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Isotopic variability of organic carbon in a phytoplankton-based, temperate estuary
Gmehtmrca (B Ptqamon
00167037/84/f3.C0
d Cosmxhimica ACUI Vol. 48, pp. 1089-1098 Rcu Ltd. 1984. Printed in U.S.A.
+ .OO
Isotopic variability of organic carbon-in a ph~op~kton-~~, temperate estuary J. N. GEARING,P. J. GEARING, D. T. RUDNICK,A. G. REQUUO and M. J. HUTCHINS Graduate School of Oceanography, University of Rhode Island, Narragansett gay Campus, Narragansett, RI 02882
(Received July 28, 1983;accepted in revisedjbrm January 3 1, 1984) Abatrmet-An isotopic survey was made of organic carbon in ph~op~n~n, sediments, zcoplankton, larvalfish,and benthic fauna from Narragansett Ray and the Marine J&systems Research Laboratory, Rhode Island; the results quantify the extent of variability in a phytoplankton-based ecosystem and elucidate some of its causes. Carbon from primary producers (phytoplankton) varied with taxon and size, ranging from -20.3 f 0.69b (mean -C 1 s.d.) for diatoms (primarily Skeletonemu cosratum) to -22.2 + 0.6%0 for nanoplankton (primarily microflagellates and non-motile ultraplankton). Planktonic isotope ratios varied little with either water temperature (0 to 2O’C) or degree of preservation (up to 2-year aerobic diagenesis in sea water). Isotopically, sediments from East and West Passages of the bay were homogeneous with location and depth, with a mean (-21.8 + 0.6%) similar to a mixture of carbon from diatoms and nanop~n~on. Providence River sediments reflected venous and an~m~nic carbon (sewage) in their isotopic ratios (-24.2 f 0.7%~). Ratios of macrozooplankton (~150 pm) were statistically separable from those of concurrently collected phytoplankton, being, on average, 0.5 to 0.65%more positive. Secondary consumers in the water column (shrimp and larval fish) were 2.4%0heavier than diatoms. Thirty-four taxa of benthic fauna had relatively positive isotope ratios (- 18. I f 1.5%) which may indicate preferential use of carbon originally from diatoms rather than nanoplankton. The wide range of benthic ratios (-22.7 to - 14.9%0)resulted from both intraspecific variability (mean range = 3%) and the variety of trophic positions occupied. Some of the intraspecific variability could be related to size. Among species, the isotope ratios increased from meiofauna (-19.5 f 0.4%) to macrofaunal non-carnivores (-18.6 + 1.3L) and carnivores (-16.6 + 0.8%). INTRODUCI-ION
(3) little or no isotopic fractionation between a consumer and its carbon supply. The extent of change STABLECARBON ISOTOPE ratios, measured as G”Cpna,’ from one trophic level to the next is difficult to dehave been widely used as a tracer of organic carbon termine since individuals of the same species grown in biogeochemical systems having two or more isoon the same diet may vary by -2%0 (DENIRO and topically distinguishable sources (PARKER, 1964; EPSTEIN,1978). Laboratory feeding studies on seventyCALDER and PARKER, 1968; NEWMAN et al., 19’73; six consumer-food pairs (Fig 13) and forty-four field HAINES, 1976a,b; GEARING et a!., 1977; FRY et al., rn~u~rnen~ of animals and their inferred food (Fig. 1977, 1978; THAYER ef al., 1978; HAINES and MONIC) revealed differences averaging +0.4 + I .4 and +0.6 TAGUE, 1979; M&ONNAUGHEY and McRou, 1979b; f I .4%0respectively. These data imply that consumers TESZENet al., 1979; BURNETT and S~HAEFFER, 1980; are slightly more positive than their food. However, PRY, 1981; RODELLI et al., in press). Several assump the range of differences is too great to generalize with tions are inherent in their use: any certainty. (1) a relatively constant isotopic composition of organic carbon produced by each plant source. Variable Deviations from these assumptions can lead to wide isotopic signatures, according to the physical and isotopic variability in systems with a single major carchemical conditions of growth, have been shown in bon source. For example, unexplained variations have laboratory and field studies for both higher plants been reported between plankton, sediments, and con(LOWDONand DYCK, 1974; GUY et al.. 1980; STE- sumer organisms of up to 5% in the Gulf of Mexico PHENSON et al.. 1983) and phytoplankton(DEGENSet (FRY and PARKER, 1979), 8960 in the Bering Sea al., 1968b; DEUSER et al., 1968; DEUSER, 1970; (MCCONNAUGHEYand McRou, 1979a), and 7% in CALDERand PARKER, 1973; PARDUEel al., 1976; the Straits of Malacca (RODELL~et al., in press). Such WONG and SACKETT, 1978); ranges reduce the resolution of studies which use S13C (2) an unch~~ng isotope ratio in plant carbon as to trace the mixing of carbon from two or more sources. it decomposes and is broken down to detritus. Among We examined in detail a temperate estuary (Narrahigher plants, ten pairs of living and dead/decomposed gansett Ray, Rhode Island) with a single major carbon plant material had an overall average difference of source (phytoplankton). and by quantifying the varionly 0.015960 (Fig. IA); ability of a13C and ascertaining its causes, focused on the three assumptions previously mentioned. All the major organic carbon reservoirs were sampled extensively at different seasons and the results were tested ‘6-&m = [(R,,+)/(R~) - I] X IOOO, where R = iQ’2C. to distinguish nonrandom isotopic differences. By 1089
I090
.I. K Gearing el al
‘1”
Difference
and August, 1980 and January. 1982. the diatom .S!&~t)ne~u t'osfarum made up most of the carbon in the water, whereas nanopiankton which pass through a 10 am Nitex mesh sieve constituted the bulk of the ph~oplank~on in samples taken during most of the period April-November, 1982. The most abundant phytoplankters in the fraction < 10 brn were uncharacterized microflagellates and nonmotile ultraplankton (FURNAS, 1982). Plankton fractions were filtered onto precornbusted (450°C) glass fiber filters, air dried. and stored in glass vials until analyzed. The samples were not routinely acidified to remove inorganic carbon as part of the work-up because tests showed acidification to have no observabie effect on the stable isotope results. Larval fish, hferzidiu menidin, were grown in flowing seawater from one of the MERL control tanks. Freshly hatched larvae were placed in small tanks in July, 1982. Approximately 25 fish were collected on days 0. 7. and 14. Several (3 to 12) individuals were combined for each isotopic measurement. The fish were dried, weighed, and stored in a desiccator. Before analysis, the whole tissue was ground, acidified to remove inorganic carbon. and tiltered.
in Isotope Ratios, %*
FIG. 1. Change in 613C(%D)resulting from (A) plant degradation,and (B, C) animal food assimilation (B, laboratory studies with known food, C, field studies with inferred food). Bar indicates mean, arrows one standard deviation. Data are from: (A) SMITH and EPSTEIN, 1970; HAINES, 1976b; FRY, 1977; HAINES, 1977; THAYER et al., 1978; HAiNESand MONTAGUE, 1979; (B) FRY, 1977; DENIRO and EPSTEIN, 1978; HAINESand MONTAGUE,1979; TEERI and S~HOELLER,1979; FRY and ARNOLD, 1982; MACKOet al.. 1982; (C) SACKEIY’ et al., 1965; DEGENSet al.. 1968a; DEUSER, 1970; MINS~N et al.. 1975; FRY, 1977; LAND et al., 1977; FRY et al., 1978; MCCONNALIGHEY and McRov, 1979a; PETELLE et al., 1979.
means of such individual ecosystem studies, our understanding of isotopic f~~ionations in nature is enhanced and we are better able to interpret results from ecosystems having more than one carbon source.
41'
410'40'
MATERIALS AND METHODS Samples for this study were collected in the Providence River, in the East and West Passages of Narragansett Bay, and in the microcosms of the Marine Ecosystems Research Laboratory (MERL), University of Rhode lsiand (Fig. 2). The biological, chemicai, geological, and physical oceanography of the Narragansett Bay estuary has been studied intensively
41’ ‘35
(e.g.HicKs, 1959;McM~srE~, ~~~~;COLLINS, 1974;OvlArr and NIXON, 1975). The MERL control microcosms are de-
signed to mimic the biological and chemical behavior of midNarragansett Bay. They consist of large, outdoor liber@ass cylinders (1.8 m diameter, 5.5 m height) with sediments, flow-through seawater, and their associated biota from midNarragansett Bay (41”351v and 71”22W). Details of these systems and effects of containment can be found in hLSON et al. (1979, ~980) and GRA.SLE et al. (1980). Plankton were isolated from net tows (69.5 gm and 150 pm mesh) and from whole water samples. During periods of high standing crops, diatoms in the size range 35-64 pm were trapped in the mr mesh nets. In the laboratory, material was size fractionated by washing through several mesh sizes of Nitex sieve, and each f-ion was examined microscop i&y. The fraction > 150 em was picked until it consisted of macrozoopiankton: caianoid copepods were usually numerically dominant. During phytoplankton blooms in January
41°
3d
-21.8 710 25'
I 7PP$
FIG. 2. Sampling location and IS’% of organic carbon in surface sediments of western NarraganseU Bay.
