Soil indicators of pre-European seabird breeding in New Zealand at sites identified by predator deposits.
Soil samples (0-15 cm) from 2 sites identified by palaeontological methods as pre-European seabird breeding sites (Annandale, Ardenest) were compared with a control site (Ngarua; no seabird breeding). All sites had moderately calcareous soils developed on limestone or marble. Breeding site Kjeldahl nitrogen (range, 0.41-1.4% N) and total phosphorus (range, 1780-5285 mg/kg P) were comparable to present-day breeding sites and higher than the control (mean, 0.20% N, 520 mg/kg P). Total cadmium (Cd) results followed the same pattern, with results from Ardenest (mean, 0.49 mg/kg) and Annandale (0.47 mg/kg) being similar to fertilised New Zealand pastoral soils. Contributions from extensive pre-European seabird breeding may therefore match superphosphate fertiliser as a Cd source. Results for [[Delta].sup.15]N and C:N were consistent with seabird breeding at both sites (Ardenest, [[Delta].sup.15]N = 8.4%, C:N = 10-6 g C/g N; Annandale, [[Delta].sup.15]N = 6.9%, C:N = 10.6). Results for [[Delta].sup.13]C were consistent with seabird breeding at Ardenest ([[Delta].sup.13]C = -22.8%) but not at Annandale ([[Delta].sup.13]C = -27.0%), indicating dilution by organic matter derived from Cs plants at Annandale. The Cd:P ratio was significantly lower (P = 0.05) at each of the seabird breeding sites (Annandale, 6.5 [+ or -] 1.5 x [10.sup.-5] mol Cd/mol P; Ardenest, 2.5 [+ or -] 0.7 x [10.sup.-5]) than the control (mean 10-0[+ or -]1.6 x [10.sup.-5]). This ratio has the potential to complement [[Delta].sup.15]N, [[Delta].sup.13]C, and C:N in identifying and mapping pre-European seabird breeding sites, although more study is required.
Additional keywords: phosphorus, nitrogen, nitrogen-15, carbon-13, cadmium, procellariid.
Pre-human New Zealand was noteworthy in that the most significant predators were birds, ranging in size from the very large Haast's eagle (Harpagornis moorei) to the New Zealand owlet-nightjar (Aegotheles novaezealandiae). Extensive seabird breeding colonies co-existed with these predators on the New Zealand mainland from the Pleistocene (Worthy and Holdaway 1993, 1995, 1996; Holdaway and Worthy 1994). However, mainland seabird breeding colonies became increasingly restricted after human contact, following a pattern common throughout the Pacific Islands (Steadman 1995). The likely cause of the elimination of the smaller burrow-nesting seabirds (along with various reptiles, amphibians, and large invertebrates) was predation by the Pacific rat (Rattus exulans) introduced about 2000 years BP (Holdaway 1996; Worthy 1997). Populations of the remaining larger seabirds became increasingly restricted following the introduction of pigs, mustelids, and cats by European colonists in the 19th century.
Determining effects of present-day seabird breeding on soil chemistry is usually straightforward. Similarly, sites where seabird breeding ceased in the recent past can be investigated from photographic or written records (Moors et al. 1988; Mizutani et al. 1991a). The problem becomes more difficult for sites where seabird breeding ceased before recording began. For these, an investigator must rely on oral traditions or on fossil deposits. Fossilisation is rare, whereas oral traditions weaken with the passing of time and are inapplicable to pre-contact times. To elucidate the ecology and geochemistry of pre-human New Zealand, it would therefore be desirable to have chemical tools which would allow unambiguous detection (and mapping) of former seabird breeding sites without the need for palaeontological confirmation.
Compared with control sites, soils at seabird breeding colonies are enriched in [[Delta].sup.15]N, [[Delta].sup.13]C, and C:N (Mizutani et al. 1986; Moors et al. 1988). These ratios have been used to identify subantarctic breeding sites of seabirds deserted over the past 30 years (Moors et al. 1988; Mizutani et al. 1991a). However, the preservation of characteristic [[Delta].sup.15]N, [[Delta].sup.13]C, and C:N ratios in soils from seabird breeding sites abandoned in pre-European times [10.sup.2]-[10.sup.3] years BP is unknown given possible dilution by terrestrial photosynthesis and nitrogen (N) fixation.
