And RICHARD A. RING Abstract

COMPARATIVE ECOLOGY OF TWO SPECIES OF HYDROPORUS (COLEOPTERA: DYTISCIDAE) IN A HIGH ARCTIC OASIS

ADMANM.H. DEBRUYN' Biology Department, McGill University, 1205 Doctor Penfield Avenue, MontrCal, Quebec, Canada H3A 1B1

and RICHARDA. RING Biology Department, University of Victoria, Box 1700, Victoria, British Columbia, Canada V8W 2Y2

Abstract The Canadian Entomologist 131: 405 - 420 (1999) At Alexandra Fiord, Ellesmere Island, the diving beetles Hydroporus morio AubC and Hydroporus polaris Fall occur in a series of shallow ponds. Detailed habitat measurements of a temporary and a permanent pond revealed a more complex and extensive organic substrate and vegetation community, longer developmental time, and greater thermal budget in the permanent pond. Hydroporus polaris was most abundant in the temporary pond, but occurred in both; this species oviposited in the absence of macrophytic vegetation, completed larval development quickly, and pupated in the drying pond substrate, and adults dispersed in fall to moister overwintering sites. Hydroporus morio was restricted to the single permanent pond; this species took longer to complete larval development, pupated in wet moss, and overwintered as adults encased in ice on the vertical pond edge. We hypothesize that H. morio is excluded from temporary ponds in the arctic by its requirement for a relatively long development time. Alternatively, H. morio may require sheltered overwintering sites that temporary ponds do not offer.

deBruyn AMH, Ring RA. 1999. ~colo~iecomparative de deux espkces d'Hydroporus (Co- leoptera; Dytiscidae) dans une oasis du Haut Arctique. The Canadian Entomologist 131 : 40.5420.

Dans le fjord d7Alexandra, dans l'ile d'Ellesmere, les dytiques Hydroporus morio AubC et H. polaris Fall habitent une sCrie d'Ctangs peu profonds. Des mesures dC- taillees de l'habitat dans un Ctang permanent et dans un Ctang temporaire ont rCvC1C que le substrat organique et la vCgCtation sont plus complexes et plus dCveloppCs dans 1'Ctang permanent et que la durCe du dCveloppement et le budget thermique y sont supCrieurs Cgalement. PrCsent dans les deux types d'Ctangs, Hydroporus polaris est abondant surtout dans 1'Ctang temporaire; l'espbce pond en l'absence de macro- phytes, la larve se dCveloppe rapidement, la nymphose se fait dans le substrat de 1'Ctang en voie de dessication et les adultes se dispersent B l'autornne pour gagner des sites plus humides pour passer l'hiver. Hydroporus morio n'a CtC trouvC que dans un seul Ctang permanent; son dCveloppement larvaire est plus long, l'espbce fait sa nymphose dans les mousses humides et les adultes passent l'hiver enfouis dans la glace sur la bordure verticale de 1'Ctang. Nous croyons qu'H. morio est ex- clu des Ctangs temporaires arctiques B cause de la durCe relativement longue de son dCveloppement. De plus, H. morio peut nCcessiter la prCsence de sites protCgCs pour passer l'hiver, ce que n'offrent pas les Ctangs temporaires. [Traduit par la Redaction]

Author to whom all correspondence should be addressed.

Downloaded from https://www.cambridge.org/core. UCL, Institute of Education, on 11 Mar 2018 at 10:33:46, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.4039/Ent131405-3 THE CANADIANENTOMOLOGIST

Introduction

High arctic ecosystems are generally considered to be biologically depauperate because of severe climatic conditions that approach the physical limits to life (Downes 1964). Lentic habitats in the arctic, however, are relatively benign. Ponds and lakes ex- hibit less seasonal and die1 temperature fluctuation than adjacent terrestrial habitats (Corbet 1972), and this moderating influence is reflected in an increasing ratio of aquatic to terrestrial insects with latitude (Danks 1987). Unfortunately, conditions and biota have been recorded in only a few arctic lakes and even fewer ponds (Corbet 1972; Hobbie 1973, 1980), and little is known about the factors that set the northern limit of distribution for aquatic species. Most authors have implicated the temperature and duration of the growing season, or the severity and stability of winter temperatures (Downes 1962; Corbet 1972; Danks 1990). Hydroporus polaris Fall (Coleoptera: Dytiscidae: Hydroporinae) is the most northerly distributed species of Dytiscidae in the Canadian high arctic archipelago (Danks 1981). The adult is small (3-3.5 mm in length) and the larva is of the creeping type. The adult of H. polaris was redescribed by Gordon (1969), but the larva has not, to our knowledge, been described. This species occurs in North America from Alaska to Baffin Island, and as far north as Lake Hazen near the northern tip of Ellesmere Is- land. The only other species of Dytiscidae reported in the Canadian high arctic is Hydroporus morio AubC (Danks 1981). The adult and larva are similar to those of H. polaris. The adult of H. morio was redescribed by Gordon (1969) and Larson (1975); the larva has been described by Jeppesen (1986) and Alarie (1991). The range of H. morio is circumpolar in the boreal zone, with scattered records in the arctic. The ranges of H. polaris and H. morio overlap at the treeline, and the two species co-occur at a few arctic sites. We report data collected at one of these sites in the Cana- dian high arctic. We combine structural, physical, and chemical data from two high arctic ponds with information on the life histories of H. polaris and H. morio to explore the factors that affect the local distribution of these species among ponds. Our objective is to generate hypotheses about the factors that determine the northern limit of distribution of these species.

