Biologically Mediated Nitrogen Dynamics in Eutrophying Esturaries. Assessing Dentrification, N2 Fixation and Primary Productivity Responses to Proposed N Loading Reduction in the Neuse River Estuary (original) (raw)
Related papers
2000
Experimental manipulations were used to predict the effects of reducing the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) (DINDIP ratio) in the Neuse River Estuary (NRE) on the abundance and activity of N2 fixing cyanobacteria. Changes in primary productivity, N2 fixation (nitrogenase activity), genetic potential for N2 fixation (presence of nifH), phytoplankton taxonomic composition (based on diagnostic photopigment concentrations) and numbers of Nz fixing cyanobacteria (microscopy) were determined. Results fiom these experiments indicate that if reduction of N loading leads to a reduced ambient DIN:DIP ratio in the NRE, rates of N2 fixation may be higher when diazotrophic cyanobacteria are present. We did not, however, detect an increase in either the diversity or abundance of N2 fixers resulting from experimental manipulations of DIN: DIP ratio. A 30% reduction in nitrogen (N), phosphorus (P) or both N and P concentrations caused a reduction in phytoplankton assimilation number at the estuarine bioassay site. There was no reduction in assimilation number at the riverine site when assessing the data for the entire experimental period. When analyzing individual experiments, N dilution reduced assimilation number as often as dilution of both N and P concentration, while P reduction alone reduced assimilation number only once. Phytoplankton taxonomic composition as measured by HPLC diagnostic photopigment analysis was not altered by any dilution at any location in this work.
2002
A variety of analyses were used to assess the structure (community composition) and function (assimilation number, nitrogen fixation) of phytoplankton in the Neuse River Estuary (NRE), NC under ambient and modified nutrient concentrations. Dilution bioassays were employed to reduce the concentration of nitrogen (N) or both N and phosphorus (P) and thus compare varied DIN:DIP ratios. Experimental manipulations created conditions that may result from mandated N load reductions to the estuary. We hypothesized that unilateral reduction of N loading to the NRE would increase the activity, abundance and diversity of N 2 fixing cyanobacteria. Changes in phytoplankton primary productivity, N 2 fixation (nitrogenase activity), genetic potential for N 2 fixation (presence of nifH), phytoplankton taxonomic composition (diagnostic photopigment concentration) and abundances of N 2 fixing cyanobacteria (microscopy) were determined. Decreasing ambient DIN:DIP ratios in NRE samples resulted in increased rates of N 2 fixation when seed populations were present and environmental conditions were amenable. Decreasing the DIN:DIP ratio did not lead to an increase in the abundance or diversity of N 2 fixing cyanobacteria. Because N 2 fixing cyanobacteria were only actively fixing nitrogen during periods of low riverine N discharge (summer and early autumn), lowering nutrient ratios may not have a major impact on the NRE. However, the maximum potential amount of N from N 2 fixation was calculated using rates from this study and was found to be approximately 3% of total riverine loading of N to the NRE. Because N 2 fixation occurs farther downstream and later in the year than riverine N loading to the NRE, there is potential for N 2 fixation to modify N dynamics. Analyses of the phytoplankton community as a whole in these relatively short term experiments indicated that reduced DIN:DIP may not have a major impact on their structure and function.
Estuaries, 2001
We examined the effects of different forms of nitrogen and mixed versus static conditions on the structure and function of natural Neuse River estuary phytoplankton communities incubated in 66-liter microcosms in March, May, August, and November 1999. Significant differences were found between effects of mixed versus static treatments in three of four experiments, but no differences were observed between effects of different forms of nitrogen. Mixed incubations resulted in higher contributions of diatoms to total community biomass (measured as chlorophyll a) than in static tanks in May. Significantly higher rates of carbon fixation were also observed, likely due to increased suspension of diatoms in surface (illuminated) layers of the tanks. In August, we found significantly higher abundances of cyanobacteria, total community biomass, and rates of carbon fixation in static tanks than in tanks that were mixed. In November, static incubations showed significantly higher abundances of cryptophytes resulting in higher total community biomass and rates of carbon fixation in static tanks than in mixed tanks. Nitrogen additions significantly increased total community biomass relative to controls in May and August, indicating that the communities were nitrogen-limited at these times. We conclude that while nitrogen additions may result in increases in phytoplankton biomass when nitrogen is limiting, phytoplankton community structure in the Neuse River Estuary may be determined more by the hydrodynamics of the system (mixing versus stratification) than by the form of nitrogen available for growth.
