Nitrogen fixation rate and phosphorus enrichment effects on diazotrophic cyanobacteria in the Gulf of Riga

The semi-enclosed Baltic Sea experiences regular summer blooms of diazotrophic cyanobacteria. Previously, it has been conclusively demonstrated that in open nitrogen-limited parts of the Baltic Sea, cyanobacteria successfully fix atmospheric N2. At the same time, diazotrophic activity is still poorly understood in Baltic Sea sub-regions where nitrogen and phosphorus are co-limiting primary production. To address this gap in research, we used the15N tracer method for in situ incubations and measured the N2-fixation rate of heterocyst-forming cyanobacteria and picocyanobacteria in the Gulf of Riga, Baltic Sea, from April to September. Physicochemical variables and phytoplankton community composition were also determined. Our results show that the dominant species of cyanobacteria for this region (Aphanizomenon flosaquae) was present in the phytoplankton community during most of the study period. We also establish that the N2-fixation rate has a strong correlation with the proportion of A. flosaquae biomass containing heterocysts (r = 0.80). Our findings highlight the importance of a heterocyst-focused approach for an accurate diazotrophic activity evaluation that is one of the foundations for future management and protection of the Baltic Sea.

Phosphorus pool variations and their relation to cyanobacteria development in the Baltic Sea: A three-year study

Variations in the elemental ratio of organic matter in the central Baltic Sea: Part I—Linking primary production to remineralization

Simulated heat waves promote the growth but suppress the N2 fixation rates of Dolichospermum spp. and cyanobacterial communities in temperate lakes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 432:17-29 (2011) - DOI: https://doi.org/10.3354/meps09147 Addition of inorganic or organic phosphorus enhances nitrogen and carbon fixation in the ­oligotrophic North Pacific Katie S. Watkins-Brandt1, Ricardo M. Letelier1, Yvette H. Spitz1, Matthew J. Church2, Daniela Böttjer2, Angelicque E. White1,* 1College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA 2Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai’i at Mãnoa, Honolulu, Hawaii, USA *Corresponding author. Email: awhite@coas.oregonstate.edu ABSTRACT: We present a spatially extensive record of dinitrogen (N2) fixation rates and distributions of N2 fixing microorganisms along with the results of exogenous phosphorus (P) addition experiments conducted during a series of cruises in the North Pacific Subtropical Gyre (NPSG). We measured the N2 and carbon (C) fixation rates of natural plankton assemblages in response to the addition of methylphosphonate (MPn), a dissolved organic phosphorus (DOP) compound, and dissolved inorganic phosphorus (DIP). Results are compared to parallel unamended controls. These experiments produced 3 major findings: (1) MPn and DIP were utilized with equal metabolic efficiency over a single photoperiod, (2) the bulk of the enhanced N2 fixation rates were within the range of those previously reported in the NPSG, suggesting that P levels in this region can be saturating but were not at the time of sampling and (3) MPn and DIP additions stimulated C fixation rates beyond estimated contributions by diazotrophs, and hence both DIP and bioavailable DOP additions could lead to enhancement of net primary productivity on short time-scales. Our results suggest that the rate of N2 fixation in our study region may have been restricted by the availability and/or the composition of the total P pool (inorganic and organic P) during our field season. KEY WORDS: Phosphorus · Nitrogen fixation · Dissolved organic matter Full text in pdf format PreviousNextCite this article as: Watkins-Brandt KS, Letelier RM, Spitz YH, Church MJ, Böttjer D, White AE (2011) Addition of inorganic or organic phosphorus enhances nitrogen and carbon fixation in the ­oligotrophic North Pacific. Mar Ecol Prog Ser 432:17-29. https://doi.org/10.3354/meps09147 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 432. Online publication date: June 27, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Salinity and temperature data from the Gulf of Riga, a semi-enclosed sub-basin of the Baltic Sea, were analyzed with a focus on interannual and seasonal variability. The data were compiled from measurements taken from 1973 to 1995, a period which includes the stagnation period in the Baltic Sea. Interannual and seasonal variations in the net inflow of saline water from the Baltic Proper were estimated from volume and salt conservation equations for the period 1973-90. The basic decreasing trend, superimposed interannual variations in salinity, and interannual and seasonal temperature variations in the Gulf of Riga coincided with corresponding changes above the halocline in the Baltic Proper. Seasonal salinity variations were notable in the Gulf of Riga as compared to the Baltic Proper, where variations were negligible. Estimated annual mean inflow varied between 2,000 and 5,000 m3/s (average 3,200 m3/s), with a notable increasing trend. A simultaneous increasing trend was extracted from annual mean river flow data. Short-term fluctuations (over 4-6 years) of annual mean inflow ran opposite to the fluctuations of the magnitude of river flow. The average salinity in the Gulf of Riga increased during strong inflow and weak river flow and decreased when inflow was weak and river flow was strong. Variations in the inflow of water salinity had a minor effect on salinity variations in the Gulf of Riga. Seasonal inflow was strongest in spring and autumn and weak in winter.

