Accumulation Of Phytoplankton Biomass Research Articles

Vitamin B1 and B12 Uptake and Cycling by Plankton Communities in Coastal Ecosystems

While vitamin B12 has recently been shown to co-limit the growth of coastal phytoplankton assemblages, the cycling of B-vitamins in coastal ecosystems is poorly understood as planktonic uptake rates of vitamins B1 and B12 have never been quantified in tandem in any aquatic ecosystem. The goal of this study was to establish the relationships between plankton community composition, carbon fixation, and B-vitamin assimilation in two contrasting estuarine systems. We show that, although B-vitamin concentrations were low (pM), vitamin concentrations and uptake rates were higher within a more eutrophic estuary and that vitamin B12 uptake rates were significantly correlated with rates of primary production. Eutrophic sites hosted larger bacterial and picoplankton abundances with larger carbon normalized vitamin uptake rates. Although the >2 μm phytoplankton biomass was often dominated by groups with a high incidence of vitamin auxotrophy (dinoflagellates and diatoms), picoplankton (<2 μm) were always responsible for the majority of B12-vitamin uptake. Multiple lines of evidence suggest that heterotrophic bacteria were the primary users of vitamins among the picoplankton during this study. Nutrient/vitamin amendment experiments demonstrated that, in the Summer and Fall, vitamin B12 occasionally limited or co-limited the accumulation of phytoplankton biomass together with nitrogen. Combined with prior studies, these findings suggest that picoplankton are the primary producers and users of B-vitamins in some coastal ecosystems and that rapid uptake of B-vitamins by heterotrophic bacteria may sometimes deprive larger phytoplankton of these micronutrients and thus influence phytoplankton species succession.

Variation in particulate C and N isotope composition following iron fertilization in two successive phytoplankton communities in the Southern Ocean

[1] Surface δ15NPON increased 3.92 ± 0.48‰ over the course of 20 days following additions of iron (Fe) to an eddy in close proximity to the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. The change in δ15NPON was associated with an increase in the >20 μm size fraction, leading to a maximal difference of 6.23‰ between the >20 μm and <20 μm size fractions. Surface δ13CPOC increased 1.18 ± 0.31‰ over the same period. After a decrease in particulate organic matter in the surface layer, a second phytoplankton community developed that accumulated less biomass, had a slower growth rate and was characterized by an offset of 1.56‰ in δ13CPOC relative to the first community. During growth of the second community, surface δ13CPOC further increased 0.83 ± 0.13‰. Here we speculate on ways that carboxylation, nitrogen assimilation, substrate pool enrichment and community composition may have contributed to the gradual increase in δ13CPOC associated with phytoplankton biomass accumulation, as well as the systematic offset in δ13CPOC between the two phytoplankton communities.

Response to Comment: Nitrogen fixation has not offset declines in the Lake 227 nitrogen pool and shows that nitrogen control deserves consideration in aquatic ecosystems

Response to Comment: Nitrogen fixation has not offset declines in the Lake 227 nitrogen pool and shows that nitrogen control deserves consideration in aquatic ecosystems

Oceanography of the subantarctic and Polar Frontal Zones south of Australia during summer: Setting for the SAZ-Sense study

This paper provides a description of the physical and chemical properties (temperature, salinity, macro-nutrient, and oxygen concentrations) and bulk biomass indicators (chlorophyll and beam attenuation) prevailing in the subantarctic zone and polar front zones south of Tasmania (Australia) during the ‘Sensitivity of the subantarctic zone to environmental change’ (SAZ-Sense) expedition carried out in the austral summer of 2007. Phytoplankton biomass showed a characteristic north–south gradient of decreasing chlorophyll from the subantarctic zone to Polar Frontal Zone, as well as a zonal gradient in the northern subantarctic zone, with an increase in chlorophyll from southwest to southeast of Tasmania. The representativeness of the observations was assessed by comparison to previous studies including satellite observation of chlorophyll biomass over a 10-year period. We consider the possible role of spatial differences in: (i) ocean water masses and frontal systems, (ii) upper mixed layer stratification at three process stations, and (iii) nutrient availability, in controlling the observed variations in phytoplankton biomass in the region. Zonal gradients of the basic oceanographic physical and chemical conditions in the subantarctic zone were relatively small and therefore unlikely to control the three-fold west-to-east differences observed in the accumulation of phytoplankton biomass. The zonal variation in subantarctic zone chlorophyll biomass appears to be driven at least partly by greater micro-nutrient (iron) supply to the waters east of Tasmania, as reported also by others ( Bowie et al., 2009; Mongin et al., 2011a). Despite this condition, the region of higher phytoplankton biomass to the southeast of Tasmania was only marginally more productive than the region of lower biomass west of Tasmania and south of the polar front, and exported less particulate carbon than the lower biomass waters ( Jacquet et al., 2011).

