INVESTIGATIONS OF TOTAL OXYGEN, METHANE AND SULFIDE FLUXES
31. PHYTOPLANKTON DIVERSITY STUDIES
Yuri B. Okolodkov Alfred-Wegener-Institut Introduction
Species diversity of unicellular plankton in general and phytoplankton in particular in the Arctic have been studied mainly in land-bound laboratories on the basis of the samples and specimens fixed with formalin or Lugol´s solution. These samples are good for the organisms with external or internal skeletons (frustules, loricae, thecae, coccoliths, etc.), however, the major part of delicate species is usually lost or drastically influenced by fixatives so that little is known about species diversity of such groups as athecate or naked dinoflagellates, ciliates, phyto- and zooflagellates. These groups may significantly contribute to the primary production, microbial loop, carbon and nitrogen vertical flux or diet of their predators. The main goal of phytoplankton diversity studies during the ARK-XXII/1c cruise onboard Polarstern was sampling and laboratory analysis in vivo. Theses studies are the second attempt to fill the gap in our knowledge about athecate dinoflagellates being complementary to those performed during the ARK-XIII/2 cruise to the Greenland Sea in July-August 1997.
Material and Methods
Phytoplankton was sampled periodically during the period of July 10 to 21, 2007, in the northeastern Greenland Sea, in the zone between Molloy Hole and Vestnesa Ridge (78o34'- 79o44'N, 2o59'- 6o05'E), at 22 oceanographic stations (PS70/144, 146, 156, 158, 161, 168, 115 Quest, 173, 178, 181, 187, 188, 193, 195, 199, 205, 208, 213, 215, 219, 220 and 224). Samples were taken with a hand plankton net of 25 cm in diam., mesh size 20 µm (Hydro-Bios, Kiel). Mostly, about 300 l of water was concentrated using the net and a DL40 DEPA air operated diaphragm pump (Alfa Laval Flow GmbH, Düsseldorf) installed permanently onboard the ship and providing sea water from 6-7 m depth in front of it. At CTD stations (158, 161, 173, 178, 195, 199 and 213), phytoplankton was sampled manually from the 25 - 30 m depth to the surface. In addition, the organisms from 1 l of water taken from the same water tube as a net sample or from a CTD 10-liter water-bottle (at 2 m depth) were concentrated using a reverse-filtration device and the nuclepore filters, pore size 1 µm, at each station (small-fractioned samples).
Samples were analyzed immediately or within several hours after sampling in a 3-ml plate chamber using a Zeiss Axiovert 40C inverted microscope equipped with phase-contrast A-Plan 10x/0.25, 20x/0.45 and 40x/0.65 objectives. Photographs were taken with an Olympus C7070 digital camera manually adjusted to an eye-piece every time before shooting; two rechargeable batteries were used not to interrupt the analysis of samples. The same cells were photographed in various focal planes preferably in ventral or dorsal view. Live observations were made with a special emphasis on the surface longitudinal striation of many athecate dinoflagellates, the apex, the position
ARK-XXII/1A-C
of nucleus and other specific organelles, presence or absence of chloroplasts when possible. Length and width of each photographed cell were measured.
Results
In total, more than 2000 images of about 600 algal cells were taken, which will be used for species identification. The relative abundance of phytoplankton species was evaluated visually.
1. Dinoflagellates
Dinoflagellates showed high species diversity at sts. 146, 161, 173, 181, 187, 205, 208 and 213. Sometimes, their relative abundance was also high. Among athecate dinoflagellates, the genera Gymnodinium, Gyrodinium, Cochlodinium (several species), Amphidinium (A. sphenoides), Katodinium (K. glaucum), Torodinium (T. robustum), Pronoctiluca (P. pelagica), Actiniscus (A. pentasterias, only as star-like silicious elements of its internal skeleton) and Warnowiaceae gen. spp. (most likely, Nematodinium or Nematodiniopsis) were presented, the former two being the most diverse in species.
Thecate dinoflagellate were mainly represented by Protoperidinium species, 2 to 4 Ceratium, 4 Dinophysis, 2 Peridiniella, 1 Prorocentrum, 1 Micracanthodinium, cf. Oxytoxum (1) species were also found. In a small-sized fraction, Prorocentrum minimum was sometimes abundant and dominant (st. 144, 161 and 188). Two species described by Meunier (1910) from the western Barents and eastern Kara seas as Echinum minus (=Polarella glacialis?) and E. majus (only at st. 187) were encountered, the former being very common and found in aggregations of up to several dozens. A curious phenomenon was observed in most of samples: many aggregations (of several cells to a few dozens) of a small naked dinoflagellate species distinguished by active behavior and covered with a common envelope. It is still questionable whether they represent a stage of life cycle of an unidentified well-known species or they represent coenobia similar to those known, for example, for many freshwater planktonic green algae. The endoparasitic dinoflagellate Amoebophrya cf. ceratii was found twice inside other dinoflagellate and unidentified algal species (sts. 146 and 181). Some empty thecae of Gonyaulax were encountered at sts. 188 and 215.
