Zooplankton Are Major Sources of Epipelagic Bioluminescence in the Southern Sargasso Sea

BULLETIN OF MARINE SCIENCE. 33(4): 855-863. 1983

ZOOPLANKTON ARE MAJOR SOURCES OF EPIPELAGIC BIOLUMINESCENCE IN THE SOUTHERN SARGASSO SEA

Elijah Swift, William H. Biggley, Peter G. Verity and Dale T. Brown

ABSTRACT In the southern Sargasso Sea, we used a pump-through bathyphotometer (15 I min-I) to investigate stimulatable bioluminescence of the epipelagic zone. Twenty-four percent (range 17-38%) in vertical profiles and 17% (range 6-24%) in discrete depth samples of the flashes were too bright to be produced by dinoflagellates. Of the total bioluminescence detected, these bright flashes contained in vertical profiles 60% (range 53-67%) and in discrete samples 56% (range 29-70%). There were enough bioluminescent dinoflagellates in the bathyphotometer effluent to account for less than 34% of the total flashes detected and less than 45% of the flashes small enough to be produced by dinoflagellates. This information, combined with plankton data, suggests that bioluminescent species of copepods, larvaceans and ostracods produce more stimulatable bioluminescence in the Sargasso Sea than dinoflagellates.

Except for the patchy occurrence of Sargassum weed, the clear blue surface waters of the southern Sargasso Sea may appear devoid of life to an observer on the deck of a ship during the day. But at night, bioluminescence caused by the ship's passage indicates the presence ofliving organisms. The continuous glow of bioluminescence in the wake is punctuated by the sporadic displays of large organisms, probably colonial tunicates and the bioluminescent clouds released by large prawns. In the present paper we explore the question-what is causing the bulk of the steady bioluminescence we see in the ship's wake? The background bioluminescence of seawater has been attributed to dinoflagellates. The reasons for this are summarized by Kelly and Tett (1978) who state that "dinoflagellates are the only luminescent organisms sufficiently abundant and widely distributed in time and space to account for the ubiquity of planktonic bioluminescence." They reported a correlation between the numbers of bioluminescent flashes and numbers of bioluminescent dinoflagellates in inshore and oceanic waters (Kelly, 1968a; 1968b; Tett, 1971). In the upper hundred meters, they found that a 240 J.lm porosity netting over the intake of a pump-through bathyphotometer decreased the number of flashes very little (Clarke and Kelly, 1965). Finally, they found that the most common flashes have a time course similar to those of dinoflagellates (Kelly and Tett, 1978). In the present study we have used a bathyphotometer which records both the flashing rate and the total amount of bioluminescence. As with previous workers, we found that dinoflagellates produce many of the flashes seen in surface waters. Although numerous, these flashes do not contain very much light. We report here that for a station in the epipelagic zone of the southern Sargasso Sea, zooplankton produce more bioluminescent light and more flashes than dinoflagellates.

MATERIALS AND METHODS

Bioluminescence in the upper 200 m was measured using a pump-through bathyphotometer. Details of the electronic circuitry and construction are available from the senior author. In brief, it consists of an underwater unit connected by 250 m of electrical cable to the shipboard unit. The underwater unit has a light-tight chamber through which seawater is pumped at a rate of 15 I min-1 by a centrifugal 855 856 BULLETIN OF MARINE SCIENCE. VOL. 33. NO.4. 1983

No. Flashes / 60 l Integrated mv/GOl 10 20 00 o 1000 3000

E 80 :I: l- n.. 120 W 0

160 Flashes ~ > IOOmv ••• ~ 100mv 200 3/6/'80 • TOTA L, mv/60L 23° 06.9' N 66°29.3'W

Figure I. The vertical profile of mechanically stimulable bioluminescence was measured by pumping seawater past a light detector (s{~etext for details). Flashes detected were divided into those producing more light than the dinoflagellate Pyrocystis noctiluca (ca. 100 mY) which were presumed to be zooplankton, and those producing less light which were presumed to be dinoflagellates and zooplankton. The left-hand panel shows the number of bioluminescent organisms producing more than 2-4 x IOlO photons. The organisms producing less light than P. noctiluca are the most frequent. The right-hand panel shows the amount of light at each depth, which is approximately proportional to the number of millivolts (mV) recorded. The righthand panel indicates that for most depths, organisms producing more than 100 mV (zooplankton), produce the majority of the bioluminescence. Station taken 3/61 80.