It has long been recognised that seabirds enrich the soils of breeding and roosting sites (e.g. Gillham 1956) via guano (Burger et al. 1978), dead birds (Williams et al. 1978), and egg material (Siegfried et al. 1978). Nutrient inputs at seabird breeding sites can be very high compared with fertiliser applications to agricultural land. For example, Mizutani (1984; cited in Mizutani et al. 1986) derived inputs for a gull colony of 530 kg N/ha. year and 450 kg phosphorus (P)/ha.year, whereas dairy farming applications in New Zealand rarely exceed 200 kg N/ha.year and 100 kg P/ha.year. Thus, seabirds (along with salmonid fish; Kline et al. 1993) reverse the commonly accepted pattern of nutrient transport from terrestrial to marine ecosystems. Although most research has emphasised seabird effects on small island ecosystems (Ure 1996), the palaeontological results discussed above imply that the New Zealand mainland also received abundant quantities of marine-derived nutrients that have essentially ceased. Since breeding colonies were widespread (Worthy and Holdaway 1993, 1995, 1996) and extended well inland (Worthy 1997), extensive enrichment of terrestrial freshwater ecosystems is also plausible.
Our study had 2 aims. The first was to identify chemical (including isotopic) markers for pre-European seabird breeding sites. The use of chemical markers should allow estimation of colony size and hence nutrient budgets. Ideally, such markers should unambiguously identify the presence or absence of seabird breeding without the need for palaeontological confirmation. The second aim was to determine whether pre-European seabird breeding contributes to present-day soil fertility.
Study sites and sampling methods
The use of predator deposits in palaeofaunal reconstruction is relatively new in New Zealand (Worthy and Holdaway 1994a; Holdaway and Worthy 1996). Two of the study sites (Fig. 1) were on limestone and identified as former seabird breeding areas from deposits left by predators. A third (control) site was on marble and identified as having no nearby seabird breeding from the absence of seabirds in a particularly rich predator deposit. Bone material for [sup.14]C dating was collected from a fourth site (Braeburn), but no soil samples were collected.
[Figure 1 ILLUSTRATION OMITTED]
At Annandale (Waiau, North Canterbury), remains of Mottled petrel (Pterodroma inexpectata) were found at a deposit accumulated by New Zealand falcons (Falco novaeseelandiae; Worthy and Holdaway 1995). In terms of the duration and diversity of seabird breeding in the local area, Mottled petrel, Cook's petrel (Pterodroma cookii), Common diving petrel (Pelecanoides urinatrix), and Hutton's shearwater//Fluttering shearwater (Puffinus huttoni/P. gavia) were found by Worthy and Holdaway (1995) in a nearby cave deposit. This deposit accumulated over at least 600 years from 2400 years BP. Combined with results from the present study, this indicates that seabirds bred in the area for several thousand years at an absolute minimum prior to their extinction. From the vegetation history presented by Worthy and Holdaway (1995), it is likely that seabirds have bred in the general area since at least the end of the Otiran glaciation (10 000 years BP). The small, extinct Scarlett's shearwater (Puffinus spelaeus) bred on the West Coast of the South Island from at least 20 000 years BP to 600 years BP (Worthy and Holdaway 1993; Holdaway and Worthy 1994), so it is conceivable that seabird breeding in the study area occurred continuously for tens of thousands of years.
At Ardenest (Waikari, North Canterbury), remains of Grey-backed storm petrel (Oceanites nereis) were found at a deposit accumulated by Laughing owls (Sceloglaux albifacies; Worthy and Holdaway 1996). Species similar to those recovered from the Annandale cave deposit were found at an undated predator deposit about 5 km from our study site. We therefore conclude that seabirds bred in the Ardenest area over the same time period as the Annandale area, i.e. several thousand years at a minimum. Formation of the Ardenest and Annandale predator deposits probably continued until at least the mid 1800s (Worthy and Holdaway 1995, 1996), well after seabirds ceased breeding.
