75

AQUATIC AREAS PROVIDE HIGH NITROGEN FORAGE FOR  
MOOSE (ALCES ALCES) IN ISLE ROYALE NATIONAL  
PARK, MICHIGAN, USA

Keren B. Tischler1, William J. Severud2, Rolf O. Peterson1, John A. Vucetich1, and 
Joseph K. Bump1,3

1School of Forest Resources and Environmental Science, Michigan Technological University, 
Houghton, Michigan 49931, USA; 2Department of Natural Resource Management, South Dakota 
State University, Brookings, South Dakota 57007, USA; 3Present address: Department of Fisheries, 
Wildlife, and Conservation Biology, University of Minnesota, Saint Paul, Minnesota 55108, USA

ABSTRACT: The distribution of ungulates reflects spatial and temporal heterogeneity in forage quality 
and quantity across the landscape. Aquatic habitats have a patchy spatial distribution and are readily used 
by moose (Alces alces) and other ecotone specialists. However, the importance of aquatic feeding to 
moose has largely been attributed to acquisition of sodium, with little consideration given to the relative 
and comparative quality of aquatic and terrestrial forage types. We show differences in forage quality as 
measured by crude protein content and carbon:nitrogen (C:N) ratios between aquatic and terrestrial sum-
mer forage in Isle Royale National Park, Michigan, USA. Aquatic macrophytes had higher crude protein 
content and lower C:N ratio than preferred terrestrial plant species of moose. Consequently, measurable 
consumption of aquatic forage may provide high quality forage in less than optimal habitats. Because the 
distribution of aquatic habitats on Isle Royale exhibits strong spatial trends, the benefits of aquatic feed-
ing may have spatial influence on the population dynamics of Isle Royale moose. 

ALCES VOL. 58: 75 – 90 (2022)

Key words: Alces alces, aquatic macrophytes, C:N ratio, crude protein, forage quality, Isle Royale, 
moose, nitrogen

Ecotone specialists that consume both 
terrestrial and aquatic resources link food 
webs between these two realms (Bartels et al. 
2012, Severud et al. 2013, Johnston 2017, 
Bump 2018). The patchy spatial and temporal 
distribution of aquatic cover types, and the 
quality and quantity of forage contained 
therein, are important factors in predicting the 
landscape distribution and density of ecotone 
specialists (Crawley 1983, McNaughton 
1988, Fryxell 1991, Wallis DeVries 1996, 
Johnston and Windels 2015). Indeed, moose 
(Alces alces) link aquatic and terrestrial 
biomes due to their extensive foraging activi-
ties in both habitats during summer (Peterson 
1955, Qvarnemark and Sheldon 2004, Peek 
2007, Tischler et al. 2019).

Moose are restricted to northern lati-
tudes characterized by a high degree of sea-
sonal variability (e.g., short growing season) 
in forage quality and quantity (Timmermann 
and Rodgers 2017). Due to the low quality 
of winter forage, moose are in negative 
energy balance during winter and rely upon 
energy reserves attained during late summer 
and autumn to maintain energy balance year-
round (DelGiudice et al. 1997, 2011, Moen 
et al. 1997, Schwartz and Renecker 2007). 
To maximize these reserves, moose face a 
trade-off between the benefits of exploiting 
high quality forage patches with potential 
costs of predation risk (Edwards 1983), high 
ambient air temperature (Renecker and 
Hudson 1986, 1990), and insect avoidance 



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(Renecker and Hudson 1990). As a result, 
moose exhibit distinctive habitat use pat-
terns including summer foraging in aquatic 
habitats. 

Aquatic habitats are typically abundant 
across boreal landscapes and readily used by 
moose in summer (Peterson 1955, 
Qvarnemark and Sheldon 2004, Peek 2007). 
Moose also use deeper water to protect 
against predation (Mech 1966, Gasaway et 
al. 1983, Jordan et al. 2010) and seek relief 
from high ambient temperatures (Renecker 
and Hudson 1986, 1990). The quality of 
aquatic forage, in terms of available energy, 
protein, and essential nutrients, has been the 
focus of many studies and some debate 
(Botkin et al. 1973, Fraser et al. 1980, 1984, 
Jordan 1987). Due to its scarcity in continen-
tal ecosystems, sodium may be a limiting 
nutrient for North American herbivores 
(Hutchinson and Deevey 1949), and 
Belovsky (1981) and Jordan (1987) noted 
the high sodium content of aquatic macro-
phytes. Several investigators have advanced 
the hypothesis that moose seek aquatic habi-
tats explicitly to satisfy sodium requirements 
(Hutchinson and Deevey 1949, Jordan et al. 
1973, Belovsky 1981, Fraser et al. 1984, 
Jordan 1987). However, sodium was not 
considered the predominant factor for moose 
consuming emergent aquatics on the Copper 
River Delta, Alaska. Rather, MacCracken et 
al. (1993) considered that the impetus for 
aquatic foraging in some systems was that 
aquatic forage was nutritious, high in digest-
ible energy and crude protein. However, few 
studies have assessed the overall nutrition, 
protein, and energy associated with aquatic 
forage for moose, and importantly, as com-
pared with terrestrial forage (but see Fraser 
et al. 1984 and MacCracken et al. 1993). 

