Evaluation of estuarine mesozooplankton dynamics at a fine temporal scale: The role of seasonal, lunar and diel cycles

Abstract

In order to study the influence of physical forcing at different temporal scales, zooplankton was sampled at a fixed station located at the mouth of Mondego estuary (southern Europe). Samples were collected during diel cycles, over neap and spring tides. Zooplankton abundance and diversity were estimated for each sampling period. Holoplankton were dominated by three taxa: Copepoda, with 48% of total zooplankton abundance, Cladocera (16%) and Medusae (12%). Meroplankton occurred mainly as barnacle and decapod larvae. Copepoda were the most diverse group, represented by 26 species followed by Decapoda larvae (21) and Medusae (16). In order to assess significant differences between seasons, a univariate analysis was carried out. Higher abundance and diversity were found in warm months, particularly at neap tides, when water temperature and salinity were higher. Multivariate analysis revealed significant seasonal differences in species composition. The estuarine community was strongly dependent on allochthonous events, such as tidal exchange and river inflow. The results of our study show that the period of higher river flow, coincident with winter, resulted in changes in the zooplankton community. Short-term temporal variations in the species composition and abundance were also attributed to tidal and diel cycles. Zooplankton reached significantly higher densities at night (P, 0.05), suggesting the occurrence of vertical migrations. By emphasizing the importance of different timescale changes in the zooplankton community of the Mondego estuary, this study will be useful for the design of more efficient sampling programs, aiming at documenting changes in the zooplankton at a broad but also at a fine temporal scale.