PLOS ONE, 2016
Nitrogen availability and form are important controls on estuarine phytoplankton growth. This study experimentally determined the influence of urea and nitrate additions on phytoplankton growth throughout the growing season (March 2012, June 2011, August 2011) in a temperate, eutrophied estuary (Neuse River Estuary, North Carolina, USA). Photopigments (chlorophyll a and diagnostic photopigments: peridinin, fucoxanthin, alloxanthin, zeaxanthin, chlorophyll b) and microscopy-based cell counts were used as indicators of phytoplankton growth. In March, the phytoplankton community was dominated by Gyrodinium instriatum and only fucoxanthin-based growth rates were stimulated by nitrogen addition. The limited response to nitrogen suggests other factors may control phytoplankton growth and community composition in early spring. In June, inorganic nitrogen concentrations were low and stimulatory effects of both nitrogen forms were observed for chlorophyll a-and diagnostic photopigment-based growth rates. In contrast, cell counts showed that only cryptophyte and dinoflagellate (Heterocapsa rotundata) growth were stimulated. Responses of other photopigments may have been due to an increase in pigment per cell or growth of plankton too small to be counted with the microscopic methods used. Despite high nitrate concentrations in August, growth rates were elevated in response to urea and/or nitrate addition for all photopigments except peridinin. However, this response was not observed in cell counts, again suggesting that pigment-based growth responses may not always be indicative of a true community and/or taxa-specific growth response. This highlights the need to employ targeted microscopy-based cell enumeration concurrent with pigmentbased technology to facilitate a more complete understanding of phytoplankton dynamics in estuarine systems. These results are consistent with previous studies showing the seasonal importance of nitrogen availability in estuaries, and also reflect taxa-specific responses nitrogen availability. Finally, this study demonstrates that under nitrogen-limiting conditions, the phytoplankton community and its various taxa are capable of using both urea and nitrate to support growth.
Limnology and Oceanography, 2015
Anthropogenic activities are altering total nutrient loads to many estuaries and freshwaters, resulting in high loads not only of total nitrogen (N), but in some cases, of chemically reduced forms, notably NH 1 4. Long thought to be the preferred form of N for phytoplankton uptake, NH 1 4 may actually suppress overall growth when concentrations are sufficiently high. NH 1 4 has been well known to be inhibitory or repressive for NO-3 uptake and assimilation, but the concentrations of NH 1 4 that promote vs. repress NO-3 uptake, assimilation, and growth in different phytoplankton groups and under different growth conditions are not well understood. Here, we review N metabolism first in a "generic" eukaryotic cell, and the contrasting metabolic pathways and regulation of NH 1 4 and NO 2 3 when these substrates are provided individually under equivalent growth conditions. Then the metabolic interactions of these substrates are described when both are provided together, emphasizing the cellular challenge of balancing nutrient acquisition with photosynthetic energy balance in dynamic environments. Conditions under which dissipatory pathways such as dissimilatory NO 2 3 / NO 2 2 reduction to NH 1 4 and photorespiration that may lead to growth suppression are highlighted. While more is known about diatoms, taxon-specific differences in NH 1 4 and NO 2 3 metabolism that may contribute to changes in phytoplankton community composition when the composition of the N pool changes are presented. These relationships have important implications for harmful algal blooms, development of nutrient criteria for management, and modeling of nutrient uptake by phytoplankton, particularly in conditions where eutrophication is increasing and the redox state of N loads is changing.
Nitrogen fixation in freshwater, estuarine, and marine ecosystems. 1. Rates and importance
Limnology and Oceanography, 1988
Nitrogen fixation is mediated by a variety of autotrophic and heterotrophic bacteria. Cyanobacteria appear responsible for most planktonic fixation in aquatic ecosystems, and rates of fixation are high only when thcsc organisms make up a major percentage of the planktonic biomass, Planktonic nitrogen fixation tends to be low in oligotrophic and mesotrophic lakes (generally <O. 1 g N m-2 yr-I) but is often high in eutrophic lakes (0.2-9.2 g N m-2 yr-I).
