M. S. 1985 Biological studies in the vicinity of a shallow-sea tidal mixing front. I. Physical and chemical background. Philosophical Transactions of the Royal Society of London B310, 407–433. Grebmeier, J. M. & Harrison, N. M. 1992 Seabird feeding on benthic amphipods facilitated by grey whale activity in the northern Bering Sea. Marine Ecology Progress Series 80, 125–133. Haney, J. C. & McGillivary, P. A. 1985 Midshelf fronts in the south Atlantic Bight and their influence on seabird distribution and seasonal abundance. Biological Oceanography 3, 401–430. Harrison, N. M., Hunt, G. L. & Cooney, R. T. 1990 Front affecting the distribution of seabirds in the northern Bering Sea. Polar Research 8, 29–31. Hill, A. E. 1993 Seasonal gyres in shelf seas. Annales Geophysicae 11, 1130–1137. Hill, A. E., James, I. D., Linden, P. F., Matthews, J. P., Prandle, D., Simpson, J. H., Gmitrowicz, E. M., Smeed, D. A., Lwiza, K. M. M., Durazo, R., Fox, A. & Bowers, D. G. 1993 Dynamics of tidal mixing fronts in the North Sea. Philosophical Transactions of the Royal Society of London A343, 431–446. Hill, A. E., Durazo, R. & Smeed, D. A. 1994 Observations of a cyclonic gyre in the western Irish Sea. Continental Shelf Research 14, 479–490. Hudson, A. V. & Furness, R. W. 1989 The behaviour of seabirds foraging at fishing boats around Shetland. Ibis 131, 225–237. James, I. D. 1978 A note on the circulation induced by a shallowsea front. Estuarine, Coastal Marine Science 7, 197–202. Kinder, T. H., Hunt, G. L., Jr., Schneider, D. & Schumacher, J. D. 1983 Correlations between seabirds and oceanic fronts around the Pribilof Islands, Alaska. Estuarine, Coastal and Shelf Science 16, 309–319. Le Fe`vre, J. 1986 Aspects of the biology of frontal systems. In Advances in Marine Biology Vol 23. (Blaxter, J. H. S. & Southward, A. J., eds). Academic Press, England, pp. 163–299. Nichols, J. H., Haynes, G. M., Fox, C. J., Milligan, S. P., Brander, K. M. & Chapman, R. J. 1990 Spring plankton surveys of the Irish Sea in 1982, 1985, 1987, 1988 and 1989: hydrography and the distribution of fish eggs and larvae. Technical report, MAFF Direc. Fish. Res., Lowestoff, 21 pp. Olson, D. B., Hitchcock, G. L., Mariano, A. J., Ashjian, C. J., Peng, G., Nero, R. W. & Podesta´, G. P. 1994 Life on the edge: marine life and fronts. Oceanography 7, 52–60.
