|Facilitation of the pacific oyster (Crassostrea gigas) in mesotidal salt marshes in Dutch estuaries|
Stueben, D. (2020). Facilitation of the pacific oyster (Crassostrea gigas) in mesotidal salt marshes in Dutch estuaries. MSc Thesis. NIOZ Royal Netherlands Institute for Sea Research: Yerseke. 81 pp.
Magallana gigas (Thunberg, 1793) [WoRMS]
The Pacific oyster Crassostrea gigas (Thunberg, 1973) is a highly resilient, adaptable and comparatively undemanding oyster species, that originates from Japan, but has – accidentally and purposely – been introduced to many parts of the world. Nowadays, it is the amongst the most globalized marine invertebrate species and the most widely cultivated oyster species for human consumption. Oyster reefs provide an abundance of ecosystem services and are commonly used as natural coastal protection measure due to their ability to attenuate hydrodynamic energy. In the south-west of the Netherlands, an estuarine area that encompasses different lakes, a bay and an estuary, is located. Here, the river Scheldt meets the North Sea in the Western Scheldt estuary. In this estuary, only very few natural oyster reefs exist, which is attributed to its high turbidity and low salinity. Contrary to expectations, dense aggregates of C. gigas have recently been found in several salt marshes in the upper intertidal of the Western Scheldt, but not on the surrounding mudflats. This discovery is not only unexpected with respect to turbidity and salinity, but also inundation frequency. Previously, C. gigas was assumed to only establish in the lower intertidal and shallow subtidal. This study aims at investigating what mechanisms enable C. gigas to establish inside the salt marshes which do not seem to be provided on the bare mudflats. As a first step, their distribution pattern was investigated followed by manipulative experiments on their spat settlement and long-term survival in the marsh. Based on the findings, the following hypotheses were suggested and investigated by means of various experiments and measurements: (i) salt marshes promote diatom growth, (ii) salt marshes decrease turbidity which improves filter-feeding efficiency, (iii) salt marshes provide stable settlement substrate. Whilst the first hypothesis remain open to investigation, the measurements and experiments conducted for the latter two have delivered conclusive results. It can be assumed that salt marshes decrease turbidity and provide stable settlement substrate for oyster spat by attenuating hydrodynamic forces. Hence, they provide an environment in which oyster spat can settle and grow undisturbed and is also able to filter-feed efficiently in all stages of life. With a view to the future, there is potential to mimic the facilitative mechanism provided by the salt marsh in order to establish artificial oyster reefs for coastal protection in areas where they have not established naturally due to environmental constraints. In the Netherlands, a country which is heavily impacted by sea level rise and coastal erosion, these approaches can serve as a promising addition to previous coastal protection measures.