Limnol. Oceanogr., 35(S), 1990, 1 182-1195 0 1990, by the American Society of Limnology and Oceanography, Inc. Oxygen isotopic composition of bottom seawater and cellulose used as indicators of water masses in the northern Bering and Chukchi Seas

Abstract -Oxygen isotopic composition of rivers, evaporated surface ocean waters, bottom seawater and tunicate cellulose were used melting glaciers, and melting sea ice can be as short-term and long-term indicators, respec- tively, of water-mass characteristics in the north- separated and water types characterized (e.g. ern Bering and Chukchi Seas. Oxygen isotopic Epstein and Mayeda 1953; Tan and Strain composition of northeastern Bering Sea waters is 1980; Bedard et al. 198 1). In contrast to the influenced by Yukon River inflows of IsO-de- variability in the surface ocean, average 180 : pleted continental water mixing with relatively 180-enriched waters contributed by the Anadyr 160 ratios for the deep (> 500 m) sea vary Current. Tunicate cellulose sampled under Alas- by < 1%~ when expressed in the conven- ka coastal water is more depleted in IsO than that tional 6 notation: collected under Bering shelf and Anadyr waters, which reflects the oxygen isotopic composition 6180 = (Rstd/R,mple- 1) X 1030/oo (1) of these waters. Tunicate cellulose collected un- der the mixed Bering shelf water displays inter- where R = 180 : l 6O and std is Standard Mean mediate 6180 values. Oxygen isotopic analyses of Ocean Water (SMOW). The low variability bottom seawater were used to determine the spa- in V80 values of waters in the deep sea has tial location and influence of continental and led to widespread use of oxygen isotopes as coastal-derived precipitation and of sea-ice for- mation on water-mass structure on the continen- a paleothermometric indicator. The a18O tal shelf of the northern Bering and Chukchi values of carbonate, silica, and phosphate Seas. Results indicate that the oxygen isotopic precipitated by both living and fossil marine composition of tunicate cellulose, averaged over organisms, such as foraminiferans, radio- multiple seasons, may serve as a long-term bio- larians, coccolithophorids, diatoms, and chemical indicator of water-mass patterns in ice- covered polar regions where continuous sampling barnacles, have been used to estimate tem- is impractical. peratures of the water in which the organism lived, based on temperature-dependent equilibrations between the oxygens of water Stable oxygen isotopes in surface marine and of the biomineralized phase of interest waters have been used to study oceanic cir- (e.g. McCrea 1950; Mikkelsen et al. 1978; culation. When combined with salinity and Moore et al. 1980; Killingley and Newman temperature data, water contributions from 1983). These methods assume relatively lit- tle variation (N 1.O%G) in the al80 values of Acknowledgments deep-ocean water over geological time-an We thank the following people for assistance at sea assumption supported by independent ev- during the study: D. Adkison, D. Veidt, T. Whitledge, idence (Ferronsky and Polyakov 1982). V. Koltun, and B. Sirenko. R. Highsmith provided The al80 values of seawater vary tem- tunicate samples collected in June 1988. D. Winter performed the mass spectrometric analyses. L. Coach- porally and spatially in portions of the ocean, man and three anonymous reviewers gave comments such as on shallow continental shelves in- that improved the manuscript. fluenced by freshwater input, particularly at Financial support was provided by NSF grants DPP high latitudes. Oxygen removed from sea- 88-l 3046, DMB 84-05003, and DMB 88-96201 and water by organisms should reflect oceanic DOE grant 87-ER60615. The U.S. Fish and Wildlife Service also provided financial and logistical assistance circulation in such circumstances. The pur- that allowed participation in the 1988 Third American- pose of our study was twofold: to analyze Soviet Joint Expedition to the Bering and Chukchi al80 values of seawater as a short-term in- Seas. Logistical and financial support was also provid- dicator of water-mass location in the shal- ed during the 1987 cruise by the ISHTAR project (NSF DPP 84-05286). We thank the Captain and crew of the low arctic and to investigate whether the RV Akademik Korolev and RV Thomas G. Thompson 6180 values of cellulose synthesized by tu- for cooperation in the field. nicates can act as a long-term (multiseason) 1182 Notes 1183 biochemical indicator of water-mass pat- St. Lawrence Island, which forms due to terns. Our intent was unconventional com- winter cooling and ice formation (Coach- pared to that of traditional oxygen isotopic man et al. 1975). studies of biosynthesized materials. Instead Salinity and 180 content are related in of relating the 6180 values of the biosyn- most ocean waters, with similar processes thesized materials to variations in water influencing both in tandem (Epstein and temperature, we sought to relate the al80 Mayeda 1953; Ferronsky and Polyakov values of tunicate cellulose to the ambient 1982). Thus, the major water masses in our 6l*O values of different water masses in a study can also be distinguished by 6180 val- shallow, polar system. ues. The salinity-al80 relationship can be- The shallow shelf of the northern Bering come decoupled when multiple freshwater and Chukchi Seas (averaging ~70 m) is ice sources of differing al80 values mix with covered for 7-8 months of the year. The saline water, leading to differences in al80 summer physical oceanographic regime in- values but not the salinity of the mixtures. cludes three major water masses that are Another deviation from the salinity-al80 re- steered bathymetrically northward across lationship can occur when sea ice forms and the northern Bering shelf into the Chukchi the resultant brine rejection increases the Sea (Coachman et al. 1975; Schumacher et underlying waters’ salinities but does not al. 1983; Walsh et al. 1989; Fig. 1). These significantly change a1*O values (Redfield water masses are defined by T’S profiles and and Friedman 1969; Vetshteyn et al. 1974; are characterized by the following average Ferronsky and Polyakov 1982). In this cir- bottom-water properties in summer: Ana- cumstance, a18O values will more accurately dyr water (AW; S > 32.5o/oo, T = - l.O- separate water masses than will salinity. 1.5”C) on the western side of the system, Thus, on shallow continental shelves in po- Bering shelfwater (BSW; S = 3 1.8 to 32.5o/oo, lar regions, where both freshwater runoff and T = 0- 1.5”C) in the middle region, and Alas- brine rejection can change seawater 180 ka coastal water (ACW, S < 3 1.8!&, T 2 4°C) composition and salinity, respectively, the near the Alaska coast (Walsh et al. 1989). combination of both measurements can dis- North of Bering Strait in the Chukchi Sea, tinguish changes or continuity in water-mass water originating from along the Siberian composition otherwise unobservable with coast to the west and north is carried to the salinity measurements alone. southeast by the Siberian Coastal Current DeNiro and Epstein (198 1) observed that (S > 33?&, T < - 1°C; Coachman et al. cellulose 6180 values from tropical and tem- 1975; Coachman pers. comm.). perate aquatic plants and were Both AW and BSW originate south of St. 27 + 3o/oomore positive than the al80 values Lawrence Island. AW originates in the of the water in which they grew. No signif- northern Bering Sea as a branch of the Be- icant temperature effects on isotopic frac- ring Slope Current (S - 33.27~), which di- tionation were observed during cellulose vides east of Cape Navarin and transits the synthesis in freshwater plants or during in Gulf of Anadyr as the Anadyr Current vitro carbonyl exchange reactions that may (Coachman et al. 1975; Walsh et al. 1989). govern the fractionation observed between The AW passing northward through Ana- cellulose and water (Sternberg and DeNiro dyr Strait contains 80-90% water from the 1983). Although tunicates have not been Bering continental slope (Bering Slope Cur- cultured under different temperature re- rent), with the rest coming from waters in gimes, the similarity of the offsets between the Gulf of Anadyr, runoff on the west side, the al80 values of water and tunicate cel- and Bering shelf water on the east side. In lulose for that lived at temperatures the central Gulf of Anadyr, water is colder differing by as much as 15°C (DeNiro and (T < -O.SOC) and less saline (S 5 32.37~) Epstein 198 1) suggests that, as in plants (Ep- than AW to the west. BSW is a mixture of stein et al. 1977; DeNiro and Epstein 1979), water from the Bering Sea mixing with the there is no significant temperature effect on less saline, cold pool of resident water on oxygen isotopic fractionation in tunicate the northern Bering Sea shelf just south of cellulose. Before this study, no research had 1184 Notes

