Northern Lake Superior Research Articles

Cyanobacteria in cold waters: A study of nearshore cyanobacteria assemblages in Lake Superior

Cyanobacterial blooms, often associated with warm, eutrophic lakes, are of widespread concern owing to their potential disruption of ecological and public health. Recently, cyanobacterial blooms have been observed in oligotrophic lakes, including Lake Superior, a large cold-water system. Despite recent developments, limited research has targeted phytoplankton assemblages in the northern Lake Superior nearshore, where isolated cyanobacteria blooms have emerged since 2019. In 2019, the cyanobacteria assemblage at 10 nearshore monitoring stations was examined to understand potential bloom-forming taxa and their association with water quality. We observed a diverse cyanobacteria assemblage, with highest densities in spring and biomass in fall. Pseudanabaena limnetica, Dolichospermum flos-aquae and Limnothrix sp. were the most prevalent species across sites and seasons. Variation partitioning analysis revealed that water quality conditions were more influential drivers of cyanobacteria density and biomass than meteorological factors, particularly in regions with elevated nutrient inputs and following stormy conditions. However, at present, cyanobacteria compose <15 % of the total phytoplankton biomass in the study areas. Our monitoring data and recent reports of cyanobacteria blooms on the north shore, suggests that current blooms are not widespread. Nonetheless, we found a diverse array of cyanobacteria taxa, with most capable of producing cyanotoxins. Considering the global uncertainty in the factors contributing to cyanobacteria blooms, in association with the large-scale climatic changes affecting Lake Superior, a proactive approach to assessing risks of blooms is suggested. This should include data-generating efforts (e.g., frequent phytoplankton monitoring and reported blooms investigations) to support future collaborative initiatives focused on managing cyanobacterial blooms.

The Ordovician meteorite event in North America: Age of the Slate Islands impact structure, northern Lake Superior, Ontario, Canada

AbstractThe Slate Islands (Ontario) is one of Canada's larger impact structures at 32 km in diameter and has been linked to the Ordovician meteorite event (OME). We report zircon U–Pb dates from two suevite and two syenite samples collected from the Slate Islands. Plagioclase 40Ar/39Ar dates were also obtained from one of the samples. The plagioclase and most zircon dates record pre‐impact ages with links to known tectonic events, including those associated with the assembly of the Superior Craton at approximately 2700 Ma. However, Neoarchean zircon grains appear to be reset at 456.1 ± 6.9 Ma (±2σ) based on the lower intercept of discordia for all dated samples. The date overlaps its previously accepted age of 450 Ma and would be 2–19 million years following the parent asteroid breakup if related to the OME.

Coastal archaeological site visibility problems and underwater prospects in the Northern Lake Superior Basin

AbstractThe early to middle Holocene was marked by considerable variability in lake levels in the Lake Superior Basin due to a combination of meltwater runoff, isostatic adjustment, shifting glacial margins, and climate change. These processes likely had a large impact on the preservation and visibility of coastal archaeological sites dating from the Early Period (Paleoindian) to the Middle Period (Shield Archaic). Of particular interest is the brief interval after 9300 cal. B.P. when ancestral Lake Superior dropped to its lowest level (Houghton) and human populations may have made incursions deeper into the basin. Elsewhere in the Upper Great Lakes, this period is associated with offshore archaeological sites submerged by rising water levels later in the Holocene. New geological data from the Thunder Bay, ON, region yield exceptional insight into the paleohydrology of the Houghton phase and, hence, the underwater archaeological prospects of this low water phase in the northern Lake Superior Basin. These data indicate that the lake reached its lowest level by at least ~9100 cal. B.P. but was highly unstable, at least initially, due to a combination of climate and meltwater runoff. Early underwater sites may be confined to two short, hydrologically closed, lowstands between ~9100 and 8700 cal. B.P. and would have been impacted by at least one lake transgression. Such sites, however, may still hold better potential for organic preservation and the visibility of large cultural features compared to their terrestrial counterparts. Coastal sites occupied when the lake was hydrologically closed may be especially well‐preserved due to rapid inundation before the gradual, and generally erosional, Nipissing transgression occurred.