2. Diatoms
Diatoms were poorly represented in the samples. Among common large-sized background species, Corethron criophilum, Chaetoceros decipiens, C. borealis, C.
concavicornis, C. convolutus, C. debilis (including cells with resting spores at st. 146), Thalassiosira cf. antarctica, Rhizosolenia hebetata f. semispina and R. styliformis were found. In several cases, Chaetoceros atlanticus (sts. 146, 205 208, 215 and 224), C. teres (only at st. 178) and Proboscia alata were present. In a small-sized fraction in some samples (especially at st. 173), diatoms prevailed numerically due to an unknown centric diatom (?Dactyliosolen sp., 3.5-7.5 µm in diam.), Cylindrotheca closterium and Pseudo-nitzschia cf. pseudodelicatissima. Eucampia groenlandica (2.5-17.5 µm in diam.) sometimes was common in nuclepore filtered samples. A number of species were rare: Chaetoceros cf. similis, Sceletonema costatum (only at st. 181), Fragilariopsis oceanica (including cells with resting spores at st. 146). A small-sized Nitzschia sp. living in the peripherical mucuous layer of Phaeocystis colonies was abundant and subdominant in the samples in which Phaeocystis cf. pouchetii was
31. PHYTOPLANKTON DIVERSITY STUDIES
the dominant species (sts. 187, 195 and 213). Resting spores of Melosira arctica and Chaetoceros furcellatus were also found (only at sts. 161, 187 and 220); M. arctica was also found at sts. 156 and 193. The presence of some typically epiphytic diatoms (sts.
168, 173, 178 and 199) are likely related to the fouling organisms which supposedly inhabit the underwater ship surface.
3. Other flagellates
The main ecological feature related to the presence of other flagellate species was high abundance of Phaeocystis cf. pouchetii which clogged the net during sampling at sts. 158, 161, 173, 178, 195 and 213. In many samples global bodies comparable in size with ova of planktonic crustaceans were found; supposedly, they are stage of life cycle of P. cf. pouchetii. Another two colonial species were sometimes relatively abundant, a crysophycean Dinobryon balticum (st. 199) and Choanoflagellata gen. sp.
(st. 213). Among chrysophyceans, some solitary cells of Dinobryon faculiferum and Meringosphaera mediterranea were also found. An unidentified cryptophyte of 15-21 µm long was numerically dominant in small-fractioned samples at sts. 161 and 188 and was present at some other stations. Among prymnesiophyceans, four morphotypes of coccolithophorids of 7.5 to 25 µm in diam. were distinguished. Emiliania huxleyi was found at low numbers and another unidentified species of 17.5 to 25 µm in diam.
was found in many samples and it was numerically dominant at st. 168 and 181. A dictyochophycean Dictyocha speculum was encountered in low numbers in almost all the samples. An unidentified phytopflagellate was numerically dominant in a small-sized fraction at st. 199. Unidentified euglenids were found at sts. 144, 220 and 224.
4. Other organisms
Among other major taxonomic groups abundantly presented in samples were ciliates (especially, tintinnids; three different genera were distinguished) and copepods (especially, nauplii). Rare specimens of nematods (sts. 144 and 173), foraminiferas ( sts. 146, 178 and 181), radiolarians (sts. 146, 161, 178, 181, 188, 195 and 208) and a filamentous cyanophycean (st. 146), were also found.
5. Indicators of physical oceanographic conditions
According to personal observations and published literature, three species, Peridiniella catenata (colonies with up to 4 cells; sts. 146, 156, 187, 193, 195, 205, 208 and 220), Polarella glacialis (at many stations, being the most common at st. 193) and Melosira arctica. seem to be related with sea ice in their life cycles, and their presence in samples can be interpreted as indicators of the presence of drifting ice in a given area during some period before sampling.
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6. Sampling during transit from HAUSGARTEN area to Tromsø
Four additional net and small-sized fraction samples were obtained during sailing from HAUSGARTEN to Tromsø (the code "T" is conventionally given for transit sampling):
T1: 76o57'N, 3o27'E - 76o47'N, 4o00'E (morning, July 23; water temperature 6.03oC);
T2: 75o24'N, 7o58'E - 75o11'N, 8o33'E (evening, July 23; water temperature 7.32oC);
T3: 73o18'N, 13o16'E - 73o08'N, 13o40'E (morning, July 24; water temperature 8.44oC);
T4: 71o53'N, 16o31'E – 71o41'N, 16o56'E (afternoon, July 24; water temperature 9.87oC).
In T1 net haul, Dictyocha speculum was dominant, and an unidentified pennate (probably, Plagiotropis sp.) was subdominant. Protoperidinium spp., especially, P.
pellucidum, were abundant; the diatom Thalasiothrix sp. was observed for the first time.
Ceratium arcticum was more common than at all previous stations in HAUSGARTEN area. T2-T4 net hauls are characterized by much higher biomass of zooplankton mainly due to the copepod nauplii and copepodite stages and Ceratium species; moreover, the latter was relatively diverse in species: C. arcticum (at all but T4), C. longipes, C. fusus, C. furca (rare cells only in T3 and T4), C. tripos and C. macroceros. In T4 net haul, C. macroceros and C. tripos was found more frequently than in previous stations, and Corethron criophilum was dominant among large-sized diatoms. Some other taxa such as Dinophysis acuta and two unidentified tintinnid and radiolarian species were observed exlusively during the transit. It is suggested that the difference in ratio between different Ceratium species in T1-T4 net hauls strongly depends on very pronounced gradients of different water masses, at least, in terms of temperature.
The relative abundance of copepods in the studied transit area and period may be interpreted as indicative of high secondary production, which can attract planktivorous sea birds and mammals.
Conclusions and recommendations
Athecate dinoflagellates are morphologically diverse and can significantly contribute to a unicellular plankton population in the study region. This group has been neglected for many years and so far it is understudied. Athecate dinoflagellates prevailed over planktonic diatoms in the number of species during the cruise. More observations in-vivo are necessary using an inverted microscope. It is recommended to have a microscope equipped with a digital camera of not less than 7.0 megapixels, a dry 60x or 63x objective of high numerical aperture with an iris diaphragm, a differential interference or relief contrast accessories to be used together with a 40x objective to observe in more detail both external and internal structures of naked dinoflagellates and to take better photographs of them, and epifluorescence facilities to discriminate reliably between photosynthetic and heterotrophic species.