The Predator Cave deposit near Ngarua (Takaka Hill, Nelson) was accumulated by Laughing owls. It contained remains of a diverse terrestrial fauna accumulated from the end of the Otiran glaciation into the late 19th century, but no seabird remains (Worthy and Holdaway 1994b). Other Laughing owl sites (e.g. on the South Island West Coast: Worthy and Holdaway 1993, 1994a) were dominated by seabird remains. Since the Laughing owl is an opportunistic predator which preys on any animal it can subdue (Holdaway and Worthy 1996), Worthy and Holdaway (1994b) concluded that seabirds have not bred in the vicinity of Predator Cave in the last 10 000 years. We used this site as a control (no seabird breeding). A description of each site is given in Table 1.
Table 1. Site descriptions for areas sampled
Study site Soil classification(A) Soil description (rainfall) Ardenest Rendic melanic soil Shallow, dark, granular; (750 mm) limestone fragments Annandale Rendic melanic soil Shallow, dark; some (700 mm) limestone fragments Ngarua Orthic melanic soil Dark, overlying marble; (2116 mm) some charcoal fragments Study site Vegetation Present landuse (rainfall) Ardenest Exotic pasture Semi-extensive (750 mm) and weeds pastoral farming Annandale Exotic pasture Semi-extensive (700 mm) pastoral farming Ngarua Exotic pasture, Semi-extensive (2116 mm) some low shrubs pastoral farming
(A) New Zealand Soil Classification.
Since burrows have long disappeared from the study areas, we chose sampling sites at Ardenest and Annandale that were likely to be favoured by breeding seabirds. Predator deposits at both sites were in recesses in limestone cliffs. Such cliffs are likely to be favoured take-off points for petrels, so most samples were collected from the gently sloping ground above the cliffs and from soil developed in the colluvium at the foot of the cliffs. The topography of the sites was similar; photographs of both sites are shown in Worthy and Holdaway (1996: figs 3 and 11). At Ngarua, there are no cliffs so 5 transects encompassing a variety of slopes and aspects were taken at right angles to a line running down-slope between the cave containing the predator deposit and a doline on one side, and a doline and a low rise on the other side.
Sampling was performed on 12 January and 20 March 1996 at Ardenest, 8 August 1996 at Annandale, and 20 September 1996 at Ngarua. Sampling was along transect lines 20 m (Ardenest and Annandale) or 15 m (Ngarua) apart near the predator sites. At Ardenest, 2 transects ran from the top of the cliff and 1 ran from the base of the cliff. At Annandale, 5 transects ran from the top of the cliff and 3 from below the cliff. At all sites, 6 samples from a depth of 0-15 cm were taken at 2-m intervals along each transect, pooled, and subsequently sieved and dried.
All samples were analysed for pH (in water), Kjeldahl N, Olsen and total P, total cadmium (Cd), and major oxides. Total P and major oxide analyses were carried out using wavelength-dispersive X-ray fluorescence by Spectrachem Analytical Ltd (Wellington, New Zealand). Total Cd was determined by GFAAS analysis of hydrochloric acid-nitric acid digests by National Chemical Residue Laboratory (Ministry of Agriculture and Fisheries, Upper Hutt, New Zealand). Olsen P, Kjeldahl N, and pH were determined in our laboratory by methods described in Blakemore et al. (1987).
The [[Delta].sup.15]N and [[Delta].sup.13]C were determined by the Institute of Geological and Nuclear Sciences Ltd (Wellington, New Zealand). Analysis for each isotope (plus total carbon (C) and total N) was carried out simultaneously on mg quantities of finely ground soil. Carbonates were not removed prior to analysis. The procedure involved converting sample C and N to N2 and [CO.sub.2] using an elemental analyser interfaced via GC to a mass spectrometer (Europa Geo 20/20). The N isotope ratio was obtained as a per mil (%o) deviation from atmospheric N:
([[Delta].sup.15]N(%o) ~ 1000 x [([sup.15]N/[sup.14]N)sample - ([sup.15]N/[sup.14]Nair)]/([sup.15]N/[sup.14]N)air
The C isotope ratio is based on the international PDB limestone standard:
([[Delta].sup.13]C(%o) ~ 1000 x [([sup.13]C/[sup.12]C)sample - ([sup.13]C/[sup.12]C)PDB]/([sup.13]C/[sup.12]C)PDB
Results were validated by comparison with flour (Europa Scientific) and NIST standards. Agreement between duplicate analyses for both isotope ratios was better than [+ or -] 0.3%. The exception was the Ngarua sample where N content was very low; agreement was [+ or -] 0.4%. The total C and total N results were used to calculate the C:N ratio.