Lipid levels and diet compositions in lake charr ecotypes at Isle Royale in northern Lake Superior

Lipid levels have been reported for lake charr Salvelinus namaycush ecotypes from Lake Superior, but have not been investigated for all four extant ecotypes concurrently in the same geographic area. In this study, a Distell fatmeter was used to measure lipid levels in the muscle in lean, humper, redfin, and siscowet lake charr collected seasonally during 2013 and 2014 on the north, east, and west sides of Isle Royale, Lake Superior from two depth strata: <80 and ≥80 m. Diet compositions and dietary similarity of lake charr were also measured. Differences in lipid level were analyzed by ANCOVA with length and age as covariates and ecotype, depth, sex, and season as factors. Lipid levels were found to be higher in lake charr inhabiting waters ≥80 m. In waters <80 m, mean lipid level was highest in large siscowets (≥703 mm; 41.2%) and lowest in old redfins (≥age 32; 11.8%). At depths ≥80 m, mean lipid levels were highest in large siscowets (64.4%) and lowest in young redfins (14.7%). In both depth strata, leans and humpers had intermediate lipid levels. Diet compositions were similar among ecotypes and did not explain the differences in lipid levels. Siscowets have the greatest depth range and vertical movement patterns among lake charr ecotypes in Lake Superior which is likely facilitated by high levels of intramuscular lipid. Lipid levels in leans and siscowets are heritable traits, but heritability of lipid levels in other ecotypes remains unknown.

Integrated geophysical analysis provides an alternate interpretation of the northern margin of the North American Midcontinent Rift System, Central Lake Superior

The Midcontinent Rift System (MRS) is a 1.1 Ga sequence of voluminous basaltic eruptions and multiple intrusions followed by widespread sedimentation that extends across the Midcontinent and northern Great Lakes region of North America. Previous workers have commonly used seismic-reflection data (Great Lakes International Multidisciplinary Program on Crustal Evolution [GLIMPCE] line A) to demonstrate that the northern rift margin in central Lake Superior developed as a normal growth fault that was structurally inverted to a reverse fault during a compressional event after rifting had ended. A prominent, curvilinear aeromagnetic anomaly that extends from Isle Royale, Michigan, to Superior Shoal in central Lake Superior, Ontario (the IR-SS anomaly), is commonly presented as a manifestation of this reverse fault. We have integrated multidisciplinary geophysical analyses (seismic-reflection, seismic-refraction, aeromagnetic, and gravity), physical-property information (density, magnetic susceptibility and remanence, and compressional-wave velocity), and geologic concepts to develop an alternate interpretation of the rift margin along GLIMPCE line A, where it intersects the IR-SS anomaly. Our new model indicates that a normal fault is the dominant structure at the northern rift margin along line A, contrary to the original rift-margin paradigm, which asserts that compressional structures are the dominant features preserved today. Integral to this alternate model is a newly interpreted, prerift sedimentary basin intruded by sills in northern Lake Superior. Our alternate model of the northern rift margin has implications for interpreting the style, scale, and timing of extension, rift-related intrusion, and compression during development of the MRS.

EARLIEST MICROBOTANICAL EVIDENCE FOR MAIZE IN THE NORTHERN LAKE MICHIGAN BASIN

There is no recorded maize (Zea maysspp.mays) from sites predating circa cal AD 800 in the northern Lake Michigan or Lake Superior basins of the western Great Lakes, despite the presence of maize microbotanicals including phytoliths and starches in Michigan, New York, and Quebec as early as 400 cal BC. To evaluate the potential for an earlier maize presence in the northern Lake Michigan basin, samples of carbonized food residues adhering to 16 ceramic vessels were obtained from the Winter site (20DE17) located on the Garden Peninsula in the northern Lake Michigan basin. Each sample was split and sent to two analysts. Both analysts identified low incidences of maize starch and phytoliths in multiple samples, with overlapping identifications on several. Three direct accelerator mass spectrometry dates on the carbonized residues reveal maize incorporated into the residues as early as the second century cal BC, 800 years before any regional macrobotanical evidence. Although the method of dispersal cannot be determined, these results support the proposition that initial northern dispersal of maize in the region may have been nearly 800 years earlier than macrobotanical evidence would suggest and is consistent with the timing of its introduction to the lower Great Lakes area.

