Perturbation dynamics of a planktonic ecosystem

Planktonic ecosystems provide a key mechanism for the transfer of carbon from the atmosphere to the deep ocean via the so-called biological pump. Mathematical models of these ecosystems have been used to predict CO2 uptake in surface waters at particular locations, and more recently have been embedded in global climate models. While the equilibrium properties of these models are well studied, less attention has been paid to their response to external perturbations, despite the fact that as a result of the variability of environmental forcing such ecosystems are rarely, if ever, in equilibrium. In this study, linear theory is used to determine the structure of perturbations to state variables of an ecosystem model describing summertime conditions at Ocean Station P (50°N 145°W) that maximize either instantaneous or integrated export flux. As a result of the presence of both direct and indirect pathways to export in this model, these perturbations involve the dynamics of the entire ecosystem. For all optimal perturbations considered, it is found that the flux to higher trophic levels is the primary contributor to export flux, followed by sinking detritus. In contrast, the contribution of aggregation is negligible. In addition, small phytoplankton contribute significantly (comparable to large phytoplankton) to the export flux through indirect pathways, primarily through the microzooplankton, even following a bloom in only large phytoplankton. While the details of these results may be specific to the particular model under consideration, the optimal perturbation framework is general and can be used to probe the dynamics of any mechanistic ecosystem model

A data science approach to understanding physical drivers of coastal primary productivity and effects on carbonate chemistry

How do ocean mixing regimes influence primary productivity and carbon dynamics? Primary productivity is a key quantity in the quality of habitat for higher trophic levels including larval salmon. Here, we analyze the physical oceanographic and primary productivity dynamics of the Salish Sea using the output of SalishSeaCast, a newly-developed biophysical model based on the NEMO framework (Olson et al, in preparation). The biophysical model estimates three classes of primary producers - diatoms, small flagellates and Mesodynium rubrum. Here, we consider daily depth-integrated biomass signals for all three organismal classes extracted from the model domain over the course of two years, as well as daily signals of halocline depth, river input, wind energy, and tidal mixing. These signals are then analyzed using a normalized hierarchical clustering approach. The analysis shows large biomass variance (~2 orders of magnitude) throughout the model domain, and clear spatial patterns in biomass correspond to regions dominated by different mixing and stratification regimes. The signal clusters demonstrate a clear boundary between the biomass patterns in the Northern and Southern Strait of Georgia, and offer a physical explanation for the difference. We then compare this output to carbonate chemistry data and the developing carbonate chemistry numerical model, to gain insight into biophysical drivers of carbonate chemistry distribution in different regions of the Strait. The study represents the first attempt at a large-scale statistical analysis of the newly-developed model, and demonstrates the unique utility of this approach in identifying discrete regions governed by various primary productivity regimes, and provides a framework for considering their effects on carbonate chemistry. In the future, such an analysis may be used to understand the impact of shifting stratification and mixing regimes on the interaction of primary productivity and carbonate chemistry under anthropogenic climate change

Concentrations and cycling of DMS, DMSP, and DMSO in coastal and offshore waters of the Subarctic Pacific during summer, 2010-2011

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 3269–3286, doi:10.1002/2016JC012465.Concentrations of dimethylsulfide (DMS), measured in the Subarctic Pacific during summer 2010 and 2011, ranged from ∼1 to 40 nM, while dissolved dimethylsulfoxide (DMSO) concentrations (range 13-23 nM) exceeded those of dissolved dimethyl sulfoniopropionate (DMSP) (range 1.3–8.8 nM). Particulate DMSP dominated the reduced sulfur pool, reaching maximum concentrations of 100 nM. Coastal and off shore waters exhibited similar overall DMS concentration ranges, but sea-air DMS fluxes were lower in the oceanic waters due to lower wind speeds. Surface DMS concentrations showed statistically significant correlations with various hydrographic variables including the upwelling intensity (r2 = 0.52, p < 0.001) and the Chlorophyll a/mixed layer depth ratio (r2 = 0.52, p < 0.001), but these relationships provided little predictive power at small scales. Stable isotope tracer experiments indicated that the DMSP cleavage pathway always exceeded the DMSO reduction pathway as a DMS source, leading to at least 85% more DMS production in each experiment. Gross DMS production rates were positively correlated with the upwelling intensity, while net rates of DMS production were significantly correlated to surface water DMS concentrations. This latter result suggests that our measurements captured dominant processes driving surface DMS accumulation across a coastal-oceanic gradient.Natural Sciences and Engineering Research Council of Canada, from the Peter Wall Institute for Advanced Studies2017-10-2

Better Regional Ocean Observing Through Cross-National Cooperation: A Case Study From the Northeast Pacific

