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Phytoplankton Seasonality Along a Trophic Gradient of
Temperate Lakes: Convergence in Taxonomic Compostion
during Winter Ice-Cover
Emon Butts1 and Hunter J. Carrick1,*
Abstract - A gap in our understanding of phytoplankton seasonality in temperate lakes exists
mainly due to the lack of information collected during the winter months. We summarized
seasonal changes of phytoplankton biomass and taxonomic composition relative to watercolumn
biogeochemical conditions in 6 lakes located on Beaver Island and 1 site in Lake
Michigan in close geographic proximity to each other (less than 20 km apart). A number of physical–
chemical parameters (e.g., temperature, DOC) were similar between lakes, but lakes towards
the interior of the island had lower pH, alkalinity, and conductivity. Moreover, lakes at the
interior of the island supported 2-fold greater phytoplankton-chlorophyll and carbon compared
with perimeter lakes, and phytoplankton taxonomic composition differed considerably
during the ice-free period (April–December). Interestingly, the winter phytoplankton assemblages
were strikingly similar in all 7 lakes, when large populations of phyto-flagellates
(Chrysophyceae and Cryptophyceae) occurred under the ice at low light and temperatures
< 4 oC. Given the mixotrophic capabilities of these phytoflagellates, we suggest seasonal
convergence reflects the community response to under-ice conditions, which promotes the
occurrence of an important component of annual phytoplankton biomass.
Introduction
The factors that regulate phytoplankton seasonality (wax and wane) can be difficult
to predict due to the many biotic and abiotic interactions that act singly or in
concert, and influence population growth, dispersal, and survival (e.g., Reynolds
2006). As with most organisms, specific phytoplankton populations are more likely
to dominate assemblages when environmental factors favor their key natural-history
requirements; in extreme cases, these peak conditions can lead to the occurrence
of seasonal blooms (Carrick 2011). In addition to increased nutrient availability,
phytoplankton blooms have shown to be induced by physical factors such as watercolumn
light availability and stability (e.g., Millie et al. 2014, Sandgren 1988).
Seasonal phytoplankton blooms and sensitivity to changing climatic conditions
have been documented in both marine and freshwater ecosystems (Winder et al.
2012), which seems logical, given that most seasonal algal blooms are associated
with reoccurring environmental conditions that promote of the growth of native
species, the presence of which, supports predictable features such as fisheries
production in lakes and marine ecosystems (see Wetzel 2001). These seasonal phytoplankton
assemblages need to be contrasted against harmful algal bloom events
1Department of Biology and Institute for Great Lakes Research, Mount Pleasant, MI 48859.
*Corresponding author - hunter.carrick@cmich.edu.
Manuscript Editor: David Halliwell
Winter Ecology: Insights from Biology and History
2017 Northeastern Naturalist 24(Special Issue 7):B167–B187
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that are assocated with declines in ecosystem aesthetics, production of secondary
metabolites harmful to vertebrates, and imbalances in primary productivity and
community respiration (Azanza et al. 2005, Burkholder et al. 1992, Kirkpatrick et
al. 2004, Rinta-Kanto et al. 2009).
The seasonal variation in phytoplankton biomass and taxonomic composition
in temperate lakes has not been fully documented or understood. Most sampling
regimes do not include winter collections because this period is assumed to be one
with biological inactivity (e.g., Salonen et al. 2009). This situation has created a bias
in the data collected to characterize the annual plankton cycle in most lakes (Twiss
et al. 2012). Data from the winter months is important because ice-cover duration
can constitute 4–6 months of the year, and thereby promotes strong changes in
physical drivers such as light and temperature, which are sustained for longstanding
periods and can be sensitive to changing climate (see Hampton et al. 2015). Given
that light and temperature are key regulators of phytoplankton growth, dispersal,
and survival (Huisman 1999, Interlandi and Kilham 2001, Litchman 1998, Sommer
1985), it stands to reason that the intensity and duration of winter conditions could
greatly influence changes in phytoplankton biomass and species c omposition.
Lakes in the temperate zone (northern hemisphere) undergo seasonal changes
that cause them to exhibit considerable variation in the timing and occurrence of
specific phytoplankton assemblages (Reynolds 2006). In many lakes, plankton
survive during ice-cover periods and can act to oxygenate the water column, apparently
because production is sufficient to outweigh respiratory losses (e.g., Phillips
and Fawley 2002). The phytoplankton that survive under ice-cover can serve as
a food for higher trophic levels, acting to sustain metazoan populations when the
overall plankton biomass is typically low. For instance, Vanderploeg et al. (1992)
showed that diatom blooms in Lake Michigan fed and sustained multi-voltine,
crustacean zooplankton assemblages under the ice. Townsend and Cammen (1988)
demonstrated that, in some shallow lakes, winter phytoplankton blooms influenced
calanoid copepod populations and that the timing of the blooms affected juvenile
fish recruitment. Decomposition of winter–spring phytoplankton blooms have
been shown to sustain future phytoplankton assemblages during much of the summer
growing season, via the slow regeneration of nutrients to the water column
(Falkowski et al. 1988), which can fuel hypoxia in some sensitive lakes (e.g.,
Lashaway and Carrick 2010, Reavie et al. 2016, Wilhelm et al. 2014). Interestingly,
physical conditions in extreme, polar environments (i.e., Arctic and Antarctic regions)
created extended periods of ice-cover, during which phytoplankton blooms
occurred in and under the ice and constituted the bulk of annual primary production
(Arrigo et al. 2012, Berman et al. 2005,).
