Interaction between shellfishing and seagrassesecological impact and social perception

Resumo

This PhD Thesis focused on the interaction between shellfishing activity and seagrass beds, paying special attention to the ecological impacts derived from this interaction and to the social perception on the significance of these coastal ecosystems. Therefore, the main objective of the research proposed in this Thesis was to assess the response of this system to the impact caused by the manual traction gear used in the collection of bivalve mollusks to gain understanding on the controlling factors of the short and long-term resilience of the seagrass ecosystems that coexist with shellfishing activities. Specifically, the effect of shellfish harvesting on the seagrass habitat fragmentation, genetic diversity and on the organic carbon sequestration of Zostera marina meadows impacted and not impacted by the shellfishing activity were assessed. In addition, the historical and present social perception of Galician population was investigated, aiming at the implementation of management and communication strategies to achieve effective conservation policies, and implementing and evaluating the impact of one of the defined actions. The meadow studied in this Thesis, the Toralla meadow, provided appropriate logistical and environmental conditions for the development of this research. The meadow, affected by the shellfishing extractive activity, is partially distributed within free shellfish banks of the Ría de Vigo. A nearby meadow not affected by clam harvesting, the Canido meadow, was also sampled as a reference in some of the investigations herewith undertaken. The main alteration associated with the shellfishing activity is the uplift of the substrate, affecting the upper sedimentary layers, breaking and eliminating shoots and rhizomes of these marine angiosperms. In a highly changing environment subjected to unavoidable and growing anthropic pressures on the coastal habitats, it is crucial to effectively address these impacts to improve ecosystem management. The meadow under study displays a discontinuous coverage in which three stages of development were identified: high density and low fragmentation areas, low density and high fragmentation area and areas devoid of vegetation. The process of habitat fragmentation refers to both the reduction of the population distribution area or to a change in its configuration, ie. the spatial arrangement of the patches in which the population is distributed is altered, as well as the distance between them and the connectivity among patches. The zone restricted to shellfishing presents a continuous coverage with high density, while the zone exposed to the clam extractive activity shows a patchy distribution of Zostera marina. In previous research it was observed that the Z. marina meadow under study was resilient to the physical disturbance caused by the harvesting of clams; a rapid response was recorded in terms of seagrass density and biomass and of the abundance of associated fauna in the affected area, reaching values similar to those measured in the non-impacted area four months after the disturbance. The potential fragmentation associated with this recurrent alteration process reduces the ability of this mechanism to completely recover bare areas. Ecosystem fragmentation is known to be associated with a potential reduction in the sexual reproductive performance of the population, which could affect genetic diversity and the presence of a seed bank and, consequently, the long-term resilience of the population to future disturbances. To that end, in the first place, the spatial and temporal dynamics of a Zostera marina seagrass meadow affected by clam harvesting was evaluated, combining photointerpretation of satellite imagery corresponding to years 2007, 2013, 2017 and 2018, with field monitoring in 2019 to assess the spatial coverage, population dynamics and genetic characterization of the Z. marina population in areas impacted and non-impacted by the shellfishing activity, reporting a highly fragmented and discontinuous seagrass matrix anthropogenically induced by the periodical disturbance associated with bottom raking, driving a continuous colonization process that characterized this seagrass landscape. A dynamic equilibrium between patch development and spatial fragmentation, which in turn depended on the intensity and amplitude of disturbance, was evidenced. The capacity of the seagrass meadow to expand by clonal growth combined with sexual reproduction and dispersal by seed production and seedling recruitment is a key factor driving seagrass seascape development. Patch growth rate and spatial patterns differ with patch size due to allometrically scaled growth rules and clone size benefits. Consequently, clearly differentiated patterns were observed in shoot density, biomass and flowering density between shellfishing-induced patches of different sizes and the long-term non-impacted areas. The medium and large patches reached total biomass values similar to those of the non-impacted areas, while the small patches did not recover shoot densities and biomass levels of the unaffected areas, thus increasing the differences with larger patches. This growth pattern is characteristic of rapid patch formation and mortality in highly fragmented seagrasses. Low disturbance, both in frequency or amplitude, can open temporal windows that allow the development of a continuous seagrass meadow. By contrast, a more intense affection or shorter time intervals between disturbances than those required for patch recovery, may led to a final stage where bare areas prevail. As reported in our preliminary investigations, all the Z. marina population variables measured were significantly higher in the non-impacted than in the impacted zone one month after the close season. As expected, a