Reproduction of Lake Trout Morphotypes at Isle Royale in Northern Lake Superior

AbstractThe reproductive biology of male and female lean, siscowet, redfin, and humper Lake Trout Salvelinus namaycush was assessed by measuring the gonadosomatic index (GSI) and by histological analysis of the gonads over 2 years from spring to fall at sites surrounding Isle Royale National Park, Lake Superior. Whole livers, otoliths, and ovaries were also collected to measure hepatosomatic index (HSI), age, and fecundity. In general, GSIs increased from spring to summer in male and female siscowets, leans, redfins, and humpers, and this was accompanied by a large increase in the number of vitellogenic (&gt;2.0 mm) follicles in females. Postspawned ovaries and regressing testes were predominant in all morphotypes by October but were also observed in September, indicating that some spawning occurred at least by September. Further, preovulatory siscowet females (GSIs &gt; 10) and siscowet males with fully mature and even regressing testes were observed in the spring and summer, indicating that spawning occurs at least in the spring and fall in siscowets at Isle Royale. For all morphotypes, average male HSIs were significantly lower than female HSIs. Absolute fecundity was positively related to size and age for all morphotypes. Humpers had the lowest absolute fecundity. Leans, redfins, and siscowets had similar fecundity relationships. The results conclusively demonstrate that deepwater siscowet Lake Trout reproduce at least during the spring and fall at Isle Royale and that leans, humpers, and redfins reproduce during the fall. In lakes that exhibit sympatric populations of deepwater and shallow‐water ecotypes of Arctic Char Salvelinus alpinus, the deepwater form also spawns in both spring and fall. Our data and those on Arctic Char suggest that there is an adaptive strategy for maintaining alternative spawning times in the deepwater ecotypes in these populations.Received June 20, 2016; accepted November 2, 2016 Published online January 31, 2017

Injury Patterns at Isle Royale National Park: An Epidemiologic Review of Injuries and Illnesses Sustained in a Remote Environment

Isle Royale National Park is a remote island in northern Lake Superior that attracts 16,000 visitors annually. The epidemiology of injuries and illnesses sustained by Isle Royale׳s visitors has not been previously studied.The purpose of this study is to examine these data and evaluate them for injury patterns. This is a retrospective observational study examining the epidemiology of injuries and illnesses sustained during the period from 2008 to 2012. Incident reports completed by park rangers were reviewed and the data sorted according to time of year, time of day, type of medical encounter, and whether the patient was stable, unstable, or required transport. Two hundred and seventy patient care reports were obtained from the National Park Service. Sixty-four percent of encounters occurred in July and August, and most patients sought care in the afternoon. Care was provided by park rangers, the majority of whom were trained to the level of emergency medical technician. Fifty-eight percent of cases were trauma related, and 20% of all cases were evacuated. The majority of incidents were trauma related. The majority of the rangers on the island are trained to the level of emergency medical technician-B and appear to offer appropriate care to the island's many visitors, utilizing the National Park Service treatment protocols and comprehensive medical kits. In addition, access to advanced medical care is readily available by air and water evacuation.

Genetic Assessment of Lake Sturgeon Population Structure in the Laurentian Great Lakes

AbstractMany populations of lake sturgeon Acipenser fulvescens have decreased in size throughout the Great Lakes basin. To implement management strategies such as stocking, it is important to understand the genetic structure of lake sturgeon spawning populations. Lake sturgeon from 27 spawning locations (25 from the Great Lakes basin and 2 from the Hudson Bay drainage) were analyzed using 12 microsatellite loci. Population structure was detected at different spatial scales. At the largest scale, consistent genetic breaks were observed among three clusters of spawning populations: (1) Hudson Bay–northern Lake Superior, (2) southern Lake Superior, and (3) the rest of the Great Lakes. These clusters were identified using a Bayesian approach that does not define the populations a priori. Within each of the three clusters, sublevels of genetic structure were detected. These sublevel clusters accounted for 8.82% of the genetic variation (P &lt; 0.000), while differences among populations within the clusters accounted for 3.72% of the genetic variation (P &lt; 0.000). At the smallest scale, significant genetic differentiation was detected between most sampled locations through pairwise genetic differentiation index (FST) tests and pairwise contingency tests. Lake sturgeon showed greater genetic differentiation in Lake Superior than elsewhere, which could be due to the lake's bathymetry. The lower genetic resolution observed elsewhere in the Great Lakes could be due to more recent colonization events. The results can be used to delineate management units and to select appropriate donor populations for supplementation or reintroductions.