Here we present data that show that phyto-flagellates dominated winter plankton
assemblages in 6 lakes of strongly divergent biogeochemistry on Beaver
Island, MI, and 1 in nearby Lake Michigan. These winter blooms occurred during
harsh conditions of darkness and low temperature, in synchrony, independent of
lake biogeochemistry and trophic state. Our research addressed 3 specific questions:
(1) What biogeochemical patterns exist along the transition from lakes in
Beaver Island’s to near-shore Lake Michigan? (2) What are the seasonal shifts
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in phytoplankton biomass and species composition among lakes that share a similar
regional climate? and (3) What is the relative contribution of winter phytoplankton
assemblages to annual phytoplankton biomass? Herein, we have restricted our
analysis to surface mixed-water assemblages, which excludes information on subsurface
assemblages that is likely to be more important in deeper, clearwater lakes
such as Lake Geneserath and Lake Michigan.
Field-site Description
Beaver Island is located in northern Lake Michigan 51 km west of Charlevoix,
MI. The island is roughly 145 km2 in surface area and 9.6 km wide x 21 km long
(Fig. 1). It offers a unique opportunity to study seasonal dynamics of phytoplankton
Figure 1. Map of the study
sites.
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because the island supports 6 inland lakes in relatively close proximity, allowing
easy access for frequent sampling. These lakes are also noteworthy in that they
experience relatively little anthropogenic impact in comparison to lakes on Michigan’s
mainland, thus making them valuable for comparisons to more-disturbed
ecosystems (see Calabro et al. 2013). We sampled northern Lake Michigan at sites
offshore of the northeast side of Beaver Island (45.55817°N, 85.47470°W). We
retrieved water samples from under the ice during the winter months.
Methods
Sampling and ambient lake-conditions
We sampled 7 lakes on 10 dates over a 1-y period at approximately monthly
intervals excluding the months of May and August (October 2011–September
2012). We sampled Lake Michigan from January to September 2012; this sampling
excluded May and August. The 6 inland lakes were sampled at a single, offshore
site, over the deep-water contour in each lake (at maximum depth); Lake Michigan
was sampled offshore of the northeast end of Beaver Island (Fig. 1). Sampling dates
covered all 4 seasons, which we defined as: winter (December, January, February);
spring (March, April, June); summer (July, August, September); and fall (October,
November). During winter ice-cover periods (January–March), we employed an ice
auger to bore 4 successive holes in the ice to create 1 large opening. We trimmed the
ice with a handheld axe and took water samples through the opening; ice thickness
was measured using a handheld measuring tape. In each lake, we collected each
sample at 0.5 m depth in 3.0-L Van Dorn bottles; immediately transferred the water
into shaded, 10-L polycarbonate carboys; and transported them to the laboratory for
further analysis and sample preservation.
We measured physical and chemical conditions in the field at each lake using a
hand-held meter (model 880, YSI, Inc., Yellow Springs, OH) to measure temperature,
conductivity, and dissolved oxygen content (DOC). We measured photosynthetically
active radiation (PAR) on 3 occasions (December, June, July) in each lake
using a Licor 1000 (Li-Cor, Lincoln, NE) equipped with underwater up-welling and
down-welling radiometers (2 pi probes). We took readings at successive depths to
estimate extinction coefficients (see Wetzel and Likens 2000). In the laboratory, we
determined several other lake-water parameters from the samples collected on all 10
sampling dates. We employed a bench-top meter (Thermo Fisher OrionVstar, Thermo
Fisher Scientific, Waltham, MA) to measure hydrogen-ion concentrations (pH). Alkalinity
was estimated as ug/L CaCO3 by titrating lake-water subsamples (100 ml)
with 0.1 N HCl (Wetzel and Likens 2000). We dispensed surface-water subsamples
into 2 bottles (250-ml polyethylene, amber): 1 preserved with 2% acid Lugol’s solution
and another with 1% gluteraldehyde; these samples were subsequently used to
estimate phytoplankton taxonomic composition under the microscope (see below).
Phytoplankton chlorophyll and phosphorus content
We determined phytoplankton biomass on each date (n = 10) using 2 independent
estimates: chlorophyll-a concentrations and estimated algal carbon from
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phytoplankton-cell counts (see below). We measured chlorophyll concentrations
fluorometrically on both raw and size-fractionated water. We conducted fractionations
by filtering raw water through membrane filters with pore sizes that selected
for organisms less than 2 μm and less than 20 μm in size. The filtrates were concentrated onto
Whatman GF/F filters (effective pore size, 0.7 μm) and pigments were extracted
using 50:50 acetone:DMSO without grinding (Carrick et al. 1991, Shoaf and Lium
1976). We measured fluorescence of the extracted pigments using a 10-AU fluorometer
(Turner Designs, San Jose, CA).
