Richard Pawlowicz
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
During 2002–2006, a comprehensive set of observations covering physical, biological, radiative and atmospheric parameters was obtained from the southern Strait of Georgia (SoG), Western Canada by the STRATOGEM program. Monthly time series of estuarine layer transports over 2002–2005 were estimated using a time-dependent 2-box model in a formal inverse approach. These transports are then consistent with the temperature and salinity fields, as well as riverine freshwater inflow (R) and atmospheric heat fluxes. Uncertainty was analyzed by resampling observations using bootstrap methods. The transport time series were then combined with observations of nutrient concentrations to construct monthly time series of nutrient uptake for nitrate, phosphate, and silicic acid.Analysis of these time series suggests that the SoG estuarine circulation is not very sensitive to the seasonal changes of R. Comparison of the surface layer transport (U₁) and R yields the first observational relationship between the SoG estuarine circulation and R. This relationship (U₁=2.68 m²s⁻²/³× 10³ R¹/³) is consistent with estuarine theories. Although the flows change slightly with the freshet, a 5-fold change in R results only in a 40% change in U₁. Based on the calculated sink of near-surface nutrients, net primary productivity is estimated to be 212 gC m⁻²yr⁻¹, which is similar to values obtained differently in similar estuaries. Comparison of the nitrate and phosphate uptake rates suggests that the primary productivity (PP) is mainly new PP during spring and summer. Thus, PP is mainly controlled by the upwelling supply of nutrients through deep inflow and entrainment. The uptake of silicic acid (Si) is almost two times larger than the uptake of nitrate during diatom spring blooms, while it is similar during the summer blooms. Such a high Si uptake suggests that spring diatoms form heavier frustules or that heterotrophic silicoflagellates compete with diatoms for Si. Speculative considerations based on comparison of the estimated production rate of near-surface oxygen and new PP also suggest that the regenerated PP is small. In addition, the summer heterotrophic respiration might be in excess by as much as2 gO m⁻² d⁻¹ relative to the net PP.
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An instrumented ferry made eight transects per day across the Fraser River plume over the years 2003 - 2006 as part of the STRATOGEM program to study biophysical coupling in the Strait of Georgia. Water temperature, salinity, chlorophyll-a fluorescence, nitrate concentration, and dissolved oxygen were measured. This thesis utilizes salinity and chlorophyll-a fluorescence to study mixing in the plume, and the impact of the plume on algal biomass. First, the effects of river discharge and tides on plume salinity and surface area are quantified. Tidal fluctuations are caused by advection of the estuarine salt field, while fortnightly variations are caused by modulation of mixing in the estuary. Tidal and fortnightly variations are strongest at high river discharge and weakest at low discharge. Plume salinity decreases quasi-linearly with river discharge. Plume surface area increases with river discharge, from about 300 km² at low river flow to about 1,200 km² at high river flow, and can be predicted by scaling the river mouth deformation radius. Second, the plume fresh water flushing time is estimated and a salinity budget is constructed. Fresh water flushing time is 2.2 days, independent of river discharge. The quasi-steady budget predicts a vertical entrainment flux which varies with river discharge. The discharge-dependent vertical entrainment velocities in the estuary and plume implied by the entrainment flux are consistent with other methods. Flow speeds at the edge of the plume estimated from this method are too weak to maintain a plume front, suggesting fronts are transient and created on tidal time-scales. Third, a time series of surface and depth-integrated chlorophyll-a biomass is constructed. Chlorophyll-a fluorometer data are corrected for fluorescence quenching with a parameterization specific to the region, and then calibrated with extracted samples. Instantaneous along-track differences in surface chlorophyll-a can be large, however, averaged over the whole time series, the distribution is nearly uniform. In contrast, depth-integrated values are about 35% lower on average in the plume compared to surrounding waters. Interannual variability in biomass is partly due to the magnitude and duration of the spring bloom, which is itself influenced by wind mixing and grazing.
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Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Surface drifter observations in coastal oceans display turbulence, but this turbulence often varies in space and it is difficult to attribute the levels of turbulence to different oceanographic processes by the typical means of studying ocean variability. In this study, I lean on an idea from chaos theory to characterise the different levels of turbulence observed, fractal dimension. Previous studies present many methods that can be used to determine fractal dimension of large-scale drifter tracks, here I present the three most popular in the literature on this matter. In this study, I first ask the question, which method is optimal for calculating fractal dimension? And secondly, I aim to determine what fractal dimensions can be expected of coastal drifter trajectories and how useful these quantities are for understanding ocean processes. To address these motivating questions, I first modify the three methods to better suit modern data (in which sources of uncertainty can play a big role) and test them on sets of known fractal dimension to determine their accuracy. Then I apply each method to drifter observations in three coastal ocean settings (the Salish Sea, the St. Lawrence, and the northern British Columbia ( BC ) continental shelf) and consider the magnitudes of fractal dimensions and how dimensions of differing magnitudes relate to different oceanographic/turbulent processes. The results of this study show that the box counting method is the most applicable method and that fractal dimensions computed by this method fall between 1 and 1.5, a range wider than those suggested by historical studies of large-scale processes. Finally, fractal dimensions are shown to be insightful metrics that can be used to determine the processes acting on the water but not in all cases. This work bridges the gap in knowledge between an historical technique and modern data, and provides scope for further exploration as to its applications.
