Shellfish filter the water column to get food. Several factors in the water, such as temperature, salinity, suspended solids and food availability, affect the uptake of particles from the water and thus the growth and condition of shellfish. These factors can vary greatly in an estuary or bay. Measurements in breeding plots in the Wadden Sea show significant differences in growth rates in mussels, as shown in Figure 1b.

A study was conducted to better understand the relationship between water composition and shellfish growth. For this purpose, two highly contrasting sites, Oosterom and Slenk (Figure 1b), in the Wadden Sea were studied ( here you can view the spatial differences in growth). Mussels are grown at both sites. On Oosterom, mussels grow on average twice as fast as in the plots at Slenk, as shown in Figure 1b. The purpose of this study was to map food conditions at these two locations.In doing so, we want to better understand why location can be such a determinant of shellfish growth. In this study, we focus primarily on mussels.

Fig.1 (A) Locations of mussel plots in the Dutch Wadden Sea with the two study sites marked (Oosterom and Slenk). (B) Growth rate of mussels at the different breeding sites in the Wadden Sea. Red for the lowest growth rates and green for the highest.

Nine measurement campaigns were conducted between November 2021 and September 2022 in the two areas (Oosterom and Slenk) aboard the MS Asterias. Water samples were taken every hour (in triplicate) and then filtered in the laboratory aboard the ship, as shown in Figure 2a. Pigments, suspended matter, organic matter, stable isotopes and C/N ratio were measured in the water samples.


The diet of mussels consists mainly of phytoplankton, which are single-celled autotrophs. The main photosynthetic pigment, chlorophyll-a, is present in all autotrophic cells. Therefore, the amount of phytoplankton in cultured areas can be directly linked to the amount of chlorophyll-a in the water. In addition to chlorophyll-a, there are other “secondary” pigments that can be used to describe phytoplankton community composition. Phytoplankton cells are classified into three groups based on their size: micro phytoplankton (20-200 μm), nano phytoplankton (2-20 μm) and pico phytoplankton (0.2-2 μm). The contribution of each group is also reflected by the pigment composition.

For total pigment analyses, water samples were filtered with a 47 mm Whatman GF/6 filter. Samples were then quickly frozen on dry ice and stored at -80°C until analyses of the photosynthetic marker pigments were performed.

Suspended matter; Organic matter; Stable Isotopes and C:N ratio

The amount of suspended solids (SPM) in the water determines how deeply light can penetrate the water. This is important because it affects phytoplankton growth. Suspended particles are small pieces in the water, some are mineral in nature, such as clay minerals, and others come from organic sources. The organic part (detritus) consists of decomposed particles of organisms or excrement. This provides a food source for microorganisms, which in turn is important as food for mussels.

The origin of the food may also influence the growth of mussels if they have a preference for certain types of phytoplankton. One way to trace the origin of food is to analyze stable isotopes. Phytoplankton from different habitats often use different types of carbon compounds, and during photosynthesis, the ratio of two stable carbon isotopes, 13C and 12C, is different among species. These differences are reflected in δ13C values. δ13C values for marine phytoplankton range between -22 and -20‰, while δ13C values for carbon sources from rural, riverine and estuarine areas are less than -27‰.

To perform these measurements, a certain amount of water was filtered through 47 mm Whatman GF/F filter. These filters were stored aboard the ship at -20°C. Particulate matter (SPM), organic matter (POC), carbon to nitrogen (C:N) ratio, and stable isotopes (δ13C) were measured in the laboratory.

In this study, analyses were also conducted to measure the ratio of carbon to nitrogen (C:N ratio). This ratio indicates how good the organic matter is that the mussels eat. It is an important measurement because it says something about the quality of the food. In marine ecosystems, a C:N ratio between 5 and 7 means that the food is very fresh and of high quality. Higher C:N ratios, around 12, show that the food is breaking down and losing nitrogen-rich substances. Mussels thrive better when fed food that has a lower C:N ratio because this means the food is of better quality and contains more nitrogen.

Fig. 2 (a) Overview of one of the sample locations; (b) water samples taken from Oosterom and (c) Water sample taken from Slenk. Already in the sample jars, the difference in turbidity can be seen between the two locations. Filtration setup can be seen at (d) with a pigment filter on (e) and an SPM filter on (f)


Significant differences were found in chlorophyll-a (Chla) and suspended particulate matter (SPM) concentrations between the Slenk and Oosterom areas (see Figures 3a and 3b). Both Chla and SPM levels were consistently higher in Slenk than in Oosterom. Moreover, a clear seasonal pattern was observed: Oosterom showed the highest values in March, while Slenk showed higher than normal values both in March and during some summer months (June and July).

The high chloropyl-a values in Slenk show that food is abundant in the water at that location. It is therefore strange that the mussels at that location are growing a lot slower than at Oosterom. However, mussels must also be able to absorb the food present efficiently. How the mussels do that is here to read and how the particles in the water affect food intake at the contrasting locations shown here is here here. The results of the secondary pigments, show that the pigment Fucoxanthin was the most present pigment at all two sites. This pigment is present in several phytoplankton groups, but especially in diatoms (diatoms). Although species of this group are distributed among the 3 size classes, most diatom species are associated with microphytoplankton (20-200 μm). This could indicate that despite the large amounts of food present in Slenk (seen by the high Chla values), the mussels are unable to feed on it.

Fig. 3. Monthly values of food-related water parameters at Oosterom and Slenk.(a) Chlorophyll a; (b) suspended matter (SPM); (c) C:N ratio; (d) fucoxanthin concentration relative to Chlorophyll a; (e) stable isotope 13 C; (f) organic fraction

The stable isotopes (Figure 3e) show a small difference between the two sites, especially in the before and after years. The δ13C values found at Slenk are more related to freshwater algae. One of the differences between freshwater algae and marine algae is their protein composition. The digestibility of proteins is essential for food intake, and freshwater algae are more poorly digestible for mussels than saltwater algae. Further, the slightly higher C:N ratios (Figure 3c) found at Slenk are an indication of more decomposing material present in the water column that may not be part of the mussels’ diet.

Food availability shows great differences between within the Wadden Sea. This affects the growth of mussels, which also show large spatial differences. The analysis of what is in the water as food shows that the difference in mussel growth at the two sites studied cannot be explained by differences in one water quality parameter. In principle, higher chlorophyll-a concentrations (proxy for food) should result in better growth. However, despite Chlorophyll a not being a limiting factor at the Slenk site and also being much higher than at the Oosterom site, this study shows that there are differences in food composition and in the total number of particles in the wtaer. How clams deal with that is here readn.

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