Alex Cunningham

Prey selection and rejection by a microflagellate; implications for the study and operation of microbial food webs

Abstract In a series of experiments, measurements were made of both numbers and biovolumes of the phototrophs Dunaliella primolecta (Butcher) (7.6 μm diameter), Isochrysis galbana (Parke) (4.5 μm), and Micromonas pusilla (Butcher) (1.5 μm), together with the phagotrophic dinoflagellate Oxyrrhis marina (Dujardin) (typically 16–20 μm). This enabled the calculation of the ‘equivalent encounter distance’ (l eq ), which gives a measure of the distance an average sized predator would have to swim in order to encounter a biovolume of prey equal to its own cell volume. If predation of a given prey type continues when its l eq is greater than that of an alternative prey item, then the predator is deemed to be demonstrating a preference for the former item. When confronted with all three phototrophs, Oxyrrhis selected Dunaliella first but, despite the 25-fold difference in cell volume, showed no preference for Isochrysis over Micromonas. Oxyrrhis may also reject Isochrysis on occasion, an event which seems to be associated particularly with elevated C:N ratios in the phototroph. Oxyrrhis has been seen to exhibit cannibalism when in the presence of Isochrysis biovolumes (biomass concentrations) an order of magnitude above that of Oxyrrhis . Such plasticity in prey selection makes it very difficult to predict the outcome of predator-prey interactions, especially where (as between Dunaliella and Isochrysis ) there are also growth interactions between the prey species. It also suggests that results obtained from short term studies of predation-kinetics, or in studies conducted under conditions such as in steady-state cultures and in continuous darkness, should not be generalised to more realistic environmental conditions.

Potential impacts of nonalgal materials on water-leaving Sun induced chlorophyll fluorescence signals in coastal waters

It has been suggested that Sun induced chlorophyll fluorescence (SICF) signals could be used to estimate phytoplankton chlorophyll concentration and to investigate algal physiology from space. However, water-leaving SICF is also a product of the ambient light field. In coastal waters both algal and nonalgal materials affect the underwater light field. In this study we examine the independent impacts of varying loads of mineral suspended solids (MSS) and colored dissolved organic materials (CDOM) on water-leaving SICF signals using Hydrolight radiative transfer simulations. We show that SICF signals in coastal waters are strongly influenced by nonalgal materials. Increasing concentrations of CDOM and minerals can reduce the water-leaving SICF per unit chlorophyll by over 50% for the concentration ranges explored here (CDOM = 0 to 1 m(-1) at 440 nm, MSS=0 to 10 g m(-3)). The moderate-resolution imaging spectroradiometer (MODIS) fluorescence line height algorithm is shown to be relatively unaffected by increasing CDOM, but performance is significantly degraded by mineral concentrations greater than 5 g m(-3) owing to increased background radiance levels. The combination of these two effects means that caution is required for the interpretation of SICF signals from coastal waters.

Role of measurement uncertainties in observed variability in the spectral backscattering ratio: a case study in mineral-rich coastal waters

The particulate backscattering ratio (b(bp)/b(p)) is a useful indicator of the angular scattering characteristics of natural waters. Recent studies have shown evidence both for and against significant spectral variability in b(bp)/b(p) in the visible domain, but most show significant variability in its magnitude. We present results from a case study in which both backscattering and scattering coefficients were measured at nine wavelengths in a region of UK coastal waters where optical scattering is strongly influenced by inorganic particles and where a wide range of turbidities is found in a small geographic area. Using a new approach based on regression analysis of in situ signals, it is shown that, for this study site, most of the apparent variability in the magnitude of the backscattering ratio can be attributed to measurement uncertainties. Regression analysis suggests that b(bp)/b(p) is wavelength dependent for these mineral-rich waters. This conclusion can only be avoided by positing the existence of undocumented, systematic, wavelength-dependent errors in backscattering measurements made by two independently calibrated sensors. These results are important for radiative transfer simulations in mineral-dominated waters where the backscattering ratio has often been assumed to be spectrally flat. Furthermore, spectral dependence also has profound implications for our understanding of the relationship between b(bp)/b(p) and particle size distributions in coastal waters since the commonly assumed power-law distribution is associated with a spectrally flat particulate backscattering ratio for nonabsorbing particles.

Evidence for wavelength dependence of the scattering phase function and its implication for modeling radiance transfer in shelf seas

More than 90% of stations from the Irish and Celtic Seas are found to have significantly higher back-scattering ratios in the blue (470 nm) than in the red (676 nm) wave band. Attempts to obtain optical closure by use of radiance transfer modeling were least successful for stations at which backscattering ratios are most strongly wavelength dependent. Significantly improved radiance transfer simulation results were obtained with a modified scattering correction algorithm for AC-9 absorption measurements that took wavelength dependency in the scattering phase function into account.

Relationships between photopigments, cell carbon, cell nitrogen and growth rate for a marine nanoflagellate

Ammonium-limited batch cultures of Isochrysis galbana were grown in both continuous light and 12-h light-dark cycles. The maximum observed specific growth rate (measured in terms of cell numbers) was similar under both illumination regimes, but the mean cell content of Chl a was higher in intermittent illumination. The N:C ratio of the culture was linearly related to relative specific growth rate. A highly correlated linear relationship was found between carotenoid and C, with a similar slope for both light regimes. In contrast, Chl a was linearly correlated with N, but the slope of the relationship changed with illumination and the coefficients of variation were higher. The Chl a:C ratio depended on nutritional state and on the illumination regime.

