Ragworms (Hediste diversicolor) limit eelgrass (Zostera marina) seedling settlement: Implications for seed-based restoration

Seagrasses are globally declining and multiple restoration efforts are undertaken to reverse these losses. However, these efforts have proven to be challenging, facing a variety of bottlenecks. We studied how predation by macroinvertebrates may form a potential bottleneck for seed-based seagrass restoration. Specifically, we questioned if the omnivorous common ragworm (Hediste diversicolor) may act as a predator on eelgrass (Zostera marina) seeds and whether that could affect seed-based eelgrass restoration trials. In a controlled lab experiment, we studied (1) how seedling establishment was affected by ragworm biomass (0, 2, 8 g DW m⁻²), (2) if the absence or presence of an additional or alternative high-protein food source (Sanikoi ® Gold Protein Plus, 52% protein) prevented potential seed predation by ragworms and (3) how ragworm size (small: 0.0029 g and 3.3× bigger: 0.0095 g DW ragworm⁻¹) affected eelgrass seedling establishment. Additionally, we questioned (4) if ragworms may provide a bottleneck for annual eelgrass restoration experiments in the Dutch Wadden Sea by combining data from a large-scale benthic survey (SIBES, Netherlands Institute for Sea Research (NIOZ), Texel) with an existing eelgrass habitat suitability map. We found that >2 g DW m⁻² ragworms completely hampered eelgrass seedling establishment, even when fed an additional, protein-rich, food source. Ragworms only seemed to target sprouted seeds rather than intact seeds. Additionally, sprouted seed consumption by ragworms was size-dependent: sprouted seeds escaped predation by smaller ragworms even when present in high biomass (2 g DW m⁻²). By extrapolating our findings to the field, we showed that 52.8% of the potential eelgrass growth sites in the Dutch Wadden Sea overlap with impeding ragworm biomass (≥2 g DW m⁻²). By consuming sprouted eelgrass seeds, ragworms may consequently strongly impede seed-based eelgrass restoration efforts, especially since both species have highly overlapping distributions. We thus provided novel insights into an unknown bottleneck for seed-based eelgrass establishment, which may have restoration implications. Especially for annual eelgrass that fully depends on successful seedling establishment for their persistence and survival.