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We used a long-term observation data set (12 years) of fish, insect and primary producer standing stocks in both reference and phosphate-fertilized reaches of the Kuparuk River located on the north slope of Alaska, USA to test a recently developed first-order approximation model. The model employs a flow analysis-type approach, but uses first-order approximations between annual mean compartment stocks and environmental drivers of temperature, discharge and solar radiation. Consequently, the model is more robust and requires fewer observations than standard process-oriented models, and can utilize observations that are difficult to incorporate into process models. Unlike standard inverse models, we show that our model is capable of prediction provided sufficient data are available for model calibration and environmental drivers are known. The results show that the inter-annual variations of several components in the Kuparuk River ecosystem, including dissolve inorganic phosphate, chironomids, black flies and Arctic grayling, can be accurately approximated as a linear function of temperature, discharge and solar radiation. In particular, the model indicates that changes in river habitat brought about by proliferation of the moss Hygrohypnum spp. in the P-fertilized reach caused a temporary shift in flow paths supporting Arctic grayling from primary producers to detrital-based pathways. However, after moss establishment, primary producer flow paths to Arctic grayling returned and detrital-based pathways weakened.

Ecosystems exhibit nonlinear dynamics that are often difficult to capture in models. Consequently, linearization is commonly applied to remove some of the uncertainties associated with the nonlinear terms. However, since the true model is unknown and the operating point to linearize the model about is uncertain, developing linear ecosystems models is non-trivial. To develop a linear ecosystem model, we assume that the annual mean state of an ecosystem is a minor bias from the long-term mean state. A first order approximation inverse model to govern the year-to-year dynamics of ecosystems whose characteristic time scales are less than 1 year is developed, through theoretically formulation, on the basis of steady state analysis, time scale separation and nondimensionalization. The approach is adept at predicting year-to-year variations and to tracking system response to changes in environmental drivers when compared to data generated with a standard nonlinear NPZD model.

We describe a nonlinear inverse technique to estimate gross primary production (GPP), community respiration (CR), and net ecosystem production in the vertically well mixed Parker River, Plum Island Sound estuarine system located in northeastern Massachusetts, USA. The approach uses a calibrated 1D advection–dispersion model to predict oxygen and salt concentrations along the estuarine length, which are compared to oxygen concentrations measured during high speed transects near dawn and dusk over a 2-day period in June 1995. The spatiotemporal shapes of the GPP and CR surfaces are represented with cubic B-splines that are deformed as needed by an algorithm that seeks to minimize error between oxygen observations and model predictions. The spatially resolving solution shows maximum instantaneous GPP of 1030 mmol O2 m −3 d −1 and CR of 340 mmol O2 m −3 d −1 in the upper portions of the estuary; however, the overall estuary is net heterotrophic ( −4.8 Mg C d −1 ). Analysis of the advection–dispersion model reveals that gas invasion and longitudinal dispersion account for up to 21 and 14% of local O2 accumulation relative to CR, respectively. However, an oxygen balance averaged over the 2-day sampling period shows that the negative net ecosystem productivity is balanced by a loss in estuarine oxygen storage. Carbon budget analysis indicates that terrestrial, marine, and marsh allochthonous inputs account for 2, 27 and 71% of the observed net heterotrophy, respectively. © 2005 Elsevier B.V. All rights reserved.

Decomposition of dissolved organic carbon, nitrogen and phosphorus (DOC, DON, DOP) was measured for surface and bottom waters of the middle Atlantic bight (MAB) and deep slope water adjacent to the MAB on two occasions in March and August 1996. We used standard bottle incubation techniques to measure the decrease in dissolved organic matter (DOM) concentrations over a 180-day interval. Generally DOM concentrations in the MAB were elevated (125mM DOC, 10.2mM DON and 0.30mM DOP) relative to the surface ocean and deep slope water (46.7mM DOC, 2.76mM DON, 0.03mM DOP). On average the C:N:P ratio of shelf DOM (431:36:1) was substantially higher than the Redfield ratio, but not nearly as high for that of deep slope water (2700:215:1). Decomposition time course data were fit to a three-pool (very labile, labile, and recalcitrant pools) multi-G model using a Marquardt fitting routine. The threepool model was superior to a simple exponential decay model assuming a single pool of DOM. We observed no significant changes in concentration of DOM in deep-water samples, attesting to the old age of this material, its recalcitrant nature, and the cleanliness of our technique for measuring decomposition. There were major differences in the relative amount of very labile, labile and recalcitrant fractions of shelf-water DOC, DON and DOP as a result of preferential remineralization of P over N and N over C. Averaged over stations, the decomposable portion of the bulk DOC, DON and DOP pools increased from 30% to 40% to 81% for C, N and P. There was a wide range in decay coefficients for the very labile and labile DOM pools: average decay coefficient for the very labile pool was 0.219 d � 1 , and 0.018 d � 1 for the labile pool. Average half-lives calculated from the decay coefficients were 4, 12 and 8 days for the very labile DOC, DON and DOP pools, and 54, 113 and 90 days for the labile DOC, DON and DOP pools. On the basis of pool turnover times relative to shelf-water residence time (B100 days) we conclude that autochthonous algal production is the source of the very labile DOM pools. Its rate of production is sufficient to sustain estimated rates of bacteria C demand in continental margins. Our results for the MAB indicate that while substantial amounts of DOM are remineralized in the same time frame as shelf-water residence time, there is substantial DOM remaining that is depleted in N and P relative to C. Strong concentration gradients in DOM occur between shelf and ocean waters and between surface and deeper waters. Coupled with appropriate vertical and horizontal advective and eddy diffusive transports, DOM export from the MAB and other shelf systems may be a significant component of ocean C dynamics. r 2002 Elsevier Science Ltd. All rights reserved.

Salinity distributions, dye release studies, and parameter estimation techniques are used to determine the longitudinal (or tidal) dispersion coefficient in the Plum Island Sound estuary in north-eastern Massachusetts, U.S.A. A one-dimensional, intertidal, advection–dispersion model is used to test the validity of the steady-state assumption employed to estimate tidal dispersion from salinity. It is found that salinity distributions in the upper Parker River can take months to relax to steady state under low freshwater discharge conditions, so that dispersion cannot be reliably estimated using the steady-state assumption. To overcome this problem, dye release studies and parameter estimation techniques are used to determine the functionality of the dispersion coefficient that produces the best fit between the observed salinity profiles, dye plumes and model output under transient conditions for the period between 28 April 1992 and 12 April 1995. The estimated dispersion coefficient is found to be a hyperbolic function of distance from the head of the estuary and varies from an average of 3·6 m 2 s −1 in the upper Parker River (0–5·2 km) to an average of 670 m 2 s −1 in Plum Island Sound (14·3–24 km). The advection–dispersion model is also used to investigate three characteristic mixing-time scales involving average age, average residence time and average transit time (also called turnover or flushing time) for freshwater and saltwater sources in defined subsections of the estuary. It is found that these time scales can vary from a few hours to 3 months depending on location and freshwater discharge. Transport in the upper Parker River can be either advection or dispersion dominated depending on discharge, while Plum Island Sound is always dominated by dispersive terms. Tidal channel morphometry leads to a very dynamic system in the upper Parker River that significantly impacts ecological processes in the region. We discuss the relevance and appropriate application of the three characteristic mixing-time scales to ecological processes.