Pingree, R. D. 1978 Cyclonic eddies and cross-frontal mixing. Journal of the Marine Biological Association U.K. 58, 955–963. Pingree, R. D. & Griffiths, D. K. 1978 Tidal fronts on the shelf seas around the British Isles. Journal of Geophysical Research 83, 4615–4622. Richardson, K., Lavı´n-Peregrina, M. F., Mitchelson, E. G. & Simpson, J. H. 1985 Seasonal distribution of chlorophyll ‘a’ in relation to physical structure in the western Irish Sea. Oceanologica Acta 8, 77–86. Russell, R. W., Harrison, N. M. & Hunt, G. L., Jr. 1998 Foraging at a front: hydrography, zooplankton and avian planktivory in the northern Bering Sea. Marine Ecology-Progress Series. In press. Savidge, G. 1976 A preliminary study of the distribution of chlorophyll ’a’ in the vicinity of fronts in the Celtic and western Irish seas. Estuarine, Coastal Marine Science 4, 617–625. Schneider, D., Harrison, N. M. & Hunt, G. L., Jr. 1987 Variation in the occurrence of marine birds at fronts in the Bering Sea. Estuarine Coastal and Shelf Science 25, 135–141. Schneider, D., Harrison, N. M. & Hunt, G. L., Jr. 1990 Seabird diet at a front near the Pribilof Islands, Alaska. Studies in Avian Biology 14, 61–66. Simpson, J. H. 1981 The shelf-sea fronts: implications of their existence and behaviour. Philosophical Transactions of the Royal Society of London A302, 531–546. Simpson, J. H. & Bowers, D. G. 1981 Models of stratification and frontal movements in shelf seas. Deep-Sea Research 28A, 727–738. Simpson, J. H., Hughes, D. G. & Morris, N. C. G. 1977 The relation of seasonal stratification to tidal mixing on the continental shelf. In A voyage of discovery (Angel, M. V., ed.). Pergamon Press, pp. 327–340. Simpson, J. H. & Hunter, J. R. 1974 Fronts in the Irish Sea. Nature (London) 250, 404–406. Tasker, M. L., Jones, P. H., Dixon, T. & Blake, B. F. 1984 Counting seabirds at sea from ships: a review of methods employed and a suggestion for a standardized approach. Auk 101, 567–577. Tasker, M. L., Webb, A., Greenstreet, S. P. R., Uttley, J. & Griffiths, A. 1985 Concentrations of auks off the north-east coast of Britain, August 1984. Nature Conservancy Council Commissioned Research Report No. 589: 1–13. Wang, D. P., Chen, D. K. & Sherwin, T. J. 1990 Coupling between mixing and advection in a shallow sea front. Continental Shelf Research 10, 123–136. Wanless, S. & Harris, M. P. 1992 Activity budgets, diet and breeding success of kittiwakes Rissa tridactila on the Isle of May. Bird Study 39, 145–154. Watson, P. S. 1981 Seabird observations from commercial trawlers in the Irish Sea. British Birds 74, 89–90. Webb, A., Tasker, M. L. & Greenstreet, S. P. R. 1984 The distribution of guillemots (Uria aalge), razorbills (Alca torda) and puffins (Fratercula arctica) at sea around Flamborough Head, June 1984. Nature Conservancy Council, Commisioned Research Report No. 590: 1–19. Yoder, J. A., Ackleson, S. G., Barber, R. T., Flament, P. & Balch, W. M. 1994 A line in the sea. Nature 371, 689–692.
Seabird Observations at a Tidal Mixing Front in the Irish Sea R. Durazoa, N. M. Harrisonb and A. E. Hillc a
UABC-Facultad de Ciencias Marinas, Apdo. Postal 453 Ensenada, B.C., Me´xico Anglia Polytechnic University, East Road, Cambridge CB1 1PT, U.K. c University of Wales, School of Ocean Sciences, Menai Bridge, Gwynedd LL59 5EY, U.K. b
Received 2 October 1997 and accepted in revised form 22 January 1998 Seabird counts were carried out along six transects across a tidal mixing front in the western Irish Sea using, for the first time, a combination of conventional visual bird observation techniques and very high-resolution mapping of frontal structure using an undulating CTD. Four seabird species were clearly aggregated at a precisely defined frontal boundary. Flocks of seabirds were found to change location over short time periods (hours), maintaining their position relative to the surface physical parameters and the subsurface thermocline. Results confirmed considerable anecdotal and limited documentary evidence that suggest fronts are important feeding areas for seabirds. 1998 Academic Press Keywords: birds; fronts; CTD; Irish Sea
Introduction Seabirds are an important and visible component of the marine fauna. Despite their ability to search over large areas of ocean in order to feed (Duffy, 1983), they tend to congregate over small conspicuous geographic locations where local concentrations of prey occur. A clear example is the aggregation of seabirds near fishing boats where they gather to make extensive use of discarded fish and offal (Watson, 1981; Hudson & Furness, 1989). Surface-feeding seabirds also aggregate where other marine predators such as whales and diving birds make prey readily available close to the surface (Schneider et al., 1990; Grebmeier & Harrison, 1992). Seabird aggregations have also been found to occur at fronts that form a boundary between mixed and