Methods

Site Description. The Alexandra Fiord lowland (7S053'N, 75'551.17) is located on the east coast of Ellesmere Island, Nunavut Territory, Canada. It is formed by a postglacial outwash plain 800 ha in area, delimited in the south by nunataks and lobes of the Twin glacier, in the east and west by 500-600 m cliffs, and in the north by Alexandra Fiord. This site and several others comprising about 2% of the high arctic archipelago are cat- egorized as "polar oases" and are characterized by more solar radiation, more mesic habitats, more extensive soil and vegetational development, and greater species richness than the rest of the high arctic (Bliss 1977; Kukal 1994; Svoboda and Freedman 1994).

Dytiscid Collection. The populations of beetles studied inhabited a group of shallow, seasonally active ponds located on raised ocean beaches at the northern edge of the lowland (Fig. 1). Of these eight ponds, two were selected for intensive study: pond A contained H. morio and H. polaris, whereas pond B, about 50 m away, contained only H. polaris. One other pond about 25 m away from pond B contained H. polaris, but this pond was studied less intensively and these data are reported elsewhere (deBruyn 1994).

FIGURE1. Location of study ponds in the Alexandra Fiord lowland, Ellesrnere Island, Nunavut Terri- tory (7go53'N, 7S055'W).

Both ponds were searched thoroughly. Larval and adult beetles were collected directly from the pond with a dip net or pipet when weather permitted, or from grab Sam- ples of flocculent algae (pond A) which were sorted immediately in the laboratory. Samples of algae were usually collected by dip net. Beetle densities in pond A were es- timated in mid-June from 16 samples collected by lowering a metal cylinder (240 cm2 cross section) to the frozen substrate and removing all of the enclosed material. All of these samples were taken from the dense algal mats, where the vast majority of beetles were observed. It was not possible to perform quantitative sampling in the coarse, rocky substrate of pond B. In early June, adults of both species were collected in both ponds, and females were dissected to check for sperm in the spermatheca; this was considered to mark the earliest point of breeding for that population. Mature larvae,-pupae, and teneral adults of H. morio were collected by hand from the vertical, mossy north bank of pond A. Mature larvae, pupae, and teneral adults of H. polaris were collected from the substrate on the northern bank of pond B.

Limnology. Study ponds were surveyed by recording elevations on a 4 m grid with a sighting level and a ruled pole. These data were used to map the ponds and determine the period of submergence of vegetation. True north was established from a Geodetic Survey of Canada benchmark at the camp. Water temperatures were recorded in 1992 with a hand-held thermocouple reader at solar noon and midnight (at 1 to 4 d intervals) at 5 m intervals on a longitudinal transect through each pond. Solar noon and midnight temperatures were close to daily maxima and minima in ponds on Bathurst Island (Danks 1971) and were therefore used to approximate the true day mean in 1992. In 1993, daily maxima and minima were recorded in the pond areas where most dytiscids were observed. In pond A, a maximum-minimum thermometer was suspended so that the bulbs were about 5 cm below the water surface and 1-2 cm from the pond edge. In pond B, the plastic thermometer casing was rested on the bottom in 5-10 cm of water near the pond edge; the bulbs were thus about 2 cm above the pond bottom and at 3-8 cm depth. Thermometers were moved as necessary to maintain these depths as water levels receded. The thermometers were in opaque brown plastic cases which provided good shading from direct insolation and approximated the colour of the substrate.

algaelsedges 'sedge meadow"

FIGURE2. Distribution of vegetation in study ponds A (permanent) and B (temporary) at Alexandra Fiord, Ellesmere Island.

