Edward P. W. Horne

Photosynthesis and photoadaptation of marine phytoplankton in the arctic

Abstract In Baffin Bay, phytoplankton populations from the 1% light level were susceptible to photoinhibition whereas populations from the 50% light level were not. Both the initial slope of the light saturation curve and the assimilation number were smaller for the deep samples than for the shallow ones. The optimal irradiance for photosynthesis by the near-surface populations was comparable to the maximum irradiance to which they might be exposed in situ , but that for the populations near the bottom of the photic zone was higher than the populations could experience in situ . The general magnitudes of the photosynthesis parameters for these late summer arctic populations fell towards the low end of the range observed in temperate waters.

Pigment transformation and vertical flux in an area of convergence in the North Atlantic

Abstract Particulate samples from the water column, to depths of 80–120 m, were collected over a 10-day period during a bloom in the North Atlantic, near 37°N, 40°W, between 12 and 22 April 1990. These samples were analysed by HPLC for their chlorin and carotenoid content. The same analysis was carried out for samples of faecal pellets collected from copepods from depths of 0–100 m, which had fed in situ, and for material from sediment traps in the 100–500 m depth range. Water column pigment compositions were consistent with observations that diatoms and Phaeocystis were major components of the bloom. They also sometimes showed high concentrations of chlorophyllide a and a phaephorbide a-like pigment, which may be due to algal die-off and cell autolysis. Faecal pellets contained high concentrations of pyrophaeophorbide a lower amounts of unidentified phaeophorbides and some chlorophyll a, but none of the phaeophorbide a-like pigment found in the water column. Sediment traps contained high concentrations of the phaeophorbides characteristics of copepod faecal pellets and lower, approximately equal amounts of the water column phaeophorbide a-like pigment and chlorophyll a. The sampling area appeared to be in a jet of current, which was moving southwesterly along a front between Eastern Basin Water (to the north) and North Atlantic Central Water (to the south) and which seemed to be drawing in water from both these water masses. Areas of downwelling, thought to be associated with these convergent currents, may have accelerated the sedimentation of dead and dying phytoplankton cells.

Turbulence dissipation rates and nitrate supply in the upper water column on Georges Bank

Abstract Measurements of velocity microstructure in the upper water column on Georges Bank are used to contrast the summertime structure of turbulent kinetic energy dissipation rate between the central mixed area and the tidal-mixing front, and to compare vertical nitrate fluxes with frontal-zone primary production demands. In the mixed area during weak winds, the dissipation rate varies strongly over the semidiurnal tidal period in close relation to the tidal current strength, varies with the monthly/fortnightly tidal modulation, and generally increases with distance below the sea surface. Collectively, these features provide strong support for the elevated vertical mixing rates on Georges Bank being primarily due to the tides, although wind forcing also contributes significantly. In the frontal zone on northern Georges Bank, the upper-ocean dissipation rates are about an order of magnitude weaker than in the mixed area, have a more complex temporal variation during the tidal period, and also vary with the monthly/fortnightly tidal modulation. The vertical eddy flux of nitrate into the frontal euphotic zone varies over the tidal period and with the tidal modulation. Averaged over the tidal period, the estimated fluxes are about one-third of the nitrogen demand estimated from concurrent primary production measurements, supportive of an important contribution from turbulent mixing to new production in the frontal zone, but also pointing to additional processes and/ or inadequate data coverage of this complex zone. The measured dissipation rates at both the mixed and frontal sites are in approximate agreement with the turbulence levels in two 3-D numerical models for summertime tidal and mean circulation on the Bank, one with and eddy-viscosity and the other an advanced turbulence closure. The latter model has more realistic vertical turbulence distributions and indicates strong sensitivity of the turbulence levels to horizontal position in the frontal zone.