Remagnetization (∼1100 Ma) of Voosges lake tuff (1542 Ma) and other mid-proterozoic rocks, Nipigon graben, northern Ontario

Recently discovered horizons of non-deformed felsic welded crystal tuff in the Proterozoic rift system of northern Ontario yield a geologically stable paleomagnetic vector. Using alternating field (AF), thermal (TD) and low-temperature demagnetization (LTD) techniques, a single stable characteristic remanence (ChRM) vector is found in all specimens (declination/inclination = 115°/−73.5°; α 95 = 2.1°, n = 58). It is overprinted by a Bruhnes’ epoch VRM. The tuffs paleopole position coincides with that of ubiquitous sub-horizontal diabase sheets which intruded at ∼1110 Ma during Late Proterozoic Keewanawan Volcanism. The latter produced numerous volcanic and intrusive rocks along the margins of the Lake Superior paleo-rift system, during the interval 1087–1120 Ma, but the sills are most extensive. Although the Voosges Lake Tuff shares the same paleopole location as the diabase sheets (1109 Ma), the nearby Seagull pluton (1112 Ma) and the Keweenawan volcanic rocks, its primary zircons yielded a Mesoproterozoic crystallization age of 1542 ± 3 Ma. This is similar to the nearby English Bay felsic intrusions dated at 1537 − 2 + 10 Ma [Davis, D.W., Sutcliffe, R.H., 1985. U-Pb ages from the Nipigon “Plate” (sic) and northern Lake Superior. Geol. Soc. Am. Bull. 96, 1572–1579]. Whereas the paleomagnetic magnetic signal appears primary in the 1542 Ma Voosges Tuff, it was completely reset by the Late Proterozoic Keewanawan paleofield (∼1100 Ma). The Tuff units are separated from the diabase sills by stratigraphic thicknesses of ≤50 m and show no traditional petrographic evidence of annealing or thermal metamorphism. Since it is improbable that the 1542 Ma tuff returned to the same paleolatitude at 1109 Ma, we postulate that metasomatic processes caused a ∼1109 Ma remagnetization. A similar event could explain unusual ChRMs reported for certain other pre-Keweenawan Proterozoic sites with adjacent Keewanawan diabase sheets. For example, some paleopoles reported for the Gunflint Formation (1878 Ma) and Sibley Formation (>1339 Ma, perhaps ∼1537 Ma by association with the English Bay intrusions) might have been acquired due to secondary remagnetization.

A post-Calumet shoreline along southern Lake Michigan

The southern shore of Lake Michigan is the type area for many of ancestral Lake Michigan’s late Pleistocene lake phases, but coastal deposits and features of the Algonquin phase of northern Lake Michigan, Lake Huron, and Lake Superior are not recognized in the area. Isostatic rebound models suggest that Algonquin phase deposits should be 100 m or more below modern lake level. A relict shoreline, however, exists along the lakeward margin of the Calumet Beach that was erosional west of Deep River and depositional east of the river. For this post-Calumet shoreline, the elevation of basal foreshore deposits east of Deep River and the base of the scarp west of Deep River indicate a slightly westward dipping water plane that is centered at ∼184 m above mean sea level. Basal foreshore elevations also indicate that lake level fell ∼2 m during the development of the shoreline. The pooled mean of radiocarbon dates from the surface of the peat below post-Calumet shoreline foreshore deposits indicate that the lake transgressed over the peat at 10,560 ± 70 years B.P. Pollen assemblages from the peat are consistent with this age. The elevation and age of the post-Calumet shoreline are similar to the Main Algonquin phase of Lake Huron. Recent isostatic rebound models do not adequately address a high-elevation Algonquin-age shoreline along the southern shore of Lake Michigan, but the Goldthwait (1908) hinge-line model does.