67 + water circulation

65

63

\ BERING SEA 61

BERING SLOPE

Fig. 1. Study area showing local water circulation, water masses, and bathymetry (modified from Coachman et al. 1975; Walsh et al. 1989). been done to investigate the l*O composi- known as a tunic, along with inhalent and tion of polar tunicate cellulose. exhalent siphons at one end and an attach- Ascidians (often called sea squirts) are ment surface at the other. The fibrous ma- sessile tunicates that occur attached to rocks, trix of the tunic is composed primarily of a pilings, or ships as well as within fine sed- type of cellulose, called tunicin, that occurs iments (Barnes 1980). These animals are in variable amounts, with proteins, poly- characterized by a tough outer body wall, saccharides, blood cells, and inorganic com- Notes 1185 pounds (Ushakov 1955; Alexander 1975; the laboratory. Oxygen isotopic ratios of Barnes 1980). Ciliary action transports wa- water samples were determined by equili- ter through the tunicate inhalent siphon and brating 1.O-ml water samples with -300 a central pharynx composed of a fine mesh, pmol of CO2 for 48 h, purifying the equil- and plankton (primarily unicellular algae) ibrated CO, cryogenically, analyzing it mass are strained from the water. Water then spectrometrically, and using mass balance passes into a surrounding cavity and is ex- to calculate the original oxygen isotopic pelled by the exhalent siphon (Alexander composition of the water (Epstein and May- 1975). Sessile tunicates filter seawater for eda 1953). Precisions of 6l*O values deter- both food and oxygen and can filter water mined for water were +O. 17~ (SD). volumes ranging from 72 to 173 liters d-l, Tunicates were collected from both O.l- although the itself is only 5-l 5 cm m2 van Veen grabs and otter trawls. Ani- long (Alexander 1975; Barnes 1980). Oxy- mals were sorted to species or lowest taxon gen is required for filter feeding, metabolic possible and frozen in Whirl-Pak bags. Tu- processes, and growth (Alexander 1975). nicates were freeze-dried in the laboratory. Solitary tunicates in tropical and temperate The body wall of solitary animals was dis- waters can live l-3 yr, colonial tunicates sected out for cellulose extraction. In the even longer (Barnes 1980). Cold tempera- case of colonial ascidians, a section of the tures and slower metabolism may be asso- outer region of the animals was excised for ciated with longer lifespans in polar re- extraction. Cellulose was extracted with a gimes, as is the case with many marine sodium chlorite-acetic acid oxidation pro- invertebrates in Antarctica (White 1977). cedure (Wise 1944). Oxygen isotopic ratios Distribution of tunicates and other macro- of cellulose were determined by pyrolyzing benthic animals in the study area has re- vacuum-dried and sealed samples in the cently been surveyed and linked to ocean- presence of HgC12 at 520°C for 5 h to form ographic processes governing pelagic CO, C02, and HCl. CO was disproportion- productivity in the Bering and Chukchi Seas ated to CO, and C by electrical discharge. (Grebmeier et al. 1988, 1989; Grebmeier HCl was removed by reaction with isoquin- and McRoy 1989). oline (Epstein et al. 1977). The CO, was Salinity, temperature, and depth data were then analyzed mass spectrometrically. Pre- collected with a Neil Brown conductivity- cisions of al80 values determined for cel- temperature-depth profiler during the cruise lulose were +0.5o/oo (SD). on the RV Thomas G. Thompson (TT214) Twenty seawater samples were collected and a Seabird SBE9 CTD/General Oceanics at 10 stations during TT2 14 (1 O-24 August rosette system during the cruise on the RV 1987): half surface and half bottom samples Akademik Korolev (AK47). Data for 1987 (Table 1, Fig. 2). Seawater was most de- (TT2 14) were collected in conjunction with pleted in l*O just offshore of the Yukon Riv- the National Science Foundation-funded er (14-m depth) in the less saline ACW, ISHTAR (Inner Shelf Transfer and Recy- where the 6l*O value was -4.9?& near the cling) project (P. McRoy and R. Tripp un- bottom. Stations farther from the Yukon publ. data). Data in 1988 (AK47) were col- River, yet still within ACW, had #*O values lected by the University of Texas Marine ranging from - 3.6 to - 3.7(?& in surface wa- Science Institute-University of Washing- ters and - 3.2 to - 3.87~ in bottom waters. ton-USSR GOSGIMET project as part of Stations in the Chirikov Basin-the region the Third American-Soviet Joint Expedi- north of St. Lawrence Island in the more tion to the Bering and Chukchi Seas (A. saline BSW- had higher l*O concentra- Amos and L. Coachman unpubl. data). Sta- tions, with a1*O values ranging from - 1.8 tion numbers are presented as the 2- or to-1.90/00atthesurfaceand-1.6to-1.8(% 3-digit cruise number followed by the 3-digit at depth. Stations north of Bering Strait had consecutive station number for that cruise. al80 values ranging from - 1.4 to -2.27~ Seawater samples were collected with for surface waters and bottom values rang- 3-liter Niskin bottles and stored in 8-ml vi- ing from - 1.4 to - 1.8%0. Comparison be- als, sealed with Parafilm, and returned to tween surface and bottom seawater samples 1186 Notes