Phosphorus is occasionally held in identifiable phosphate minerals (Lindsay et al. 1989). To examine this possibility, XRD analysis of the 2 samples with highest total P was carried out by Landcare Research Ltd (Palmerston North, New Zealand).
Estimates of the time at which seabird breeding ceased were obtained by [sup.14]C dating of bone gelatin from the most recent seabird material from undisturbed deposits near Braeburn (described in Worthy 1997) and at Annandale. Accelerator mass spectrometer dating and calibration (including a marine correction) was carried out by the Institute of Geological and Nuclear Sciences Ltd; methodology is summarised in Worthy and Holdaway (1993, 1995).
Results and discussion
[sup.14]C dating of predator deposit material
Dating of the most recent seabird material (Mottled petrel) from Annandale yielded a [sup.14]C age ([+ or -] 68% CI) of 1115 [+ or -] 70 years BP (IGNS Rafter Laboratory reference number NZA 6967). Eight dates obtained by Worthy and Holdaway (1993) on Scarlett's shearwater (Puffinus spelaeus) from near Charleston on the South Island West Coast ranged from 17340 [+ or -] 140 to 594 [+ or -] 56 years BP (NZA 2142, 2146, 2247-2250, 2318, 2850). The most recent seabird material (2 samples) from the Braeburn site in South Canterbury (Worthy 1997) was also dated, to provide another estimate of the cessation of seabird breeding in the South Island. The [sup.14]C ages were 610 [+ or -] 66 (NZA 7226) and 738 [+ or -] 67 years BP (NZA 7240) on Hutton's shearwater Puffinus huttoni, or Fluttering shearwater Puffinus gavia (it was not possible to differentiate between these 2 species at this site). These conventional radiocarbon ages translate via a correction for marine reservoir effect into the following calendar ages: Annandale, 1216-1314 AD; Braeburn, 1481-1642 AD and 1634-1710 AD; Charleston, 1670-1806 AD (most recent date only). It was not possible to estimate the most recent date at which seabirds were breeding at Ardenest, because the deposit had been disturbed by rabbits and sheep (Worthy and Holdaway 1996).
Interpretation of the dating results is constrained by the relative rarity of fossilisation, and species-dependent differences in extinction dates. Although cursory examination of the results might suggest that seabird breeding ceased at Annandale before the 2 other South Island sites, the species available for dating differed. Instead, the results can be interpreted as showing Hutton's/Fluttering shearwater extinction subsequent to Mottled petrel and Scarlett's shearwater. This is consistent with the present-day status. Scarlett's shearwater is extinct. The formerly widespread Mottled petrel has been largely eliminated from the South Island, in contrast to Hutton's shearwater which has a (slowly declining) breeding population of [10.sup.5] pairs (Heather and Robertson 1996). Fluttering shearwaters are confined to islands off the north of the South Island, and around the North Island. For moderately-sized species such as Mottled petrel and Hutton's shearwater, our data suggest that mainland extinction from lowland sites would have occurred within the last 300-700 years. Smaller species such as the storm petrels and diving petrel(s) would undoubtedly have disappeared sooner but after the introduction of kiore about 2000 years BP (Holdaway 1996). At Ardenest, the only seabird species present in the predator deposit was a storm petrel so that seabird breeding at this site may have ceased over 1000 years ago. Thus, nutrient input from seabird breeding is likely to have occurred at all sites for [10.sup.3]-[10.sup.4] years until the colonies were extinguished.
Major oxide composition
All sites had relatively high levels of CaO (Table 2). Rainfall sequence was Ngarua [is greater than] Annandale = Ardenest (Table 1). These results are consistent with the soil descriptions (see Table 1).