The effect of vitamin B12 on phytoplankton growth and community structure in the Gulf of Alaska

A majority of eukaryotic phytoplankton species require an exogenous source of vitamin B12 for growth and recent field studies in some coastal and polar regions indicate that the addition of vitamin B12 alone, or with another limiting nutrient can influence the accumulation of phytoplankton biomass. We quantified the concentrations and uptake rates of vitamin B12, characterized phytoplankton community composition, and examined the ability of vitamin B12 to alter the growth and composition of phytoplankton communities in the Gulf of Alaska. Picoplankton (0.2–2 μm) were responsible for the majority of vitamin B12 uptake in both coastal and high‐nutrient low‐chlorophyll (HNLC) regions and B12 concentrations and uptake rates were higher in HNLC regions compared to coastal regions with higher iron (Fe) concentrations. During vitamin amendment experiments, B12 alone or in conjunction with other limiting nutrients (N or Fe) significantly enhanced algal biomass and increased the growth rates of multiple groups of larger (&gt; 2 μm) phytoplankton. This included ecologically significant, B12 auxotrophs such as Gymnodinium sp. and Alexandrium sp. (in the costal experiment) as well as Chaetoceros sp. and Gymnodinium sp. (in the HNLC experiment). The ability of vitamin B12 to shape algal community composition in coastal and HNLC areas of the Gulf of Alaska, even in cases where it does not limit total phytoplankton production, suggests that it may influence carbon export in this and other polar ecosystems.

Ratio of nitrogen to phosphorus in the Pearl River and effects on the estuarine coastal waters: Nutrient management strategy in Hong Kong

The Pearl River is the second largest river in China, and has a 454,000 km 2 drainage basin. Excess nutrients can result in algal blooms, or even harmful algal blooms and subsequent dissolved oxygen (DO) consumption can lead to hypoxia. However, not all nutrients are equal; only one nutrient relative to other nutrients is the most limiting for algal biomass production and the other nutrients that are in excess cannot be used to produce a further increase in an algal bloom. Therefore, the strategy of nutrient pollution control is to remove the most limiting nutrient from the sewage effluent to minimize eutrophication impacts on the receiving waters. This, in turn, determines the type and level of sewage treatment. In the Pearl River, nitrogen (N) is very high and phosphorus (P) is relatively low, leading to a very high N:P ratio. The Pearl River flows into coastal waters in the South China Sea and heavily influences Hong Kong waters located to the east of the Pearl River estuary. When the Hong Kong government planned to upgrade the domestic sewage facility to biological treatment, this triggered the scientific question of which nutrient, N or P is the most limiting nutrient and the answer to this question became critical in making the management decision on the treatment facilities for removal of N or P, which bears a huge financial implication. In the past, because N is high in southern waters, it was thought that any addition of N would exceed the environmental assimilation capacity and result in algal blooms. Therefore, N has been typically considered for removal from sewage effluent. However, evidence revealed that P was the most limiting nutrient in the southern waters of Hong Kong and it actually limits phytoplankton biomass accumulation and potentially limits bacterial DO consumption. Hence, the removal of P has been suggested to receive priority over N removal, if there is a need for the future elevation of treatment levels. However, as this conclusion is reached with respect to the present condition of the Pearl River, it may be prudent to retain flexibility such that treatment levels can be adjusted to accommodate future changes in the estuarine conditions, in particular if changes in the nitrogen loads entering the Pearl River decrease and cause the N:P ratio to decrease.

