Short-term responses of estuarine mesozooplankton to maintenance dredging activities

Maintenance dredging is a common practice in estuaries to ensure navigability and support port operations, but it also represents a widespread anthropogenic disturbance. Its effects on planktonic communities, however, remain poorly understood, particularly for mesozooplankton, a key component linking primary producers and higher trophic levels. This study evaluated the short-term responses of mesozooplankton communities and environmental variables to a maintenance dredging operation in the Guadalquivir estuary (SW Spain). Sampling was conducted at two sites with contrasting salinity conditions (Salinas: polyhaline and Puntalete: mesohaline) across three distinct phases: pre-, during, and post-dredging. Among the environmental variables analysed, pH showed a significant but transient decrease during dredging, while chlorophyll-a varied significantly among sampling phases. Turbidity tended to increase during dredging, although differences were not significant, and most other physicochemical variables remained largely unchanged. Overall, the observed environmental patterns were compatible with a short-term dredging signal but could not be confidently distinguished from the high natural temporal variability characteristic of the estuary. Biologically, the mesozooplankton community exhibited contrasting site-specific responses. In the polyhaline zone, species richness declined significantly and community composition shifted, with only partial recovery after dredging. Conversely, the mesohaline zone maintained a stable community composition but showed a marked decline in total abundance, representing the clearest biological response observed during dredging. These contrasting responses observed between sites suggest that the local ecological context may influence how estuarine ecosystems respond to short-term dredging-related disturbances.