Dusenberry
Jeffrey A.
Dusenberry
Jeffrey A.
No Thumbnail Available
Search Results
Now showing
1 - 3 of 3
-
ArticleAssessment of skill and portability in regional marine biogeochemical models : role of multiple planktonic groups(American Geophysical Union, 2007-08-02) Friedrichs, Marjorie A. M. ; Dusenberry, Jeffrey A. ; Anderson, Laurence A. ; Armstrong, Robert A. ; Chai, Fei ; Christian, James R. ; Doney, Scott C. ; Dunne, John P. ; Fujii, Masahiko ; Hood, Raleigh R. ; McGillicuddy, Dennis J. ; Moore, J. Keith ; Schartau, Markus ; Spitz, Yvette H. ; Wiggert, Jerry D.Application of biogeochemical models to the study of marine ecosystems is pervasive, yet objective quantification of these models' performance is rare. Here, 12 lower trophic level models of varying complexity are objectively assessed in two distinct regions (equatorial Pacific and Arabian Sea). Each model was run within an identical one-dimensional physical framework. A consistent variational adjoint implementation assimilating chlorophyll-a, nitrate, export, and primary productivity was applied and the same metrics were used to assess model skill. Experiments were performed in which data were assimilated from each site individually and from both sites simultaneously. A cross-validation experiment was also conducted whereby data were assimilated from one site and the resulting optimal parameters were used to generate a simulation for the second site. When a single pelagic regime is considered, the simplest models fit the data as well as those with multiple phytoplankton functional groups. However, those with multiple phytoplankton functional groups produced lower misfits when the models are required to simulate both regimes using identical parameter values. The cross-validation experiments revealed that as long as only a few key biogeochemical parameters were optimized, the models with greater phytoplankton complexity were generally more portable. Furthermore, models with multiple zooplankton compartments did not necessarily outperform models with single zooplankton compartments, even when zooplankton biomass data are assimilated. Finally, even when different models produced similar least squares model-data misfits, they often did so via very different element flow pathways, highlighting the need for more comprehensive data sets that uniquely constrain these pathways.
-
ArticleClimate-mediated changes to mixed-layer properties in the Southern Ocean : assessing the phytoplankton response(Copernicus Publications on behalf of the European Geosciences Union, 2008-05-19) Boyd, Philip W. ; Doney, Scott C. ; Strzepek, R. ; Dusenberry, Jeffrey A. ; Lindsay, Keith ; Fung, Inez Y.Concurrent changes in ocean chemical and physical properties influence phytoplankton dynamics via alterations in carbonate chemistry, nutrient and trace metal inventories and upper ocean light environment. Using a fully coupled, global carbon-climate model (Climate System Model 1.4-carbon), we quantify anthropogenic climate change relative to the background natural interannual variability for the Southern Ocean over the period 2000 and 2100. Model results are interpreted using our understanding of the environmental control of phytoplankton growth rates – leading to two major findings. Firstly, comparison with results from phytoplankton perturbation experiments, in which environmental properties have been altered for key species (e.g., bloom formers), indicates that the predicted rates of change in oceanic properties over the next few decades are too subtle to be represented experimentally at present. Secondly, the rate of secular climate change will not exceed background natural variability, on seasonal to interannual time-scales, for at least several decades – which may not provide the prevailing conditions of change, i.e. constancy, needed for phytoplankton adaptation. Taken together, the relatively subtle environmental changes, due to climate change, may result in adaptation by resident phytoplankton, but not for several decades due to the confounding effects of climate variability. This presents major challenges for the detection and attribution of climate change effects on Southern Ocean phytoplankton. We advocate the development of multi-faceted tests/metrics that will reflect the relative plasticity of different phytoplankton functional groups and/or species to respond to changing ocean conditions.
-
ThesisPicophytoplankton photoacclimation and mixing in the surface oceans(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1995-02) Dusenberry, Jeffrey A.Fluctuations in light intensity due to vertical mixing in the open ocean surface layer will affect phytoplankton physiology. Conversely, indicators of phytoplankton photoacclimation will be diagnostic of mixing processes if the appropriate kinetics are known. A combination of laboratory and field experimental work, field observations, and theoretical models were used to quantify the relationship between vertical mixing and photoacclimation in determining the time and space evolution of single cell optical properties for the photosynthetic picoplankton, Prochlorococcus spp. Diel time-series observations from the Sargasso Sea reveal patterns in single-cell fluorescence distributions within Prochlorococcus spp. populations which appear to correspond to decreasing mixing rates and photoacclimation during the day, and increased mixing at night. Reciprocal light shift experiments were used to quantify the photoacclimation kinetics for Prochlorococcus spp. fluorescence. A laboratory continuous culture system was developed which could simulate the effects of mixing across a light gradient at the level of the individual cell. This system was operated at four different simulated diffusivities. Prochlorococcus marinus strain Med4 fluorescence distributions show distinct patterns in the mean and higher moments which are consistent with a simple quasi-steady turbulent diffusionphotoacclimation model. In both, daytime photoacclimation drove the development of a gradient in mean fluorescence, a decrease in variance overall, and skewing of distributions away from the boundaries. These results suggest that picophytoplankton single-cell fluorescence distributions could prove to be a useful diagnostic indicator of the mixing environment.