The response of phytoplankton to iron enrichment in Sub-Antarctic HNLCLSi waters: Results from the SAGE experiment

Areas of high nutrients and low chlorophyll a comprise nearly a third of the world’s oceans, including the equatorial Pacific, the Southern Ocean and the Sub-Arctic Pacific. The SOLAS Sea-Air Gas Exchange (SAGE) experiment was conducted in late summer, 2004, off the east coast of the South Island of New Zealand. The objective was to assess the response of phytoplankton in waters with low iron and silicic acid concentrations to iron enrichment. We monitored the quantum yield of photochemistry (F_v/F_m) with pulse amplitude modulated fluorometry, chlorophyll a, primary productivity, and taxonomic composition. Measurements of F_v/F_m indicated that the phytoplankton within the amended patch were relieved from iron stress (F_v/F_m approached 0.65). Although there was no significant difference between IN and OUT stations at points during the experiment, the eventual enhancement in chlorophyll a and primary productivity was twofold by the end of the 15-day patch occupation. However, no change in particulate carbon or nitrogen pools was detected. Enhancement in primary productivity and chlorophyll a were approximately equal for all phytoplankton size classes, resulting in a stable phytoplankton size distribution. Initial seed stocks of diatoms were extremely low, <1% of the assemblage based on HPLC pigment analysis, and did not respond to iron enrichment. The most dominant groups before and after iron enrichment were type 8 haptophytes and prasinophytes that were associated with ∼75% of chlorophyll a. Twofold enhancement of biomass estimated by flow cytometry was detected only in eukaryotic picoplankton, likely prasinophytes, type 8 haptophytes and/or pelagophytes. These results suggest that factors other than iron, such as silicic acid, light or physical disturbance limited the phytoplankton assemblage during the SAGE experiment. Furthermore, these results suggest that additional iron supply to the Sub-Antarctic under similar seasonal conditions and seed stock will most likely favor phytoplankton <2 ??m. This implies that any iron-mediated gain of fixed carbon will most likely be remineralized in shallow water rather than sink and be sequestered in the deep ocean.     

Bacterioplankton responses to iron enrichment during the SAGE experiment

We studied the microbial food web in the upper 100 m of the water column in iron-limited sub-Antarctic HNLC waters south-east of New Zealand in the SAGE experiment in 2004, with focus on bacterioplankton. Samples were collected daily from inside and outside the iron enriched patch. Short term enrichment experiments were conducted on board in 4 L polycarbonate bottles with water outside the iron enriched patch to study single and combined effects of micronutrient additions on microbial food web. Low bacterial growth was recorded in the study area with community turnover times of 50 h or more during the study period. Measurements of bacterial standing stocks and production rates in the study show minor responses to the large scale iron enrichment, with increase in rates and stocks after the first enrichment and at the end of the study period after the third iron enrichment when solar radiation increased and wind mixing decreased. The average daily bacterial production rates were 31.5 and 33.7 mgCm⁻² d⁻¹ for the OUT and IN stations, respectively; thus overall there was not a significant difference between the control and the iron-enriched patch. In the bottle experiments bacterial thymidine incorporation showed responses to single iron and silicic acid enrichments and a major growth response to the combined iron and sucrose enrichments. Phytoplankton chlorophyll-a showed clear stimulation by single additions of iron and silicic acid and silicic acid enhanced the iron impact. Cobalt additions had no effect on bacteria growth and a negative effect on phytoplankton growth. Low bacterial in situ growth rates and the enrichment experiments suggest that bacteria are co-limited by iron and carbon, and that bacterial iron uptake is dependent on carbon supply by the food web. With the high iron quota (??mol Fe mol C⁻¹) bacteria may scavenge considerable amounts of the excess iron, and thus influence the relative importance of the microbial food web as a carbon sink.     

