Vertical flux regulation and plankton composition in a simple ecological system: snapshots from the small marine Lake Rogoznica (Croatia)

Abstract

        Vertical flux regulation was investigated in the small, shallow, marine and partly anoxic Lake Rogoznica (eastern coast of the Adriatic Sea) by studying plankton composition, zooplankton fecal pellet (FP) production and vertical carbon flux. The lake is naturally eutrophicated with high nutrient and particulate organic carbon (POC) concentrations and relatively low species diversity. However, the few species found can be highly abundant. Due to a simple ecological structure and low physical forcing, Lake Rogoznica may be regarded as a natural laboratory especially suitable for plankton studies. Only one copepod species, Acartia italica, was found in the lake and had a maximal abundance of 140 animals L^-1. The mixotrophic dinoflagellate Ceratium furca and the diatom Chaetoceros curvisetus accounted for ~90% of the phytoplankton biomass. Phytoplankton made up 30% of the POC flux at 5 m and 10% at 10 m depth and was dominated by C. curvisetus and C. furca. Average export of POC was 730 ± 40 mg m^-2 d^-1 at 5 m and 750 ± 90 mg m^-2 d^-1 at 10 m depth, and detritus comprised 68 and 86 % at 5 and 10 m depth, respectively. Despite high copepod abundance and high suspended FP concentration, FP only contributed 4 – 5% of the vertical POC flux. The highest contribution to vertical carbon flux was in terms of detritus, and high retention was likely due to FP grazing or fragmentation by A. italica copepodites above 5 m depth. It is concluded that Lake Rogoznica is a productive system where organic material is rapidly transformed to detritus.