Our goal was to measure and compare 
the relative nutritional value of aquatic and 
terrestrial moose forages as measured by 
crude protein content and carbon:nitrogen 

(C:N) ratios at Isle Royale National Park, 
Michigan, USA. We predicted that aquatic 
macrophytes contain more protein and have 
lower C:N ratios than terrestrial plants. In 
this system, sodium is a seasonally import-
ant nutrient for moose (Jordan 1987), with 
aquatic habitats dominated by submergent 
(plants either free-floating or entirely sub-
merged beneath water surface) rather than 
emergent (plant parts emergent above water 
surface) species. Apart from vegetation, 
sodium is also available at mineral licks and 
springs on Isle Royale which may be in suf-
ficient abundance to meet the nutritional 
needs of moose given that these sources are 
well-used by moose (Risenhoover and 
Peterson 1986). We additionally compared 
forage quality among plant species compos-
ing the principal terrestrial summer diet of 
moose on Isle Royale, and evaluated spatial 
differences in forage quality between eastern 
and western sides of the island due to differ-
ent glacial history (Huber 1973; see Study 
Area). 

STUDY AREA
Isle Royale is a 544 km2 island archipelago 
located in the boreal forest region of north-
western Lake Superior, USA, 24 km from the 
nearest shoreline (48° N, 89 °W). The pri-
mary island consists of Precambrian-aged 
basalt and conglomerate bedrock shaped by 
the last glaciation into a series of parallel 
ridges and valleys including numerous 
water bodies (Huber 1973). Lakes and ponds 
(n = 84 ≥ 1 ha) comprise 36 km2 of the sur-
face area, with an additional 8 km2 of palus-
trine emergent wetlands. Additional shoreline 
is found in numerous bays of Lake Superior, 
particularly at the east end of the island.

As a result of glacial activity, soils are 
more developed on the west end of Isle 
Royale (Huber 1973). Fire has historically 
burned the entire east end, and relatively 
recent fires (1936 and 1948) have burned the 



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midsection of the island. Moose have been on 
Isle Royale since the early 1900s, with winter 
density ranging from 1 to 4 moose/km2 across 
the island in the past ~50 years (Vucetich and 
Peterson 2004). The legacy of the island’s 
disturbance regime has resulted in forest suc-
cession following different trajectories on the 
east and west ends of the island. Forests on 
the west end are in a late successional stage 
and dominated by deciduous species, while 
forests on the east end are younger and coni-
fer-dominated (Janke et al. 1978). 

Mean daily high temperature is 20°C in 
summer and −3°C in winter (De Jager et al. 
2020). Snow and ice cover persist from 
November through April, and the islands 
receive ~750 mm of precipitation annually 
(Risenhoover and Maass 1987). In winter, 
moose concentrate along shoreline areas 
where balsam fir (Abies balsamea) is ~ 60% 
of the diet, with the remainder woody browse 
and arboreal lichens (Risenhoover 1987, 
Parikh et al. 2017, Tischler et al. 2019). The 
summer spatial distribution and local density 
of moose on Isle Royale is unknown, but the 
spring diet of moose includes newly emer-
gent leaves and the summer diet is largely 
composed of current leaf growth of decidu-
ous plants and aquatic macrophytes 
(Ackerman 1987, Tischler et al. 2019). 

METHODS

Sample Design
We collected aquatic macrophytes and 
leaves of terrestrial plant at the east and west 
ends of the island (hereafter E and W, respec-
tively). The E and W sampling sites were 
delineated by the boundary of the 1936 and 
1948 fires, leaving the central portion of the 
island unsampled. We collected samples 
between 13 July and 3 August 2002 when 
plants were mature, as opposed to emergent 
or senescent. Samples were collected at ≥ 5 
E and W sites from the 6 terrestrial species 

composing the principal summer diet of 
moose on Isle Royale: mountain maple (Acer 
spicatum), sugar maple (A. saccharum), 
mountain-ash (Sorbus spp.), paper birch 
(Betula papyrifera), yellow birch (B. 
alleghaniensis), and beaked hazelnut 
(Corylus cornuta) (Ackerman 1987). Sites 
were separated by >200 m and at each we 
collected 5 green leaves of each species 
(including the petiole and excluding twigs) 
at browse height (0.5–3.0 m) from separate 
but neighboring stems; samples were pooled 
for analysis.