Table 2. Soil analyses for total phosphorus (P), Olsen P, Kjeldahl nitrogen (N), pH (water), total cadmium (Cd), and CaO
Values are means [+ or -] 95% CI for Ngarua and Annandale, and ranges (3 transects only) for Ardenest
Site Total P Olsen P (mg/kg) (mg/kg) Ngarua 520 [+ or -] 50 0.3 [+ or -] 0.3 (control) Annandale 2250 [+ or -] 400 16 [+ or -] 13 Ardenest 5147-5285 31-173 Site Kjeldahl N Total Cd (%) (mg/kg) Ngarua 0.20 [+ or -] 0.01 0.19 [+ or -] 0.04 (control) Annandale 0.60 [+ or -] 0.16 0.49 [+ or -] 0.05 Ardenest 0- 60-1.44(A) 0.43-0.50 Site pH CaO (water) (%) Ngarua 5.7 [+ or -] 0.4 3 [+ or -] 1 (control) Annandale 6.3 [+ or -] 0.5 1.9 [+ or -] 0.3 Ardenest 7.11-7.54 9.1-17.0
(A) Mass spectrometric result.
Phosphorus, nitrogen, and cadmium chemistry
Phosphorus concentrations (Table 2) at Ardenest were particularly high, at up to 5285 mg/kg total P and 173 mg/kg Olsen P. Although P results from Annandale were lower (mean total P, 2252 mg/kg; mean Olsen P, 16 mg/kg), they were substantially higher than those for the Ngarua control (mean total P, 520 mg/kg; mean Olsen P, [is less than] 1 mg/kg) notwithstanding differences in rainfall. Although the Olsen P concentrations from Annandale and Ardenest reflect the magnitudes of the total P concentrations, plant growth at these sites is water-limited because of the shallow soils and only moderate rainfall. Although there is a trend of increasing Olsen P with pH, the Ngarua results were consistently lower than those from Ardenest and Annandale at overlapping pH. Comparisons of Olsen P should be valid because the major oxide compositions of these soils are similar. Kjeldahl N concentrations (Table 2) were much higher at the seabird breeding sites (means: Annandale, 0.60%; Ardenest, 1.1%) than at the Ngarua control (mean, 0.20%).
Remarkably, the total P and Kjeldahl N results were in the same range as analyses from some present-day breeding sites. For example, Mizutani et al. (1986) reported Kjeldahl N values of 0.5-2.7% for seabird breeding sites at both polar and temperate locations. Okazaki et al. (1993) reported Kjeldahl N values of 0.4-1.1% from a Sooty shearwater (Puffinus griseus) breeding colony in the Marlborough Sounds (New Zealand). For total P, Morris (1994) reported results from a Banks Peninsula (Canterbury) penguin breeding site of 1300-6600 mg/kg. However, Moors et al. (1988) reported extremely high total P concentrations in the range of 20 000-36 000 mg/kg from penguin breeding sites on Campbell Island which have been progressively abandoned over 30 years. Kjeldahl N levels in the study of Moors et al. (1988) were in the range of 0.4-1.4%, consistent with the other results. The relative similarity of the total P and Kjeldahl N results from Annandale and Ardenest to those from most present-day sites suggests that the rate of loss of nutrients from the study sites has been relatively slow. Physical factors which may have minimised losses are the relatively low rainfall and the gentle slope. Chemical factors would exert a range of influences. Solubility of Ca-P phases decreases with increasing pH, while phosphate adsorption on Fe oxides decreases. Thus, the high CaO content and pH would retain the P in association with Ca at the expense of adsorption on Fe and Al surfaces. The high pH might be expected to increase the rate of ammonia volatilisation, although soil pH was much less than the p[K.sub.a] of [NH.sub.4]+ (9.2). The [[Delta].sup.15]N results do not support such a possibility (see below).
Cadmium results (Table 2) also reflected the presence or absence of seabird breeding. A mean concentration of 0.19 mg/kg recorded from Ngarua is typical of native (non-pastoral) New Zealand soils examined by Roberts et al. (1994). Concentrations in the vicinity of 0.5 mg/kg (Ardenest, Annandale) were typical of the pastoral soils in the study of Roberts et al. (1994). The Ardenest and Annandale results were higher than Cd results (Loganathan and Hedley 1997) from a New Zealand fertiliser trial where different phosphate fertilisers were applied to a yellow-grey earth soil for 10 years at 30 or 60 kg P/ha. year. The breeding site results were also typical of agricultural soils in a range of industrialised countries (see review by McLaughlin et al. 1996). Anthropogenic sources of Cd (typically phosphate fertilisers) are therefore likely to be matched in many New Zealand locations by contributions from seabirds that formerly bred in these areas.