Detailed structure of currents and hydrography on the northern side of Georges Bank

A suite of observations from July 2–3, 1988, is used to describe the spatial structure and temporal evolution over the tidal period of currents and hydrography across the northern side of Georges Bank in summer under light winds. The data set includes moored current and hydrographic observations at four cross-bank positions, fast response thermistor chain observations at two of the sites, a conductivity-temperature-depth section, 10 repeated sections over the tidal period using a towed Batfish and ship-mounted acoustic Doppler current profiler, and two surface drifter trajectories. The observations provide a detailed description of previously identified features such as the strong semidiurnal tidal currents, an internal tide, a tidal front, and a residual current jet and also reveal a hierarchy of energetic smaller-scale structures. These include an internal hydraulic jump during off-bank tidal flow and subsequent internal waves propagating onto the bank and also a surface convergence in the frontal zone. The physical oceanographic regime on the northern side of Georges Bank during spring-fall can be conceptualized as a hybrid of a stratified shelf break with strong tidal advection and a tidal(mixing) front. Key factors to the regime are the strong tidal currents and abrupt topographic variation over the banks side. The result is a nonlinear and baroclinic tide-topography interaction at the bank edge and a frontal zone with strong variability over the tidal period associated with tidal advection and large-amplitude internal waves. The along-bank transport in the residual jet is estimated to be 0.91 Sv, indicating that it may be the largest summertime transport feature on the northwestern Atlantic shelf between Cape Hatteras and the Grand Bank.

The western North Atlantic bloom experiment

An investigation of the spring bloom was carried out in the western North Atlantic (40–50°W) as one component of the multi-nation Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment (NABE). The cruise track included an extended hydrographic section from 32 to 47°N and process studies at two week-long time-series stations at 40 and 45°N. Biological and chemical data collected along the transect indicated that the time-series stations were located in regions where the spring bloom was well developed; algal biomass was high and surface nutrient concentrations were reduced from maximum wintertime levels. Despite similarities in the vertical structure and magnitude of phytoplankton biomass and productivity, the two stations clearly differed in physical, chemical and other biological characteristics. Detailed depth profiles of the major autotrophic and heterotrophic microplankton groups (bacteria, phytoplankton, microzooplankton) revealed a strong vertical coherence in distribution at both sites, with maximum concentrations in the upper 50 m being typical of the spring bloom. Ultraplankton (< 10 μm) were an important component of the primary producers at 40°N, whereas larger netplankton (diatoms, dinoflagellates) were more important at 45°N. Silicate depletion was clearly evident in surface waters at 45°N, where diatoms were most abundant. Despite the relative importance of diatoms at 45°N, dinoflagellates dominated the biomass of the netplankton at both sites; however, much of this community may have been heterotrophic. Bacterial biomass and production were high at both stations relative to phytoplankton levels, particularly at 45°N, and may have contributed to the unexpectedly high residual ammonium concentrations observed below the chlorophyll maximum layer at both stations. Microzooplankton grazing dominated phytoplankton losses at both stations, with consumption as high as 88% of the daily primary production. Grazing losses to the mesozooplankton, on the other hand, were small (<10%), but mesozooplankton contribution to the vertical flux of organic matter (fecal pellets) was important at 45°N. F-ratios estimated by 15N tracer methods and sediment trap fluxes were similar and suggeste that ∼30% of the daily primary production was lost by direct sedimentation during the observation period. Numerous similarities in bloom characteristics were noted between the western and eastern Atlantic study sites.

Carbon Fixation and Oxygen Evolution by Phytoplankton in the Canadian High Arctic

SummaryIn the Canadian high arctic, in summer, in situ profiles of oxygen evolution and carbon fixation by phytoplankton showed excellent qualitative and quantitative agreement with an apparent photosynthetic quotient in the range from 1.3 to 1.8. Water column photosynthesis was linear in the available light at the surface. Daily rates photosynthesis exceeded 1 mg C m-2 d-1, implying a minimal estimate for annual production in the range from 35 to 70 g C m-2 y-1.

Circulation, Hydrographic Structure and Mixing at Tidal Fronts: The View from Georges Bank [and Discussion]

The steep slope on the northern side of Georges Bank and its location in the Fundy-Maine tidal system result in a persistent summertime frontal system comprising a tidal-mixing front and a stratified tide-topography interaction at the Bank edge. Recent field studies have provided a high-resolution description of the circulation, hydrographic structure and mixing in the region. Frontal features include an along-front residual jet, a surface convergence zone, regular variations in frontal structure and position over the tidal period and tidal modulation cycle, largeamplitude internal waves, and strong spatial and temporal variations in small-scale turbulence. The observations suggest that the magnitude of cross-front and vertical exchange in frontal regions can be site-specific depending on the relative importance of the underlying physical processes.