Meso- and microzooplankton responses to an in situ iron fertilization experiment (SEEDS II) in the northwest subarctic Pacific

Abstract A mesoscale iron fertilization experiment was carried out in the western subarctic Pacific during summer 2004. The iron-patch was traced for 26 days after the enrichment, and the abundance and behavior of meso- and microzooplankton was compared with those outside of the patch. The surface chlorophyll-a concentration in the patch was high between days 10 and 13 (2.5 mg m−3) and decreased to the initial level after day 20. Microzooplankton grazing rates, estimated by a dilution method, was mostly balanced with phytoplankton growth rates throughout the observed period. Dominant mesozooplankton species in the upper 200 m were copepods: dominated by Eucalanus bungii, Neocalanus plumchrus and Metridia pacifica. Species composition did not change in the patch over the observation period. The copepod biomass was 3–5 times higher than in Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS), the previous iron-enrichment experiment in the same area, before the bloom, and exponentially increased both inside and outside the patch, which was mainly brought by the development of N. plumchrus. The development rates of N. plumchrus were not significantly different between inside and outside the patch. Estimated grazing rate suggest that the copepod grazing was main cause of the low accumulation of phytoplankton biomass, and dominance of grazing-resistant organisms such as large ciliates, large diatoms and diatoms with extremely long setae. “Arrested migration” for M. pacifica and upward shift of vertical distribution by E. bungii were observed during the bloom period, even if the accumulation of phytoplankton biomass was very low compared to other iron-enrichment experiments. These results indicate that the copepod grazing shaped the food-web structure of the lower trophic levels (biomass and species composition) in SEEDS II.

Base-line variations in stable isotope values in an Arctic marine ecosystem: effects of carbon and nitrogen uptake by phytoplankton

Stable isotope values are useful for elucidating C and N cycling and pathways in marine and aquatic ecosystems. Variations in the base-line isotope values, the δ13C and δ15N values of phytoplankton, put constraints on their usefulness as tracers for trophic interactions and sources of organic matter in food web studies, however. We investigated the C and N stable isotope values of suspended particulate organic matter in relation to uptake of total dissolved inorganic carbon and nitrate, chlorophyll a concentration and the isotope composition of dissolved inorganic carbon in an Arctic marine environment (northern Barents Sea) in order to improve the understanding of factors regulating the variation in stable isotope values at the base of the marine food web. The stable isotope values of water-column suspended particulate organic carbon (δ13Corg) and nitrogen (δ15Norg) varied from −28.3‰ to −20.2‰ and 2.9‰ to 8.3‰, respectively, among stations sampled during spring and summer. δ13Corg was not linearly related to carbon uptake, but the values were on average 3‰ higher at stations in a late-bloom stage, characterised by higher carbon uptake compared to early-bloom stations. Accumulation of phytoplankton biomass had a strong impact on δ13Corg values, reflected in a positive relationship between δ13Corg and chlorophyll a concentration. δ15Norg was positively related to the percentage of nitrate taken up from initial (winter) concentrations. These results indicate a strong relationship between bloom progression and isotope composition of particulate organic C and N pools. Synoptic data on stable isotope compositions, nutrient concentrations and phytoplankton biomass therefore improve the interpretation of isotope values when these are compared across pools with different turnover times, such as phytoplankton and consumers or suspended and sedimentary organic matter.

Spatial and temporal variations in nutrient and chlorophyll- a concentrations in the northern East China Sea surrounding Cheju Island

Nutrients, chlorophyll- a (Chl -a), and environmental conditions were extensively investigated in the northern East China Sea (ECS) near Cheju Island during five research cruises from 2003 to 2007. In the eastern part of the study area, surface waters were characterized only by the Tsushima Current Water (TCW) during all five cruises. However, the western surface waters changed with season and were characterized by the Yellow Sea Cold Water (YSCW) in spring, the Changjiang Diluted Water (CDW) in summer, and the Yellow Sea Mixed Water (YSMW) in autumn. In spring and autumn, relatively high concentrations of nitrate and phosphate were observed in the surface waters in the western part of the study area, where vertical mixing brought large supplies of nutrients from deep waters. Changes in wind direction occasionally varied the inflow of the Changjiang plume in summer, clearly causing the annual variation in surface nitrate and phosphate concentrations in summer. In summer, the surface distribution of nitrate and phosphate did not coincide with that of silicate in the study area, which probably resulted from the significant drop in the Si:N ratio in the Changjiang plume since construction of the Three Gorges Dam (TGD). Despite large temporal and spatial variations in surface Chl- a concentrations, depth-integrated Chl- a concentrations exhibited little variation temporally and spatially. In the study area, surface Chl- a concentration did not well reflect the standing stocks of phytoplankton. The vertical distribution of Chl- a showed large temporal and spatial variations, and the main factor controlling the vertical distribution of Chl- a in summer was the availability of nitrate. The thermohaline front may play an important role for accumulation of phytoplankton biomass in spring and autumn.