stratified waters (Schneider et al., 1987; Harrison et al., 1990; Russel et al., unpubl.). As a result of the spatial variation of tidal stirring, fronts occur during the summer over regions of the European continental shelf (ECS), and their positions are predictable (Simpson & Hunter, 1974; Pingree & Griffiths, 1978). On the ECS, the transition zone between stratified and tidally-mixed waters is typically about 5–10 km and the horizontal extent is about 1500 km (Hill et al., 1993). Typical cross-front gradients of surface temperature are about 1 C km 1 (Simpson, 1981). Slicks, ripples, surface convergences, frontal instabilities and eddies shedding from the frontal boundary are common features of the 0272–7714/98/020153+12 $30.00/0
highly active transition zone (Hill et al., 1993; Farmer et al., 1995). Frontal systems may be areas of greater phytoplankton concentration due to accumulation (convergence) or greater productivity (exchange of nutrients) (James, 1978; Simpson, 1981; Yoder et al., 1994). Shelf-sea fronts have long been recognized as areas of increased primary productivity (Savidge, 1976; Pingree, 1978; Kinder et al., 1983; Richardson et al., 1985; Le Fe`vre, 1986; Davenport & Rees, 1993; Olson et al., 1994). High productivity might be of both local origin, by the growth of phytoplankton induced by continuous replenishment of nutrients, or by the mixing of two different water masses, one stratified and less tidally-energetic and the other homogeneous, highly energetic (mixed) water. Fronts have been found to be areas of enhanced biological productivity and also areas with a high biomass at higher trophic levels (Fogg et al., 1985; Le Fe`vre, 1986). The importance of shelf-sea fronts as feeding areas for marine predators has been stressed by numerous authors [see Le Fe`vre (1986) for a review]. The occurrence of large numbers of seabirds near fronts in the South Atlantic Bight has been documented by Haney and McGillivary (1985), and in the Bering Sea by Kinder et al. (1983), Schneider et al. (1987, 1990), and Harrison et al. (1990). On the ECS, observations have been made of seabirds associated with shelfsea fronts in the Irish Sea (Fogg et al., 1985) as well 1998 Academic Press
154 R. Durazo et al.
F 1. The study area in the western Irish Sea. Insert shows where seabirds were counted and the front was mapped along the six transects. The zig-zag line denotes the front. Broken lines denote depth contours in metres.
as in the North Sea (Webb et al., 1984; Tasker et al., 1985). Considerable interest in the distribution of biomass at tidal mixing fronts is reflected in a number of studies (Le Fe`vre, 1986), but rarely have physical and biological investigations been conducted simultaneously. Previous attempts to correlate oceanographic and biological observations have been based on conventional, coarsely spaced CTD profiling (Kinder et al., 1983; Schneider et al., 1990; Davenport & Rees, 1993). Most of these studies have used mean hydrographic measurements and have been too coarse to detect short-term variability. A shelf-sea front may exhibit variations in the position of the transition boundary depending on the mixing regime over the seasons, or with tidal strength over a month (Simpson & Bowers, 1981), or as a result of changes in tidal stirring at the semi-diurnal period, over a matter of hours (Allen et al., 1980). Thus, while associations of marine life with fronts are numerous (Le Fe`vre, 1986), fine-resolution studies are required if the mechanisms driving these patterns are to be understood. Attempts to describe in detail the biological properties of fronts have been severely hampered by
the limitations of instruments in measuring complex and dynamically active environments. This study reports counts of seabirds feeding in the Irish Sea at a tidal mixing front; cross-frontal hydrographic structure was mapped with a continuous, rapid sampling towed undulating CTD. Aggregations of seabirds have been noted at the Irish Sea front (Fogg et al., 1985), however, this represents the first detailed description of the seabird aggregations relative to the frontal circulation. Irish Sea front This study was conducted across a thermal transition zone between the deep western Irish Sea (WIS) and the shallow eastern Irish Sea (Figure 1). The transition zone known as the western Irish Sea front (Simpson & Hunter, 1974) is a permanent feature of the summer. The dynamics of the eastern area are dominated by tidally-driven water movements, mainly the semi-diurnal tides, which generate currents of up to 1·2 m s 1. Here, the strong tidal currents assure that the water column remains vertically mixed throughout the year. In the WIS, tidal currents are weak (0·2 ms 1) which permits the development of
Seabird observations in the Irish Sea 155