Breccia formation at a complex impact crater: Slate Islands, Lake Superior, Ontario, Canada

The Slate Islands impact structure is the eroded remnant of a ∼30–32 km-diameter complex impact structure located in northern Lake Superior, Ontario, Canada. Target rocks are Archean supracrustal and igneous rocks and Proterozoic metavolcanics, metasediments, and diabase. A wide variety of breccias occurs on the islands, many of which contain fragments exhibiting shock metamorphic features. Aphanitic, narrow and inclusion-poor pseudotachylite veins, commonly with more or less parallel boundaries and apophyses branching off them, represent the earlist breccias formed during the compression stage of the impact process. Coarse-grained, polymictic clastic matrix breccias form small to very large, inclusion-rich dikes and irregularly shaped bodies that may contain altered glass fragments. These breccias have sharp contacts with their host rocks and include a wide range of fragment types some of which were transported over minimum distances of ∼2 km away from the center of the structure. They cut across pseudotachylite veins and contain inclusions of them. Field and petrographic evidence indicate that these polymictic breccias formed predominantly during the excavation and central uplift stages of the impact process. Monomictic breccias, characterized by angular fragments and transitional contacts with their host rocks, occur in parautochthonous target rocks, mainly on the outlying islands of the Slate Islands archipelago. A few contain fragmented and disrupted, coarse-grained, polymictic clastic matrix breccia dikes. This is an indication that at least some of these monomictic breccias formed late in the impact process and that they are probably related to a late crater modification stage. A small number of relatively large occurrences of glass-poor, suevitic breccias occur at the flanks of the central uplift and along the inner flank of the outer ring of the Slate Islands complex crater. A coarse, glass-free, allogenic breccia, containing shatter-coned fragments derived from Proterozoic target rocks (upper target strata), observed at two locations may be analogous to the ‘Bunte Breccia’ of the Ries crater in Germany. At one of these locations, this breccia lies close to a crater suevite deposit. At the other, it overlies parautochthonous, monomictic breccia. The Slate Islands impact breccias are superbly exposed, much better than breccias in most other terrestrial impact structures. Observations, including those indicative of multiple and sequential processes, provide insight on how impact breccias form and how they relate to the various phases of the impact process. Eventually they will lead to an improved understanding of planetary impact processes.

New constraints on the Slate Islands impact structure, Ontario, Canada

The Slate Islands in northern Lake Superior represent the eroded remains of a complex impact crater, originally approximately 32 km in diameter. New field studies there reveal allogenic crater fill deposits along the eastern and northern portions of the islands indicating that this 500-800 Ma impact structure is not as heavily eroded as previously thought. Near the crater center, on the western side or Patterson Island, massive blocks of target rocks, enclosed within a matrix of fine-grained polymict breccia, record the extensive deformation associated with the central uplift. Shatter cones are a common structural feature on the islands and range from less than 3 cm to over 10 m in length. Although shatter cones are powerful tools for recognizing and analyzing eroded impact craters, their origin remains poorly constrained.

Transparency Calibrations for Lake St. Clair and Lake Michigan

Extensive measurements of water transparency using a Sea Tech transmissometer and total suspended solids (TSM) were made in the southern basin of Lake Michigan and in Lake St. Clair. The relationship between transparency and TSM in each lake can be expressed very well by a single equation. The similarity in the slopes of the lines for the two lakes suggests that the physical properties of the particles suspended in the water are very similar, while the large difference between the intercepts is due to the presence in Lake St. Clair of very fine material (particle diameter less than one micron) which is not present in Lake Michigan. Attempts to use the transparency measurements to identify different particle populations in Lake Michigan as a function of time, station, and sample depth were unsuccessful. Limited data from northern Lake Michigan, Lake Huron, and Lake Superior suggest that the material in Lake Huron is similar to that in the southern basin of Lake Michigan while the observations in northern Lake Michigan more closely resemble those from Lake Superior.