Interactions between nitrogen dynamics and the phytoplankton community in Lake George, Florida, USA

Piehler, M.F., J. Dyble, P.H. Moisander, A.D. Chapman, J. Hendrickson and H.W. Paerl. 2009. Interactions between nitrogen dynamics and the phytoplankton community in Lake George, Florida, USA. Lake Reserv. Manage. 25:1–14. Nutrient addition bioassays were conducted to examine the relationship between inorganic nutrient enrichment and phytoplankton community structure and function in Lake George in the St. Johns River System, Florida, USA. Additionally, a nitrogen budget for the lake was developed and included data from the period of this study. We identified the factors that affected cyanobacterial productivity, prevalence, and nitrogen (N2) fixation. The importance of N2 fixation to the nitrogen (N) budget of the system was also assessed. We hypothesized that N2 fixation significantly contributed to the Lake George N budget and that changing the nutrient conditions in manipulative experiments would affect rates of N2 fixation and composition of the phytoplankton, particularly the N2-fixing community. Phytoplankton primary productivity in Lake George was stimulated by the addition of both N and phosphorus (P). Phytoplankton biomass accumulation was most often enhanced by the combined addition of N and P; however, N alone was also often stimulatory. When detected, N2 fixation was always stimulated by P additions. Short-term changes in phytoplankton community composition included taxonomic shifts in response to nutrient manipulations. Fixation of N2 appeared to be a significant contributor to the N load to the lake. Human impacts that change the loading of N, P, or both N and P to Lake George may affect phytoplankton community structure (composition and biomass) and function (primary productivity and N2 fixation). These changes could have consequences for biogeochemical cycling in Lake George and potentially through the freshwater-marine continuum. Continuing nutrient management efforts in this and other similar systems must account for the activity of N2-fixing cyanobacteria, as products (carbon fixation) and drivers (sources of new nitrogen) of eutrophication.

Modelling the temperature and the phytoplankton distributions at the Aveiro near coastal zone, Portugal

The main objective of this paper is to implement a coupled three-dimensional physical and ecological model for the Aveiro coast, and to apply it to study the temperature and the phytoplankton biomass spatial distributions along the coastal ecosystem. The Aveiro coast is located at Portugal within the upwelling system of the Atlantic Iberian coast, characterized by nutrients availability and phytoplankton biomass accumulation, from April to October. In order to implement the ecological model, its validation was assessed by comparing simulations to data relative to the horizontal and vertical distributions of the temperature, nutrients and phytoplankton biomass, obtained during the CICLOS I survey off the Portuguese coast [Moita, M.T., 2001. Estrutura, Variabilidade e Dinâmica do Fitoplâncton na Costa de Portugal Continental. PhD Thesis. Faculdade de Ciências da Universidade de Lisboa, 272 pp.]. A sensitivity analysis of the model has been performed in order to assess the influence of the main ecological model variables. The simulation results show that the model is capable of predicting realistic the temperature, the nutrients and the chlorophyll-a distributions for the study area. The scenarios evidence the setup of a thermal stratification pattern resulting from the upwelling of deep and rich in nutrients water to the surface layer and a chlorophyll-a maxima extending offshore, along the picnocline and the nutricline. The results confirm the crucial role played by the physical processes in the phytoplankton bloom along the Aveiro coast. They also evidence the close link between the surface phytoplankton distribution and the surface temperature distribution.

Influence of a buoyant river plume on phytoplankton nutrient dynamics: What controls standing stocks and productivity?