pump impeller. Organisms pass into the light-tight chamber and are maximally stimulated as they are drawn through an orifice into the pump impeller. The residence time of water in the chamber is I sec. Organisms leave the chamber through a 1.9-cm diameter hose and are retained by a 25/lm porosity net tied to the end of the hose. The impeller is rotated by a 6 volt DC submersible motor powered by a rechargeable Pb-Ca battery immersed in a mineral oil bath. The mineral oil expands and contracts due to temperature and pressure changes in an expansion chamber fitted with a piston. In a pressure housing above the agitation chamber, the bioluminescence is sensed by a photomultiplier tube (Gencon-EMI 9824A operated at -700 V). The photomultiplier current is passed through a resistance and the voltage change is amplified by an electrometer amplifier. The output (0-10 V) is passed up the electrical cable to a deck unit where it is recorded (as an analogue of intensity) using a recorder with a pen response of <0.0 I sec (Gould-Brush model 220). This voltage signal is also integrated electronically over time to indicate the total light in each bioluminescent flash. The integrated signal is recorded on a second channel. When the deck unit is turned on, it supplies 7 V to the imput side of a high voltage power supply in the underwater unit for the photomultiplier. The same current activates relays initiating battery power for the amplifier and for the pump motor. The signals from the bathyphotometer are calibrated at sea by immersing the bathyphotometer in a test tank and introducing a small number of dark-adapted cells of the dinoflagellate Pyrocystis noctiluca (Murray) Schuett, 1885. These cells are collected late in the afternoon, picked out of net tows by pipette, and used for calibration two or more hours after sundown and at least several hours after they have been in the dark. P. noctiluca produce 2-7 x 10lOphotons when vigorously stimulated. For this cruise, the cells of P. noctiluca passing through the bathyphotometer produced 100 mV response in the integrated signal. The samples and profiles described here were taken on cruise 049 of R/V ENDEAVOR while the ship stayed with a drogue launched at Lat. 23°07'N and Long. 66°27'W on 6 III 80 and recovered on II III 80 at Lat. 23°11.9'N and Long. 66°20.6'W. During this period the daily photon flux of photosyn- SWIFT ET AL: EPIPELAGIC BIOLUMINESCENCE 857

No. Flashes /45 L Integrated Light, mv/45 L 10 20 30 40 o 1000 2000 3000

E 80 J: t- a.. 120 w a Flashes 160 " > 100 mv • :s 100 mv • TOTAL, mv/45L 200 3/9/'80 23°12.87' N 66°31.2' W

Figure 2. As in Figure I. Station taken 3/8/80.

thetically active radiation (vacuum wave lengths of 400-700 nm) was 44.3 Einsteins m-2 sec on 5 III 80, and on subsequent days through II III 80, 43.7, 29.6, 28.4, 42.4, 45.6 and 42.6 Einsteins m-2• Generally, the depth to which I% of this photon flux penetrated was 120 m (range 115- 130 m). Surface temperature was 24-25°C, the mixed layer was 80-90 m deep and the temperature at 200 m was 20·C. Salinity increased from surface values of 36.30/00to 36.70/00at 200 m.

RESULTS We investigated the vertical profile of bioluminescence in the upper 200 m by lowering our bathyphotometer either 10 or 20 m at a time and pumping 3-5 minutes (45-75 I) at each depth (Figs. I, 2, 3, Table I). In the profiles, organisms producing more light than P. noctiluca were in the minority (Table 1). Above 100 m they were 13-29% of the bioluminescent organisms and below 100 m they were 7-33% of the bioluminescent organisms. This minority of highly luminous organisms produced 53-63% of the bioluminescence above 100 m, 49-81 % of the bioluminescence below 100 m (Table 1). As P. nocti/uca has the greatest bioluminescent capacity of oceanic dinoflagellates (Swift et aI., 1973; Esaias, 1973; Tett and Kelly, 1973), we assume that the flashes containing more light than this are from zooplankton. Flashes from organisms producing less light than a P. noctiluca were 71-87% of the flashes above 100 m and 67-93% of the flashes below 100 m. We also lowered the bathyphotometer to discrete depths, pumped water past the photomultiplier tube and collected the organisms in the effluent with a fine mesh (25 ~m porosity) plankton net. We observed the same trends as with the more extensive profiles, 17% of the flashes observed contained more light than a P. nocti/uca flash (100 mY) and contained 56% (range 29-70%) of the total light in the sample (Table 2). There were not enough bioluminescent dinoflagellates collected by the 25 ~m net to account for the number of flashes smaller than 100 mV (Table 3). A comparison of Tables 2 and 3 suggests that bioluminescent dinoflagellates account for 45% (range 22-72%) on average of the dim flashes, and 34% of all the flashes. 858 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.4, 1983

