Anya M. Waite

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.

Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Physical and chemical signatures of a developing anticyclonic eddy in the Leeuwin Current, eastern Indian Ocean

A multidisciplinary cruise aboard the R/V Southern Surveyor was conducted in May 2006 to sample a developing anticyclonic eddy of the Leeuwin Current off Western Australia. The eddy formed from a meander of the Leeuwin Current in mid-April 2006 and remained attached to the current until mid-August. In this study, a combination of satellite data (altimeter, sea surface temperature, and chlorophyll a) and shipboard measurements (acoustic Doppler current profiler and conductivity-temperature-depth) were used to characterize the physical and chemical signatures of the eddy. The temperature-salinity properties of the mixed layer waters within the anticyclonic eddy and on the shelf were both connected to that of the Leeuwin Current, indicating the water mass in the eddy is mainly derived from the Leeuwin Current and the modified Leeuwin Current water on the shelf. Above the salinity maximum near the eddy center, there was a regionally significant concentration of nitrate (>0.9 μmol L-1), and the maximum (2 μmol L-1) was at 150 in depth, below the photic zone. Nitrification within the eddy and/or local upwelling due to the forming eddy could be responsible for this high concentration of nitrate near the eddy center which potentially makes the eddy a relatively productive feature in the Leeuwin Current.

Sedimentation of phytoplankton during a diatom bloom : Rates and mechanisms

ABSTRACT Phytoplankton blooms are uncoupled from grazing and are normally terminated by sedimentation. There are several potential mechanisms by which phytoplankton cells may settle out of the photic zone: sinking of individual cells or chains, coagulation of cells into aggregates with high settling velocities, settling of cells attached to marine snow aggregates formed from discarded larvacean houses or pteropod feeding webs, and packaging of cells into rapidly falling zooplankton fecal pellets. We quantified the relative significance of these different mechanisms during a diatom bloom in a temperate fjord, and evaluated their potential to control phytoplankton population dynamics. Overall specific sedimentation rates of intact phytoplankton cells were low during the 11 -day study period, averaging ca. 0.1 d-l, and mass sedimentation and bloom termination did not occur. Most cells settled attached to marine snow aggregates formed from discarded larvacean houses, whereas settling of unaggregated cells was insignificant. Formation rates of phytoplankton aggregates by physical coagulation was very low, and losses by this mechanism were co.07 d-t; phytoplankton aggregates were neither recorded in the water column (by divers) nor in sediment traps. The low coagulation rates were due to a very low ‘stickiness’ of suspended particles. The dominant diatom, Thalassiosira mendiolana, that accounted for up to 75% of the phytoplankton biomass, was not sticky at all, and did not turn sticky upon nutrient depletion in culture experiments. The low particle stickiness recorded may be related to low formation rates by diatoms of transparent exopolymeric particles (TEP), that occurred in low concentrations throughout the study period. Zooplankton grazing rate did not respond to the development of the bloom and accounted for a loss term to the phytoplankton populations comparable to sinking of intact cells; fecal pellets accounted for 3&50% of settled phytoplankton and phytodetritus. While coagulation may give rise to density-dependent losses to phytoplankton populations and, hence, control blooms, neither of the other mechanisms 1. Danish Institute for Fisheries Research, Charlottenlund Castle, DK-2920 Charlottenlund, Denmark (email: tk@dfu.min.dk) 2. Marine Biological Laboratory, Strandpromenaden 5, DK-3000 Helsinger, Denmark. 3. Marine Sciences Institute, University of California, Santa Barbara, California, 93106, U.S.A. 4. Department of Oceanography, Texas A&M University, College Station, Texas, 77843, U.S.A. 5. Department of Marine Sciences, University of Connecticut, Groton, Connecticut, 06340-6097, U.S.A. 6. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02143, U.S.A. 7. Present address: School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Welling- ton, New Zealand. 8. Universidad de Cadiz, Department de Biologia Animal, Vegetal y Ecologia, Aptdo 40, E-l 1510 Puerto Real (Cadiz), Spain. 1123

