Raleigh R. Hood

Assessment of skill and portability in regional marine biogeochemical models: Role of multiple planktonic groups

[1] 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 onedimensional 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.

Detecting Trichodesmium blooms in SeaWiFS imagery

A multispectral classification scheme was developed to detect the cyanobacteria Trichodesmium spp. in satellite data of the sea-viewing wide field-of-view sensor (SeaWiFS). The criteria for this scheme were established from spectral characteristics derived from (1) SeaWiFS imagery of a Trichodesmium bloom located in the South Atlantic Bight and (2) modeled remote sensing reflectances of Trichodesmium and other phytoplankton. The classification scheme, which is valid for moderate chlorophyll concentrations of Trichodesmium in coastal waters, is based on the magnitude of the 490-channel reflectance and the spectral shape of remote sensing reflectance at 443, 490 and 555 nm. Analysis suggests that the spatial structure of Trichodesmium populations at sub-pixel scales must be considered when employing spectral characteristics to detect their presence in satellite imagery. This study demonstrates the potential of mapping Trichodesmium from space using spectral observations, even in waters as optically complex as the South Atlantic Bight. Future efforts, which will incorporate ancillary data such as wind speeds and water temperature, will improve the likelihood of correct identification.

A four-component ecosystem model of biological activity in the Arabian Sea

Abstract A coupled, physical-biological model is used to study the processes that determine the annual cycle of biological activity in the Arabian Sea. The physical model is a 2 1 2 - layer system with a surface mixed layer imbedded in the upper layer, and fluid is allowed to move between layers via entrainment, detrainment and mixing processes. The biological model consists of a set of advective-diffusive equations in each layer that determine the nitrogen concentrations in four compartments: nutrients, phytoplankton, zooplankton and detritus. Coupling is provided by the horizontal-velocity, layer-thickness, entrainment and detrainment fields from the physical solution. Surface forcing fields (such as wind stress and photosynthetically active radiation) are derived from monthly climatological data, and the source of nitrogen for the system is upward diffusion of nutrients from the deep ocean into the lower layer. Our main-run solution compares favorably with observed physical and biological fields; in particular, it is able to simulate all the prominent phytoplankton blooms visible in the CZCS data. Three bloom types develop in response to the physical processes of upwelling, detrainment and entrainment. Upwelling blooms are strong, long-lasting events that continue as long as the upwelling persists. They occur during the Southwest Monsoon off Somalia, Oman and India as a result of coastal alongshore winds, and at the mouth of the Gulf of Aden through Ekman pumping. Detrainment blooms are intense, short-lived events that develop when the mixed layer thins abruptly, thereby quickly increasing the depth-averaged light intensity available for phytoplankton growth. They occur during the fall in the central Arabian Sea, and during the spring throughout most of the basin. In contrast to the other bloom types, entrainment blooms are weak because entrainment steadily thickens the mixed layer, which in turn decreases the depth-averaged light intensity. There is an entrainment bloom in the central Arabian Sea during June in the solution, but it is not apparent in the CZCS data. Bloom dynamics are isolated in a suite of diagnostic calculations and test solutions. Some results from these analyses are the following. Entrainment is the primary nutrient source for the offshore bloom in the central Arabian Sea, but advection and recycling also contribute. The ultimate cause for the decay of the solutions spring (and fall) blooms is nutrient deprivation, but their rapid initial decay results from grazing and self shading. Zooplankton grazing is always an essential process, limiting phytoplankton concentrations during both bloom and oligotrophic periods. Detrital remineralization is also important: in a test solution without remineralization, nutrient levels drop markedly in every layer of the model and all blooms are severely weakened. Senescence, however, has little effect: in a test solution without senescence, its lack is almost completely compensated for by increased grazing. Finally, the models detrainment blooms are too brief and intense in comparison to the CZCS data; this difference cannot be removed by altering biological parameters, which suggests that phytoplankton growth in the model is more sensitive to mixed-layer thickness than it is in the real ocean.

Phosphorus deficiency in the Atlantic: An emerging paradigm in oceanography

Nitrogen, iron, and silica are widely considered to be the most important nutrients that limit phytoplankton growth in the worlds oceans. Though clearly important in lakes, the role of phosphorus has been largely ignored in the ocean. In part, this is because of early studies that suggested there was excess phosphate (P) relative to the needs of the phytoplankton in open ocean waters. Thanks to recent studies at the Hawaiian Ocean Time (HOT) series station (Station ALOHA) in the North Pacific subtropical gyre [Karl et al., 2001, and references therein], there is a growing appreciation of the potential importance of phosphorus as a limiting nutrient in subtropical Pacific waters.