Attached Filamentous Algae of Northern Lake Superior: Field Ecology and Biomonitoring Potential During 1983

A survey of attached filamentous algae along the Canadian shoreline of northern Lake Superior was carried out in late August, 1983. Consistent with earlier surveys of the American northwestern shoreline, Ulothrix zonata was identified as the dominant taxon, although most substrata were largely free of attached algae, as is characteristic of highly oligotrophic conditions. A variety of minor green and blue-green algae were also identified. It is expected that the basinwide distribution of U. zonata will directly reflect any future increases in nearshore nutrient status. In contrast, it is not expected that Cladophora will ever widely occur in Lake Superior, irrespective of nearshore enrichment, because water temperatures are apparently prohibitively low. U. zonata samples from three Lake Superior shoreline sites were analyzed for elemental and polychlorinated biphenyl (PCB) concentrations. In general, Lake Superior water-to-algae elemental concentration factors (10 -2-10 -5 ×) were similar to those from other locations in the Great Lakes. Elevated concentrations of Fe (1.40 mg/g), Al (0.54 mg/g), and V(16.0 μg/g) in the vicinity of Thunder Bay Harbour and the mouth of the Kaministikwia River are indicative of point source loadings, while detection of PCBs (<20–40 ng/g) at all three Lake Superior sites is suggestive of widespread low-level contamination.

Statistical estimation of a shock center: Slate Islands astrobleme

Statistical models are proposed for the distribution of directions in three dimensions that are thought to point toward a single source. These models are based on the Fisher distribution. The method of maximum likelihood is used to obtain an estimate of position of the source and to provide corre-sponding confidence regions. When applied to shatter cone data from the Slate Islands, northern Lake Superior, the method yields estimates comparable with those obtained by Stesky and Halls (1982), as well as statistically valid confidence regions.

Atmospheric deposition of polycyclic aromatic hydrocarbons to water surfaces: A mass balance approach

A mass balance model was developed to explain the movement of polycyclic aromatic hydrocarbons (PAH) into and out of Siskiwit Lake, which is located on a wilderness island in northern Lake Superior. Because of its location, the PAH found in this lake must have originated exclusively from atmospheric sources. Using gas Chromatographie mass spectrometry, 11 PAH were quantified in rain, snow, air, lake water, sediment core and sediment trap samples. From the dry deposition fluxes, an aerosol deposition velocity of 0.99 ± 0.15 cm s −1 was calculated for indeno[1,2,3- cd]pyrene and benzo[ ghi]perylene, two high molecular weight PAH which are not found in the gas phase. The dry aerosol deposition was found to dominate the wet removal mechanism by an average ratio of 9:1. The dry gas flux was negative, indicating that surface volatilization was taking place; it accounted for 10–80 % of the total output flux depending on the volatility of the PAH. The remaining PAH were lost to sedimentation. From the dry gas flux, an overall mass transfer coefficient for PAH was calculated to be 0.18 ± 0.06 m d −1. In this case, the overall mass transfer is dominated by the liquid phase resistance.

Regional heat flow variations in the northern Michigan and Lake Superior region determined using the silica heat flow estimator

Conventional heat flow data are sparse for northern Michigan. The groundwater silica heat flow estimator expands the database sufficiently to allow regional variations in heat flow to be examined. Heat flow shows a pattern of alternating highs and lows trending ESE across the Upper Peninsula and Lake Superior. The informal names given to these features, their characteristic heat flow and inferred causes are listed: southwestern high > 40 mW m −2 High heat generation of basement rocks; central low < 40 mW m −2 Geohydrology (groundwater recharge) possibly coupled with low heat generation of basement rocks; northeastern high > 40 mW m −2 Geohydrology (groundwater discharge); northeastern low < 40 mW m −2 Low heat generation of basement rocks. The results suggest that, for the study area, regional variations in heat flow cannot be interpreted solely in terms of regional variations of the heat generation rate of basement rocks.