Seasonal production regimes off south-western Australia : influence of the Capes and Leeuwin Currents on phytoplankton dynamics

Temporal primary production dynamics were investigated off south-western Australia, where the summer upwelling regime of the Capes Current was compared with early winter conditions characterised by strengthened near-shore Leeuwin Current flow. Seasonal upwelling in this region sourced nitrate levels of ≥1 μm from the nutricline at the base of the Leeuwin Current’s mixed layer, with total water column production reaching a maximum of ~950 mg C m−2 day−1 in the Capes Current. Stable isotope signatures of particulate matter indicated that productivity off south-western Australia was heavily reliant on nitrate as a nitrogen source, with mean δ15N ranging from ~4 to 5 ‰ under both upwelling and non-upwelling (winter) conditions. Unexpectedly, significant nutrient enrichment within the Leeuwin Current (up to 3.1 μm nitrate) occurred during winter, likely as a result of the meandering Leeuwin Current flooding the inner shelf north of the study area and entraining relatively high-nutrient shelf waters in its southwards flow. However, early winter production under these nutrient-replete conditions (mean ± s.d. 310 ± 105 mg C m−2 day−1) was significantly lower than in summer (695 ± 140 mg C m−2 day−1) due to light limitation, both as a result of reduced surface irradiance characteristic of the winter months and significantly higher light attenuation within the water column as compared with summer conditions.

A Biomechanical Analysis of Four Humeral Fracture Fixation Systems

A biomechanical study was initiated to compare four fracture fixation devices: the AO dynamic compression plate, a distal fin locking nail, a solid locked intramedullary nail, and paired flexible nails for humeral fracture fixation. Eighteen pairs of fresh-frozen, intact humeri were harvested, standardized midshaft transverse osteotomies were created in each specimen, and left and right specimens were fixed with plates and nails, respectively. The bending properties of the plate-fixed humeri were significantly greater than the nailed humeri in both the anteroposterior and mediolateral planes on a paired basis. The distal fin nail and solid locked nail had comparable bending properties, and both had bending properties superior to those measured for the paired flexible nails. The torsional properties of humeri fixed with plates and solid locked nails were equivalent, except for rigidity and stiffness, which were superior for the nail. Both fixation methods resulted in torsional properties significantly greater to those measured for humeri fixed with paired flexible nails or a distal fin nail.

Determining the Diet of Larvae of Western Rock Lobster (Panulirus cygnus) Using High-Throughput DNA Sequencing Techniques

The Western Australian rock lobster fishery has been both a highly productive and sustainable fishery. However, a recent dramatic and unexplained decline in post-larval recruitment threatens this sustainability. Our lack of knowledge of key processes in lobster larval ecology, such as their position in the food web, limits our ability to determine what underpins this decline. The present study uses a high-throughput amplicon sequencing approach on DNA obtained from the hepatopancreas of larvae to discover significant prey items. Two short regions of the 18S rRNA gene were amplified under the presence of lobster specific PNA to prevent lobster amplification and to improve prey amplification. In the resulting sequences either little prey was recovered, indicating that the larval gut was empty, or there was a high number of reads originating from multiple zooplankton taxa. The most abundant reads included colonial Radiolaria, Thaliacea, Actinopterygii, Hydrozoa and Sagittoidea, which supports the hypothesis that the larvae feed on multiple groups of mostly transparent gelatinous zooplankton. This hypothesis has prevailed as it has been tentatively inferred from the physiology of larvae, captive feeding trials and co-occurrence in situ. However, these prey have not been observed in the larval gut as traditional microscopic techniques cannot discern between transparent and gelatinous prey items in the gut. High-throughput amplicon sequencing of gut DNA has enabled us to classify these otherwise undetectable prey. The dominance of the colonial radiolarians among the gut contents is intriguing in that this group has been historically difficult to quantify in the water column, which may explain why they have not been connected to larval diet previously. Our results indicate that a PCR based technique is a very successful approach to identify the most abundant taxa in the natural diet of lobster larvae.

