The microzooplankton community structure (species abundance, biomass, diversity) was investigated at a coastal marine station on the South-West Atlantic Ocean (34°23′S–53°45′W, Uruguay). This is a hydrographically complex site within the Subtropical Convergence zone of the SW Atlantic where knowledge of the microzooplankton is particularly scant. The main goal was to perform a first characterization of that community and evaluate its association to environmental drivers along an annual cycle. Oceanographic variables (temperature, salinity, irradiance, nutrients, chlorophyll-a) and ciliates (aloricate and loricate), and dinoflagellates were recorded monthly from July 2019 to June 2020. Over 100 microzooplankton taxa belonging to approximately 30 families and 40 genera were identified, including several subtropical and subantarctic species. Community structure followed wide transitions at the seasonal scale – particularly between summer and winter as subtropical taxa alternated with euryhaline taxa from colder subantarctic waters. The core environmental variables (temperature, salinity and dissolved inorganic nitrogen [DIN]) explained overall variance in microzooplankton community assembly. During summer, high temperatures (20.3, 16.3–22.4°C) and low nutrients (DIN: 3.5, 0.7–6.7 μM; PO4: 1.0, 0.8–1.5 μM) benefited the development of aloricate ciliates. A nutrient pulse in winter posed favourable stoichiometric conditions and the numerical abundance was dominated by dinoflagellates and loricate ciliates in the following months, while diversity remained highest (taxonomic richness: 36 [22–46]; Shannon–Wiener index: 2.5 [1.9–2.8]). Results suggested that the microzooplankton community at the study site is mainly structured by hydrographic variability linked to the seasonal replacement of offshore water masses that differed in thermohaline properties and nutrient levels, and local processes.

Spatial distribution patterns of microzooplankton (0.055–0.3 mm) biomass and abundance were studied in relation to the hydrographic situation and the local flow field in the waters off Ampère and Senghor, two shallow seamounts in the subtropical and tropical NE Atlantic, in comparison with unaffected open ocean reference sites. Ampère was sampled during November/December 2010 and Senghor during December 2011 and February 2013. The study includes taxonomic composition, abundance of meroplanktonic larvae and an estimation of the respiratory carbon demand. Biomass (dry weight) standing stocks of microzooplankton in the upper 100 m ranged between 30–120 mg m−2 over Ampère and 140–260 mg m−2 over Senghor Seamount, corresponding to 33 and 24% of the total zooplankton (0.055–20 mm). Highest total abundance was always found in the upper 50 m with numbers of 1070–5060 Ind m−3 at Ampère and 5050–20,000 Ind m−3 at Senghor with microzooplankton contributing 70–95%. Zooplankton accumulated mainly at the thermocline coincident with the deep fluorescence maximum and was ascertained by food supply rather than by oxygen limitation. The microzooplankton contribution to the total respiratory carbon demand was ~50% in the subtropical waters off Ampère and ~30% at Senghor, reflecting the important role of microzooplankton in the waters of the NE Atlantic subtropical gyre. Clear evidence of local seamount effects resulting in enhanced microzooplankton biomass compared with the unaffected reference sites were not detected. However, we confirmed Senghor as a hotspot for meroplanktonic larvae, suggesting a retention potential that results in significantly enhanced larval abundance in the seamount waters as compared with the open ocean.

We investigated the occurrence and seasonal variation of the biomass of rotifers, tintinnids, the heterotrophic dinoflagellate Gyrodinium fusus and copepod nauplii in the Bahía Blanca Estuary (38°42′S61°50 ′W), Argentina, during an annual cycle. The rotifers fauna comprised three species, while the tintinnids were represented by sixteen species. The biomass of the rotifers fluctuated between 0.62 and 8.90 µgC l−1. The biomass of the tintinnids fluctuated between 0.13 and 9.37 µgC l−1, the biomass of the nauplii stages between 1.78 and 7.65 µgC l−1; while the biomass of G. fusus varied from 0.26 and 7.94 µgC l−1, these results are compared to estimates of microzooplankton in other regions. We analysed the presence of the different groups in relation to the environmental variables, based on point-biserial correlation. Salinity fluctuated between 25.14 and 36.64; temperature between 7.5 and 23.2°C, solar radiation between 0.9 and 30.8 MJ m−2d−1 and Secchi distance between 0.25 and 1.43 m. Rotifers were correlated positively with temperature, chlorophyll-a and Secchi depth and negatively with salinity. The tintinnids were positively correlated with salinity. Gyrodinium fusus was positively correlated with Secchi depth, and chlorophyll-a, and negatively with temperature and solar radiation. Nauplii stages were negatively correlated with chlorophyll-a. Based on the occurrence of the microzooplankters in relation to the physico-chemical variables, it was possible to establish two seasonal assemblages: (a) the co-occurrence of the rotifers and the heterotrophic dinoflagellate G. fusus during the winter–spring; and (b) the tintinnids and nauplii larvae during the summer. We conclude that, in this estuary, physico-chemical variables are the forcing factors that directly, or indirectly, influence the seasonal assemblages of the microzooplankton.

Grazing experiments targeting the determination of in situ grazing rates are standard. In two separate experiments the effect of the frequently used siphon filling technique on the abundance of microzooplankton during the set-up of grazing experiments was investigated and compared to results from an alternative filling method. Hereby, water containing natural communities from Helgoland Roads, Germany (54°11.3′N 7°54.0′E), was transferred into incubation bottles using a funnel system (funnel-transfer technique (FTT)). The impact of pre-screening with a 200 µm net for excluding larger mesozooplankton grazers from the incubations was evaluated. Results show that the ciliate community was strongly affected by siphoning and pre-screening, leading to significant differences in abundance and Margalef diversity. The most affected ciliates were Lohmanniella oviformis and Myrionecta rubra, both important species in the North Sea. Dinoflagellates did not show any significant response to either siphoning or pre-screening with the exception of one athecate species. Such artificial bias in ciliate assemblages is very problematic for biodiversity consideration and grazing investigations. Simply changing the method of filling during the experimental set-up can ensure the measurement of accurate grazing rates of field abundances of microzooplankton. We thus recommend using conservative filling approaches like the FTT in experiments, especially when sensitive species are present, in order to avoid shifts in the overall microzooplankton community. Furthermore, we recommend introducing a control to evaluate the degree of changes in the target community due to the experimental set-up.

Tintinnid ciliates were present throughout the upper (100m) water column of the ice-edge zone when sampled in autumn 1986 in the Weddell Sea. Biomass ranged from 0.02 μgC1−1 under the sea-ice to 1.3 μgC1−1 in the ice-free water column. Cymatocylis, Codonellopsis, Laackmaniella and a small Salpingella were the most abundant and/or largest biomass contributors. The under ice assemblage was characterized by low biomass and dominated by small species (Salpingella and Codonellopsis); the ice edge stations were dominated by these same taxa but in higher abundances while the open water assemblage was characterized by high biomass and dominated by Cymatocylis, the largest taxa. All taxa exhibited maximum concentrations in the upper 50m of the water column. Both krill and salps grazed upon the Cymatocylis and Codonellopsis without preference in both the ice covered and open water regimes.