The results show that nutrients contributed by pre-European seabird breeding remain as highly significant contributors to soil fertility. The only colonial non-seabirds found in New Zealand do not inhabit hill country sites. Kjeldahl N and total P concentrations should therefore be suitable as indicators of seabird breeding. Consistent with this, Aveling (1997) proposed soil P as an indicator of stock enclosure in a structure occupied 3800-4000 BC in Britain. Total Cd is also an indicator of previous seabird breeding in non-pastoral soils although the difficulty (or cost) of analysis is much higher than for N or P.
Mass spectrometry results
Analytical results for [[Delta].sup.15]N and [[Delta].sup.13]C are given in Table 3. The [[Delta].sup.15]N results for Annandale (6.9%) and Ardenest (8.4%) were very close to soil values from present-day seabird breeding sites at the same latitude (Mizutani et al. 1991b). Since our results fit into the same trend with latitude as the results of Mizutani et al. (1991b), it would appear that dilution of the [[Delta].sup.15]N signal by N fixation at Annandale and Ardenest has not been significant. Such a conclusion is consistent with observations (Mizutani et al. 1991a) that [[Delta]].sup.15]N at seabird colonies stabilises about 10 years after the colony is deserted. The relatively small [[Delta].sup.15]N enrichment from the Ngarua control site (2.9%o) may result from burning around the turn of the century, given the charcoal fragments present in the soil samples, or from farming activities. As discussed earlier in the paper, the richness of the predator deposit gives us substantial confidence in our inference of no seabird breeding at Ngarua.
Table 3. Results of mass spectrometry analyses
All results are given as the mean of duplicate analyses on pooled samples from each site
Site [[Delta].sup.15]N [[Delta].sup.13]C C:N (%0) (%0) (g C/g N) Ngarua (control) 2.9 -25.8 16.9 Annandale 6.9 -27.0 10.6 Ardenest 8.4 -22.8 10.6
The [[Delta].sup.13]C results from Ngarua and Annandale were indistinguishable at -26% to -27%. These results are similar to those reported for [C.sub.3] plants (Bender 1971; Moors et al. 1988), which is reasonable for the control site but not for sites with significant marine inputs. We conclude that the marine contribution at Annandale has been diluted out by organic matter derived from [C.sub.3] plants. However, the Ardenest result (-22.8%) was similar to that obtained for past and present Rockhopper Penguin breeding sites on Campbell Island (Moors et al. 1988). The Ardenest result could also be explained by a significant carbonate contribution since carbonates enrich the [[Delta].sup.13]C value of soil. The C:N ratios at Annandale and Ardenest (both 10.6 g C/g N) were substantially lower than at the control site (16.9), as observed at Campbell Island.
In another attempt to find a robust predictor of former seabird breeding sites, mineralogical analysis of 2 samples from Ardenest (containing 5285 and 5147 mg/kg total P, respectively) was carried out. One phosphate mineral, taranakite, is sometimes associated with seabird breeding sites (J. Whitton, pers. comm.). However, the analysis yielded no significant phosphate minerals in any of the fractions.
Cadmium: phosphorus ratio as an indicator of seabird breeding
Although Cd (a metal) and P (a non-metal) are chemically very different, localised relationships between these 2 elements are well known (e.g. seawater, Frew and Hunter 1995; phosphate rock, McLaughlin et al. 1996). These relationships led us to evaluate Cd:P ratios for our study sites. Mean Cd:P values ([+ or -] 95% CI) were 10.0 [+ or -] 1.6 x [10.sup.-5] mol Cd/mol P at the Ngarua control site, 6.5 [+ or -] . 5 x [10.sup.-5] mol Cd/mol P at Annandale, and 2-5 [+ or -] 0.7 x [10.sup.-5] mol Cd/mol P at Ardenest. The mean Cd: P ratio was therefore significantly higher (P = 0.05) for the Ngarua control than the mean for each of the breeding sites. The global Cd:P ratio for sedimentary rock (shale) is 10.7 x [10.sup.-5] mol Cd/mol P (Reeves and Brooks 1978), consistent with the Ngarua result.