We measured the total phosphorus (TP) and intracellular phosphorous concentration,
in the form of intracellular polyphosphates bodies (poly-P), by concentrating
the seston in lake water samples collected from each lake–date combination onto
0.2-μm membranes (Millipore GWSP; EMD Millipore, Billerica, MA) or analyzing
whole lake-water, respectively. We determined total P content by treating subsamples
of whole lake-water to a potassium persulfate extraction (final concentration
of 2.4 mM), followed by autoclaving at 100 oC for 60 min. The poly-P content of
plankton material was measured by heating samples at 100 oC for 60 min, thereby
liberating soluble reactive P (PO4
-3) from the condensed, inorganic polyphosphate
compounds (poly-P) that can occur in either cyclic, linear, or cross-linked bonds
with oxygen (Fitzgerald and Nelson 1966, Harold 1966). We then estimated both
poly-P and total P concentrations as soluble reactive P measured colorimetrically
using a spectrophotometer (method 365.1; USEPA 1997, 2002).
Phytoplankton biomass taxonomic composition
We estimated phytoplankton biomass (as cellular carbon) and taxonomic
composition using complementary enumeration techniques (Booth 1993, Carrick
and Schelske 1997). We enumerated the abundance and general taxonomic
composition of phototrophic picoplankton from subsamples preserved with 1%
gluteraldehyde that were filtered onto 0.2-μm black nuclepore membrane filters.
The filters were then mounted onto glass slides with immersion oil, stored at -20
°C, and counted within one week to reduce fading of fluorescence (see Carrick
and Fahnenstiel 1989). We used a Leica DMR 5000 research-grade microscope
equipped for chlorophyll-a fluorescence (blue light 450–490-nm excitation and
>515 nm emission) to performed the counts at 1000x magnification, and enumerated
250 individuals from 2 duplicate slides to determine phycobilin proteins
(green light 530-560-nm excitation and >580 nm emission). We assigned general
taxonomic (division) position and morphological categories (e.g., spherical,
rod-shaped, colonial) based on dominant pigment fluorescence of individual picoplankton
cells.
We enumerated the abundance and taxonomic composition of nano- and microsized
phytoplankton from subsamples preserved with 2% Acid Lugol’s; aliquoits
were dispensed into settling chambers (10–50 ml volume) and allowed to settle
for 24 h on coverslips (Utermöhl 1958). We counted a total of 300–400 cells by
random fields under a Leica DMI 5000 research-grade, inverted microscope at both
400x and 630x magnification. The appropriate taxonomic references were used to
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enumerate, to their lowest taxonomic position, the phytoplankton taxa encountered
(Prescott 1962, Skuja 1956). Water-column cell densities and species-specific carbon
were calculated using standard equations and conversion factors (e.g., Carrick
and Fahnenstiel 1989, 1990). For both methods of microscopy, we calculated cell
biovolumes (μm- 3) for each taxon by the cellular dimensions of at least 10 cells on
at least 2 dates, took the average, and expressed the result as the equivalent spherical
diameter (μm- 3). We converted taxon-specific biovolumes to carbon using equations
of Strathmann (1967) for diatoms (0.1 pg C per μm- 3), the Verity et al. (1992)
equation for nanoplankton (0.433 [μm- 3]0.863), and the Laws et al. (1984) conversion
factor for picoplankton (0.28 pg C per μm- 3).
Statistical analyses
We used factor analysis to evaluate environmental variation among lakes, where
water-column sampling events (lake-date combinations) were considered observations
and key biogeochemical parameters were considered variables. These data
were log transformed to meet assumptions of normality, and then assembled into a
66 x 4 data matrix through an iterative process, whereby variables with the greatest
explanatory power were retained for final analysis. We conducted factor analysis
on the correlation matrix among variables (principal components analysis, PCA),
retained the resulting factors with eigenvalues > 1.0 for interpretation, following
an axis rotation procedure using the Varimax method (see Manly 1986). We scored
the observations (sampling events) into the space defined by the newly derived
factors, and grouped them by their proximity to one another according to visual
inspection and subsequent statistical evaluation (see below). Differences in 9 major
biogeochemical parameters (physical, chemical, biological) were evaluated among
lake types (perimeter, interior) through a series of pair-wise comparions using a
Mann-Whitney U test (significancnt at alpha = 0.05). Spatio-temporal variation
in chlorophyll and phytoplankton carbon among lakes (perimeter, interior) and
sampling periods (winter, spring, summer, fall) were evaluated using 2-way, multivariate
analysis of variance (MANOVA; Zar 2009). We made pairwise comparisons
with Tukey’s multiple means comparisons to isolate pair-wise differences (alpha =
0.05). The data was log transformed to attain a normal distribution, and the assumption
of equal variance was broken.