The influence of the Columbia River plume on phytoplankton rates and biomass accumulation was examined using multiday deckboard incubations as part of the coastal ocean processes River Influences on Shelf Ecosystems field program in August 2005. At a set of five stations encompassing the near‐field plume (three stations) on the Oregon and Washington coasts, treatments consisting of control, added nitrate, and added nitrate with 0.45 μm filtered plume water were used to assess the phytoplankton community response to macronutrients and micronutrients. For a subset of these stations, nutrient (nitrate, ammonium, and ammonium inhibition of nitrate) kinetics were obtained, as well as carbon‐based estimates of productivity. For all experiments, nitrogen (nitrate) was clearly controlling both biomass accumulation and growth rates. Despite the apparent poleward trend toward increasing biomass in this region, there were no obvious differences in phytoplankton physiological capacity, nor were there any symptoms of iron limitation in the short term. We conclude that phytoplankton in this region are predominantly nitrogen limited but that upon release from this limiting factor, phosphorous and/or silicic acid (in waters not influenced by the Columbia River plume) would quickly become limiting. Evidence suggests that the mesoscale differences in phytoplankton biomass between the Oregon and Washington coasts result from a combination of enhanced grazing downstream and the physically retentive and dispersive effects of the plume itself.

The effects of seston food quality on planktonic food web patterns

In planktonic food webs, the conversion rate of plant material to herbivore biomass is determined by a variety of factors such as seston biochemical/elemental composition, phytoplankton cell morphology, and colony architecture. Despite the overwhelming heterogeneity characterizing the plant–animal interface, plankton population models usually misrepresent the food quality constraints imposed on zooplankton growth. In this study, we reformulate the zooplankton grazing term to include seston food quality effects on zooplankton assimilation efficiency and examine its ramifications on system stability. Using different phytoplankton parameterizations with regards to growth strategies, light requirements, sinking rates, and food quality, we examined the dynamics induced in planktonic systems under varying zooplankton mortality/fish predation, light conditions, nutrient availability, and detritus food quality levels. In general, our analysis suggests that high food quality tends to stabilize the planktonic systems, whereas unforced oscillations (limit cycles) emerge with lower seston food quality. For a given phytoplankton specification and resource availability, the amplitude of the plankton oscillations is primarily modulated from zooplankton mortality and secondarily from the nutritional quality of the alternative food source (i.e., detritus). When the phytoplankton community is parameterized as a cyanobacterium-like species, conditions of high nutrient availability combined with high zooplankton mortality led to phytoplankton biomass accumulation, whereas a diatom-like parameterization resulted in relatively low phytoplankton to zooplankton biomass ratios highlighting the notion that high phytoplankton food quality allows the zooplankton community to sustain relatively high biomass and to suppress phytoplankton biomass to low levels. During nutrient and light enrichment conditions, both phytoplankton and detritus food quality determine the extent of the limit cycle region, whereas high algal food quality increases system resilience by shifting the oscillatory region towards lower light attenuation levels. Detritus food quality seems to regulate the amplitude of the dynamic oscillations following enrichment, when algal food quality is low. These results highlight the profitability of the alternative food sources for the grazer as an important predictor for the dynamic behavior of primary producer–grazer interactions in nature.

Shallow water processes govern system-wide phytoplankton bloom dynamics: A modeling study

A pseudo-two-dimensional numerical model of estuarine phytoplankton growth and consumption, vertical turbulent mixing, and idealized cross-estuary transport was developed and applied to South San Francisco Bay. This estuary has two bathymetrically distinct habitat types (deep channel, shallow shoal) and associated differences in local net rates of phytoplankton growth and consumption, as well as differences in the water column's tendency to stratify. Because many physical and biological time scales relevant to algal population dynamics decrease with decreasing depth, process rates can be especially fast in the shallow water. We used the model to explore the potential significance of hydrodynamic connectivity between a channel and shoal and whether lateral transport can allow physical or biological processes (e.g. stratification, benthic grazing, light attenuation) in one sub-region to control phytoplankton biomass and bloom development in the adjacent sub-region. Model results for South San Francisco Bay suggest that lateral transport from a productive shoal can result in phytoplankton biomass accumulation in an adjacent deep, unproductive channel. The model further suggests that turbidity and benthic grazing in the shoal can control the occurrence of a bloom system-wide; whereas, turbidity, benthic grazing, and vertical density stratification in the channel are likely to only control local bloom occurrence or modify system-wide bloom magnitude. Measurements from a related field program are generally consistent with model-derived conclusions.