different behaviour during the colonization process was found in the fragmented areas in comparison with the continuous established meadow. All patch sizes at the fragmented meadow showed higher rates of increase in total and aboveground biomass and shoot density with respect to the non-impacted areas. Three main pathways describe seagrass meadow development from discontinuous seagrass patches after an existing patch become fragmented: 1) rooting of displaced ramets imported via tides or currents followed by recovering from the seed bank, 2) surviving rhizomes in recently denuded areas and 3) dispersion of a single seed or a set of seeds from the seed bank to new locations. The sampled area is seasonally exposed to bottom tracking with a 6-month temporal window of clam extraction closure coinciding with the life-spam of the species. The simultaneous density and biomass maximum observed at the end of the growing season in the fragmented meadow is characteristic of developing meadows where no space limitations constrain the internal packing of shoots during growth, as opposed to the control meadows where density peaked before biomass, facilitating the regulation of ramet formation to prevent the overproduction of shoots. Processes at the seascape scale can also affect reproductive strategies, as reproductive effort and output in seagrass meadows seem to be influenced by habitat fragmentation. The evolutionary potential of the Z. marina meadow was evaluated, addressing the study of the genetic diversity of the population affected by this impact with respect to control non-impacted populations experiencing similar environmental conditions. A genetic differentiation among the impacted and non-impacted populations is reported, driving to a potential genetic isolation of the impacted meadow related to the shellfishing impacts and the fragmentation of the meadow. Thus, long term genetic diversity should be monitored as flowering shoots in isolated patches within fragmented beds could be experiencing pollen limitation, hence reducing seed production per shoot. In this connection, an Allee effect driven by pollen limitation in a fragmented seagrass meadow could drive a lower fruit set in fragmented populations as compared to continuous population even at similar reproductive density plants. Our results showed different population dynamics of the patches composing the seagrass seascape as a consequence of clam harvesting disturbance and reinforce the need for a combined design involving patch dynamics with a historical perspective integrating population genetic structure in order to assess the impact of disturbances on seagrass populations. Within the ecosystem services that seagrass meadows provide, its highlighted its contribution to climate mitigation and ecosystem resilience in coastal environments, being recognised among the most effective carbon sink ecosystems on Earth. The high primary production rates characteristic of seagrass meadows, their capacity to filter out particles from the water column combined with the resistance to erosion provided by the below ground network preventing resuspension of sedimentary deposits and the low organic matter decomposition rates characteristic of the anoxic sediment conditions, that slow down the remineralisation process, are key factors responsible for the high efficiency of seagrass meadows to store carbon. As a result, organic materials stored in seagrass sediments derive from autochthonous and allochthonous sources, which include terrestrial organic matter, seston particles dominated by phytoplankton and Z. marina tissues. Although seagrass meadows are declining worldwide at alarming rates, direct measurements of the consequences of habitat degradation on the sedimentary carbon stock remains still scarce. The capacity of the Zostera marina meadow to accumulate organic matter and sequester carbon was evaluated, thus, biotic (seagrass cover, density and biomass) and abiotic factors (water depth and sediment grain size distribution) of the studied meadow were characterized and carbon stocks in control and impacted areas of vegetated and bare sand zones were compared. Clam harvesting activity represents a high level of physical disturbance in the studied area, with consequent destabilization of the sediment and partial removal and uprooting of vegetation. This research has shown that the meadow exposed to the harvesting activity showed a decreasing capacity to sequester carbon by reducing seagrass standing stock and carbon preservation in the associated sediments. C-isotope signatures determined in sediments of the investigated seagrass meadow suggested that the import and burial of allochthonous organic C, mainly from planktonic origin is the main source of the accumulated sediments in the meadow, which is consistent with the massive blooms of phytoplankton biomass associated to the coastal wind-driven upwelling processes dominating in the studied area. The physical disturbance resulted in a significant reduction in shoot density (63%) and biomass (64%) in the impacted area with respect to the adjacent zone not affected by the disturbance, whereas the sedimentary carbon stock was reduced by 50%. In summary, clam harvesting activity not only eroded the historical carbon stock accumulated over decades but also endangers further potential accumulation. Therefore, the results presented in this chapter demonstrated that the mechanical impact derived from clam harvesting activity triggers the erosion of carbon stock inventories and endangers further potential carbon sequestration. The δ13C values of organic matter measured in the seagrass meadows suggested that the capacity of this meadow to sequester carbon is mainly driven by its ability to filter out and trap suspended particles from the water column. Although the carbon stocks measured in the undisturbed meadow were relatively