Processes governing the carbon chemistry during the SAGE experiment

Measurements of pCO₂, pH and alkalinity in the surface waters of an iron fertilised patch of sub-Antarctic water were made during SAGE (SOLAS SAGE: Surface-Ocean Lower Atmosphere Studies Air-Sea Gas Experiment). The iron addition induced a minor phytoplankton bloom, however the patch dynamics were dominated by physical processes which suppressed and masked the biological effects. The Lagrangian nature of the experiment allowed the carbonate chemistry in the patch to be followed for 15.5 days, and the relative importance of the biological and physical factors influencing the surface water pCO₂ was estimated. The pCO₂ of the surface waters of the patch increased from 327 ??atm prior to iron addition to 338 ??atm on Day 14, effects of vertical and horizontal mixing offset the 15 ??atm drawdown that would have occurred had the induced biological uptake been the sole factor to influence the pCO₂. The air-sea carbon flux calculated using the measured skin temperature and a piston velocity parameterisation determined during SAGE (Ho et al., 2006) was 98.5% of the flux determined using conventional bulk temperature measurement and the Wanninkhof (1992) piston velocity parameterisation. The skin temperature alone contributed to an 8% increase in the flux compared with that determined using bulk temperature.     

Did dilution limit the phytoplankton response to iron addition in HNLCLSi sub-Antarctic waters during the SAGE experiment?

An in situ iron addition experiment (SAGE) was carried out in high-nitrate low-chlorophyll low-silicic acid (HNLCLSi) sub-Antarctic surface waters south-east of New Zealand. In contrast to other iron addition experiments, the phytoplankton response was minor, with a doubling of biomass relative to surrounding waters, with the temporal trends in dissolved iron and macronutrients instead dominated by physical factors such as mixing and dilution. The initial increase in patch surface area indicated a lateral dilution rate of 0.125 d⁻¹, with a second estimate from a model of the decline in peak SF₆ concentration yielding a higher lateral dilution rate of 0.16-0.25 d⁻¹. The model was tested on the SOIREE SF₆ dataset and provided a lateral dilution of 0.07 d⁻¹, consistent with previous published estimates. MODIS ocean colour images showed elevated chlorophyll coincident with the SF₆ patch on day 10 and 12, and an elevated chlorophyll filament at the SAGE experiment location 3-4 days after ship departure, which provided additional lateral dilution estimates of 0.19 and 0.128 d⁻¹. Dissolved iron at the patch centre declined by 85% within two days of the initial infusion, of which dilution accounted for 50-65%; it also decreased rapidly after the 2nd and 3rd infusions but remained elevated after the fourth infusion. Despite decreases in nitrate and silicic acid from day 7 and 10, respectively, the final nutrient concentrations in the patch exceeded the initial concentrations due to supply from lateral intrusion and mixed-layer deepening. The low Si:N loss ratio suggested that the observed limited response to iron was primarily by non-siliceous phytoplankton. Algal growth rate exceeded the minimum dilution rate during two periods (days 3-6 and 10-14), and coincided with net chlorophyll accumulation. However, as the ratio of algal growth to dilution was the lowest reported for an iron addition experiment, dilution was clearly a significant factor in the SAGE experiment recording the lowest phytoplankton response to mesoscale iron addition.     

Distribution and contribution of major phytoplankton groups to carbon cycling across contrasting conditions of the subtropical northeast Atlantic Ocean