Flashes /60 L Integrated Light. mv /60 L a a 20 30 a 1000 2000 3000 4000 5000

:c I- 0.. W Cl Flashes t> > 100mv •• $IOOmv • TOTAL, mv/60 L

200 3/81'80 23° 13.1'N 66°34.I'W

Figure 3. As in Figure 1. Station taken 3/9/80.

In fact, they probably account for even less of the dim flashes than that (see Discussion). We assume the flashes larger than 100 mV are due to zooplankton and some P. nocti/uca cells. A much smaller number of organisms (4%) produced flashes containing three times as much light as P. noctiluca. These rare flashes produced 24% of the bioluminescence in these waters. As their numbers are small, the Poisson (or counting) error associated with their occurrence or absence led to a great deal of variability in the total light found at anyone depth. This variability in bioluminescence due to the sporadic occurrence of a few very luminous organisms would also cause the apparent structure in the vertical profiles (Figs. 1, 2, 3). Table 4 shows the relationship between the bioluminescent organisms in the net of the bathyphotometer and the bioluminescent signals they produced as they passed through the viewing chamber. The largest flashes correlate best with the sum of the number of ostracods and the bioluminescent copepods in the genus Pleuromamma (Clarke et a1., 1962; Artemkin et a1., 1969; Rudyakov and Vo-

Table 1. The number of bioluminescent organisms at night producing more light than a Pyrocystis noctiluca (100 mY) and their relative contribution to the total bioluminescence in vertical profiles

Depth Range % of Flashes % of Light in % of Flashes % of Light in Date (m) >100 mV Flashes> 100 mV >300 mV Flashes> 300 m V

3/6/80 21-92 29 63 2 7 ] 15-195 33 81 8 21 21-195 38 67 4 13 3/8/80 10-98 18 62 4 3] ]16-162 14 49 ] 14 10-162 17 60 3 28 3/9/80 10-91 17 53 2 12 121-185 17 50 3 20 10-185 17 53 2 13 SWIFf ET AL.: EPIPELAGIC BIOLUMINESCENCE 859

Table 2. The number of flashes from bioluminescent organisms in bathyphotometer samples pumped at discrete depths into a 25 ILmporosity net show the majority were in small flashes. However, more than half the light was generally found in the flashes> 100 mV. Light per m-3 can be converted to an approximate value in photons as each mV represents 2-7 X 108 photons

Yol. Total # Total Depth Pumped Rashes % Flashes % Flashes Light (mY) % Light % Light Date (m) (m-') (m-') > 100 mY >300 mY (m-') > 100 mY >300 mY 3/6/80 70 0.160 206 21 0 21,375 41 0 3/8/80 50 0.225 604 24 4 50,290 64 18 3/9/80 30 0.225 520 20 5 42,960 62 24 3/11/80 30 0.225 511 21 3 34,800 58 20 3/11/80 30 0.225 493 13 5 37,220 63 36 3/11/80 30 0.225 449 16 5 35,220 61 30 3/11/80 120 0.225 276 6 2 11,730 29 13 3/11/80 120 0.225 293 17 4 23,730 70 43

ronina, 1967). There are more flashes containing> 100 and <300 mV than there are recognized bioluminescent organisms collected in the net. These include larvaceans, larvacean houses, Pyrocystis species and copepods in the genus Corycaeus and Oncaea. One group we have not enumerated which may help explain this discrepency are colonial bioluminescent radiolaria and foraminfera. Also, our pump impeller and use of formalin as a preservative may destroy gelatinous zooplankton.

