Seasonal variation in marine C:N:P stoichiometry: can the composition of seston explain stable Redfield ratios?

Seston is suspended particulate organic matter, comprising a mixture of autotrophic, heterotrophic and detrital material. Despite variable proportions of these components, marine seston often exhibits relatively small deviations from the Redfield ratio (C:N:P = 106:16:1). Two time-series from the Norwegian shelf in Skagerrak are used to identify drivers of the seasonal variation in seston elemental ratios. An ordination identified water mass characteristics and bloom dynamics as the most important drivers for determining C:N, while changes in nutrient concentrations and biomass were most important for the C:P and N:P relationships. There is no standardized method for determining the functional composition of seston and the fractions of POC, PON and PP associated with phytoplankton, therefore any such information has to be obtained by indirect means. In this study, a generalized linear model was used to differentiate between the live autotrophic and non-autotrophic sestonic fractions, and for both stations the non-autotrophic fractions dominated with respective annual means of 76 and 55%. This regression model approach builds on assumptions (e.g. constant POC:Chl-a ratio) and the robustness of the estimates were explored with a bootstrap analysis. In addition the autotrophic percentage calculated from the statistical model was compared with estimated phytoplankton carbon, and the two independent estimates of autotrophic percentage were comparable with similar seasonal cycles. The estimated C:nutrient ratios of live autotrophs were, in general, lower than Redfield, while the non-autotrophic C:nutrient ratios were higher than the live autotrophic ratios and above, or close to, the Redfield ratio. This is due to preferential remineralization of nutrients, and the P content mainly governed the difference between the sestonic fractions. Despite the seasonal variability in seston composition and the generally low contribution of autotrophic biomass, the variation observed in the total seston ratios was low compared to the variation found in dissolved and particulate pools. Sestonic C:N:P ratios close to the Redfield ratios should not be used as an indicator of phytoplankton physiological state, but could instead reflect varying contributions of sestonic fractions that sum up to an elemental ratio close to Redfield

Grazing by the heterotrophic dinoflagellate Protoperidinium steinii on a Ceratium bloom

Variations in heterotrophic dinoflagellate populations at a station in the inner Oslofjord, Norway, were studied by sampling at ca 4 d intervals. Cells were concentrated from 1 l samples by filtration before being counted in an inverted microscope. Additional data include autotrophic carbon biomass estimates based on microscopy of the phytoplankton, chlorophyll a (chl a) concentrations, and hydrography. A modest (2 to 4 µg chl a l-1) diatom bloom in September was followed by a large (up to 128 µg chl a l-1) dinoflagellate bloom in October, dominated by Ceratium furca. Altogether 25 thecate heterotrophic dinoflagellate species were recorded in this study. Their total biomass at all times was 2000 cells l-1. P. pyriforme, P. brevipes, P. curtipes, and Oblea rotunda showed more modest increases, while no significant response was seen in any of the other 20 heterotrophic dinoflagellates. In incubated plankton samples, we recorded 81 instances of P. steinii feeding on C. furca or on other dinoflagellates. Our study confirms previous laboratory findings suggesting that P. steinii belongs to the limited selection of Protoperidinium species capable of exploiting dinoflagellate prey in the natural environment

Spring bloom nutrient dynamics in the Oslofjord

Primary production and uptake rates of ammonium, nitrate and silicic acid were measured during the 1996 spring bloom in the Oslofjord, southern Norway, using isotope methodology (14C, 15N and 32Si). Chlorophyll a (chl a) standing stock peaked at 38 mg chl a m-3. Diatoms, especially Thalassiosira nordenskioeldii, dominated the plankton community. The diatom bloom followed a sigmoid growth pattern until it abruptly declined. Both nitrate and silicic acid were depleted from the surface layer during the bloom. Nitrate was the major nitrogen-source for phytoplankton growth, the f-ratio was >0.9 until the bloom declined. Virtually all the silicic acid was converted into diatom frustules (biogenic silica), and silicon controlled the primary production during the bloom

Monitoring of the outer Oslo fjord - inputs and surveys in the watermasses in 2015. Technical report

Overvåkningsprogrammet av vannmassene i Ytre Oslofjord fremskaffer informasjon om miljøtilstand og tilførsler, med fokus på næringssalter (eutrofi). Rapporten beskriver tilførsler for 2015 samt undersøkelser og resultater for planteplankton, hydrofysiske og hydrokjemiske forhold i 2016. Det fant ikke sted noen større utskiftninger bunnvannet i sidefjordene i løpet av vinteren og våren 2016. I sidefjordene med grunne terskler eller flere bassenger (Iddefjorden, Frierfjorden) har det ikke funnet sted noe utskiftning av bunnvannet siden vinteren 2010. I november 2016 fant det sted en utskiftning av bunnvannet i Hvaler området som resulterte i forbedret oksygenforhol i området helt inn til Iddefjorden. I 2016 var planteplanktonbiomassen moderat, der fureflagellater var svært tallrike. Unntaket var i fjorder sterkt påvirket av ferskvann, der kiselager var mest tallrike på grunn av tilførsel av silikat og nitrogen. Konsentrasjon av nitrogen var noe høyere i vinterperioden, med enkelte unntak. Det ble derimot målt lavere konsentrasjoner av næringssalter på sommeren ved de fleste stasjonene. I sommerperioden og høsten 2016 ble det registret færre perioder med avrenning enn i de foregående årene. Utskiftningen av bunnvannet i Hvaler i november førte til høye konsentrasjoner av fosfat i overflaten i Iddefjorden og Ringdalsfjorden da fosfat rikt vann kom til overflaten. Rapporten omhandler også næringssalttilførsler til Ytre Oslofjord. De nasjonale rapporteringsrutinene for kildespesifikke data og etterfølgende bearbeiding gjør at resultater fra det enkelte år først foreligger sent på høsten året etter det år som rapporteres. Figurene for næringssalter gjelder derfor året 2015.Water column monitoring in the outer Oslo Fjord is performed to obtain information about the environmental condition and inputs to the area, focusing on nutrients (eutrophication). The report describes inputs to the fjord in 2015 and results from the monitoring regarding phytoplankton, hydrophysical and hydro-chemical conditions in 2016. The national reporting routines for source-specific data, and subsequent processing, of inputs to the outer Oslofjord mean that results from the individual year will not be available until the autumn of the year after the year reported. The figures for nutrient salts in this report therefore apply for the year 2015. There was no major replacement of bottom water in sidefjords during the winter and spring of 2016. In sidefjords with shallow sills or more pools (Iddefjorden, Frierfjord) no replacement of bottom waters has taken place since the winter of 2010. However, in November 2016 there were observed replacement of the bottom water in Hvaler all the way in to Iddefjorden. In 2015 was phytoplankton biomass moderate. The phytoplankton were dominated by dinoflagellates in in the open sidefjords, whereas diatoms dominated in the fjord influenced by freshwater. Concentration of nitrogen was similar or higher compared with 2015 for the winter period. During the summer period the concentration of nitrogen was lower in 2016 than in 2015. During the summer of 2016, fewer periods of runoff was recorded than in the previous years, result in lower silicate and nitrogen concentrations. The replacement of bottom water in November in Hvaler result in high concentration of phosphate in the surface.Fagråd for Ytre OslofjordpublishedVersio