Pronounced Diel Pattern Research Articles

Spatial Dynamics and Fine-Scale Vertical Behaviour of Immature Eastern Australasian White Sharks (Carcharodon carcharias).

Knowledge of the 3-dimensional space use of large marine predators is central to our understanding of ecosystem dynamics and for the development of management recommendations. Horizontal movements of white sharks, Carcharodon carcharias, in eastern Australian and New Zealand waters have been relatively well studied, yet vertical habitat use is less well understood. We dual-tagged 27 immature white sharks with Pop-Up Satellite Archival Transmitting (PSAT) and acoustic tags in New South Wales coastal shelf waters. In addition, 19 of these individuals were also fitted with Smart Position or Temperature Transmitting (SPOT) tags. PSATs of 12 sharks provided useable data; four tags were recovered, providing highly detailed archival data recorded at 3-s intervals. Horizontal movements ranged from southern Queensland to southern Tasmania and New Zealand. Sharks made extensive use of the water column (0-632 m) and experienced a broad range of temperatures (7.8-28.9 °C). Archival records revealed pronounced diel-patterns in distinct fine-scale oscillatory behaviour, with sharks occupying relatively constant depths during the day and exhibiting pronounced yo-yo diving behaviour (vertical zig-zag swimming through the water column) during the night. Our findings provide valuable new insights into the 3-dimensional space use of Eastern Australasian (EA) white sharks and contribute to the growing body on the general ecology of immature white sharks.

The diel pattern in harbour porpoise clicking behaviour is not a response to prey activity

Wild harbour porpoises (Phocoena phocoena) mainly forage during the night and, because they rely on echolocation to detect their prey, this is also when they are most acoustically active. It has been hypothesised that this activity pattern is a response to the diel behaviour of their major prey species. To test this hypothesis, we monitored the acoustic activity of two captive harbour porpoises held in a net pen continuously during a full year and fed by their human keepers during daylight hours, thus removing the influence of prey activity. The porpoises were exposed to similar temperature and ambient light conditions as free-ranging animals living in the same region. Throughout the year, there was a pronounced diel pattern in acoustic activity of the porpoises, with significantly greater activity at night, and a clear peak around sunrise and sunset throughout the year. Clicking activity was not dependent on lunar illumination or water level. Because the porpoises in the pen are fed and trained during daylight hours, the results indicate that factors other than fish behaviour are strongly influencing the diel clicking behaviour pattern of the species.

Circadian regulation of grapevine root and shoot growth and their modulation by photoperiod and temperature

Some plant species demonstrate a pronounced 24 h rhythm in fine root growth but the endogenous and exogenous factors that regulate these diel cycles are unclear. Photoperiod and temperature are known to interact with diel patterns in shoot growth but it is uncertain how these environmental factors are interrelated with below-ground growth. In this particular study, the fine root system of two grapevine species was monitored over a period of ten days with a high resolution scanner, under constant soil moisture and three different photoperiod regimes. Pronounced diel rhythms in shoot and root growth rates were apparent under a fixed 14 h photoperiod. Maximal root growth rate occurred 1–2 h prior to- and until 2 h after the onset of darkness. Subsequently, during the latter part of the dark period, root growth rate decreased and reached minimal values at the onset of the light period. Relative to 22 °C, exposure to a 30 °C air and soil temperature halved root growth but stimulated shoot growth. Notably, the shoot extension rate peak shifted from late afternoon to midnight at this higher temperature zone. When plants were exposed to a delayed photoperiod or progressively shortening photoperiod, the diel changes in root growth rate followed the same pattern as in the fixed photoperiod, regardless of whether the plant was in light or dark. This suggests that light was not the predominant trigger for stimulating root elongation. Conversely, shoot growth rates were not fixed to a clock, with minimum growth consistently at the completion of the dark period regardless of the time of day. In summary, fine root growth of grapevines was found to have a pronounced diel pattern and an endogenous circadian clock appears to orchestrate this rhythm. Soil temperature modified the amplitude of this pattern, but we argue here that, as evidenced from exhausted starch reserves within root tips by early morning, carbon supply from photosynthesis is also required to maintain maximum root growth.