Guano is the largest source of mineral inputs associated with seabird breeding (Burger et al. 1978). We do not know of any analyses of guano for both Cd and P. Uptake of Cd by seabirds is probably dietary and/or from seawater (Warham 1996). Water masses found around New Zealand have Cd: P ratios in the range of 1 x [10.sup.-2] to 3 x [10.sup.-4] mol Cd/mol P (Hunter and Ho 1991; Frew and Hunter 1995). The Cd content (and hence Cd: P ratio) of planktonic prey varies with zooplankton species, geographical location and/or size (Rainbow 1989). Cd: P ratios for seabird carcasses calculated from Smith's (1978) study of seabirds breeding at Marion Island (southern Indian Ocean) ranged between 4.0 x [10.sup.-4] (Salvin's prion, Pachyptila salvini) and 1.5 x [10.sup.-7] mol Cd/mol P (Black-bellied storm petrel, Fregetta tropica). Thus, one might expect Cd:P in guano to also depend on both seabird species and location. This expectation is borne out in the range of Cd:P ratios found in guano-derived phosphate rock used in superphosphate manufacture [7.7 x [10.sup.-5] (Christmas Island) to 1.8x [10.sup.-4] (Nauru) on a molar basis; McLaughlin et al. 1996]. Thus, the range of Cd:P ratios we found at former breeding sites is within the general range of that reported for seabird carcasses and phosphate rock. Although the Cd:P results reported in our study appeared to be consistent indicators of seabird breeding, the Cd:P ratio for guano is almost certainly widely variable. This could lead to difficulties in interpreting Cd:P ratios of soil at breeding sites. Interpretation may also be compromised by present-day addition of superphosphate. [The ratio in surface soil at the superphosphate trial site reported by Loganathan and Hedley (1997) was 13 x [10.sup.-5] mol Cd/mol P, higher than any of the results from our study but similar to the ratio of 9.8 x [10.sup.-5] in the fertiliser itself.] Studies at present-day breeding sites will be carried out to examine these possibilities.
Seabird breeding at the study sites probably ceased at least 300-700 years BP, depending on species. Despite the relatively long time interval since nutrient input ceased, significant contributions to present-day N, P, and Cd concentrations were found at the 2 former breeding sites. Consistent indications of former seabird breeding were also provided by [[Delta].sup.15]N, C: N, and Cd: P. The [[Delta].sup.15]N results indicated that N cycling at the former breeding sites remains dominated by the seabird input, with subsequent N fixation being of little significance. Total Cd results from the breeding sites were similar to those reported recently for fertilised New Zealand pastoral soils. Thus, contributions from pre-European seabird breeding may match anthropogenic sources of Cd (e.g. superphosphate fertilisers) in many New Zealand locations.
Special thanks are due to the landowners whose properties were included in the study (J. and N. Dalmer, Annandale; R. McMillan, Arden; M. Endres, Ngarua). J. Whitton (Landcare Research Ltd) carried out and interpreted the mineralogical analyses. J. Warham (University of Canterbury) drew our attention to a number of pertinent references, and provided a copy of Okazaki et al. (1986). Analyses were paid for by grants from the Christchurch Polytechnic Academic Research Committee, and the School of Science. R.N.H. acknowledges financial support from the New Zealand Foundation for Research, Science and Technology under contract PLC501.
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Manuscript received 26 May 1998, accepted 7 August 1998
D. J. Hawke(A), R. N. Holdaway(B), J. E. Causer(A), and S. Ogden(A)
(A) School of Science, Christchurch Polytechnic, PO Box 22-095, Christchurch, New Zealand.
(B) Palaecol Research, PO Box 16-569, Hornby, Christchurch, New Zealand.
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|Author:||Hawke, D. J.; Holdaway, R. N.; Causer, J. E.; Ogden, S.|
|Publication:||Australian Journal of Soil Research|
|Date:||Jan 1, 1999|
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