Nutrient and phytoplankton dynamics of the Hunter River estuary

Observational studies and nutrient amendment experiments were conducted to better understand the nutrient and phytoplankton dynamics of the Hunter River estuary. Eutrophic conditions above ANZECC guidelines for estuaries dominate the Hunter River estuary. The upper Hunter estuary, upstream of its confluence with the Williams River, had the highest concentrations of nutrients and chlorophyll a. The major source of nutrients appears to be riverine discharge. Discharge from WWTP in the upper Hunter potentially contributes an important secondary source of phosphorus. Processes such as bank erosion and resuspension may also be important in explaining variation in nutrient concentrations. Light and turbidity were the main factors limiting phytoplankton growth in the upper estuary. The nutrient amendment experiments showed that when light limitation was alleviated, phytoplankton were either nitrogen limited or remained unlimited by nutrients (suggesting nutrients were in surplus for growth). The expression of nitrogen limitation is likely due to low N:P in the estuary. Organic nitrogen dominates the nitrogen pool within the Hunter estuary. The bioavailability of organic nitrogen in the estuary is unknown which may explain the lack of relationship between phytoplankton and nitrogen concentrations within the estuary. Diatoms and green algae dominated phytoplankton. There were occasions when toxic cyanobacteria was in high abundance in the upper estuary, however a longer data set of phytoplankton assemblage is needed to more adequately assess the risk of toxic cyanobacteria. Comparison of data from the monthly, twice-weekly, and hourly sampling intervals demonstrated the five-year monthly sampling data appeared to mostly capture the variability of nutrient and chlorophyll a concentrations in relation to their main explanatory factors (discharge and light). There were some examples of chlorophyll a and nitrogen concentrations that fell outside of predicted ranges. Overall the results suggest any increase in nitrogen loads to the estuary may lead to increased phytoplankton growth. Improved light climate may also lead to increased phytoplankton growth. Reducing inputs of both nitrogen and phosphorus to the upper Hunter estuary should be a priority action to increase ecosystem health.

Biophysical controls on seasonal changes in the structure, growth, and grazing of the size-fractionated phytoplankton community in the northern South China Sea

The size-fractionated phytoplankton growth and microzooplankton grazing are crucial for the temporal change of community size structure, regulating not only trophic transfer but also the carbon cycle of the ocean. However, the size-dependent growth and grazing dynamics on a monthly or an annual basis are less addressed in the coastal ocean. In this paper, the seasonal responses of the size-fractionated phytoplankton growth and grazing to environmental change were examined over 1 year at a coastal site of the northern South China Sea. We found a nanophytoplankton-dominated community with strong seasonal variations in all size classes. Phytoplankton community growth rate was positively correlated to nutrients, with community grazing rate correlating to the total chlorophyll a at the station, reflecting a combined bottom-up and topdown effect on phytoplankton population dynamics. Further analyses suggested that the specific growth rate of microphytoplankton was significantly influenced by phosphate, and that of nanophytoplankton was influenced by light, although picophytoplankton growth was controlled by both nitrate and temperature. In addition, the specific grazing rate of nanophytoplankton was well correlated to phytoplankton standing stock, while that of micro- and pico-compartments was negatively influenced by ciliate abundance and salinity. Finally, a lower grazing impact for micro-cells (38 %) than nano- and pico-cells (72 % and 60 %, respectively) may support size-selective grazing of microzooplankton on small cells at this eutrophic system.

Increased Nitrogen Loading Boosts Summer Phytoplankton Growth by Alterations in Resource and Zooplankton Control: A Mesocosm Study

The effectiveness of controlling nitrogen (N) to manage eutrophication of aquatic ecosystems remains debated. To understand the mechanisms behind phytoplankton growth in shallow lakes (resource and grazing effects) under contrasting N loading scenarios, we conducted a 70-days mesocosm experiment in summer. The mesocosms contain natural plankton communities deriving from a 10-cm layer of lake sediment and 450 L of lake water. We also added two juvenile crucian carp (Carassius carassius) in each mesocosm to simulate presence of the prevailing omni-benthivorous fish in subtropical lakes. Our results showed that N addition increased not only water N levels but also total phosphorus (TP) concentrations, which together elevated the phytoplankton biomass and caused strong dominance of cyanobacteria. Addition of N significantly lowered the herbivorous zooplankton to phytoplankton biomass ratio and promoted the phytoplankton yield per nutrient (Chl-a: TP or Chl-a: TN ratio), indicating that summer N addition likely also favored phytoplankton growth through reduced grazing by zooplankton. Accordingly, our study indicates that summer N loading may boost eutrophication via both changes in resource and grazing control in shallow lakes. Thus, alleviation of eutrophication in shallow eutrophic lakes requires a strategic approach to control both nutrients (N and P) appropriately.

