Planktonic cnidarian responses to contrasting thermohaline and circulation seasonal scenarios in a tropical western boundary current system

Abstract. In western boundary current systems (WBCSs), strong
currents flow coastward carrying oceanic water masses and their associated
planktonic fauna. Variation in the intensity of these currents and in the
continental runoff may affect the dynamic interplay between oceanic and
coastal communities. In addition, changes in the continental runoff and the
thermohaline structure modulate the primary production, adding complexity to
the dynamics of these oligotrophic systems. These dynamics likely shape the
planktonic cnidarian communities. To further understand such relationships,
we used a comprehensive dataset encompassing samples collected above the
shelf and slope and around oceanic seamounts and islands of the Fernando de
Noronha Ridge in the western tropical South Atlantic, in two seasons
characterised by distinct thermohaline structure and circulation patterns.
Results show that in the tropical South Atlantic and, likely, other western
boundary systems with narrow continental shelves, coastward currents spread
oceanic waters and their associated cnidarian species over the continental
shelf. However, while both coastal and oceanic communities co-occur when the
continental runoff is notable, oceanic species dominate almost the entire
shelf during the dry season characterised by a stronger boundary current
intensity. We also conclude that when the mixed-layer depth and associated
nutricline are shallower, the enhanced primary productivity supports larger
populations of planktonic cnidarian species through a bottom–up control.