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Phytoplankton C isotopes Sediments were co&ted with a gravity corer We day) or open-bar&d, hand-h& corer (MERL micrWosrns; GE,XRWSet ui., 1980; FRITHSEN d al., 1983) and were stored fioren. Depth sections of each extruded core were washed ~a0.85mmmeshsicvemremoveshcllsando~ acidified with HCl to remove carbonate, tiltered, washed, dried at 4O”C, and powdered. The majority of benthic organisms were collected by box core (0.25 m2 USNEL spade corer) on May 14, 1981 and Bay. Small macrofauna July 13,1982, from mid-N_ and meiofauna were isolated from open-barrel corer taken by divers. gome benthic organisms were coikcted by openbarreled, hand-held corers in 1979-1983 from MERL microcosms containing mid-bay sediments. The benthos were washed through sieves (2.0 mm mesh for box core samples, 100 pm mesh for other samples) with seawater. Meiofauna were picked from the top half centimeter of sediment. Organisms retained on sieves were identified separated into groups or species, and held live in clean seawater for l-2 days to clear their guts before being stored frozen. After being thawed, each sampk was examined under a dissecting microscope; tissue with no evidence of gut material was selected for analysis. Organ&s with shells were shucked or treated with dilute HCI to remove carbonate. Tissues were then filtered, dried, and stored. All samples were combusted in a LECO radio-frequency furnace (PARI(ERet al., 1972). The sediments were weighed before combustion and the volume ofCOr evolved (measured manometricaUy)was used to determine catbonat&ee otganic carbon contents. The organiccarbon of some sediments was also measured using a Carlo Erba Model 1106 elemental analyzer. The CO, produced from each sample was purified and its stabIe carbon isotope ratio (&r%)was measured using a Micromass 602 mass spectrometer. Details of the mass spectrometry and standard are given in CRAIG ( t957), NIER er al. ( 1962)and SCAXAN and MORGAN( 1970).Analyses of replicate samples and comparisons with other laboratories have shown our analytical precision (1 standard deviation) to be fi.31 (L%NERand PHILLIPS,1981; &rNERand Bttls,
1982;MORSELLI and BAGC~ET~, in press).Sets of values from various carbon reservoirs were compared using the non-parametric Mann-Whitney “U”-test (LCIR~AHL, i967), which does not require the assumption of equal ~p~ation variances. RESULTS AND DEXXJSSION Samples for thii survey were collected both directly from Narragansett Bay and from MERL control microcosms with bay water, sediments, and organisms. Comparisons of results (Table 1) showed no differences at the a = 0.1 level. On this basis, the two sampling locations are assumed to be isotopically equivalent.
a
13
6 Dstc
Phyropluskton
z.x+ankt.nb
-20.6 t 0.4(12)= -19.7 t 0.3 tsj= -20.2 L o.7ao)d
-20.0 t I.O(l2) -19.3 f 0.3 (5)
co.5 t 1.1 +0.4 ? 0.3
-20.3 t 0.6(27)
-19.8 f 0.9(17)
co.5 2 1.0
T.-C
l/80 8180
1
20 1
l/S2
1182 4182 5182 6f82 7102
1 4 12 16 19
'1..
CPW
AZ-P
-21.8 t 0.3 (7)' -22.2 -22.0 -22.2 -23.2 -22.5 -22.4 -21 . 5
i:0.4 L 0.2 t.0.2 t 0.4 t 0.7
(3)f (3)f (3)f (S)f
-21 -20.9 .8 -22.7 -22.9 -21.8 -20.9 49.8
* 0 1s I:;:
f 1 f t t
.
0 6 0.3 1.1 1.6 0.6
I:; (3) (31 (4) (3) (2)
Cl.2 +0.4 -0.5 +O., +0.6 +I.6 +2.0
? t t i t
0.3 0.3 0.7 1.4 0.3
-22.2 * 0.6(29)
-21.7 ? 1.3(19)
+0.6 t 1.0
-21.3 f l.l(SS)
-20.8 f 1.5(36)
+0.6 f 1.0
that of plankton ratios reported in the literature for other temperate regions (Table 3). The variability of these literature data, however, is large enou@r to cause considerable uncertainty when attempting to trace the mixing of carbon from planktonic and other sources (where the difference of means is usually 5 to 10%). The causes of the variance have not been determined, but isotopic changes have been correlated with species composition (WONG and SACKS?; 1978), temperature (SACKETT et al., 1965; FONTUGNEand DUPLESSY, 198 I), water masses (FONTUGNRand DUPLESSY,1978), latitude (RAU et al., 1982), and &“C of inorganic carbon fixed (PARKJZR, 1964; DEUSER, 1970; SMITH and K.RooPNICK, 198 1f. In this study, samples were taken to examine some of these relationships. Most of the variability could be explained by differences in phytoplankton types (standard deviation reduced from 1.1 to 0.6). Temperature had little or no discernible effect on phytoplankton isotope ratios. Likewise, extensive aerobic decomposition in seawater caused no 613Cchange in the one planktonic sample examined. Variations of phytoplankton 813C brought about by changes in the isotope ratio of the source inorganic carbon (ICC) are not specifically addressed in this paper. Preliminary data indicate little variation in IOC
Primary producers Overall, the fifty-six samples of phytoplankton averaged-21.3+ l.l%owitharangeof-23.4to-19.3969 (Table 2). The avenge and distribution are similar to
Table 3.
Type of Sample ParLiCUlaCe or*. ellrbonb Partlculstt
‘ 613C (iample#
* lrna mrandsrd
U7D.L
SW
Phyroplankton, Janury Sedimats, nid-Pa, Saathic Or~niu, Hay e -n
.I.*
PDB-
-20.6 f 0.4 (12) -21.7 t 0.3 (13) -18.6 t 1.4 (30)
dw4ation
(nu~aar
cf
-20.2 t 0.7 (10) -21.5 2 0.5 (27) -19.1 t 2.1 (10,
uaplasf
111otoPeratMe
org.
edsn=
P.itieti.,teorg. eatbond Particulste org. c*rhone PartiCUZaCC cxg. cart& Hat plantrtoof Ner plazxkto& Wet Planktonff Net plmktonh Net plmktoa~ Nat P1.nkto.f Nat ,,l.nkronk
b+'CP,)
of plankton uorldvide.
Lc.c.tian 45-N
to 50-N
to 59.1 133-N 18% Le 29-N -ulS*W 6O'N to Lt=N 45% to 50% %33=N 28.N to 39-s 21-N 19.S 16.N to 37.9 3'8 to 13-S 28.N
to
n
Hsrn
11 5 5 23 12 4 5 12 12 71 27 18
-23.9 -20.6 -23.2 -22.0 -21.8 -19.6 -23.8 -20.9 -21.9 -19.7 -20.1 -19.3
S.D.. 0.8 0.4 0.8 2.1 I.0 1.3 1.2 0.9 1.6 0.6 0.7 1.2
RanSs 3.0 1.0 2.3 8.9 3.2 2.3 3.1 2.7 4.9 4.2 2.6 4.7
pY" (1974) SACKSTY et al. (1965) ~lilwsl!~tl~wPLSSSY (1981) 'FOHTUCNE aad DUPLSSSY (1978) %T%XS s &. (1968.)
I092
.I. N.
Gearing e[ al
#3C in this well-mixed estuary except during the largest winter-spring blooms. Note that the standard deviation of samples collected over a month’s period during the 1982 winter-spring bloom was larger than that of samples taken on a single day during the 1980 bloom or during a smaller bloom in August, 1980 (0.7 versw 0.4 and 0.3%0, Table 2). Variation caused by the presence of zooplankton in samples will be discussed in a following section. The greatest isotopic variation was between samples in which diatoms p~ominated (-20.3 ?Z 0.6%, n = 27) and those where nanopiankton were the most abundant phytoplankters (-22.2 + 0.6%. n = 29). The two sets of &‘%Zvalues are significantly different (a < 0.001). There was a similar difference between nanoplankton (< 10 pm fraction) and net samples of Skeletonema costatum collected at the same time (Table 2, January 1982). The result is a seasonal difference in overall planktonic isotope ratios, since nanoplankton are the primary Narragansett Bay phytopfankton in summer whereas diatom blooms occur in the winter-spring and sometimes in the fall (PRATT, 1959; DURBIN et al., 1975). This change is potentially useful for tracing carbon seasonally and for determining food sources in this plankton-based ecosystem. This finding also suggests that the method by which ph~oplankton are sampled may sibilantly affect the isotope ratio obtained. Samples from which nanoplankton and picoplankton are effectively excluded, such as collections from nets of large mesh, may have ratios more positive than samples containing smaller size fractions such as particulate organic carbon from filtered water. In this study, filtered whole water samples taken in August, 1980 during a diatom bloom averaged -20.9 f OX% (n = 6), signi~cantIy more negative (CU= 0.01) than samples of ph~opIankton from net tows collected at the same time (Table 2). The largest size fractions (> 150 pm, macrozooplankton) from the same tows were even more positive isotopically (see section on water column consumers). Similarly, the smallest size fractions of water collected from a tank in March, 1982 (~15 pm) were more negative than fmctions between I5 and 64 pm (-20.9 + 0.7%, n = 5, versus - 19.8 + 0.5% n = 6, LY= 0.05). On the whole, the values of POC reported in the literature are more negative than those of net plankton (Table 3) although some of the differences may be due to the presence of zooplankton. One would of course expect the collection method to matter less when there are few large plankton present. TAN and STRAIN (1983), for instance, found no significant difference between POC and net samples collected in late summer in the St. Lawrence Estuary. We suggest that care be taken in describing the plankton cotlection method used, particularly the suite of sizes sampled, and that microscopic determination of sample contents be made when possible. Various reasons for species-specific isotope variations have been postuiated. WONG and SACKETY
(1978), growing phytoplankton in culture, reported greater fractionation of carbon isotopes by flagellated nanoplankton than by centric diatoms and ascribed the difference to species. The slower growth rates of the major nanoplankton groups relative to diatoms in Narragansett Bay reported by FURNAS ( 1982) might also lead to greater fractionation (PARDUEet ui , 1976). Species-related differences in 613C may be a result of variations in the relative contents of storage products and other biochemicals with differing d”C values (PARKER, 1964: DE(;ENS d nl., 1968a; WONG and SACKETT, 1978). Isotope variations with water temperature have been postulated on the basis of laboratory and field data (SACKETTet al., 1965: DEUSERet al., 1968; FON-IIJGNE and DUPLESSY,198 1). In particular, diatoms such as Skeietonema costatum have been reported to have more negative isotope ratios when grown in cold water (DEGENS et al., 1968k WONG and SACKETT, 1978). Other studies have found no evidence for a direct temperature effect (CALDER and PARKER, 1973: FONTUGNE and DUPLESSY. 1978; RAU et al., 1982). In this study, samples were taken during natural blooms of Skeletonema costatum: their isotope ratios did not correlate significantly with temperature (slope of +0.005% per degree C. n = 27, correlation coefficient = 0.33). The correlation of nanoplan~on isotope ratios with temperature was signi~cant (n = 29, correlation coelhcient = 0.56) but the slope was small (-0.04%0 per degree C) and in the opposite direction to that postulated. We conclude that temperature has little or no (< 1%o)effect on the isotope ratio of phytoplankton in temperate waters under natural conditions. except possibly indirectly through species succession. Finally, we examined the possibility of changes in planktonic isotope ratio during d~om~sition. There was no difference (-22.5 % 0.3%. 12= 3 vcr.rsus-22.4 ~fr0.2%0, n = 3) between a fresh sample of mixed plankton collected from Long Island Sound during an almost pure bloom of the diatom Skefetonema costatum and an aliquot of the same tow which had been allowed to decompose for nearly two years in the laboratory ( 1 kg plankton in 50 L aerated seawater). During this time the material went from 16.7 to 5.3 percent organic carbon. No evidence for isotopic change due to selective degradation (EADIF.and JEFFREY, 1973) was found. We conclude that there is little isotopic change in planktonic carbon during aquatic decomposition to detritus, just as little has been reported for higher plants (Figs 1‘4). Sediments
In the East and West Passages, sedimentary carbon isotope ratios reflected those of total phytoplankton, averaging -2 1.8 Z?Z 0.6% (n = 26) with no clear trends with location or depth (Fig 2, Table 4). Samples of mid-bay sediments from both the bay and MERL control tanks had similar values, -2 I .5 rt 0.5% (n = 701, within the range reported for marine sediments
1093
Phytoplankton C isotopes
613Ct 'f..
