Are tropical coastal reefs sinks or sources of mesozooplankton? A case study in a Brazilian marine protected area

In spite of the paramount ecological and socioeconomic relevance of tropical reef ecosystems, the dynamics of their meroplankton abundance remain poorly characterized. The small-scale distribution and detailed analysis of individual biomass of mesozooplankton were studied in the coastal reefs of Tamandare (Brazil). Mesozooplankton (> 300 mu m) was collected during nocturnal ebb tides at new moon, using three different devices to sample at three different environments: a standard ring net that was towed at subsurface, the Channel Midwater Neuston Net that collected at midwater in channels between patch reefs and the Reef Edge Net that captured organisms that are washed by ebb currents from reef tops toward the reef edge. Samples were analyzed using a ZooScan to obtain abundances and biovolume of each taxonomic group. Specific biomass measurements were taken to obtain allometric equations used to calculate zooplankton biomass from biovolume. The mesozooplankton were significantly more abundant at subsurface and at the reef edge compared to channel environments. The high abundance of organisms at reef edges suggests a low predation pressure on zooplankton at near-bottom areas, since the reefs of Tamandare present a low coverage of planktivorous corals, being dominated by macroalgae. These results show that rather than sinks these ecosystems may be considered important sources of zooplankton available for planktivorous species. Regarding zooplankton composition, we found large amounts of initial stages of meroplanktonic larvae and newly hatched fish eggs, which presented consistently greater abundances compared to holoplankton and emergent benthic taxa. Decapod larvae were the most abundant group of the meroplankton, but cirripedian nauplii, stomatopod larvae, fish larvae and fish eggs were also abundant. More than 50% of the total biomass was due to meroplankton taxa, mainly composed of decapod larvae. This study indicates that the contribution of meroplankton to mesozooplankton composition and biomass off tropical reefs may have been underestimated.