Seasonal abundance and distribution of gelatinous zooplankton in Oslofjorden, Norway

87 Pages·2012·3.04 MB·English

by  Katrine Selsø

Seasonal abundance and distribution of
gelatinous zooplankton in Oslofjorden,
Norway
An ecological snapshot

Katrine Selsø

Master Thesis
Marine Biology Program, Department of Biology
UNIVERSITY OF OSLO
December 2012
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Seasonal abundance and distribution of
gelatinous zooplankton in Oslofjorden,
Norway: An ecological snapshot
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Copyright Katrine Selsø
2012
Seasonal abundance and distribution of gelatinous zooplankton in Oslofjorden, Norway: An
ecological snapshot
Katrine Selsø
http://www.duo.uio.no
Print: Reprosentralen, Universitety of Oslo
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Preface
This master thesis was carried out in 2010-2012 at the Marine Biology Program of the
Department of Biology at the University of Oslo. The thesis was intended to be the beginning
of a larger project on zooplankton in Oslofjorden, but due to unforeseen circumstances, this
project is now in hibernation. Thus, the field and laboratory work performed is a much larger
data set than the one included in the thesis. I hope that these data can be used in future
research at the University of Oslo.
This thesis would not have been possible without help from a large number of people. First, I
would like to thank my primary supervisor, Josefin Titelman (UiO) for giving me the
opportunity to do a field study on gelatinous zooplankton. Thank you for all your guidance, a
good collaboration, and especially for allowing me to take part in the course in gelatinous
impact. I would also like to thank my co-supervisor Aino Hosia (IMR) for teaching me about
jellyfish and taxonomy. Without you, I would still be in the lab trying to understand the
identification literature. Thank you, as well, for helping me with finishing the writing
process, and discovering the discrepancies in my thesis. Rita Amundsen, our Head engineer,
you have been a tremendous help on cruises and in the laboratory. Your help and support was
invaluable. Thank you, Senior engineer Sissel Brubak, for teaching me and helping me
analyze the chlorophyll a samples. Karl Inne Ugland and Andreas Lindén, thank you both for
saving the day and for your great help with the statistical analyses in the thesis. Your quick
and thorough responses to questions and your support are what ultimately brought this thesis
to its conclusion. Sindre Holm and the rest of the crew aboard Trygve Braarud, thank you so
much for all your help and extra work, you made the cruises a great experience. I will never
forget the efforts you went to so I could obtain all my samples, even giving me an extra
cruise day.
Thank you to Asbjørn Vøllestad (UiO) Julie Døvle Johansen, Signe Røysland Sørlie and
Christopher Hinchcliffe for taking the time to proofread and comment on my thesis. I also
want to thank my fellow students for making these years unforgettable and for all the support.
Last, but not least, I want to thank my family and friends for supporting me and always being
there. The biggest thank you goes to my boyfriend, Sigurd. You supported me every day,
believed in me when I couldn´t and were incredibly patient with me throughout.

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Abstract
Gelatinous zooplankters are thought to be important members of the pelagic ecosystem. Their
abundance is known to vary with season, food availability and hydrographic conditions,
sometimes forming massive blooms thought to be detrimental to socio-economic installations
like aquaculture and power stations. There is, however, a severe lack of research on the
gelatinous zooplankton community in Oslofjorden, Norway. In this study, three stations
(Missingene, Elle and Steilene) along a geographical transect were sampled 10 times from
January 2011 to January 2012. Environmental variables were analyzed to portray the
hydrographic seasonal cycle during the year. A month was chosen to represent each of the
four seasons. Gelatinous zooplankton was identified, measured and enumerated while non-
gelatinous zooplankton was weighed, giving an estimate of biomass. These abundances were
compared according to station and season, and showed that Missingene and Steilene were
more similar in gelatinous community composition and abundance than Elle. The distribution
of a formerly highly abundant hydromedusa in Oslofjorden, Aglantha digitale, was
tentatively found to have decreased since the 1970´s. The abundance of six groups of
gelatinous zooplankton (hydromedusae, siphonophores, ctenophores, chaetognaths,
appendicularians, and holopelagic polychaetes, i.e., Tomopteris helgolandica) was modeled
as a function of environmental variables using a new multivariate modeling-tool for
ecological community research,  “Mvabund”. Significant variables were station, density,
depth and fluorescence, while season did not appear to have any significant effect on the
abundance of gelatinous zooplankton. The model had better fit for the groups hydromedusae,
siphonophores and Chaetognatha than the groups Ctenophora, Appendicularia, and
Tomopteris.
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Table of contents
1 Introduction ............................................................................................................................... 2
1.1 Gelatinous zooplankton ............................................................................................................... 3
1.1.1 Gelatinous terminology ........................................................................................................................ 3
1.1.2 History of coelenterate research ...................................................................................................... 3
1.1.3 Gelatinous life history and ecologic role ....................................................................................... 4
1.2 Fjord ecosystems ............................................................................................................................ 8
1.2.1 Fjord hydrography ................................................................................................................................. 8
1.2.2 Fjord biology ............................................................................................................................................. 9
1.2.3 Oslofjorden ................................................................................................................................................ 9
1.3 Purpose of the master thesis .................................................................................................... 11
2 Materials and Methods ......................................................................................................... 12
2.1 Sampling sites ................................................................................................................................ 12
2.1.1 Missingene .............................................................................................................................................. 12
2.1.2 Elle ............................................................................................................................................................. 13
2.1.1 Steilene ..................................................................................................................................................... 13
2.2 Field and lab methods ................................................................................................................. 15
2.2.1 Environmental variables .................................................................................................................. 15
2.2.2 Zooplankton net sampling ............................................................................................................... 17
2.2.3 Preliminary sample preparations and preservation ............................................................. 17
2.2.4 Zooplankton identification, enumerating, and measuring ................................................. 18
2.3 Statistical analysis ........................................................................................................................ 21
2.3.1 Environmental variables .................................................................................................................. 21
2.3.2 Zooplankton statistical analysis .................................................................................................... 21
3 Results ........................................................................................................................................ 26
3.1 Environmental variables ............................................................................................................ 26
3.2 Zooplankton analyses ................................................................................................................. 31
3.2.1 Gelatinous identification and enumeration .............................................................................. 31
3.2.2 Gelatinous zooplankton vs. non-gelatinous zooplankton ................................................... 33
3.3 Multivariate abundance analyses ........................................................................................... 36
3.3.1 Mvabund for all depth layers at Missingene and Elle (Zoo ) ............................................. 36
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3.3.2 Mvabund model of all stations, two upper strata (Zoo )..................................................... 40
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4 Discussion ................................................................................................................................. 46
4.1 Gelatinous zooplankton ............................................................................................................. 46
4.1.1 Abundances and gelatinous community composition ......................................................... 46
4.1.2 The curious case of Aglantha digitale .......................................................................................... 47
4.2 Environmental variables ............................................................................................................ 49
4.2.1 Seasonality .............................................................................................................................................. 49
4.2.2 Station differences ............................................................................................................................... 51
4.3 Sampling method and study design ....................................................................................... 52
4.4 Multivariate models ..................................................................................................................... 54
4.4.1 Ecological interpretation of the model results ........................................................................ 54
4.5 Choice of model and model tests ............................................................................................. 58
4.5.1 Different aspects of model evaluation......................................................................................... 59
5 Conclusions and future directions ................................................................................... 60

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