The annual cycle of plankton was studied over 14 years from 1984 to 2000 at a coastal station in the Gulf of Naples, with the aim of assessing seasonal patterns and interannual trends. Phytoplankton biomass started increasing over the water column in February-early March, and generally achieved peak values in the upper layers in late spring. Another peak was often recorded in autumn. Diatoms and phytoflagellates dominated for the largest part of the year. Ciliates showed their main peaks in phase with phytoplankton and were mainly represented by small (< 30 mm) naked choreotrichs. Mesozooplankton increased in March-April, reaching maximum concentrations in summer. Copepods were always the most abundant group, followed by cladocerans in summer. At the interannual scale, a high variability and a decreasing trend were recorded over the sampling period for autotrophic biomass. Mesozooplankton biomass showed a less marked interannual variability. From 1995 onwards, phytoplankton populations increased in cell number but decreased in cell size, with intense blooms of small diatoms and undetermined coccoid species frequently observed in recent years. In spite of those interannual variations, the different phases of the annual cycle and the occurrence of several plankton species were remarkably regular.

We tracked temporal changes in protist diversity at the Long Term Ecological Research (LTER) station MareChiara in the Gulf of Naples (Mediterranean Sea) on eight dates in 2011 using a metabarcoding approach. Illumina analysis of the V4 and V9 fragments of the 18S rDNA produced 869 522 and 1 410 071 sequences resulting in 6517 and 6519 OTUs, respectively. Marked compositional variations were recorded across the year, with less than 2% of OTUs shared among all samples and similar patterns for the two marker tags. Alveolata, Stramenopiles and Rhizaria were the most represented groups. A comparison with light microscopy data indicated an over-representation of Dinophyta in the sequence dataset, whereas Bacillariophyta showed comparable taxonomic patterns between sequence and light microscopy data. Shannon diversity values were stable from February to September, increasing thereafter with a peak in December. Community variance was mainly explained by seasonality (as temperature), trophic status (as chlorophyll a ), and influence of coastal waters (as salinity). Overall, the background knowledge of the system provided a sound context for the result interpretation, showing that LTER sites provide an ideal setting for high-throughput sequencing (HTS) metabarcoding characterisation of protist assemblages and their relationships with environmental variations.

The response of phytoplankton biomass, growth rates and primary production to seasonally varying physical forcing was studied at a station southeast of Bermuda over an 18 month period. Phytoplankton growth rates and primary production were measured using the pigment-labeling method, and phytoplankton biomass was calculated from these measurements. Phytoplankton carbon biomass varied systematically over the year. Highest values were observed during the winter and spring. Seasonal variations of chlorophyll (Chi) a in the surface layer could primarily be attributed to variations in phytoplankton biomass and secondarily to photoacclimation. During the summer period, average values of carbon (C)/Chl ratios (g C g⁻¹ Chi) ranged from 160 at the surface to 33 at the 1.6% light level, changes attributed to photoacclimation of the phytoplankton, consistent with the observation that phytoplankton biomass did not vary as a function of depth. Phytoplankton growth rates in the surface layer did not vary systematically over the year, ranging from 0.15 to 0.45 day⁻¹, in spite of seasonally varying concentrations of nitrate. Growth rates varied as a function of depth from average values of 0.3 day⁻¹ in the surface layer to <0.1 day∼¹ at the 1.6% light level. Thus, the primary response of the phytoplankton community to nutrient enrichment during the winter period was an increase in phytoplankton biomass rather than an increase in growth rates. A simple nutrient-phyto-plankton-zooplankton model was used to explore this phenomenon. The model demonstrated that the observed response of the phytoplankton to nutrient enrichment is only possible when phytoplankton growth is not severely limited by nutrients.

Skeletonema comprises a planktonic genus of centric diatoms occurring in coastal and brackish waters worldwide. The most commonly reported species, Skeletonema costatum, was believed to be ubiquitous, morphologically plastic and euryhaline. Yet, recent studies showed that this perceived taxon consists of a series of species, including a more narrowly defined Skeletonema costatum sensu stricto. Here we investigated the effect of salinity on growth rate and cell morphology. Ten strains belonging to six Skeletonema species were incubated at salinities between 0 and 35; their growth response was measured and their cell morphology observed using light and electron microscopy. All strains grew well between salinities of 10 and 35, whereas the lowest salinity at which growth was observed differed among species and even within the same species: Skeletonema marinoi strains deriving from distinct environments (Baltic and North Sea) showed different lower tolerance limits. Two species, Skeletonema costatum and Skeletonema subsalsum, showed growth between salinities of 0 and 35 and exhibited very short intercellular processes when cultured in freshwater. Skeletonema subsalsum underwent cell enlargement when exposed to a salinity of 35.