Improving seed‐based restoration in Eelgrass ( Zostera marina ) in combined aquarium and field experiments

Abstract Introduction Seagrass restoration efforts are being implemented globally to accelerate the slow natural recolonization success. Among the various existing methods, seed‐based approaches offer the greatest scalability for species like Zostera marina , yet these methods remain undeveloped for a number of sub‐tidal systems like the Baltic Sea. Objectives To address this, parallel field and lab experiments were conducted to examine the effect of six factors on Z. marina seed germination and establishment. Methods Laboratory experiments focused on seed coat color, collection site, temperature, and salinity, while field studies explored seed coat color, sowing timepoint, and Arenicola marina bioturbation. Results Seed coat color and salinity emerged as the two most significant factors influencing germination success. Gray seeds consistently outperformed dark and light seeds by two and 14 times in the lab, and two and five times in the field. Sowing time was also important, with seeds sown later in spring germinating faster and achieving higher germination rates (5 ± 2 to 11 ± 3 seedlings from 100 gray seeds sown). In the lab, salinity significantly below natural conditions (5 vs. 18–20 psu) was detrimental to germination success. Despite complex temperature effects (e.g. at 10°C, gray seeds showed both their highest and lowest germination success across different salinities), seeds at 10°C reached their peak germination faster, within 40 days, whereas those at 5°C needed 50 days. Optimal germination occurred under 18–20 psu and 10°C conditions, with a maximum of 45 ± 6 seedlings (out of 100 gray seeds sown). Field germination in combination with early survival was four times lower than in controlled lab conditions. Conclusions Our findings suggest that Z. marina seed germination can be optimized by sowing them in late spring, aligning with higher temperatures and natural salinity levels.