Results
Ambient conditions
The 7 lakes varied considerably in terms of their biogeochemistry and trophic
status (Table 1). The range of pH among lakes was 3 units (Min–max = pH 5.47–
8.56); conductivity values typically ranged almost 10-fold in magnitude (min–max
= 29–286 μS cm-1; see Table 1 for mean values). Mean alkalinity varied from 6.1
to 138.0 mg CaCO3 L-1 among lakes, indicating broad differences in their buffering
capacities; alkalinity was lowest in Greene’s Lake and the highest in Barney’s
Lake. The average DOC varied from 8.84 mg L-1 to 10.20 mg L-1. Total phosphorus
concentrations ranged about 2-fold among lakes, with mean values from 10.61 to
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19.77 μg L-1 (Table 1) and polyphosphate concentrations varied from 2.69 to 6.29
μg L-1. Additionally, variation in the amount of visual color and productivity created
nonuniform light penetration among the lakes, with attenuation coefficients
of 0.13 to 2.77 m-1 (Table 1). During the winter period (December–February), ice
thickness among all the Beaver Island lakes varied from 1.9 to 32 cm in thickness;
however, mean ice thickness was not very different among the 6 island lakes (overall
mean ± 1 SD = 15.6 ± 8.4 cm). Ice cover on Lake Michigan was variable due
to movement of ice sheets throughout the lake. The mean water temperature, TP,
and poly-P concentrations were not statistically different across the 7 lakes, which
provided evidence that they experience similar climate conditions (1-way ANOVA:
F = 0.01, P > 0.05).
The ordination of water-column variables produced 2 principal components
(PC) with eigenvalues > 1.00 (Table 2, Fig. 2A). Both components were correlated
strongly with original variables and collectively accounted for > 84% of the
variation in the dataset. Once scored into the space defined by the 2 PCs, the 66
sampling events clustered into 3 discernable groups that corresponded well with the
sampling periods (summer, spring/fall, winter; Fig. 2A). PC-1 accounted for 47.1%
of the variation; this axis correlated positively with pH and conductivity (r > 0.92
for both). In general, sampling events were broadly distributed along PC-1; this
result was expected given the wide range among lakes in their dissolved inorganic
substances as reflected in the pH and conductivity measurements (carbon and total
substances; Fig. 2, Table 2). PC scores among seasons exhibited complete overlap
along this axis suggesting that variation was consistent among seasons (see Fig.
2A). Samples with higher pH and higher conductivity scored positively with PC-
1; these samples were collected from Barney’s Lake, Font Lake, Lake Geneserath,
and Lake Michigan (lakes located around the perimeter of Beaver Island; Table 2).
Samples with lower-conductance water had negative scores along this axis; these
observations corresponded with samples collected from Greene’s, Fox, and Egg
Lakes (interior; Table 2). The pH of these lakes was generally less than 7 (Table 2). PC-2
Table 1. Mean values of key biogeochemical conditions measured from October 2011 to September
2012 among 7 lakes: 6 lakes on Beaver Island and nearshore Lake Michigan. Attenuation coefficients
were measured on 3 occasions (December, June, July) , while ice thickness was measured on 3 dates
(December, January, February). Water depths are estimates of the average depth. Temperature is represented
as minimum and maximum values observed.
Ligtht
Water Total attenuation Ice
depth Temp. Phosphorus Conductivity coeff. thickness
Lake Type (m) (°C) (μg L-1) (μS cm-1) pH (m-1) (cm)
Greene’s Lake Interior 3 5.4, 8.3 15.1 28.7 6.00 2.33 16.0
Fox Lake Interior 6 4.9, 28.0 10.6 31.1 6.71 2.33 16.1
Egg Lake Interior 2 5.3, 26.2 16.9 93.1 7.87 2.77 11.5
Lake Geneserath Perimeter 10 5.1, 27.1 11.7 165.3 7.83 0.26 19.8
Font Lake Perimeter 3 4.5, 25.4 15.5 182.1 8.14 1.12 10.9
Barney’s Lake Perimeter 3 4.9, 25.6 19.8 200.2 8.29 0.48 19.3
Lake Michigan Perimeter 100 7.7, 23.5 10.9 228.9 8.14 0.13 Open
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accounted for 33.4% of the variation. This axis correlated positively with temperature
and internal phosphorus storage (as poly-P) in the plankton (r > 0.81 for both).
In general, PC-2 seemed to define a phytoplankton physiological gradient defined
by changes in the water-column temperature and storage of poly-P. This result
makes sense given that the summer assemblages occurred at higher temperatures
and stored larger concentrations of poly-P compared with the winter period (lowest
scores). Interestingly, the spring and fall assemblages transitioned between the
Figure 2. (A) Principal component analysis (PCA) ordination, where PC-1 accounted for
47.1% of the variation and was correlated positively with pH and conductivity. PC-2 accounted
for 33.4% of the variation and correlated positively with temperature and internal
phosphorus storage (as poly-phosphorus; Poly-P). Site–date samples were plotted against
the PCs and distinguished by season. (B) Log chlorophyll plotted against component PC-1.
Triangles represent interior lakes; circles represent perimeter lakes.