The ocean knows no political borders. Ocean processes, like summertime wind-driven upwelling, stretch thousands of kilometers along the Northeast Pacific (NEP) coast. This upwelling drives marine ecosystem productivity and is modulated by weather systems and seasonal to interdecadal ocean-atmosphere variability. Major ocean currents in the NEP transport water properties such as heat, fresh water, nutrients, dissolved oxygen, pCO2, and pH close to the shore. The eastward North Pacific Current bifurcates offshore in the NEP, delivering open-ocean signals south into the California Current and north into the Gulf of Alaska. There is a large and growing number of NEP ocean observing elements operated by government agencies, Native American Tribes, First Nations groups, not-for-profit organizations, and private entities. Observing elements include moored and mobile platforms, shipboard repeat cruises, as well as land-based and estuarine stations. A wide range of multidisciplinary ocean sensors are deployed to track, for example, upwelling, downwelling, ocean productivity, harmful algal blooms, ocean acidification and hypoxia, seismic activity and tsunami wave propagation. Data delivery to shore and observatory controls are done through satellite and cell phone communication, and via seafloor cables. Remote sensing from satellites and land-based coastal radar provide broader spatial coverage, while numerical circulation and biogeochemical modeling complement ocean observing efforts. Models span from the deep ocean into the inland Salish Sea and estuaries. NEP ocean observing systems are used to understand regional processes and, together with numerical models, provide ocean forecasts. By sharing data, experiences and lessons learned, the regional ocean observatory is better than the sum of its parts

An investigation of possible phytoplankton seeding in the Strait of Georgia from Nanoose Bay

This thesis investigates the possibility of bays providing the seed population for the spring phytoplankton bloom to larger adjacent bodies of water via advective transport. The study area was Nanoose Bay, Vancouver Island and the adjacent region of the Strait of Georgia. In 1992 and 1993 data were collected 2-3 times weekly and a mooring with an array of 5 current meters was placed at the mouth of the bay during the 1992 study. Interannual variability was tremendous. In 1992 seeding from Nanoose Bay was not possible as the net transport was into the bay at the surface and middle depths. The influence of the Fraser River seemed to dominate as low density water with high silicate concentrations was present at the surface and density profiles were generally well stratified. Although nutrients were not limiting and light availability appeared high, phytoplankton concentrations were low until March 5 when they began to increase and a bloom occurred. It is suggested that horizontal advection and flushing of the bay were responsible for suppressing a bloom prior to March 5 in 1992. In 1993 phytoplankton concentrations were high inside the bay from the beginning of February onward. In the Strait no periods of high phytoplankton concentration occurred although there were two small increases which appear to be due to advective transport, although it is possible that the first was due to reduced wind mixing. It is suggested that seeding of the Strait from Nanoose Bay was possible in this year, although it is also possible that seeding occurred from other locations depending on time and conditions. The conservation equation for a scalar was used to investigate advective transport as balanced by biological sources and sinks. With no current measurements available in 1993, estimates were made from the above equation and compared to wind direction in the Strait and density changes at the mouth of the bay. In 1993 profiles were well mixed with respect to 1992 and overall salinity was higher. It is suggested that light was usually limiting to phytoplankton growth in the Strait due to vertical mixing throughout the study, while in the bay the depth of the water column limited vertical mixing thus allowing phytoplankton to bloom. To continue experiments of this type it is suggested that daily sampling be done as temporal changes can occur quickly. As evidenced from the 1993 data, spatial resolution is also valuable. Current measurements are necessary and their absence in the 1993 data set was unfortunate. It is suggested that drogues may be useful for measuring currents. They could be used to attempt to track phytoplankton when concentrations begin to increase.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

A carbon and nitrogen flux model in a coastal upwelling region

An improved understanding of oceanic biogeochemical cycles is critical for predicting global climate. In coastal upwelling regions fluxes of both carbon and nitrogen are disproportionately large relative to the global ocean. This is especially true of the downward flux of organic matter that transports carbon away from the ocean surface layer causing absorption of atmospheric carbon dioxide (i.e. the 'biological pump'). In addition to biologically limiting nutrients, upwelling brings inorganic carbon to the surface, potentially providing a source of carbon to the atmosphere. However, global models seldom resolve the complex, non-homogeneous coastal ocean. I have developed a carbon and nitrogen flux model for coastal upwelling regions that considers all important processes both within and below the euphotic zone over time scales of days to decades. Physical circulation is represented by six boxes, an upper and lower box for three horizontal regions: the continental shelf, slope and open ocean. Dissolved inorganic, dissolved organic and particulate organic forms of both carbon and nitrogen as well as salinity are modelled. The model is parameterized and physically forced to apply to the west coast of Vancouver Island, Canada. In addition, a field study during July 1998 was undertaken to complement and constrain the model. The model predicts annual primary production, net air-sea CO₂ gas exchange and net export fluxes between the open ocean and the model system under different physical forcing scenarios (including El Niño-Southern Oscillation Events). Model results suggest that coastal upwelling regions do not contribute to the oceanic sequestration of CO₂; rather, they provide a conduit for subsurface inorganic carbon to the surface ocean. There is net annual air-sea CO₂ gas influx over the shelf and slope, but it is small. In winter, subsurface waters enriched in inorganic carbon and nitrogen are mixed into the surface, causing gas evasion that almost balances the summer invasion. On the other hand, there is a large flux of inorganic carbon from the lower ocean to the surface ocean via the model system, probably leading to CO₂ outgassing offshore of the shelf. Meanwhile, fluxes of organic carbon from the model system to the open ocean are small (compared with the inorganic carbon flux), especially in the lower layer. Thus, temperate coastal upwelling regions do not operate as strong biological pumps.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