Phytoplankton growth rates estimated from depth profiles of cell concentration and turbulent diffusion

We develop a theory for estimating the net population growth rate (reproductive rate minus mortality rate) of a phytoplankton population in situ from biological observations of cell concentrations and physical measurements of turbulance. High resolution vertical profiles of cell concentration and turbulent diffusion rates from the central North Atlantic measured on successive days were used to study depth-dependent growth rates for categories of ultraphytoplankton defined by flow cytometry. The four categories and their equivalent spherical diameters were cyanobacteria (≈1 μm), the abundant newly discovered very small red fluorescing bodies (prochlorophytes <1 μm), small eukaryotes (<2 μm) large eukaryotes (mean 2–4 μm, mode 2–3 μm). For all groups, the highest net population growth rates occurred in areas of peak cell concentration near the top of the nitracline and the bottom of the euphotic zone around the 1% light level. Cyanobacteria grew most rapidly between 85 and 95 m, whereas eukaryotes grew most rapidly between 100 and 115 m; the maximum population growth rates were 0.48 ± 0.07 d−1 for cyanobacteria, and 1.43 ± 0.32 d−1 and 0.20 ± 0.03 d−1 for small and large eukaryotes, respectively. Oriented motility toward the region of peak cell concentration may have contributed to the apparently high maximum growth rate of the small eukaryotes. The most negative rates of population growth occurred adjacent to the regions of peak cell concentration, consistent with previous observations of densest concentrations of zooplankton grazers around and above the deep chlorophyll maximum. Below about 150 m, at light intensities <0.1% of surface values, cell concentrations of all ultraphytoplankton groups decreased exponentially with depth, corresponding to net population growth rates ranging from −0.5 to −2.0 d−1 in this region.

A multispectral remote sensing study of coastal waters off Vancouver Island

In March 1996, a multispectral aircraft survey of the coastal waters off Vancouver Island was carried out using a Compact Airborne Spectrographic Imager (CASI). This survey was combined with in situ measurements of water properties (phytoplankton composition, phytoplankton pigments, absorption spectra of phytoplankton, and concentration of dissolved organic carbon, or DOC). Comparison of the phytoplankton absorption data from this experiment with similar data from other regions shows that phytoplankton community has a significant impact on the spectral form and magnitude of absorption spectra, when normalized to unit chlorophyll-a. Concurrent measurements of in situ properties and aircraft data were obtained at eight stations. The in situ measurements of phytoplankton absorption and estimates of downwelling irradiance based on a clear-sky atmospheric-transmission model are used as inputs to a model of water-leaving irradiance. The modelled irradiances are compared with the remotely sensed values of water-leaving radiances. The observed differences between model and observation are used to evaluate the potential influence of DOC on water-leaving radiance. Practical difficulties of separating the phytoplankton signal from that of the coloured component of DOC (also known as yellow substance) are examined. Algorithms for estimation of the concentration of chlorophyll-a (the major phytoplankton pigment) can be based on their absorption or fluorescence properties. The distribution of chlorophyll-a in the study area is estimated using both these approaches, and possible causes for the observed discrepancies are discussed.

Physiological adaptations of an under-ice population of Pseudocalanus in Barrow Strait (N.W.T) to increasing food supply in spring

SummaryZooplankton and water samples were collected at weekly intervals between April 25 and May 30, 1986 in Barrow Strait, N.W.T. (Canadian Arctic Archipelago). In tows from 0–30 m, the zooplankton community (>202 μm) was dominated by Pseudocalanus. The population was apparently growing and developing as shown by an increase in the proportion of adults (stage VI) and decreases in the proportion of stages III, IV, and V as the season progressed. Respiration and excretion rates of the Pseudocalanus populations were probably linked, there being an immediate increase in excretion rate, accompanying an increase in feeding rate when chlorophyll concentrations increased, which was followed by a smaller increase in respiration rate after a time lag. Hence, there was a large decrease in the O∶N ratio. Increased metabolism coincided with changes in the population structure, as did protease and bodily protein, but could not be clearly linked to dietary acclimation. Only laminarinase activity could be statistically related to an identifiable fraction of potential nutritional value in the water, particulate soluble carbohydrate, but neither showed overall seasonal change.