The maximum-minimum midpoint was used to estimate the true day mean of these microhabitats. Cumulative thermal budgets were calculated by summing these day mean estimates over the period in which the pond contained water; 1993 data were smoothed for plotting. Daily air temperature extremes were measured with a maximum-minimum thermometer housed in a meteorological screen at 1.5 m height. Pond day mean temperatures were compared with screen values with t tests paired by date. Pond pH was recorded daily with a hand-held digital pH meter at temperature sampling sites. Water depth was measured daily relative to a benchmark; absolute water depth was then calculated from the topographical survey. Depth of permafrost was measured on 29 June 1992 and weekly in 1993 with a permafrost probe at 10 m intervals on a longitudinal transect through each pond. Permafrost depths under each pond on 28 June 1993 were compared with those on 29 June 1992 with t tests paired by sampling site.

Results and Discussion

Pond Morphometry. Detailed pond morphometry is reported elsewhere (deBruyn 1994). Pond A was roughly oval in shape and about 58 m x 26 m (Fig. 2). Maximum depth was 45 cm. This pond resembled a very shallow tarn (sensu Oliver and Corbet 1966). The edges were nearly vertical, with a lush growth of mosses; the bottom was obscured by a thick layer of fine organic mud. Thickness of this mud layer varied as the active-layer depth changed, reaching a maximum thickness of about 30 cm in early Au- gust. In the northeastern part of the pond, this layer was very thin and the substrate there was a coarse mixture of sand and gravel. In the centre of pond A was a small island with typical hydric-terrestrial vegetation ("sedge meadow"). Pond B had two distinct oval basins (Fig. 2). The larger western basin was 62 m x 32 m, and the eastern basin 30 m x 21 m. Maximum depth was 67 cm. The substrate

- Pond B

15 June 1 July 15 July 1 Aug. 15 Aug. Date FIGURE3. Water level in 1992 (fine lines) and 1993 (bold lines) in study ponds A (permanent) and B (temporary) at Alexandra Fiord, Ellesmere Island.

was primarily bare, medium to large angular rocks, but patches of silt-sand occurred near the edges.

Water Level and Permafrost. Pond A is a permanent pond. This pond has never dried up in 17 years of observations (G. Henry, Department of Geography, University of Brit- ish Columbia, personal communication). Snow began to melt from the north edge of this pond prior to the first week of June in both years of the study. Ice melted away from the edges, then down from the surface; the last traces of ice disappeared by the last week of June in both years. The water level in pond A rose rapidly as the ice melted, reaching maximum depth on 14 June 1993 (Fig. 3). This level remained constant until late July in both years, when it began to decrease slowly. Permafrost under pond A began to melt first in 1993 at the west end, under the algal mats. This deepening of the active layer occurred slowly and broadened to form a substantial basin under the pond. The active basin expanded through June and July, began to stabilize in early Au- gust, and by mid-August had begun to shrink slightly under the centre of the pond. Pond B is a temporary pond; it dried up completely, well before the first persistent snow in August (Fig. 3). As in pond A, the snow cover melted in spring from north to south. Pond B had scattered snowmelt pools by 5 June 1993, and filled gradually with lowland meltwater. The well-developed active layer under this pond, combined with a loosely packed substrate, provided good drainage and it dried up by 17 August 1993. Pond B developed an extensive active layer well before it received meltwater. Precipitation is generally very low in the high arctic, and the exposed aspect of the raised beach ridges results in a thinner than average snow cover. The dark substratum of

Pond A

15 June 1 July 15 July 1 Aug. Date FIGURE4. Solar noon (open circles) and midnight (solid circles) water temperatures for 1992 in study ponds A (permanent) and B (temporary) at Alexandra Fiord, Ellesmere Island. Maximum and minimum screen air temperatures are also shown (upper and lower lines, respectively).

this pond probably received significant amounts of insolation even before it was snow free. The permafrost under pond B melted down much earlier than that under pond A, reaching depths of 40 cm in early June 1993 and providing effective drainage for spring meltwater from the surrounding lowland. This basin deepened slowly through June and July and began to shrink again by mid-August. Active-layer depths were an average of 8 cm shallower under pond A on 29 June 1992 (paired sample t test against 28 June 1993 data: t0a5(2)6= 2.59, P = 0.041) and 11 cm shallower under pond B (to.05(2)s= 2.85, P = 0.021). The 1992 profiles most closely resembled those collected several weeks earlier in 1993 (1-2 weeks for pond A and 2-3 weeks for pond B).