The relation between trophic regime and phytoplankton composition and function in oceanic systems is well accepted in oceanography. However, the relative dynamics and carbon cycling contributions of different phytoplankton groups across gradients of ocean richness are not fully understood. In this work we investigated phytoplankton dynamics along two transects from the NW African coastal upwelling to open-ocean waters of the north Atlantic subtropical gyre. We adopted a pigment-based approach to characterize community structure and to quantify group-specific growth and grazing rates and associated carbon fluxes. Changes in pigment cell concentration during the incubation experiments due to photoadaptation were corrected to obtain reliable rates. The oceanic region was dominated by Prochlorococcus (PRO) (45±7% of total chlorophyll a) while diatoms dominated in upwelling waters (40±37%). Phytoplankton grew faster (μ=0.78±0.26 d⁻¹) and free of nutrient limitation (μ/μ_n=0.98±0.42) in the coastal upwelling region, with all groups growing at similar rates. In oceanic waters, the growth rate of bulk phytoplankton was lower (μ=0.52±0.16 d⁻¹) and nutrient limited (μ/μ_n=0.68±0.19 d⁻¹). Diatoms (0.80±0.39 d⁻¹) and Synechococcus (SYN) (0.72±0.25 d⁻¹) grew faster than Prymnesiophyceae (PRYMN) (0.62±0.26 d⁻¹) and PRO (0.46±0.18 d⁻¹). The growth rates of PRO and SYN were moderately nutrient limited (μ/μ_n=0.81 and 0.91, respectively), while the limitation for diatoms (μ/μ_n=0.71) and PRYMN (μ/μ_n=0.37) was more severe. Microzooplankton grazing rate was higher in upwelling (0.68±0.32 d⁻¹) than in oceanic waters (0.37±0.19 d⁻¹), but represented the main loss pathway for phytoplankton in both systems (m/μ=0.90±0.32 and 0.69±0.24, respectively). Carbon flux through phytoplankton, produced and grazed, increased from offshore to coastal (∼2 to ∼200 μg C L⁻¹ d⁻¹), with diatoms dominating the flux in the upwelling region (52%) while PRYMN (40%) and PRO (30%) dominated in the open ocean.     

Top-down control of spring surface phytoplankton blooms by microzooplankton in the Central Yellow Sea,China

Phytoplankton growth and microzooplankton grazing were studied during the 2007 spring bloom in Central Yellow Sea. The surveyed stations were divided to pre-bloom phase (Chl a concentration less than 2 μg L⁻¹), and bloom phase (Chl a concentration greater than 2 μg L⁻¹). Shipboard dilution incubation experiments were carried out at 19 stations to determine the phytoplankton specific growth rates and the specific grazing rates of microzooplankton on phytoplankton. Diatoms dominated in the phytoplankton community in surface waters at most stations. For microzooplankton, Myrionecta rubra and tintinnids were dominant, and heterotrophic dinoflagellate was also important in the community. Phytoplankton-specific growth rates, with an average of 0.60±0.19 d⁻¹, were higher at pre-bloom stations (average 0.62±0.17 d⁻¹), and lower at the bloom stations (average 0.59±0.21 d⁻¹), but the difference of growth rates between bloom and pre-bloom stations was not statistically significant (t test, p=0.77). The phytoplankton mortality rate by microzooplankton grazing averaged 0.41±0.23 d⁻¹ at pre-bloom stations, and 0.58±0.31 d⁻¹ during the blooms. In contrast to the growth rates, the statistic difference of grazing rates between bloom and pre-bloom stations was significant (after removal of outliers, t test, p=0.04), indicating the importance of the top-down control in the phytoplankton bloom processes. Average potential grazing efficiency on primary productivity was 66% at pre-bloom stations and 98% at bloom stations, respectively. Based on our results, the biomass maximum phase (bloom phase) was not the maximum growth rate phase. Both phytoplankton specific growth rate and net growth rate were higher in the pre-bloom phase than during the bloom phase. Microzooplankton grazing mortality rate was positively correlated with phytoplankton growth rate during both phases, but growth and grazing were highly coupled during the booming phase. There was no correlation between phytoplankton growth rate and cell size during the blooms, but they were positive correlated during the pre-bloom phase. Our results indicate that microzooplankton grazing is an important process controlling the growth of phytoplankton in spring bloom period in the Central Yellow Sea, particularly in the “blooming” phase.     

