Population dynamics of a fragmented subtidal Zostera marina population affected by shell fishing

This investigation illustrates the spatial and temporal dynamics of a Zostera marina seagrass meadow affected by clam harvesting. Photointerpretation of satellite imagery corresponding to years 2007, 2013, 2017 and 2018, combined with field monitoring in 2019 allowed assessing the spatial coverage, population dynamics and genetic characterization of the Z. marina population in areas impacted and non-impacted by the shellfishing activity. The impacted meadow displayed a highly fragmented and discontinuous seagrass matrix anthropogenically induced by the periodical disturbance associated with bottom raking. A continuous colonization process characterized the seagrass landscape, where the area occupied by the meadow varied by a two-fold factor, with changes even exceeding 86% in some years. Only 740 m2 (ca. 15%) of the seagrass matrix remained vegetated in the four years monitored in this investigation. The number of patches showed a large interannual variability, exceeding 100% in the four years studied, ranging from 58 to 199, while the border effect perimeter/area indicator showed a two-fold variation ranging between 1 and 2. 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. A significant pattern of genetic differentiation among impacted and control populations were also found. Our results showed that population dynamics varied as a function of Z. marina patch-sizes, thus reinforcing the need for a combined approach involving seascape structure and patch dynamics with population dynamics and genetic structure to assess the impact of disturbances on seagrass ecosystems.