OS54A-01 16:00h
On Modeling the Biomass Energy Density Cascade Through An Ecosystem
Following the earlier ideas of Silvert and Platt (1978), a thermodynamic formalism for ecosystem models is described in terms of biomass energy density cascading through a size spectrum from small to larger sizes rather than traditional box type kinematic ecosystem models. The biomass energy model takes account of the differing dynamics of autotrophs, heterotrophs and detritus and dissolved material. Given a size-dependent initial energy density distribution, the model describes the generalized diffusion and dissipation of energy through the ecosystem. The detritus and dissolved material energy equation has storage and loss terms, apart from a gain term from autotrophs and heterotrophs, but no growth term. The autotroph biomass energy equation has strorage, loss, long-range scale separated diffusion due to interaction with heterotrophs, and energy input from the environment via physical factors (e.g. light). The biomass energy equation for heterotrophs features the storage, loss, growth term, as well as complement of the long-range scale separated diffusion energy (autotroph-heterotroph interaction), and energy diffusion within heterotrophs. This work will present some preliminary ideas concerned with such an ecosystems view, including how ocean physics affects the energy density cascade.
OS54A-02 16:15h
Large Eddy Simulation of Particle Settling in the Ocean Mixed Layer
The settling velocity of suspended particles in the ocean mixed layer, which plays an important role in the biogeochemical process in the ocean, was calculated by analyzing the Lagrangian motion of a large number of particles in the three-dimensional turbulent structure of the ocean mixed layer simulated by LES. Two different types of the ocean mixed layer, with and without Langmuir circulation (LC), and two different types of particles with different terminal velocity in the still fluid ws were investigated. In the presence of LC, particles are swept down rapidly following the downward jets of LC, while the uniform downward propagation of particles characterized by diffusion occurs in the absence of it. Settling velocity is smaller than ws in all experiments, but the magnitude of the decrease is larger in the presence of LC, which is consistent with the Stommel's theory on the inhibition of particle settling by a vortex motion. The vertical distribution of particles and the probability distribution of the vertical velocity of particles also show significant effects of LC. Besides, it was found that the effect of turbulent flows on the particle settling becomes stronger for smaller ws.
OS54A-03 16:30h
Simulation of plankton ecosystem dynamics and upper ocean biogeochemistry in the California Current system
Eastern boundary current systems are characterized by strong upwelling of nutrient-rich deep waters and high biological productivity, yet the coupling of physical processes and ecosystem response is not well understood, partly because of the importance of small-scale, short-term variability. We address this issue by investigating these processes in the California Current system with the Regional Oceanic Modeling System (ROMS). Our U.S. west coast setup of ROMS has a horizontal resolution of about 15 km with a nested model covering the central California coast with a resolution of 5 km. The biogeochemical model is an NPZD-type model with nitrogen as the limiting nutrient. The model results (e.g. Chlorophyll concentrations) agree fairly well with observations from remote sensing (SeaWiFS) and shipboard measurements (CalCOFI). However, the simulated productivity is less than estimates derived from remotely sensed or other observed data. We will discuss possible reasons for this discrepancy. We studied the fluxes of nitrogen through the system under climatological forcing. The coupling of ocean physics and biogeochemistry has a strong impact on the dynamics of the system. New production is fueled by the upwelling of nitrate along the coast. It is highest in jets and filaments originating from capes and ridges, with the f-ratio reaching values of 0.6. Further offshore, regenerated production becomes more dominant with f-ratios as low as 0.2. Both new and regenerated production undergo a distinct seasonal cycle, with production peaking between spring (Southern California Bight) and summer (central California). Zooplankton exerts a strong control on the maximum concentration of phytoplankton. Over large scales in time and space, new and export production equal each other. However, strong decoupling between the two is observed on small temporal and spatial scales as discussed in detail by Plattner et al.
OS54A-04 16:45h
Decoupling of Export From New Production: The Role of Lateral Transport
Since the work of Eppley and Peterson in the late 1970s, it has been a widely applied concept in ocean biogeochemical studies to numerically equate export production with new production in equilibrium situations. This concept allows the use of estimates of new production as a substitute for the often more difficult measurements of export production. The underlying assumption is that horizontal transport of organic matter is negligible compared to vertical export. We investigate the validity of this assumption using an eddy-resolving coupled physical-biogeochemical model of the central Californian coast forced with climatological winds as discussed by Frenzel et al. The central Californian coast is dominated by intense coastal upwelling, highly turbulent flow, and high biological production. Meso- and submesoscale phenomena such as eddies and filaments are common features in this environment and are key processes controlling the input of nutrients into the euphotic zone as well as the vertical and lateral export of organic matter. We find that transport associated with such circulation structures leads to a substantial local decoupling of export from new production, which is largest in the very dynamic near-shore region. The distribution of new production is primarily determined by the vertical supply of nitrate and shows a relatively smooth on-offshore gradient caused by the near-shore upwelling and the ensuing lateral supply of nitrate. By contrast, export production shows a complicated pattern with both negative and positive values, even in the annual mean, determined primarily by the convergence and divergence of the flow and the associated vertical transport of organic matter in and out of the euphotic zone. These annual mean divergences and convergences are associated with the fact that eddies and other meso-scale phenomena along the central Californian coast tend to be organized into standing features primarily related to undulations in topography and shoreline. In summary, our results indicate that the concept of numerically equal new and export production has to be used with great care, particularly in dynamic oceanic environments.
OS54A-05 17:00h
Biogeochimical modelling study of Tropical Instability Waves in Pacific ocean.