Biophysical responses of the upper ocean to major Gulf of Mexico hurricanes in 2005

The biophysical responses of the upper ocean to three major Gulf of Mexico (GoM) hurricanes in 2005 were analyzed using satellite observations. The degree and orientation of the responses exhibited by Hurricanes Katrina, Rita, and Wilma were greatly affected by the oceanic processes that occur within the GoM, as well as the translation speed of each hurricane. Maximum sea surface temperature change was 6–7°C, 4–5°C, and 5–6°C; whereas, chlorophyll‐a (chl‐a) enhancement was 3 mg m−3, 2 mg m−3, and 4 mg m−3, respectively. Areas of chl‐a enhancement (i.e., increases in phytoplankton biomass accumulation at the ocean's surface) were observed days after passage of each hurricane. Comparison of thermocline displacement estimates and depth approximations of the nitracline and deep chl‐a maximum suggests that the increases in surface phytoplankton biomass associated with Hurricanes Katrina and Rita were the result of new production from nutrient influx, as well as entrainment of phytoplankton from the deep chlorophyll maximum, while chl‐a enhancement linked with Hurricane Wilma appears to be attributable to the latter.

Nutrient Bioassays of Plankton Biomass and Metabolism in an Urbanized Drowned River-mouth Lake (Mona Lake, Michigan)

ABSTRACT A variety of short-term responses were observed in inorganic nutrient enrichment bioassays on the plankton of Mona Lake—an urbanized and eutrophic drowned river-mouth lake that drains directly to Lake Michigan (USA). Chlorophyll a and phytoplankton production data indicated phosphorus (P) limitation in spring, nitrogen (N) and P co-limitation in summer, and no N or P limitation in autumn. Heterotrophic bacterial abundance, dissolved organic carbon levels, and plankton respiration were not stimulated by nutrient enrichment. Autotrophic biomass and production, but not heterotrophic biomass or respiration, were seasonally stimulated by the availability of P and/or N. Such imbalance between autotrophic and heterotrophic processes may contribute to the accumulation of phytoplankton biomass during the growing season. Cyanobacterial abundance in the lake increased from 22% of the plankton biomass in the spring when the TN:TP ratio was high to 68% in the autumn when the TN:TP ratio was low, suggesting that P availability helps drive a seasonal succession towards cyanobacterial dominance.

Variations of the abundance and nucleic acid content of heterotrophic bacteria in Beaufort Shelf waters during winter and spring

Depth profiles of heterotrophic bacteria abundance were measured weekly over a 6-month period from December to May in Franklin Bay, a 230 m-deep coastal Arctic Ocean site of the southeastern Beaufort Sea. Total bacteria, low nucleic acid (LNA) and high nucleic acid (HNA) bacteria abundances were measured using flow cytometry after SYBR Green I staining. The HNA bacteria abundance in surface waters started to increase 5–6 weeks after phytoplankton growth resumed in spring, increasing from 1 × 10 5 to 3 × 10 5 cells mL − 1 over an 8-week period, with a net growth rate of 0.018 d − 1 . LNA bacteria response was delayed by more than two months relative to the beginning of the phytoplankton biomass accumulation and had a lower net growth rate of 0.013 d − 1 . The marked increase in bacterial abundance occurred before any significant increase in organic matter input from river discharge (as indicated by the unchanged surface water salinity and DOC concentrations), and in the absence of water temperature increase. The abundance of bacteria below the halocline was relatively high until January (up to 5 × 10 5 cells mL − 1 ) but then decreased to values close to 2 × 10 5 cells mL − 1 . The three-fold bacterial abundance increase observed in surface waters in spring was mostly due to HNA bacteria, supporting the idea that these cells are the most active.