Table 1. Summary of hydrographic and stable oxygen isotopic measurements for surface and bottom waters collected during cruise TT214 (12-3 1 August 1987).

Bottom water Depth a-t 6’80sMow St% Location (ml e-4 64 002 63”58SN, 165”30.5’W 5 29.53 6.81 23.13 -3.7 17 29.63 6.67 23.23 -3.8 003 63”30. l’N, 165”35.4’W 5 30.00 11.48 23.00 -3.6 22 30.29 5.71 23.84 -3.8 004 62”45.0’N, 165”56.O’W 5 29.84 12.09 22.55 -4.8 14 29.88 12.05 22.58 -4.9 007 62”19.0’N, 167”08.O’W 5 29.33 11.12 22.44 -3.6 31 30.40 2.31 24.24 -3.2 028 64”23.2’N, 169”2O.O’W 5 31.39 9.38 24.3 1 -1.9 39 32.10 0.36 25.72 -2.1 050 64”46.0’N, 168”38.O’W 5 30.19 10.45 23.32 -1.8 45 32.16 0.59 25.76 -1.6 128 65”58.0’N, 168”56.O’W 5 32.32 1.10 25.89 -1.8 45 32.9 1 2.05 26.27 -1.4 144 67”30.0’N, 168”55.O’W 5 32.04 2.91 25.44 -1.4 49 32.75 1.63 26.23 -1.4 148 67”46.1’N, 167”lO.O’W 9 31.24 5.86 24.68 -1.9 44 32.17 1.11 25.84 -1.8 156 68”00.O’N, 168”55.O’W 5 30.37 5.76 24.25 -2.2 45 32.75 0.89 25.65 -1.5 at individual stations indicated no signifi- - 1.89m) under BSW and in the BSW-ACW cant difference in l*O *. 160 ratios with a Wil- frontal zone (- 1.8 to - 3.09~). The most coxon signed-ranks test (P = 0.173). There- ‘*O-depleted values occurred near the Alas- fore, all values were combined to compare ka coastline in ACW (- 3.0 to - 5.09~). The the relationship of 6180 values to salinity. pattern is similar in the western portion of Since water masses in the area are defined the southern Chukchi Sea (excluding the by bottom salinity values and tunicates grow westernmost protrusion of more negative benthically, we designed the second field al80 values), with P*O values becoming season to investigate water-mass location more negative at stations closer to the Alas- with bottom-water samples only. ka coastline. Fifty bottom-water samples for stable ox- We observed a significant relation in bot- ygen isotopic composition were collected in tom seawater salinity-stable oxygen isotope the central Bering Sea, Gulf of Anadyr, in the study area. Different water masses can northern Bering Sea, and southern Chukchi be distinguished (Fig. 3) based on both the Sea during AK47 (25 July-3 September salinity definitions provided in the intro- 1988; Table 2). These data, in combination duction and the spatial distribution of P*O with P*O bottom-water values from cruise values (Fig. 3): ACW (S = 29-31.59m, P*O TT2 14, show a pattern of more enriched = -3.0 to -5.09~), BSW (S = 31.8-32.59~, I80 seawater in the central Bering Sea with V*O = - 1.6 to -2.19~), AW (S = 32.5- P*O values becoming more negative on the 33 1900 6180 = - 1.4 to - 1.69~), Gulf of continental shelf of the northern Bering and Anady; water (GAW; S = 32.49~ ?jl*O = Chukchi Seas (Fig. 2). In the Gulf of Anadyr, - 1.9 to - 2.09~), and Siberian coasial water the stable oxygen isotopic values ranged (SCW; S > 339m, 6180 = - 1.7 to - 1.89~). from - 1.4 to - 1.69m in AW, with the most Stations in the Bering Slope Current water negative P*O values for this area observed (BSCW) had bottom salinity values ranging in the outer boundary of the gulf (Figs. 1, from 33.0 to 33.19~ and al80 from - 1.3 to 2). Bottom 6 l *O values generally ranged from - 1.49~. The least negative al80 value - 1.4 to - 1.69~ in the western portion of (- 0.89m) occurred in deep Bering Sea water the Chirikov Basin under AW, becoming (3,215-m depth). more negative in the central basin (- 1.6 to Twenty-one benthic tunicate samples were Notes 1187

69

67

65

63

61

-0.8 . I I I I 180 175 170 165 Fig. 2. Distribution of bottom-water P*O values for stations occupied during RV Akudemik Korolev cruise 47 (0) and RV Thomas G. Thompson cruise 2 14 (0). Isotope ratio units: ‘SO.

collected at 18 stations in the Gulf of Ana- family contained three species. The dyr, northern Bering Sea, and southern P*O values for tunicate cellulose ranged Chukchi Sea in 1988 (Table 3). Five of the from +24.8 to + 3 1.49~. The distribution six tunicate families identified at all stations of tunicate cellulose P*O values in the were represented by only one species; the northern Bering Sea (Fig. 4) is similar to the 1188

Table 2. Summary of hydrographic and stable oxygen isotopic measurements for bottom waters collected during cruise AK47 (26 July-3 September 1988). In addition, one station from RV Alpha Helix cruise 113 (10 July 1988) is included (nd-no data).