a thermocline from early April–May until the end of October. A front extends from the southern tip of the Isle of Man to Dublin in Eire (Figure 1). The front is visible in infra-red images (Simpson, 1981) and instabilities and eddies shedding from the front are manifest in the surface (Pingree, 1978). The seasonal stability of the Irish Sea front is explained by recent oceanographic studies of the WIS (Hill et al., 1994). Lagrangian current measurements have indicated mean currents of the order of 0·05 m s 1, which flow as a cyclonic gyre around the centre of the WIS completing a revolution in about 15 days. The outstanding importance of the gyre on the ecology of the western Irish Sea has been summarized by Hill (1993) and Hill et al. (1994), who suggested that the cyclonic circulation is an effective physical retention mechanism permitting fish larvae to circulate through the region ensuring their return and recruitment into the adult population. Recruitment through seasonal stability may bring a year-to-year secure supply of food to higher trophic levels such as that of seabirds. The area of the WIS front is important for a number of fish stocks and supports commercial fisheries. Studies have shown that high productivity is associated with the front. Davenport and Rees (1993) found maximum concentrations of neuston at the front; Richardson et al. (1985) concluded that over the entire season of thermal stratification, the mean chlorophyll a concentration in surface waters was highest near the front as compared to adjacent waters. Methods Observations were made from the RRS Challenger between 16 and 18 July 1990. The WIS front was repeatedly traversed at a speed of 3·1–3·6 m s 1 (6–7 knots) along six transects, spaced 5 km apart (Figure 1), which had been chosen so as to cross the front at approximately 90 to its mean orientation, as observed using CTD casts. Seabird counts were carried out during daylight hours at 10-min intervals (about 2 km spatial resolution) and the number and species of seabirds on the water or feeding in a 300 arc from bow to beam were recorded (Tasker et al., 1984). The counts were made 300 m forward, whereas the undulating CTD was towed behind the ship. While fishing boats frequent the Irish Sea front, during the observations reported here there were no fishing vessels which could have biased the distribution of the surface feeding species. The ship position was obtained at 30 s intervals using a Decca navigation system. The cross-frontal variability of temperature and salinity was measured
with an undulating CTD (Searover) (Bauer et al., 1985). The descent and ascent of the instrument was computer controlled from the ship’s deck, and the instrument sampled from the surface down to 10 m above the sea-bed. Typical spatial resolution was 400 m in the horizontal and 1 m (after low-pass filtering) in the vertical. Flat seas throughout the measurements allowed visual detection of 100–200 m wide slicks and ripple bands at the time of crossings. Surface temperature was recorded continuously from a seawater source 3 m below the surface using a thermo-salinograph. Vertical casts carried out simultaneously with Searover and a conventional CTD indicated that the mean offset of Searover temperature readings was 0·1 C. Partial correlation was used to test for significant correlation between bird numbers and surface temperature, surface salinity, temperature and salinity gradients and visible sea-surface features. Water column stability was identified using a stratification parameter, the potential energy anomaly (PEA) defined as (Simpson, 1981):
where z is the vertical co-ordinate, g is gravity, h is depth and ñ is water density with mean ñ. The PEA represents the work which would be done to bring about complete vertical mixing. Hence, PEA is zero for a fully-mixed water column and becomes positive for stable (stratified) conditions. For the WIS, the transition boundary is known to occur close to the point where PEA drops from 10 joules m 3 to virtually zero (Simpson et al., 1977). In the present study, a depth of h=45 m, the maximum descent of the undulating instrument on the shallow edge of each transect, was taken to calculate the stratification parameter. A Kruskall-Wallis non-parametric analysis of variance was used to test for patterns in seabird distribution relative to water-column stability. Results Each of the transects illustrated in Figure 1 was run at least four times. A total of 11 runs were in full daylight over most of the transect, making bird counts possible. All 11 transect runs crossed the shallow stratification at the edge of the front, with only three of the 11 transects crossing into fully-mixed water (mixed top to bottom). The selected transects included here are those traversed at least twice with bird counts covering most of the section (Figures 2–6). On each figure, vertical arrows mark the bounds of the front at PEA=10 and 5, respectively.