The effect of vertical and horizontal dilution on fertilized patch experiments

A great deal of attention, both negative and positive, has been directed at the potential of large-scale iron fertilization schemes to sequester carbon by inducing phytoplankton blooms that would, in theory, result in significant export of organic carbon to the deep ocean in high nitrogen - low chlorophyll regions. A suite of iron manipulation or `patch' experiments have been performed over length-scales of 10s of km. Here, we use a physical-ecological-chemical model, with prognostic nitrogen, silica and iron dynamics, to study one of the most successful of these experiments,the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES), focusing on the vertical export of organic material, which is difficult to observe in the field. The implications of large-scale fertilization, i.e. increasing patch size, are investigated. Our results agree with the general conclusions obtained from the field experiments. Only a modest export of organic carbon occurs (less than 25% of carbon uptake by phytoplankton) at the base of the mixed layer. Furthermore, we show that lateral and vertical supply of silicic acid is necessary to fuel a sustained phytoplankton bloom. Increasing patch size results in less lateral nutrient supply relative to patch area and so a decrease, not only in total production (per unit area), but in the contribution by large phytoplankton due to silica limitation. Most importantly, the export of organic carbon (per unit area) decreases substantially, by nearly an order of magnitude as scales of 1000 km are approached

Mechanisms that influence pH and aragonite saturation state in the Strait of Georgia

A 1-D vertical biophysical coupled model is used to investigate the seasonal, interannual, and long-term variability of pH and aragonite saturation state in the southern Strait of Georgia. The model is initialized using casts from local sampling programs, and continuously forced with local meteorological and river discharge observations. Dissolved inorganic carbon (DIC) and total alkalinity are modeled as scalar quantities and used to calculate pH and aragonite saturation state. Model results show an aragonite saturation horizon at ~20 m that shoals to the surface during winter and sometimes in summer during large freshets from the Fraser River. pH is high (\u3e 8) near the surface in spring/summer/fall and low (\u3c 7.7) below 10 m due to entrainment of DIC-rich water from the intermediate layer. Sensitivity studies suggest a seasonal succession of forcing dominance on surface pH and aragonite saturation state. In spring, pH is strongly anticorrelated to windspeed due to mixing across the large, shallow pH gradient. In summer, pH and river discharge are anticorrelated due to reduced primary productivity near the Fraser River plume. The deepening of the aragonite saturation horizon below the surface in early spring appears to coincide with the onset of the spring diatom bloom, and the summer surface aragonite undersaturation duration is clearly a function of Fraser River discharge. This study demonstrates the importance of local forcing in determining the interannual variability of near-surface pH and aragonite saturation state in the Strait of Georgia

Local Inorganic Carbon Dynamics: Acidification Status in the Georgia and Juan de Fuca Straits

We present findings from recent carbon modeling studies and 5 years of carbon sampling in the Georgia and Juan de Fuca Straits. Near-surface pH and aragonite saturation state (ΩA) are highly variable (temporally and with depth) in stratified areas like the Strait of Georgia, and vary less in well-mixed areas like Juan de Fuca Strait and the San Juan Islands. Strong seasonal productivity increases Strait of Georgia surface pH and ΩA during summer, but these waters are naturally corrosive (aragonite-undersaturated) during winter and below 20 m depth. Several potentially acidification-sensitive species of plankton and shellfish reside at least partially within these corrosive zones. Strong Fraser River freshets can offset surface pH and ΩA increases because of low-salinity carbon dynamics, river shading of phytoplankton, and calcium dilution. Estuarine-driven inflows from the Pacific Ocean characterize the deep Juan de Fuca Strait and are carbon-rich during the upwelling season. However, these inflows effectively increase pH and ΩA in the Strait of Georgia because of local carbon retention. These findings may provide insights into several areas of marine management including preservation area planning, aquaculture best practices, and seasonal fisheries forecasting