Evidence for the grazing hypothesis: Grazing reduces phytoplankton responses of the HNLC ecosystem to iron enrichment in the western subarctic pacific (SEEDS II)

A mesoscale iron-enrichment study (SEEDS II) was carried out in the western subarctic Pacific in the summer of 2004. The iron patch was traced for 26 days, which included observations of the development and the decline of the bloom by mapping with sulfur hexafluoride. The experiment was conducted at almost the same location and the same season as SEEDS (previous iron-enrichment experiment). However, the results were very different between SEEDS and SEEDS II. A high accumulation of phytoplankton biomass (∼18 mg chl m−3) was characteristic of SEEDS. In contrast, in SEEDS II, the surface chlorophyll-a accumulation was lower, 0.8 to 2.48 mg m−3, with no prominent diatom bloom. Photosynthetic competence in terms of Fv/Fm for the total phytoplankton community in the surface waters increased after the iron enrichments and returned to the ambient level by day 20. These results suggest that the photosynthetic physiology of the phytoplankton assemblage was improved by the iron enrichments and returned to an iron-stressed condition during the declining phase of the bloom. Pico-phytoplankton (<2 µm) became dominant in the chlorophyll-a size distribution after the bloom. We observed a nitrate drawdown of 3.8 µM in the patch (day 21), but there was no difference in silicic acid concentration between inside and outside the patch. Mesozooplankton (copepod) biomass was three to five times higher during the bloom-development phase in SEEDS II than in SEEDS. The copepod biomass increased exponentially. The grazing rate estimation indicates that the copepod grazing prevented the formation of an extensive diatom bloom, which was observed in SEEDS, and led to the change to a pico-phytoplankton dominated community towards the end of the experiment.

Factors controlling the seasonal development and distribution of the phytoplankton community in the lowland course of a large river in Northern Italy (River Adige)

The principal environmental factors influencing the seasonal dynamics of phytoplankton were examined from September 1997 to July 1998 in three stations along a 26-km stretch of the lowland course of River Adige (northeast Italy). Nutrient concentrations did not appear to be limiting for the phytoplankton growth. Annual minimum concentrations of reactive and total phosphorus, and dissolved inorganic nitrogen were 22 μg P l−1, 63 μg P l−1 and 0.9 mg N l−1, respectively. The most critical forcing factors were physical variables, mainly water discharge and other variables related to hydrology, i.e. suspended solids and turbidity, which acted negatively and synchronously by diluting phytoplankton cells and decreasing light availability. Higher algal biomass was recorded in early spring, in conditions of lower flow velocity and increasing water temperature. In late spring and summer, higher water discharge caused a decrease in phytoplankton biomass. Conversely, low algal biomass in late autumn and winter, during low discharge, was mainly related to low water temperatures and shorter photoperiod. Physical constraints had a significant and measurable effect not only on the development of total biomass, but also on the temporal dynamics of the phytoplankton community. Abiotic and biotic variables showed a comparable temporal development in the three sampling stations. The small number of instances of spatial differences in phytoplankton abundance during the period of lower flow velocity were related to the increasing importance of biological processes and accumulation of phytoplankton biomass.

Iron–light interactions during the CROZet natural iron bloom and EXport experiment (CROZEX) I: Phytoplankton growth and photophysiology

The CROZet natural iron bloom and EXport experiment (CROZEX) investigated phytoplankton blooms in the vicinity of the Crozet Plateau in the polar frontal zone (PFZ) of the Southern Ocean. Peak chlorophyll concentrations reached during an intense bloom within naturally iron (Fe)-fertilised regions north of the plateau were an order of magnitude higher than those observed in deeper mixed layers and low-Fe waters to the south. To establish the factors influencing phytoplankton dynamics, a suite of in situ phytoplankton physiological measurements and shipboard Fe–light perturbation experiments was performed. Addition of Fe in experiments performed during bloom decline north of the plateau resulted in increased accumulation of phytoplankton biomass and changes in a number of phytoplankton physiological characteristics. In particular photosystem II (PSII) photochemical efficiencies (Fv/Fm) measured by fast repetition rate fluorometry increased above in situ values within 24 h of Fe amendment, suggesting that Fe stress had contributed to bloom termination. In contrast, responses to Fe amendment were minor within an experiment initiated in low-silicic acid, post-bloom waters south of the Plateau. Within the intense bloom in the north, light limitation due to self-shading may have constrained the peak phytoplankton standing stock. However, in the absence of Fe amendment, incubation at higher than in situ irradiance levels had little influence on phytoplankton biomass accumulation for declining bloom populations. Instead reduced Fv/Fm, reflecting increased photoinhibitory damage to PSII, was observed in high-light incubations and was also apparent in situ. Interactions between Fe and light availability thus influenced phytoplankton physiology and growth and potentially contributed to bloom longevity during CROZEX.