The ISME Journal, 2014
Nitrogen (N) fixation is fueling planktonic production in a multitude of aquatic environments. In meso-and poly-haline estuaries, however, the contribution of N by pelagic N 2 fixation is believed to be insignificant due to the high input of N from land and the presumed absence of active N 2-fixing organisms. Here we report N 2 fixation rates, nifH gene composition and nifH gene transcript abundance for key diazotrophic groups over 1 year in two contrasting, temperate, estuarine systems: Roskilde Fjord (RF) and the Great Belt (GB) strait. Annual pelagic N 2 fixation rates averaged 17 and 61 mmol N m À 2 per year at the two sites, respectively. In RF, N 2 fixation was mainly accompanied by transcripts related to heterotrophic (for example, Pseudomonas sp.) and photoheterotrophic bacteria (for example, unicellular diazotrophic cyanobacteria group A). In the GB, the first of two N 2 fixation peaks coincided with a similar nifH-expressing community as in RF, whereas the second peak was synchronous with increased nifH expression by an array of diazotrophs, including heterotrophic organisms as well as the heterocystous cyanobacterium Anabaena. Thus, we show for the first time that significant planktonic N 2 fixation takes place in mesohaline, temperate estuaries and that the importance of heterotrophic, photoheterotrophic and photosynthetic diazotrophs is clearly variable in space and time.
Contribution of nitrate to the uptake of nitrogen by phytoplankton in an ocean margin environment
Hydrobiologia
Rates of nitrate and ammonium uptake by phytoplankton were measured fromJuly 1990 to March 1995 in the surface waters at several stations locatedalong the continental margin of the NE Atlantic Ocean. Total inorganicnitrogen assimilation ranged from 2.3 to 95 nM h–1 andexhibited two maxima during the spring bloom and in fall at the beginning ofthe vertical mixing of the water column. Seasonal and spatial changes in thenitrogen uptake regime (f-ratios) were estimated (1) by correcting ammoniumuptake rates with an isotope dilution model, and (2) by evaluating theinhibition of nitrate uptake by ammonium, using a variation of theMichaelis-Menten equation. Overall, nitrate uptake rates paralleled carbonfixation rates, and f-ratios followed the well-known function of nitrate.During spring, new production, sensu Dugdale & Goering (1967), accountedfor 46 to 85% of the total inorganic nitrogen production. It can gainin importance through vertical mixing in fall (0.29 < f="">&l...
The effect of inorganic nitrogen speciation on primary production in the San Francisco Estuary
Estuarine, Coastal and Shelf Science, 2012
We describe the results of a series of 96-h enclosure experiments conducted using water from stations in the northern San Francisco Estuary (SFE) along a gradient in ammonium (NH 4) and nitrate (NO 3) concentrations. Using dual-labeled 13 C/ 15 N tracers, we followed the timing and sequence of primary (carbon, C) production and phytoplankton nitrogen (N) use during experimental phytoplankton blooms. Our results show that diatoms consistently drive the phytoplankton blooms in the enclosures. By tracing both C and N uptake we provide clear evidence that high rates of C uptake are linked to phytoplankton NO 3 , and not NH 4 , use. Results from kinetics experiments demonstrated higher specific uptake rates (V MAx) for NO 3 compared to NH 4 in the SFE. Finally, dissolved inorganic carbon and nutrient drawdown ratios in the enclosures from the chronically high NH 4 regions of the SFE were substantially lower than predicted from the Redfield ratio, suggesting suppressed C uptake, in relation to other elemental uptake. Our conceptual model of the DIN interactions that lead to higher primary production and phytoplankton blooms in the SFE suggests that higher rates of primary production that accompany phytoplankton NO 3 uptake are sufficient to outpace phytoplankton losses, leading to blooms, compared to the lower rates associated with NH 4 uptake (only 20% of that based upon NO 3). Historical changes in wastewater practices have increased the proportion of NH 4 to the DIN pool in the SFE leading to reduced access to NO 3 by phytoplankton. This may help to explain some of the reduced primary production and phytoplankton biomass observed there since the 1970s.