CO2 evolution from standing litter of the emergent macrophyte Deyeuxia angustifolia in the Sanjiang Plain, Northeast China

Microbial decay of emergent macrophyte litters in the standing-dead phase is an essential component of litter decomposition and thus carbon (C) cycle in wetland ecosystems. In this study, we examined the amount of microbial respiration from standing litter in a Deyeuxia angustifolia-dominated marsh in the Sanjiang Plain by integrating field studies with laboratory experiments. In the field, CO2 evolution from standing litter exhibited a pronounced diel pattern, with high evolution rates concurring with increasing litter water potential at night. Notably, significant CO2 evolution even existed at temperature of −4°C. Overall, rates of CO2 evolution showed strong positive correlations with relative humidity and litter water potential. In the laboratory, rates of CO2 evolution from wetted culms and leaves increased with increasing temperature, and the relationship between CO2 evolution rates and temperature could be well described with an exponential model. Based on extrapolation from temperature-adjusted mean CO2 evolution rates and standing litter mass, annual CO2 fluxes of culms (3.07gCm−2yr−1) and leaves (6.81gCm−2yr−1) accounted for 2.32% (culms, 132.30gCm−2yr−1) and 6.94% (leaves, 98.20gCm−2yr−1) of the corresponding annual net aboveground production, respectively. In total, microbial mineralization of standing litter only contributed to 1.12% of the ecosystem respiration (880gCm−2yr−1). Our results highlight the importance of standing-dead phase in litter decomposition and C cycle in wetlands, and also suggest that CO2 flux from standing litter could even be neglected in assessing C budget in the D. angustifolia-dominated freshwater marshes in the Sanjiang Plain, Northeast China.

Atmospheric turbulence triggers pronounced diel pattern in karst carbonate geochemistry

Abstract. CO2 exchange between terrestrial ecosystems and the atmosphere is key to understanding the feedbacks between climate change and the land surface. In regions with carbonaceous parent material, CO2 exchange patterns occur that cannot be explained by biological processes, such as disproportionate outgassing during the daytime or nighttime CO2 uptake during periods when all vegetation is senescent. Neither of these phenomena can be attributed to carbonate weathering reactions, since their CO2 exchange rates are too small. Soil ventilation induced by high atmospheric turbulence is found to explain atypical CO2 exchange between carbonaceous systems and the atmosphere. However, by strongly altering subsurface CO2 concentrations, ventilation can be expected to influence carbonate weathering rates. By imposing ventilation-driven CO2 outgassing in a carbonate weathering model, we show here that carbonate geochemistry is accelerated and does play a surprisingly large role in the observed CO2 exchange pattern of a semi-arid ecosystem. We found that by rapidly depleting soil CO2 during the daytime, ventilation disturbs soil carbonate equilibria and therefore strongly magnifies daytime carbonate precipitation and associated CO2 production. At night, ventilation ceases and the depleted CO2 concentrations increase steadily. Dissolution of carbonate is now enhanced, which consumes CO2 and largely compensates for the enhanced daytime carbonate precipitation. This is why only a relatively small effect on global carbonate weathering rates is to be expected. On the short term, however, ventilation has a drastic effect on synoptic carbonate weathering rates, resulting in a pronounced diel pattern that exacerbates the non-biological behavior of soil–atmosphere CO2 exchanges in dry regions \\mbox{with carbonate soils}.

Diel movements of out-migrating Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (Oncorhynchus mykiss) smolts in the Sacramento/San Joaquin watershed

We used ultrasonic telemetry to describe the movement patterns of late-fall run Chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (O. mykiss) smolts during their entire emigration down California’s Sacramento River, through the San Francisco Bay Estuary and into the Pacific Ocean. Yearling hatchery smolts were tagged via intracoelomic surgical implantation with coded ultrasonic tags. They were then released at four upriver locations in the Sacramento River during the winters of 2007 through 2010. Late-fall run Chinook salmon smolts exhibited a nocturnal pattern of migration after release in the upper river. This is likely because individuals remain within a confined area during the day, while they become active at night and migrate downstream. The ratio between night and day detections of Chinook salmon smolts decreased with distance traveled downriver. There was a significant preference for nocturnal migration in every reach of the river except the Estuary. In contrast, steelhead smolts, which reside upriver longer following release, exhibited a less pronounced diel pattern during their entire migration. In the middle river, Delta, and Estuary, steelhead exhibited a significant preference for daytime travel. In the ocean Chinook salmon preferred to travel at night, yet steelhead were detected on the monitors equally during the night and day. These data show that closely related Oncorhynchus species, with the same ontogenetic pattern of out-migrating as yearlings, vary in migration tactic.