Aeolian Iron and Its Contribution to Phytoplankton Production in McMurdo Sound, Southwest Ross Sea, Antarctica

<p>Each summer the waters in McMurdo Sound (Lat. 77.5ºS; Long. 165ºE), south-western (SW) Ross Sea encounter vast phytoplankton blooms. This phenomenon is stimulated by the addition of bio-available iron (Fe) to an environment where phytoplankton growth is otherwise Fe-limited. One possible source of such Fe is aeolian sand and dust (ASD) which accumulates on sea ice and is released into the ocean during the summer melt season. The amount of bio-available Fe (i.e. the amount of Fe immedately accessible to phytoplankton) potentially supplied to the ocean by ASD depends on a number of factors including; the ASD flux into the ocean, its particle size distribution and Fe content. However, none of these parameters are well constrained in the SW Ross Sea region and, as a result, the significance of this Fe source in the biogeochemical cycle of phytoplankton growth remains to be quantified. This study focuses on an area (7400 km²) of Southern McMurdo Sound, one of the few areas where direct sampling of ASD that has accumulated on sea ice is possible. To evaluate the flux and solubility of Fe contained in ASD into McMurdo Sound, the mass accumulation rate and particle size of 70 surface snow samples and 3 shallow (3 m) firn cores from the nearby McMurdo Ice Shelf covering the period 2000 - 2008 have been analysed. Selected samples were also measured for total and soluble Fe, Sr and Nd isotopic ratios and mineralogy as a guide to Fe-fertilisation potential and provenance, respectively. Mass and particle size data show an exponential decrease in mass accumulation rate (from 26.00 g m⁻² yr⁻¹ to 0.70 g m⁻² yr⁻¹) and a decrease in modal particle size (from 130 to 69 μm) over a distance of 120 km from Southern McMurdo Sound northwards to Granite Harbour. Both these trends are consistent with ASD being dispersed northwards across the sea ice by southerly storms from an area of the McMurdo Ice Shelf, where submarine freezing and surface ablation have resulted in a surface covered with debris from the sea floor, known as the 'dirty ice' or 'debris bands' (Lat. 77.929ºS; Long. 165.505ºE) in Southern McMurdo Sound. This assertion is further supported by the Sr and Nd isotopic signature of ASD matching local source rocks and the presence of vesicular glass of Southern McMurdo Sound in all samples which also points to the debris bands as the origin of ASD in McMurdo Sound. Bio-available Fe is extremely difficult to quantify hence Fe solubility was used as an approximation in this thesis. Analysis of both total (i.e. particulate and soluble) and the percentage of soluble Fe in the 0.4 - 10 μm dust size fraction (i.e. the fraction most likely to become bio-available) by solution ICP-MS shows a narrow range of values; 3.84 ± 1.99 wt % and 9.42 ± 0.70 % respectively. Combining these values with mass accumulation rate estimates for the particles 0.4 - 10 μm in size, gives an annual soluble Fe flux for the region 500 km² north of the debris bands in McMurdo Sound of 0.55 mg m⁻² yr⁻¹ (9.89 μmol m⁻² yr⁻¹), with spatial variability largely determined by differences in mass accumulation rate. These fluxes are at least an order of magnitude greater than predicted in global dust deposition models for the Southern Ocean and measured in snow samples from East Antarctica. Furthermore, these values exceed the Fe threshold, estimated as 0.2 nM (Boyd and Abraham, 2001), required for phytoplankton growth following the simple dust-biota model of Boyd et al. (2010) and assuming the release of captured ASD in snow is instantaneous. Whilst not constrained in the present study, ASD sourced from the debris bands may be sufficiently widely dispersed, particularly during storm years, to contribute to Fe-fertilisation up to 1200 km from Southern McMurdo Sound. Short, ~10 year long, firn core records of mass accumulation and methylsuphonate concentration, a proxy for phytoplankton productivity, shows a close correspondence between the two during particularly stormy years. Whilst not demonstrating a cause-and-effect relationship, this observation suggests coastal ice cores may contain an important record of the interplay between climate, dust supply, Fe-fertilisation of near shore waters and phytoplankton productivity on decadal and longer timescales.</p>