Depth, CP
Lacar1on
x O.C.
PROVIDENCERIVER 41'49'N 71'23'W 41'48.5'N71'24'W 41947' N 71'23'W 41*45.5'N71'23'W 41'44.5'N71=22'W 71'22'W 41'44% 1,
41'43.5'N71'22'W 11 EAST
O-5 o-5 O-5 O-5 O-1 O-5 10-41 O-l l-20
-24.5 -24.5 -23.2 -24.7 -21.5 -22.3 -22.6 + 1.0 (3) -22.0 -22.3 2 0.4 (5)
3.2 9.5 6.2 7.9 1.2 4.4 3.1 5.2 4.4
-23.6 -21.6 -22.5 -22.1 * 0.3 (3) -21.7 -21.7 f 0.3 (12) -20.6 -21.5 -21.2 -21.8 -22.2 f 0.1 (3)
0.9 4.4 2.5 2.3 1.9 1.6 4.6 0.8 1.0 1.0 1.0
AND USST PASSAGES
41'41'N
71'22.W O-1 71.19'W o-2.5 41'38'8 71*18'w o-2.5 II 7.5-32.5 41'35'8 71'22'W O-2.5 II 3-152.5 41'34.5'N71*1S'W o-2.5 41.30.5.N71'20.5'W o-2.5 41*28*~ 71*25'W o-1 41*25'N 71'26'U O-1 41’40’N
11
l-4
throughout the world (Table 5). The mid-bay is only 0.25% more negative than the seasonally integrated planktonic ratio, assuming (I) half of the yearly production by diatoms (-20.3%) and half by nanoplankton (-22.2%) as estimated by DURBIN et al. (197% and (2) equal sedimentation and preservative of both types of carbon. The sediment average could also be arrived at by greater sedimentation (SMAYDA, 19701, followed by preferential utilization by the benthos, of diatom detritus (PRATT and CAMPBELL,1956). Thus isotopic fractionation during decomposition and diagenesis does not need to be invoked to explain sediment-plankton differences found in a single sampling day (up to 3% in this study). Corresponding to the seasonal change in planktonic isotope ratios, there is some evidence for a slight shift in the ratios of sedimentary carbon. Surface (O-l cm) sediments in MERL control tanks ranged from -2 1.7 to -20.9% in April and May (-21.3 + 0.3960,n = 6) to -22.5 to -21.7% (-22.1 It 0.3960, n = 4) in JulySeptember, the sets being disting~~ble with a = 0.05. More data are needed to confirm or reject this trend, but grouping the present data seasonally reduces the
standard deviation of sediment S13C values from 0.5 to 0.3%* In the upper Providence River and some nearshore stat&&, more negative isotope ratios were measured, probably due to inputs of terxigenous organic matter such as sewage. The percentage of terrigenous organic carbon can be roughly estimated by using a simple mixing equation and assuming the isotope ratios of terrigenous and marine organic carbon to be -26.0 and -2 1.8%0reqectively (CALL~ER and PARKE%,1968; RURN~ and §CXAEFFER, 1980; S~ENEY ef al., 1980). It ranges from up to 80% near the river mouths to ~20% at the southern end of the Providence River. More information on the ratios of sewage is needed before better calculations can be made. Consumers-water
column
It has been postulated and some studies have reported that animals have more positive isotope ratios (by about 1%) than that of the organisms on which they feed, while other observations have shown the opposite trend (Fig. 1B and C). Knowing the extent of isotopic variability due to trophic levels in the field is important both for increasing the accuracy of stable isotopes as tracers and for using isotopic composition as an indication of trophic position. We examined zooplankton collected concurrently with phytoplankton; the two populations were separable with a = 0.063. Zooplankton were more positive in 25 out of 35 sets of samples, with the differences averaging 0.6k (Table 2, Fig. 3). This value is similar to those reported in the literature, but has less variability associated with it (s.d. of 1.O versus 1.4%~). When samples from diatom-dominated and nanoplanktondominat~ periods were considered separately (so as to obtain more isotopically homogenous sets), zooplankton values remained 0.5 to 0.6% more positive than those of phytoplankton (Table 2, a = 0.008 and 0.087). These differences should be considered as minima because of the impossibility of completely eliminating microzooplankton from phytoplankton samples. We conclude that zooplankton are more positive than their fbod by at least 0.5%, and also that they show a seasonal change in isotope ratio (two populations separable, a < 0.001) which correlated with that of phytoplankton. In July 1982, we obtained larval fish grown in water
Numberof
Difference
l08GUNG Ct 11. (1977) bTdJl and xsiin (1479) =INCZXSt. 11982)
dBuRmrr ."d SCIIAEFFSP. fBsO) +xxdN~~. (1973) cpuLI"G(1975)
in Isotope Ratios. 7~
FIG. 3. Difference in 613C between concurrently collected phytoplankton Bar indicates mean, arrows . . .and I zooplankton. . one aanaam aevumon.
1094
J. N.
Gearing et al
from a MERL tank. Atlantic silversides, Menidia menidia, are thought to feed on zooplankton (MULKANA, 1966). Directly after hatching, the larvae avemged - 19.5 + I. I %I (n = 4). Their isotope ratios increased with body weight, nearing an asymptote of - 18.0 + 0.5% fn = 4) at - 1 mg, or 300% of initial body weight. Ph~oplankton (>80% in the less than IQ pm size fraction) and zooplankton (primarily copepods) collected from the feeder tank around this time averaged -23.5 f OXi!&(n = 4). Either the isotopic fractionation with trophic level was unusu~ly large for these fish or they selectively assimiIated carbon derived from the iarge diatoms {estimated Si3C - 19.7% in summer, Table 2) rather than from totaI phytoplankton. These hypothesized routes of carbon flow are diagrammed in Fig. 4. Another mobile consumer sampled from MERL control tanks, the omnivorous shrimp Crungon sepfem~p~no~a (PRICE, 1962) was simi~riy positive. Four organisms cohected between March and 3uIy averaged - 17.9 + 3.04i;o.There was no trend to more negative vaiues during the summer when nanopIankton dominated the primary production. Consumers-benthos The isotope ratios of 140 samples of bentbic animah are shown in Table 6, The 34 taxa averaged - 18. I t 1.5%. Despite iarge intraspecific variability, two points are apparent from the data: f I ) 9 1%of the species averages, as well as the overall mean, are more positive
than -2OL, the upper limit for their presumed sources of carbon, phytoplankton and bulk sedimentary organic matter, and (2) the isotope ratios of species generahy fah into logicat groups corresponding to their postulated feeding behaviors. Intraspecific ranges of up to 2k have been reported for animals grown in the laboratory on isotopic&y constant diets (DENIRO and EPSTEIN, 197X: FRY and ARNOLD, 1982). The benthic organisms examined in our study had intraspecific standard deviations averaging 1%. with a mean range of 3Y0a.Sex. size, and bi~hern~~ composition have been postulated to affect &i3C vafues (FRY and PARKER, I979; HUGHES and SHERR, 1983), We found positive correlations with size for three bivalve species (Fig. 51, the larger animals tending to have more carbon- 13. The same tendency was noted for some other animals, primarily Ccriantheopsis americanus, but too few data were available for analysis. In other species the importance of size could not be assessed because of its narrow range and the practice of pooling individu~s. A positive correlation between size and 613C could arise because of changes in size of particles ingested, in biochemical make-up, or in the ratio of respiration to growth. The result is a difference of I to 2’pm,between the smallest and largest representatives of these three species. HUGHES and SHERR ( 19g3) found a positive trend of di3C with size for three inve~ebmte species from a Georgia marsh. Another possible cause of intraspecific variability is a temporal change in the isotope ratio of the animals’
PhytO~lQnktCk’! r&W. Nanoplankton
Shrtmp and Larval Fat? ?I=8 -17.9 L 2.0%
Non-carnivores
FIG. 4. Summary of organic carbon 613Cvalues (no. of samples, mean 2 one standard deviation) in a plankton-&ed -em. A, change in 6°C through carbon transfer. 1yindicates level of significantdifference based on a two-sided Mann-Whitney U-test.
1095
Phytopiankton C isotopes
‘1.. 613sr33.
XFmPAoNA
H.rpscricoid coprpodm (4/,1500) Kir.orh,nchs(11'200) Nutodea (Z/>lOOO)
-19.0 1:1.6 -19.7 -19.8
-21.0 to -17.2 -20.2 to -19.3
pucROPAuNA MmP.PACX
PesDEps -16.6 a9.7 -16.0 -21.0 -18.3 -19.5 -19.8 -17.3 -17.2 -20.3 -18.9
SIJ3SOILPACE
DEPOSIT
t 1.1 f.2.7
-21.7 Lo -18.3 -16.4 t* -15.5 -22.7 to -15.5
f 1.1
-21.8 LO -16.8 -17.6 to -15.8 -19.4 to -15,s
1:1.2
-19.8 to -17.2
-17.7 * 0.5 -21.1 -17.9 -19.5 -38.9 -18.8 t 1.3 -18.1 t 0.6 -18.1 ).1.2 -18.3 -17.6 t 0.3 -18.1 t 1.0
-18.3 LB -17.2
t 1.8
F8EDX88
wrd-rrus Ub&. (P. 4/>50) linoa .isr%~~f20f Hucul. annula& (B. 11/>200) Pnctaaria wuldit (P. 212) Pharolion strombi (S, 319) yoldia liurula (II.24/250)
-14.9 -16.9 -17.6 -16.3 -16.4 -17.7 -16.6 -16.3 -16.5
-20.1 -19.0 -20.7 -19.4 -17.9 -20.0
to to to to to to
-17.1. -17.0 -16.2 -17.2 -17.3 -15.9
5 0.7 f 0.4
-16.4 to -17.0 -17.0 LO -16.1
* 1.2
-19.4 LO -14.9 -16.6 fo -16.5
* 0.1
-16.6 to -16.4
food. Since the di3C of plankton varied seasonally, we examined the isotope ratios of benthic organisms with season. There was no consistent pattern for either the total fauna or individual species. A linear regression of 6i3C versus time gave a slope of 0.004960 per day and a correlation coefficient of only 0.17 (n = 9). More information is needed on animals of the same species, size, and growth rate to determine whether any seasonal change exists. These results suggest that a large proportion of intraspecific variability is caused by factors other than individual size and isotopic changes in food resources. To minimize this va~ability, we aualyzed as many organisms as possible and have used the species’ average b’% values for evaluation of interspecific differences. This is not a completely satisfactory solution; for example, even some species which were extensively sampled and were quite uniform in size, e.g. Mediomastus am&eta and Nassarius trivittatus, had standard deviations of over la. We suggest, therefore, that as much info~ation as possible (number of organisms analyzed, time of collection, size, biological condition, etc.) be included in future reports of isotopic studies to establish the uncertainty of the data and to facilitate understanding of the causes of intraspecific variability.