Depth a-t 6’80sMow sta. Location (m) (94 (94 004 58”3l.l’N, 174’27.l’W 150 33.00 3.50 26.25 -1.4 005 58”30.0’N, 175”29.5’W 140 33.07 3.20 26.33 -1.3 006 59”29.4’N, 179’14.l’W 3,215 34.64 1.58 27.73 -0.8 007 60”32.0’N, 177’46.O’W 140 33.06 1.90 26.42 -1.4 009 61”20.8’N, 176’05.3’W 107 32.40 -0.95 26.04 -1.6 011 61”39.5’N, 178’39.5’W 144 33.05 1.28 26.46 -1.5 013 62”11.3’N, 179’52.3’W 148 33.01 0.98 26.45 -1.4 015 62”34.0’N, 179’3 1.2’W 97 32.55 - 1.04 26.17 -1.9 018 62”00.6’N, 175’02.6’W 75 32.38 - 1.53 26.04 -2.0 019 62”28.l’N, 174’00.6’W 63 32.35 -1.60 26.01 -2.0 022 62”35.4’N, 175’04.8’W 88 32.70 0.48 26.23 -1.8 024 63”42.6’N, 178’28.8’W 81 33.10 1.25 26.50 -1.6 027 64”44.8’N, 178”46.O’W 64 33.09 0.72 26.52 -1.4 031 64”20.3’N, 174”59.O’W 72 32.95 0.64 26.42 -1.5 032 63”58,l’N, 174’57.3’W 76 32.89 0.75 26.36 -1.5 035 62”58.5’N, 173’00.6’W 63 32.45 -1.59 26.10 -2.0 041 64”01.7’N, 172”12.8’W 52 32.63 0.03 26.19 -1.5 045 67”44.9’N, 172’47.9’W 45 33.65 0.06 27.02 -1.8 047 68”06.0’N, 170’53.8’W 49 32.88 0.72 26.36 -1.6 049 68”28.0’N, 169’08.O’W 55 32.40 2.31 25.87 -1.8 050 68”39.7’N, 168”2O.O’W 48 32.13 4.13 25.49 -1.9 052 68”03.6’N, 176”ll .O’W 46 32.37 3.89 25.7 1 nd 053 67”37.5’N, 165’43.7’W 38 32.33 2.52 25.79 -1.9 054 67”45.0’N, 167’19.O’W 51 32.76 1.92 26.18 -1.6 055 67”44.6’N, 168*25.4’W 46 32.77 1.57 26.22 -1.4 057 68”44.0’N, 17 l”20.8’W 43 33.05 0.57 26.50 -1.8 059 67”09.2’N, 172”59.8’W 38 33.50 0.18 26.89 -1.7 064 67”18.3’N, 166’41.5’W 45 32.62 1.86 26.07 -1.4 065 67”20.9’N, 164’58.4’W 29 32.00 3.66 25.43 nd 067 66”57.4’N, 166’47.l’W 35 32.09 3.73 25.50 -1.8 069 67”00.O’N, 168’43.9’W 42 32.27 2.08 25.78 -1.7 071 66”54.0’N, 170’0 1.O’W 38 33.28 1.01 26.66 nd 072 66”31.8’N, 170’01.2’W 49 32.73 1.64 26.18 -1.5 074 66”33.0’N, 168”36.6’W 45 32.24 2.47 25.72 -1.6 083 65”40.3’N, 168”29.9’W 49 32.09 3.48 25.52 -1.8 087 65”24.3’N, 170’24.3’W 34 32.85 2.09 26.24 -1.7 088 65”21.4’N, 170”21.4’W 39 32.82 1.63 26.25 -1.4 089 65”13.8’N, 169’21.2’W 45 32.40 1.69 25.9 1 -1.8 090 65”10.5’N, 168’39.5’W 47 32.30 1.37 25.86 -1.8 091 65”14.6’N, 167’59.O’W 33 30.19 10.20 23.17 -3.3 092 64”40.6’N, 167”40.5’W 27 31.46 2.83 25.08 -3.3 093 64”45.3’N, 168’27.O’W 40 32.32 1.62 25.85 nd 094 64”5l.O’N, 169’11.7’W 42 32.54 1.03 26.06 -1.5 095 64”58.2’N, 169’58.6’W 44 32.71 2.06 26.14 -1.6 096 65”06.4’N, 170’45.l’W 42 32.82 1.45 26.27 -1.6 097 65”45.0’N, 171’29.8’W 42 32.94 1.43 26.36 -1.6 098 64”43.2’N, 170’53.l’W 44 32.72 1.89 26.15 -1.5 099 64”32.2’N, 170°02.1’W 41 32.44 1.79 25.99 -1.7 100 64”22.6’N, 169’10.9’W 37 32.51 1.79 26.00 nd 101 64”22.6’N, 169’10.9’W 34 32.16 2.79 25.64 -1.7 102 64”05.2’N, 167’23.3’W 28 32.13 2.31 25.65 -2.5 103 63”39,5’N, 168’2 1.7’W 28 31.81 4.98 25.15 -1.8 104 63”50.7’N, 170’12.4’W 30 32.20 4.00 25.56 -1.7 105 64”02.2’N, 170°04.2’W 29 31.95 5.41 25.21 -1.6 106 64”14.0’N, 170”54.7’W 32 32.11 0.80 25.80 -1.8 113-034 65”07.0’N, 168”23.6’W 45 32.73 -1.60 nd nd Notes 1189