DISCUSSION The evidence that we have collected suggests that zooplankton rather than dinoflagellates are responsible for at least two-thirds of the flashes and at least 60% of the bioluminescence stimulated in the several cubic meters of water we investigated. We feel that zooplankton at this station would produce most of the light seen in a ship's wake as a steady plume of light. Of course, at slow speed on a small vessel, even small organisms would appear as point sources, whereas at high speed on large ships, even very large bright displays would tend to be blended into a seemingly continuous glow in the wake. These results from the southern

Table 3. Dinoflagellate abundance and the number of species found in the 25 /lm porosity net collection from the effluent of the bathyphotometer. Volumes pumped and samples are the same as in the other tables. Of the number of dinoflagellates found (#DF), the bioluminescent dinoflagellates (#BDF) were only a small percentage. Of the species of dinoflagellates present (#DF spp.), the number of bioluminescent dinoflagellates species (#BDF spp.) were also a small percentage. Dinoflagellates which were considered bioluminescent in these samples were Ceratium fusus. Peridinium conicum. P. dil'ergens. P. globulus. P. oceanicum, Ceratacorys horrida and all Pyrocystis species

Depth #DF #BDF Date (m) (m-') (m-') %BDF #DF spp. #BDFspp. % 6 March 70 1,204 36 3 26 2 8 8 March 50 1,076 133 12 47 5 II 9 March 30 1,876 129 7 44 7 15 II March 30 3,609 298 8 46 4 9 II March 30 1,276 151 12 39 2 5 11 March 120 2,222 147 7 37 4 II II March 120 938 151 16 31 2 6 860 BULLETIN OF MARINE SCIENCE. VOL 33. NO.4. 1983

0-0000- I I

r-r-O'\--t"tiOON---

000-00-

000"',,",00

~ 'C -0;:lo. o~ •..•o0. U SWIFT ET AL: EPIPELAGIC BIOLUMINESCENCE 861

Sargasso Sea may be applicable to other tropical oceanic regions where a balanced ecosystem is reflected in a higher number of bioluminescent zooplankton than bioluminescent dinoflagellates. Support for the idea that zooplankton are probably at least as important as dinoflagellates comes from Russian work with a bathyphotometer which, like ours, measures bioluminescence per unit water volume (Gitel'zon et a!., 1973). Rudyakov and Voronina (1967) suggested that zooplankton were probably at least as important as dinoflagellates in surface waters of the Gulf of Aden and the Red Sea, and that bioluminescent zooplankton were highly correlated (r = 0.83) with the bioluminescence in vertical profiles. Gitel'zon et al. (1973) observed peaks in the amount of bioluminescence associated with the thermocline in the tropical Pacific. They noted that different organisms might be causing the bioluminescence at different depths, because correlation of the hydrographic parameters with bioluminescence changed with depth. The horizontal patterns of bioluminescence exhibited characteristic patch lengths from 100-150 m to 1,000-1,200 m, similar to those found for zooplankton in the North Sea by Mackas and Boyd (1979). Murray (1885) suggested that Pyrocystis spp. were responsible for some of the most spectacular surface bioluminescence seen during the cruise of HMS CHAL- LENGER, especially in calm weather. In our subsurface samples, Pyrocystis is never a predominant component of bioluminescence (Swift et aI., 1973; Table 4). Dinoflagellates outside of Pyrocystis spp. and some large ceratia would not produce enough light to be detected in our bathyphotometer. The limit of detection was 2-4 X 109 photons during this cruise. Thus, while on average there were enough dinoflagellates to produce 45% of the small flashes « 100 m V) found on the bathyphotometer records, most of those small flashes were presumably due to organisms other than dinoflagellates. We have multiplied the numbers of dinoflagellates found by typical values for light emitted by similar dinoflagellates (Esaias, 1973; Table 3). This calculation suggests that dinoflagellates other than Pyrocystis do not contribute more than a few percent of the total stimulable bioluminescence at the station we have examined in the southern Sargasso Sea. The remainder of the organisms producing less light than P. noctiluca may be zooplankton. Larvaceans, small ostracods, and the developmental stages of copepods may be dim enough to contribute to this fraction of the small flashes. Ifanything, our bathyphotometer underestimates the contribution of zoo plankton. As Tett and Kelly (1973) have indicated, pump-through bathyphotometers only collect smaller zooplankton which cannot or do not swim away from the suction current of the intake hose. This may explain why our data do not give more emphasis to bioluminescent members of the euphausids, prawns, salps, tunicates, cephalopods and fish. However, in a ship's wake, all but the euphausids would presumably produce enough light to be noticed as individual displays rather than the more steady glow we are trying to analyse. We would like to extend our results to other regions using abundance data on larvaceans, copepods, ostracods and possibly euphausids. Unfortunately, there is insufficient information on the distribution of bioluminescence among developmental stages and inadequate data on the color and intensity of bioluminescence in these forms. However, it seems for the Sargasso Sea near Bermuda there are more bioluminescent copepods, ostracods, and larvaceans present than Pyrocystis species. While bioluminescent Ceraiium. Ceratocorys and Peridinium species may be an order of magnitude more common than Pyrocystis spp., they are also several orders of magnitude dimmer, so their contribution is not large compared to Pyrocystis. Apstein (1909) estimated 630-2,055 Pyrocystis species m-2 in the Sargasso Sea. 862 BULLETLN OF MARINE SCIENCE, VOL. 33, NO.4, 1983