Secondary Sex Ratios and Egg Sequence in Herring Gulls

Variation in the secondary (hatching) sex ratio has been documented for many bird species. Significant deviations from the 1:1 ratio have led to theoretical speculation about adaptive facultative manipulation of the sex ratio (Trivers and Willard 1973). Seasonal changes in the nestling sex ratio of the Common Grackle (Quiscalus quiscula; Howe 1977) and the Redwinged Blackbird (Agelaius phoeniceus; Fiala 1981) and the biased production of male Zebra Finches (Poephila guttata; Burley 1981) by females mated with highquality males provide some support to the TriversWillard hypothesis. Recently, Ankney (1982) found that in newly hatched Lesser Snow Geese (Chen caerulescens) the first two eggs of 4-egg clutches produced more males (64%) than females and the last two eggs produced more females (72%) than males. Similarly, Ryder (1983) reported in Ring-billed Gulls (Larus delawarensis) the first-laid eggs of 3-egg clutches hatched predominantly more males (66%) than females and the second-laid eggs hatched more females (61 %) than males. This phenomenon suggested a facultative manipulation of the sex produced, although the adaptive significance is unexplained. Weatherhead (1985) examined the occurrence of a sex-sequence relationship in the Red-winged Blackbird after Fiala's (1981) documentation that the last-laid egg of this species produced significantly more females than males. Although Weatherhead (1985) found a sex-sequence relationship, the pattern was not consistent among different clutch sizes. Stimulated by these studies, we examined the secondary sex ratio in relation to laying sequence in newly hatched Herring Gull (L. argentatus) chicks on Granite Island (48?43'N, 88?29'W) in Black Bay, northern Lake Superior (see Ryder 1976 for a description of the colony). During the 1983-1986 nesting seasons, newly constructed Herring Gull nest sites were marked numerically with spray paint and the nests checked daily for the presence of new eggs. Each egg in a clutch was marked in sequence laid by placing dots of nail enamel on the blunt end of the egg (1 dot = first egg laid, 2 dots = second, 3 dots = third). The latest date we arrived at Granite Island was 11 May in 1983. We thus used 11 May as the standard date to compare the progress of Herring Gull egg laying in the three nesting seasons (1984 data were not available). Close to the expected time of hatching, nests were checked twice daily for signs of pipping eggs. Chicks hatched from 3-egg clutches were collected and their sex determined by gonadal inspection. We used only those clutches from which all eggs hatched and where the position of each chick in the sequence laid was known for statistical analyses. We compared clutch sizes using ANOVA and Scheffe a posteriori tests. The sex-ratio data were analyzed using Chi-square and G-tests (Sokal and Rohlf 1981). Significance was accepted at P < 0.05. Upon our arrival at Granite Island each year, laying in several nests had already commenced. The difference in the proportions of initiated and completed clutches by 11 May each year (Fig. 1) indicates that Herring Gulls breeding in 1983 did so later in the season than those nesting in 1985 and 1986. The proportion of nests with 3 eggs was much lower in 1983 than in the other years (Table 1). This low 3-egg clutch-size frequency was manifested in the significantly lower average clutch size for 1983 (F = 5.02, P < 0.05). The secondary sex ratio favored females in 1983 and 1985 and males in 1984 and 1986 (Table 2). The deviation from a 1:1 sex ratio, however, was not significant in any of these years. The G-statistic also indicated no significant relationship between sex and sequence for any year. Similar results were obtained from the combined data. Our results suggest that the allocation of sex chromosomes by the heterogametic female Herring Gull is a random process. A late season, lower average clutch size and hatch of 12% more females than males occurred in 1983. Although it was tempting, at first, to invoke the predictions of the Trivers-Willard (1973) hypothesis, whereby the later-starting females may have been in relatively poor physiological condition

Intraclutch Variation in the Size, Mass and Composition of Ring-Billed Gull Eggs

In 1984, 94 complete three-egg clutches of Ring-billed Gull (Larus delawarensis) eggs were collected on Granite Island, northern Lake Superior. The fresh eggs (i.e., collected within 12 hr of being laid) were weighed, measured and frozen for later chemical analysis to determine if relationships existed between egg size, composition, and sequence. Egg size and mass decreased through the laying sequence. The average composition of fresh eggs was 35% yolk, 56% albumen, and 9% shell. Water accounted for 69% of fresh egg mass. Approximately 8% of fresh egg mass was yolk lipid. Dry yolk consisted of approximately 40% protein and 60% lipid. Caloric content averaged 1.55 kcal/g fresh egg mass. Within clutches, third-laid eggs contained absolutely less albumen and nutrients than earlier laid eggs. Albumen increased in proportion to the 1.2 power of egg mass, and the yolk in proportion slightly greater than the 0.66 power of egg mass. Results agreed well with egg composition studies of other semiprecocial species.