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- Population-level effects of clam harvesting on the seagrass Zostera noltiiPublication . Cabaço, Susana; Alexandre, Ana; Santos, RuiSeagrass declines have been reported worldwide, mostly as a consequence of anthropogenic disturbance. In Ria Formosa lagoon, southern Portugal, the intertidal meadows of Zostera noltii are highly disturbed by clam harvesters. The most common technique used to collect the clams consists of digging and tilling the sediment with a modified knife with a large blade. Here we present both descriptive and experimental evidence of the negative effects of clam harvest on the Z. noltii populations of Ria Formosa. A comparison between disturbed and undisturbed meadows suggests that clam harvesting activities change the species population structure by significantly reducing shoot density and total biomass, particularly during August, when the harvest effort is higher. Experimental harvest revealed a short-term impact on shoot density, which rapidly recovered to control levels during the following month. An experimental manipulation of rhizome fragmentation revealed that plant survival is reduced only when fragmented rhizomes are left with 1 intact internode. Shoot production and rhizome elongation and production of fragmented rhizomes having 2 to 5 internodes were not significantly affected, even though growth and production were lower when only 2 internodes were left. Experimental shoot damage at different positions along the rhizome had a significant effect on plant survival, rhizome elongation, and production only when the apical shoot was removed. Our results show that clam harvest can adversely affect Z. noltii meadows of Ria Formosa while revealing a low modular integration that allows the species to rapidly recover from physical damage.
- Effects of clam harvesting on sexual reproduction of the seagrass Zostera noltiiPublication . Alexandre, Ana; Santos, Rui; Serrão, EsterThe sexual reproduction of Zostera noltii in the Ria Formosa lagoon (southern Portugal) was assessed by quantifying the variation in both flowering effort and evolution of maturation stages during the fertile season. The effects of clam harvesting on the sexual reproduction of Z. noltii were investigated using both descriptive and experimental approaches. Meadows disturbed by clam harvesting were compared with undisturbed meadows in 2 sectors of the lagoon. In both sectors, disturbed meadows showed significantly lower vegetative shoot density and significantly higher reproductive effort. The reproductive effort in the western disturbed meadows was 2-fold higher than in undisturbed meadows, whereas in eastern disturbed meadows it was 4-fold higher. In disturbed meadows, early maturation stages were found until the end of the flowering season and higher seed production was observed. Both the negative effect of clam harvesting on Z. noltii density and the positive effect on its reproductive effort were confirmed by manipulative field experiments. Plots that were experimentally harvested during the fertile season showed significantly lower vegetative shoot density and higher reproductive effort than unharvested plots. Results suggest that Z. noltii responds to clam harvesting disturbance by both increasing its reproductive effort and extending its fertile season.
- Timing and success of reproductive stages in the seagrass Zostera noltiiPublication . Alexandre, Ana; Cabaço, Susana; Santos, Rui; Serrão, EsterThe timing and success of sexual reproduction of the seagrass Zostera noltii was investigated at the Ria Formosa lagoon, Portugal. Thirty plants were tagged and monitored individually through time to determine in situ the development time of each maturation stage, from the emergence of the flowers to the production of seeds. The overall process of flowering and fruiting lasted 47 ± 4 days, during which formation and maturation of the fruits was the most time-consuming stage (27 ± 2 days). Spathe success, i.e. the percentage of spathes that produced seeds, was 22 ± 4% while spathe mortality was 34 ± 6%. A considerable percentage of spathes (37 ± 7%) was lost through leaf detachment, but some of these may still add to the reproductive success of the species by contributing to the species recruitment within the source meadow or elsewhere. Meadow seed production (MSP) of Z. noltii was estimated to be 312 ± 66 seeds m−2, whereas the potential seed production of the studied meadow was 2623 seeds m−2. Under laboratory conditions, 70% of Z. noltii seeds germinated within 26 days, but only 10% reached the seedling stage due to the high mortality of germlings. Fertility, defined as the probability of a seed to originate a new plant, was estimated to be 14 × 10−4, which is higher than what is expected for most seagrasses.
- Impacto da actividade de marisqueio na floração e no esforço de reprodução sexual em Zostera noltii Hornemann na Ria FormosaPublication . Alexandre, Ana; Santos, RuiA angiospérmica marinha Zostera noltii cresce e reproduz-se sexualmente nas zonas intertidais da Ria Formosa. A época de floração ocorre entre Março e Novembro. O desenvolvimento das flores ao longo da época de floração, desde a formação da inflorescência até à produção da semente, foi observado e organizado numa escala de maturação. A razão entre o número de flores masculinas e femininas determinado para a Z. noltii foi de 1:1 (4±0.14 S.E. flores masculinas para 4±0.13 S.E. flores femininas). O impacto da actividade de marisqueio na reprodução sexual da espécie foi avaliado. Valores mais elevados de esforço reprodutivo (6.38±3.91%), bem como da densidade de rebentos reprodutores (2200 rebentos por m2), foram observados nos campos mariscados em relação aos campos de controlo (1.85±1.14% de esforço reprodutivo; 1062 rebentos por m2). Os resultados obtidos mostram que a Z. noltii aumenta o seu esforço na reprodução sexual como resposta ao distúrbio provocado pela actividade de marisqueio. No entanto, devido ao rápido crescimento clonal da Z. noltii e aos constrangimentos existentes no seu ciclo reprodutor, a recuperação dos campos mariscados na Ria Formosa é provavelmente efectuada através do desenvolvimento vegetativo da espécie.