Temperature and pH. Pond temperatures (Figs. 4, 5) were consistently higher than those measured in the meteorological screen. Daily mean temperatures of both ponds exceeded screen day means by an average of 7°C in 1992 (pond A: t0,05(2)14= 13.76, P < 0.001; pond B: t0.05(2)18= 17.20, P < 0.001); microhabitat day means exceeded screen day means by an average of 10°C in 1993 (pond A: t0.05(2)68= 25.01, P < 0.001; pond B: to,05(2)55= 25.06, P < 0.001). For the period in which both ponds were active, their thermal regimens were similar. Both ponds experienced a maximum day mean temperature of around 22°C in late July and a minimum day mean temperature of 4.5"C in early August. The ponds had comparable season means (14-15°C). July day mean temperatures have been reported from ponds at several other sites in the high and low arctic: means range from 3 to 8°C

Pond A

Pond B

15 June 1 July 15 July 1 Aug. 15 Aug. Date FIGURE5. Maximum and minimum pond microhabitat temperatures for 1993 in study ponds A (permanent) and B (temporary) at Alexandra Fiord, Ellesmere Island (bold lines; see text for details of thermometer placement). Maximum and minimum screen air temperatures are also shown (upper and lower fine lines, respectively).

in permanent ponds on Bathurst Island, Northwest Territories (75"43'N, 98"28'W) (Danks 1971); from 4 to 13°C in permanent ponds near Cape Thompson (68"08'N, 165'58W) (Watson et al. 1966; Kalff 1968) and Barrow, Alaska (7l020'N, 156'46'W) (Stanley 1976; Miller et al. 1973); and from 6 to 14°C in both a temporary and a permanent pond near Lake Hazen, Ellesmere Island (81°49'N, 71°18W) (Oliver and Corbet 1966). Comparable values were recorded at Alexandra Fiord in 1992. In the microhabitats that were monitored in 1993, however, July day means ranged from 9 to 22°C. The extreme maximum water temperature reported for ponds near Barrow was 20°C (Stanley 1976; Miller et al. 1973), and the highest temperature reported from ponds near Cape Thompson was 16.4"C (Watson et al. 1966). Extreme maxima in microhabitats at Alexandra Fiord ranged from 33.0°C (pond B) to 373°C (pond A) on a bright, warm day in 1993. The Alexandra Fiord lowland has generally been described as a particularly warm, lush site (Svoboda and Freedman 1994), and these data illustrate that this description can be extended to the lowland's freshwater habitats as well. The cumulative thermal budget (in degree-days above 0°C) available to completely aquatic organisms such as larvae of Hydroporus spp. is a function of both daily mean temperature and site-water duration. Pond A was warmest in 1993, became active first, and remained active longest (Fig. 3); it therefore had a cumulative thermal budget 25% greater than that of pond B by 22 August (Table 1).

Pond A Pond B 1992 1993 1992 1993 Extreme maximum 37.5 33.0 (31 July) (27 July) Maximum day mean 15.1 21.8 15.0 20.8 (26 July) (31 July) (26 July) (31 July) Minimum day mean 6.7 4.5 7.3 4.5 (30 July) (9 Aug.1 (30 July) (9 Aug.) Extreme minimum -3.5 - -1.5 (21 Aug.) (7 Aug.) Season-mean 14.3 14.0 Accumulated degree-dayst 986.2 785.8 Maximum die1 amplitude 37.0 28.5 Season-mean amplitude 22.2 16.8

NOTE:Pond A values do not encompass the entire period of pond activity. * Missing values were not calculated due to incomplete data. t Threshold of 0°C. The study ponds were alkaline, with pH ranging from 7.9 to 8.9 in 1992 and from 8.5 to 10.2 in 1993 (Fig. 6). These values are considerably more alkaline than those reported in ponds at Barrow (pH 6.7 to 7.2; Kalff 1968; Prentki et al. 1980) or Cape Thompson (pH 5.8 to 7.7; Watson et al. 1966) and somewhat higher than those reported from ponds near Lake Hazen, where most temporary and permanent ponds ranged from pH 7.2 to 8.6 (Oliver and Corbet 1966). The trend of gradually increasing pH in both years suggests that these high values may reflect the contribution of carbonate to meltwater by the dolomitic shelf supporting the shallow granitic overburden of the Alexandra Fiord lowland. The difference in mean pH between ponds was far exceeded by the range of variation within each pond, both within and between years.