Moose commonly forage in aquatic hab-
itats during summer and appear to consume 
aquatic species in proportion to abundance 
at Isle Royale (Qvarnemark and Sheldon 
2004). Consequently, we opportunistically 
collected dominant (i.e., most abundant) 
aquatic macrophyte species (identified to the 
genus) at 3 E and 3 W sites (lakes) used by 
moose. At each site, 5 subsamples of each 
species along the shoreline were collected 
(where available) and pooled for analysis. 
Since moose are not known to discriminate 
among aquatic plant parts (i.e., rhizome, 
stem, flower), we attempted to collect the 
entire plant, excluding only large and well-
rooted rhizomes. To minimize the collection 
of benthic sediment, we rinsed samples in 
lake water to remove loose debris prior to 
placing in plastic sample bags. Due to the 
paucity of inland lakes in the W, all 3 W sites 
were in bays of Lake Superior, versus 1 E 
site. To reduce the potential effect of sam-
pling in Lake Superior, in 2003 we expanded 
aquatic plant sampling (6–18 July) to include 
5 E and 5 W inland aquatic sites. The W sites 
included lakes, small ponds, or wetland hab-
itats containing open water where moose 
were observed feeding or evidence of use 
was identified (e.g., tracks, fecal pellets). To 
minimize degradation prior to analyses, we 
cooled samples until freezing them at −20°C 
within 12 h of collection.



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We collected winter forage samples at 7 
E and 7 W sites spaced ≥ 200 m apart between 
12 January and 10 February 2003. At each 
site, we collected 5 twigs (current annual 
growth) from adjacent stems of individual 
plants from balsam fir, white cedar (Thuja 
occidentalis), mountain-ash, red-osier dog-
wood (Cornus stolonifera), paper birch, and 
quaking aspen (Populus tremuloides). We 
clipped twigs at the average diameter for each 
species eaten by moose in winter (Risenhoover 
1987). At each site, arboreal lichens of the 
genera Usnea and Parmelias were collected 
from the branches/bark of standing or newly 
fallen white spruce (Picea glauca) and paper 
birch. Samples were handled and frozen as 
with the aquatic macrophytes.

Metrics of Quality and Analysis
Indices of forage quality are based upon 
either the presence of essential plant nutri-
ents (e.g., water, carbohydrate, fat, protein, 
vitamins, and minerals) or the absence of 
indigestible structural carbon (C) com-
pounds and toxins (Crawley 1983). Nitrogen 
(N) availability is considered the most limit-
ing aspect of herbivore nutrition (Crawley 
1983). Since rumen microbes can incorpo-
rate both organic and inorganic sources of N 
into the synthesis of amino acids, crude pro-
tein (N × 6.25) is a sufficient metric of 
digestible protein in ruminants (Schwartz 
and Renecker 2007). The elemental ratio of 
C:N is also a useful index of gross forage 
quality as it provides a measure of the rela-
tive investment in C structural compounds 
(associated with reduced quality) per atom 
of N (associated with enhanced quality) 
(Crawley 1983, Sterner and Elser 2002). We 
report crude protein content (%) and C:N 
mass ratios of forage types as metrics of 
overall forage quality.

We oven-dried plant tissue at 60 °C for 
48 h to constant mass (or longer as needed for 
aquatic macrophytes) and ground it to fine 

powder in a ball mill (Spex CertiPrep Inc., 
Metuchen, New Jersey, USA). All samples 
were re-dried overnight and stored in a dessi-
cator until subsamples (C: 1.5 ± 0.1 mg, N: 
3.0 ± 0.1 mg) were weighed into tin cups. 
Subsamples were combusted in a Costech 
Elemental Combustion System 4010 elemen-
tal analyzer (Costech Analytical Technologies, 
Valencia, California, USA) to measure C and 
N content. The instrument was calibrated 
with acetanilide and internal organic check 
standards were analyzed every 10 samples; 
analytical precisions were %C ± 0.20 and 
%N ± 0.05. Duplicate samples were analyzed 
every 5 samples and results were accepted 
only if the variance between duplicates was 
less than that of the standards.