156 R. Durazo et al.
10 Kittiwake 5 0 30 Other surface feeders 15
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F 2. Top to bottom: number of birds at the surface for transect A2–B2 (run 5, 17 July 1990, 06.19–08.15h), potential energy anomaly (PEA) (thick line) and surface temperature (TS) (thin line), and the cross-frontal temperature structure. Contour intervals are 0·25 C. The broken line denotes periods over which no bird observations were carried out. Note that the horizontal axis for PEA graph has been displaced so as to cross the vertical axis at PEA=5. In the same graph, the symbols ` indicate the transition zone where PEA values drop from 10 to 5 J m 3.
Seabird observations in the Irish Sea 157 10 Kittiwake 5 0 30 Other surface feeders 15
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F 3. As in Figure 2 for transect A2–B2 (run 15, 18 July 1990, 05.20–07.04h). The inverted triangle indicates an area of surface convergence, where large quantities of scum and debris were observed.
Four species were associated with the front: three subsurface-feeding species, Manx shearwaters (Puffinus puffinus), guillemots (Uria aalge), razorbills (Alca torda) and one surface-feeding species, the
kittiwake (Rissa tridactyla). A fifth group is depicted in the figures to represent the assembly of all other surface-feeding species; included in this group are fulmar (Fulmarus glacialis), European storm-petrel
158 R. Durazo et al. 10 Kittiwake 5 0 30 Other surface feeders 15
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F 4. As in Figure 2 for transect A2–B2 (run 17, 18 July 1990, 09.46–11.52h).
(Hydrobates pelagicus), northern gannet (Morus bassanus), herring gull (Larus argentatus) and lesser black-backed gull (Larus fuscus).
The hydrographic profiles in Figures 2–6 show a broad frontal zone of 10 km or more over which horizontal gradients in temperature and water column
Seabird observations in the Irish Sea 159 10 Kittiwake 5 0 30 Other surface feeders 15
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F 5. As in Figure 2 for transect B3–A3 (run 8, 17 July 1990, 13.25–15.18h).
stability (PEA) were evident. The transects illustrated in Figures 2, 3–4 crossed into fully-mixed water at the shallow end (surface temperature <14 C and PEA <5). Over this broad zone, the shallow stratification
was pushed to the surface by the strong tidally-driven flow below. The surface expression of the front varied on the crossings illustrated, with some transects showing a relatively abrupt transition from shallow
160 R. Durazo et al. 10 Kittiwake 5 0 30 Other surface feeders 15
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F 6. As in Figure 2 for transect B3–A3 (run 18, 18 July 1990, 12.18–14.12h).
stratification to mixed water, and a marked drop in surface temperature (Figure 3). On other transects (Figure 2), the transition was gradual. On all 11 transects there were high numbers of birds aggregated over areas where the thermocline
rose close to the surface. Very large peaks in seabird numbers were found at the surface expression of the front, which was characterized by a marked change in surface temperature (TS) on four out of 11 transects (e.g. Figures 3 and 4).