Bearing in mind the extent of this variability, we can still deduce with confidence some interspecific isotopic differences. The most apparent aspect of the data is the -overall heaviness of the benthos. All 34 taxa had averages more positive than the average values for sedimentary organic matter, nanopl~~on, or nanoplankton-consuming zooplankton. Twenty-nine (85%) had averages more positive than those of diatoms or the zooplankton eating them. This is in accord with the concept of isotopic shifts to more positive ratios with increasing trophic level. The consistently positive ratios of benthic organisms permit the speculation that they selectively assimilated carbon produced by diatoms. The o~isms might be deriving their carbon from: (1) bulk sedimentary organic matter, particularly in the case of the deposit-fders; (2) all freshly deposited planktonic carbon (MARE, 1942; SANDERS,1960; YINGST, 1976); (3) a particular fraction of the phytoplanlcton production such as diatoms (PRA’ITand CAMPBELL,1956) or nanoplankton; (4) a mixture of the above. If the first possible carbon source were important, the isotopic difference between non-carnivores and their food would be -3460. This difference is rather large but not unheard of (Fig. 1B and 1C), and utilization of this source would account for the lack of seasonal changes in cP3Cof the benthos. Preferential use of the second carbon source could lead to a seasonal change,
0
20
40
, 60
Let’yth Ol Shsll.mm
0
lo LmgthOl
20 StWl.m
30
FIG. 5. Variation in 6°C with siz.ein (A) Pitar morrhuana. (B) NucuIaanmdata, and (C) Yoldia lima&a.
1096
J. N. Gearing ef a/
with the benthos being about 2k more negative in the summer than in the spring. Slowly growing organisms might, however, integrate out seasonal variations in food supply. This isotopic shift was not noted and, in fact, there may even have been a slight trend to more positive ratios in the summer and fall-winter (prior to winter-spring diatom bloom). The third possible source fits the available data best, with diatoms being the preferred food. The isotopic differences between non-carnivores and their food would then average 1.6%0. This source would also account for the lack of seasonal changes, with any possible minor shift to more positive values in summer being attributable to reworking of carbon deposited during the winterspring bloom. The final possibility, a mixture of carbon sources, cannot be ruled out, but it does not fit well with postulated biological behavior or with the isotopic evidence. In addition to the overall positive character of the benthic isotope ratios, there were isotopic differences which correspond to the animals’ hypothesized feeding patterns. Macrofauna (Table 6) were separated into groups according to their feeding behavior: (1) interface feeders ingesting particles on or suspended above the sediment surface (SMITH, 1950; SANDERS, 1956; RHOADSet al., 1978; WHITLATCH, 1980); (2) subsurface deposit feeders primarily ingesting particles below the flocculent layer at the sediment-water interface (BLEGVAD, 19 14; SANDERS, 1960; SANDERS et al., 1962; FAUCHALD and JUMARS, 1979; WHITLATCH, 1980; LOPEZ and CHENG, 1983); (3) carnivores using living metazoan biomass and carrion to a significant extent, along with detritus and associated microbes (FRETTER and GRAHAM, 1949; GRAHAM, 1955; MANZI, 1970; BARNES.1974: FAUCHALDand JUMARS, 1979). The most notable of the differences with known feeding behavior is the 2.1 %a difference between noncarnivores and carnivores (Fig. 4). The averages of the two sets of species are separable with LY< 0.001. Although individuals had widely ranging b13Cvalues, the carnivores were a fairly homogeneous group isotopitally with a standard deviation of 0.8% and a range of averages of 2.8%0. This clustering may result from general consumption of a variety of animals based upon their abundance and availability. The non-carnivores overall were more heterogeneous isotopically (- 18.7 f 1.3560,Fig. 4). Within this group, meiofauna averaged - 19.5 + 0.46, interfacefeeding macrofauna - 18.6 + 1.6% and subsurface deposit-feeding macrofauna - 18.6 It 1.O%. The similarity of the averages (none is significantly different from the others at a = 0.05) and the large standard deviations among the macrofauna suggest that this categorization inadequately portrays the complex flow of carbon in this benthic community. For some individual species where feeding behavior is well known, the isotope ratios varied as expected. For example the deposit-feeding bivalves Nucuia anmdata and Yoldia limatula were - 1.5% more positive than Mu/inia la-
teralis and Pandora gouldiana, filter-feeding bivalves. The meiofauna were generally more negative than the macrofauna. Ceriantheopsis americanus. an anemone which may be feeding on copepods, was one of the most negative animals, and Asychis efonguta, a maldanid polychaete which always feeds below about 5 cm, presumably on reworked organic matter from the upper layers of sediment, was one of the most positive organisms. A more comprehensive interpretation of the 6°C values of non-carnivores, however, must await improvements in both biological information and knowledge of intraspecific isotope variations. The overall pattern is of increasing isotope ratios with trophic level, progressing from diatoms (-20.3%0) to zooplankton (- 19.8’%), meiofauna (- 19.5%0),noncarnivorous macrofauna (- 18.6%0),and benthic predators (- 16.6%). Similar values and trends have been reported worldwide (Table 7). The consistency of these numbers is strong evidence of an isotopic shift to more positive values with increasing trophic level. The data also provide a good picture of the isotope ratios and their variations for most phytoplankton-based ecosystems. Acknowledgements-We thank Susan Brown-Leger, Jeffrey Frithsen, Eric Klos, Chip McCarty and Andrew McLaughlan for their aid in collecting and identifying samples. Larval fish were grown and provided by BarbaraSullivan and Chris Powell. Samples of fresh and degraded plankton were obtained from Joseph Westrich and Robert Berna (Yale University). The aid and suggestions of Candace Oviatt, Michael Piison,
and the staff at MERL are gratehtlly acknowledged. P. L. Parker, E. B. Sherr, and P. M. Strain improved the paper with their suggestions in review. This work was supported by EPA Cooperative Agreement CR-807795 and NSF Grant OCE-80-18831.
REFERENCES BARNEY R. (1974) InvertebruteZoology. Toppan, Singapore. BLEGVAD H. (1914) Food and conditions of nourishment among the communities of invertebrate animals found on or in the sea bottom in Danish waters. Rep. Dunish Biof. Sm. 22, 41-78.
BURNETT W. C. and SCHAEFFER 0. A. (1980) Effectof ocean dumping on 13C/‘v ratios in marine sediments from the New York Bight. Estuarine Coastal Mar. Sci. 11, 60561 1.
CALDER J. A. and PARKERP. L. (1968) Stable carbon isotope ratios as indices of petrochemical pollution of aquatic systerns. Environ. Sci. Technol. 2, 535-539. CALDERJ. A. and PARKERP. L. (1973) Geochemical im-
Phytopiankton C isotopes
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(
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Differences in natural carbon isotope ratios of milk and hair from cattle grazing tropical and temperate pastures. Nature 256, 602. MORSELLIM. and BAGGETTK. L. ( 1984) Mass spectrometric determination of cane sugar and corn syrup in maple syt-uti by use of “C/t% ratio: collaborative study. J. Assoc. Us/: Anal. Chem. (in press). MULKANAM. S. (1966) The growth and feeding habits of juvenile fishes in two Rhode Island estuaries. Gul/Re.s. Reports 2, 97-168. MYERS E. P. (1974) The concentration and isotopic composition of carbon in marine sediments affected by a sewage discharge. Ph.D. dissertation, California Institute of Technology. NEWMANJ. W., PARKERP. L. and BEHRENSE. W. (1973) Organic car&m isotope ratios in Quatemary cores from the Gulf of Mexico. G~h~rn. Cosmochim. Acta 37,225238. NIER A. O., ECKELMANN W. R. and Lupro~ R. A. (1962) A mass -meter for isotope ratio analyses. Anal. Chem. 34, 1358-1360. OV~ATTC. A. and NIXONS. W. (1975) Sedunent resuspension and deposition in Narragansett Bay. Estuan’ne CoastalMar. Sci. 3, 201-217. PARDUEJ. W., SCALANR. S., VAN BAALENC. and PARKER P. L. (1976) Maximum carbon isotope ~~onation in phot~ynth~s by blue-green aigae and a green a&a. Geu c&m. Cosmochim. Acta 40, 309-3 13. PARKERP. L. ( 1964) The biogeoehemistry of the stabie isotopes of carbon in a marine bay. Geochim. Cosmochim. Acta 28, 1155-l 164. PARKERP. L., BEHRENS E. W., CALDERJ. A. and SHULTZ D. (1972) Stable carbon isotope ratio vatiations in the organic carbon from the Gulf of Mexico. Contrib. Mar. Sci. Univ. Texus 16, 139-147. PETELLEM., HAIN= B. and I-&N= E. (1979) Insect food preferences anaiysed using isC/t2C ratios. Oecobgia {BerU 3% 159- 166. OLSON M. E. Q., OVIA~ C. A., VARGOG. A. and VARGO S. L. (1979) Replicability of MERL microcosms: initial observations, In Advances in Marine Environmental Research (ed. F. S. JACOFT),pp. 359-381, U.S. Environmental Protection Agency, Washington, DC. PILSONM. E. Q., OVIA~ C. A. and NIXON S. W. (1980) Annual nutrient cycles in a marine microcosm. In bficrocosms in Ecalagi~a~Research (cd. J. P. GIESY),DOE Symposium seties 52, CONF78iiO1, pp. 753-778, Nationai Technical Information Service, Springfield, Va. PRATTD. M. ( 1959) The phytopIankton of Narragansett Bay. Limnol. Oceanogr. 4,425-440. PRAY D. M. and CAMPBELLD. A. (1956) Environmental factors affecting growth in Venus mercenaria. Limnol. Oceanogr. 1, 2- 17. PRICEK. S. JR. (1962) Biology of the sand shrimp, Crurzgon septemspinosa in the shore zone of the D&mare Bay region. Chesupeake Sci. 3.244-255. RAU G. W., SWEENEYR E. and &PLAN 1. R (1982) PIankton ‘%?*C ratio changes with iatitude: differences between northern and southern oceans. Deep Sea Res. 2% 10351039. RHOADS D. C., MCCALL P. L. and YINGST J. Y. (1978) Disturbance and production on the estuarine seafloor. Amer. Sci. 66, 577-586. RODELLIM. R., GEARINGJ. N., GEARINGP. J., MARSHALL N. and SASEKUMARA. (1984) Carbon sources used by organisms in Malaysian mangrove swamps and nearsimm waters as determined by &t’C values. OecoIogju (Berl.1. (in Pm=). SACKE-IT W. M., ECKELMANNW. R., BENDER M. L. and