Aeolian Iron and Its Contribution to Phytoplankton Production in McMurdo Sound, Southwest Ross Sea, Antarctica

<p>Each summer the waters in McMurdo Sound (Lat. 77.5ºS; Long. 165ºE), south-western (SW) Ross Sea encounter vast phytoplankton blooms. This phenomenon is stimulated by the addition of bio-available iron (Fe) to an environment where phytoplankton growth is otherwise Fe-limited. One possible source of such Fe is aeolian sand and dust (ASD) which accumulates on sea ice and is released into the ocean during the summer melt season. The amount of bio-available Fe (i.e. the amount of Fe immedately accessible to phytoplankton) potentially supplied to the ocean by ASD depends on a number of factors including; the ASD flux into the ocean, its particle size distribution and Fe content. However, none of these parameters are well constrained in the SW Ross Sea region and, as a result, the significance of this Fe source in the biogeochemical cycle of phytoplankton growth remains to be quantified. This study focuses on an area (7400 km²) of Southern McMurdo Sound, one of the few areas where direct sampling of ASD that has accumulated on sea ice is possible. To evaluate the flux and solubility of Fe contained in ASD into McMurdo Sound, the mass accumulation rate and particle size of 70 surface snow samples and 3 shallow (3 m) firn cores from the nearby McMurdo Ice Shelf covering the period 2000 - 2008 have been analysed. Selected samples were also measured for total and soluble Fe, Sr and Nd isotopic ratios and mineralogy as a guide to Fe-fertilisation potential and provenance, respectively. Mass and particle size data show an exponential decrease in mass accumulation rate (from 26.00 g m⁻² yr⁻¹ to 0.70 g m⁻² yr⁻¹) and a decrease in modal particle size (from 130 to 69 μm) over a distance of 120 km from Southern McMurdo Sound northwards to Granite Harbour. Both these trends are consistent with ASD being dispersed northwards across the sea ice by southerly storms from an area of the McMurdo Ice Shelf, where submarine freezing and surface ablation have resulted in a surface covered with debris from the sea floor, known as the 'dirty ice' or 'debris bands' (Lat. 77.929ºS; Long. 165.505ºE) in Southern McMurdo Sound. This assertion is further supported by the Sr and Nd isotopic signature of ASD matching local source rocks and the presence of vesicular glass of Southern McMurdo Sound in all samples which also points to the debris bands as the origin of ASD in McMurdo Sound. Bio-available Fe is extremely difficult to quantify hence Fe solubility was used as an approximation in this thesis. Analysis of both total (i.e. particulate and soluble) and the percentage of soluble Fe in the 0.4 - 10 μm dust size fraction (i.e. the fraction most likely to become bio-available) by solution ICP-MS shows a narrow range of values; 3.84 ± 1.99 wt % and 9.42 ± 0.70 % respectively. Combining these values with mass accumulation rate estimates for the particles 0.4 - 10 μm in size, gives an annual soluble Fe flux for the region 500 km² north of the debris bands in McMurdo Sound of 0.55 mg m⁻² yr⁻¹ (9.89 μmol m⁻² yr⁻¹), with spatial variability largely determined by differences in mass accumulation rate. These fluxes are at least an order of magnitude greater than predicted in global dust deposition models for the Southern Ocean and measured in snow samples from East Antarctica. Furthermore, these values exceed the Fe threshold, estimated as 0.2 nM (Boyd and Abraham, 2001), required for phytoplankton growth following the simple dust-biota model of Boyd et al. (2010) and assuming the release of captured ASD in snow is instantaneous. Whilst not constrained in the present study, ASD sourced from the debris bands may be sufficiently widely dispersed, particularly during storm years, to contribute to Fe-fertilisation up to 1200 km from Southern McMurdo Sound. Short, ~10 year long, firn core records of mass accumulation and methylsuphonate concentration, a proxy for phytoplankton productivity, shows a close correspondence between the two during particularly stormy years. Whilst not demonstrating a cause-and-effect relationship, this observation suggests coastal ice cores may contain an important record of the interplay between climate, dust supply, Fe-fertilisation of near shore waters and phytoplankton productivity on decadal and longer timescales.</p>