Vegetation and Invertebrate Richness. A dense hydric sedge community bordered much of the margin of pond A (Fig. 2). Along the northern edge the sedges overlapped with thick algal mats, while along the western and southwestern edges the sedges were interspersed within a mat of aquatic mosses. A dense, flocculent mass of Chlorophyta and Cyanophyta nearly filled the western half of the pond. Desmids were occasional, and diatoms rare (Table 2). A minimum richness of 18 animal morphospecies was recorded in pond A (Table 3). Cladocera were abundant in the open-water zone. A few chironomid morphospecies were abundant within the algal mats, as were Acarina and Cladocera. Few individuals of other taxa were observed. A sparse growth of hydric sedges occurred along the southern margin of pond B and in small patches elsewhere (Fig. 2). There was a sparse growth of filamentous Chlorophyta and several patches of aquatic moss, also along the southern pond margin. A mixture of Cyanophyta and Chlorophyta occurred in thin mats over much of the pond substrate. Desmids were occasional, and diatoms rare (Table 2). A minimum richness of 14 animal morphospecies was recorded in pond B (Table 3). The most abundant taxa in the water column were fairy shrimp, Cladocera, and, to a lesser extent, mosquito larvae. Enchytraeid worms were abundant in the substrate, as were Ostracoda and chironomid larvae.

Pond A

Pond 6

15 June 1 July 15 July 1 Aug. 15 Aug. Date FIGURE6. pH in 1992 (fine lines) and 1993 (bold lines) of study ponds A (permanent) and B (temporary) at Alexandra Fiord, Ellesmere Island.

Hydroporus rnorio. Hydroporus rnorio has been recorded from a variety of standing waters, including bogs, marshes, ponds, and lakes in Scandinavia and Greenland (reviewed in Bijcher 1988), stagnant upland mires in England (Eyre et al. 1986), and peatlands in North America (Larson 1987; Larson and House 1990). At Alexandra Fiord, H. rnorio was found only in study pond A. Hydroporus adults were most abundant in the algal mats at the north edge of pond A, frequently found perched within or on the underside of floating pieces of flocculent algae, or swimming to the surface. Adults were patchily distributed: densities in mid-June ranged from 1 to 72 H. rnorio (mean = 16.2, SD = 18.9) and 0 to 4 H. polaris (mean = 0.87, SD = 1.4) per 240 cm2 sample. The proportion of H. morio adults in all samples from pond A was high compared with that of H. polaris. In 23 grab samples of algae (a total of 841 beetles, most of which were returned to the pond), 96.8% of the adults collected in pond A were H. rnorio. This proportion was relatively constant throughout the season. For the purposes of determining phenology, it was therefore as- sumed that all larvae observed in pond A were H. rnorio. Larvae of Hydroporus spp. found in pond A were perched on the submerged parts of algal mats or sedges. On warm, sunny days, larvae were often observed crawling actively over the upper surface of the algal mats. In late autumn, third-instar larvae were often observed crawling on the mossy pond edge. Hydroporus rnorio overwinters in the adult stage. Adults were observed swimming in the newly melted pond A on 17 June 1992, and sperm were observed in the spermatheca of a female collected 2 days later (Fig. 7). The phenology of this species in 1993 was slightly earlier than in 1992. Adults were observed swimming and in copulo

TABLE2. Plant taxa observed in study ponds A (permanent) and B (temporary) at Alexandra Fiord, Ellesmere Island

Pond A Pond B Terrestrial mosses (Bryophyta) Campylium stellatum (Hedwig) C. Jensen (Amblystegiaceae) Dicranum sp. Hedwig 1782 (Dicranaceae) Aquatic mosses (Bryophyta) Aulacomnium turgidum (Wahlenberg) Schwaegr. (Aulacomniaceae) Aulacomnium turgidum Drepanocladus revolvens (Sw.) Warnstorf (Amblystegiaceae) Drepanocladus revolvens Brachythecium sp. Schimper 185 1 (Brachytheciaceae) Brachythecium sp. Sedges Carex aquatilus Wahlenberg (Cyperaceae) Carex aquatilus Eriophorum angustifolium Honck. (Cyperaceae) Eriophorum angustifolium Eriophorum scheuzcheri Hoppe (Cyperaceae) Eriophorum scheuzcheri Algae Mat-forming Cyanophyta Zygnemopsis sp. (Skuja) Transeau 1929 (Chlorophyta: Zygnematales) Nostoc sp. Vaucher 1803 (Cyanophyta: Spirogyra sp. Link 1820 (Chlorophyta: Nostocales) Zygnematales) Myxosarcina sp. Printz 1921 (Cyanophyta: Pleurocapsales) Nostoc sp. Oscillatoria sp. Vaucher 1803 Colonial Aphanocapsa sp. Nageli 1849 (Cyanophyta: Nostocales) (Cyanophyta: Chroococcales) Cosmarium sp. Corda 1834 (Chlorophyta: Desmidales) Cosmarium sp. Euastrum sp. Ehrenberg 1832 (Chlorophyta: Desmidales) Euastrum sp. Diatoms (Bacillariophyta) Diatoms