Modeling the effect of nitrogen fixation on carbon and nitrogen fluxes at BATS

Recent geochemical estimates of N2-fixation in the North Atlantic ocean indicate rates that are significantly higher than those derived from direct observations. In this paper different N2-fixation rate scenarios are explored using a one-dimensional, biogeochemical model that includes an explicit representation of Trichodesmium. This model reproduces most of the observed interannual variability in phytoplankton production and generates seasonal Trichodesmium biomass and N2-fixation cycles similar to those observed at BATS. Two solutions are presented, one where the N2-fixation rate is increased enough to reproduce the observed summertime drawdown of DIC, and a second where it is tuned to reproduce the observed sediment trap fluxes. The high N2-fixation solution reproduces the seasonal and interannual variability in DIC concentrations quite accurately and generates N2-fixation rates that agree with direct rate measurements from 1990 and recent geochemical estimates. However, this solution generates export fluxes that are more than 4 times higher than those observed, and predicts the development of DON and DOC anomalies in late summer/early fall that have not been observed. In contrast, the low N2-fixation solution generates trap fluxes that are approximately correct, but overestimates the summertime DIC concentrations by 20–View the MathML source. Both solutions indicate that there is significant interannual variability in N2-fixation at BATS and that the rates were much lower in 1995–1996 than in the previous six years. It is suggested that this variability is linked to decadal-scale fluctuations in the North Atlantic climate.

The impact of mixotrophy on planktonic marine ecosystems

Abstract Mixotrophic protists, which utilize a nutritional strategy that combines phototrophy and phagotrophy, are commonly found in fresh, estuarine, and oceanic waters at all latitudes. A number of different physiological types of mixotrophs are possible, including forms which are able to use both phototrophy and phagotrophy equally well, primarily phototrophic phagocytic ‘algae’, and predominantly heterotrophic photosynthetic ‘protozoa’. Mixotrophs are expected to have important effects on the trophic dynamics of ecosystems, but the exact nature of these effects is not known and likely varies with physiological type. In order to study the impact that mixotrophs may have on the microbial food web, we developed mathematical formulations that simulate each of the three aforementioned physiological types of mixotrophs. These were introduced into idealized, steady-state open ocean and coastal/estuarine environments. Our results indicate that mixotrophs compete for resources with both phytoplankton and zooplankton and that their relative abundance is a function of the feeding strategy (physiological type and whether or not they feed on zooplankton) and the maximum growth and/or grazing rates of the organisms. In our models coexistence of mixotrophs with phytoplankton and zooplankton generally occurs within reasonable parameter ranges, which suggests that mixotrophy represents a unique resource niche under summertime, quasi-steady state conditions. We also find that the introduction of mixotrophs tends to decrease the primary production based on uptake of nitrogen from the dissolved inorganic nitrogen pool, but that this decrease may be compensated for by mixotrophic primary production based upon organic nitrogen sources.

Influences of diurnal and intraseasonal forcing on mixed‐layer and biological variability in the central Arabian Sea

A three-dimensional, physical-biological model of the Indian Ocean is used to study the influences of diurnal and intraseasonal forcing on mixed-layer and biological variability in the central Arabian Sea, where a mooring was deployed and maintained from October 1994 to October 1995 by the Woods Hole Oceanographic Institution Upper Ocean Processes group. The physical model consists of four active layers overlying an inert deep ocean, namely, a surface mixed layer of thickness h1, diurnal thermocline layer, seasonal thermocline, and main thermocline. The biological model consists of a set of advective-diffusive equations in each layer that determine nitrogen concentrations in four compartments: nutrients, phytoplankton, zooplankton, and detritus. Both monthly climatological and “daily” fields are used to force solutions, the latter being a blend of daily-averaged fields measured at the mooring site and other products that include intraseasonal forcing. Diurnal forcing is included by allowing the incoming solar radiation to have a daily cycle. In solutions forced by climatological fields, h1 thickens steadily throughout both monsoons. When h1 detrains at their ends, short-lived, intense blooms develop (the models spring and fall blooms) owing to the increase in depth-averaged light intensity sensed by the phytoplankton in layer 1. In solutions forced by daily fields, h1 thins in a series of events associated with monsoon break periods. As a result, the spring and fall blooms are split into a series of detrainment blooms, broadening them considerably. Diurnal forcing alters the mixed-layer and biological responses, among other things, by lengthening the time that h1 is thick during the northeast monsoon, by strengthening the spring and fall blooms and delaying them by 3 weeks, and by intensifying phytoplankton levels during intermonsoon periods. Solutions are compared with the mixed-layer thickness, phytoplankton biomass, and phytoplankton production fields estimated from mooring observations. The solution driven by daily fields with diurnal forcing reproduces the observed fields most faithfully.