Diel echolocation activity of harbour porpoises (Phocoena phocoena) around North Sea offshore gas installations

Abstract Todd, V. L. G., Pearse, W. D., Tregenza, N. C., Lepper, P. A., and Todd, I. B. 2009. Diel echolocation activity of harbour porpoises (Phocoena phocoena) around North Sea offshore gas installations. – ICES Journal of Marine Science, 66: 734–745. Echolocation clicks of harbour porpoises (Phocoena phocoena) were detected with T-PODs, autonomous, passive, acoustic-monitoring devices, deployed from an offshore-exploration-drilling-rig and gas-production-platform complex in the Dogger Bank region of the North Sea from 2005 to 2006. Echolocation-click trains were categorized into four phases of the diel cycle: morning, day, evening, and night. Porpoises were present near (<200 m) the platform, and there was a pronounced diel pattern in echolocation activity; the number of porpoise encounters (visits) was greater by night than by day. The number of click trains with a minimum inter-click interval of <10 ms also increased at night. This was confirmed by a comparison of the ratios of feeding buzzes to search-phase clicks (feeding buzz ratios) and an analysis of the changes in pulse-repetition frequencies within each train. A reasonable interpretation of this pattern was that porpoises were feeding below or around the platform at night. The evidence for changes in activity during the morning and evening was less clear, so these may be transitional phases. The pattern of porpoise-echolocation behaviour around this platform is related most probably to the diel activity of their prey. If porpoises cluster regularly around such installations within 500-m shipping exclusion zones, they may be omitted from population surveys. We conclude that offshore installations may play an important role as nocturnal porpoise-feeding stations in an overfished environment, but that further replicated and controlled studies are required. These findings should be taken into consideration during offshore-installation-decommissioning decisions in the North Sea.

Estimation of the phytoplankton productivity in three Estonian lakes

INTRODUCTION Light provides the energy necessary for the transformation of inorganic matter into organic matter by the planktonic algae and all other photoautotrophic plants. Primary production is the direct product of photosynthesis, and primary productivity is the sum of all photosynthetic rates in an ecosystem (Fee, 1998). Information on the primary production enables to improve the understanding of food web relationships in aquatic ecosystems. Because of changing light conditions, primary production has a pronounced diel pattern. In order to acquire integrated results over longer time periods (days, months, years), many consecutive measurements of instantaneous photosynthesis rate should be carried out and integrated. In some studies (Joniak et al., 2003; Yoshida et al., 2003; Forget et al., 2007) the values of daily primary production integrated over the photic zone were estimated from in situ incubations. However, such approach gives reliable results only in clear waters, while in highly productive waters incubation cannot be performed during a long period (e.g. from morning to evening) as part of the [sup.14]C-label gets lost from the cells during long-term incubation due to respiration of photosynthetic products (Lancelot & Mathot, 1986) and release of extracellular products (holler Jensen, 1985). Bio-optical model calculations could provide an alternative to the time-consuming 14C method. Several studies estimate primary production from light intensity and abundance of phytoplankton pigments (cited in Arst et al., 2008a). In cases the processes of interest occur on a longer time-scale, it is common to ignore diurnal variations and use mean daily photosynthetically available irradiance (PAR) to force models of primary producers. Widely used methods include those where the dependence of photosynthesis on available light is expressed by an equation containing two parameters: the initial slope [[alpha].sup.[[beta] and the assimilation number [P.sup.B.sub.m] (Sathyendranath et al., 1989). In these models also data on the vertical profiles of the photosynthetically active radiation (PAR) (Einst [h.sup.-1] [m.sup.-2]) are needed. This is a rather complicated way and gives results on the basis of radiation integrated over the PAR region. For reliable description of primary production profiles, however, it is preferable to use a 'spectral approach', in which the model is based on spectral data of underwater quantum irradiance and absorption coefficients of phytoplankton (Sathyendranath et al., 1989; Smith et al., 1989; Schofield et al., 1990; Kyewalyanga et al., 1992; Kirk, 1994; Sosik, 1996; Arst et al., 2006, 2008a). Two versions (spectral and integral) of a semi-empirical model for calculation of the vertical profiles of primary production in lakes were elaborated by Arst et al. (2008a). The main difference between the models resides in the data on underwater irradiance (spectral or integral). Quantification of these models was performed using the data of in situ measurements of bio-optical parameters in three turbid Estonian lakes (Peipsi, Vortsjarv, and Harku) in 2003-2005. The objectives of the present study were (1) to demonstrate the variability of underwater quantum irradiance by calculating its spectral and diurnal variations at different depths of three lakes for all measurement days; (2) to compare in situ primary production measurement results obtained in 2007 2008 with corresponding model results; and (3) using model calculations to estimate the diurnal variability of the vertical profiles of primary production as well as the corresponding integrated (over water column) values. MATERIALS AND METHODS Study sites The measurements were carried out in three Estonian lakes: Peipsi, Vortsjarv, and Harku. Morphometric data and other parameters of the lakes are shown in Table 1. Lake Peipsi is a large shallow lake on the border of Estonia and Russia. …