Roles of Iron Limitation in Phytoplankton Dynamics in the Western and Eastern Subarctic Pacific

The subarctic Pacific is one of the major high-nitrate, low-chlorophyll (HNLC) regions where marine productivity is greatly limited by the supply of iron (Fe) in the region. There is a distinct seasonal difference in the chlorophyll concentrations of the east and west sides of the subarctic Pacific because of the differences in their driving mechanisms. In the western subarctic Pacific, two chlorophyll concentration peaks occur: the peak in spring and early summer is dominated by diatoms, while the peak in late summer and autumn is dominated by small phytoplankton. In the eastern subarctic Pacific, a single chlorophyll concentration peak occurs in late summer, while small phytoplankton dominate throughout the year. In this study, two one-dimensional (1D) physical–biological models with Fe cycles were applied to Ocean Station K2 (Stn. K2) in the western subarctic Pacific and Ocean Station Papa (Stn. Papa) in the eastern subarctic Pacific. These models were used to study the role of Fe limitation in regulating the seasonal differences in phytoplankton populations by reproducing the seasonal variability in ocean properties in each region. The results were reasonably comparable with observational data, i.e., cruise and Biogeochemical-Argo data, showing that the difference in bioavailable Fe (BFe) between Stn. K2 and Stn. Papa played a dominant role in controlling the respective seasonal variabilities of diatom and small phytoplankton growth. At Stn. Papa, there was less BFe, and the Fe limitation of diatom growth was two times as strong as that at Stn. K2; however, the difference in the Fe limitation of small phytoplankton growth between these two regions was relatively small. At Stn. K2, the decrease in BFe during summer reduced the growth rate of diatoms, which led to a rapid reduction in diatom biomass. Simultaneously, the decrease in BFe had little impact on small phytoplankton growth, which helped maintain the relatively high small phytoplankton biomass until autumn. The experiments that stimulated a further increase in atmospheric Fe deposition also showed that the responses of phytoplankton primary production in the eastern subarctic Pacific were stronger than those in the western subarctic Pacific but contributed little to primary production, as the Fe limitation of phytoplankton growth was replaced by macronutrient limitation.

Phytoplankton of the Вarents sea

On the basis of the analysis of summer plankton phytocenosis structure, 4 areas representing various stages of a succession cycle are allocated for water areas of the Barents Sea. In the most productive places of the water area the level of phytoplankton growth corresponded to indicators of mesotrophic-eutrophic waters and was maximum in the northern area. Concentration of phosphates was the main regulator of bloom of coccolithophore Emilianiahuxleyi, besides water temperature. The presence in the modern plankton phytoсenosis structure in the northern part of sea (80ºN) of the Atlantic species, along with annual bloom of E. huxleyi in the southwest part of the sea, are the indicators of increased «atlantification» of the Arctic Region.

Mixing and Phytoplankton Growth in an Upwelling System

Previous studies focused on understanding the role of physical drivers on phytoplankton bloom formation mainly used indirect estimates of turbulent mixing. Here we use weekly observations of microstructure turbulence, dissolved inorganic nutrients, chlorophyll a concentration and primary production carried out in the Ría de Vigo (NW Iberian upwelling system) between March 2017 and May 2018 to investigate the relationship between turbulent mixing and phytoplankton growth at different temporal scales. In order to interpret our results, we used the theoretical framework described by the Critical Turbulent Hypothesis (CTH). According to this conceptual model if turbulence is low enough, the depth of the layer where mixing is active can be shallower than the mixed-layer depth, and phytoplankton may receive enough light to bloom. Our results showed that the coupling between turbulent mixing and phytoplankton growth in this system occurs at seasonal, but also at shorter time scales. In agreement with the CTH, higher phytoplankton growth rates were observed when mixing was low during spring-summer transitional and upwelling periods, whereas low values were described during periods of high mixing (fall-winter transitional and downwelling). However, low mixing conditions were not enough to ensure phytoplankton growth, as low phytoplankton growth was also found under these circumstances. Wavelet spectral analysis revealed that turbulent mixing and phytoplankton growth were also related at shorter time scales. The higher coherence between both variables was found in spring-summer at the ~16–30 d period and in fall-winter at the ~16–90 d period. These results suggest that mixing could act as a control factor on phytoplankton growth over the seasonal cycle, and could be also involved in the formation of occasional short-lived phytoplankton blooms.