Table 2. Pair-wise comparisons for biological, chemical, and physical variab les measured among perimeter
(Barneys, Font, Geneserath, Michigan) and interior (Egg, Fox, Greene’s) lakes on Beaver Island,
MI. Lakes were sampled on 10 occasions during October–September 2011–2012. Values (± SD)
are averages for each lake type and comparions were performed using a series of Mann-Whitney U
Tests (where, **P < 0.01; ns = not significant).
Parameter (units) Perimeter lakes Interior lakes Comparison
Biological
Chlorophyll-a (μg/L) 5.1 ± 0.8 11.5 ± 1.4 Perimeter < interior**
Phytoplankton C (μg/L) 158.1 ± 19.0 390.5 ± 88.8 Perimeter < interior**
Chemical
Alkalinity (μ/S/cm) 107.2 ± 5.5 19.7 ± 4.3 Perimeter > interior**
Conductivity (μ/S/cm) 184.6 ± 8.8 49.6 ± 5.5 Perimeter > interior**
Oxygen (mg/L) 9.7 ± 0.5 9.3 0.5 Perimeter = interior ns
pH 8.2 ± 0.1 6.9 ± 0.2 Perimeter > interior**
Poly-P (μg/L) 3.1 ± 1.3 4.3 ± 0.5 Perimeter = interior ns
Total P (μg/L) 14.6 ± 0.3 14.2 ± 1.2 Perimeter = interior ns
Physical
Temperature (°C) 13.4 ± 1.2 13.3 ± 1.5 Perimeter = interior ns
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summer (high scores) and winter conditions (low scores). When PC-1 was correlated
with chlorophyll concentrations, it showed that interior lakes supported
greater phytoplankton biomass relative to the perimeter lakes and was statistically
significant (Fig. 2B, Table 2).
Spatio-temporal variation in phytoplankton
The chlorophyll-a concentration among the 7 lakes (n = 10 for each) varied
from 0.8 to 31.1 μg L-1, indicating a trophic status spanning from oligotrophic
to eutrophic (Table 2, Fig. 2). Lake Michigan (average ± 1 standard deviation,
1.18 ± 0.11 μg L-1) had the lowest average concentrations of chlorophyll, whereas
Greene’s Lake (17.7 ± 2.9 μg L-1) had the greatest chlorophyll concentrations. In
terms of size structure, >60% of the chlorophyll in Lake Michigan was <2 μm in
size; in the other 6 lakes, this fraction contributed <20% to the assemblages’ overall
mean biomass. In Barney’s, Egg, Font, and Geneserath lakes, the the 2–20 μm and
>20-μm fractions contributed similarly to the chlorophyll totals. The assemblage
in Greene’s Lake was dominated by chlorophyll from phytoplankton in the 2–20-
μm size range, while the chlorophyll from cells in the >20 μm fraction was most
prevalent in Fox Lake.
Total phytoplankton biomass (as estimated cellular carbon) exhibited a similar
trophic gradient among lakes (range = 8.6–2451.7 μg C L-1) and showed good
agreement when compared with chlorophyll concentrations determined from paired
samples (r = 0.76 P < 0.0001, n = 66). In general, phytoplankton biomass and
chlorophyll-a were greater in the interior lakes than in the perimeter lakes (Tables
2, 3). Total phytoplankton carbon varied significantly among the 4 seasons; carbon
levels were greater in the spring and fall periods compared with those in the summer
and winter (Table 3).
Table 3. Two-way MANOVA results coupled with Tukey’s pairwise comparisons that assessed variation
in phytoplankton biomass (as celluar carbon, ug L-1) among taxonomic categories, and for estimates
of total phytoplankton. Lake type (perimeter or interior, see Table 1) and season (winter, spring,
summer, fall) were considered fixed factors, where ns = not significantly different, * = P < 0.05, ** =
P < 0.01, and *** = P < 0.001. No significant interactions were observed.
Factor Group F-value Tukey’s pairwise tests
Lake Diatom 1.55 ns No difference
Chlorophyte 0.06 ns No difference
Chrysophyte 0.01 ns No difference
Cyanobacteria 0.02 ns No difference
Other 6.24* Perimeter < interior
Total phytoplankton 6.21* Perimeter < interior
Total chlorophyll 6.96* Perimeter < interior
Season Diatom 3.68* Winter = summer < fall = spring
Chlorophyte 2.40 ns No difference
Chrysophyte 4.71** Summer < fall = winter = spring
Cyanobacteria 10.97*** Winter < spring = fall = summer
Other 5.96** Winter < summer = spring = fall
Total phytoplankton 6.02** Summer = winter < spring = fall
Total chlorophyll 0.22 ns No difference
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Key taxonomic groups (algal divisions) were not different between interior
and perimeter lakes, although the biomass in 3 of the 5 groups varied among seasons
(Table 3). The biomass of cyanobacteria, cryptophytes, dinoflagellates, and
euglenoids was lowest in the winter compared with the other 3 seasons, whereas
chrysophyte biomass was lowest in the summer compared with levels present in
the other 3 seasons, and no seasonal differences among seasons were observed for
diatoms and chlorophytes (Table 3, Fig. 3). Seasonal trends in phytoplankton taxonomic
composition appeared to be lake-specific and showed considerable compositional
changes from month to month (Fig. 3). Greene’s, Fox, and Barney’s lakes and
Lake Michigan showed unimodal peaks in phytoplankton biomass over the year.