Diving behavior and movements of juvenile hawksbill turtles Eretmochelys imbricata on a Caribbean coral reef

As historically abundant spongivores, hawksbill turtles Eretmochelys imbricata likely played a key ecological role on coral reefs. However, coral reefs are now experiencing global declines and many hawksbill populations are critically reduced. For endangered species, tracking movement has been recognized as fundamental to management. Since movements in marine vertebrates encompass three dimensions, evaluation of diving behavior and range is required to characterize marine turtle habitat. In this study, habitat use of hawksbill turtles on a Caribbean coral reef was elucidated by quantifying diel depth utilization and movements in relation to the boundaries of marine protected areas. Time depth recorders (TDRs) and ultrasonic tags were deployed on 21 Cayman Islands hawksbills, ranging in size from 26.4 to 58.4 cm straight carapace length. Study animals displayed pronounced diel patterns of diurnal activity and nocturnal resting, where diurnal dives were significantly shorter, deeper, and more active. Mean diurnal dive depth (±SD) was 8 ± 5 m, range 2–20 m, mean nocturnal dive depth was 5 ± 5 m, range 1–14 m, and maximum diurnal dive depth was 43 ± 27 m, range 7–91 m. Larger individuals performed significantly longer dives. Body mass was significantly correlated with mean dive depth for nocturnal but not diurnal dives. However, maximum diurnal dive depth was significantly correlated with body mass, suggesting partitioning of vertical habitat by size. Thus, variable dive capacity may reduce intraspecific competition and provide resistance to degradation in shallow habitats. Larger hawksbills may also represent important predators on deep reefs, creating a broad ecological footprint over a range of depths.

The tunicate Salpa thompsoni ecology in the Southern Ocean. I. Distribution, biomass, demography and feeding ecophysiology

Distribution, density, and feeding dynamics of the pelagic tunicate Salpa thompsoni have been investigated during the expedition ANTARKTIS XVIII/5b to the Eastern Bellingshausen Sea on board RV Polarstern in April 2001. This expedition was the German contribution to the field campaign of the Southern Ocean Global Ocean Ecosystems Dynamics Study (SO-GLOBEC). Salps were found at 31% of all RMT-8 and Bongo stations. Their densities in the RMT-8 samples were low and did not exceed 4.8 ind m−2 and 7.4 mg C m−2. However, maximum salp densities sampled with the Bongo net reached 56 ind m−2 and 341 mg C m−2. A bimodal salp length frequency distribution was recorded over the shelf, and suggested two recent budding events. This was also confirmed by the developmental stage composition of solitary forms. Ingestion rates of aggregate forms increased from 2.8 to 13.9 μg (pig) ind−1 day−1 or from 0.25 to 2.38 mg C ind−1 day−1 in salps from 10 to 40 mm oral-atrial length, accounting for 25–75% of body carbon per day. Faecal pellet production rates were on average 0.08 pellet ind−1 h−1 with a pronounced diel pattern. Daily individual egestion rates in 13 and 30 mm aggregates ranged from 0.6 to 4.8 μg (pig) day−1 or from 164 to 239 μg C day−1. Assimilation efficiency ranged from 73 to 90% and from 65 to 76% in 13 and 30 mm aggregates, respectively. S. thompsoni exhibited similar ingestion and egestion rates previously estimated for low Antarctic (~50°S) habitats. It has been suggested that the salp population was able to develop in the Eastern Bellingshausen Sea due to an intrusion into the area of the warm Upper Circumpolar Deep Water