A numerical study of circulation in the Gulf of Riga, Baltic Sea. Part I: Whole-basin gyres and mean currents

The availability of iron (Fe) and phosphorus (P) have been shown to be key factors regulating rates of nitrogen fixation in the western Subtropical Pacific. However, their relative importance at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by carrying out measurements of (i) finer scale spatial variabilities in N2 fixation rates and diazotroph abundances throughout these regions, and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates were lower in the NSCS than around the Luzon Strait and the western North Pacific, which we hypothesize was due to lower Fe-to-fixed nitrogen supply ratios that decrease their competitive ability with non-diazotrophic phytoplankton. The nutrient addition bioassay experiments demonstrated that nitrogen fixation rates in the central northern South China Sea (NSCS) were co-limited by Fe and P, whereas in the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in nitrogen fixation rates, and the largest responses were always dominated by either Trichodesmium or UCYN-B. In general, nutrient addition had a relatively restricted impact on diazotroph community structure apart from on UCYN-B, which showed increased contribution to the diazotroph community following P addition at sites where N2 fixation rates were P-limited. We further hypothesize the importance of absolute Fe supply rates in regulating spatial variability in diazotroph community structure across the study area.

The availability of iron (Fe) and phosphorus (P) have been shown to be key factors regulating rates of nitrogen fixation in the western Subtropical Pacific. However, their relative importance at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by carrying out measurements of (i) finer scale spatial variabilities in N2 fixation rates and diazotroph abundances throughout these regions, and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates were lower in the NSCS than around the Luzon Strait and the western North Pacific, which we hypothesize was due to lower Fe-to-fixed nitrogen supply ratios that decrease their competitive ability with non-diazotrophic phytoplankton. The nutrient addition bioassay experiments demonstrated that nitrogen fixation rates in the central northern South China Sea (NSCS) were co-limited by Fe and P, whereas in the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in nitrogen fixation rates, and the largest responses were always dominated by either Trichodesmium or UCYN-B. In general, nutrient addition had a relatively restricted impact on diazotroph community structure apart from on UCYN-B, which showed increased contribution to the diazotroph community following P addition at sites where N2 fixation rates were P-limited. We further hypothesize the importance of absolute Fe supply rates in regulating spatial variability in diazotroph community structure across the study area.

The availability of iron (Fe) and phosphorus (P) have been shown to be key factors regulating rates of nitrogen fixation in the western Subtropical Pacific. However, their relative importance at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by carrying out measurements of (i) finer scale spatial variabilities in N2 fixation rates and diazotroph abundances throughout these regions, and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates were lower in the NSCS than around the Luzon Strait and the western North Pacific, which we hypothesize was due to lower Fe-to-fixed nitrogen supply ratios that decrease their competitive ability with non-diazotrophic phytoplankton. The nutrient addition bioassay experiments demonstrated that nitrogen fixation rates in the central northern South China Sea (NSCS) were co-limited by Fe and P, whereas in the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in nitrogen fixation rates, and the largest responses were always dominated by either Trichodesmium or UCYN-B. In general, nutrient addition had a relatively restricted impact on diazotroph community structure apart from on UCYN-B, which showed increased contribution to the diazotroph community following P addition at sites where N2 fixation rates were P-limited. We further hypothesize the importance of absolute Fe supply rates in regulating spatial variability in diazotroph community structure across the study area.

The availability of iron (Fe) and phosphorus (P) have been shown to be key factors regulating rates of nitrogen fixation in the western Subtropical Pacific. However, their relative importance at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by carrying out measurements of (i) finer scale spatial variabilities in N2 fixation rates and diazotroph abundances throughout these regions, and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates were lower in the NSCS than around the Luzon Strait and the western North Pacific, which we hypothesize was due to lower Fe-to-fixed nitrogen supply ratios that decrease their competitive ability with non-diazotrophic phytoplankton. The nutrient addition bioassay experiments demonstrated that nitrogen fixation rates in the central northern South China Sea (NSCS) were co-limited by Fe and P, whereas in the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in nitrogen fixation rates, and the largest responses were always dominated by either Trichodesmium or UCYN-B. In general, nutrient addition had a relatively restricted impact on diazotroph community structure apart from on UCYN-B, which showed increased contribution to the diazotroph community following P addition at sites where N2 fixation rates were P-limited. We further hypothesize the importance of absolute Fe supply rates in regulating spatial variability in diazotroph community structure across the study area.

