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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.
- 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.
- Recent trend reversal for declining European seagrass meadowsPublication . de los Santos, Carmen B.; Krause-Jensen, Dorte; Alcoverro, Teresa; Marbà, Nuria; Duarte, Carlos M.; Van Katwijk, Marieke; Pérez, Marta; Romero, Javier; Sánchez Lizaso, José Luis; Roca, Guillem; Jankowska, Emilia; Perez-Llorens, Jose Lucas; Fournier, Jérôme; Montefalcone, Monica; Pergent, Gérard; Ruiz, Juan M.; Cabaço, Susana; Cook, Kevan; Wilkes, Robert J.; Moy, Frithjof E.; Trayter, Gregori Muñoz-Ramos; Arañó, Xavier Seglar; Jong, Dick J. de; Fernández-Torquemada, Yolanda; Auby, Isabelle; Vergara, Juan J.; Santos, RuiSeagrass meadows, key ecosystems supporting fisheries, carbon sequestration and coastal protection, are globally threatened. In Europe, loss and recovery of seagrasses are reported, but the changes in extent and density at the continental scale remain unclear. Here we collate assessments of changes from 1869 to 2016 and show that 1/3 of European seagrass area was lost due to disease, deteriorated water quality, and coastal development, with losses peaking in the 1970s and 1980s. Since then, loss rates slowed down for most of the species and fast-growing species recovered in some locations, making the net rate of change in seagrass area experience a reversal in the 2000s, while density metrics improved or remained stable in most sites. Our results demonstrate that decline is not the generalised state among seagrasses nowadays in Europe, in contrast with global assessments, and that deceleration and reversal of declining trends is possible, expectingly bringing back the services they provide.
- Impact of anthropogenic activities on the seagrass Zostera noltiiPublication . Cabaço, S.; Santos, RuiO declínio de ervas-marinhas é um fenómeno documentado à escala global, principalmente devido a distúrbios de origem antropogénica. Esta tese tem como objectivo avaliar o impacto das principais actividades antropogénicas que afectam a erva-marinha Zostera noltii na Ria Formosa. O distúrbio por nutrientes originado pela descarga de efluentes urbanos afecta claramente a estrutura da população, a morfologia das plantas e o conteúdo de N das folhas. As elevadas concentrações de amónia (158-663 μM) na estação mais próxima do efluente de descarga (270 m) revelou ser tóxica para esta espécie, reduzindo a sua biomassa e o tamanho das folhas e internós. Dois dos principais processos bióticos identificados pela análise multivariada nas populações de Z. noltii, i.e. o tamanho geral das plantas e a dinâmica da biomassa-densidade, estão significativamente correlacionados com os processos abióticos claramente relacionados com o efeito do efluente urbano, i.e. com os nutrientes e anóxia do sedimento e com o contraste entre a salinidade e a concentração de nutrientes na água, respectivamente. Os efeitos adversos do efluente urbano nas pradarias de Z. noltii da Ria Formosa parecem estar espacialmente restritos a áreas até 600 m de distância da descarga. As variações nas relações da biomassa-densidade reflectem os gradientes antropogénicos de nutrientes, descrevendo os parâmetros derivados da estrutura da população um padrão de variação com a disponibilidade de nutrientes. As estações impactadas por nutrientes apresentaram correlações significativas entre a biomassa e a densidade, enquanto que nas estações não-impactadas, os dados de biomassa-densidade não se correlacionaram. O marisqueio afectou negativamente as pradarias de Z. noltii, reduzindo significativamente a densidade de rebentos e a biomassa total. O marisqueio experimental revelou um impacto a curto prazo na densidade, que recuperou rapidamente para níveis controlo em um mês. A recuperação pode ocorrer mesmo que plantas com apenas 1 ou 2 módulos, incluindo o rebento apical, permaneçam no sedimento. O nível crítico de soterramento tolerado por esta espécie é extremamente baixo (entre +4 e +8 cm), devido ao seu pequeno tamanho e falta de rizomas verticais. Apesar desta ser uma espécie de crescimento rápido, a sua recuperação não ocorreu durante o período experimental (2 meses), mas plantas completamente soterradas sobreviveram em laboratório por 1-2 semanas. As ervas-marinhas mostraram ser extremamente sensíveis a alterações do nível do sedimento. A sua capacidade para resistir ao soterramento é significativamente dependente do seu tamanho. O comprimento das folhas e o diâmetro dos rizomas são as características que melhor predizem o impacto do soterramento nas ervas-marinhas. Os distúrbios antropogénicos estudados revelaram ser adversos para a erva-marinha Z. noltii, representado uma séria ameaça para esta espécie. No entanto, o seu rápido crescimento e elevada plasticidade permitem-lhe sustentar, até certo ponto, o distúrbio.
