Katja Fennel

Testing a marine ecosystem model: sensitivity analysis and parameter optimization

A data assimilation technique is used with a simple but widely used marine ecosystem model to optimize poorly known model parameters. A thorough analysis of the a posteriori errors to be expected for the estimated parameters was carried out. The errors have been estimated by calculating the Hessian matrices for different problem formulations based on identical twin experiments. The error analysis revealed inadequacies in the formulation of the optimization problem and insufficiencies of the applied data set. Modifications of the actual problem formulation, which improved the accuracy of the estimated parameters considerably, are discussed. The optimization procedure was applied to real measurements of nitrate and chlorophyll at the Atlantic Bermuda site. The parameter optimization gave poor results. We suggest this to be due to features of the ecosystem that are unresolved by the present model formulation. Our results emphasize the necessity of an error analysis to accompany any parameter optimization study.

Denitrification effects on air-sea CO2 flux in the coastal ocean: Simulations for the northwest North Atlantic

[1] The contribution of coastal oceans to the global air-sea CO2 flux is poorly quantified due to insufficient availability of observations and inherent variability of physical, biological and chemical processes. We present simulated air-sea CO2 fluxes from a high-resolution biogeochemical model for the North American east coast continental shelves, a region characterized by significant sediment denitrification. Decreased availability of fixed nitrogen due to denitrification reduces primary production and incorporation of inorganic carbon into organic matter, which leads to an increase in seawater pCO2, but also increases alkalinity, which leads to an opposing decrease in seawater pCO2. Comparison of simulations with different numerical treatments of denitrification and alkalinity allow us to separate and quantify the contributions of sediment denitrification to air-sea CO2 flux. The effective alkalinity flux resulting from denitrification is large compared to estimates of anthropogenically driven coastal acidification. Citation: Fennel, K., J. Wilkin, M. Previdi, and R. Najjar (2008), Denitrification effects on air-sea CO2 flux in the coastal ocean: Simulations for the northwest North Atlantic, Geophys. Res. Lett., 35, L24608, doi:10.1029/2008GL036147.

A box model approach for a long-term assessment of estuarine eutrophication, Szczecin Lagoon, southern Baltic

Abstract We develop a layered “box model” to evaluate the major effects of estuarine eutrophication of the Szczecin lagoon which can be compared with integrating measures (chlorophyll a (Chl a ), sediment burial, sediment oxygen consumption (SOC), input and output of total nutrient loads) and use it to hindcast the period 1950–1996 (the years when major increase in nutrient discharges by the Oder River took place). The following state variables are used to describe the cycling of the limiting nutrients (nitrogen and phosphorus): phytoplankton (Phy), labile and refractory detritus (D N , D Nref , D P , D Pref ), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and oxygen (O 2 ). The three layers of the model include two water layers and one sediment layer. Decrease of the carrying capacity with respect to the increased supply of organic matter of the system with advancing eutrophication over the period studied is parameterized by an exponential decrease of the sediment nitrogen fluxes with increasing burial, simulating changing properties from moderate to high accumulating sediments. The seasonal variation as well as the order of magnitude of nutrient concentrations and phytoplankton stocks in the water column remains in agreement with recent observations. Calculated annual mean values of nutrient burial of 193 mmol N m −2 a −1 and 23 mmol P m −2 a −1 are supported by observed values from geological sediment records. Estimated DIN remineralization in the sediments between 100 and 550 mmol N m −2 a −1 corresponds to SOC measurements. Simulated DIP release up to 60 mmol P m −2 a −1 corresponds to recent measurements. The conceptual framework presented here can be used for a sequential box model approach connecting small estuaries like the Szczecin lagoon and the open sea, and might also be connected with river box models.

Domoic acid uptake and elimination kinetics in oysters and mussels in relation to body size and anatomical distribution of toxin.

Toxin accumulation by suspension-feeding qualifier depends on a balance between processes regulating toxin uptake (i.e. ingestion and absorption of toxic cells) and elimination (i.e. egestion, exchange among tissues, excretion, degradation and/or biotransformation) during exposure to toxic blooms. This laboratory study compares the size-specific uptake and elimination kinetics of domoic acid (DA) from Pseudo-nitzschia multiseries in two co-occurring bivalves, the oyster Crassostrea virginica and the mussel Mytilus edulis. Domoic acid concentrations were measured in visceral and non-visceral tissues of different-sized oysters and mussels during simultaneous long-term exposure to toxic P. multiseries cells in the laboratory, followed by depuration on a non-toxic algal diet. Mussels attained 7-17-fold higher DA concentrations than oysters, depending on the body size and exposure time, and also detoxified DA at higher rates (1.4-1.6 d(-1)) than oysters (0.25-0.88 d(-1)) of a comparable size. Small oysters attained markedly higher weight-specific DA concentrations (maximum=78.6 μg g(-1)) than large, market-sized individuals (≤ 13 μg g(-1)), but no clear relationship was found between body size and DA concentration in mussels (maximum=460 μg g(-1)). Therefore, differential DA accumulation by the two species was, on average, approximately 3-fold more pronounced for large bivalves. An inverse relationship between DA elimination rate and body size was established for oysters but not mussels. Elimination of DA was faster in viscera than in other tissues of both bivalves; DA exchange rate from the former to the latter was higher in oysters. The contribution of viscera to the total DA burden of mussels was consistently greater than that of other tissues during both uptake (>80%) and depuration (>65%) phases, whereas it rapidly decreased from 70-80% to 30-40% in oysters, and this occurred faster in smaller individuals. Residual DA concentrations (≤ 0.25 μg g(-1)) were detected at later depuration stages (up to 14 d), mainly in viscera of oysters and non-visceral tissues of mussels, suggesting that a second, slower-detoxifying toxin compartment exists in both species. However, a simple exponential decay model was found to adequately describe DA elimination kinetics in these bivalves. The lower capacity for DA accumulation in oysters compared to mussels can thus only be explained by the formers comparatively low toxin intake rather than faster toxin elimination.

