Effects of hydrodynamics on mussel larvae settlement and mussel distribution - with a focus on fine-scale velocities in Port Alfred Marina, South Africa

Abstract: Mussels, like other intertidal organisms, depend on the motion of water throughout their entire life cycle. Hydrodynamic forces are essential for the transportation and settlement of mussel larvae, as well as feeding and growth for adult mussels. In this study physical factors, such as local water velocities and wave characteristics, were measured over an eight-week period in Port Alfred Marina, on the south coast of South Africa. These measurements were paired with a biological assessment of mussel settlement and adult mussel distribution, to determine potential correlations. Due to a lack of identified settling larvae, the physical measurements were exclusively linked to the adult mussel distributions. Although not significant, the results indicate that higher flow velocities were recorded at sites where no mussels were residing, suggesting that too high velocities may increase the detachment risk for both mussel larvae and adult mussels. Decreased velocities could however lead to silt depositing on the mussel populations and thereby impeding their growth. An average water velocity of 0,15 m/s was, according to measurements, the ideal velocity for mussel communities within Port Alfred Marina. The direction of flow could also have a potential effect on mussels, as a less parallel flow to the shoreline is proposed to be advantageous. Regarding ambient wave forces, the study found that a greater wave height was linked to more spatially dense mussel communities, increasing the nutrient input and larval contact with settlement sites. The results are highly uncertain, since only a limited number of sites are incorporated and other factors could interfere with the data, such as chemical cues, roughness, salinity, temperature, heavy metal concentration, parasites, predators and lack of food.