We calculated standard errors of C:N 
ratios using error propagation, which derives 
a composite error from that of its component 
parts (Sterner and Elser 2002). Differences 
in crude protein and C:N ratios among pooled 
forage types (terrestrial, aquatic), terrestrial 
species, sampling location (E and W), and 
sampling year (2002, 2003) of aquatic mac-
rophytes were tested separately using univar-
iate analysis of variance (ANOVA) (SAS 
Institute Inc., Cary, North Carolina, USA) 
due to the unbalanced nature of the data with 
respect to aquatic sampling location. 

We examined crude protein content and 
C:N ratios among all known moose forage 
types (i.e., summer and winter) for correla-
tion without a priori predictions as to the 
nature of the relationship. We included data 
on winter forage types (terrestrial plant twigs 
and lichens) in this analysis to increase the 
range of values used to model this correla-
tion. We used the best-fit model describing 
the correlation for only summer terrestrial 
leaves as a baseline for comparing the 
observed and predicted relationship for sub-
mergent and emergent aquatic macrophytes 
using univariate ANOVA. We determined 
the best fit model by visual assessment and 



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79

improvements in R2. Where ANOVA results 
were significant (P < 0.05), we used Tukey’s 
Honestly Significant Difference (HSD) to 
determine which samples differed. All tests 
were considered significant at  the α = 0.05 
level and assumptions of normal distribution 
and homogeneous variance were tested. 
Where the assumption of homogeneous vari-
ance was violated, individual comparisons 
were made with two-sample t-tests assuming 
unequal variance. We report means ± stan-
dard errors, unless otherwise noted.

RESULTS
The aquatic macrophyte samples (n = 26; 17 
Lake Superior, 9 inland) were from 7 genera 
in 7 families in 2002, increasing to 88 sam-
ples from 27 genera in 15 families in 2003 
(Table 1). Samples collected from Lake 
Superior in 2002 had lower C:N ratios  
(−x = 13.7 ± 0.7) than those collected from 
inland lakes (= 18.0 ± 1.2; F1,24 = 11.07, 
P = 0.0028); crude protein content did 
not  differ between lake types (t22 = 0.10; 
P = 0.92). We detected no annual difference 
in crude protein content of aquatic plants by 
sample year (F1,86 = 1.01, P = 0.3172); how-
ever, the C:N ratio of aquatic samples was 
higher (t76 = −2.95, P = 0.0043) in 2003 (C:N 
= 18.3 ± 0.8) than in 2002 (C:N = 15.2 ± 0.7). 
The C:N ratio distribution was non-normal in 
the 2003 aquatic samples, in large part due to 
two outliers that were emergent taxa that con-
tain more structural compounds and expected 
to have a higher C:N ratio than submerged 
plants. Removal of the emergent species 
(n = 15, all collected in 2003) from the analy-
sis resulted in a normally distributed dataset 
with no difference in crude protein and C:N 
ratios between years. Therefore, the aquatic 
macrophyte data we present in comparisons 
among forage types represent only submergent 
aquatic data pooled across years, and includes 
both inland and Lake Superior samples. 
Alternative analyses using unpooled aquatic 

data including all taxa (submergent and emer-
gent) did not alter the statistical significance of 
the comparison (Supplemental Table).

Submergent aquatic macrophytes had 
higher crude protein (T110 = 2.9, P < 
0.0001; Fig. 1) and lower C:N ratios  
(T104 = −10.6, P = 0.004; Fig. 1) than terres-
trial plant leaves. Since aquatic macrophyte 

Table 1. Mean crude protein content (%) and 
carbon:nitrogen (C:N) ratios (±SD) of aquatic 
macrophyte taxa sampled at Isle Royale National 
Park during 2002–2003.

Taxon n
Crude  
protein C:N

Emergent
Juncus 1 17.7 15.4
Sagittaria 1 14.8 14.6
Asteraceae: unk. 1 14.0 15.1
Lysimachia 2 13.7 ± 2.7 18.5 ± 2.1
Eupatorium 1 13.1 17.4
Eliocharus 2 13.0 ± 3.1 14.9 ± 3.4
Menyanthes 1 12.0 24.3
Poaceae: unk. 3 10.4 ± 1.9 25.3 ± 7.9
Equisetum 1 9.6 25.3
Dulichium 1 9.5 28.1
Carex 1 5.8 49
Submerged
Nuphar 5 22.9 ± 2.7 12.0 ± 1.0
Potamogeton 28 15.3 ± 3.0 16.9 ± 3.4
Brasenia 2 14.9 ± 7.3 17.8 ± 5.1
Elodea 1 14.2 17.0
Myriophyllum 5 14.0 ± 1.7 14.6 ± 2.0
Lemna 1 13.8 18.4
Najas 3 13.7 ± 4.3 13.7 ± 1.6
Sparganium 10 13.4 ± 0.6 19.4 ± 0.9
Utricularia 7 12.1 ± 1.1 15.0 ± 1.8
Megalodonta 1 11.6 17.0
Scirpus 2 11.4 ± 1.2 20.3 ± 1.7
Isoetes 1 11.3 16.5
Vallisneria 1 8.9 21.5
Sclerolepis 1 7.0 11.7
Chara 3 6.8 ± 0.7 17.5 ± 1.8
Ranunculus 1 4.5 15.0