Interestingly, the perimeter lakes supported spring or early summer phytoplankton
blooms (March, April, June), all of which were composed of diatoms and chrysophytes.
All 7 lakes supported mixed assemblages during the summer months (June,
July, September). In the late summer–early fall period, cyanobacteria dominated in
6 of 7 lakes.
Despite the apparent seasonal differences in phytoplankton observed among
lakes, a distinct environmental gradient of increasing temperature and poly-P
storage (PC-2) were evident among the lakes, which corresponded with shifts
in phytoplankton biomass and taxonomic composition (Fig. 4). The biomass of
Figure 3. Estimates of phytoplankton biomass and taxonomic composition (as cellular carbon)
plotted for the 10 seasonal sampling dates. The red box denoted the 3-month winter
period when the lakes were ice covered (the other category includes Cryptophyta and Pyrrophyta).
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cyanobacteria, chlorophytes, and the cryptophyte/dinoflagellate group all increased
with temperature and poly-P concentrations in the plankton assemblage (Fig. 4).
The biomass of diatoms increased with pH and conductivity, although this relationship
was not significant (r = 0.25, P=0.57, n =57). In contrast, chrysophyte carbon
exhibited an abrupt decline along a gradient of increasing temperature and poly-P
storage (Fig. 4). This result made sense because the winter (December–February)
Figure 4. Phytoplankton biomass (as carbon) for each of the 4 major taxonomic groups
(Chrysophyta, Chlorophyta, Cyanobacteria) and Others (Cryptophyta, Pyrrophyta) plotted
against PC-2. Triangles represent interior lakes; while circles represent peri meter lakes.
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phytoplankton assemblages in all 7 lakes were dominated by chrysophyte phytoflagellates
that reached their greatest biomass in 4 lakes during this time (Fig. 4).
Interestingly, the 6 inland lakes were completely frozen during this period, while
Lake Michigan was inundated with floating ice sheets at our sampling locations
during the winter 2011–2012 (NOAA-GLERL 2017).
Contribution of winter assemblages
Our data showed that the biomass of winter phytoplankton assemblages was
contributed mainly by the presence of phyto-flagellates (Fig. 3). The dominant
flagellates occurring in the inland lakes were composed of chrysophytes, specifically
several species of Dinobryon (D. cylindricum O.E Imhof, D. bavaricum O.E
Imhof, D. divergens O.E Imhof, and D. sertularia Ehrenberg). In Lake Michigan,
several phyto-flagellate species dominated the winter assemblage, including the
cryptophytes Rhodomonas minuta Skuja and dinoflagellate Gymnodinium varians
Maskell. This trend was surprising given that the lakes were ice covered, creating a
seemingly harsh set of environment conditions for 3 months, when the environment
was typified by low light (no light penetrated under the snow and ice in the 6 inland
lakes) and low temperature (1.5–3.5 oC).
During ice cover, cumulative chlorophyll biomass was considerable in all of
the lakes. When comparing phytoplankton biomass in the 6 island lakes over the
entire year versus the biomass in chlorophyll that occurred during ice cover (December–
February), winter assemblages contributed 24.8 ± 7.2% (mean ± 1 SD)
of the annual biomass (Fig. 5). The winter assemblage in Fox Lake showed the
smallest contribution (12% of its annual biomass), while Barney’s Lake displayed
the largest contribution (31.5%) (Fig. 5). In Lake Michigan, the contribution by
Figure 5. Compilation of all biomass in carbon of sampling dates highlighting the contribution
by winter assemblages to the annual biomass.
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winter assemblages was about ~7 %, but this total also excluded the 3 months (October–
December) that were taken into account for the island lakes (Fig. 5). Thus,
the winter phytoplankton community may play a key, but understudied, role in the
overall function of the microbial food-web dynamics.
Discussion
Spatial patterns in lake biogeochemistry and phytoplankton biomass
The Beaver Island lakes share a historic relationship with Lake Michigan because
their formation was thought to be regulated by the geologic history of the
Laurentian Great Lakes some 12,600 years ago (see Lewis et al. 2010). Thus,
the Beaver Island archipelago and specific landscape features were likely produced
during the last North American glaciation, which formed the Laurentian Great
Lakes through sequential fluctuations in lake water levels and the isostatic rebound
of the landmass (Colman et al. 1994, Ewert et al. 2004). This scenario seems
reasonable given the timeline for the formation of other landscape and geologic
features that currently exist on Beaver Island (e.g., Angeline’s Bluff and Bonner’s
Bluff; see Dietrich 1988). Furthermore, the lakes on Beaver Island were formed
during different geologic periods, indicating that their ages differ, which may explain
the divided spatial variation in biogeochemistry observed between the interior
and perimeter lakes. For instance, the interior lakes include Greene’s, Fox, and Egg
lakes (Dietrich 1988) that were formed earlier in the natural history of the island,
while the perimeter lakes— Barney’s, Font and Geneserath Lakes—consisted of
more recently formed embayments of Lake Michigan (Leuck et al. 2007).