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The Strait of Georgia (SoG) is a productive, temperate, and coastal ecosystem in the Northeast Pacific Ocean. Throughout a year in the SoG, plankton productivity and community structure have strong seasonal variations. However, the diverse range of plankton species, trophic processes, and interactions are often over-simplified in food web models. The objectives of this study were to: a) describe the seasonal trends in plankton community composition, biomass, and vertical distribution; b) represent the seasonal food web structure of the plankton community; and c) compare this complex and seasonal modelling approach to a more common and simplified approach. In the spring, diatoms dominate the plankton biomass in the SoG, leading to high primary productivity and fueling zooplankton growth. In summer, primary production is nutrient limited, leading to a decline in diatom biomass and increased mixotrophic flagellate dominance. Zooplankton biomass, dominated by large crustaceans including copepods, amphipods, and euphausiids, peaks and their diets shift to become more omnivorous. In winter, plankton biomass and productivity are low. Based on these seasonal trends, I constructed a food web model for each season (spring, summer, and winter), with the plankton community represented by ten mesozooplankton groups, two mixotrophic microzooplankton groups, one phytoplankton group, one heterotrophic bacteria group, and one detrital group. The spring food web is mostly fueled by high diatom primary production. However, the microbial loop, consisting of detritus, heterotrophic bacteria, and microzooplankton, shifts to become an important energy source for the system under limiting conditions in the summer and winter food webs. These structural changes throughout a year could have important implications and insights for higher trophic levels, such as into the seasonal food availability and quality for plankivorous fish. Finally, I compared this detailed seasonal approach to plankton modelling with an aggregated and year-averaged approach. The microbial loop is often excluded from coastal ecosystem models but is an important component, influencing trophic positions and transfer efficiencies. However, aggregating plankton groups appears to be an adequate approach to ecosystem modelling in the SoG, but modelling decisions should be driven by the research question.
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The Fraser River plume in the Strait of Georgia, BC, is significantly influenced by the tide. Here we use 17 years of daily MODIS observations of suspended particulate matter to understand the tidal variability of the plume. Our results show a consistent negative correlation between the Fraser River plume area and the tidal elevation with a phase lag at about one hour from two independent methods. The plume area routinely increases/decreases by about 20% during the ebb/flood tides, and a lower river flowrate typically leads to a more dramatic tidal variation in the plume area. A tidal harmonic analysis is performed on the HF-radar derived surface currents, and the difference between the extents of diurnally and semi-diurnally driven river influence suggests two distinct dynamical regions in the horizontal plume structure. A simple analytical model based on the volume conservation and salinity balance equations is built to analyze the mechanism of the tidal variability in the plume size. The observed tidal patterns of the plume area variation are partly reproduced using tidally modulated plume salinity (observed from instrumented ferries) and river flowrate (from numerical model outputs). These new findings will improve our understanding of the short-term dynamics of the Fraser River plume.
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Double-diffusive convection or Double Diffusion is an interaction within a fluidwhose density is governed by two constituents of different molecular diffusivities.Double diffusion in the ocean appears to create unique structures that look likestaircases in vertical profiles of temperature and salinity. Many oceanographersbelieve that double diffusion can affect the water masses and the circulation of theocean. However, in the current literature the detailed physics behind the formationof this staircase are still unclear. In sea salt each ion has different diffusion rate andbecause of that modelling salt diffusion is actually more complicated since thereis no single ”salt diffusivity”. Therefore in order to describe the effects of doublediffusion in seawater we have to consider a multicomponent system where each ionis reacting differently than the other ones. To simulate this system we use MIN3Pa multicomponent diffusion model. Our approach is primarily numerical, but in orderto test the conclusions of our model we compare against observations in PowellLake. We see that Multicomponent Diffusion can change the chemical compositionof seawater and should be considered an important transport mechanism in theocean.
