Kenneth L. Denman

A Time-Dependent Model of the Upper Ocean

Abstract A model describing the time-dependent modification of the upper mixed layer of the ocean by meteorological influences is developed. The turbulent mixing and the radiative heating within the mixed layer are expressed so that only simple parameters available from routine meteorological measurements are required as input. The model is sensitive to the rate of production by the wind stress of energy available for mixing, and to the rate of absorption with depth of the solar radiation. Analytic and numerical results of the model for conditions of large constant winds and typical summer heating are consistent with laboratory results. The mixing response to a wind normally distributed in time is also presented. Finally, the model simulates the physical behavior of the upper mixed layer in response to diurnally varying heating: results for several different values of wind speed indicate that, even in low winds and typical summer heating, the daily fluctuations in sea surface temperature in the open ocean...

Organisation in the pelagic ecosystem

A continuous, steady-state theory has been developed for the abundance of organisms in the pelagic ecosystem as a function of their body weight. It is based on accepted relationships for the weight-dependence of metabolism and growth, in a context where individual organisms are assigned to one of a series of size classes for which the nominal weights increase in a geometric progression. Analysis of the biomass flow in such a representation leads to the conclusion that, in the steady state, the total biomass in any given size class decreases in a regular manner with increasing size. Explicitly,b(w2)/b(w1)~(w2/w1)0.22, whereb(w2) andb(w1) are the total biomasses in the size classes characterised by weightsw2 andw1, respectively. The exponent (−0.22) represents a balance between catabolism and anabolism, based on published reviews concerning the revelant parameters. This result agrees favourably with data collected by other workers in the subtropical oceans. The theory can be used to draw conclusions about the functional dynamics of the pelagic ecosystem, such as community respiration and rate of biomass flow.

Modelling planktonic ecosystems: parameterizing complexity

This paper explores several simplified representations of complexity or ecological ‘texture’ in models of the marine planktonic ecosystem. It is relatively straightforward to formulate more complex models to include explicitly different functional groups of phytoplankton, zooplankton and bacteria, and to include regulation by multiple nutrients such a nitrate, ammonium, silica, and iron. However, the number of parameters that must be specified from observations increases approximately as the square of the number of compartments and quickly surpasses our ability to constrain them properly from observations. Moreover, ecosystem models often become unstable for small changes in parameter values, and increasing complexity may not lead to increased stability. Here we consider alternative formulations for ecosystem models that try to represent complex interactions, such as the microbial loop, in simpler, less explicit ways. Results are presented demonstrating diagnostic methods to represent (1) multiple size classes of phytoplankton according to total biomass of a single phytoplankton compartment, and (2) partitioning a single compartment for nitrogen into nitrate and ammonium according to the origin of incoming fluxes, nitrification by bacteria, and a relative preference algorithm for uptake by phytoplankton. Ambient concentrations of ammonium are simulated with this model and evaluated against observations from Ocean Station P in the subarctic Northeast Pacific Ocean.

Covariability of chlorophyll and temperature in the sea

Abstract Series of chlorophyll a and temperature obtained from multiple-depth horizontal tows have been treated by spectral analysis. Composite power or variance spectra show the chlorophyll spectra to be similar but not identical in shape to the temperature spectra. Temperature and chlorophyll at the same depth usually were significantly coherent at wavelengths greater than 100 m. Parameters measured 4 to 5 m apart in depth showed no significant coherence. In general, for runs with low chlorophyll variance and high coherence with temperature, most of the observed variance in the chlorophyll is attributed to internal wave and vertical mixing effects. However, in several runs with high chlorophyll variance, the observed variance is more than 10 times that which could be accounted for by internal wave effects. Such ‘patchy’ observations are believed to indicate regimes of greater biological activity, as characterized by higher growth rates.

The response of two coupled one-dimensional mixed layer/planktonic ecosystem models to climate change in the NE subarctic Pacific Ocean

Abstract In this paper, we report on simulations of ecosystem responses to climate change with two planktonic ecosystem models, both coupled to a one-dimensional mixed-layer model run with annual wind and solar heating from Ocean Station P (50°N, 145°W) in the NE subarctic Pacific. The first ecosystem model is a four-component model previously tested with extensive observations from OSP (Deep-Sea Res. II 46 (1999) 2877). The second ecosystem model is more complex, including phytoplankton partitioned into two size classes, and imposed grazing by mesozooplankton, which varies in time according to long-term observations from OSP. Both models include temperature dependence of physiological rates. Two possible climate change scenarios are considered: (i) increasing ocean temperatures by 2°C (and 5°C) applied only to the ecological component, and (ii) changing the availability of iron to phytoplankton in the subarctic Pacific. Responses of the two models are similar, indicating that they are not primarily model-dependent. In the warming cases, annual behavior and average standing stocks decrease marginally (⩽10% for T=2°C, and ⩽22% for T=5°C, second model only), ecosystem recycling increases with warming, and losses of organic particles to the ocean interior decrease (∼10%) in the simpler model or increase slightly (

Rapid underway profiling of chlorophyll with an in situ fluorometer mounted on a ‘Batfish’ vehicle

Abstract A modified version of the Variosens fluorometer employed in the in situ measurement of chlorophyll a fluorescence has been examined as a potential research and survey tool. The instrument was transported on a towed undulating vehicle, with simultaneous measurement of water temperature, conductivity and depth. Frequency response studies, instrument calibration procedures, measurement errors and operating procedures at sea are discussed. The capability of the system is illustrated by examples of chlorophyll and physical data contoured in the vertical plane and sampled from the coastal shelf region adjacent to Nova Scotia.

