11 - Influence of Intra-Seasonal Variability of Metabolic Rates on the Output of a Steady-State Food Web Model

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Abstract

The work aims at assessing the effect of intra-seasonal variability of metabolic rates on the output of linear steady-state food web models. The methodology was tested on a simple food web, which could be representative of a coastal lagoon where filter feeders are the most important predators of the plankton community. Results indicate that seasonal fluctuations of water temperature can have remarkable effects on the estimation of energy and matter flows across the food web, and that systematic changes in forcing functions and age/size structure of the populations may be important components of the overall food web model uncertainty.

Introduction

The estimation of energy and mass fluxes which flow through food webs is an important topic in system ecology, since they allow one to calculate ecological indexes, which, besides providing insights into the ecosystem functioning, can also be used for assessing the status of marine ecosystems (CEC, 2000). Steady-state linear food web models are regarded as useful tools for providing the above estimates on the basis of partial information embodied in field data. Therefore, in most instances, they represent the only way to derive ecological indexes from incomplete data sets. Furthermore, these models play a crucial role in nonlinear dynamic simulations also, since they can be used for supplying consistent initial conditions (Belgrano et al., 2005). Unfortunately, the application of steady-state food web models requires comprehensive data sets. In principle, these data should be representative of the “average” condition of the system within the selected time frame. However, in the large majority of cases, data are being collected at a given point in time which: (i) may not be the same for all compartments; and (ii) may not provide data which are representative of the whole time frame (e.g., summer or spring–summer in Degré et al. (2006) and Savenkoff et al. (2007)). Furthermore, care should be taken in using literature data for setting important input parameters, since these estimates may be obtained in environmental conditions rather different from those of the studied ecosystem. In particular, specific production rates (P/B) are among the most important input parameters of a food web model. These production rates are strongly affected by water temperature, since metabolic rates are strongly dependent on the thermal requirements of organisms (Kooijman, 2000) and also depend on the size of individuals. Furthermore, in both cases the relationship is nonlinear: therefore estimating the the production rate from an average value of temperature and size, to be regarded as representative of the time frame of application of the steady-state model, may lead to incorrect estimates of energy and matter fluxes. Thus far, the potential role of the above factors in determining the result of food web models has not been investigated. The present study aims at filling this gap, by proposing the use of dynamic individual bioenergetic models for estimating the specific production of single-species compartments. The approach proposed here was tested on a simplified food web, with the following objectives:

  • Assessing the effect of seasonal fluctuations of water temperature on the estimation of energy and matter flows across a food web.

  • Testing the sensitivity of food web model results to interannual variability of water temperature and size structure of the population.

The work was carried out by combining two models:

  • the individual model presented in Solidoro et al. (2000), which is based on a Scope for Growth formulation;

  • a linear food web model, in which the constrained inverse problem was solved by means of two open-access R packages (van Oevelen et al., 2009).

The main features of these models and the methodology for their coupling are presented in the following section. Details concerning their application are given in Section 11.2.2, followed by the presentation and discussion of our main findings.

Section snippets

Methods

The approach proposed here is based on the use of dynamic bioenergetic models for estimating some of the most important input parameters of a food web model, namely the specific productions P/B, which is the ratio between the rate of increase of the biomass and the biomass standing stock of a given compartment. In fact, the specific production of most marine organisms depends strongly on water temperature (Mann and Lazier, 2006) and, in general, decreases with the size/age of individuals (

Results

Average daily values of air temperature for the period May 1–October 31, 1993, and water temperatures estimated from the site-specific correlation model are shown in Fig. 11.3a. In accordance with the typical evolution in the Lagoon of Venice (Pastres et al., 2004), the water temperatures ranged between 13.6 and 29.1 °C. The highest values were reached at the beginning of August. Afterward, the temperature steadily decreased, falling below 18 °C at the end of October. Equations (2), (4) were

Discussion

In the first numerical experiment carried out in this work, we compared the results obtained, using as input two P/B values computed by means of the growth model and a reference value taken from the literature. Such comparison shows that the use of P/B deduced from dynamic bioenergetic models leads to markedly different estimates of energy fluxes in the network. The literature value considered here was the result of a detailed set of short-term feeding and respiration measurements, carried out

Conclusions

The comparison carried out shows that fluctuations of water temperature can induce remarkable effects on food web model estimations, and that metabolic rate estimates obtained from an average water temperature and body size may lead to estimate energy fluxes which are not representative for the whole time frame of application of a steady-state model. Offline coupling of dynamic growth models and food web models provides a way to test the sensitivity of the result of the food web model to

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  • Cited by (3)

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