Mark Gall

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

Role of iron, light, and silicate in controlling algal biomass in subantarctic waters SE of New Zealand

Phytoplankton processes in subantarctic (SA) waters southeast of New Zealand were studied during austral autumn and spring 1997. Chlorophyll a (0.2–0.3 μg L−1) and primary production (350–650 mg C m−2 d−1) were dominated by cells 1 nmol kg−1, there was little evidence of Fe-stressed algal populations, and Fυ/Fm approached 0.60 at the STC. In addition to these trends, waters of SA origin were occasionally observed within the STC and north of the STC, and thus survey data were interpreted with caution. In vitro Fe enrichment incubations in SA waters resulted in a switch from flavodoxin expression to that of ferredoxin, indicating the alleviation of Fe stress. In another 6-day experiment, iron-mediated increases in chlorophyll a (in particular, increases in large diatoms) were of similar magnitude to those observed in a concurrent Si/Fe enrichment; ambient silicate levels were 4 μM. A concurrent in vitro Fe enrichment, at irradiance levels comparable to the calculated mean levels experienced by cells in situ, resulted in relatively small increases (approximately twofold) in chlorophyll a. Thus, in spring, irradiance and Fe may both control diatom growth. In contrast, during summer, as mean irradiance increases and silicate levels decrease, Fe limitation, Fe/Si colimitation, or silicate limitation may determine diatom growth.

Size‐fractionated phytoplankton standing stocks and primary production during austral winter and spring 1993 in the Subtropical Convergence region near New Zealand

Abstract Size‐fractionated phytoplankton standing stocks and potential primary production (PPP) off the west and east coasts of South Island, New Zealand, were evaluated in austral winter and spring 1993. These are the first size‐fractionated primary production data to be reported from the southern Subtropical Convergence (STC) and in oceanic New Zealand waters. Picophytoplankton ( 30% of integrated chlorophyll a and daily PPP in most water types and seasons, except when the 20–200 μm size class dominated in west coast waters in spring and in the STC in winter and spring. In subtropical and STC waters, PPP was 30% higher than at similar latitudes in the North Atlantic Ocean. Interpretation of whether or not there was accumulation of chlorophyll a in the various water types and seasons depends on microzooplankton grazing, because mesozooplankton grazing and the proportion of chlorophyllous particulate matter sinking out of the upper water column was low. In the STC in spring, phytoplankton ...

Phytoplankton assemblages and photosynthetic pigments during winter and spring in the Subtropical Convergence region near New Zealand

The distribution of phytoplankton on both the east, in the region near the Subtropical Con- vergence (STC), and the west coast of South Island, New Zealand was investigated in winter and spring 1993. Water samples were analysed for the cell car- bon biomass, species composition, and pigment characteristics. In both winter and spring, the over- all biomass/standing stocks consistently were great- est in the STC, intermediate in the subtropical (ST) both on the west (ST(W)) and east (ST(E)) coasts, and the least in the subantarctic (SA) waters. In spring, organic cell carbon and chlorophyll a meas- ured in the STC were up to 6-fold greater than in winter. In the STC in both seasons, phytoplankton assemblages were dominated by diatoms, and these species were more similar to those found in ST than in SA waters. The domination of diatoms in phytoplankton assemblages in the STC is consistent with the predominance of fucoxanthin observed in this region. The highest diatom concentrations (both in terms of cell carbon and chlorophyll a) detected during the spring bloom in the STC were located either at 10 m below the surface, or below the euphotic zone (close to the bottom of mixed layer

Validation of SeaWiFS data from around New Zealand

Abstract The near-surface concentration of chlorophyll- a ( C a ) was measured during 11 research voyages around New Zealand between 1998 and 2000. Over 400 measurements of spectral normalised water-leaving radiance were made using a high precision profiling spectroradiometer. The data were used to assess the quality of remotely sensed measurements of C a by the NASA ocean colour sensor, SeaWiFS, in New Zealand waters. The results in the open-ocean regions are consistent with a combination of negative biases in the C a algorithm of ca. 20% (some regional dependency), and positive biases due to under-estimation of water reflectance towards the blue end of the spectrum by SeaWiFS.

Pigment fluxes from the Subtropical Convergence region, east of New Zealand: Relationships to planktonic community structure

Abstract A preliminary sediment trap study using photo synthetic pigments as biomarker tracers of pelagic food web processes was conducted in three different water types (subantarctic, Subtropical Convergence, and subtropical), east of New Zealand, in winter and spring 1993. The presence of undegraded pigments in trap samples from water depths of 100–550 m indicate that phytoplankton cells were removed rapidly from surface waters, presumably mainly as sinking intact phytoplankton chains, marine aggregates, or as unconverted pigments in zooplankton waste products. Average pigment budget estimates indicate that <1% day‐1 of chlorophyll a standing stock and <4% of primary production were exported during winter and spring. Microzooplankton grazing (4–92% day‐1 chlorophyll a standing stock and 20–194% primary production) was potentially the most important process affecting particle retention in the upper water column and hence pigment fluxes across all three water types. Bacterial degradation, mesozooplankton ...