Bg A. W. (1965)Temperature dependence of carbon isotope composition in marine plankton and sediment samples. Science 148, 235-237. SANDERSH. L. (1956) Oceanography of Long Island Sound. X. The biology of marine bottom communities. Bnll. B~nghum Oceunogr. Coil. 15, 345-414. SANDERSH. L. (i960) Benthic studies in Buzzards Bay. IV. The structure of the z&bottom community Limnni. Oceanogr. 5, 138-f 53. SANDEM H. L., G~UIXMIT E. M., MIL.~~E. L. and HAMPSON G. E. (1962) A study of the intertidal fauna of Barnstable Harbor, Massachusetts. Limnol. Oceanogr. 7, 63-79. SCALANR. S. and MORGANT. D. (I 970) Isotope ratio mass spectrometer instrumentation and application to organic matter contained in recent sediments. Int. J. Mass Spectrom. Iort Phys. 4, 267-281. SMAYDAT. J. (1970) The suspension and sinking of phytoplankton in the sea. Oceanogr. Mar. Biol. Ann. Rev. 8, 3.53-414. SMITHB. N. and EPSTEINS. (I 970) Biogeochemistry of the stable isotopes of hydrogen and carbon in salt marsh biota. Plant Physiol. 46, 738-742. SMITH F. E. (1950) The benthos of Block Island Sound. 1. The invertebrates, their quantities and their relations to the fishes Ph.D. dissertation, Yaie Univ. SMITHS. V. and KROOPNICKP. (1981) Carbon-13 isotopic f~ctionation as a measure of aquatic metabolism. Nutwe 294,252-253. SPIES R. B. and DESMARAISD. J. (1983) Naturai isotope study of trophic enrichment of marine bentbic communities by petroleum seepage. Mar. Biol. 73, 67-71. STEPHENSONR. L., TAN F. C. and MANN K. H. (1983) Isotopic variability in marine macrophytes and its implications to food web studies. Esfuaries 6, 304. SWEENEYR. E., KALILE. K. and KAPLANI. R. f i980) Characterization of domestic and industrial sewage in southern California coastai sediments using nitrogen, carbon, sulfur and uranium tracers. Mar. Environ. Res. 3, 225-243. TAN F. C. and STRAIN P. M. (1979) Organic carbon isotope ratios in recent sediments in the St. Lawrence Estuary and the Gulf of St. Lawrence. Estuarirte Coastal Mar. Sci. 8. 213-225. TAN F. C. and STRAINP. M. (I 983) Sources, sinks and distribution of organic carbon in the St. Lawrence Estuary, Canada. Geochim. Cosmochim. Rctu 47, 125- 132. TEERI J. A. and SCHOELLERD. A. (1979) &t3Cvalues in an herbivore and the ratio of Cs to C, piant carbon in its diet. Oecologiu (BerI.1 39, f 97-200. THAYERG. W., PARKERP. L., LA CRO~XM. W. and FRY B. ( 1978) The stable carbon isotope ratio of some components of an eelgrass, Zostera marina, bed. Oecologia (Ber/.) 35, I- 12. TIESZENL. L., HEIN D., QVORTRUPS. A., TROUGHTON J. H. and IMBAMBAS. K. (1979) Use of 6°C values to determine vegetation selectivity in East African herbivores. Oecoiogia (Be&] 37, 35 i-359. WHITLATCHR. B. (1980) Patterns of resource utiii~tion and coexistence in marine intertidal deposit-feeding communities. J. Mar. Res. 38, 743-765. WILLIAMSP. M. and GO-N L. 1. ( 1970)Carbon- 13:carbonI2 ratios in dissolved and particulate organic matter in the sea. Deep Sea Res. 17, 19-27. WONG W. W. and SACKETTW. M. (1978) Fractionation of stable carbon isotopes by marine phytopiankton. Geochim. Cosmochim. Acta 42, 1809- 1815. YINCST J. Y. (1976) The utilization of organic matter in shallow marine sediments by an epibenthic deposit-feeding holothurian. f. Exp. Mar. Biol. Ecol. 23, 55-69.
00167037/84/f3.C0
d Cosmxhimica ACUI Vol. 48, pp. 1089-1098 Rcu Ltd. 1984. Printed in U.S.A.
+ .OO
Isotopic variability of organic carbon-in a ph~op~kton-~~, temperate estuary J. N. GEARING,P. J. GEARING, D. T. RUDNICK,A. G. REQUUO and M. J. HUTCHINS Graduate School of Oceanography, University of Rhode Island, Narragansett gay Campus, Narragansett, RI 02882
(Received July 28, 1983;accepted in revisedjbrm January 3 1, 1984) Abatrmet-An isotopic survey was made of organic carbon in ph~op~n~n, sediments, zcoplankton, larvalfish,and benthic fauna from Narragansett Ray and the Marine J&systems Research Laboratory, Rhode Island; the results quantify the extent of variability in a phytoplankton-based ecosystem and elucidate some of its causes. Carbon from primary producers (phytoplankton) varied with taxon and size, ranging from -20.3 f 0.69b (mean -C 1 s.d.) for diatoms (primarily Skeletonemu cosratum) to -22.2 + 0.6%0 for nanoplankton (primarily microflagellates and non-motile ultraplankton). Planktonic isotope ratios varied little with either water temperature (0 to 2O’C) or degree of preservation (up to 2-year aerobic diagenesis in sea water). Isotopically, sediments from East and West Passages of the bay were homogeneous with location and depth, with a mean (-21.8 + 0.6%) similar to a mixture of carbon from diatoms and nanop~n~on. Providence River sediments reflected venous and an~m~nic carbon (sewage) in their isotopic ratios (-24.2 f 0.7%~). Ratios of macrozooplankton (~150 pm) were statistically separable from those of concurrently collected phytoplankton, being, on average, 0.5 to 0.65%more positive. Secondary consumers in the water column (shrimp and larval fish) were 2.4%0heavier than diatoms. Thirty-four taxa of benthic fauna had relatively positive isotope ratios (- 18. I f 1.5%) which may indicate preferential use of carbon originally from diatoms rather than nanoplankton. The wide range of benthic ratios (-22.7 to - 14.9%0)resulted from both intraspecific variability (mean range = 3%) and the variety of trophic positions occupied. Some of the intraspecific variability could be related to size. Among species, the isotope ratios increased from meiofauna (-19.5 f 0.4%) to macrofaunal non-carnivores (-18.6 + 1.3L) and carnivores (-16.6 + 0.8%). INTRODUCI-ION
(3) little or no isotopic fractionation between a consumer and its carbon supply. The extent of change STABLECARBON ISOTOPE ratios, measured as G”Cpna,’ from one trophic level to the next is difficult to dehave been widely used as a tracer of organic carbon termine since individuals of the same species grown in biogeochemical systems having two or more isoon the same diet may vary by -2%0 (DENIRO and topically distinguishable sources (PARKER, 1964; EPSTEIN,1978). Laboratory feeding studies on seventyCALDER and PARKER, 1968; NEWMAN et al., 19’73; six consumer-food pairs (Fig 13) and forty-four field HAINES, 1976a,b; GEARING et a!., 1977; FRY et al., rn~u~rnen~ of animals and their inferred food (Fig. 1977, 1978; THAYER ef al., 1978; HAINES and MONIC) revealed differences averaging +0.4 + I .4 and +0.6 TAGUE, 1979; M&ONNAUGHEY and McRou, 1979b; f I .4%0respectively. These data imply that consumers TESZENet al., 1979; BURNETT and S~HAEFFER, 1980; are slightly more positive than their food. However, PRY, 1981; RODELLI et al., in press). Several assump the range of differences is too great to generalize with tions are inherent in their use: any certainty. (1) a relatively constant isotopic composition of organic carbon produced by each plant source. Variable Deviations from these assumptions can lead to wide isotopic signatures, according to the physical and isotopic variability in systems with a single major carchemical conditions of growth, have been shown in bon source. For example, unexplained variations have laboratory and field studies for both higher plants been reported between plankton, sediments, and con(LOWDONand DYCK, 1974; GUY et al.. 1980; STE- sumer organisms of up to 5% in the Gulf of Mexico PHENSON et al.. 1983) and phytoplankton(DEGENSet (FRY and PARKER, 1979), 8960 in the Bering Sea al., 1968b; DEUSER et al., 1968; DEUSER, 1970; (MCCONNAUGHEYand McRou, 1979a), and 7% in CALDERand PARKER, 1973; PARDUEel al., 1976; the Straits of Malacca (RODELL~et al., in press). Such WONG and SACKETT, 1978); ranges reduce the resolution of studies which use S13C (2) an unch~~ng isotope ratio in plant carbon as to trace the mixing of carbon from two or more sources. it decomposes and is broken down to detritus. Among We examined in detail a temperate estuary (Narrahigher plants, ten pairs of living and dead/decomposed gansett Ray, Rhode Island) with a single major carbon plant material had an overall average difference of source (phytoplankton). and by quantifying the varionly 0.015960 (Fig. IA); ability of a13C and ascertaining its causes, focused on the three assumptions previously mentioned. All the major organic carbon reservoirs were sampled extensively at different seasons and the results were tested ‘6-&m = [(R,,+)/(R~) - I] X IOOO, where R = iQ’2C. to distinguish nonrandom isotopic differences. By 1089
I090
.I. K Gearing el al
‘1”
Difference
and August, 1980 and January. 1982. the diatom .S!&~t)ne~u t'osfarum made up most of the carbon in the water, whereas nanopiankton which pass through a 10 am Nitex mesh sieve constituted the bulk of the ph~oplank~on in samples taken during most of the period April-November, 1982. The most abundant phytoplankters in the fraction < 10 brn were uncharacterized microflagellates and nonmotile ultraplankton (FURNAS, 1982). Plankton fractions were filtered onto precornbusted (450°C) glass fiber filters, air dried. and stored in glass vials until analyzed. The samples were not routinely acidified to remove inorganic carbon as part of the work-up because tests showed acidification to have no observabie effect on the stable isotope results. Larval fish, hferzidiu menidin, were grown in flowing seawater from one of the MERL control tanks. Freshly hatched larvae were placed in small tanks in July, 1982. Approximately 25 fish were collected on days 0. 7. and 14. Several (3 to 12) individuals were combined for each isotopic measurement. The fish were dried, weighed, and stored in a desiccator. Before analysis, the whole tissue was ground, acidified to remove inorganic carbon. and tiltered.
in Isotope Ratios, %*
FIG. 1. Change in 613C(%D)resulting from (A) plant degradation,and (B, C) animal food assimilation (B, laboratory studies with known food, C, field studies with inferred food). Bar indicates mean, arrows one standard deviation. Data are from: (A) SMITH and EPSTEIN, 1970; HAINES, 1976b; FRY, 1977; HAINES, 1977; THAYER et al., 1978; HAiNESand MONTAGUE, 1979; (B) FRY, 1977; DENIRO and EPSTEIN, 1978; HAINESand MONTAGUE,1979; TEERI and S~HOELLER,1979; FRY and ARNOLD, 1982; MACKOet al.. 1982; (C) SACKEIY’ et al., 1965; DEGENSet al.. 1968a; DEUSER, 1970; MINS~N et al.. 1975; FRY, 1977; LAND et al., 1977; FRY et al., 1978; MCCONNALIGHEY and McRov, 1979a; PETELLE et al., 1979.
means of such individual ecosystem studies, our understanding of isotopic f~~ionations in nature is enhanced and we are better able to interpret results from ecosystems having more than one carbon source.