In comparison, Lohmann (1896) found about 10,000-36,000 Oikopleura m-2 in the Sargasso Sea near Bermuda. Deevey (1971) reported an annual average of27 1arvaceans m-3 in the upper 500 m (13,500 m-2) of the Sargasso Sea near Bermuda. These larvacean concentrations are not very different from those reported for Pyrocystis species (Swift et al., 1981). Galt (1978) has shown for some species, that not only the larvaceans themselves, but also the mucous filtering houses that they build and discard several times a day are bioluminescent. Pelagic ostracods occur year round in subtropical waters and rank third in annual abundance of zooplankton in the Sargasso Sea after cope pods and larvaceans (Deevey, 1971; Deevey and Brooks, 1971). At least 36 species of Conchoecia were described for the Sargasso Sea near Bermuda with a mean annual concentration of 17 m-3 between the surface and 500 m (8,500 m-2) (Deevey, 1968), and several of these are known to be bioluminescent (Angel, 1968; Harvey, 1952; Rudyakov and Voronina, 1967). The common bioluminescent copepod species in the Sargasso Sea are in the genera Pleuromamma. Lucicutia, Corycaeus, Euchaeta and Oncaea (Giesbrecht, 1895; Artemkin et aI., 1969; Rudyakov and Voronina, 1967; Clarke et aI., 1962). Using Deevey's (1971) copepod counts near Bermuda, this suggests that 17% of the copepods are bioluminescent, or that there are about 32 bioluminescent copepods m-3 in the upper 500 m (16,000 m-2). These adult copepods would be associated with an unknown number of bioluminescent copepodites and nauplii. There appear to be enough bioluminescent zooplankton in the southern Sargasso Sea near Bermuda to produce more light than dinoflagellates, particularly ifmany of these organisms are as bright or brighter than Pyrocystis, as our and other data indicate (Tett and Kelly, 1973). To the extent that conditions at Bermuda are representative of the Sargasso Sea, our conclusions about the importance of zooplankton bioluminescence might be generalized to that oligotrophic oceanic re- gIOn.

ACKNOWLEDGMENTS

We wish to thank Professor H, H. Seliger (Johns Hopkins University) without whose support it would not have been possible to initiate these bioluminescence studies. Professor T. A. Napora helped with the development of the bathyphotometer and Ms. R. Schenck helped deploy the photometer at sea. This work was partially supported by a contract from the Office of Naval Research NOOO14-81- C-0062 and a grant from the Oceanography Division of the National Science Foundation, OCE-78- 09457.