Diel cycles in viral infection of bacterioplankton in the North Sea

Diel variations in substrate availability (largely due to variations in phytoplankton production) can lead to pronounced diel patterns in prokaryotic activity in the euphotic zone. We examined short-term changes in viral infection of bacterioplankton and its relation to bacterial activ- ity in 3 distinct masses of North Sea surface waters marked by drifting buoys in June 2001 and April 2002. The water masses were sampled every 4 to 6 h for a period of 20 to 36 h. The frequency of infected cells (FIC) estimated by a virus dilution approach varied from 5 to 64% at the western site, 17 to 55% at the southern site, and 10 to 22% at the northern site and was generally higher during the night than at daytime. Furthermore, FIC was negatively related to bacterial activity at all sites. Bacterial activity, measured via ( 14 C)-leucine incorporation, was ca. 1.5- to 5-fold higher during the day than at night. Our results indicate that viral lysis of bacteria occurs around noon to afternoon, and infection mainly during the night. Moreover, lysis and viral production seem to take place during high bacterial activity; this could be a strategy to increase the number of newly produced viruses.

Urban aerosol radiative properties: Measurements during the 1999 Atlanta Supersite Experiment

As part of the Atlanta Supersite 1999 study, aerosol radiative and related physical and chemical properties are examined on the basis of measurements of PM2.5 (aerosol particles with aerodynamic diameters, Dp, less than 2.5 μm) in urban Atlanta. In addition to potential compliance issues with proposed regulatory standards, PM2.5 concentrations in Atlanta and the surrounding region are large enough to have an important impact on atmospheric radiative transfer and hence visibility and potentially regional climate. Arithmetic means and standard deviations of the light scattering by PM2.5 (σsp at 530 nm) and absorption coefficients (σap at 550 nm) measured at a controlled relative humidity of 49 ± 5% are 121 ± 48 and 16 ± 12 Mm−1, respectively. Though the mean light extinction coefficient (σep) in Atlanta is much larger than background sites, it is comparable to nonurban areas in the interior southeast United States highlighting the contribution of a regional haze here. The single scattering albedo (ωo) in Atlanta is 0.87 ± 0.08 and is ∼10% lower than reported in nonurban polluted sites, likely a result of the emission of elemental carbon (EC) from mobile sources. A pronounced diel pattern in aerosol properties is observed with clear influences from mobile sources (morning rush hour maxima in concentrations, particularly soot‐related indicators) and atmospheric mixing (afternoon minima). A strong linear relationship between σsp and PM2.5 is observed, and using several techniques, gives a range of mean mass scattering efficiencies (Esp) from = 3.5 to 4.4 m2 g−1. EC and σap are observed to have a relationship though less strongly correlated than σsp and PM2.5. Four methods of determining the mass absorption efficiency of EC give Eap ranging from 5.3 to 18.3 m2 g−1. This wide range of values is a result of the variability in aerosol properties, uncertainties in the light absorption method, and in particular, differences in the EC measurement techniques. Best agreement was found using measured EC mass distributions using a multistage impactor in comparison to σap calculated with a Mie code yielding Eap = 9.5 ± 1.5 m2 g−1, while EC mass summed from the impactor stages in comparison to measured σap gives Eap = 9.3 ± 3.2 m2 g−1. Mie light‐scattering calculations using inputs of measured mass and EC size distributions give geometric mean light scattering and absorption Dp = 0.54 and 0.13 μm, respectively, and show the dominance of the submicrometer diameter particles to light extinction in the urban environment. Based on the measured aerosol optical depth in Atlanta, δa (500 nm) = 0.44 ± 0.22, and other radiative measurements, a best estimate of the average direct aerosol radiative forcing at the top of the atmosphere (a measure of the climate significance) is ΔF = −11 ± 6 W m−2 in Atlanta. This value is an order of magnitude greater than global mean estimates for aerosols underscoring the influence of aerosol particles on radiative transfer in the urban environment.

