Do harvesting activities in Zostera noltii beds affect nematode and macrofauna communities? A field experiment approach

Seagrass beds are highly productive ecosystems, typical of intertidal and shallow subtidal coastal environments, which support high abundance and diversity of organisms. They are important habitats to a large set of fauna, providing nutrients, shelter against predators and nursery for the juveniles. In these areas, bivalve harvesting is a very common activity and, in Portugal, bivalve harvesting has a long tradition, with an estimated consumption rate per capita of 58.5 kg/year. The digging activity causes physical disturbances in the sediment, with effects on the biological assemblages by exposing benthic species to dessication, predation and by altering the oxygenation of the sediments. In the Mira estuary (SW Portugal), harvesting practices of bivalve molluscs for human consumption are frequent in the seagrass beds of the estuary. These seagrass beds were denser in the past, but nowadays the vegetation is in a natural recovery process after a major collapse with still unknown causes. The goal of this research was to investigate if the digging activity during the harvesting could have triggered and stimulated the habitat loss of the estuarine intertidal seagrass beds, through the study of the nematode and macrofauna assemblage responses to the physical disturbances as bioindicators for physical stress. An experimental field design was developed to compare nematode and macrofauna assemblages’ responses to the physical disturbance effects caused by the sediment digging related with hand bivalve harvesting. Two treatments (control and digging) were analyzed at three different times (T0-before digging simulation; T1 –14 days after and T2 – 45 days after digging). Nematode assemblages registered similar density and diversity throughout the experiment in both control and digging locations, indicating the absence of digging effects. Macrofauna communities registered a reduction on richness in digging after digging (T1) but a recovery of abundances and number of species after 45 days (T2). This study revealed some differences on the responses of nematode and macrofauna assemblages to digging effects, supporting the discussion of their functional traits as bioindicators of physical disturbances