Tropical Instability Waves (TIW) are prominent features of the equatorial Atlantic and Pacific Oceans. They have been suggested to induce profound modifications of the marine ecosystem with implications for production and carbon fluxes. Here, we use a coupled dynamical-biogeochemical model of the tropical Pacific to document and understand how TIWs affect the ecosystem over the 1992-2000 time period. The model is forced with scatterometer winds and produces realistic TIW variability. The Ecosystem model includes, PO4, NO3, SI and Iron colimitations, 2 classes of phyto and zooplankton and micronekton. The patterns of nutrients, phyto, zooplankton, and micronekton resembles those observed during PICOLO. First, we dicuss the mecanisms at work to produce such patterns and their implication on the production and carbon fluxes at these scales. Second, we force the ecosystem model off line without the TIW dynamical variability to infer the seasonal ecosystem budgets induced by the TIW over the 1992-2000 period.
OS54A-06 17:15h
Biogeochimical modelling study of 1997-1999 ENSO.
The 1997-1999 ENSO was among the largest ENSO on records. It had a dramatic impact of the marine ecosystem in the equatorial Pacific and produced striking features such as large unexpected phytoplankton blooms in the central Pacific when El Nino switched to La Nino in May 1998. Here, we use a coupled dynamical-biogeochemical model of the tropical Pacific to document and understand how the tropical Pacific variability during that ENSO is modified. The model is forced with scatterometer winds over 1992-2000 and produces realistic dynamical and biogeochemical features. We document the biogeochemical onset, development and demise of the El Nino and the return of La Nina with a particular focus on the contrast between the warm pool oligotrophic waters and the upwelling waters, the influence of Kelvin waves onto the ecosystem during that event and the spectacular bloom associated with La Nina's return.
OS54A-07 17:30h
Spatial and Temporal Patterns of Phytoplankton and Sea Surface Temperature during CoOP WEST (northern California)
A dominant physical feature affecting the flux of carbon in coastal regions are zones of high biomass and warm water that represent retention zones for phytoplankton (organic carbon). Retention zones occur in association with capes and bays, and stand out as relatively warm features, with higher chlorophyll content than surrounding waters. Such topographic features exist within the study area of the NSF Coastal Ocean Processes (CoOP) "Wind Events and Shelf Transport" study (WEST) which extends from the Gulf of the Farallones to Point Arena. Through the use of empirical orthogonal function (EOF) analysis of satellite imagery for 2000 to 2003, patterns of variability for chlorophyll and sea surface temperature are identified for this area. Mooring and shipboard data further identify and explain these patterns by providing a three-dimensional view of the apparent retention and longer residence times for phytoplankton over the continental shelf. During spring and summer, this region experiences the strongest wind stress along the US west coast, yet it is still highly productive. There is a lack of understanding of processes that can retain organic particles over the shelf region, perhaps even during strong wind events. While patterns of warm SST and coherent high chlorophyll both north and south of Pt Reyes reflect long residence times, a distinction is made between circulation patterns north and south of the cape - with retentive circulation occurring south of the cape. These areas of longer residence time have a spatial extent of 20-30km, visible in chlorophyll a (SeaWiFS) and sea surface temperature (MODIS) satellite radiometry data during periods of upwelling. In this presentation we explain some of the causes and consequences of these retention zones using satellite EOF analysis, mooring data, and shipboard data.
OS54A-08 17:45h
Storms in the Sargasso Sea: Rapid Particle Export and DIC Drawdown
Increasingly, biogeochemical studies are showing that event-scale phenomena, $\sim$3-10 days in duration, have disproportionately large impacts on phytoplankton biomass and export production in oligotrophic ocean basins. During March 2004, we observed the passage of a storm front that significantly impacted biogeochemical cycling in the Sargasso Sea. The passage of the storm resulted in significant shoaling of the 19$^{o}$C isotherm and the 1.0 $\mu$M nitracline. During the four days after the storm, integrated (0-140 m) primary production ($\sim$1500 mg C m$^{-2}$ d$^{-1}$) was 3-4x greater than the mean water column production previously observed by BATS for this time of year. These high rates of primary production preceded a drawdown in depth-averaged, salinity-normalized DIC of 7-8 $\mu$mol kg$^{-1}$. Estimated rates of CO$_{2}$ exchange would have added 0.05 to 0.2 $\mu$mol kg$^{-1}$ d$^{-1}$ to the mixed layer, counteracting the observed drawdown of DIC. These high rates of primary production and DIC drawdown are congruent with elevated rates of silicate uptake, which averaged ~7.5 mmol Si m$^{-2}$ d$^{-1}$, almost 20x the mean rate reported during non-bloom periods at BATS. There was no significant accumulation of phytoplankton biomass as Synechococcus, picoeukaryotes, and $>$5 $\mu$m eukaryotes densities all remained largely constant and suspended biogenic silica concentrations (indicative of diatoms) increased only marginally, suggesting a significant loss term. Indeed, total biogenic silica sedimentation rates at 200 m (486 to 508 $\mu$mol m$^{-2}$ d$^{-1}$) were as large as any rate previously observed in the Sargasso Sea. Rates of particulate organic carbon and nitrogen export also exceeded prior estimates determined as part of the BATS program. This singular, very short-lived ($\sim$3.5 days) event, following a stratification/destratification event in the upper ocean, represents a significant contribution to annual export production, prior to the traditional spring bloom and is therefore excluded from most estimates of annual new and export production. We hypothesize that the high rates of sedimentation with little accumulation in phytoplankton biomass were due to a very active zooplankton grazing community that rapidly consumed phytoplankton as they grew in response to the storm-induced nutrient injection.