Mark J. Doubell

Microscale patchiness of virioplankton

The microscale spatial distributions of viruses were investigated in three contrasting environments including oligotrophic open ocean, eutrophic coastal and estuarine habitats. The abundances of two discrete populations of both viruses and heterotrophic bacteria were measured at spatial resolutions of between 1 and 5 cm using purpose-designed microscale sampling equipment and £ow cytometric sample analysis. Within open water samples, virus distributions were characterized by non-normal distributions and by ‘hotspots’ in abundance where concentrations varied by up to 17-fold. In contrast to patterns generally observed at larger spatiotemporal scales, there was no correlation between bacterial and viral abundance or correspondence between bacteria and virus hotspots within these samples. Consequently, strong hotspots and gradients in the virus:bacteria ratio (VBR) were also apparent within samples. Within vertical pro¢les taken from above the sediment ^ water interface within a temperate mangrove estuary, distributions of planktonic viruses were characterized by gradients in abundance, with highest concentrations observed within the 1^2 cm immediately above the sediment surface, and virus distributions were correlated to bacterial abundance (P50.01). The patterns observed in these contrasting habitats indicate that microscale patchiness of virus abundance may be a common feature of the marine environment. This form of heterogeneity may have important implications for virus ^ host dynamics and subsequently in£uence microbial trophodynamics and nutrient cycling in the ocean.

Physical and biological controls of vertical gradients in phytoplankton

Small-scale vertical heterogeneity in phytoplankton distributions is common in coastal waters and may be a critical feature influencing trophic coupling in planktonic systems. Here we develop a model to investigate the biological and physical dynamics that control vertical gradients in phytoplankton abundance. The model includes phytoplankton layer formation and layer destruction through mixing and predicts that the local maximum scaled phytoplankton gradient is controlled by the relative strengths of these dynamics. We compare the predictions of this model to highly resolved profiles of phytoplankton concentration and fluorescence collected using a free-falling planar laser imaging fluorometer (FIDO-F) and turbulence microstructure profiler data (TurboMAP-L). From these profiles, we estimate the model parameters: the maximum rate of layer formation and minimum possible layer thickness. The maximum rate of layer formation ranged from 0.46 to 0.94d 21 , which is comparable to maximum reported growth rates of the most common phytoplankton taxa found in our samples. The minimum layer thickness estimated from our data suggests that persistent phytoplankton layers thinner than approximately 0.5m may be rare in coastal waters. This study provides a mechanistic explanation for some of the underlying dynamics governing phytoplankton layer formation, maintenance, and destruction and will allow us to better predict the magnitude and occurrence of these ecologically important structures in the field.

High-resolution fluorometer for mapping microscale phytoplankton distributions

ABSTRACT A new high-resolution, in situ profiling fluorometer maps fluorescence distributions with a spatial resolution of 0.5 to 1.5 mm to a depth of 70 m in the open ocean. We report centimeter-scale patterns for phytoplankton distributions associated with gradients exhibiting 10- to 30-fold changes in fluorescence in contrasting marine ecosystems.

The Role of Diatom Nanostructures in Biasing Diffusion to Improve Uptake in a Patchy Nutrient Environment

Background Diatoms are important single-celled autotrophs that dominate most lit aquatic environments and are distinguished by surficial frustules with intricate designs of unknown function. Principal Findings We show that some frustule designs constrain diffusion to positively alter nutrient uptake. In nutrient gradients of 4 to 160 times over <5 cm, the screened-chambered morphology of Coscincodiscus sp. biases the nutrient diffusion towards the cell by at least 3.8 times the diffusion to the seawater. In contrast, the open-chambers of Thalassiosira eccentrica produce at least a 1.3 times diffusion advantage to the membrane over Coscincodiscus sp. when nutrients are homogeneous. Significance Diffusion constraint explains the success of particular diatom species at given times and the overall success of diatoms. The results help answer the unresolved question of how adjacent microplankton compete. Furthermore, diffusion constraint by supramembrane nanostructures to alter molecular diffusion suggests that microbes compete via supramembrane topology, a competitive mechanism not considered by the standard smooth-surface equations used for nutrient uptake nor in microbial ecology and cell physiology.

Multilayer biological structure and mixing in the upper water column of Lake Biwa during summer 2008

We carried out a 24-h station experiment at Lake Biwa (Japan) to measure mixing events and concurrent biological signals using a free-fall microstructure profiler (TurboMAP-L), conventional hydrographic measurement device (F-probe), and the Tracker acoustic profiling system (TAPS). A clearly defined three-layer physical system was observed. Two layers were actively mixed: the surface-mixed layer and the subsurface-mixed layer. Both winds and night-time convection create the surface-mixed layer, and vertical shear due to a counterclockwise gyre maintains turbulence in the subsurface mixing layer. A strongly stratified layer between these two mixing layers is almost turbulence free, so no material flux is expected. A local oxygen maximum layer, a local oxygen minimum layer, and layers of increased chlorophyll and zooplankton abundance are all located in this strongly stratified layer. The data show the intricate influence of physical processes on the structure of biological systems and their combined influence on biogeochemical and trophic transfers in aquatic systems.

Evaluation of MODIS Products in the Great Australian Bight and Adjacent Coastal Waters

Satellite remote sensing data can produce global environmental data and is easily accessible and widely used by the scientific and non-scientific community. However, to use satellite data, it is important to know its limitations and how it validates against in situ measurements for the different regions. Here, field measurements of chlorophyll-a concentration and euphotic depth within the Great Australian Bight, Gulf St Vincent and Spencer Gulf were used to validate ocean colour products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite. The field data include in situ and in vivo chlorophyll-a concentration, which were compared against MODIS chlorophyll-a products derived from three algorithms (OC3M, Carder, and Garver-Siegel-Maritorena (GSM)), as well as euphotic depth measurements derived from photosynthetically active radiation (PAR) profiles, which were compared against two MODIS euphotic depth products (derived semi-analytically and from surface chlorophyll-a). The OC3M product performed well in open waters, with errors below the 35% NASA accepted limit, but it overestimated chlorophyll-a values in shallow (<50 m) waters. The GSM product produced the lowest errors, but also showed a smaller dynamic range, while the Carder product produced higher errors than GSM and it also showed small dynamic range. The relationships between the MODIS and in situ euphotic depth were robust, with errors lower than 20%. MODIS products showed weaker or no significant relationships to in situ measurements in the Eastern Great Australian Bight. This is thought to be due to the summertime subsurface upwelling pool that is characteristic of the area. Based on these results, the OC3M product provides the most reliable estimates of chlorophyll-a, and is recommended for further applications of MODIS imagery, if the limitations in shallow waters are taken into account. Alternatively, the GSM product could be a better option if the algorithm were locally adjusted. Changes in the sampling methodology to improve the algorithms are discussed. Derived euphotic depth products can be used with confidence in applying MODIS products for monitoring water clarity, ecosystem health or primary productivity in the region.