Grethe Rytter Hasle’s research while affiliated with University of Oslo and other places

Since February 1980 surface net-phytoplankton samples are being collected on a monthly schedule at three locations (58°23′ N 08°49′ E, 58°08′ N 09°11′ E and 57°42′ N 09°45′ E) along a transect that crosses the Skagerrak from the Norwegian coast (Torungen) to the Danish coast (Hirtshals). A total of 300 samples were examined (including collections prior to 1980) with the goal of classifying the diatoms according to their occurrence in the Skagerrak plankton during different seasons. High species diversities were observed from October to March. Diversity drops in the spring when a Thalassiosira - Chaetoceros - Skeletonema community is established, and is lowest in June and during the summer when Proboscia alata dominates. Autumn is the period of major advection of foreign species introduced into the Skagerrak by the Jutland Current and/or the Dooley Current. Nonplanktonic diatoms released from the bottom by turbulence are most common in winter. Species new to the area, and the possible response of the diatom flora to climatic fluctuations is discussed. A list of the diatoms recorded and their common synonyms is given.

The morphological similarity of Coscinodiscus asteromphalus, C. centralis, and C. concinnus may cause identification problems, as stated in the first phytoplankton reports from the Oslofjord and the Skagerrak at the turn of the century. Light and electron microscope examinations of clonal cultures and field samples showed the distance of the single ring of labiate processes from the margin, the distance between the processes, the presence or absence of wide interstriae, and the structure of the valve centre to be distinctive features at the specific level. Observations made on plankton samples collected during the last 30 years have shown that C. radiatus, C. centralis, C. concinnus, C. granii and C. asteromphalus are regular members of the plankton flora of the Oslofjord and the Skagerrak, the two latter less common in number and occurrence than the others. Coscinodiscus wailesii was first observed in the area in the autumn of 1979, two years after its arrival in the English Channel. Our investigation includes examination of type material of C. wailesii.

In polar areas, Fragilariopsis are among the most abundant phytoplankton organisms in plankton and ice. Four Fragilariopsis species, F. oceanica, E reginae-jahniae, F. atlantica and F. pacifica sp. nov., recorded from and apparently restricted to the Northern Hemisphere have been explored with regard to morphology, distribution and phylogeny. Morphologically the examined species are pair-wise very similar and easily confused. Hence, the numerous records of F. oceanica from sea ice and plankton in Arctic waters most likely include F reginae-jahniae. Fragilariopsis atlantica and the sibling species F. pacifica were found in the plankton, not in ice. All four species exhibit a pronounced valve variation related to the diminution of the apical length through vegetative reproduction (stadial variation). In F. oceanica and F. reginae-jahniae this is expressed by a variation in valve outline from being more linear to almost elliptical and in F. atlantica and F. pacifica by heteropolarity of the greater specimens. Fragilariopsis oceanica and F. reginae-jahniae differed in cell length and width, valve and girdle structures as did E atlantica and F. pacifica. In the phylogenetic tree of Fragilariopsis, a biogeographical subdivision appeared with the Antarctic Fragilariopsis taxa forming one clade, the bipolar taxa a second chide and the Arctic taxa were found in a third clade and basal in the tree. The phylogenetic analyses support the morphological data and show F. oceanica and E reginae-jahniae to be sister taxa. The morphologically defined E oceanica were found to be paraphyletic, representing two cryptic species.

Electron microscope investigations of the siliceous frustule show that the diatom described by Hustedt as Stephanodiscus subsalsus (A. Cleve) Hust. is not Skeletonema subsalsum (A. Cleve) Bethge (Melosira subsalsa A. Cleve) but is Microsiphona potamos Weber. This species is so similar to Skeletonema costatum (Grev.) Cleve and Skeletonema subsalsum that the combination Skeletonema potamos (Weber) Hasle is suggested. Present records classify Skeletonema potamos as a freshwater species of lakes and rivers. In Sandusky Bay, Lake Erie (U.S.A.) and in River Wümme, a tributary of the River Weser (Germany) it grows with Skeletonema subsalsum. In nature, and when grown in cultures at a salinity of 0%, the processes are extremely short; when grown at salinities of 2% or more, the processes are much longer.