The large degree of variation in lake biogeochemistry that we observed among
the lakes on Beaver Island was atypical considering their relative proximity (see
Wetzel 2001). This result suggests a relatively unusual scenario in comparison to
those in other regions in North America. Specifically, lakes in the interior of Beaver
Island exhibited low pH and conductivity, and appeared to be dystrophic in their
trophic status, with higher levels of dissolved organic carbon and lower lightpenetration
compared to the perimeter lakes (see Tables 1, 2); these lake conditions
are typical of older and more productive lakes (see Williamson et al. 1999). Interestingly,
we detected little difference among lakes in terms of total phosphorus,
oxygen, or temperature, while average chlorophyll and phytoplankton biomass was
significantly greater in the interior compared with the perimeter lakes (Table 2, Fig.
2B). These findings are somewhat surprising, given that phytoplankton biomass
often correlates with increasing total phosphorous content (Dillon and Rigler 1974,
Filstrup et al. 2014, Van Nieuwenhuyse and Jones 1996). We expected the lack of
temperature variation among lakes because of their proximity and similar climate
regimes. Although we did not measure nitrogen during this study, we acknowledge
the role of N, P, and trace elements in contributing to the seasonal changes in phytoplankton
biomass and taxonomic composition that were observed in other lakes
in the Great Lakes region, such as Lake Erie (Moon and Carrick 2007).
In looking at chlorophyll as a proxy for lake trophic state (Nürnberg 1996), the
interior lakes contained more chlorophyll compared with the perimeter lakes, and
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our data showed that the interior lakes and perimeter lakes were different from one
another in terms of their mean chlorophyll concentrations ranging from oligotrophic
to eutrophic (range = 0.8–31.1 μg L-1). Phytoplankton carbon from cell counts
showed the same pattern, thereby corroborating the chlorophyll data (r = 0.76
P < 0.0001, n = 66). The idea that lake trophic status increases with geologic age
has been well established in work on succession theory (Wetzel 2001). Moreover,
Kalff (2002) and Nünberg and Shaw (1998) compared 600 freshwater lakes and
found that lakes with higher dissolved organic carbon content (stained) generally
exhibited higher primary production and bacteria productivity compared with clearwater
lakes. More recently, Solomon et al. (2015) suggested several mechanisms
that can regulate variation in phytoplankton biomass and productivity among lakes
of varying DOC concentrations, including trade-offs between light penetration and
higher nutrients with increasing DOC. In either case, enhanced plankton biomass
and production in higher DOC lakes, which can support enhanced biodiversity (del
Georgio and Peters 1994, Young et al. 2005), has been attributed to the alternative
energy source that added DOC provides. Although our results are first-order estimates,
they support this idea.
Seasonal phytoplankton blooms and their contribution to taxonomic
composition
Many temperate lakes support relatively discrete phytoplankton bloom events
during thermal-mixing periods, with coinciding shifts in taxonomic composition
(Reynolds 2006, Wetzel 2001). This type of temporal variation in phytoplankton assemblages
likely reflects the balance between many interacting forces that impinge
on individual populations (Reynolds 2006). Of the 7 lakes we evaluated, only nearshore
Lake Michigan supported a well-defined spring bloom, which was composed
mostly of diatoms. Historical trends in that lake indicate that predictable spring
diatom blooms generally made up over 50% of the carbon in the phytoplankton assemblage
during the March–April period (Carrick et al. 2001). This annual spring
diatom bloom in Lake Michigan has been shown to fuel benthic production in the
lake (Gardner et al. 1990). Interestingly, this spring bloom has not been observed in
southern Lake Michigan since 2004, due mainly to the expansion of invasive mussels
(Fahnenstiel et al. 2010). Our data suggest that specific areas in Lake Michigan
may act as refugia where spring diatom blooms still occur, and that this disappearance
may not be a complete, basin-wide feature (Carrick et al. 2015).
Perhaps the most stiking result we observed in contrasting the phytoplankton
among lakes was their temporal dissimilary from month to month, particularly
during the ice-free months, when a mixed assemblage was present, the specific
composition of which was unique to each lake (Figs. 3, 4). Despite these considerable
shifts in taxonomic composition during the ice-free period, we unexpectedly
observed an overwhelming dominance by phyto-flagellates in all 7 lakes during
winter. The physical conditions in each lake were harsh; the light and temperature
conditions hardly seemed conducive to supporting phytoplankton under the ice
(Table 1). However, all of the inland lakes exhibited dominance by chrysophytes,
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2017 Vol. 24, Special Issue 7
specifically 4 species of Dinobryon (D. bravaricuum, D. divergens, D. cylindricum,
D. sertularia), while Lake Michigan was dominated by the cryptophyte, Rhodamonas
minuta. Thus, these conditions appeared to select for chrysophytes like
Dinobryon regardless of the biogeochemical conditions and trophic states of each
lake. Interestingly, Dinobryon is commonly found in ice-covered phytoplankton
communities, where it has exhibited blooms and is capable of grazing on pelagic
bacteria (Abgeti and Smol 1995, Berninger et al. 1992, Thomas et al. 1991, Watson
et al. 2008). Our data suggest that this lake feature may be widespread and occur
with some level of temporal synchrony. Our data were not of high-enough resolution
to evaluate short-term variation among lakes or changes in abundance with
residence time under the ice. However, it was evident that, as observed elsewhere
(Vanderploeg et al. 1992), conditions under the ice lacked any significant light penetration,
likely due to the snow cover on top of the ice.