Bottom water Siberian coastal water

PO SMOW (O/00)

m offshore Yukon River (14 m depth) I I I I I I 30 31 32 33 34 35

Bottom water salinity (o/oo) Fig. 3. Relation between bottom-water PO values and salinity for stations in the northern Bering and Chukchi Seas. Water masses are defined by both salinity and location.

pattern of P *O values in the overlying water Fig. 5). The one sample collected in ACW (Fig. 2). P*O values are most positive (+ 30.0 had a P80 value of +26.4%~, although the to + 32.07~) in the western region, normally lowest tunicate cellulose 6180 value oc- covered by AW, with the least positive curred just north of Bering Strait in BSW. tunicate cellulose P80 values (+25.0 to Tunicates collected under SCW also had +28.07~) in the eastern basin under ACW lower cellulose 180 content (P80 = +26.8 and the ACW-BSW frontal zone. Tunicates and 27.9Y&~).No significant relationship was inhabiting the transition zone of BSW be- found between tunicate cellulose P80 and tween AW and ACW exhibit P80 cellulose salinity when all stations were used in the values ranging from +28.0 to + 30.0%. The analysis (Spearman’s p = 0.29, P > 0.05, n pattern is similar in the central and eastern = 21). part of the southern Chukchi Sea (Fig. 4). The distribution of bottom-water P*O Stations in the southwestern section of the values in the study area indicates the dis- Chukchi Sea, corresponding in location to tribution and spatial flow of water masses SCW, have less positive tunicate cellulose and freshwater inputs in the northern Bering al80 values (+26.0 to +28.0%~), however, and Chukchi Seas. The P*O-S relationship than animals underlying the adjacent AW for all stations analyzed (Fig. 3) separates and BSW (+28.0 to + 30.0%) to the east. the major water masses in the area: AW, The relation is significant between tuni- BSW, and ACW. In addition, our results cate cellulose P80 values and 6 180 values support the conclusion of Coachman et al. of the water from which the tunicates were (1975) that AW passing through Anadyr obtained (Spearman’s p = 0.505, 0.025 < Strait (mean 6180 = - 1.5&O. I?&, n = 9) is P < 0.05, n = 15; Fig. 5). (Due to sampling composed of both BSCW (mean P*O = limitations, only 15 of the 2 1 tunicate sam- - 1.4+0.1?& n = 5) and GAW (mean al80 ples collected had simultaneous water col- = -2.OfO.1%0, n = 4). Solving a simple lections.) Tunicate cellulose values in AW mixing equation, showed the lowest variability (mean 6180 = +30.7*0.8%) among four species com- x(V80 BSCW)+ (1 - xW8%4w) pared to a wider spread in BSW (mean = + 28.5 + 1.87~) among five species (Table 3, = (~18Q4wh (2) 1190 Notes

Table 3. Oxygen isotopic composition of tunicate effects on salinity (Coachman et al. 1975) cellulose for samples collected in the Gulf of Anadyr, and al80 values. Local effects may account northern Bering Sea, and southern Chukchi Sea on cruise AK47. The WO value for a sample obtained for the more negative P*O value just south from RV Alpha Helix cruise 113 is also included. Pre- of Bering Strait along the Siberian coast (Fig. vious family names given in parentheses. 2). The Yukon River provides a large l*O-

Family depleted continental water source to the northern Bering Sea system, producing the Clavelinidae most depleted al80 value (- 4.87~) mea- Clavelina sp. 102 +2&o sured at the shallowest station (14 m) in the Molgulidae study (Figs. 2 and 3). The y-intercept of the Molgula sp. 065 +26.8 salinity (x)-P*O@) relation, -24.9o/oo (Fig. Molgula sp. 100 +28.5 Molgula sp. 102 + 30.2 3), indicates the isotopic composition of the Molgula sp. 113-034 +28.9 continental input to the area. This y-inter- Pyuridae cept P*O value is close to al*O values for echinata 052 +26.8 Yukon River water collected at the Dalton 041 +31.2 Highway Bridge on 3 and 29 June 1989 B. ovifera 088 +31.3 (-22.4 and -2 1.7o/oo;L. Cooper unpubl. B. ovifera 106 +30.4 data). This bridge is upstream of the Tanana aurantium 053 +29.3 H. aurantium 105 +30.0 River confluence, which drains much of the high-elevation Alaska Range, where alti- Corellidae (Rhodosomatidae) tude effects should intensify depletion of 1*O Chelyosoma orientale 031 +31.4 in precipitation. The Tanana is the largest Styelidae single tributary of the Yukon, so the v-in- Pelonaia corrugata 059 +26.8 tercept value of - 24.9?& is probably close P. corrugata 071 +27.9 P. corrugata 072 +29.9 to the actual average al80 value for fresh- P. corrugata 093 +29.5 water at the Yukon River delta. Polyclinidae (Synoicidae) Variations from this salinity (x)-P*O@) Synoicum sp. 041 +29.6 relationship @l*O = 0.71x + -24.97~) in- Synoicum sp. 074 +24.8 dicate intrusions of waters distinct from the Synoicum sp. 083 +27.7 two end-members: continental freshwater Synoicum sp. 092 +26.4 (-24.9ym) and deep Bering Sea water Synoicum sp. 106 +27.6 (-0.8o/oo; Fig. 3). Deviations just north of St. Lawrence Island (Fig. 3) indicate the in- trusion of different freshwater. Inflows of indicates that BSCW provides 0.80 (x) of coastal precipitation are less depleted in ‘*O AW and GAW the remaining 0.20(1 - x) than continental freshwater precipitation of the total water forming AW. This simple (Gat 1980). For example, the annual aver- model is consistent with the conclusion of age al80 value for coastal precipitation on Coachman et al. that AW contains 80-90% Adak Island (52”N) in the western Aleutian of the incoming Bering Slope Current water Islands is -9%~ (Hage et al. 1975). Thus, and lo-20% Gulf of Anadyr water. local freshwater flowing off St. Lawrence Is- ACW is a combination of Yukon River land may provide a less ‘*O-depleted pre- inflow and input from other large rivers to cipitation signature to these stations relative the south, such as the Kuskokwim, and the to the more ‘*O-depleted continental fresh- Kobuk and Noatak Rivers north of Bering water dilution influencing BSW. Another Strait (Coachman et al. 1975). In compar- possibility is that this region is a mixing ison, freshwater inflow on the Siberian side zone for AW and BSW that is concurrently is much less important. The largest river in diluted by sea-ice known to accumulate the region, the Anadyr, contributes only a north of St. Lawrence Island (Coachman fourth the annual discharge of the Yukon pers. comm.), thus reducing the salinity of River. Thus, the freshwater inputs are large the water but not its l*O content. The vari- enough to produce local, but not regional, ations of points within ACW and opposite Notes 1191

