Online Resource 2
Landscape-scale food webs of fish nursery habitat along a river-coast mixing zone
Joel C. Hoffman*, John R. Kelly, Greg S. Peterson, and Anne M. Cotter
US Environmental Protection Agency, Office of Research and Development, National Health
and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, 6201
Congdon Blvd, Duluth, Minnesota 55804 USA
*corresponding author; email: hoffman.joel@epa.gov, phone: 218-529-5420, fax: 218-529-5003
Comparison of Methods for Estimating Phytoplankton 13C Values. We evaluated three
methods to calculate the phytoplankton 13C value: using periphyton 13C values to estimate 13C
fractionation (13C) and then adjust this value for known differences in 13C between
periphyton and phytoplankton; using consumer 13C values (planktivorous fish larvae) to
estimate phytoplankton 13C; and using particulate organic carbon 13C values (13CPOC) by
correcting for algal biomass based on Chl a content. The latter method yields comparable
estimates to isolated phytoplankton samples (Marty and Planas 2008). For the first two methods,
we included a relationship between 13C and temperature (Temp, °C) because Lake Superior
stations generally had surface temperatures 3-12° C cooler than the tributaries. In the oceans,
there is pronounced temperature (or latitudinal) dependence of marine POM 13C due to covariance among factors affecting fractionation, such as DIC concentration, phytoplankton growth
rate and cell size, and nutrient availability (Goericke et al. 1994; Goericke and Fry 1994).
First, we calculated fractionation (13C) between periphyton and DIC as the difference
between the mean periphyton 13C value and the corresponding station mean 13CDIC value (i.e.,
13Cperiphyton = 13Cperiphyton – 13CDIC), assuming CO2 uptake by algae. For river thalweg
periphyton samples, the least-squares linear regression between 13C (13Cthalweg periphyton) and
mean station temperature (Temp) was significant (p <0.0001, r2 = 0.67): 13Cthalweg periphyton = 26.47‰ + 0.56 x Temp (Online Resource 2, Fig. 1). We similarly examined 13C estimates for
samples from embayments and side channels (13Cembayment periphyton); the five samples had a
small temperature range and similar 13C, ca. -10‰ (Online Resource 2, Fig. 1). The 13C
values from the warmest sampling period (20° - 22° C) were similar to fractionation values
reported for benthic periphyton (ca. -9‰) and epiphytic periphyton (ca. -14‰) in temperate
freshwater ecosystems (Online Resource 2, Fig. 1). Freshwater phytoplankton 13C values are ca.
-7‰ less than benthic periphyton 13C values (France 1995); thus, we would expect
13Cphytoplankton = -33.47‰ + 0.56 x Temp.
Second, we similarly examined the relationship between fractionation based on
planktivorous fish larvae (13Cfish) and temperature, where 13Cfish = 13Cfish – 13CDIC, which
was estimated using the mean 13C value from stations dominated by planktivorous fish larvae
(average of species’ means; Online Resource 2, Fig. 1). The regression was significant (p
<0.0001, r2 = 0.86): 13Cfish = -33.14‰ + 0.55 x Temp. The result was remarkably similar to
that predicted based on the periphyton samples. Assuming trophic fractionation was +0.8‰, the
corresponding equation to estimate phytoplankton 13C is: 13Cphytoplankton = 13CDIC – 33.9‰ +
0.55 x Temp.
Third, we estimated phytoplankton 13C values based on 13CPOC values by correcting for
algal biomass: 13Cphytoplantkon = (13CPOC – (1- x) x 13Cterr) / x, where x is the algal fraction based
on C: Chl a and 13Cterr is the value of terrestrial OM, essentially vascular plant-derived C
(Marty and Planas 2008). We used C: Chl a = 80, the mean value reported by Marty and Planas
(2008), and 13Cterr = -26.0‰, the typical POM value in riverine stations when Chl a was <1.0
g L-1. We estimated 13Cphytoplankton values for stations in which the river fraction was 1.0 or 0.0
(i.e., lake water) to meet the model assumption that the only contributing sources are
phytoplankton and terrestrial OM. We subsequently compared 13Cphytoplankton estimates based on
either 13CPOC values or 13CDIC and 13Cfish values (i.e., the second method). The two methods
yielded similar results; the regression was highly significant with a slope of nearly 1 (p <0.0001,
r2 = 0.80, slope = 0.90 [standard error 0.11]). The mean difference between the two approaches
was +1.3‰; the method using 13CPOC values estimated a slightly higher value for
13Cphytoplankton. Because all three approaches strongly concurred, we estimated 13Cphytoplankton
values for all stations and cruises: 13Cphytoplankton = 13CDIC + (0.55 x Temp – 33.9‰).
-5
Plant - DIC = -9 (benthic periphyton)
13C, ‰ (biota - DIC)
-10
Plant - DIC = -14 (epiphytic periphyton)
-15
-20
Plant - DIC = -21 (phytoplankton)
-25
planktivorous fish
periphyton, river channel
periphyton, embayment
-30
8
10
12 14 16 18 20 22
Mean water temperature (°C)
24
Online Resource 2, Figure 1 13C (difference between the station mean 13C values of various
ecosystem components and the station mean 13CDIC values) as a function of station mean
water temperature. The dotted lines indicate typical 13C fractionation values for
phytoplankton (-21‰, Peterson and Fry 1987), epiphytic periphyton (-14‰, France 1995),
and benthic periphyton (-9‰, France 1995). For the two least-squares regression lines
shown, river channel periphyton: 13C = -26.47‰ + 0.56*Temp, r2 = 0.67, p = 0.007; and
planktivorous fish: 13C = -33.14‰ + 0.55*Temp, r2 = 0.86, p<0.0001