- Effects of nutrient enrichment on seagrass population dynamics: evidence and synthesis from the biomass-density relationshipsPublication . Cabaco, Susana; Apostolaki, Eugenia T.; Garcia-Marin, Patricia; Gruber, Renee; Hernandez, Ignacio; Martinez-Crego, Begona; Mascaro, Oriol; Perez, Marta; Prathep, Anchana; Robinson, Cliff; Romero, Javier; Schmidt, Allison L.; Short, Fred T.; van Tussenbroek, Brigitta I.; Santos, RuiThe available data from experimental and descriptive studies on seagrass biomass and density responses to nutrient enrichment were analysed to assess the intraspecific mechanisms operating within seagrass populations and whether biomass-density relationships can provide relevant metrics for monitoring seagrasses. The response of shoot biomass and density to nutrient enrichment was dependent on the type of study; the short-term positive response of biomass and density in experimental studies reveals context-specific nutrient limitation of seagrasses. The long-term negative response of descriptive studies probably results from ecosystem-scale events related to nutrient enrichment such as increased turbidity, algal blooms, epiphyte loads and anoxia. Most seagrass species analysed lie in the nonthinning part of the theoretical biomass-density curves. A simultaneous increase in biomass and decrease in density, evidence of self-thinning, were only observed in 4 of 28 studies. The analysis of both the static and the dynamic biomass-density relationships revealed that the slopes increase under nutrient enrichment. Surprisingly, the species-specific slopes (log B-log D) were higher than one, revealing that the B/D ratio, that is, the average shoot biomass, increases with density in all seagrass species analysed. Nutrient enrichment further enhanced this effect as biomass-density slopes increased to even higher values. The main drivers behind the increasing biomass-density slopes under nutrient enrichment were the increase in shoot biomass at densities above a species-specific threshold and/or its decrease below that threshold. Synthesis. Contrasting short- and long-term responses of both biomass and density of seagrasses to nutrient enrichment suggest that the former, positive ones result from nutrient limitation, whereas the later, negative ones are mediated by whole ecosystem responses. In general, shoot biomass of seagrasses increases with density, and nutrient enrichment enhances this effect. Experimental testing of facilitation processes related to clonal integration in seagrasses needs to be done to reveal whether they determine the low incidence of self-thinning and the intriguing biomass-density relationships of seagrass species. The increasing slopes and decreasing intercepts of the species-specific dynamic biomass-density relationships of seagrasses and the decreasing coefficients of variation of both biomass and density constitute relevant, easy-to-collect metrics that may be used in environmental monitoring.
- Extending the life history of a clonal aquatic plant: dispersal potential of sexual and asexual propagules of Zostera noltiiPublication . Berković, Buga; Cabaço, Susana; Barrio, Juan M.; Santos, Rui; Serrão, Ester; Alberto, F.The dispersal potential of sexual and asexual propagules of Zostera noltii was experimentally quantified in still water and under different currents in microcosm and mesocosm facilities. We estimated sinking rates of seeds and changes over time in shoot buoyancy, shoot viability, fragment breakage, shoot growth rate and spathe release rate of floating fragments. The fast sinking rates of detached Z. noltii seeds suggest a small spatial scale of dispersal insufficient to connect fragmented populations, whereas the traits of floating fragments, particularly shoot buoyancy and shoot survival (>55 d), suggest a potential for long distance dispersal (>2300 km) that could connect distant patches and allow colonization of new areas. We showed that the Z. noltii vegetative and reproductive fragments have the potential to extend the dispersal achieved by detached seeds alone.