Enrichment of alkanes within a phytoplankton bloom during an in situ iron enrichment experiment in the western subarctic Pacific

During the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study ΙΙ (SEEDS-II), we monitored variations in the concentrations of non-methane hydrocarbons (NMHCs), CH₃Cl, N₂O, and CH₄ within a phytoplankton bloom. Stable isotopic compositions were also determined to evaluate the sources of the variations. Although there was little variation in either the concentrations or the stable isotopic compositions of alkenes, CH₃Cl, N₂O, and CH₄ during the 23-day observation period, alkane concentrations increased substantially as the phytoplankton bloomed. The column-integrated quantities of alkanes increased to 3 times pre-bloom levels for C₂H₆, 5 times for C₃H₈, and 20 times for n-C₄H₁₀. The δ¹³C values of both C₂H₆ and C₃H₈ remained almost constant while concentrations increased, whereas that of n-C₄H₁₀ increased by about 12‰. To evaluate the sources of the alkanes produced during the bloom, we compared their δ¹³C values with those of alkanes produced in axenic phytoplankton cultures in our laboratory. We concluded that during the SEEDS-ΙΙ experiment the major portions of C₂H₆ and C₃H₈ were produced during the autolysis of diatoms cells, whereas n-C₄H₁₀ was produced during autolysis of other phytoplankton cells such as cryptophytes and dinoflagellates.     

Community Structure and Dynamics of Phytoplankton in the Western Subarctic Pacific Ocean: A Synthesis

The phytoplankton community in the western subarctic Pacific (WSP) is composed mostly of pico- and nanophytoplankton. Chlorophyll a (Chl a) in the <2 μm size fraction accounted for more than half of the total Chl a in all seasons, with higher contributions of up to 75% of the total Chl a in summer and fall. The exception is the western boundary along the Kamchatka Peninsula and Kuril Islands and the Oyashio region where diatoms make up the majority of total Chl a during the spring bloom. Among the picophytoplankton, picoeukaryotes and Synechococcus are approximately equally abundant, but the former is more important in term of carbon biomass. Despite the lack of a clear seasonal variation in Chl a concentration, primary productivity showed a large seasonal variation, and was lowest in winter and highest in spring. Seasonal succession in the phytoplankton community is also evident with the abundance of diatoms peaking in May, followed by picoeukaryotes and Synechococcus in summer. The growth of phytoplankton (especially >10 μm cell size) in the western subarctic Pacific is often limited by iron bioavailability, and microzooplankton grazing keeps the standing stock of pico- and nano-phytoplankton low. Compared to the other HNLC regions (the eastern equatorial Pacific, the Southern Ocean, and the eastern subarctic Pacific), iron limitation in the Western Subarctic Gyre (WSG) may be less severe probably due to higher iron concentrations. The Oyashio region has similar physical condition, macronutrient supply and phytoplankton species compositions to the WSG, but much higher phytoplankton biomass and primary productivity. The difference between the Oyashio region and the WSG is also believed to be the results of difference in iron bioavailability in both regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Single particle analysis of oceanic suspended matter during the SEEDS II iron fertilization experiment

Suspended particles collected from surface seawater during the SEEDS II (Subarctic Iron Experiment for Ecosystem and Dynamics Study II) experiment were analyzed individually using an electron probe X-ray micro analyzer and characterized by size and elemental composition. Their numbers, relative abundances, and relative particle volume all showed clear differences between samples collected inside vs. outside the phytoplankton bloom that developed following the addition of iron. Throughout the study, Si-rich, Ca-rich and Organic particles were dominant and their number increased inside the fertilized patch; these particles accounted for 21%, 13% and 58% of the particles examined, respectively. Si-rich or Ca-rich particles commonly consisted of fragments of diatom frustules and coccolithophorids. There was consistently greater percentage of Ca-rich particles and lower percentage of Si-rich particles inside the patch than outside of it in number, but both types of these particles apparently occupied a larger volume inside the patch than outside of it. Organic particles, that showed having peaks in smaller diameter particles, increased apparently inside the patch with time after iron fertilization. The Organic particles had a more diverse mixture of both bio-related and crustal trace elements than the other types of particles. These results suggest that the increase in suspended particles following the iron enrichment was due to a combination of detrital material and live phytoplankton.