as early as 5 June 1993; sperm were recorded from the spermatheca of a female on 6 June 1993. Pupae were found in the vertical, mossy north edge of pond A, several centi- metres above the current water level. The sampling procedure required to uncover these pupae was destructive, and consequently few were collected. The first was collected on 6 August 1993, but pupae may have been present prior to this date. No newly emerged adults were found in this habitat in autumn, but many live adults were found in spring, trapped by ice under a thin layer of algae on the surface of the mossy bank.

Hydroporus polaris. Larson (1987) reported examining specimens of H. polaris collected from marshes. A more general description of the habitat of this species has not, to our knowledge, been reported previously in the literature. At Alexandra Fiord, H. polaris was collected from both study ponds and another temporary pond similar to pond B. Adults were most abundant in the shallow water at the north and west edges of pond B. On warm, sunny days, adults were often observed swimming actively along the sand-mud substrate among pieces of periphyton. On cooler days, adults were frequently

TABLE3. Number of macroinvertebrate morphospecies observed in study ponds A (permanent) and B (temporary) at Alexandra Fiord, Ellesmere Island

Taxon Pond A Pond B

Turbellaria: Tricladida 1 1 Nematoda - 1 Oligochaeta: Enchytraeidae 1 1 Cmstacea: Branchiopoda: Cladocera 1 1 Crustacea: Branchiopoda: Anacostraca - 1 Crustacea: Copepoda 1 - Crustacea: Ostracoda - 1 Arachnids: Acari 1 - Insecta: Diptera: Culicidae 2 1 Insecta: Diptera: Chironomidae 7 4 Insecta: Diptera: Empididae 2 1 Insecta: Diptera: Dolichopodidae - 1 Insecta: Coleoptera: Dytiscidae 2 1

...... ,993 ; >...... ?-.I,'.' <(_ ...... L...... -...... ,.:__.-.-.-_ ...... _/" ...... -

......

- .* ...... 1992 '.A...... - .. L......

......

15 June 1 July 15 July 1 Aug. 15 Aug. Date FIGURE7. Phenology of Hydroporus morio in 1993 and 1992 in study pond A (permanent) at Alexandra Fiord, Ellesmere Island. M, first mating; I, 11, and 111, larval instars; PP, prepupae; P, pupae; A, newly emerged adults. The dotted lines represent the relative water level in the pond (Fig. 3).

found in the interstitial spaces in gravel or under flat-bottomed rocks at the water's edge. Beetles were only observed in the deep, central pond water in early spring; the few individuals recorded here were the only ones ever observed to surface for air. Larvae were found only in the shallow water at the north and west edges of pond B. On warm, sunny days, they were observed perched on sloping rock faces in shallow water. On cooler days, larvae were found only in the interstitial spaces in gravel at the water's edge. Hydroporus polaris also overwinters in the adult stage. Adults were observed swimming in small snowmelt puddles in pond B as early as 13 June, and in copulo on 26 June 1992; in 1993, adults were observed swimming and in copulo as early as 5

......

15 June 1 July 15 July 1 Aug. 15Aug. Date FIGURE8. Phenology of Hydroporus polaris in 1993 and 1992 in study pond B (temporary) at Alexan- dra Fiord, Ellesmere Island (symbols as in Fig. 7). The dotted lines represent the relative water level in the pond (Fig. 3).

June (Fig. 8). Oviposition sites were not found. No first-instar larvae were observed in this species, although the habitat was searched thoroughly from the end of June in both years. As the water level receded in pond B, third-instar larvae were observed to remain in the exposed substrate, where they formed spherical pupation cells about 3 mm in di- ameter in the moist sand under rocks. The larvae pupated and had begun to emerge as adults in these cells by 8 August in 1992 and as early as 23 July in 1993. Many adults were observed leaving these pupation cells. Subsequently, aggregations of newly emerged adults were found in moist, alga-filled depressions in the substrate of another pond, several metres from the nearest abandoned pupation cells.