41'
410'40'
MATERIALS AND METHODS Samples for this study were collected in the Providence River, in the East and West Passages of Narragansett Bay, and in the microcosms of the Marine Ecosystems Research Laboratory (MERL), University of Rhode lsiand (Fig. 2). The biological, chemicai, geological, and physical oceanography of the Narragansett Bay estuary has been studied intensively
41’ ‘35
(e.g.HicKs, 1959;McM~srE~, ~~~~;COLLINS, 1974;OvlArr and NIXON, 1975). The MERL control microcosms are de-
signed to mimic the biological and chemical behavior of midNarragansett Bay. They consist of large, outdoor liber@ass cylinders (1.8 m diameter, 5.5 m height) with sediments, flow-through seawater, and their associated biota from midNarragansett Bay (41”351v and 71”22W). Details of these systems and effects of containment can be found in hLSON et al. (1979, ~980) and GRA.SLE et al. (1980). Plankton were isolated from net tows (69.5 gm and 150 pm mesh) and from whole water samples. During periods of high standing crops, diatoms in the size range 35-64 pm were trapped in the mr mesh nets. In the laboratory, material was size fractionated by washing through several mesh sizes of Nitex sieve, and each f-ion was examined microscop i&y. The fraction > 150 em was picked until it consisted of macrozoopiankton: caianoid copepods were usually numerically dominant. During phytoplankton blooms in January
41°
3d
-21.8 710 25'
I 7PP$
FIG. 2. Sampling location and IS’% of organic carbon in surface sediments of western NarraganseU Bay.
1091
Phytoplankton C isotopes Sediments were co&ted with a gravity corer We day) or open-bar&d, hand-h& corer (MERL micrWosrns; GE,XRWSet ui., 1980; FRITHSEN d al., 1983) and were stored fioren. Depth sections of each extruded core were washed ~a0.85mmmeshsicvemremoveshcllsando~ acidified with HCl to remove carbonate, tiltered, washed, dried at 4O”C, and powdered. The majority of benthic organisms were collected by box core (0.25 m2 USNEL spade corer) on May 14, 1981 and Bay. Small macrofauna July 13,1982, from mid-N_ and meiofauna were isolated from open-barrel corer taken by divers. gome benthic organisms were coikcted by openbarreled, hand-held corers in 1979-1983 from MERL microcosms containing mid-bay sediments. The benthos were washed through sieves (2.0 mm mesh for box core samples, 100 pm mesh for other samples) with seawater. Meiofauna were picked from the top half centimeter of sediment. Organisms retained on sieves were identified separated into groups or species, and held live in clean seawater for l-2 days to clear their guts before being stored frozen. After being thawed, each sampk was examined under a dissecting microscope; tissue with no evidence of gut material was selected for analysis. Organ&s with shells were shucked or treated with dilute HCI to remove carbonate. Tissues were then filtered, dried, and stored. All samples were combusted in a LECO radio-frequency furnace (PARI(ERet al., 1972). The sediments were weighed before combustion and the volume ofCOr evolved (measured manometricaUy)was used to determine catbonat&ee otganic carbon contents. The organiccarbon of some sediments was also measured using a Carlo Erba Model 1106 elemental analyzer. The CO, produced from each sample was purified and its stabIe carbon isotope ratio (&r%)was measured using a Micromass 602 mass spectrometer. Details of the mass spectrometry and standard are given in CRAIG ( t957), NIER er al. ( 1962)and SCAXAN and MORGAN( 1970).Analyses of replicate samples and comparisons with other laboratories have shown our analytical precision (1 standard deviation) to be fi.31 (L%NERand PHILLIPS,1981; &rNERand Bttls,
1982;MORSELLI and BAGC~ET~, in press).Sets of values from various carbon reservoirs were compared using the non-parametric Mann-Whitney “U”-test (LCIR~AHL, i967), which does not require the assumption of equal ~p~ation variances. RESULTS AND DEXXJSSION Samples for thii survey were collected both directly from Narragansett Bay and from MERL control microcosms with bay water, sediments, and organisms. Comparisons of results (Table 1) showed no differences at the a = 0.1 level. On this basis, the two sampling locations are assumed to be isotopically equivalent.
a
13
6 Dstc
Phyropluskton
z.x+ankt.nb
-20.6 t 0.4(12)= -19.7 t 0.3 tsj= -20.2 L o.7ao)d
-20.0 t I.O(l2) -19.3 f 0.3 (5)
co.5 t 1.1 +0.4 ? 0.3
-20.3 t 0.6(27)
-19.8 f 0.9(17)
co.5 2 1.0
T.-C
l/80 8180
1
20 1
l/S2
1182 4182 5182 6f82 7102
1 4 12 16 19
'1..
CPW
AZ-P
-21.8 t 0.3 (7)' -22.2 -22.0 -22.2 -23.2 -22.5 -22.4 -21 . 5
i:0.4 L 0.2 t.0.2 t 0.4 t 0.7
(3)f (3)f (3)f (S)f
-21 -20.9 .8 -22.7 -22.9 -21.8 -20.9 49.8
* 0 1s I:;:
f 1 f t t
.
0 6 0.3 1.1 1.6 0.6
I:; (3) (31 (4) (3) (2)
Cl.2 +0.4 -0.5 +O., +0.6 +I.6 +2.0
? t t i t
0.3 0.3 0.7 1.4 0.3
-22.2 * 0.6(29)
-21.7 ? 1.3(19)
+0.6 t 1.0
-21.3 f l.l(SS)
-20.8 f 1.5(36)
+0.6 f 1.0
that of plankton ratios reported in the literature for other temperate regions (Table 3). The variability of these literature data, however, is large enou@r to cause considerable uncertainty when attempting to trace the mixing of carbon from planktonic and other sources (where the difference of means is usually 5 to 10%). The causes of the variance have not been determined, but isotopic changes have been correlated with species composition (WONG and SACKS?; 1978), temperature (SACKETT et al., 1965; FONTUGNEand DUPLESSY, 198 I), water masses (FONTUGNRand DUPLESSY,1978), latitude (RAU et al., 1982), and &“C of inorganic carbon fixed (PARKJZR, 1964; DEUSER, 1970; SMITH and K.RooPNICK, 198 1f. In this study, samples were taken to examine some of these relationships. Most of the variability could be explained by differences in phytoplankton types (standard deviation reduced from 1.1 to 0.6). Temperature had little or no discernible effect on phytoplankton isotope ratios. Likewise, extensive aerobic decomposition in seawater caused no 613Cchange in the one planktonic sample examined. Variations of phytoplankton 813C brought about by changes in the isotope ratio of the source inorganic carbon (ICC) are not specifically addressed in this paper. Preliminary data indicate little variation in IOC
Primary producers Overall, the fifty-six samples of phytoplankton averaged-21.3+ l.l%owitharangeof-23.4to-19.3969 (Table 2). The avenge and distribution are similar to
Table 3.
Type of Sample ParLiCUlaCe or*. ellrbonb Partlculstt
‘ 613C (iample#
* lrna mrandsrd
U7D.L
SW
Phyroplankton, Janury Sedimats, nid-Pa, Saathic Or~niu, Hay e -n
.I.*
PDB-
-20.6 f 0.4 (12) -21.7 t 0.3 (13) -18.6 t 1.4 (30)
dw4ation
(nu~aar
cf
-20.2 t 0.7 (10) -21.5 2 0.5 (27) -19.1 t 2.1 (10,
uaplasf
111otoPeratMe
org.
edsn=
P.itieti.,teorg. eatbond Particulste org. c*rhone PartiCUZaCC cxg. cart& Hat plantrtoof Ner plazxkto& Wet Planktonff Net plmktonh Net plmktoa~ Nat P1.nkto.f Nat ,,l.nkronk
b+'CP,)
of plankton uorldvide.
Lc.c.tian 45-N
to 50-N
to 59.1 133-N 18% Le 29-N -ulS*W 6O'N to Lt=N 45% to 50% %33=N 28.N to 39-s 21-N 19.S 16.N to 37.9 3'8 to 13-S 28.N
to
n
Hsrn
11 5 5 23 12 4 5 12 12 71 27 18
-23.9 -20.6 -23.2 -22.0 -21.8 -19.6 -23.8 -20.9 -21.9 -19.7 -20.1 -19.3
S.D.. 0.8 0.4 0.8 2.1 I.0 1.3 1.2 0.9 1.6 0.6 0.7 1.2
RanSs 3.0 1.0 2.3 8.9 3.2 2.3 3.1 2.7 4.9 4.2 2.6 4.7
pY" (1974) SACKSTY et al. (1965) ~lilwsl!~tl~wPLSSSY (1981) 'FOHTUCNE aad DUPLSSSY (1978) %T%XS s &. (1968.)
I092
.I. N.