Abstract. N2 fixation rates were measured daily in large (∼ 50 m3) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N2 fixation rates in relation with environmental parameters and study the fate of diazotroph-derived nitrogen (DDN) in a low-nutrient, low-chlorophyll ecosystem. The mesocosms were fertilized with ∼ 0.8 µM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N2 fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol N L−1 d−1 over the 23 days, and increased by a factor of 2 during the second half of the experiment (days 15 to 23) to reach 27.3 ± 1.0 nmol N L−1 d−1. These later rates measured after the DIP fertilization are higher than the upper range reported for the global ocean. During the 23 days of the experiment, N2 fixation rates were positively correlated with seawater temperature, primary production, bacterial production, standing stocks of particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP), and alkaline phosphatase activity, and negatively correlated with DIP concentrations, DIP turnover time, nitrate, and dissolved organic nitrogen and phosphorus concentrations. The fate of DDN was investigated during a bloom of the unicellular diazotroph UCYN-C that occurred during the second half of the experiment. Quantification of diazotrophs in the sediment traps indicates that ∼ 10 % of UCYN-C from the water column was exported daily to the traps, representing as much as 22.4 ± 5.5 % of the total POC exported at the height of the UCYN-C bloom. This export was mainly due to the aggregation of small (5.7 ± 0.8 µm) UCYN-C cells into large (100–500 µm) aggregates. During the same time period, a DDN transfer experiment based on high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labeling revealed that 16 ± 6 % of the DDN was released to the dissolved pool and 21 ± 4 % was transferred to non-diazotrophic plankton, mainly picoplankton (18 ± 4 %) followed by diatoms (3 ± 2 %). This is consistent with the observed dramatic increase in picoplankton and diatom abundances, primary production, bacterial production, and standing stocks of POC, PON, and POP in the mesocosms during the second half of the experiment. These results offer insights into the fate of DDN during a bloom of UCYN-C in low-nutrient, low-chlorophyll ecosystems.

Trophic Status of the South-Eastern Baltic Sea: A Comparison of Coastal and Open Areas

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 191:1-18 (1999) - doi:10.3354/meps191001 Occurrence and bacterial cycling of dissolved nitrogen in the Gulf of Riga, the Baltic Sea Niels O. G. Jørgensen1,*, Lars J. Tranvik2,**, Gry Mine Berg3,*** 1Department of Ecology, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark 2Department of Ecology, Limnology, Lund University, 223 62 Lund, Sweden 3Horn Point Laboratory, University of Maryland, Center for Environmental Science, Cambridge, Maryland 21613, USA *Corresponding author. E-mail: nogj@kvl.dk Present addresses: **Department of Limnology, Evolutionary Biology Centre, Uppsala University, 752 36 Uppsala, Sweden ***Institut für Meereskunde an der Universität Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany ABSTRACT: Occurrence, composition and biolability of total dissolved nitrogen (TDN, including dissolved inorganic and organic N ([DIN and DON]) were examined in May and July 1996 in the northern (Köiguste transect) and southern part (Saulkrasti transect) of the Gulf of Riga. In the Saulkrasti transect, the Daugava River was a major source of TDN as indicated by concentrations of up to 60 µM TDN in the river plume, compared to about 20 µM TDN in the open gulf. DON made up 80 to 90% of the TDN, but on May 16 a nitrate-rich river plume lowered the proportion of DON to 65%. Riverine DIN and DON stimulated the biological activity along the transect in May, but apparently not in July. In the Köiguste transect, concentrations of TDN were 0.5-fold lower than at Saulkrasti. In May, input of terrestrial nutrients to the inner part of the transect probably increased the biological production. In both transects, the labile DON fraction, defined as bacterial DON degradation over a week, was estimated at 4 to 29%, with an average of 13%. Dissolved combined amino acids (DCAA) made up 10 to 30% of the DON. A high DON lability coincided with a large proportion of DCAA, relative to TDN, or a high algal biomass. The l/d ratio of especially dissolved combined glutamic acid appeared to be a potential indicator of DON lability. DCAA, as well as dissolved free amino acids (DFAA), were dominant nutrient sources to the bacteria, on average sustaining 77% (range 8 to 136%) of the bacterial N demand. In addition to amino acids, the bacteria incorporated or released DIN and urea. The effect of solar radiation on DON biolability was tested, but no consistent evidence for a light effect on transformation or bacterial utilization of recalcitrant DON was found. Our results show that there is an active microbial cycling of DON in the gulf, driven in part by input of N from the Daugava River. In summer, availability of assimilable N appears to limit the microbial activity as indicated by an estimated C/N ratio of 43 of labile DOM, as compared to a ratio of 4 in May. The microbial N dynamics in the Gulf of Riga are comparable to low-eutrophic estuaries and do not indicate direct eutrophication effects. KEY WORDS: Gulf of Riga · Total dissolved nitrogen · DON · DIN · DON lability · Amino acids · l/d ratio · Urea · UV radiation Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 191. Publication date: December 30, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Directional variation of return periods of water level extremes in Moonsund and in the Gulf of Riga, Baltic Sea