Winter ice-cover conditions and the role of phyto-flagellates: An hypothesis
When phyto-flagellates dominate winter assemblages, they have the ability
to employ mixotrophy, which should afford them the physiological flexibility
to enhance their likelihood of surviving harsh conditions of low light and
temperature (Fig. 6). In Lake Michigan, although there was not 100% ice-cover,
phyto-flagellates still dominated during cold temperatures. This unique adaptation
could provide a competitive advantage, thereby expanding their tolerance
so that they could supplement limited resources through the consumption of
bacteria. Mixotrophy has relatively high metabolic costs in order to maintain the
necessary enzymes and cellular structures that facilitate both modes of nutrition
(Tranvik 1989). Its wide geographic and taxonomic distribution suggest that mixotrophy
confers an adaptive advantage despite these costs (Bird and Kalff 1987,
Raven 1997). Mixotrophy has been observed and documented across 5 classes of
phytoplankton, among ciliates (Boraas et al. 1988), and in ecosystems of varying
climate. As such, we developed a heuristic model to visually display how
phyto-flagellates might employ mixotrophy in temperate lakes in both scenarios
of ice-cover and ice-free conditions (Fig. 6). In scenario A, winter conditions in
ice-covered lakes result in low (or no) light that limits photosynthesis. Under
these conditions, phyto-flagellates decrease the size of their chloroplast, decrease
their uptake of dissolved inorganic carbon (DIC) due to depressed photosynthesis,
and instead enhance their intake of bacteria. The carbon source that fuels excretion
under these conditions originates from particulate organic matter in the form
of living bacteria in the water column. Under typical ice-free lake conditions (scenario
B), ample light is available and phyto-flagellates increase their chloroplast
size, increase DIC intake, and decrease their consumption of bacteria. The carbon
source fueling excretion under these conditions originates from dissovled inorganic
matter that has been remineralized by bacteria in the water column.
Many lakes around the world have been shown to exhibit algal growth during
ice cover (Hegseth 1998, Legendre et al. 2011, Smith and Nelson 1986,),
although few studies have documented synchrony among lakes of varying condiNortheastern
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tion. Additionally, we appear to have identified a unique phyto-flagellate assemblage
present in these lakes that is particularly adapted to these highly selective
conditions (mainly chrysophytes). Previous studies have demonstrated that phytoflagellates
were able to survive longstanding periods of little or no light (Berge
et al. 2008, Tittel et al. 2003, Watson et al. 2008) because they employ unique
adaptations to perform heterotrophy or mixotrophy to maintain viable populations
(see Fay et al. 2013). Recent studies have shown mixotrophy to be a significant
component of carbon cycling in the oligotrophic ocean, with plastid-bearing protists
experimentally exhibiting higher rates of bacterivory than aplastidic protists
(Hartmann et al. 2012, Moorthi et al. 2009). Arenovski et al. (1995) and Hartmann
et al. (2012) hypothesized that plastidic protists compensate for the insufficient
amount of inorganic nutrients in oligotrophic ecosystems by consuming bacteria.
Thus, it is possible that the scarcity of resources during harsh winter conditions
has caused selection for planktonic organisms with physiological capabilities to
not only withstand harsh conditions, but that may actually act opportunistically on
the conditions of low light and temperature.
Figure 6. A heuristic model outlining the potential importance of mixotrophy during icecover
and ice-free periods. (A) During ice-cover period, phyto-flagellates rely most heavily
on bacterivory relative to photosynthesis; ice cover reduces light availability leading to
increased uptake of bacteria with a reduction in the size of plastids. (B) During ice-free
periods, light availablity is relatively high, prompting the full development of plastids and
enhanced uptake of dissolved inorganic carbon (DIC) by phyto-flagellates with little to no
bacterivory (where, POC = particulate organic matter, DOC = dissolved organic matter).
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Acknowledgments
Discussions with P. Lavrenteyv were helpful in developing some of our ideas. We thank
K. Carrick, C. Frazier, D. Schuberg, and A. Stimetz for their generous technical assistance in
the field and laboratory. S. Taylor completed the poly-phosphate analyses. Travel and some
material costs for E. Butts were supported through the Biology Undergraduate Research
Mentoring Program (BUMP) at Central Michigan University. Laboratory space, boat use, and
housing were provided through an internal grant from the Institute for Great Lakes Research
(Central Michigan University Biological Station) to H.J. Carrick. We thank A. Monfils and C.
Damer for their support though the BUMP Program. The paper is contribution number 82 of
the Institute for Great Lakes (IGLR) at Central Michigan University. We also appreciate the
comments of 3 anonymous reviewers on an earlier draft of this manuscript.
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