Seabird observations in the Irish Sea 161 T 1. Occurrence of bird peaks (>20 birds) over maximum ÄT at surface, to one side of ÄT toward the mixed side, or seawards over stratified water
Peak at ÄT Peak to mixed side Peak to stratified side
Manx shearwater
Guillemot
Razorbill
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Manx shearwaters, guillemots, razorbills and kittiwakes all contributed to the numbers in the largest aggregations, but overall the seabird species differed in their distribution along the transects. The differences between the three subsurface-feeding species can be described by counting the aggregations of each species across three regimes: fully stratified water, the boundary defined as the maximum ÄTS along each transect, and over fully-mixed waters (Table 1). An aggregation was defined as 20 or more birds on a 10 min transect section. Peak numbers of shearwaters, often hundreds of birds, appeared to congregate along maximum temperature gradients at the surface. Smaller peaks occurred over the shallow stratification immediately adjacent to the area of maximum ÄTS. The largest aggregations of guillemots were also at the maximum ÄTS (Figures 3 and 4), but aggregations were found in stratified water and mixed water as well. Guillemots were the most widely dispersed species of those observed, and the only species to occur at all in the deeply stratified water at a distance from the front. Razorbills appeared in large aggregations only on the mixed side of the front. Temperature gradients at the surface appear to be a good predictor of seabird aggregations, indicating the extent to which the thermocline has broken down along the transect. These temperature gradients are not very large, but are a marker of flow gradients which shape the circulation at the frontal boundary. To test the relationship between the seabirds and stability, the 10-min bird observations were categorized into: PEA <5; PEA 5–10; PEA 10–25; and PEA >25. Razorbills were significantly more abundant in mixed water with a PEA of <5 (Kruskall-Wallis ANOVA, P <0·05). For guillemots and Manx shearwaters there were no significant differences (guillemot; ns, P=0·07, Manx shearwater; ns, P=0·53). The results reflect the fact that there is not a simple decrease in stability across the frontal boundary. The PEA values vary substantially over relatively short distances along the transects (Figures 4–6).
Spatial and temporal changes were evident in the pattern of bird aggregation. Changes took place on very short time-scales apparently in response to changes in thermocline depth and surface temperature gradients. Figures 2–4 illustrate the temporal evolution during three runs of transect A2–B2, and show a typical frontal structure, with the surface expression of the front at around 20 km and sharp changes in surface temperature (TS) in the vicinity of the front. While similarities in the hydrographic structure were evident, short-term changes were seen in the thermocline depth. The first two of these runs (Figures 2 and 3) were carried out approximately 24 h apart. The two were similar, with the thermocline reaching the surface at about 12 km. The third run (Figure 4) was traversed 5–6 h after the run in Figure 3, and shows the thermocline to have been displaced downwards. This phenomenon may reflect the influence of internal waves, as is suggested by the shape of the thermocline. Figures 5 and 6 show two runs along transect A3–B3, with differences in surface temperature and stability. Surface temperature was generally 0·5 C or more warmer on the run in Figure 5 than on that in Figure 6. On both runs a very shallow, strong thermocline can be seen, buckling toward the surface; fullymixed water was never reached. Figure 5 shows noisy fluctuations in the surface temperature and stability, and aggregations of birds distributed across the wide frontal boundary. Where there was a more regular drop in PEA and surface temperature (Figure 6), there was a different pattern, with many of the same bird species present, but only Manx shearwaters forming a very large aggregation (at 25 km). The same transect A3–B3 was crossed 10 h earlier (not shown) and revealed a surface–temperature distribution similar to that in Figure 6, suggesting a predictable shift between the conditions illustrated in Figures 5 and 6. The distributions of the most abundant birds were compared to patterns in surface temperature and salinity (Table 2). All species except for fulmar were significantly associated with temperature gradients. All species were found over more saline surface waters (e.g. away from the strongest stratification). However, changes in salinity were very small in the cross-frontal direction, and no patterns were found in seabird distribution relative to salinity gradients. There was no significant correlation with time of day. Slicks are a good indication of property changes at the surface, such as where internal waves come to the surface, or the surface expression of fronts. For all crossings, the null hypothesis of a random distribution relative to these small-scale structures was tested. The results presented in Table 2 indicate that birds were more often associated with streaky waters than
162 R. Durazo et al. T 2. Results of partial correlation for five seabirds
Manx shearwater Guillemot Razorbill Kittiwake Fulmar
Surface temperature
ÄT
Surface salinity
ÄS
Streaky water
Time of day
<0·01 ns <0·01 0·059 ns
<0·01 <0·01 <0·01 0·03 ns
<0·01 <0·01 <0·01 <0·01 <0·01
ns ns ns ns ns
0·05 0·02 ns <0·01 ns
ns ns ns ns 0·022
ns, non-significant. ÄT and ÄS are cross-frontal temperature and salinity gradients, respectively.