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taxa were collected opportunistically, sample 
sizes were too small to compare forage qual-
ity among taxa. Nevertheless, yellow pond 
lily (Nuphar) had the highest mean protein 
content and a low C:N ratio (Table 1). Among 
terrestrial forage species, leaves from yellow 
birch, beaked hazelnut, and paper birch had 
higher crude protein (F5,54 = 6.86, P < 0.0001; 
Fig. 2a) and lower C:N ratio (F5,54 = 8.53, P < 
0.0001; Fig. 2b) than leaves from sugar 
maple. The trend in C:N ratios across these 
terrestrial species mirrored that of crude pro-
tein content with no exceptions (Figs. 2a, b).

Crude protein content of aquatic macro-
phytes was higher (F1,86 = 6.56, P = 0.012) 
at W sites (−x = 15.1% ± 0.5) than E sites  
(−x = 12.6% ± 0.9), whereas the C:N ratio was 
lower at W sites (T2,24  =  −3.3, P = 0.003). 
Among summer terrestrial species, only sugar 
maple differed by location, with crude 
 protein higher (F1,10 = 5.35, P = 0.046) at  
W (−x = 10.3% ± 0.5) than E sites  
(−x = 8.5% ± 0.5). The C:N ratio of sugar maple 
at W sites (−x = 29.4 ± 1.7) was correspondingly 
lower than at E sites (−x = 35.0 ± 2.2), but not 
different (F1,10 = 3.67, P = 0.088).

The crude protein content and C:N ratios 
of terrestrial forage types (i.e., summer 
leaves, winter twigs, and winter lichens) 

were strongly correlated (R2 = 0.97; Fig. 3a). 
This relationship was best explained by a 
negative exponential model: y = β0 × e − β1x, 
where x = percent crude protein and y = C:N 
ratio. Aquatic macrophytes did not fit the 
exponential model describing summer ter-
restrial leaves (y = 69.006 × e −0.0843x, SEb1 = 
0.228, F2,145 = 23.7, P < 0.0001), showing 
instead a high degree of variability in C:N 
ratios, particularly at low levels of protein 
(Fig. 3a). Among aquatic macrophytes, the 
relationship between crude protein content 
and C:N ratio of emergent taxa followed the 
terrestrial curve more closely than submer-
gent taxa (Fig. 3b), but was not different 
(t-test, P = 0.065). Sample variances among 
forage types were non-homogenous, 
although the ANOVA and t-test results 
agreed.

DISCUSSION
Because crude protein content and C:N 
ratios are indicative of forage quality, our 
results support the hypothesis that aquatic 
macrophytes provide high quality summer 
forage to moose, complementing their con-
sumption of terrestrial plants. On Isle 
Royale, aquatic macrophytes have ~20% 
higher crude protein and 40% lower C:N 

Fig. 1. Boxplot of crude protein content and carbon:nitrogen (C:N) ratios of aquatic and terrestrial 
moose forage types from Isle Royale National Park, Michigan, USA, 2002. Boxes depict interquartile 
range, dark lines are median values, circles are outliers, and whiskers are 1.5× interquartile range. 
Aquatic macrophytes are submergent species pooled across sampling years. 



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ratios than terrestrial plant leaves collected 
in mid-summer. Additionally, the quality of 
aquatic forage found in bays of Lake 
Superior appears to be higher than in inland 
lakes – similar crude protein content and 
lower C:N ratios. These data support studies 
on the Copper River Delta in Alaska sug-
gesting that submergent aquatic plants repre-
sent an important protein source for moose 
during summer (MacCracken et al. 1993). In 
Ontario, moose in a “cafeteria” food trial 
preferred aquatic species with higher 

sodium, phosphorus, and crude protein 
(Fraser et al. 1984). However, crude protein 
content of terrestrial and aquatic plants did 
not differ in the Alaskan study, perhaps due 
to highly variable data, low sample size, and 
the relatively high protein content (16%) of 
one terrestrial species, pin cherry (Prunus 
pennsylvanica) (MacCracken et al. 1993). 
The crude protein content of terrestrial plants 
in our study was generally similar to levels 
reported in other studies; however, the 
aquatic macrophytes had lower crude 

Fig. 2. Boxplot of crude protein content and carbon:nitrogen (C:N) ratios among leaves of summer 
terrestrial forage species from Isle Royale National Park, Michigan, USA, 2002. Boxes depict 
interquartile range, dark lines are median values, circles are outliers, and whiskers are 1.5× 
interquartile range. Letters indicate species that are significantly different.



