The river Scheldt and the Western Scheldt estuary are vital to the economies of Belgium and The Netherlands and these systems are subjected to severe anthropogenic stress. As a matter of fact, important loads of organic matter and nutrients (NH₄⁺, NO₃^-, PO₄^3-) are released from wastewaters and by fertilizers leaching into the upstream waters and, hence, the estuary. It follows an eutrophic situation that is often the source of an imbalance in carbon cycle since it allows phytoplancton to grow excessively from early spring until september. The respiration of the decayed algal biomass by bacteria leads rapidly to a depletion of the dissolved oxygen in the water column and this results in the death of the higher organisms and the deterioration of the water quality as a tool for human activities.

The purpose of this work is firstly to evaluate, by means of experimental ground measures, the main parameters controlling the intensity of the primary production in the Scheldt estuary (e.g.: light intensity, nutrients, salinity, temperature, pH, alcalinity, dissolved O₂...); and secondly to establish, on the basis of the harvested datas, experimental laws describing the main biological processes (e.g.: gross & net primary production, algal mortality, respiration, chemoautotrophic production, grazing...). This will allow us to build up a biological model centered on the primary production of the phytoplancton that will be coupled to the previously developped hydrodynamical model of the Scheldt estuary «CONTRASTE» (Coupled Networked Transient-Reaction Algorithm for Strong Tidal Estuaries).

The Scheldt estuary dynamic is dominated by strong tides and low inflows of fresh waters. An important consequence of it is that the water column is vertically well mixed. It ensues a long residence time of the freshwater in the estuary and a large throw-in of the deposits in suspension causing a high turbidity in the water column. Hence, the primary production by phytoplancton is fairly limited by light availability during the bloom whereas nutrients are in excess of growth demand.

The influence of light intensity on phytoplancton growth is studied by means of H¹⁴CO₃^- incorporations under variable light. Platt’s exponential equation is used to fit the experimental curves and the photoinhibition term is neglected since it is not observed.

m^chl = m^chl _max [1 - exp (-a I / m^chl _max)]

Where: m^chl is de specific growth rate;
           m^chl_max is the maximum photosynthetic capacity;
           a is the photosynthetic efficiency and
            I  is the photosynthetic available light intensity.

In our case, the photosynthetic efficiency (a), reflecting the physiological structure of the phytoplancton light-harvesting pigments, is the main parameter controlling the gross primary production (GPP) during the bloom because under low light conditions, the specific growth rate depends linearly on a ,as shown below:

m^chl = a . I              With I <<<

GPP = m ^chl .C_chl  . Z  . t

Where:  C_chl is chlorophyll-a concentration;
             Z is the euphotic depth and
             t  is the photoperiod.

As an example of our first results, the evolutions of photosynthetic efficiency (a) and integrated gross primary production (GPP) during 1999/2000 are shown for the upper estuary (Antwerp), where maxima of chlorophyll-a and minima of dissolved O₂ occur (see: fig. 1 and 2).
We see an obvious correlation between the graphs during the bloom period from june to late september (day 150 to 270). Outside the bloom period, low light availability and low temperatures, among other environmental factors, limit the gross primary production despite high values of photosynthetic efficiency.

Figure 1:Photosynthetic efficiency of the phytoplancton during 1999/2000 in the upper Scheldt estuary.The origin corresponds to 1/1/1999.

Figure 2:

 

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