Diel variations in Prochlorococcus optical properties

Prochlorococcus is an important component of phototrophic biomass in oligotrophic areas. In such systems, diel variations in optical properties have been reported. In order to better understand these natural variations, the optical (absorption, attenuation, and scattering) and biochemical (carbon and pigment) properties of an axenic clone of Prochlorococcus, PCC 9511, grown in a turbidostat set under light conditions that simulated those found near the ocean surface, were monitored every 2 h for several consecutive days. All optical parameters showed pronounced diel patterns except the divinyl‐chlorophyll a (Dv‐Chl a) specific absorption coefficient at 676 nm, a*(676), which remained stable (0.019 m2 mg Dv‐Chl a21). The diel oscillations of the Dv‐Chl a specific absorption coefficient at 440 nm, a*(440) (43% increase between sunrise and sunset), were essentially governed by variations in the zeaxanthin to Dv‐Chl a ratio (52% increase of this ratio between sunrise and sunset). The scattering cross section at 555 nm, σb(555), showed oscillations with the largest amplitude (182% increase between sunrise and sunset). Finally, the carbon‐specific attenuation coefficient at 660 nm, cc*(660) (1.04 m2 gC−1) is less than half that of the other algal groups. This estimation, together with the diel fluctuations in cc*(660) highlighted in the present study, is discussed in the context of using in situ measurements of optical properties to infer biogeochemical stocks (vegetal or detrital biomass) or processes (primary production).

DIEL PATTERNS IN LIGHT ABSORPTION AND ABSORPTION EFFICIENCY FACTORS OFISOCHRYSIS GALBANA(PRYMNESIOPHYCEAE)1

The chlaspecific absorption coefficients [a* (λ), m2·mg chla−1] were examined in chemostat culture of the PrymnesiophyceaeIsochrysis galbana(Parke) under a 12:12‐h light:dark cycle at low light (75 μmol photons·m−2·s−1) and high light (500 μmol photons· m−2·s−1) conditions. Other associated measurements such as pigment composition, cell density, and diameter as the measure of cell size were also made at the two light regimes every 2 h for 2 days to confirm the periodicity. A distinct diel variability was observed for thea* (λ) with maxima near dawn and minima near dusk. The magnitude of diel variation ina* (440) was 15% at low light and 22% at high light. Pronounced diel patterns were observed for cell size with minima near dawn and maxima near dusk. The magnitude of diel variation in cell size was 9.3% at low light and 21% at high light. The absorption efficiency factors [Q a(440)] were determined by reconstruction using intracellular concentrations of pigments and cell size. TheQ a(440) also showed a distinct diel variability, with minima near dawn and maxima near dusk. The diel variation ina* (λ) andQ a(λ) was primarily caused by changes in cell size due to growth, although there was some influence from diel variations in the intracellular pigment concentrations. The results presented here indicated that diel variation ina* (λ) was an important component of the optical characterization of phytoplankton.

Diel patterns in optical properties of the chlorophyte Nannochloris sp.: Relating individual‐cell to bulk measurements

To investigate how cell growth and division affect the optical properties of phytoplankton, cultures of the chlorophyte Nannochloris were sampled over a diel cycle to measure cell size and concentration, light‐beam attenuation and absorption, flow cytometric forward light scattering and chlorophyll fluorescence, and carbon content. Refractive index was calculated using the anomalous diffraction approximation. At six different light levels, ranging from 60 to 1,500 µmol photons m−2 s−l, cell division was tightly phased to the light: dark cycle, occurring soon after dark. There were pronounced diel patterns, with minima near dawn and maxima near dusk, in cell size, cell‐specific beam attenuation and absorption, flow cytometric forward light scatter, and carbon content. The diel variations in attenuation cross section were primarily caused by changes in cell size due to growth and division, although there was some influence from changes in refractive index. Because the major constituents of many phytoplankton communities are small eukaryotic cells similar to Nannochloris in cell size and division pattern, the results presented here have important consequences for the interpretation of diel variations in optical properties observed in the ocean.

Mineral licks as a sodium source for Isle Royale moose.

Natural mineral licks and their use by moose (Alces alces) on Isle Royale National Park, Michigan, were studied during 1982-85. The distribution of known licks suggested that they occurred in association with glacial debris, primarily in the western portions of the island. Moose utilized mineral springs extensively during the spring-summer period, and at least 5 licks were used year-round. During summer, a pronounced diel pattern of moose visitation was apparent, with peak use occurring between 0400-0800 h. Although daytime lick use declined by late June, morning and evening use continued to be relatively high throughout the study period. Peak lick use coincided with leaf-emergence in spring. Moose continued to utilize mineral licks despite the availability of ponds containing aquatic plants. Sodium appeared to be the element attracting moose to licks where they ingest copious amounts of water. Observed sodium ingestion rates (0.35 g/min) at licks indicate that licks provide a more concentrated source of sodium compared to aquatic plants (0.023 g/min). Based on the data presented, we reject the conclusions of earlier workers that aquatic plants constitute the only significant source of sodium for Isle Royale moose.