Boat-based videographic monitoring of an Adriatic lagoon indicates increase in seagrass cover associated with sediment deposition

Within the scope of a seagrass monitoring program in the Novigrad Sea, Central Croatian Adriatic, we predicted that the annual variability in coverage of seagrasses (Zostera marina, Zostera noltii, and Cymodocea nodosa) can be partially explained by the annual variability in sediment translocation. From 23 fixed DGPS-referenced monitoring video transects followed over three years (June 2007–2009), we calculated annual (i) changes in interior bed seagrass coverage, (ii) gain in seagrass at the lower edge of the bed and seagrass bed expansion, and (iii) accumulation of sediment, its depth dependence, and the associated changes in transect slope. We found that in 2007 to 2008, the year with net sediment accumulation, seagrass coverage increased and the bed expanded. In both years seagrass cover within the seagrass bed increased with increasing sediment accumulation, while seagrass bed expansion was highest under intermediate sedimentation rates. Boat-based videographic monitoring can document both natural sediment movement along the depth gradient, and species-specific responses necessary for informed management of submerged aquatic vegetation in the Adriatic Sea.

► Boat-based underwater videography is a rapid and inexpensive method of monitoring small changes in seagrass cover. ► This method was applied to a mixed-species sward of seagrasses in a Croatian Adriatic lagoon. ► Seagrass cover increased over a two year period coincident with a net sediment influx. ► Sediments can increase seagrass habitat by filling in areas too deep for sufficient light penetration. ► Sediment movement may also eliminate species relatively intolerant of burial (such as Zostera marina or Zostera noltii) and favor those more tolerant (such as Cymodocea nodosa).