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Recent studies have shown increasing Zostera noltei meadows in areas modified by anthropogenic activities. However, it is not entirely clear whether this trend of expansion could be linked to a greater reproductive effort in the species. Anthropogenic stressors can induce the reproductive effort of seagrass meadows as a response to stress, but other variables, such as seagrass biometrics or environmental factors, can also influence their sexual reproduction. To increase the knowledge regarding this issue, we monitored the flowering effort, seagrass biometrics and abiotic parameters of three Z. noltei meadows in an area that has been highly modified by anthropogenic activities during the past decades. Results showed that silt and clay content in the sediment (strongly correlated with organic matter) and seagrass vertical shoot density explained 54% of the variability in the flowering effort of the meadows. This study suggests that stress-induced flowering of Z. noltei may occur under determinate environmental conditions, such as silty environments with organic enrichment.

Posidonia oceanica is one of the few seagrasses that can colonize hard and soft substrates. To test whether substrate could affect root development of the seedlings, with a legacy effect upon transplantation to sand, we germinated seeds on hard (glass slide) versus soft (sand) substrates in microcosms. We found that sand favored root system development, with a compensatory slowing of leaf development, while glass had the opposite effect. After 4 months, we transplanted all seedlings to sand and tested for a legacy effect of initial substrate type. Leaves of seedlings germinated on sand and glass slides reached approximately the same length, but roots from seedlings germinated on glass did not develop fully. Seed burial (0–1.5 cm) did not affect seedling development. These results suggest that the culture of P. oceanica on sand prior to transplantation could enhance seedlings survival in restoration programs.

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The seagrass Posidonia oceanica is considered one of the oldest and largest living organisms on Earth. Notwithstanding, given the difficulty of monitoring its fruits and seeds in the field, the development of P. oceanica during its sexual recruitment is not completely understood. We studied the stages of development of P. oceanica seeds from their dispersion in the fruit interior to their settlement in sediment through histological, ultrastructural and mesocosm experiments. P. oceanica sexual recruitment can be divided into three main stages that focus on maximising photosynthesis and anchoring the seedlings to the sediment. In the first stage (fruit dispersion), seeds perform photosynthesis while being transported inside the fruit along the sea surface. In the second stage (seed adhesion), seeds develop adhesive microscopic hairs that cover the primary and secondary roots and favour seed adhesion to the substrate. In the last stage (seedling anchorage), roots attach the seedlings to the substrate by orienting them towards the direction of light to maximise photosynthesis. The adaptations observed in P. oceanica are similar to those in other seagrasses with non-dormant seeds and fruits with membranous pericarps, such as Thalassia sp. and Enhalus sp. These common strategies suggest a convergent evolution in such seagrasses in terms of sexual recruitment. Understanding the sexual recruitment of habitat-forming species such as seagrasses is necessary to adequately manage the ecosystems that they inhabit.

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Marine litter can end up deposited on sandy beaches and become entangled in the natural wrack, threatening its roles in ecosystems. However, it is currently unknown whether the storage of both artificial and natural accumulations on sandy beaches is correlated. Here, we quantified and compared, by first time, the litter and natural wrack on five sandy beaches in the north of Portugal. Results showed that the amount of marine litter and natural wrack were not correlated. Most of the sandy beaches had more litter than wrack and both artificial and natural accumulations disclosed high spatio-temporal variability. In summer, annual and opportunistic macroalgae dominated the wrack, while the litter was mainly formed by cigarette butts and leftover food. In winter, perennial taxa were more abundant in the wrack and plastics from mussel farming and cotton bud sticks dominated the litter. The macroalga Fucus spp., plastic pieces and materials from fishing were frequent in both periods. This study confirms that, currently, more litter than natural wrack reaches the Northern Portuguese sandy beaches, evidencing the need to take urgent measures against this contamination. Future management measures should consider this spatio-temporal variability to quantify both depositions.

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