Gearing e[ al
#3C in this well-mixed estuary except during the largest winter-spring blooms. Note that the standard deviation of samples collected over a month’s period during the 1982 winter-spring bloom was larger than that of samples taken on a single day during the 1980 bloom or during a smaller bloom in August, 1980 (0.7 versw 0.4 and 0.3%0, Table 2). Variation caused by the presence of zooplankton in samples will be discussed in a following section. The greatest isotopic variation was between samples in which diatoms p~ominated (-20.3 ?Z 0.6%, n = 27) and those where nanopiankton were the most abundant phytoplankters (-22.2 + 0.6%. n = 29). The two sets of &‘%Zvalues are significantly different (a < 0.001). There was a similar difference between nanoplankton (< 10 pm fraction) and net samples of Skeletonema costatum collected at the same time (Table 2, January 1982). The result is a seasonal difference in overall planktonic isotope ratios, since nanoplankton are the primary Narragansett Bay phytopfankton in summer whereas diatom blooms occur in the winter-spring and sometimes in the fall (PRATT, 1959; DURBIN et al., 1975). This change is potentially useful for tracing carbon seasonally and for determining food sources in this plankton-based ecosystem. This finding also suggests that the method by which ph~oplankton are sampled may sibilantly affect the isotope ratio obtained. Samples from which nanoplankton and picoplankton are effectively excluded, such as collections from nets of large mesh, may have ratios more positive than samples containing smaller size fractions such as particulate organic carbon from filtered water. In this study, filtered whole water samples taken in August, 1980 during a diatom bloom averaged -20.9 f OX% (n = 6), signi~cantIy more negative (CU= 0.01) than samples of ph~opIankton from net tows collected at the same time (Table 2). The largest size fractions (> 150 pm, macrozooplankton) from the same tows were even more positive isotopically (see section on water column consumers). Similarly, the smallest size fractions of water collected from a tank in March, 1982 (~15 pm) were more negative than fmctions between I5 and 64 pm (-20.9 + 0.7%, n = 5, versus - 19.8 + 0.5% n = 6, LY= 0.05). On the whole, the values of POC reported in the literature are more negative than those of net plankton (Table 3) although some of the differences may be due to the presence of zooplankton. One would of course expect the collection method to matter less when there are few large plankton present. TAN and STRAIN (1983), for instance, found no significant difference between POC and net samples collected in late summer in the St. Lawrence Estuary. We suggest that care be taken in describing the plankton cotlection method used, particularly the suite of sizes sampled, and that microscopic determination of sample contents be made when possible. Various reasons for species-specific isotope variations have been postuiated. WONG and SACKETY
(1978), growing phytoplankton in culture, reported greater fractionation of carbon isotopes by flagellated nanoplankton than by centric diatoms and ascribed the difference to species. The slower growth rates of the major nanoplankton groups relative to diatoms in Narragansett Bay reported by FURNAS ( 1982) might also lead to greater fractionation (PARDUEet ui , 1976). Species-related differences in 613C may be a result of variations in the relative contents of storage products and other biochemicals with differing d”C values (PARKER, 1964: DE(;ENS d nl., 1968a; WONG and SACKETT, 1978). Isotope variations with water temperature have been postulated on the basis of laboratory and field data (SACKETTet al., 1965: DEUSERet al., 1968; FON-IIJGNE and DUPLESSY,198 1). In particular, diatoms such as Skeietonema costatum have been reported to have more negative isotope ratios when grown in cold water (DEGENS et al., 1968k WONG and SACKETT, 1978). Other studies have found no evidence for a direct temperature effect (CALDER and PARKER, 1973: FONTUGNE and DUPLESSY. 1978; RAU et al., 1982). In this study, samples were taken during natural blooms of Skeletonema costatum: their isotope ratios did not correlate significantly with temperature (slope of +0.005% per degree C. n = 27, correlation coefficient = 0.33). The correlation of nanoplan~on isotope ratios with temperature was signi~cant (n = 29, correlation coelhcient = 0.56) but the slope was small (-0.04%0 per degree C) and in the opposite direction to that postulated. We conclude that temperature has little or no (< 1%o)effect on the isotope ratio of phytoplankton in temperate waters under natural conditions. except possibly indirectly through species succession. Finally, we examined the possibility of changes in planktonic isotope ratio during d~om~sition. There was no difference (-22.5 % 0.3%. 12= 3 vcr.rsus-22.4 ~fr0.2%0, n = 3) between a fresh sample of mixed plankton collected from Long Island Sound during an almost pure bloom of the diatom Skefetonema costatum and an aliquot of the same tow which had been allowed to decompose for nearly two years in the laboratory ( 1 kg plankton in 50 L aerated seawater). During this time the material went from 16.7 to 5.3 percent organic carbon. No evidence for isotopic change due to selective degradation (EADIF.and JEFFREY, 1973) was found. We conclude that there is little isotopic change in planktonic carbon during aquatic decomposition to detritus, just as little has been reported for higher plants (Figs 1‘4). Sediments
In the East and West Passages, sedimentary carbon isotope ratios reflected those of total phytoplankton, averaging -2 1.8 Z?Z 0.6% (n = 26) with no clear trends with location or depth (Fig 2, Table 4). Samples of mid-bay sediments from both the bay and MERL control tanks had similar values, -2 I .5 rt 0.5% (n = 701, within the range reported for marine sediments
1093
Phytoplankton C isotopes
613Ct 'f..
Depth, CP
Lacar1on
x O.C.
PROVIDENCERIVER 41'49'N 71'23'W 41'48.5'N71'24'W 41947' N 71'23'W 41*45.5'N71'23'W 41'44.5'N71=22'W 71'22'W 41'44% 1,
41'43.5'N71'22'W 11 EAST
O-5 o-5 O-5 O-5 O-1 O-5 10-41 O-l l-20
-24.5 -24.5 -23.2 -24.7 -21.5 -22.3 -22.6 + 1.0 (3) -22.0 -22.3 2 0.4 (5)
3.2 9.5 6.2 7.9 1.2 4.4 3.1 5.2 4.4
-23.6 -21.6 -22.5 -22.1 * 0.3 (3) -21.7 -21.7 f 0.3 (12) -20.6 -21.5 -21.2 -21.8 -22.2 f 0.1 (3)
0.9 4.4 2.5 2.3 1.9 1.6 4.6 0.8 1.0 1.0 1.0
AND USST PASSAGES
41'41'N
71'22.W O-1 71.19'W o-2.5 41'38'8 71*18'w o-2.5 II 7.5-32.5 41'35'8 71'22'W O-2.5 II 3-152.5 41'34.5'N71*1S'W o-2.5 41.30.5.N71'20.5'W o-2.5 41*28*~ 71*25'W o-1 41*25'N 71'26'U O-1 41’40’N
11
l-4
throughout the world (Table 5). The mid-bay is only 0.25% more negative than the seasonally integrated planktonic ratio, assuming (I) half of the yearly production by diatoms (-20.3%) and half by nanoplankton (-22.2%) as estimated by DURBIN et al. (197% and (2) equal sedimentation and preservative of both types of carbon. The sediment average could also be arrived at by greater sedimentation (SMAYDA, 19701, followed by preferential utilization by the benthos, of diatom detritus (PRATT and CAMPBELL,1956). Thus isotopic fractionation during decomposition and diagenesis does not need to be invoked to explain sediment-plankton differences found in a single sampling day (up to 3% in this study). Corresponding to the seasonal change in planktonic isotope ratios, there is some evidence for a slight shift in the ratios of sedimentary carbon. Surface (O-l cm) sediments in MERL control tanks ranged from -2 1.7 to -20.9% in April and May (-21.3 + 0.3960,n = 6) to -22.5 to -21.7% (-22.1 It 0.3960, n = 4) in JulySeptember, the sets being disting~~ble with a = 0.05. More data are needed to confirm or reject this trend, but grouping the present data seasonally reduces the
standard deviation of sediment S13C values from 0.5 to 0.3%* In the upper Providence River and some nearshore stat&&, more negative isotope ratios were measured, probably due to inputs of terxigenous organic matter such as sewage. The percentage of terrigenous organic carbon can be roughly estimated by using a simple mixing equation and assuming the isotope ratios of terrigenous and marine organic carbon to be -26.0 and -2 1.8%0reqectively (CALL~ER and PARKE%,1968; RURN~ and §CXAEFFER, 1980; S~ENEY ef al., 1980). It ranges from up to 80% near the river mouths to ~20% at the southern end of the Providence River. More information on the ratios of sewage is needed before better calculations can be made. Consumers-water
column
It has been postulated and some studies have reported that animals have more positive isotope ratios (by about 1%) than that of the organisms on which they feed, while other observations have shown the opposite trend (Fig. 1B and C). Knowing the extent of isotopic variability due to trophic levels in the field is important both for increasing the accuracy of stable isotopes as tracers and for using isotopic composition as an indication of trophic position. We examined zooplankton collected concurrently with phytoplankton; the two populations were separable with a = 0.063. Zooplankton were more positive in 25 out of 35 sets of samples, with the differences averaging 0.6k (Table 2, Fig. 3). This value is similar to those reported in the literature, but has less variability associated with it (s.d. of 1.O versus 1.4%~). When samples from diatom-dominated and nanoplanktondominat~ periods were considered separately (so as to obtain more isotopically homogenous sets), zooplankton values remained 0.5 to 0.6% more positive than those of phytoplankton (Table 2, a = 0.008 and 0.087). These differences should be considered as minima because of the impossibility of completely eliminating microzooplankton from phytoplankton samples. We conclude that zooplankton are more positive than their fbod by at least 0.5%, and also that they show a seasonal change in isotope ratio (two populations separable, a < 0.001) which correlated with that of phytoplankton. In July 1982, we obtained larval fish grown in water
Numberof
Difference
l08GUNG Ct 11. (1977) bTdJl and xsiin (1479) =INCZXSt. 11982)
dBuRmrr ."d SCIIAEFFSP. fBsO) +xxdN~~. (1973) cpuLI"G(1975)
in Isotope Ratios. 7~
FIG. 3. Difference in 613C between concurrently collected phytoplankton Bar indicates mean, arrows . . .and I zooplankton. . one aanaam aevumon.