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protein than measured in Alaska and Ontario 
(Table 2; Fraser and Hristienko 1983, 
MacCracken et al. 1993). Indeed, crude pro-
tein content was highly variable within and 
between lakes on Isle Royale with the min-
ima and maxima ranging 16 and 18%, 
respectively. The lower crude protein con-
tent may reflect differences in species com-
position, local nutrient inputs at sampling 
sites, and the sampling period. It is possible 
that our small sample sizes were not entirely 
representative of the average crude protein 
or C:N ratios for certain species (Table 1). 

Summer diets of moose in the region of 
Isle Royale and northeastern Minnesota 
include measurable amounts of aquatic 
plants (13–40%) as estimated via stable 

isotope analysis (Berini 2019, Tischler 
et al. 2019). Further, moose inhabiting rela-
tively warmer areas of northeastern 
Minnesota consumed higher amounts of 
aquatic vegetation (Berini 2019). 
Consumption of aquatic plants certainly 
provides highly nutritional forage based on 
the crude protein and C:N ratios we mea-
sured, and moose simultaneously address 
other nutritional requirements including 
sodium balance. However, presumably 
moose diets are necessarily balanced with 
aquatic and terrestrial vegetation, in part, 
because aquatic foraging is believed lim-
ited by gut fill due to the high water content 
of aquatic macrophytes and incidental 
water consumption (Belovsky 1978).

Fig. 3. Correlation between crude protein content (nitrogen [N] × 6.25) and carbon:nitrogen (C:N) 
ratio among moose forage types (A), and among types of aquatic macrophytes (B) from Isle Royale 
National Park, Michigan, USA, 2002.



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The efficiency with which ingested C is 
converted into heterotrophic biomass is neg-
atively correlated with forage C:N ratios 
(Elser et al. 2000). In general, the C:N stoi-
chiometry of freshwater aquatic autotrophs 
is lower and less variable than that of terres-
trial autotrophs up to a magnitude of three 
between freshwater seston and terrestrial 
plants (Elser et al. 2000). At Isle Royale, 
moose feeding on aquatic plants during sum-
mer would acquire 1.5 times more N per C 
atom consumed than acquired through ter-
restrial foraging during summer, suggesting 
that the assimilation efficiency of aquatic 
macrophytes is greater than that of terrestrial 
plants. This pattern may largely be attribut-
able to physiological constraints that obli-
gate terrestrial plants to a large structural C 
investment rather than differences in N con-
tent per se (Sterner and Elser 2002). Indeed, 
we found that terrestrial plants had a high 
and relatively fixed (46–49%) C content 
while the C content of aquatic macrophytes 
was highly variable (11–48%), perhaps 
reflecting that submerged and emergent 
aquatic macrophytes were pooled for analy-
sis. Furthermore, we found that the C:N stoi-
chiometry of all terrestrial forage types 

(summer leaves, winter twigs, lichens) fol-
lowed a tight pattern of exponential decay 
with increasing crude protein content, 
whereas aquatic macrophyte C:N ratios were 
comparatively low and much less predict-
able across a wide range of crude protein 
contents. Interestingly, the crude protein-C:N 
relationship of emergent aquatic macro-
phytes, which require more structural sup-
port than submerged plants, was intermediate 
that of terrestrial and submergent aquatic 
plants. 

Our results suggest that higher C:N 
ratios of terrestrial plants are due to greater 
structural C allocation and lower N content. 
Thus, even if the difference in crude protein 
content is not biologically significant, 
aquatic macrophytes are a higher quality for-
age than terrestrial plants due to lower con-
centration of structural C which hinders 
digestibility. In support, Belovsky and 
Jordan (1978) reported higher digestibility 
for aquatic plants (94%) than deciduous 
leaves (72%) on Isle Royale; albeit, digest-
ibility of both is considered high and decidu-
ous leaves are the principle component of 
the summer diet of moose. Furthermore, N 
content was negatively correlated with the 

Table 2. Comparison of crude protein content (SD) of summer terrestrial and aquatic forage among studied 
moose populations.

