Effect of natural iron fertilization on carbon sequestration in the Southern Ocean.

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial-interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization--an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below--as invoked in some palaeoclimatic and future climate change scenarios--may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

Temporal changes in phytoplankton biomass and cellular properties; implications for the IMO ballast water convention.

In the Wadden Sea, the Netherlands, and at L4 in the English Channel, UK, the size class distribution of phytoplankton was investigated with respect to the size range >10-≤50 µm identified by the IMO Ballast Water Convention. Size fractionation using 10 µm mesh filtration showed considerable size bias; 23.1% of >10 µm cells were still present in the <10 µm, but 21.8% of the smaller size cells were also retained on the mesh, resulting in an overestimated number of cells/mL by as much as a factor of 5.4. Flowcytometry measurements indicated that the phytoplankton in the size range 2-50  µm was dominated by the smaller size (<10 µm) at both sites. For the >10-≤50 µm size, these were on average 3.6% and 2% in the Wadden Sea and at L4, respectively. In terms of chlorophyll biomass, they represented 28.7% and 12%, respectively. The filtration method resulted in much higher chlorophyll values for 10-50  µm size range: 53.7% in the Wadden Sea and 38% at L4. This overestimation appears to be caused by cells in 6-10  µm size range being retained on the mesh. These findings are relevant in the context of the size class distribution based on flowcytometry and semi-quantification using chlorophyll as proxy for cell density.

Different approaches and limitations for testing phytoplankton viability in natural assemblies and treated ballast water.

Shipping is recognised as an unintentional efficient pathway for spreading non-native species, harmful organisms and pathogens. In 2004, a unique IMO Convention was adopted to control and minimize this transfer in ship's ballast water. This Convention entered into force on 8th September 2017. However, unlikely the majority of IMO Conventions, the Ballast Water Management Convention requires ships to comply with biological standards (e.g. concentration of organisms per unit of volume in ballast water discharges). This study aimed to apply different techniques developed to measure concentrations of viable phytoplankton in natural and treated ballast water samples and compare them with the established flow cytometry method and vital staining microscopy. Samples were collected in the English Channel over one year and on-board during ballast water shipboard efficacy tests. Natural abundance of live phytoplankton varied from 23% to 89% of the total, while for cells larger than 10 µm (a size defined by the BWM Convention) the percentage varied from 3% to 60%. An overall good correlation was seen between the measurements taken with the two fluorometers and in comparison with the flow cytometry analysis, as found in previous studies. Analysis of treated ballast water samples showed a large variation in the number of viable cells, however indicating a low level of risk on all occasions for regulatory purposes. One of the key aspects to bear in mind when sampling and analysing for compliance is to be aware of the limitations of each technique.

Variations in spatial and temporal distribution of Archaea in the North Sea in relation to environmental variables.

The spatial and temporal distribution of pelagic Archaea was studied in the southern North Sea by rRNA hybridization, sequencing and quantification of 16S rRNA gene and membrane lipid analyses and related to physical, chemical and biological parameters to determine the factors influencing archaeal biogeography. A clear temporal variability was observed, with marine Crenarchaeota (Group I.1a) being relatively more abundant in winter and Euryarchaeota dominating the archaeal assemblage in spring and summer. Spatial differences in the lateral distribution of Crenarchaeota were also evident. In fact, their abundance was positively correlated with the copy number of the gene encoding the alpha subunit of crenarchaeotal ammonia monooxygenase (amoA) and with concentrations of ammonia, nitrate, nitrite and phosphorus. This suggests that most Crenarchaeota in the North Sea are nitrifiers and that their distribution is determined by nutrient concentrations. However, Crenarchaeota were not abundant when larger phytoplankton (>3 microm) dominated the algal population. It is hypothesized that together with nutrient concentration, phytoplankton biomass and community structure can predict crenarchaeotal abundance in the southern North Sea. Euryarchaeotal abundance was positively correlated with chlorophyll a concentrations, but not with phytoplankton community structure. Whether this is related to the potential of Euryarchaeota to perform aerobic anoxygenic phototrophy remains to be shown, but the conspicuous seasonal distribution pattern of Crenarchaeota and Euryarchaeota suggests that they occupy a different ecological niche.