1094
J. N.
Gearing et al
from a MERL tank. Atlantic silversides, Menidia menidia, are thought to feed on zooplankton (MULKANA, 1966). Directly after hatching, the larvae avemged - 19.5 + I. I %I (n = 4). Their isotope ratios increased with body weight, nearing an asymptote of - 18.0 + 0.5% fn = 4) at - 1 mg, or 300% of initial body weight. Ph~oplankton (>80% in the less than IQ pm size fraction) and zooplankton (primarily copepods) collected from the feeder tank around this time averaged -23.5 f OXi!&(n = 4). Either the isotopic fractionation with trophic level was unusu~ly large for these fish or they selectively assimiIated carbon derived from the iarge diatoms {estimated Si3C - 19.7% in summer, Table 2) rather than from totaI phytoplankton. These hypothesized routes of carbon flow are diagrammed in Fig. 4. Another mobile consumer sampled from MERL control tanks, the omnivorous shrimp Crungon sepfem~p~no~a (PRICE, 1962) was simi~riy positive. Four organisms cohected between March and 3uIy averaged - 17.9 + 3.04i;o.There was no trend to more negative vaiues during the summer when nanopIankton dominated the primary production. Consumers-benthos The isotope ratios of 140 samples of bentbic animah are shown in Table 6, The 34 taxa averaged - 18. I t 1.5%. Despite iarge intraspecific variability, two points are apparent from the data: f I ) 9 1%of the species averages, as well as the overall mean, are more positive
than -2OL, the upper limit for their presumed sources of carbon, phytoplankton and bulk sedimentary organic matter, and (2) the isotope ratios of species generahy fah into logicat groups corresponding to their postulated feeding behaviors. Intraspecific ranges of up to 2k have been reported for animals grown in the laboratory on isotopic&y constant diets (DENIRO and EPSTEIN, 197X: FRY and ARNOLD, 1982). The benthic organisms examined in our study had intraspecific standard deviations averaging 1%. with a mean range of 3Y0a.Sex. size, and bi~hern~~ composition have been postulated to affect &i3C vafues (FRY and PARKER, I979; HUGHES and SHERR, 1983), We found positive correlations with size for three bivalve species (Fig. 51, the larger animals tending to have more carbon- 13. The same tendency was noted for some other animals, primarily Ccriantheopsis americanus, but too few data were available for analysis. In other species the importance of size could not be assessed because of its narrow range and the practice of pooling individu~s. A positive correlation between size and 613C could arise because of changes in size of particles ingested, in biochemical make-up, or in the ratio of respiration to growth. The result is a difference of I to 2’pm,between the smallest and largest representatives of these three species. HUGHES and SHERR ( 19g3) found a positive trend of di3C with size for three inve~ebmte species from a Georgia marsh. Another possible cause of intraspecific variability is a temporal change in the isotope ratio of the animals’
PhytO~lQnktCk’! r&W. Nanoplankton
Shrtmp and Larval Fat? ?I=8 -17.9 L 2.0%
Non-carnivores
FIG. 4. Summary of organic carbon 613Cvalues (no. of samples, mean 2 one standard deviation) in a plankton-&ed -em. A, change in 6°C through carbon transfer. 1yindicates level of significantdifference based on a two-sided Mann-Whitney U-test.
1095
Phytopiankton C isotopes
‘1.. 613sr33.
XFmPAoNA
H.rpscricoid coprpodm (4/,1500) Kir.orh,nchs(11'200) Nutodea (Z/>lOOO)
-19.0 1:1.6 -19.7 -19.8
-21.0 to -17.2 -20.2 to -19.3
pucROPAuNA MmP.PACX
PesDEps -16.6 a9.7 -16.0 -21.0 -18.3 -19.5 -19.8 -17.3 -17.2 -20.3 -18.9
SIJ3SOILPACE
DEPOSIT
t 1.1 f.2.7
-21.7 Lo -18.3 -16.4 t* -15.5 -22.7 to -15.5
f 1.1
-21.8 LO -16.8 -17.6 to -15.8 -19.4 to -15,s
1:1.2
-19.8 to -17.2
-17.7 * 0.5 -21.1 -17.9 -19.5 -38.9 -18.8 t 1.3 -18.1 t 0.6 -18.1 ).1.2 -18.3 -17.6 t 0.3 -18.1 t 1.0
-18.3 LB -17.2
t 1.8
F8EDX88
wrd-rrus Ub&. (P. 4/>50) linoa .isr%~~f20f Hucul. annula& (B. 11/>200) Pnctaaria wuldit (P. 212) Pharolion strombi (S, 319) yoldia liurula (II.24/250)
-14.9 -16.9 -17.6 -16.3 -16.4 -17.7 -16.6 -16.3 -16.5
-20.1 -19.0 -20.7 -19.4 -17.9 -20.0
to to to to to to
-17.1. -17.0 -16.2 -17.2 -17.3 -15.9
5 0.7 f 0.4
-16.4 to -17.0 -17.0 LO -16.1
* 1.2
-19.4 LO -14.9 -16.6 fo -16.5
* 0.1
-16.6 to -16.4
food. Since the di3C of plankton varied seasonally, we examined the isotope ratios of benthic organisms with season. There was no consistent pattern for either the total fauna or individual species. A linear regression of 6i3C versus time gave a slope of 0.004960 per day and a correlation coefficient of only 0.17 (n = 9). More information is needed on animals of the same species, size, and growth rate to determine whether any seasonal change exists. These results suggest that a large proportion of intraspecific variability is caused by factors other than individual size and isotopic changes in food resources. To minimize this va~ability, we aualyzed as many organisms as possible and have used the species’ average b’% values for evaluation of interspecific differences. This is not a completely satisfactory solution; for example, even some species which were extensively sampled and were quite uniform in size, e.g. Mediomastus am&eta and Nassarius trivittatus, had standard deviations of over la. We suggest, therefore, that as much info~ation as possible (number of organisms analyzed, time of collection, size, biological condition, etc.) be included in future reports of isotopic studies to establish the uncertainty of the data and to facilitate understanding of the causes of intraspecific variability.
Bearing in mind the extent of this variability, we can still deduce with confidence some interspecific isotopic differences. The most apparent aspect of the data is the -overall heaviness of the benthos. All 34 taxa had averages more positive than the average values for sedimentary organic matter, nanopl~~on, or nanoplankton-consuming zooplankton. Twenty-nine (85%) had averages more positive than those of diatoms or the zooplankton eating them. This is in accord with the concept of isotopic shifts to more positive ratios with increasing trophic level. The consistently positive ratios of benthic organisms permit the speculation that they selectively assimilated carbon produced by diatoms. The o~isms might be deriving their carbon from: (1) bulk sedimentary organic matter, particularly in the case of the deposit-fders; (2) all freshly deposited planktonic carbon (MARE, 1942; SANDERS,1960; YINGST, 1976); (3) a particular fraction of the phytoplanlcton production such as diatoms (PRA’ITand CAMPBELL,1956) or nanoplankton; (4) a mixture of the above. If the first possible carbon source were important, the isotopic difference between non-carnivores and their food would be -3460. This difference is rather large but not unheard of (Fig. 1B and 1C), and utilization of this source would account for the lack of seasonal changes in cP3Cof the benthos. Preferential use of the second carbon source could lead to a seasonal change,
0
20
40
, 60
Let’yth Ol Shsll.mm
0
lo LmgthOl
20 StWl.m
30
FIG. 5. Variation in 6°C with siz.ein (A) Pitar morrhuana. (B) NucuIaanmdata, and (C) Yoldia lima&a.
1096
J. N. Gearing ef a/
with the benthos being about 2k more negative in the summer than in the spring. Slowly growing organisms might, however, integrate out seasonal variations in food supply. This isotopic shift was not noted and, in fact, there may even have been a slight trend to more positive ratios in the summer and fall-winter (prior to winter-spring diatom bloom). The third possible source fits the available data best, with diatoms being the preferred food. The isotopic differences between non-carnivores and their food would then average 1.6%0. This source would also account for the lack of seasonal changes, with any possible minor shift to more positive values in summer being attributable to reworking of carbon deposited during the winterspring bloom. The final possibility, a mixture of carbon sources, cannot be ruled out, but it does not fit well with postulated biological behavior or with the isotopic evidence. In addition to the overall positive character of the benthic isotope ratios, there were isotopic differences which correspond to the animals’ hypothesized feeding patterns. Macrofauna (Table 6) were separated into groups according to their feeding behavior: (1) interface feeders ingesting particles on or suspended above the sediment surface (SMITH, 1950; SANDERS, 1956; RHOADSet al., 1978; WHITLATCH, 1980); (2) subsurface deposit feeders primarily ingesting particles below the flocculent layer at the sediment-water interface (BLEGVAD, 19 14; SANDERS, 1960; SANDERS et al., 1962; FAUCHALD and JUMARS, 1979; WHITLATCH, 1980; LOPEZ and CHENG, 1983); (3) carnivores using living metazoan biomass and carrion to a significant extent, along with detritus and associated microbes (FRETTER and GRAHAM, 1949; GRAHAM, 1955; MANZI, 1970; BARNES.1974: FAUCHALDand JUMARS, 1979). The most notable of the differences with known feeding behavior is the 2.1 %a difference between noncarnivores and carnivores (Fig. 4). The averages of the two sets of species are separable with LY< 0.001. Although individuals had widely ranging b13Cvalues, the carnivores were a fairly homogeneous group isotopitally with a standard deviation of 0.8% and a range of averages of 2.8%0. This clustering may result from general consumption of a variety of animals based upon their abundance and availability. The non-carnivores overall were more heterogeneous isotopically (- 18.7 f 1.3560,Fig. 4). Within this group, meiofauna averaged - 19.5 + 0.46, interfacefeeding macrofauna - 18.6 + 1.6% and subsurface deposit-feeding macrofauna - 18.6 It 1.O%. The similarity of the averages (none is significantly different from the others at a = 0.05) and the large standard deviations among the macrofauna suggest that this categorization inadequately portrays the complex flow of carbon in this benthic community. For some individual species where feeding behavior is well known, the isotope ratios varied as expected. For example the deposit-feeding bivalves Nucuia anmdata and Yoldia limatula were - 1.5% more positive than Mu/inia la-
teralis and Pandora gouldiana, filter-feeding bivalves. The meiofauna were generally more negative than the macrofauna. Ceriantheopsis americanus. an anemone which may be feeding on copepods, was one of the most negative animals, and Asychis efonguta, a maldanid polychaete which always feeds below about 5 cm, presumably on reworked organic matter from the upper layers of sediment, was one of the most positive organisms. A more comprehensive interpretation of the 6°C values of non-carnivores, however, must await improvements in both biological information and knowledge of intraspecific isotope variations. The overall pattern is of increasing isotope ratios with trophic level, progressing from diatoms (-20.3%0) to zooplankton (- 19.8’%), meiofauna (- 19.5%0),noncarnivorous macrofauna (- 18.6%0),and benthic predators (- 16.6%). Similar values and trends have been reported worldwide (Table 7). The consistency of these numbers is strong evidence of an isotopic shift to more positive values with increasing trophic level. The data also provide a good picture of the isotope ratios and their variations for most phytoplankton-based ecosystems. Acknowledgements-We thank Susan Brown-Leger, Jeffrey Frithsen, Eric Klos, Chip McCarty and Andrew McLaughlan for their aid in collecting and identifying samples. Larval fish were grown and provided by BarbaraSullivan and Chris Powell. Samples of fresh and degraded plankton were obtained from Joseph Westrich and Robert Berna (Yale University). The aid and suggestions of Candace Oviatt, Michael Piison,
and the staff at MERL are gratehtlly acknowledged. P. L. Parker, E. B. Sherr, and P. M. Strain improved the paper with their suggestions in review. This work was supported by EPA Cooperative Agreement CR-807795 and NSF Grant OCE-80-18831.
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