Factors Affecting the Local Distribution of H. norio and H. polaris. Dytiscidae are well adapted to the constraints imposed by the temporary nature of many pond habitats, but species seem to have different requirements regarding site-water duration. De- gree of habitat permanence has been cited by several authors as an important factor in the distribution of water beetle species, and by some authors as the most important (Galewski 1971). Wiggins et al. (1980) considered varying site-water duration to be the most important biological determinant among all temporary pond habitats. Hydroporus polaris was found only in the three most persistent ponds at Alexandra Fiord (A.M.H.D., personal observation,); H. morio was found only in the single permanent pond. This pattern suggests an effect of site-water duration on distribution. It is uncertain, however, whether this effect is direct or indirect. For example, degree of permanence affects total thermal budget, vegetation and substrate development, and, directly and indirectly, the occurrence of invertebrate prey species (Millar 1973; Driver 1977). Alternatively, the length of time a pond is active may constrain species based solely on the time in which they are able to complete their development. Both species studied were overwintering spring recruits (group 2 of Wiggins et al. 1980; type 1 life cycle of Nilsson 1986a). Hydroporus morio in pond A, despite a "head start" of nearly a week in 1993, completed its larval development later than H. polaris

in pond B. First- and second-instar H. rnorio were still present in pond A in early and late July, respectively, whereas all H. polaris in pond B had developed to third instar sometime before 1 July. Third-instar H. rnorio larvae appeared several days later, and were still active in late August, when no H. polaris larvae could be found. This indi- cates a minimum developmental time constraint on H. rnorio which is longer than that for H. polaris. Data from 1992 seem to indicate the reverse: third-instar larvae and prepupae were observed in H. morio earlier in the season than in H. polaris. It is important to note, however, that the period of observation in 1992 did not encompass the whole period of pond activity. Furthermore, that year was widely recognized as having an atypically cool and compressed summer. Pond B became active later than in 1993 and was slower to fill; it is unknown when pond A became active. Both species reached third instar later than in 1993. This between-years difference was greatest for H. polaris, suggesting that whatever combination of conditions was responsible for this delay may have been most severe in pond B. Degree of permanence also affects the nature of the environment in which invertebrates remaining at the site must overwinter. Braasch (1989) classified adult overwintering dytiscids into three types: (1) those that overwinter totally or almost totally in water, (2) those that overwinter either in water or in a wet habitat near water, and (3) those that hibernate exclusively on land (often far from water). He placed all 14 species of Hydroporus for which he had information into the first category. In autumn, newly emerged H. polaris adults were observed to leave their pupation cells and aggregate in still-moist parts of the pond B basin. In early spring, adults were collected from the dry substrates of several temporary ponds in the study area, including pond B, despite their absence from any but two for the remainder of the season. Over winter, then, these beetles inhabit sites where they must resist both low temperature and desiccation. Hydroporus rnorio adults were only collected from the recently melted edges of pond A in spring. Hydroporus polaris adults were also collected there in spring, but it is unknown whether these adults had overwintered there or dispersed to this pond in spring. Several northern terrestrial Coleoptera have been shown to be freezing tolerant (Ring 1981), but Moore and Lee (1991) note that very few aquatic insects are freezing tolerant, and few show more than a limited ability to supercool. Behav- ioural avoidance of ice or the capacity to remain unfrozen while encased in ice may be particularly important for these species. Unfortunately, no cold-hardiness data are available for these aquatic beetles. Water temperature is important in determining mortality, growth rate, fecundity, survival, and distribution of aquatic invertebrates (Vannote and Sweeny 1980; Williams 1991). Danks (1981) noted that prolonged life cycles in arctic insects have been attrib- uted primarily to temperature. Dytiscidae at Alexandra Fiord, however, were semivoltine. Development of many aquatic invertebrates can occur at or near 0°C (Downes 1964); the insects studied here have access, during their developmental period, to generally high habitat temperatures. In pond B, temperatures were almost al- ways above 0°C when water was present. Minimum temperatures in pond A were slightly below freezing from early August onwards, but maxima and day means were comparable to those measured in July and probably remained high enough to permit development until shortly before freezeup. The developmental window available to the fauna of these ponds is primarily, therefore, a function of site-water duration. Hydrogen ion concentration has been cited by many authors as an important factor in the distribution of many aquatic insects, including Dytiscidae (Cuppen 1983, 1986; Eyre et al. 1986, 1990). However, Juliano (1991) found no effect of pH on species richness or evenness, and no clear relationship of individual species' abundances with pH. In addition, he reported a lack of agreement between individual species' abun- dance patterns with respect to pH and those documented by Cuppen (1986). Similar