-69 CHUKCHI SEA

0 0 BERING SEA

.67

CHUKOT PENINSULA

.65

GULF OF ANADY R

63

BERING SEA

61

180 175 170 165 Fig. 4. Distribution of tunicate cellulose 6’*0 values for stations occupied during RV Akademik Korolev cruise 47 (0) and RV Alpha Helix cruise 113 (*). Isotope ratio units: o/00.

the Yukon River delta are probably due to in the mixing zone between BSW (mean P*O entrainment of coastal water from the south = -1.8%)andACW(meanP80= -4.0%). with Yukon River water. The station la- SCW stations are at variance with the S- beled frontal station BSW-ACW (Fig. 3) is ai80 relationship (Fig. 3), indicating the in- 1192 Notes

32- 3QG3 2 Tunicate 30- cellulose 18 1 28- ’ OSMOW I (O/00)

Bering Strait

-3.0 -2.5 -2.0 -1.5

Bottom water $ 80sMow (“/,,)

Fig. 5. Relation between PO values of tunicate cellulose and bottom water at the same station. The symbols refer to water-mass designations based on salinity values: * -ACW (<31.8o/oo); l -BSW (31.8-32.5%); A-AW (>32.5-33.1%0); 0-SCW (>33.1?&). A line connecting two points indicates two different species collected at the same station. The numbers refer to the species analyzed: 1 - Clavelina sp.; 2-A4olgula sp.; 3 -Boltenia ovifera; 4 - ; 5 - Chelyosoma orientale; 6 - Pelonaia corrugata; 7 -Synoicum sp.

trusion of another water type with high sa- column with salinity of 32?&, the salinity of linity and l*O depletion (Fig. 2). Resident the remaining water would increase to Chukchi Sea water occurs in the south-cen- 3 3.1?&. Unlike salinity, the a1*O value of tral Chukchi Sea (near Wrangel Island) dur- the remaining seawater does not change sig- ing summer as remnant water from winter nificantly from the initial P*O value of the within a stagnant flow regime (Coachman seawater before ice formation, although the et al. 1975). If, as Coachman et al. (1975) newly formed ice is enriched by 2o/ooin l*O propose, this remnant water is a cooled mix- relative to the initial seawater (Redfield and ture of Anadyr and Bering shelf waters (re- Friedman 1969; Tan and Strain 1980; Be- ferred to as Bering Sea water in their book), dard et al. 198 1). The above example, which the salinity would increase due to brine re- is a typical depth in the study region, sug- jection to the bottom water during ice for- gests that if 4% of the water column (2 out mation, and the l*O content would be in- of 50 m) freezes and becomes enriched by fluenced by both water types. The 2o/oo,the remaining 96% that has a 1.17~ entrainment of this water type in the Sibe- increased salinity will decrease in its al80 rian Coastal Current would transport the value by