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The spatial and temporal properties of naturally occurring double diffusive (DD) structures present in the bottom waters of Powell Lake, British Columbia were investigated. Observations were obtained from four annual surveys consisting of vertical cm-resolution conductivity-temperature-depth (CTD) profiles along the 9 km length of the lake, and from a month-long mooring consisting of thirty-eight temperature sensors and two current meters. DD layers were identified by isolating clusters on temperature-salinity (T-S) diagrams, and tracked spatially and temporally throughout each of the CTD surveys. The layers were observed to be persistent over four years, and horizontally coherent over the entire lake length at depths of 336-347 m. In this region the vertical density ratio (a non-dimensional measure of the relative strengths of vertical temperature and salinity gradients), Rρz, and buoyancy frequency, N, were near constant at Rρz = 2.2 ± 0.2 and N = (2.3 ± 0.3) x 10‐³ s‐¹, and the Rayleigh number reached a peak at Ra ≈ 10⁷. Layers just above and below this region were less horizontally-coherent and with larger values of Rρz and N. Spatial variations in layer depth and the background temperature/salinity distribution showed persistent trends throughout the study period. These trends indicated that layer slope and horizontal property gradients are linked and that the horizontal density ratio may be an indicator of the mean layer slope. Linear fits to the layer properties indicated that a horizontal density ratio of Rρx = -0.35 ± 0.17 was accompanied by a mean layer slope of ∆z/ ∆x = 0.05 ± 0.02 m/km. An individual DD step within one of the stable and horizontally coherent DD layers was identified within the moored temperature time series and tracked over the course of a week. The convective regime within the DD step was observed to be composed of intermittent thermal plumes emitted from the bottom diffusive interface. The features appeared as a common peak in the mean DD step temperature and horizontal velocity power spectra. The plumes had a period of ∼22 minutes (coinciding well with the buoyancy period within the diffusive interface), a temperature scale of T' ≈ 0.2 m°C, and horizontal and vertical velocity scales of u' = w' ≈ 0.5 mm/s.
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The oxygen budget in the top 50 m of lower Strait of Georgia, British Columbia is investigated using high resolution measurements of dissolved oxygen concentration and other oceanographic and meteorological properties from an instrumented ferry. An budget equation is established to describe the oxygen balance in the surface Strait of Georgia. The budget equation consists of 4 parts, which includes (1) the storage rate term, which is calculated with the ferry oxygen measurements using a 2-point differential scheme; (2) advective and vertical transport, which is estimated using a box model; (3) air-sea gas transfer, which is estimated using a bulk parametrization of air-sea gas flux; and (4) net community productivity, which is estimated by taking the residual of the budget equation. To further investigate the productivity level in the Strait of Georgia, daily community respiration rate is estimated by extracting the diurnal variation signal of oxygen, and gross productivity is estimated by combining net community productivity with the community respiration rate. Results suggest that gross productivity in the lower Strait of Georgia varies from 1.4 to 11.8 gC.m-²day-¹ and averages at 4.4 gC.m-²day-¹, slightly higher than historical measurements.
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High resolution measurements of temperature and electrical conductivity in Powell Lake, British Columbia provide an extensive set of layer and interface observations of a double diffusive staircase found between 325–350 m depth. Powell Lake is an ex-fjord with a quiescent salt layer at thermal steady state in which double diffusion is naturally isolated from turbulent and advective processes. Layers are coherent on the basin scale and their characteristics have a well defined vertical structure. The steady state heat flux is estimated from the large-scale temperature profile and agrees with an earlier estimate of the flux in thesediments. These estimates are compared to a 4/3 flux parameterization which agrees with the steady state flux to within a factor of 2. The discrepancy is explained by testing the scaling underlying the parameterization directly, and it is found that the assumed power law deviates systematically from the observations. Consequently, a different scaling which better describes the observations is presented. The assumption that interfacial fluxes are dominated by molecular diffusion is tested by comparing the interfacial gradient to that expected from the steady state heat flux; at low density ratios, the average interfacial gradient is not sufficiently large to account for transport by molecular diffusion alone, indicating that double diffusive fluxes cannot generally be estimated from bulk interface properties. Salinity interfaces are only marginally (9%) smaller than temperature interfaces, and a simple model to describe the observed difference is presented and shown to be consistent with the observations.
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A hydrographic dataset from the 2008-2009 Rivers Inlet Ecosystem Study (RIES) field program was used (a) to provide a more complete oceanographic description of Rivers Inlet, British Columbia and (b) to develop the first quantitative estimates of estuarine circulation and new production for this system. Water column observations show a highly stratified two-layer estuarine structure, particularly in the spring and summer months when river discharge and atmospheric heat inputs were high. The net air-sea heat flux had a seasonal range of approximately 220 Wm⁻² and peaked almost a month earlier in 2008 than in 2009. The main source of river input comes from the Wannock River. As temperatures begin to rise in the spring, the river discharge can suddenly increase by an order of magnitude (from about 100 m³ s⁻¹ to almost 1000 m³ s⁻¹) in less than two weeks. Residence times (ie. first-order estimates of estuarine circulation) were estimated for every cruise using salinity and temperature budgets in a two-layer box model parameterization of the flow structure. The results show that upper box residence times vary seasonally with river discharge; dropping from about 14 days in the winter to as low as 4 days in the spring at the freshet onset. An earlier flushing event in 2009 caused residence times to drop earlier and could have caused higher advection losses for phytoplankton in the early spring. Overall, residence times averaged to about 7 days for the upper layer and about 165 days for the lower layer during periods of high river discharge, and about twice that during periods of low river discharge. Deep water in the lower layer below the sill was renewed almost once a year in summer and was affected only by vertical diffusion during the rest of the year. Finally, a spring/summer new production estimate of 0.6-1.7 gCm⁻²d⁻¹ (which implies about 110-300 gCm⁻²y⁻¹ assuming no production during the other months) was obtained by combining transport estimates with observations of nutrients to infer a surface nitrate sink. This range compares well with independent estimates made in nearby regions.
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