inconsistencies were found in this study. In northeast England, H. rnorio inhabits upland mires and has been referred to as an acid-water species (Eyre et al. 1986); in North America, this species is generally regarded as characteristic of peatlands and is associ- ated with accumulated organic material and low pH (Larson 1987; Larson and House 1990). The strongly alkaline waters at Alexandra Fiord, however, seem to provide suit- able habitat. This suggests that pH may be less important than other factors in the distribution of this species; almost certainly, the very small pH differences between our study ponds are negligible. Galewski (1971) emphasized the importance of aquatic vegetation to water beetles, especially those species with endophytous oviposition requirements. Emergent vegetation in pond B generally follows the contour 20 cm below maximum water level; this region was submerged between 30 June and 4 August in 1992 and 1993. Hydroporus polaris eggs hatched well prior to 30 June and must therefore have been laid on some other substrate, possibly in the extensive mats of algae. Hydroporus rnorio almost certainly cannot complete its development in 35 days, and is therefore excluded from pond B if it cannot oviposit on other substrates. The oviposition sites of H. polaris and H. rnorio have not been reported and were not discovered in this study. Galewski (1971) also noted that substrate type affects the distribution of Dytiscidae, because larvae of many species of Hydroporinae tend to hide in organic debris. In our study, larval H. polaris were found in the interstitial spaces of gravel as often as among organic debris. As in other studies (Eyre et al. 1986; Larson 1987; Larson and House 1990), larval H. rnorio were found only in association with organic material, but it is uncertain whether they would be able to survive in a simpler, less organic substrate. Mature larvae of H. polaris were observed to build pupation cells in the exposed muddy substrate as the water level of pond B receded. Pond A offers no similar substrate in which to build these cells. If H. polaris larvae are unable to pupate in the mossy banks of pond A, they may not persist there from year to year and the few H. polaris adults observed in this permanent pond may have come from elsewhere. Another possible explanation for the rarity of H. polaris in pond A is exclusion by competition with the more abundant H. rnorio. However, competitive exclusion between ecologically similar dytiscids is generally considered to be of minor importance in temporary habitats (Larson 1985; Nilsson 1986b). Temporary ponds have intense, short-term pulses of productivity in spring and early summer; prey are abundant and predators are probably not food limited (Larson 1985). Nilsson and Svensson (1994) placed all 15 Hydroporus species for which they had information, including H. rnorio, into guild 1 (sensu Nilsson 19863): adults feed mainly on culicid and chironomid larvae; larvae feed on cladocerans, copepods, and small chironomid larvae. Although prey densities were not quantified, these taxa were obviously abundant in pond A, suggesting that Hydroporus may not be food limited. Furthermore, the few studies of field evidence reviewed by Lawton and Hassell (1984) provide only weak and indirect evidence of interspecific competition in predatory Coleoptera. To explain the pattern of occurrence of Hydroporus species in ponds at Alexandra Fiord, and the restricted distribution of H. rnorio in the arctic, we propose the following testable hypotheses that could be the focus of future experimental or manipulative studies on these species: 1. Hydroporus rnorio is restricted to permanent ponds in the arctic by a relatively long development time requirement. This species is thus excluded from arctic sites that lack permanent standing waters. 2. Hydroporus rnorio requires a sheltered overwintering site such as that found between a mossy pond edge and a large volume of frozen pond water and is thus excluded from sites that do not offer this protection (such as temporary ponds).

3. Mature larvae of H. polaris require an appropriate substrate in which to build pupation cells. A pond with a muddy or sandy bottom provides this substrate, but a pond with mossy edges does not. Hydroporus polaris cannot, consequently, persist in such a pond and will occur there only if adults disperse from a nearby pond with appropriate pupation sites.

Acknowledgments We are grateful to Olga Kukal and Greg Henry for support and collaboration at Alexandra Fiord. We also thank Alan Austin, Cori Barraclough, Darcy Grant, and Pat- rick Lucey for identifying algae, and Mike Ryan for identifying mosses. The manuscript was much improved by suggestions from two anonymous reviewers. This work was supported financially by a Postgraduate Scholarship from the Natural Sciences and En- gineering Research Council of Canada (NSERC) and Northern Scientific Training Pro- gram awards to A.M.H.D. Logistical support was supplied by the Polar Continental Shelf Project. Additional financial support was provided by NSERC and the University of Victoria to R.A.R.

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(Date received: 20 May 1998; date accepted: 16 December 1998)