Fine-scale variability in phytoplankton community structure and inherent optical properties measured from an autonomous underwater vehicle

The relationship between inherent optical properties (IOPs), phytoplankton community structure and the abundance of suspended particles in the size range 3–500 Am was studied near the Isles of Scilly (UK) in May 2000. Autosub, an autonomous submersible vehicle specifically designed for science missions, was used as an instrument-positioning platform. It carried a CTD system, an ac-9+ dual tube spectrophotometer, a prototype submersible flow cytometer and an Aqua-monitor water sampler. The vehicle made a 10-km transect at constant depth across a boundary between water masses with contrasting remote sensing reflectance, which was located using SeaWiFs ocean colour imagery. This boundary corresponded to a sharp (1 km) transition between one phytoplankton community consisting of coccolithophores, flagellates and dinoflagellates, and a second community dominated by diatoms and flagellates. Inherent optical properties measured along the autonomous underwater vehicle (AUV) track showed marked changes in magnitudes, ratios, spectral shapes and fine-scale spatial variability. These changes were well correlated with variations in the composition of the suspended particle assemblage measured by microscopy and in situ flow cytometry. D 2003 Elsevier B.V. All rights reserved.

Inherent and apparent optical properties in coastal waters: a study of the Clyde Sea in early summer

Profiles of absorption and attenuation coefficients, downward irradiance and upward radiance were measured at 14 stations in the Clyde Sea during May 2000. The absorption coefficient at 676 nm was linearly correlated with chlorophyll concentration (R2=0.88), and the ratio of scattering to absorption in this waveband could be used to discriminate between phytoplankton and other suspended material. The radiance and irradiance measurements showed rapid attenuation of blue and red wavelengths near the surface, while the red component was augmented by chlorophyll fluorescence in deeper waters. Throughout the Clyde Sea the relationship between absorption coefficients a(λ), scattering coefficients b(λ), and diffuse attenuation coefficients for downward irradiance Kd(λ) was well described by a function proposed by Kirk (Limnol. Oceanogr. 29 (1984) 350). A procedure was devised for retrieving inherent optical properties from measurements of the radiance reflectance and diffuse attenuation coefficient that was able to successfully predict measured absorption coefficients throughout the study area (R2=0.95). Backscattering ratios estimated using this procedure varied with depth and location according to changes in the relative concentration of phytoplankton to other particles. Tables of measured inherent and apparent optical properties and derived backscattering ratios for near-surface water are provided for modelling radiative transfer in the Clyde Sea and other coastal areas subject to freshwater influence.

Impact of measurement uncertainties on determination of chlorophyll‐specific absorption coefficient for marine phytoplankton

Understanding variability in the chlorophyll-specific absorption of marine phytoplankton, aph*Chl (λ), is essential for primary production modelling, calculation of underwater light field characteristics, and development of algorithms for remote sensing of chlorophyll concentrations. Previous field and laboratory studies have demonstrated significant apparent variability in aph*Chl (λ) for natural samples and algal cultures. However, the potential impact of measurement uncertainties on derived values of aph*Chl (λ) has received insufficient study. This study presents an analysis of measurement uncertainties for a data set collected in the Ligurian Sea in Spring and assesses the impact on estimates of aph*Chl (λ). It is found that a large proportion of apparent variability in this set of aph*Chl (λ) can be attributed to measurement errors. Application of the same analysis to the global NOMAD data set suggests that a significant fraction of variability in aph*Chl (λ) may also be due to measurement errors.

Fluorescence pulse shape as a morphological indicator in the analysis of colonial microalgae by flow cytometry

Abstract Individual algal cells and colonies were entrained in a fluid stream and passed through a tightly focused (200 × 10 μm) laser beam to generate chlorophyll-fluorescence pulses. Simple pulse-shape parameters (duration, height and integral) were extracted by analogue circuitry and the relationship between these parameters analysed digitaly. Pulse shape analysed in this way provided low-resolution morphological information at a throughput rate up to 1000 cells·s −1 : such information is particularly useful in the flow analysis of larger algal cells and colonial species. Successful implementation of the technique required that the flow regime oriented the cells uniformly and stably in the laser-beam waist and it worked best with a wide-bore (300 μm) hydrodynamic focusing nozzle which resisted clogging and avoided disrupting colonies by shearing. As a demonstration of feasibility, the technique was used to measure the length distribution of filaments of the cyanobacterium Anabaena solitaria and to discriminate between these filaments and globular colonies of the green alga Eudorina unicocca . Since a low power (100 mW) Ar laser was found to provide adequate illumination, the method can be implemented using low-cost cytometric equipment.

Semi-empirical correction algorithm for AC-9 measurements in a coccolithophore bloom

Values for the coefficients of absorption (a) and attenuation (c) obtained from AC-9 measurements in coccolithophore blooms do not provide satisfactory inputs for radiance transfer models. We have therefore modified the standard AC-9 scattering correction algorithm by including an extra term, F(lambda, lambda(r)), which allows for possible wavelength dependence in the scattering phase function. We estimated the magnitude of F(lambda, lambda(r)), which is unity in the standard algorithm, by adjusting the absorption and scattering values in Hydrolight radiance transfer calculations until the depth profiles of downward irradiance (E(d)) and upward radiance (L(u)) matched those measured in situ. The modified algorithm was tested with data from a phytoplankton bloom dominated by the coccolithophore Emiliania huxleyi, which occurred in the western English Channel in May 2001. In this paper, we only have sufficient data to adequately constrain the radiance transfer model in one wave band centered on 488 ma. A single value of F(lambda, lambda(r)) = 1.4 was found to produce satisfactory agreement between modeled and observed profiles at four widely spaced stations within the bloom. Measurements of the ratio of backscattering (b(b)) to total scattering (b) showed significant wavelength dependence at these stations.