Phytoplankton community and physical-chemical data measured in the Gulf of Trieste (northern Adriatic Sea) over the period March 2006–February 2007

Biological, hydrological and chemical data were acquired at monthly intervals from March 2006 to February 2007, at the Long-Term Ecological Research site C1 in the Gulf of Trieste, in the northernmost part of the Adriatic Sea. The biological dataset comprises total chl a and phaeopigment concentrations, and the distinction of the total phytoplankton biomass into three photoautotrophic community fractions, i.e. cyanobacteria, nano- and microphytoplankton, collected at discrete depths. Hydrological data encompass the thermohaline properties of the water column (temperature and salinity profiles from CTD casts). Chemical data consist of silicate and phosphate concentrations obtained from discrete seawater samples collected with Niskin bottles at four depths (0.5–5–10–15 m). Data presented here are related to the paper “Structural and functional response of phytoplankton to reduced river inputs and anomalous physical-chemical conditions in the Gulf of Trieste (northern Adriatic Sea) by Cibic et al. (2018) [1].


Subject area
Marine ecology More specific subject area Physical-, chemical-and biological oceanography Type of data Value of the data Data provide information on the phytoplankton biomass, in terms of chl a, and its division into three photoautotrophic communities of different size-classes.
Data on cyanobacteria, nanophyto-and microphytoplankton, obtained synoptically, may be used as a baseline for future studies.
Data on temperature and salinity highlight anomalous thermohaline features in a shallow basin and may be used for future comparisons with similar temperate semi-enclosed seas.
Inorganic nutrient data, highlighting silicate depletion for diatom growth, could be valuable to researchers investigating coastal oligotrophic ecosystems.
Data here presented may be used to study the effects of environmental factors on phytoplankton biomass and community structure.

Data
The biomass of three different size-classes of phytoplankton, i.e. cyanobacteria (0.2-2 mm), nano-(2-20 mm) and microphytoplankton (20-200 mm), expressed as percentage of the total phytoplankton, is presented in Table 1. Along the water column, cyanobacteria were the prevalent phototrophs in late summer-early autumn (September and October).

Inorganic nutrient and pigment analyses
Samples for the determination of dissolved inorganic nutrient (phosphate, P-PO 4 ; and silicate, Si-Si(OH) 4 ) concentrations were prefiltered through 0.7 mm pore size glass-fibre filters (Whatmann GF/F), stored at À 20°C and analysed by a flow injection spectrophotometric method on a five-channel Bran-Luebbe Autoanalyzer 3 using standard procedures [2]. To highlight silicate limitation, the Redfield-Brzezinski nutrient ratio of C:Si:N:P ¼ 106:15:16:1 for diatoms was applied to our dataset [3]. Subsamples for chl a analysis were stored in the dark and kept at 4°C until filtration through 47 mm Whatman GF/F filters that were then stored frozen ( À 20°C) until laboratory analysis. Pigments were extracted overnight (4°C) with 90% acetone and determined spectrofluorometrically [4]. The measurements of chl a and phaeopigments were performed, respectively, before and after acidification with two drops of HCl 1N using a PERKIN ELMER LS-50B spectrofluorometer.

Determination of different phytoplankton size-classes
The cyanobacteria (0.2-2 mm) abundance was estimated from 50 mL-samples, preserved in 0.2 mm pre-filtered formaldehyde (2% v/v final concentration) in the dark at 4°C and processed within 48 h. Samples were filtered in triplicate (3-15 mL per subsample) through 0.2 mm pore-size black-stained polycarbonate membranes (Ø 25 mm, Nuclepore). Filters were mounted on microscope slides using non-fluorescent oil and stored at À 20°C. The enumeration was carried out using an Olympus BX51 Table 3 Chlorophyll a (chl a) and phaeopigment (phaeo) concentrations, expressed as mg L À 1 , and their ratios, at the four sampling depths during the study period.
For nanophytoplankton (2-20 mm) analysis, water samples were collected in 100 mL-dark bottles, fixed with prefiltered glutaraldehyde (1% final concentration) and stored in the dark at 4°C until the analyses. Subsamples (30 mL) were filtered at low pressure (max 100 mmHg) through 0.8 mm pore-size black polycarbonate membranes (Ø 25 mm, Nuclepore). Filters were stained with DAPI (4 0 6 0 -diamidino-2-phenylindole) and mounted on glass slides in three replicates for each sample [6]. A minimum of 200 nanophytoplankton cells per filter were counted in randomly selected fields at a 1000 Â final magnification using an Olympus BX51 microscope equipped with a mercury burner light. The set of filters for chlorophyll fluorescence (BP450-490/FT 510/LP520) was used. The biovolume was estimated and converted to carbon content using a conversion factor of 0.14 pg C mm À 3 [7].
For microphytoplankton (20-200 mm) analysis, samples were collected in 500 mL-dark bottles and preserved with prefiltered and neutralized 1.6% formaldehyde [8]. Cell counts of the microphytoplankton were performed following the Utermöhl method [9]. A variable volume of seawater (25-50 mL) was settled depending on cell concentrations. Counting was performed in random fields (20-40) using an inverted microscope (Leitz Fluovert FS) equipped with phase contrast, at a final magnification of 320 Â . In addition, one half of the Utermöhl chamber was also examined at a magnification of 200 Â , to obtain a more correct evaluation of less abundant microphytoplankton taxa. The biovolume of the microphytoplankton cells was calculated from cell-size and shape by using appropriate geometric formulas [10,11]. Cell volumes were then converted to carbon content using the formula introduced by Menden-Deuer and Lessard [12].