Impacts of iron control on phytoplankton production in the modern and glacial Southern Ocean

Abstract Paleoceanographic evidence points to the Southern Ocean as a strong sink for atmospheric CO2 during the Last Glacial Maximum (LGM), but no consensus about the responsible mechanism has yet been reached. Martin (Paleoceanography 5 (1990) 1) postulated that greater iron input during the LGM could have stimulated phytoplankton to consume the surface nutrients in the Southern Ocean, increasing carbon export substantially. We use a simple ecological model to elucidate the extent to which iron availability affects export production in the southwest Pacific sector. The model includes the effect of iron in a semi-explicit way. Based on the physiological response of the photosynthetic apparatus, the simulated phytoplankton growth rates are explicitly dependent on iron. The cycling of iron in the food web is not tracked, since uncertainties persist about the dynamics of iron uptake, transformation and release processes within the pelagic community. A simulation of the present ocean that uses the semi-explicit approach to include iron is compared with a simulation that accounts for iron only implicitly by using model parameters that are typical for low iron conditions. The simulations agree within the range of available observations. Glacial scenarios are simulated (assuming an increase in iron supply) and compared to the modern ocean simulation. Primary and export production increase in the glacial simulations, in particular if we assume an adaptation of the Si:N cell quota of diatoms to the higher iron levels. In this case the export doubles north of the Polar Front and in the Seasonal Ice Zone.

A reevaluation of the magnitude and impacts of anthropogenic atmospheric nitrogen inputs on the ocean

We report a new synthesis of best estimates of the inputs of fixed nitrogen to the world ocean via atmospheric deposition and compare this to fluvial inputs and dinitrogen fixation. We evaluate the scale of human perturbation of these fluxes. Fluvial inputs dominate inputs to the continental shelf, and we estimate that about 75% of this fluvial nitrogen escapes from the shelf to the open ocean. Biological dinitrogen fixation is the main external source of nitrogen to the open ocean, i.e., beyond the continental shelf. Atmospheric deposition is the primary mechanism by which land-based nitrogen inputs, and hence human perturbations of the nitrogen cycle, reach the open ocean. We estimate that anthropogenic inputs are currently leading to an increase in overall ocean carbon sequestration of ~0.4% (equivalent to an uptake of 0.15 Pg C yr−1 and less than the Duce et al. (2008) estimate). The resulting reduction in climate change forcing from this ocean CO2 uptake is offset to a small extent by an increase in ocean N2O emissions. We identify four important feedbacks in the ocean atmosphere nitrogen system that need to be better quantified to improve our understanding of the perturbation of ocean biogeochemistry by atmospheric nitrogen inputs. These feedbacks are recycling of (1) ammonia and (2) organic nitrogen from the ocean to the atmosphere and back, (3) the suppression of nitrogen fixation by increased nitrogen concentrations in surface waters from atmospheric deposition, and (4) increased loss of nitrogen from the ocean by denitrification due to increased productivity stimulated by atmospheric inputs.

What proportion of riverine nutrients reaches the open ocean

Globally, rivers deliver significant quantities of nitrogen (N) and phosphorus (P) to the coastal ocean each year. Currently, there are no viable estimates of how much of this N and P escapes biogeochemical processing on the shelf to be exported to the open ocean; most models of N and P cycling assume that either all or none of the riverine nutrients reach the open ocean. We address this problem by using a simple mechanistic model of how a low-salinity plume behaves outside an estuary mouth. The model results in a global map of riverine water residence times on the shelf, typically a few weeks at low latitudes and up to a year at higher latitudes, which agrees well with observations. We combine the map of plume residence times on the shelf with empirical relationships that link residence time to the proportions of dissolved inorganic N (DIN) and P (DIP) exported and use a database of riverine nutrient loads to estimate the global distribution of riverine DIN and DIP supplied to the open ocean. We estimate that 75% of DIN and 80% of DIP reaches the open ocean. Ignoring processing within estuaries yields annual totals of 17 Tg DIN and 1.2 Tg DIP reaching the open ocean. For DIN this supply is about 50% of that supplied via atmospheric deposition, with significant east-west contrasts across the main ocean basins. The main sources of uncertainty are exchange rates across the shelf break and the empirical relationships between nutrient processing and plume residence time.