LITERATURE CITED

Angel, M. V. 1968. Bioluminescence in planktonic halocyprid ostracods. J. Mar. BioI. Assoc. U.K. 48: 255-257. Apstein, C. 1909. Die Pyrocysteen der Plankton-Expedition. Ergebnisse Plankton-Exped. Hum- boldt Stiftung Bd. IV M. c. 27 pp. Artemkin, A. S., E. P. Baldina. V. N. Grese and V. S. Filimonov. 1969. Resultados preliminares sobre zooplankton su luminesciencia en la region oriental del Mer Cariba. Ser. Oceanol. Inst., Acad. Cienc. Cuba 2: 1-77. Clarke, G. L. and M. G. Kelly. 1965. Measurements of dirunal changes in bioluminescence from the sea surface to 2000 meters using a new photometric device. Limnol. Oceanogr. IO(Suppl.): R54-R66. --, R. J. Conover, C. N. David and J. A. C. Nicol. 1962. Comparative studies ofluminescence in cope pods and other pelagic marine animals. J. Mar. BioI. Assoc. U.K. 42: 541-564. Deevey, G. B. 1968. Pelagic ostracods of the Sargasso Sea off Bermuda. Peabody M:..s. Natur. History Bull. 26. 125 pp., 65 figs. --. 1971. The annual cyde in quantity and composition of the zooplankton of the Sargasso Sea off Bermuda. I. The upper 500 m. Limnol. Oceanogr. 16: 219-240. -- and A. L. Brooks. 1971. The annual cycle in quantity and composition of the zooplankton of the Sargasso Sea off Bermuda. II. The surface to 2,000 m. Limnol. Oceanogr. 16: 925-943. SWIFfETAL.:EPIPELAGICBIOLUMINESCENCE 863

Esaias, W. E. 1973. Studies on the occurrence, physiology, and ecology of bioluminescence in dinoflagellates. Ph.D. Thesis, Oregon State University. 76 pp. Galt, C. P. 1978. Bioluminescence: dual mechanisms in a planktonic tunicate produces brilliant surface display. Science 200(4337): 70-72. Giesbrecht, W. 1895. Mitteilungen ueber Copepoden. 8. Ueber das Leuchten der pelagischen Co- pepoden und das tierische Leuchten im Allgemeinen. MitL Zool. Sta. Neapel II: 648-689. Gitel'zon, I. S., L. A. Levin, A. P. Shevymogov, R. N. Utyushev and A. S. Artemkin. 1973. Pelagic bathymetric sounding and its possible application to studies of the spatial structure of biocenoses. Pages 51-66 in M. E. Vinogradov, ed. Life activity of pelagic communities in the ocean tropics. Akademiya Nauk SSSR (Translated from Russian, Israel Program for Scientific Translations). Harvey, E. N. 1952. Bioluminescence. Academic Press, New York. 649 pp. Kelly, M. G. 1968a. The occurrence of dinoflagellate bioluminescence at Woods Hole. BioI. Bull. Mar. BioI. Lab. Woods Hole 135: 279-295. 1968b. Oceanic bioluminescence and ecology of dinoflagellates. Ph.D. Thesis, Harvard University. 186 pp. -- and P. B. TetL 1978. Bioluminescence in the ocean. Pages 349-417 in P. J. Herring, ed. Bioluminescence in action. Academic Press, London. Mackas, D. L. and C. M. Boyd. 1979. Spectral analysis of zooplankton spatial heterogeneity. Science 204: 62-64. Murray, J. 1885. Pyrocystis noctiluca. Rep. Sci. Res. Exp]or. Voyage H.M.S. "Challenger," Narr. vol. 1(2): 935-938. Rudyakov, Yu. A. and N. M. Voronina. 1967. Plankton and bioluminescence in the Red Sea and the Gulf of Aden. Oceanology 7: 838-848. Swift, E., w. H. Biggley and H. H. Seliger. 1973. Species of oceanic dinoflagellates in the genera Dissodinium and Pyrocystis: interclonal and interspecific comparison of the color and photon yield of bioluminescence. J. Phycol. 9: 420-426. ---, v. A. Meunier, W. H. Biggley, J. Hoarau and H. Barras. 1981. Factors affecting bioluminescent capacity in oceanic dinoflagellates. Pages 95-105 in K. H. Nealson, ed. Bioluminescence: Current perspectives. Burgess Publ. Co., Minneapolis. Tett, P. B. 1971. The relation between dinoflagellates and the bioluminescence of sea water. J. Mar. BioI. Assoc. U.K. 51: 183-206. --- and M. G. Kelly. 1973. Marine bioluminescence. Oceanogr. Mar. BioI. II: 89-173.

DATEACCEPTED: March 16, ]982.

ADDRESSES: (E.s.. D.T.B .. P. V.) Graduate School of Oceanography. University of Rhode Island. Kingston, Rhode Island, 02881; (W.H.B.) Biology Department, The Johns Hopkins University, Bal- timore, Maryland 21218.