Source

% Crude protein

Location Terrestrial† Aquatic

This study Michigan 12 (0.3) 14 (0.6)*
MacCracken et al. (1993) Alaska 13 (1)# 17 (1)
Fraser et al. (1984) Ontario 13 (1)# 16 (1)
Crete and Jordan (1982) Quebec 14 (0.3)‡ na
Renecker and Hudson (1985) Alberta 13 (0.4)§ na
Oldemeyer et al. (1977) Alaska 13 na

Note: Samples collected between 30 June and 2 August unless otherwise noted.
†Samples include deciduous leaves and exclude twigs unless otherwise noted.
*Submergent species only.
#Samples include both leaves and twigs.
‡Only beaked hazelnut and mountain maple were sampled; does not represent principal summer diet of moose.
§A composite sampled to reflect diet.



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content of phenolics (anti-herbivory com-
pounds) (Jones and Hartley 1999), although 
the degree to which aquatic macrophytes 
have evolved chemical defense against her-
bivory has been little studied (but see Parker 
et al. 2006). 

Pond lilies have historically been identi-
fied as an important aquatic forage for moose 
in North America (Peterson 1955, Cobus 
1972). Murie (1934) provided anecdotal evi-
dence of near extirpation of abundant pond 
lilies in the 1930s by an irrupting moose 
population on Isle Royale, which may be 
due to their preference by moose or sensitiv-
ity to disturbance (Fraser and Hristienko 
1983). More recently, cover of watershield 
(Brasenia schreberi), a previously abundant 
aquatic macrophyte, has declined in many of 
Isle Royale’s water bodies during periods of 
high beaver and moose density that coin-
cided with low wolf abundance (Hoy et al. 
2019). It is not surprising that we found pond 
lilies to be a high-quality moose forage 
(mean crude protein content = 22.9%; 
Table 1). Among terrestrial species, our 
results suggest that sugar maple is a 
low-quality summer moose forage. Sugar 
maple appears to be an important species in 
the spring diet of moose on Isle Royale 
(Ackerman 1987), perhaps due to early leaf 
emergence and high calcium concentration 
relative to other terrestrial leaves, but its use 
declines as forage and diet diversity increase 
through spring and summer (Belovsky et al. 
1973, Krefting 1974, Miquelle and Jordan 
1979, Belovsky 1981, Ackerman 1987). 

As with northern ungulates, the protein 
content of the winter diet of moose is insuf-
ficient to meet maintenance protein require-
ments (5–7%, Fig. 3) (Schwartz et al. 1988). 
Compensation of this “deficit” is achieved 
principally through catabolizing fat  and lean 
body mass stored during late summer and 
autumn when forage is up to 3 X more nutri-
tious than in winter (Renecker and Hudson 

1986), recycling urea (van Hoven and 
Boomker 1985), and limiting fetal growth 
and gestational costs during early-mid win-
ter (Schwartz 2007). Thus, the abundance of 
high quality forage consumed during sum-
mer influences pre-winter body condition 
and survival (Parker 2003). It follows that 
consumption of high quality aquatic forage 
used throughout summer aids post-winter 
recovery, pre-winter nutritional condition, 
and winter survival of Isle Royale moose. 
Surprisingly, in nearby northeastern 
Minnesota moose in relatively warmer areas 
consumed poorer diets characterized as high 
in aquatic forage and low in high-preference 
terrestrial forage. Further, moose dying 
overwinter consumed diets higher in aquatic 
forage than surviving moose (Berini 2019). 

The spatial patterns we identified in for-
age quality among aquatic and terrestrial 
plant species is consistent with the large-
scale spatial (E-W) differences in soil rich-
ness and plant species composition on Isle 
Royale (see Study Area). Spatial heteroge-
neity in resource quality is widely known to 
influence browsing behavior and the distri-
bution of herbivores across landscapes 
(Crawley 1983, Renecker and Hudson 1985, 
1986, McNaughton 1988, Fryxell 1991, 
Wallis DeVries 1996, Parker 2003). Many 
ungulate populations “track the pulse of pro-
duction” and “green waves” (Mattson 1980) 
via seasonal migration, thereby exploiting 
nutritious forage and maximizing the time 
period to access such forage when available 
(Festa-Bianchet 1988, Merkle et al. 2016). 