Progress made in study of ocean's calcium carbonate budget

Many of the uncertainties in diagnostic and prognostic marine carbon cycle models arise from an imperfect understanding of the processes that control the formation and dissolution of calcium carbonate (CaCO3). On the production side of the equation, the factors that control the abundances of calcifying phytoplankton or zooplankton are largely unknown. On the dissolution side, changes in the depth of CaCO3 saturation horizons for both calcite and aragonite may produce large-scale changes in dissolution of shelf and slope sediments and reefs, with potentially significant implications for atmospheric carbon dioxide concentration and climate change, as well as for coralline organisms themselves. In recent years, concern about the long-term fate of anthropogenic CO2 in the oceans has re-ignited scientific interest in the fundamental abiotic and biotic processes that control the marine CaCO3 budget, since biological CaCO3 production and export are important mechanisms by which carbon is exported from the oceans surface to its abyss. CaCO3 precipitation releases CO2 to solution, while CaCO3 dissolution takes up CO2 from solution.

Phytoplankton and photosynthetic light response in the Coastal Transition Zone off northern California in June 1987

In June 1987 the geostrophic flow in the coastal transition zone off northern California (between 50 and 150 km off the coast from Point Reyes to just north of Cape Mendocino) was dominated by a well-defined, southward-meandering current. Three vertical sections are presented that show the hydrographic structure of the current down to 100 m and its relationship to the distribution of phytoplankton biomass. The sections show that the geostrophic adjustment brought cold, saline, deep water up to the surface on the low steric height, or cold side of the flow, and that this upwelled water supported a relatively large diatom biomass (chlorophyll-a concentrations between 1 and 10 mg m−3). We present particle size spectra and photomicrographs of the phytoplankton that show that the diatom biomass was dominated by chain-forming species (e.g., Skeletonema costatum, Chaetoceros spp., Thalassiosira spp., and Rhizosolenia alata, but also a single-celled Actinocyclus sp.). Photosynthetic light response measurements reveal that these diatom communities were capable of high photosynthetic rates (Pmax between 5 and 25 mg C mg Chla−1 h−1). Although most of the diatoms were located in cold, slow-moving water on the low side of the current, some were being carried downstream. High chlorophyll concentrations were observed at depths > 75 m in and along the cold edge of the flow in all of our sections; we show evidence that in two out of three cases this was the result of water mass subduction.

The influence of wind and river pulses on an estuarine turbidity maximum: Numerical studies and field observations in Chesapeake Bay

The effect of pulsed events on estuarine turbidity maxima (ETM) was investigated with the Princeton Ocean Model, a three-dimensional hydrodynamic model. The theoretical model was adapted to a straight-channel estuary and enhanced with sediment transport, erosion, deposition, and burial components. Wind and river pulse scenarios from the numerical model were compared to field observations before and after river pulse and wind events in upper Chesapeake Bay. Numerical studies and field observations demonstrated that the salt front and ETM had rapid and nonlinear responses to short-term pulses in river flow and wind. Although increases and decreases in river flow caused down-estuary and up-estuary (respectively) movements of the salt front, the effect of increased river flow was more pronounced than that of decreased river flow. Along-channel wind events also elicited non-linear responses. The salt front moved in the opposite direction of wind stress, shifting up-estuary in response to down-estuary winds and vice-versa.Modeled pulsed events affected suspended sediment distributions by modifying the location of the salt front, near-bottom shear stress, and the location of bottom sediment in relation to stratification within the salt front. Bottom sediment accumulated near the convergent zone at the tip of the salt front, but lagged behind the rapid response of the salt front during wind events. While increases in river flow and along-channel winds resulted in sediment transport down-estuary, only reductions in river flow resulted in consistent up-estuary movement of bottom sediment. Model predictions suggest that wind and river pulse events significantly influence salt front structure and circulation patterns, and have an important role in the transport of sediment in upper estuaries.