Examination of original material from the Ehrenberg collection plus clonal cultures from the Gulf of Mexico and net hauls from widely separated stations in the world's oceans confirms that Coscinodiscus eccentricus is correctly placed as Thalassiosira eccentrica (Ehrenb.) Cleve. A closely related species, T. symmetrica, is described, with, the main differences being found in the value processes and distribution patterns. The former species is more abundant in inshore waters; the latter has been found in oceanic waters. Comparisons are drawn between these 2 species, T. mendiolana and Planktoniella sol.

Examination of type material showed that 8 taxa must be reduced to synonomy with Thalassiosira punctigera (Castr.) comb, nov., an extremely variable species as to size and valve structure. Preliminary experiments indicated a certain relationship between the areola size and the temperature at which clonal cultures had been grown. While previously known from the North Pacific and the South Atlantic Oceans in addition to a single record from the Caribbean Sea it has since 1979 been recorded regularly in the Skagerrak (between Denmark and Norway).

The ecologically important cold-water diatom species Fragilariopsis cylindrus sensu lato was studied using morphological and molecular data in order to examine whether it is a bipolar species as has been hitherto reported. Six strains from Arctic and Antarctic regions as well as a number of field samples were examined. The morphological studies revealed the presence of two separate species: F cylindrus and F nana within the concept of Fragilariopsis cylindrus. Fragilariopsis nana differs from E cylindrus in having a smaller valve width, a higher poroid density in the striae and a tendency to have more rows of poroids. Both species are found in polar and subpolar regions in the Arctic and in the Antarctic and in open water as well as in ice. No morphological differences were detected between the species from the two hemispheres and we therefore cannot reject that the two species are actually bipolar. Unfortunately the Cultures used in the phylogenetic analyses appeared to be Arctic E cylindrus and Antarctic E nana. The phylogenetic analyses supported the morphological studies but could not be used to support or reject the hypothesis about bipolarity.

Examination of 17 samples collected by a 20 μm meshed meshed net in Kongsfjorden, Svalbard, 8–19 July 1988, showed a dominance of dinoflagellates and the chrysophyte Dinobryon Balticum in the surface layers, whereas the diatom and the haptophyte Phaecystis pouchetii abundance increased with depth. The diatom Pseudo-nitzschia granii appeared together with P. pouchetii through the whole water column, and Actinocyclus curvatulus was one of the few diatoms present also in the surface samples. Two samples, from 15 and 50 m, respectively, were cleaned of organic material and mounted in Naphrax for a more critical identification of the diatoms. We were able to group the species according to habitats, especially types of ice. The planktonic Thalassiosira antarctica var. borealis, T. hyalina, T. nordenskioeldii, Bacterosira bathyomphaia, Chaetoceros furcellatus, C. socialis and Fragilariopsis oceanica were present mainly as resting stages representing a post-bloom situation. These species and T. gravida appear early in the season and may have started to grow already under the ice. Fragilariopsis cylindrus and F. oceanica seem to have a closer affinity to ice than Thalassiosira and Chaetoceros spp. although they are common in the plankton. Some Nitzschia species which are usually regarded as typical sea-ice diatoms and have thicker and older ice as the main habitat were present only in small cell numbers in the plankton samples. The last component, evidently introduced from Atlantic water in the Norwegian Sea, consisted of diatoms with a more oceanic distribution, e. g. Fragilariopsis pseudonana and a small form of Thalassiosira bioculata.

Diatom species formerly referred to the genus Symbolophora Ehrenb. are herein assigned partly to Symbolophora sensu Hasle & Sims and partly to Stellarima nom. nov. The generitype for Symbolophora is established as S. trinitatis Ehrenb. and that for Stellarima as S. microtrias (Ehrenb.) comb. nov. Symbolophora has been recorded so far from deposits of middle Eocene to middle Miocene age, whereas Stellarima contains extant as well as extinct species.