Variability in Isotope Discrimination Factors in Coral Reef Fishes: Implications for Diet and Food Web Reconstruction

Interpretation of stable isotope ratios of carbon and nitrogen (δ13C and δ15N) is generally based on the assumption that with each trophic level there is a constant enrichment in the heavier isotope, leading to diet-tissue discrimination factors of 3.4‰ for 15N (ΔN) and ∼0.5‰ for 13C (ΔC). Diet-tissue discrimination factors determined from paired tissue and gut samples taken from 152 individuals from 26 fish species at Ningaloo Reef, Western Australia demonstrate a large amount of variability around constant values. While caution is necessary in using gut contents to represent diet due to the potential for high temporal variability, there were significant effects of trophic position and season that may also lead to variability in ΔN under natural conditions. Nitrogen enrichment increased significantly at higher trophic levels (higher tissue δ15N), with significantly higher ΔN in carnivorous species. Changes in diet led to significant changes in ΔN, but not tissue δ15N, between seasons for several species: Acanthurus triostegus, Chromis viridis, Parupeneus signatus and Pomacentrus moluccensis. These results confirm that the use of meta-analysis averages for ΔN is likely to be inappropriate for accurately determining diets and trophic relationships using tissue stable isotope ratios. Where feasible, discrimination factors should be directly quantified for each species and trophic link in question, acknowledging the potential for significant variation away from meta-analysis averages and, perhaps, controlled laboratory diets and conditions.

Particle transformations and export flux during an in situ iron-stimulated algal bloom in the Southern Ocean

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite of biogeochemical measurements (water column 234Th and δ13Corg inventories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particulate organic carbon (POC) is enhanced due to iron-induced increases in phytoplankton production. During the 13-days that the SOIREE bloom was monitored, export fluxes within the iron-fertilised patch were not substantially different to those in waters outside the bloom. On days 11–13, iron enrichment may have caused particle transformations that could lead to elevated future export via particle aggregation and/or diatom chain formation. The unknown time-lag between increased production and export, the longevity of the SOIREE bloom, and the absence of nutrient limitation over days 1–13, however, prohibit prediction of any iron-induced export. This conclusion highlights the difficulties of fully testing the “Iron Hypothesis” and for evaluating the implications for global climate change.

Stable isotope analysis reveals community-level variation in fish trophodynamics across a fringing coral reef

In contrast to trophodynamic variations, the marked zonation in physical and biological processes across coral reefs and the concomitant changes in habitat and community structure are well documented. In this study, we demonstrate consistent spatial changes in the community-level trophodynamics of 46 species of fish across the fringing Ningaloo Reef, Western Australia, using tissue stable isotope and fatty acid analyses. Increasing nitrogen (δ15N) and decreasing carbon (δ13C) isotope ratios in the tissues of herbivores, planktivores and carnivores with increasing proximity to the ocean were indicative of increased reliance on oceanic productivity. In contrast, detritivores and corallivores displayed no spatial change in δ15N or δ13C, indicative of the dependence on reef-derived material across the reef. Higher δ13C, as well as increased benthic- and bacterial-specific fatty acids, suggested reliance on reef-derived production increased in back-reef habitats. Genus-level analyses supported community- and trophic group-level trends, with isotope modelling of species from five genera (Abudefduf sexfasciatus, Chromis viridis, Dascyllus spp., Pomacentrus spp. and Stegastes spp.), demonstrating declining access to oceanic zooplankton and, in the case of Pomacentrus spp. and Stegastes spp., a switch to herbivory in the back-reef. The spatial changes in fish trophodynamics suggest that the relative roles of oceanic and reef-derived nutrients warrant more detailed consideration in reef-level community ecology.