low as compared to global estimations, mainly associated to the biotic and abiotic features of the study area, were significantly higher than bare sand, supporting the premise that seagrasses are hotspots of sediment carbon content. Effective conservation strategies require the recognition of the problems associated with the degradation of ecosystems. Implementation of effective management and conservation policies benefit from a wide range of knowledge sources on the social-ecological system under study, as the ultimate driver of policy is human values and perceptions. In this regard, quantification of ecosystem services and analysis of their social perception are considered as missing layers for the integration of sociocultural values into coastal management and conservation policies. Thus, one of the crucial challenges for global seagrass conservation is to engage society and deliver behavior changes by designing and implementing more effective communication plans, which may eventually improve the effectiveness of conservation strategies. In this context, communication plans need to be supported by local studies on social-ecological knowledge and perceptions and be oriented towards the design of informative techniques leading people to make up their own minds on the basis of scientific evidence. For such, it is necessary to identify target audiences and their current knowledge level on the ecosystem services provided by seagrasses and to develop segmented communication actions. Therefore, this Thesis aimed at assessing the existing knowledge on seagrass ecology by the Galician population and the recognition of the services, barriers to knowledge, preconceptions and the most relevant conflicts related to seagrass ecosystems by integrating the historical narrative and the current social perception. The ultimate goal of our research was to design a communication strategy to achieve effective conservation policies centered on the shellfishing-seagrass interaction. The proposed plan addresses the main values, impacts, interactions and conflicts identified in the study. To that end, the knowledge of the Galician population on the ecology and of seagrass meadows and the recognition of the benefits and pressures provided by these ecosystems was evaluated by integrating the historical narrative and the current social perception. The low visibility of seagrasses in the Galician media was confirmed, as shown by the low number of news registered on this topic over the last 160 years, as well as their low social profile, with a generally negative image for more than a century. The current social perception of these habitats reflected an ample level of ignorance on their functions, which makes them one of the most forgotten coastal ecosystems on Earth. Based on the historical and current social perception data, Grunig's situational communication theory was applied to identify the key environmental publics on which communication strategies should be focused. The Grunig’s Situational Theory was used in this investigation to identify and study the communicative behavior of audiences aimed at improving the social perception of these ecosystems from the understanding of which are the motivations of people to communicate and when they are most likely to do so, thereby designing actions that ultimately lead to better management and conservation procedures. We identified six key audiences on which communication and training actions should be focused. Involvement of direct users, mainly local fishers and shellfishers, is essential to gather their ecological knowledge and to remove barriers, so that facilitating the creation of a shared understanding of how degradation of seagrass meadows can impact the economic, cultural and ecological values of exploitable marine resources. A further audience’s assessment allowed the proposal of a communication action plan including specific actions to be implemented by each one of the identified strategic audiences. Finally, one of the actions identified in the communication plan was developed, directed to the school community, including both students and teachers to improve the knowledge of young people about their natural environment and the socioeconomic interactions associated with one of the key ecosystems of coastal habitats. Likewise, an initial and post-program evaluation was carried out to assess the success of the program in relation to the understanding of new knowledge about seagrass beds and its relationship with the human activities developed in the environment. The designed program combined theoretical sessions and practical activities that lead to a significant increase in the socio-ecological knowledge of these ecosystems and the services they provide. This thesis has demonstrated the negative effect of shellfishing on the integrity of seagrass meadows and on some of the services they provide, such as the ability to sequester carbon. It has also been demonstrated the effect of this extractive activity on the seascape dynamics, resulting in reduced areal coverage and higher proportion of edge habitat, but also on the ability of this system to recover after disturbances. It also draws attention on the possible population’s isolation and the potential effect of clam harvesting on reducing genetic diversity. It has also shown the low social perception on seagrass ecosystems of both the general public and scholars, with a historic negative insight due to its low charisma and association with unhealthy environments. To confront that situation, a communication plan has been elaborated in this thesis. Such communication plan included strategic actors to improve the social perception, but also direct users and policy makers in order to create the appropriate scenario where the conflicts emerging from this socioecological interaction could be discussed and eventually overcome.