Species-specific traits and the environment drive ichthyoplankton fluxes between an intermittently closed-open lagoon and adjacent coastal waters

Abstract

This study evaluated the connectivity between an intermittently closed-open lagoon (LR) and adjacent coastal waters (CW), and the influence of that connection on ichthyoplankton assemblages. We performed field surveys along a year in LR and CW to capture different sandbar states (open/closed) and lagoon-sea connectivity processes (e.g. lagoon discharges, marine intrusion, overwashing). Results showed differences in environmental conditions and ichthyoplankton communities between LR and CW, and also evidenced different mechanisms of exchange of water and ichthyoplankton. At least three estuarine species were found to spawn and use LR as a nursery ground. Larval stages of six marine species were found at CW. Environmental factors (e.g. wind and runoff) and species-specific traits (e.g. life cycle strategy, spawning season) determined ichthyoplankton exchange vs. retention. Marine estuarine-related species entered LR mostly during summer-autumn, at their peak abundance in CW. Sandbar closure did not impede the entrance of these species. Overwashing events (closed bar) and light onshore winds during the open-sandbar period were the most important process that promoted the transport of these larvae into LR. Conversely, there was no evidence of entrance of marine estuarine-independent larvae to LR. Eggs and larvae of estuarine species that spawned within LR were effectively retained there, except when heavy rains led to severe lagoon discharges. Similarly, unusual strong onshore winds advected eggs from CW to LR, possibly reducing their survival. Generally, eggs were more vulnerable to drift than larvae. Many of the estuarine larvae that drifted out of the lagoon seemed to survive, suggesting a loss of individuals from the lagoon population but a potential contribution to the genetic flux among coastal populations.