Chattonella globosa is a member of Dictyochophyceae: reassignment to Vicicitus gen. nov., based on molecular phylogeny, pigment composition, morphology and life history

Chang F.H., McVeagh M., Gall M. and Smith P. 2012. Chattonella globosa is a member of Dictyochophyceae: reassignment to Vicicitus gen. nov., based on molecular phylogeny, pigment composition, morphology and life history. Phycologia 51: 403–420. DOI: 10.2216/10-104.1 Chattonella globosa is a heterokont flagellate that has been previously assigned to the Class Raphidophyceae. Phylogenetic analyses of the nuclear encoded SSU, partial LSU, ITS1 rDNA and mitochondrial encoded COI clearly indicated that it is closely related to Dictyocha speculum and Dictyocha octonaria, Order Dictyochales, in the Class Dictyochophyceae. Moreover, High Performance Liquid Chromatography (HPLC) pigment analysis of Chattonella globosa revealed the presence of two acyl-oxyfucoxanthin derivatives, 19′-butanoyl-oxyfucoxanthin and 19′-hexanoyl-oxyfucoxanthin, pigments not found in members of the Raphidophyceae. In the late exponential growth phase a large number of smaller, less pigmented cells of Chattonella globosa, which acted like gametes, fused to give rise to a large multi-nucleate cell. At times two or more of these cells fused further to form a ‘massive’, plasmodium-like aggregate (up to 500 µm long). Chattonella globosa is also characterised by swift transformation of the globular, either small, uni-nucleate, or large, multi-nulceate motile cells, into amoeboid form. The multi-nucleate cell, plasmodium-like aggregate and amoeboid form are three major cell types shared by many dictyochophytes in their polymorphic life history but not raphidophytes. Based on the pigment composition, morphological and life history data we transfer Chattonella globosa to a new genus, Vicicitus gen. nov. of the Order Dictyochales, Class Dictyochophyceae, as Vicicitus globosus (Y. Hara et Chihara) F. H. Chang comb. nov.

Predicting rates of primary production in the vicinity of the Subtropical Convergence east of New Zealand

Abstract Water column integrated productivity of phytoplankton in three water masses east of New Zealand (Subtropical, Subtropical Convergence, and Subantarctic) varied both seasonally and spatially, ranging across two orders of magnitude. Productivity was highest in spring and autumn, and lowest in winter. Subtropical and Convergence waters were generally more productive than Subantarctic waters, but during summer the 2–3‐fold range of production was similar in each of the three water masses. Multiple regression analysis identified that most of the variability in production was associated with differences in integrated chlorophyll a concentrations (56%). By considering both surface chlorophyll a and incident irradiance we derived a relationship which explained 74% of observed spatial and seasonal variability in integrated production. These surface variables can be sensed remotely, suggesting the potential for broad spatial mapping of the seasonal cycles of production in these water masses in the New Zeal...

Light attenuation - a more effective basis for the management of fine suspended sediment than mass concentration?

Fine sediment continues to be a major diffuse pollution concern with its multiple effects on aquatic ecosystems. Mass concentrations (and loads) of fine sediment are usually measured and modelled, apparently with the assumption that environmental effects of sediment are predictable from mass concentrations. However, some severe impacts of fine sediment may not correlate well with mass concentration, notably those related to light attenuation by suspended particles. Light attenuation per unit mass concentration of suspended particulate matter in waters varies widely with particle size, shape and composition. Data for suspended sediment concentration, turbidity and visual clarity (which is inversely proportional to light beam attenuation) from 77 diverse New Zealand rivers provide valuable insights into the mutual relationships of these quantities. Our analysis of these relationships, both across multiple rivers and within individual rivers, supports the proposition that light attenuation by fine sediment is a more generally meaningful basis for environmental management than sediment mass. Furthermore, optical measurements are considerably more practical, being much cheaper (by about four-fold) to measure than mass concentrations, and amenable to continuous measurement. Mass concentration can be estimated with sufficient precision for many purposes from optical surrogates locally calibrated for particular rivers.

Photosynthetic parameters in water masses in the vicinity of the Chatham rise, south pacific ocean, during late summer

The relationship between photosynthesis (P) and light (E) was determined for phytoplankton from the mixed layer and the deep chlorophyll maximum (DCM) at 23 stations off the east coast of New Zeala...