dicating the potential value of these Walsh et al. 1989). The horizontal expanse organisms for defining water masses and of BSW is directly influenced by the volume hence possibly inferring ocean circulation and mixing of Bering Sea water from south in polar regions. of St. Lawrence Island with AW from the For AW, which arises in the Gulf of Ana- west and ACW from the east, all flowing dyr, the distribution of tunicate cellulose northward through the region and perhaps 61 *O values (Fig. 4) is very similar in spatial explaining the wide range of water and distribution to the overlying bottom-water tunicate cellulose P*O values (Figs. 2 and V*O values (Fig. 2), indicating a potential 5). Tunicate cellulose P*O values ranged relationship between the two. Tunicate cel- from + 26.4Ym in the east to + 30.2o/ooin the lulose 6l*O values in AW averaged west, giving an average BSW value of + 30.7 +0.8%0 (n = 5) for both solitary (Bol- +28.5* 1.8?& (n = 8; Fig. 5). The larger tenia ovifera, Chelyosoma orientale, Pelo- variability in the stable oxygen isotopic val- naia corrugata) and colonial (Clavelina sp., ues of tunicate cellulose compared to sam- Synoicum sp.) ascidians (Table 3, Fig. 5). ples collected in AW may be indicative of The fact that four tunicate species had sim- either the large spatial variability in BSW ilar cellulose V*O values indicates minimal over the open-water season due to mixing species effects on oxygen isotopic fraction- with AW to the west and ACW to the east ation in this water mass. ACW, in the east- or interspecific physiological differences that ernmost region of the study area, has bot- could influence oxygen isotopic fractiona- tom-water P*O values ranging from - 3.0 tion during tunicate cellulose production. to -5.O?& (Figs. 2 and 3). Tunicates were Further investigation is also needed to collected at only one station in ACW, but study isotopic effects influenced by physi- by comparison with AW had a less positive ological differences between solitary and co- P*O value, +26.4% (Table 3, Fig. 5). lonial ascidians on oxygen fractionation The least positive value of tunicate cel- during cellulose production. When both sol- lulose P*O (+24.8%) was observed at sta- itary and colonial species were obtained at tion AK47-074 just north of Bering Strait individual stations in both BSW and AW, (Figs. 4 and 5), which, based on bottom- colonial ascidians (Clavelina sp., Synoicum water P*O and salinity values, was covered sp.) consistently had cellulose #*O values with BSW. Longer term, seasonal analyses less positive than solitary species (Molgula of the location of ACW indicate that it oc- sp., B. ovzjkra; Fig. 5). This difference may cupies this area most of summer (Fig. 1; be due to physiological differences between Coachman et al. 1975), suggesting that the solitary tunicates, which build individual lower P*O value recorded in the tunicate body walls, and colonial tunicates, which cellulose is an indicator of tunicate growth share a common outer tunic and fused in- when ACW normally overlies this station, terior chambers. Another possibility may be particularly earlier in the season when fresh- that solitary and colonial organisms have water runoff is maximal. If we exclude this different growth rates or periods of growth. station, based on the a priori assumption Nevertheless, if most interspecific differ- that ACW normally overlies this station, ences are caused by growth at different times the significance of the correlation between of year, rather than by biochemistry, tuni- tunicate cellulose P*O values and 6 l*O val- cates collected in AW probably are a better ues of bottom seawater increases from 0.025 indicator of the magnitude of biochemical < P < 0.05 (Spearman’s p = 0.505, n = 15) variability because BSW is a mixed water to 0.01 < P < 0.025 (Spearman’s p = 0.60, mass with a more variable salinity and iso- n = 14). topic composition. Additional work is also BSW had P*O values ranging from - 1.6 needed to determine the lifespan of the tu- to -2.l%~, intermediate between those of nicate species in the study area in order to AW and ACW (Figs. 2 and 3). The location use tunicate cellulose as a seasonal indicator and composition of BSW is dependent on of water-mass location. water transport events through Anadyr and Despite various uncertainties, there is a Shpanberg straits (Coachman et al. 1975; distinct gradient of more positive P*O val- 1194 Notes ues of water and tunicate cellulose in AW Department of Geological Sciences in the west to intermediate values in BSW, University of Southern California with the least positive values in ACW to the Los Angeles 90089 east (Figs. 2 and 4). Tunicate cellulose P80 Lee W. Cooper2 values are significantly related to the al80 Michael J. DeNiro values of the overlying bottom water throughout the study area, but not to bot- Department of Earth Sciences tom salinity. As discussed earlier, however, University of California higher salinity in SCW is apparently caused Santa Barbara 93 106 by sea-ice formation, which does not sig- nificantly affect bottom seawater P80 val- ues, so tunicates collected at two stations in that water mass show a relationship to sea- References water P80 values, even though the seawater ALEXANDER,R. M. 1975. The . Cambridge. 6l 8O-S relationship has been disturbed (Figs. BARNES, R. D. 1980. Invertebrate zoology, 4th ed. 2 and 4). If we exclude SCW stations from Saunders. the correlation between tunicate cellulose B~DARD, P., C. HILLAIRE-MARCEL,AND P. PAGE. 198 1. 6180 and salinity for all samples collected, l*O modelling of freshwater inputs in Baffin Bay and Canadian Arctic coastal waters. Nature 293: the relationship becomes significant (Spear- 287-289. man’s p = 0.5 15, 0.01 < P < 0.025, n = COACHMAN,L. K., K. AAGAARD, AND R. B. TRIPP. 19), as it is between tunicate cellulose and 1975. Bering Strait: The regional physical ocean- bottom-seawater 6180 values. This finding ography. Univ. Washington. demonstrates the conservative nature of P80 DENIRO, M. J., AND S. EPSTEIN. 1979. Relationship between the oxygen isotope ratios of terrestrial values as opposed to salinity in character- plant cellulose, carbon dioxide and water. Science izing water masses at high latitudes, even 204: 5 l-53. when tunicate cellulose values are used. -, AND -. 198 1. Isotopic composition of Given the indications from this study that cellulose from aquatic organisms. Geochim. Cos- tunicate cellulose P*O values can be used mochim. Acta 45: 1885-l 894. EPSTEIN,S., AND T. MAYEDA. 1953. Variations of 018 to characterize water-mass locations at high content of waters from natural sources. Geochim. latitudes over time scales that are expensive Cosmochim. Acta 4: 2 13-224. and difficult to sample, we think that de- -, P. THOMPSON,AND C. J. YAPP. 1977. Oxygen tailed studies of the basis for the tunicate and hydrogen isotopic ratios in plant cellulose. Science 198: 1209-l 2 15. cellulose-water oxygen isotope relationship FERRONSKY,V. I., AND V. A. POLYAKOV. 1982. En- are in order. Work is needed to elucidate vironmental isotopes in the hydrosphere. Wiley. the biochemical processes involved in cel- GAT, J. R. 1980. The isotopes of hydrogen and ox- lulose synthesis in tunicates, focusing spe- ygen in precipitation, p. 21-48. In P. Fritz and J. C. Fontes [eds.], Handbook of environmental iso- cifically on the issue of whether cellulose tope geochemistry. V. 1. Elsevier. al80 values are determined solely by the in GREBMEIER,J. M., H. M. FEDER,AND C. P. MCROY. situ isotopic composition of the seawater in 1989. Pelagic-benthic coupling on the shelf of the which the animal lives, or whether the P80 northern Bering and Chukchi Seas. 2. Benthic values of organic filtrate available to tuni- community structure. Mar. Ecol. Prog. Ser. 51: 253-268. cates or temperature also affect cellulose -, AND C. P. McRou. 1989. Pelagic-benthic composition. Once the magnitude of these coupling on the shelf of the northern Bering and potentially interfering signals is evaluated, Chukchi Seas. 3. Benthic food supply and carbon the widespread distribution of benthic tu- cycling. Mar. Ecol. Prog. Ser. 53: 79-9 1. - - AND H. M. FEDER. 1988. Pelagic- nicate populations may prove useful in benthic coupling on the shelf of the northern Be- characterizing temporal variation of water- ring and Chukchi Seas. 1. Food supply source and mass distributions in polar regions. benthic biomass. Mar. Ecol. Prog. Ser. 48: 57-67. HAGE, K. D., J. GRAY, AND J. C. LINTON. 1975. Iso- Jacqueline M. Grebmeier 1 topes in precipitation in northwestern North America. Mon. Weather Rev. 103: 958-966.

