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Knowledge - Cause - effect chain modelling of sand mining using mussels

Abstract: Dynamic Energy Budget (DEB) model is presented to describe the effect of fine sediment (dredging) on the on filter-feeding bivalve populations (e.g. the blue mussel Mytilus edulis)

Technology Readiness Level: 5 (validated in relevant environment)

Environment: Sandy shores, Estuaries, Lakes and rivers
Keywords: Eco-physiology, Bivalves, Environmental impact, Marine mining, Silts and sediments, Water quality, Dynamic Energy Budget Model

 

 

About

Every year large amounts of sand are extracted from the North Sea to meet the demand for construction and nourishment activities. Potential ecological effects of these activities have to be examined and reported in Environmental Impact Assessments (EIAs). Sand mining cause an increase in fine sediment concentrations. Increased concentrations of fines can increase turbidity, change the relative composition of organic and inorganic particulate matter in the water column and limit the light penetration, thus affecting water quality and reducing primary production. Most of the bivalves are filter feeders and extensive knowledge of how increased sediment concentrations influence their physiological organisation is essential if an impact assessment is to be made. In order to understand the effect of suspended sediment concentrations on the activity of filter-feeding bivalves, deterministic model is presented using the model species, the blue mussel Mytilus edulis.

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Effects of increased suspended solid concentrations

During the dredging process a sand-water mixture is pumped up from the seabed into the hold of the dredging vessel. Although most of the sandy part of this mixture settles in the hold, the excess transport water containing fine sand particles flows back into the sea causing a ‘dredging plume’ near the dredging vessel. As a result of tidal currents and wave action, this dredging plume will be spread over a larger area. A direct effect of increased suspended particle concentrations in the water column is a higher turbidity level and less light penetration. This may affect phytoplankton productivity and consequently food supply to higher trophic levels. The actual impact may be local and restricted in time, although when dredging takes place at the seaward end of an estuary, for instance, a larger water mass may be affected. Potential ecological implications consist of significant decreases in biomass and productivity of phytoplankton, zooplankton and filter-feeding benthos such as Mytilus edulis. For instance, deposition of sediment layers of 1-2 cm thickness can result in increased mortality.

 

1. Effects of dredging on blue mussel Mytilus edulis

2. Ecology of blue mussel Mytilus edulis

3. Feeding and food selection by filter feeders

 

 

Energy Budget Model

Functional responses of filter-feeding bivalves to variations in seston have been discussed by numerous authors and implemented into physiological models (for detailed references see the Case Study Mussels - Modelling the effect of dredging on filter-feeding bivalves). Some of these models are based on the DEB theory, and this will be discussed further.

The Dynamic Energy Budget (DEB) model has been developed by Kooijman 30 years ago (Kooijman 1986; Kooijman 2000; Kooijman 2010). The DEB model describes the energy flow through an organism as a function of its size, its development stage and environmental conditions. An individual organism state is represented by three state variables: structural volume (V, cm3), reserves (E, Joule) and reproduction (R, Joule). The organism acquires energy from food by assimilation. The energy that is not assimilated by the organism is released as faeces. The assimilated energy is first stored into reserves (blood, glycogen). From the reserves, the energy is mobilized into growth, development and maintenance. A fixed fraction (κ) of the energy flow from the reserves is used for growth and somatic maintenance, but the priority is given to the maintenance. The rest of the energy flow from the reserves (1-κ) is spent on maturity maintenance and reproduction (gamete production and spawning). Juveniles spend this energy to maturation, while adults use it for gamete production and spawning. A key process is the assimilation rate of the food. In DEB, this is generally described by a Holling type II functional response. The scaled functional response (f) is the ratio between food uptake rate (in Joules) and the maximum food uptake rate (in Joules) and varies between 0 (no food uptake) and 1 (maximum food uptake rate). At a food concentration X = Xk, the functional response equals 0.5.

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DEB model: results for varying environmental conditions

A baseline model simulation was run with a standard DEB model for mussels, extended with the formulation of clearance rates (volume of water cleared of particles per unit time) and pseudofaeces production. The model was forced with environmental conditions (temperature, chlorophyll a and particulate inorganic matter) from model calculations for the North Sea (Schellekens, 2012) at location Schouwen 4. The model was run from May 1st, 2007 to December 31st, 2011. The temperature varied more or less sinusoidally between about 6˚C in winter and about 18˚C in summer. The Chl a concentration had a peak in early spring and a second peak in summer. Particulate inorganic matter is highest in winter due to the wind conditions. The resulting scaled functional response fluctuates between about 0.1 in winter and 0.8 in spring and summer.

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Lessons learned

In DEB models, a relatively simple description using a scaled functional response is applied. The scaled functional response describes the energy uptake rate as a function of food concentration. The effect of suspended sediment concentration can be incorporated in the functional response. The low concentrations of particulate inorganic matter have an effect on the food uptake rate. Filter feeding mussels need to invest time and energy processing the inorganic matter. Short-term increases of particulate inorganic matter have less impact on the growth performance of the shellfish compared to a continuous increase. The timing of dredging activities is also important. In the winter period, when the activity of the mussels is low, the impact of increased suspended sediment concentration is much less than during summer time. Because mussels are able to adapt to different silt concentrations in the water column, it is important that impact studies take into account the natural variability of silt concentrations the area, as well as the time spans during which these conditions may remain modified. Short term increases in suspended sediment concentration will have less impact on the food intake of shellfish than a continuous release.

This study is primarily focused on blue mussels as a model species for suspension-feeding lamellibranchiate bivalves. It is assumed that the processes will be comparable for other species and that only the values of the parameters will differ. However, it is good to check this assumption using literature data from other species.The models that were used in this case study were not directly calibrated with field observations and literature information on filtration rates and pseudofaeces production. It would be an improvement to perform an additional calibration with the appropriate data.

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References

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