Assuming a similar distribution of mac-
rophytes among aquatic habitats on Isle 
Royale, ~ 75% of aquatic biomass occurs on 
the east half of the island based on the length 
of shoreline available for aquatic foraging. 
Unfortunately, little is known about or 
whether moose migration is common on Isle 
Royale; however, only 2 of 22 radio-collared 
moose migrated between the east and 



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85

west ends of the island in the late 1980s 
(unpublished data of author, R. O. Peterson). 
Perhaps this lack of migration can be 
explained by our observation that while 
overall forage quality appears to be better on 
the west end of the island, the biomass of 
aquatic macrophytes is higher at the east 
end. Because no large patches of landscape 
are noticeably devoid of vegetation, except 
where fire removed it, we recognize that 
individuals meet their summer-autumn 
nutritional requirements through a varied 
diet. Forage quality (protein content), 
although variable among species in summer, 
is on a continuum where the majority of ter-
restrial leaves are nutritious (protein con-
tent), highly digestible, and a mainstay of 
the spring-summer diet across moose range. 

The evolution of muzzle anatomy in 
moose is believed a morphological adapta-
tion for efficient underwater feeding, a 
behavior unique to moose among cervids 
(Hofmann 1989, Geist 1998, Clifford and 
Witmer 2004). Aquatic feeding is undoubt-
edly an important source of sodium for many 
moose populations (Botkin et al. 1973, 
Jordan et al. 1973, Fraser et al. 1984); how-
ever, moose on Isle Royale (and elsewhere) 
can meet sodium requirements at mineral 
licks which contain much higher sodium 
concentrations (on a wet-weight basis) than 
aquatic plants (Risenhoover and Peterson 
1986). We propose that aquatic foraging by 
moose at Isle Royale is also a mechanism to 
exploit relatively N-rich microsites (aquatic 
habitats) in an otherwise N-limited land-
scape (White 2012). Regardless, aquatic 
habitats provide moose summer forage high 
in digestible protein critical to physical 
recovery and growth, while reducing their 
post-winter sodium deficit, insect harass-
ment, risk of predation, and thermal stress 
(Morris 2014). 

Given the patchy spatial distribution of 
aquatic habitats on Isle Royale, the positive 

influence of aquatic feeding on the pre-win-
ter nutritional condition of moose could 
affect spatial dynamics of winter population 
density, mortality, and predation rate. We 
suggest that increased consumption of high 
quality aquatic macrophytes on the east end 
of Isle Royale might supplement the lower 
quality winter forage, thereby foregoing the 
need to migrate. From this perspective, it is 
understandable that aquatic foraging by 
moose is prevalent (Tischler et al. 2019); 
however, it is unknown if the pre-winter 
condition of moose differs at the island ends 
or the time associated with developing a 
migratory strategy. We encourage further 
research to test such assumptions and to bet-
ter understand the relative use and role of 
aquatic plants on moose population 
dynamics.

ACKNOWLEDGEMENTS
We thank K. Pregitzer, L. Vucetich, J. Kaplan, 
D. McCormick, M. Romanski, C. Lawler, J. 
Deutsch, and N. Hambel for field assistance, 
and K. Raisanen-Schourek, B. Allshouse, B. 
Baibak, and D. Donaldson for assistance with 
sample preparation. J. Marr provided expertise 
in aquatic macrophyte identification. Forage 
plant carbon and nitrogen content was deter-
mined by Jennifer Eikenberry at the School of 
Forest Resources and Environmental Science, 
Michigan Technological University. J. Oelfke 
and M. Romanski of the National Park Service 
provided logistical support in the field. C. 
Giardina, P. Hurley, and L. Kruger, provided 
valuable comments on earlier versions of this 
manuscript. T. Drummer and J. Pickens pro-
vided useful advice on data analysis. This 
research was funded by the Ecosystem Science 
Center at Michigan Tech and the National 
Science Foundation, and support to R. P. from 
the Robbins Chair in Sustainable Management 
of the Environment at Michigan Technological 
University. J.K.B. was supported by grants 
NSF ID#1545611 and NSF ID#1556676. All 



AQUATIC AREAS AND HIGH NITROGEN FORAGE – TISCHLER ET AL. ALCES VOL. 58, 2022

86

necessary permits from the National Park 
Service were obtained for the described field 
studies. We additionally thank 2 anonymous 
reviewers for comments that improved the 
clarity of the manuscript.

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SUPPLEMENTAL TABLE

Comparison of ANOVA test results for the effect of forage type on plant crude protein content or C:N ratio 
when aquatic macrophytes are either pooled or separated by sample year (2002, 2003) and type 
(submergent and emergent, submergent only).

Aquatic dataset n†

Crude protein C:N ratio

F P F P

2002 26 149.2 <0.0001 229.0 <0.0001
2003 62 118.1 <0.0001 289.8 <0.0001

2002, 2003 combined 88 122.4 <0.0001 367.9 <0.0001

2002, 2003 combined; submergent only 73 124.6 <0.0001 373.7 <0.0001
†Sample size of aquatic macrophyte dataset used for analysis.