’ Present address: Graduate Program in Ecology, 2 Present address: Department of Geological Sci- University of Tennessee, Knoxville 37996- 119 1. ences, University of Tennessee, Knoxville 37996- 1410. Notes 1195

KILLINGLEY, J. S., AND W. A. NEWMAN. 1983. O-18 atoms of acetone and water. Geochim. Cosmo- fractionation in barnacle calcite. A barnacle pa- chim. Acta 47: 227 l-2274. leotemperature equation. J. Mar. Res. 40: 893- TAN, F. C., AND P. M. STRAIN. 1980. The distribution 902. of sea-ice melt-water in the eastern Canadian arc- MCCREA, J. M. 1950. On the isotopic chemistry of tic. J. Geophys. Res. 85: 1925-1932. carbonates and paleotemperature scale. 9. Chem. USHAKOV, P. V. 1955. Subphylum Tunicata, class Phys. 18: 849-857. Ascidiae, p. 307-3 12. Zn E. N. Pavlov&ii [ed.], MIKKELSEN,N., L. LABEYRIE, JR., AND W.H. BERGER. Atlas of the invertebrates of the far eastern seas 1978. Silica oxygen isotopes in diatoms: A 20,000 of the USSR. Akad. Nauk SSSR. Zool. Inst. [Isr. yr record in deep-sea sediments. Nature 271: 536- Program Sci. Transl., 1966.1 538. VETSHTEYN, V.Y.,G.A. MALYUK,AND V.P. RUSANOV. MOORE, T. C., JR., AND OTHERS. 1980. The recon- 1974. Oxygen- 18 distribution in the central arctic struction of sea surface temperatures in the Pacific basin. Oceanology 14: 5 14-5 19. Ocean of 18,000 B.P. Mar. Micropaleontol. 5: 2 15- WALSH, J. J., AND OTHERS. 1989. Carbon and nitrogen 247. cycling within the Bering/Chukchi Seas: Source REDFIELD, A.C., ANDLFRIEDMAN. 1969. Theeffect regions for organic matter effecting AOU demands of meteoric water, melt water and brine on the of the Arctic Ocean. Prog. Oceanogr. 22: 279-36 1. composition of polar sea water and of the deep WHITE, M. G. 1977. Ecological adaptions by Ant- waters of the ocean. Deep-Sea Res. 16: 197-214. arctic poikilotherms to the polar marine environ- SCHUMACHER, J. D., K. AAGAARD, C. H. PEASE, AND ment, p. 197-208. Zn G. A. Llano [ed.], Adaptions R. B. TRIPP. 1983. Effects of a shelf polynya on within Antarctic ecosystems. Gulf Publ. flow and water properties in the northern Bering WISE, L. E. 1944. Wood chemistry. Reinhold. Sea. J. Geophys. Res. 88: 2723-2732. Submitted: 12 April I989 STERNBERG,L. D. S. L. O., AND M. J. D. DENIRO. 1983. Biogeochemical implications of the isotopic equi- Accepted: 4 December 1989 librium fractionation factor between the oxygen Revised: 17 April 1990

Lmnol. Oceanogr., 3549, 1990, 1195-1200 0 1990, by the AmericanSociety of Limnology and Oceanography, Inc. Predicting diel vertical migration of zooplankton

Abstract-Amplitude of diel vertical migration 0.880. The residual ofthe migration-water clarity is predicted by water clarity measured by Secchi relationship is significantly and inversely corre- depth. The model assumes that vertical migra- lated with percent illumination of the moon. Wa- tion serves to minimize mortality from visually ter clarity and moon intensity together account feeding fish and to maximize grazing rate within for 84% of the variation in migration amplitude this predation context. Three of the 24 obser- for the 21 observations. vations of diel vertical migration are outliers which are either ultraoligotrophic, or have minimal populations of plankton-eating fish, or both. The The usual behavior pattern of diel vertical other 2 1 observations in lakes with average pho- tosynthetic rates 2300 mg C m-2 d-’ and more migration is for a population of zooplank- than -2 g m-2 of plankton-eating fish showed ton to spend the daylight hours deep in a diel vertical migration proportional to Secchi lake and then to rise toward the surface for depth, with a correlation coefficient of about a few hours at night. This behavior has been studied for nearly two centuries (Lampert Acknowledgments 1989). Several recent reviews identify a few I thank Virginia Dodson, Ken Parejko, Charles factors as important causes of the behavior Ramcharan, Bart DeStasio, and Peter Jumars for com- in both marine and freshwater habitats: in- ments on the manuscript. Thanks to Charles Ram- tensity of visual predation on zooplankton, charan (Licht Pond), Trevor Downie (Muskellunge Lake), and the 1987 University of Wisconsin summer light intensity, temperature, and food levels limnology class (Lake Mendota), who helped sample (e.g. Clark and Levy 1988; Gabriel and these lakes. I especially thank Steve Carpenter who Thomas 1988; Lampert 1989; Wurtsbaugh allowed me to use unpublished data from Long Lake, and Neverman 19 8 8). Although differing in Michigan, Ami Litt who went out and sampled Lake Washington when I asked if there were any data, and details, a common theme runs through these Jane and Wes Licht, who allowed me to sample their studies. Diel vertical migration is thought pond in Wisconsin. of as being primarily determined by a com-