Nitrogen fixation by filamentous cyanobacteria supplies significant amounts
of new nitrogen (N) to the Baltic Sea. This balances N loss processes such
as denitrification and anammox, and forms an important N source supporting
primary and secondary production in N-limited post-spring bloom plankton
communities. Laboratory studies suggest that filamentous diazotrophic
cyanobacteria growth and N
Nitrogen (N) is an essential element for cell functioning in the biosphere
due to its presence in many important biomolecules such as nucleic acids and
proteins. However, in many marine ecosystems N is considered the limiting
nutrient for important cellular processes in phytoplankton (Vitousek and
Howarth, 1991), as indicated through stimulation carbon fixation and pigment
synthesis through addition of inorganic N (e.g. Moore et al., 2008, 2013).
This low N availability also prevails in post-spring bloom plankton
communities in the Baltic Sea, as the nitrate pool is exhausted during the
spring bloom, leaving behind an excess of dissolved inorganic phosphorus
(Wasmund et al., 2001). Consequently, filamentous diazotrophic
(N
Changes in seawater carbonate chemistry due to increased atmospheric
CO
In this mesocosm study, our aim was to assess diazotroph growth and rates of
N
The study took place in the period between June and August 2012 in
Tvärminne Storfjärden, which is situated in the Archipelago Sea on
the southwestern tip of Finland. Six pelagic mesocosms (total volume
A gradient of CO
Depth-integrating water samplers (IWS, HYDRO-BIOS, Kiel) were used to collect water from 0 to 17 m depth in each mesocosm for analysis of particulate matter, dissolved inorganic and organic matter, phytoplankton pigments, phytoplankton abundances and carbonate chemistry variables. Samples of carbonate chemistry variables were taken directly from the IWS on board the sampling boat, whereas all other samples were pooled in 10 L plastic carboys and stored on board in the dark until subsampling onshore (Paul et al., 2015). Particulate matter collected in the sediment trap was pumped to the surface and collected in sampling bottles (Boxhammer et al., 2016).
Particulate matter (C, N, P) and phytoplankton pigment samples were
collected onto GF/F filters (nominal pore size of 0.7
Temporal development in
Phosphate excess (P*, Deutsch et al., 2007) was calculated from the
dissolved inorganic phosphate, nitrate and ammonium concentrations according
to
Incubations for determination of N
Water samples for N
Incubations were terminated after 24 h by filtration through a combusted
(6 h at 450
Counts of phytoplankton cells > 20
A linear regression analysis was applied to determine the relationship
between mean
Three experimental phases after initial CO
There were low concentrations of inorganic N present throughout the study
period, with inorganic nitrate concentrations in the range of 3–107 nmol L
There was an excess of inorganic phosphate to inorganic N in all mesocosms
(P* > 0 nmol L
Diatoms were mostly abundant at the beginning of the experiment, with the
species
Dissolved organic nitrogen (DON) concentrations ranged between 20 and 25
Temporal development in
The abundance of filamentous diazotrophic cyanobacteria remained low
throughout the experiment, with no significant bloom development
(< 6
Summary of linear regression analyses of
Variables indicating abundance and activity of filamentous
diazotrophic cyanobacteria:
Rates of N
The natural abundance
Assessment of in situ N
Low filamentous diazotrophic cyanobacteria abundances exacerbated the
inherent sampling error in both microscopy and pigment analyses due to
patchy distribution and the tendency of filaments to aggregate. Hence,
unfortunately no reliable statistical analyses on the effect of higher
Bioavailable N was present in low concentrations and was probably the
limiting macronutrient in the plankton community. Hence, higher phytoplankton
biomass and lower phosphate concentrations at higher CO
The absence of any detectable effect may of course be influenced by the
relatively low abundances of filamentous diazotrophic cyanobacteria in this
study, as temperatures were mostly below temperatures thought to stimulate
bloom development (16
In addition to these highly visible filamentous N
In this area of the Baltic Sea, plankton communities, containing filamentous
diazotrophic cyanobacteria, are exposed to large diurnal and seasonal
changes in pH (Almén et al., 2014; Brutemark et al., 2011). In addition,
filamentous cyanobacteria form characteristic surface aggregations. Inside
these aggregations, microenvironments can create substantially different
conditions compared to the surrounding water with large diurnal fluctuations
in pH (7.4 vs. 9.0) and O
Productivity in this plankton community appeared to be dominated by
regenerative production (sensu Dugdale and Goering, 1967) under low nitrate
availability during Phase I, as has been observed in summer plankton
communities in the Baltic Sea (Kuparinen, 1987; Sahlsten and Sörensson,
1989; Tamminen, 1995). DON appeared to be a more important N source than N
derived from N
Neither a significant effect of CO
Plankton biomass build-up in this study was limited by low inorganic N
availability; therefore organic N pools were utilised supporting regenerative
production during the more productive period in Phase I, with diatoms
benefitting from this N turnover. Estimated N
We detected no significant differences in N pool sizes between CO
Nonetheless, it appears that increased CO
We would like to thank Douglas Campbell and one anonymous referee for their
constructive comments, which improved the manuscript during the review
process. We thank the KOSMOS team and all of the participants in the
mesocosm campaign for their contribution to the mesocosm sampling during the
experiment. In particular, we would like to thank Andrea Ludwig for
co-ordinating the campaign logistics and assistance with CTD operations, the
diving team, as well as Kerstin Nachtigall for analyses. Thank you also to
Dana Hellemann, Francois-Eric Legiret, Jana Meyer, Michael Meyerhöfer,
Jehane Ouriqua and Michael Sswat for assistance in sampling and analyses. We
would also like to sincerely thank the Tvärminne Zoological Station for
their warm hospitality, support and use of facilities for this experiment.
We also gratefully acknowledge the captain and crew of R/V