expected by chance, which suggests that birds were using these clues at the surface to position and feed. Discussion The results show persistent seabird aggregations at a well-defined frontal boundary, and prompt several questions on how seabirds locate fronts, and how they benefit from frontal circulations. The data suggest that seabird flocks form and disperse over very short time-scales (hours), and that different species either prefer to use different parts of the circulation or have different capacities to locate the best feeding areas. In the case of the surface-feeding species, their distributions may reflect the concentrations of prey in fine-scale convergences at the surface. Diving species may instead benefit most from areas with strong vertical flow, making prey available closer to the surface or in a predictable location. The significant relationship between low stability and razorbill numbers suggests that strong vertical flow may bring prey near the surface. The fact that there is no such significant relationship between stability and guillemots or Manx shearwaters suggests they have a complex response to the circulation along the frontal boundary. The Irish Sea front is characterized by a complex circulation, with eddies and internal waves being two ways through which seabird prey could be concentrated in the surface waters. The changes in the distribution of birds through time may provide some clues as to how birds locate frontal boundaries. The correlation of most species with visible streaking at the surface suggests this may be an important clue for locating fronts. In weak winds it is common to observe the surface manifestation of internal waves as streak bands of different colour or roughness, oriented parallel to the frontal boundary. The rapid sampling instrument was able to detect vertical displacements of the thermocline from one pass to the next, and internal waves are
clearly visible in figures. Other features of the surface include along-front lines of scum and debris. Convergence zones were noticeable as lines of scum and floating jellyfish. Figure 3 presents the most striking example of convergence: at distance run 13 where the thermocline reaches the surface (), large quantities of floating material and a large number of surface-feeding birds were observed. Another reason for convergences may be the presence of eddy instabilities (Farmer et al., 1995). These are shed from the front and may trap high concentrations of prey in surface convergences. Eddies in the WIS front are common features during the summer (Pingree, 1978; Simpson, 1981) and a sequence of daily satellite images presented by Pingree (1978) suggest that these structures last for about 3–5 days. However, at the time of the observations it was not possible to obtain clear satellite images of the study area to further explore the importance of eddy features to seabird aggregations. The occurrence of big flocks of seabirds at the front and to the mixed side of the front is consistent with work of Schneider and his colleagues (1990) in the Bering Sea. Schneider (1990) attributed this pattern to the increased concentrations of chlorophyll a in the mixed region compared to stratified waters. In the WIS front, large concentrations of chlorophyll a (Savidge, 1976; Richardson et al., 1985) and high productivity (Beardall et al., 1982) have been frequently reported for the transition region. Similarly, larger biomass measurements have been reported for areas closer to the front and landwards (Nichols et al., 1990). The high productivity is reflected in the very large amount of biological activity observed: persistent large aggregations of birds and basking sharks. The exact location of the birds must relate to the circulation concentrating their prey at the frontal boundary. Changes in surface temperature, thermocline shape and depth as well as variations in the strength of the
Seabird observations in the Irish Sea 163