Collection and Analysis of Underway Data and Related Physical Measurements

Our ability to describe and understand the marine environment is limited both by the type and detail of the environmental data we can acquire, and by the power and resolution of the analysis methods we can apply to that data. Much of our recent understanding of the spatial variability in the planktonic community has been achieved from the analysis of series of essentially continuous measurements, taken either at a point in space or along a vertical or horizontal line. For a few parameters, well-established methods allow resolution of spatial structure over all length scales from centimeters to thousands of kilometers. Measurements of physical (and some chemical) scalar variables (e. g. temperature and salinity) provide the best examples. However, for biological parameters, many of the desired spatial “windows” (see Angel, 1977) are less adequate. Because these windows have in general been opened only recently by new methodologies, their technical limitations are poorly known. They also tend to be narrower (Figure 1) and yield data that are even more subject than the physical scalars to the confounding effects of variations that occur along the temporal and unsampled spatial axes. For this reason various authors (e. g. Denman, 1976) have suggested that one-dimensional sampling schemes are inadequate to describe the spatial structure fully. As discussed in the introductory chapter (Steele), horizontal structure often cannot be separated from vertical structure, either observationally or in terms of operative mechanisms, and there is a serious need for the collection of continuous biological, physical and chemical data in both dimensions simultaneously.

Progress and Challenges in Biogeochemical Modeling of the Pacific Arctic Region

At this early stage of modeling marine ecosystems and biogeochemical cycles in the Pacific Arctic Region (PAR), numerous challenges lie ahead. Observational data used for model development and validation remain sparse, especially across seasons and under a variety of environmental conditions. Field data are becoming more available, but at the same time PAR is rapidly changing. Biogeochemical models can provide the means to capture some of these changes. This study introduces and synthesizes ecosystem modeling in PAR by discussing differences in complexity and application of one-dimensional, regional, and global earth system models. Topics include the general structure of ecosystem models and specifics of the combined benthic, pelagic, and ice PAR ecosystems, the importance of model validation, model responses to climate influences (e.g. diminishing sea ice, ocean acidification), and the impacts of circulation and stratification changes on PAR ecosystems and biogeochemical cycling. Examples of modeling studies that help place the region within the context of the Pan-Arctic System are also discussed. We synthesize past and ongoing PAR biogeochemical modeling efforts and briefly touch on decision makers’ use of ecosystem models and on necessary future developments.

Quantitative stimulation of phytoplankton productivity by deep water admixture in a coastal inlet

Abstract Batch culture experiments in the laboratory have been made to determine the potential enhancement of surface primary production in a coastal inlet by artificial enrichment with bottom water. An optimal enrichment rate of less than 1% bottom water was found, suggesting that in nature newly upwelled water might not sustain maximum productivity until it had been diluted approximately 100-fold. The time scale for this is estimated to be ≲ 10 days. The enrichment mechanism is sufficiently subtle that prediction of productivity in artificially upwelled water requires more information than the amount of inorganic nutrient added. For a coastal inlet, relatively undepleted at the surface in inorganic nutrients, surface primary production at optimal enrichment was increased on the average by a factor of 4, chlorophyll biomass by a factor of 3 and maximum P B by a factor of 1.4.

Euphotic Zone Study moves forward

The Global Ocean Euphotic Zone Study (GOEZS), a potential core program of the International Geosphere-Biosphere Programme (IGBP) being planned jointly with the Scientific Committee on Oceanic Research (SCOR), was recently given the go-ahead by IGBPs Scientific Committee to move on to the next level of developing its scientific program. The GOEZS program will focus on the coupled physical, biological, and chemical processes operating in the euphotic zone, which is the ocean surface layer where sufficient light penetrates for photosynthesis by phytoplankton to exceed their metabolic energy losses. The upper ocean is extremely important to understanding the atmosphereocean system because it mediates exchanges of heat, momentum, carbon dioxide, sulphur, and nitrogen between the atmosphere and the ocean interior. For the major greenhouse gas carbon dioxide for example, there is more carbon in the upper ocean than in the whole atmosphere. Essentially all carbon dioxide from the atmosphere that passes from the upper ocean to the ocean interior has been transformed chemically or biologically in the upper ocean. Moreover, the upper ocean is the site of all marine shipping and most recreation and industrial activity and contains the planktonic food chain and most fish stocks.