Geochemical and Biological Consequences of Phytoplankton Evolution

Publisher Summary This chapter examines the role of marine photoautotrophs on Earths carbon cycle, with an emphasis on the impact of these organisms on the redox changes inferred from the isotopic signals preserved in the geological record. The ““biological”” or “fast” carbon cycle is biologically driven and is based on redox reactions, which are at the core the fundamental chemistry of life. A critical point in the discussion of the Wilson cycle is the exchange between oceanic crust and cratons. This is followed by a review of the macroevolutionary trends of marine phytoplankton. There is a clear link between the history of phytoplankton evolution and the carbon cycle. The third section of the chapter considers the carbon isotopic records in carbonates and organic matter throughout the Phanerozoic. In the chapter, the author has suggested that there may be a causal relationship between the large-scale tectonics of the current Wilson cycle and biogeochemical cycles driven, at least in part, by changing phytoplankton community structure. The chapter gives a closer look at past Wilson cycles, macroevolutionary changes, and biogeochemical cycles. Carbon isotope records provide critical information that can be used to reconstruct changes in redox conditions and biological processes that affected past atmospheric and seawater chemistry. Carbon isotope records of carbonates and organic matter are used in conjunction with sulfur isotopes of sulfates, in model simulations to reconstruct carbon burial, pCO 2 , and pO 2 . These model results indicate that organic carbon burial and pO 2 have increased since the Early Jurassic, whereas pCO 2 has decreased since the Early Cretaceous.

Modeling controls of phytoplankton production in the southwest Pacific sector of the Southern Ocean

Abstract The Southern Ocean is the largest high-nutrient, low-chlorophyll region in the worlds ocean and a potentially important site for the sequestration of carbon. We present a one-dimensional physical/biogeochemical model that integrates biogeochemical measurements obtained during the AESOPS (U.S. JGOFS) study in the southwest Pacific sector to elucidate the controls of primary productivity and export. The model is applied to a series of four stations along 170°W spanning the different biogeochemical subsystems in the Polar Frontal Zone, the Polar Front, and the Seasonal Ice Zone south of the Polar Front. Since horizontal fluxes of heat, freshwater and nutrients are found to be important but cannot be resolved explicitly in a one-dimensional model, we employ a restoration of temperature, salinity and nutrients. The surface fluxes of light and momentum are modified during ice-covered periods to account for the effects of sea ice. The biological model component includes the elemental cycles of nitrogen and silica. Diatoms are represented as a separate phytoplankton group, and small phytoplankton and zooplankton are tightly coupled. The effect of the low iron availability in the region is implicitly taken into account by using typical phytoplankton growth rates and a typical, high Si:N stoichiometry of 4 for the diatoms. The model captures the essential features of the distinct subsystems including the low-chlorophyll condition north of the Polar Front, the diatom blooms in the vicinity of the Polar Front and to its south, the differential drawdown of nitrate and silicic acid, and the seasonal patterns of biogenic silica, primary production and vertical particle flux. “Top-down” control of the small phytoplankton by efficient microzooplankton grazing and “bottom-up” control of diatoms by light and silicic acid are the main factors for the simulated behavior. A sensitivity analysis of the biological model component shows that the growth parameters for the two phytoplankton groups are most important in constraining primary productivity and overall model behavior. This implies that changes in growth rates induced by variations in iron supply as assumed over glacial–interglacial transitions can affect primary and export production substantially.

A modeling study of physical controls on hypoxia generation in the northern Gulf of Mexico

The Louisiana shelf (LA shelf) in the northern Gulf of Mexico experiences hypoxic conditions every summer due to the combination of eutrophication and strong water column stratification. Here we use a three-dimensional circulation model coupled with a simple oxygen model to examine the physical controls on hypoxia generation on the LA shelf. The model assumes a constant oxygen utilization rate in the water column and a sediment oxygen consumption rate that depends on the bottom water oxygen concentration and temperature. Despite its simplicity, the model reproduces the observed variability of dissolved oxygen and hypoxia on the LA shelf, highlighting the importance of physical processes. Model results demonstrate that both river discharge and wind forcing have a strong effect on the distribution of the river plume and stratification, and thereby on bottom dissolved oxygen concentrations and hypoxia formation on the LA shelf. The seasonal cycle of hypoxia is relatively insensitive to the seasonal variability in river discharge, but the time-integrated hypoxic area is very sensitive to the overall magnitude of river discharge. Changes in wind speed have the greatest effect on the simulated seasonal cycle of hypoxia and hypoxic duration, while changes in wind direction strongly influence the geographic distribution of hypoxia. Given that our simple oxygen model essentially reproduces the evolution of hypoxia simulated with a full biogeochemical model and that physical processes largely determine the magnitude and distribution of hypoxia, a full biogeochemical model might not be necessary for short-term hypoxia forecasting.