frontal circulation may be more or less predictable over a tidal cycle. Similar behaviour has been depicted in an intensive study of frontal variability in the WIS during June by Allen et al. (1980). The predictability of the WIS front in time and space may explain the high numbers of birds; earlier studies have suggested ephemeral fronts are less important for birds than strong predictable features (Schneider et al., 1987). It is likely that the importance of the WIS front to seabird populations is due to its geographical and temporal persistence and the magnitude or strength of cross-frontal gradients. The WIS front develops during spring each year, reaches maximum cross-frontal gradients by mid July to early August, and its position remains more or less fixed every year (Simpson & Hunter, 1974; Allen et al., 1980; Richardson et al., 1985; Hill, 1993). Strong parallel and cross-frontal flows are predicted with strong across-the-boundary gradients of physical parameters. Persistence and seasonal strengthening make for predictable feeding sites for breeding seabirds and may determine breeding success. Persistence and short-term strengthening of cross-frontal gradients may not only determine the position of prey and feeding birds, but may also promote interaction between predators. Attraction to the strongest part of the circulation may be by other seabirds (Schneider et al., 1990) or by large predators below the surface (Grebmeier & Harrison, 1992). Species interactions will influence the distributions of the most abundant species in the area of the front. The very large aggregations of Manx shearwaters could disrupt feeding by other species, or alternatively surface-feeding species may benefit from the active feeding by diving species. Schneider et al. (1990) found auks were making food available at the surface for kittiwakes; dead and disoriented euphausiids were floating up from a subsurface feeding frenzy and accumulating in fine-scale surface convergences. The main aim of this work was to determine whether there was a predictable association of seabirds with the tidal-mixing front in the WIS. Results showed that peak numbers occurred close to the transition boundary and that flocks of birds moved rapidly according to the changing conditions of the physical environment. Thus, the extensive shelf-sea fronts in the ECS (Pingree & Griffiths, 1978; Hill et al., 1993) must play a key role in seabird ecology. Time spent at foraging may be directly linked to the presence of frontal areas at a suitable distance from the breeding site (Wanless & Harris, 1992). Systematic counts in the North Sea (Dunnet et al. 1990) have found a large number of seabirds during the breeding season close to the shore along the eastern coast of Scotland, a region where a persistent
along-coast tidal front is formed during the summer season. Offshore, peak seabird densities also appear to coincide with the eastern extension of the Flamborough front (Webb et al., 1984; Tasker et al., 1985), and may explain the seabird colony at Flamborough Head and the southern boundary of breeding for many species. It is possible that similar patterns in seabird distribution may be found along the ECS whenever a shelf-sea front develops during the summer. Seabird breeding success depends largely on an abundant food source. Food may not be available when rough weather results in turbulent seas and prey concentrations become dispersed, reducing a seabird’s chance of success in a breeding season (Dunn, 1973; Wanless & Harris, 1992). Observations have shown that shelf-sea fronts are eroded from the surface to the depth of the mixed layer by the passing of storms, but that conditions prevailing prior to the storm are rapidly restored afterwards (Allen et al., 1980; Wang et al., 1990; Hill, 1993). Thus, there may be much greater success in seabird colonies which have access to predictable, persistent fronts. Colony growth and success may depend on the capacity of individual birds to identify the physical clues which indicate the position of fronts, where prey may be easily available. Acknowledgements We would like to thank to officers and crew of the RRS Challenger for their valuable help during the observations. N. Mathers and D. Boon maintained the Searover in working order. R.D. acknowledges support through scholarships from Conacyt and UABC in Me´xico and from the Overseas Research Studentship Committee in Great Britain. The authors would also like to thank M. Whitehouse and J. Reid for their help reviewing the manuscript. Comments from two anonymous reviewers greatly improved the manuscript. This work was supported by NERC under research grant GST/02/272. The seabird survey was made possible through support from the Joint Nature Conservation Committee and grants from the Departments of Transport, Energy and the Environment (Northern Ireland), BP, Shell, Esso, Hydrocarbons GB and Chevron. References Allen, C. M., Simpson, J. H. & Carson, R. M. 1980 The structure and variability of shelf sea fronts as observed by an undulating CTD system. Oceanologica Acta 3, 59–68. Bauer, J., Fischer, J., Leach, H. & Woods, J. D. 1985 SEA ROVER data report I—North Atlantic summer 1981—NOA’81. Ber. Inst. fu¨r Meereskunde, Kiel 143, 155 pp.
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