Short-term impact of bait digging on intertidal macrobenthic assemblages of two south Iberian Atlantic systems

The present study aimed to assess short-term effects of intertidal bait digging on macrobenthic assemblages of two south Iberian Atlantic coastal systems: the Ria Formosa (RF) lagoon and the Bay of Cلdiz (BC). Within each system, two different areas were selected. In each area, six plots were randomly located at least 1 m apart from each other. Half of the plots were left undisturbed (control) and the other three were dug (disturbed). Three macrobenthic core samples were taken from each plot, immediately before, one, four and seven days after digging. Samples for analyses of grain-size and organic matter content (OM) were also collected from the top 2 cm sediment layers in each sampling period. The effects of bait digging were site specific and were related to sediment and biological composition of the areas before disturbance. Macrobenthic assemblages from the area with proportionally less mud content (RF1), which presented the greatest infaunal diversity and evenness values before disturbance, showed minor effects of digging and assemblages generally recovered within 7 days. On the other hand, the areas located in the Bay of Cلdiz, which before disturbance presented the greatest mud content and macrobenthic assemblages dominated by only a few species, were the most affected and did not show signs of total recovery within 7 days after digging. The abundance of sedentary polychaetes from families Spionidae, Capitellidae and Cirratulidae decreased after digging. In contrast, in some areas the abundance of the gastropods Bittium spp., Haminoea hydatis and Retusa truncatula increased after bait digging indicating that they are less affected by this activity. Differences were detected in the response of benthic assemblages to the same intensity, frequency and nature of disturbance between and within coastal systems. Therefore, generalizations of the effects of bait digging and in the recovery of macrobenthic assemblages in the south of Iberian Peninsula are not possible.