Elsevier

Ecological Modelling

Volume 102, Issues 2–3, 29 October 1997, Pages 259-272
Ecological Modelling

Long term simulations of population dynamics of Ulva r. in the lagoon of Venice

https://doi.org/10.1016/S0304-3800(97)00060-4Get rights and content

Abstract

The dynamic of macroalgae is implemented in a 3D transport-water-quality model of the central part of the lagoon of Venice. Ulva biomass density and nitrogen concentration in Ulva tissue have been added to the set of state variables previously considered, that is to phytoplankton and zooplankton densities, concentrations of nutrients in water, detritus and dissolved oxygen. The model shows that Ulva succeeds in the competition with the phytoplanktonic community in the shallower areas, where water temperature and irradiance levels reaching the bottom are sufficient to sustain growth. Long term evolutions of Ulva colonies, under different scenarios of forcing functions, show that adverse meteoclimatic conditions can be more effective in reducing Ulva biomass than a consistent decrease in the loads of Nitrogen.

Introduction

In the early eighties, the macroalgae community became an important component of the ecosystem in the Venice lagoon, reaching standing crops at an order of magnitude higher than those of the phytoplanktonic pool in 1987 and 1989. High levels of biomass production have been maintained from early spring to late autumn in large areas of the lagoon, with biomass density of up to about 20 kg/m2 (wet weight). Under particular conditions, anoxic crises are likely to occur, especially during the summer, followed by sharp collapses in population, and resultant releases of hydrogen sulphide (Sfriso et al., 1987, Sfriso et al., 1989). The massive presence of Ulva rigida, by far the most dominant species (Sfriso, 1987), has heavily affected the ecosystem, and has prompted new field and model studies, aimed at understanding the reasons for such a change and at forecasting the long-term evolution of the system.

A few models of the dynamic of Ulva r have already been proposed, both zero dimensional (Bendoricchio et al., 1994; Pecenik et al., 1991) and one dimensional (Solidoro et al., 1997b), but a full understanding of the seasonal cycle of macroalgae and of the mechanisms underlying the behaviour of the ecosystem requires the development of a 3D model. In fact, only in this way one can describe the transport phenomena and the interactions between biotic and abiotic components, as well as the competition with other communities, such as that of phytoplankton. This paper presents the implementation of Ulva r. dynamic (Solidoro et al., 1997b) in a 3D model (Dejak and Pecenik, 1987), whose major features are briefly outlined below.

Section snippets

General features of the 3D model

The 3D finite-difference model covers the central part of the lagoon of Venice and includes the most important industrial area and the city of Venice. This area is divided on a mesh of 100 m by 100 m, with a vertical step of 1 m, in order to reproduce the bathometry in sufficient detail (Fig. 1). The model, developed during the eighties, couples transport processes with the dynamics of the communities of primary producers and that of zooplankton (Dejak and Pecenik, 1987, Dejak et al., 1990),

Major features of the trophic submodel

The formulations here proposed, Table 1, take into account the characteristics of two communities of primary producers, phytoplankton and macroalgae. The governing equations for the two groups will be briefly discussed together, in order to highlight their similarities and differences. Formulations concerning the dynamics of Ulva and their experimental basis are discussed in detail in Solidoro et al. (1997b).

Phytoplanktonic species are pooled, and the composition of the pool in terms of major

Analysis of the evolution in the reference scenario of forcing functions

The 3D model allows one to study the ecosystem in realistic conditions of inflows and outflows of nutrients. In a first simulation, the initial density of macroalgae has been taken as a constant for the whole area, in order to check if the model would succeed in giving rise to temporal and spatial patterns. This simulation confirms that Ulva colonies survive only in shallower areas, less than 2 meters deep, whereas the distribution of phytoplankton is not correlated with the bathometry. The

Conclusions

The model here discussed represents the first attempt at including the dynamic of macroalgae in a 3D water quality model of a coastal basin. The application to the lagoon of Venice is particularly interesting, because this ecosystem has been studied from many points of view, due to its peculiarity. The model has proved to be an effective tool for studying primary production in an eutrophic environment, where macro and microalgae are both present. A 3D model gives one the possibility to

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