Ringkøbing Fjord is a coastal lagoon system in western Jutland (Denmark), with a north-south orientation. The fjord is approximately 30 kilometres long and 10-15 kilometres wide. It has an area of almost 300 square kilometres and is on average just under 2 metres deep. The fjord is connected to the North Sea through a sluice and receives freshwater inputs from the Skjern River and smaller streams. Ringkøbing Fjord drains a basin of about 3 500 km2, where agriculture, which covers 65-70% of the catchment, is the dominant activity.
The Water Framework Directive (WFD – 2000/60/EC) classifies Ringkøbing Fjord as having poor ecological potential due to eutrophication.
In light of this situation, the Ringkøbing-Skjern Municipality brought together relevant stakeholders to promote the development of an ecosystem modelling framework (Fig. 1) for the fjord.
The purpose of this framework was to (i) provide a thorough understanding of the interactions among the catchment, the fjord, and the sluice; (ii) to offer insights into how the stakeholders can work together effectively to achieve the targets set by the WFD; and (iii) to support policy makers.
Approach
Pelagic primary production in the fjord ecosystem is determined by both bottom-up and top-down control—the latter is due to the softshell clam Mya arenaria, known locally as sand mussel. This has the potential to become a managed fishery, while at the same time reducing eutrophication symptoms in the fjord and enhancing seagrass recovery.
The well-tested SUCCESS (System for Understanding Carrying Capacity, Ecological, and Social Sustainability) framework (e.g. Ferreira et al., 2023) was applied to Ringkøbing Fjord (Fig. 1). The modelling framework includes (i) a hydrological model to quantify the discharge of water and nutrients into the fjord, including source apportionment; (ii) a sluice model to simulate the exchange between the fjord and the North Sea (Nielsen et al., 2005); (iii) a hydrodynamic model (Delft3D) to simulate the 3D circulation inside the fjord; (iv) a system-scale ecological model (EcoWin) that includes sub-models for both the soft-shell clam Mya arenaria and the eelgrass Zostera marina.
The key features of the bivalve model are:
· Simulation of key physiological functions;
· Integration of relevant physical and biogeochemical components;
· Environmental feedback of shellfish growth, e.g. removal of phytoplankton and detritus, production of particulate organic waste, excretion of dissolved nitrogen, and oxygen consumption.
Sand mussels are not commercially important in Denmark, but in other markets such as the US (Beal et al., 2002) the soft-shell clam constitutes a valuable managed fishery, with a farmgate price of about 10 USD per pound of meat (National Marine Fisheries Service, 2021), or about 15 € per kg total fresh weight. There is no comparable European market that approaches those valuations.
The modelling framework was run for a period of ten years, and various scenarios were tested, including bottom-up (various changes to agriculture loading) and top-down (with and without soft-shell clams) control. Changes in phytoplankton biomass (chlorophyll as an indicator), dissolved inorganic nitrogen (DIN), and benthic macrophyte biomass were used for comparison of management strategies, and a risk matrix was developed to support management.
Results and discussion
The ecosystem dynamics of Ringkøbing Fjord (Fig. 2) are complex. Under normal discharge conditions and sluice operation, a significant population of soft-shell clams exists in the fjord. In EcoWin, this is simulated through a process of annual seeding (equivalent to recruitment) of about 120 animals (0.5 g each) per m2 and leads to a simulated annual harvest of about 21,000 tonnes fresh weight, or about 9 animals per m2, partly due to an annual mortality of 70%, partly because the animals have a two-year growth cycle, so the model simulates crop rotation, and partly because animals below harvestable weight remain in the fjord.
Salinity can fluctuate due to natural events such as high precipitation, and/or management policies such as reduction in water exchange with the North Sea through the sluice. Such events can lead to low salinity conditions, which result in high clam mortality.
If clam biomass is high, top-down control reduces chlorophyll (chl) and increases water clarity, leading to an increase in benthic vegetation.
However, since there is less nutrient drawdown by phytoplankton, excessive nutrients can also result in epiphyte growth on benthic plants. A reduction in nutrient loading coupled with higher clam biomass provides a sweet spot with low chl, eelgrass recovery, and less nuisance epiphytes. This is illustrated in Fig. 3 and was used to interpret EcoWin outputs for the various scenarios tested in this work.
Table 1 shows the effect of soft-shell clams on various indicators relevant to the WFD classification of Ringkøbing Fjord. The top-down control of phytoplankton biomass is evident in all areas of the fjord, while no effect can be seen on winter DIN, as would be expected. The response of benthic plants to the presence of soft-shell clams is more pronounced in box 47 (2.2 m water column depth); in the shallow area of box 29 (1.2 m), light penetration to the bottom is less of an issue, and in the deepest box (45), where the water depth is 4.0 m, the underwater light climate is less suitable for benthic vegetation to thrive. This also decreases the epiphyte risk score since the substrate (eelgrass) available to epiphytes is reduced.
Among the various management recommendations instrumental in the recovery of Ringkøbing Fjord is the maintenance of an adequate salinity throughout the year to avoid mass mortality of clams, which result in chlorophyll peaks and addition of organic matter to the system. The potential value of Mya arenaria in the fjord can be estimated for provisioning services using a fraction of the US cost, since there is no European market for this bivalve. Even so, considering 10% of the US price, the potential annual revenue is of the order of 30 million euros.
The value of regulatory ecosystem services can be calculated in two ways: the conventional approach would be to determine the N removal associated with the potential clam harvest. However, since there is no clam fishery at the present time, the alternative is to determine the avoided cost of nutrient removal on land associated with chlorophyll reduction through top-down control (Ferreira & Bricker, 2019). The development of such a fishery could potentially have added value through a nutrient credit trading mechanism (e.g. (CDEEP, 2018)) similar to those developed in the United States.
References
Beal, B.F., 2002. Adding value to live, commercial size soft-shell clams (Mya arenaria L.) in Maine, USA: results from repeated, small-scale, field impoundment trials. Aquaculture, 210, 1–4, 119-135.
Connecticut Department of Energy and Environmental Protection (CDEEP), 2018. Report of the Nitrogen Credit Advisory Board for Calendar Year 2018 To the Joint Standing Environment Committee of the General Assembly Concerning the Nitrogen Credit Exchange Program. CT DEEP, 79 Elm Street, Hartford CT. https://portal.ct.gov/-/media/DEEP/water/municipal_wastewater/NitrogenReport2018Final.pdf
Ferreira, J.G., Bricker, S.B., 2019. Assessment of Nutrient Trading Services from Bivalve Farming. In: Goods and Services of Marine Bivalves. Aad C. Smaal, Joao G. Ferreira, Jon Grant, Jens K. Petersen, Øivind Strand, Editors. p. 551-584.
Ferreira, J.G., Bernard-Jannin, L., Cubillo, A., Lencart-Silva, J., Diedericks, G.P.J., Moore, H., Service, M., Nunes, J.P., 2023. From soil to sea: an ecological modelling framework for sustainable aquaculture. Aquaculture 557, 1-14.
National Marine Fisheries Service, 2021. Fisheries of the United States, 2019. U.S. Department of Commerce, NOAA Current Fishery Statistics No. 2019. 167 pp.
Nielsen, M.H., Rasmussen, B., Gertz, F., 2005. A simple model for water level and stratification in Ringkøbing Fjord, a shallow, artificial estuary. Estuarine, Coastal and Shelf Science 63, 235-248.