Title of the dataset:
Data underlying the publication: “Spatial and seasonal variation in phytoplankton abundance and composition in Lake Victoria’s Mwanza Gulf, compared to northern parts of the lake”

Creators:
Leopold A.J. Nagelkerke: 	Aquaculture and Fisheries Group, Wageningen University and Research, P.O. box 338, 6700 AH Wageningen, the Netherlands; ORCID: 0000-0003-1130-749X.
Theo H. Frank: 			Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB, Wageningen, the Netherlands.
Ilse J.M. Cornelissen: 		Aquaculture and Fisheries Group, Wageningen University and Research, P.O. box 338, 6700 AH Wageningen, the Netherlands; Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB, Wageningen, the Netherlands; ORCID: 0000-0001-5122-0300.
Jacobus Vijverberg: 		Netherlands Institute of Ecology (NIOO-KNAW), Droevendaalsesteeg 10, 6708 PB, Wageningen, the Netherlands; ORCID: 0000-0003-2791-3646.

Related publication: Spatial and seasonal variation in phytoplankton abundance and composition in Lake Victoria’s Mwanza Gulf, compared to northern parts of the lake.

Description:
The dataset contains the raw data of chlorophyll content, and of taxonomy, numbers, and sizes of phytoplankton samples collected in Mwanza Gulf of Lake Victoria (Africa) in 2011-2011. The R-script used for data analysis is also included.

Keywords:
Africa
algae
chlorophyll a content
Lake Victoria
limnology
phytoplankton
Tanzania

Spatial coverage:
Mwanza Gulf (South of Lake Victoria, Tanzania)

Temporal coverage:
2010-2011

This dataset contains the following files:
VictChlorophyll.csv: contains chlorophyll-concentrations in samples from all stations and seasons
VictPhytoRaw.csv: contains every individually identified and measured phytoplankton organism, its station, season, the subsample it was analysed in, and its dimensions
VictPhytoTaxonomy.csv: contains taxonomic information (checked on AlgaeBase: Guiry, M.D. & Guiry, G.M. 2022. AlgaeBase. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org; searched on January 11, 2022.), and information on morpho-functional groups (MFGs), following the method of Salmaso and Padisák (2007) Morpho-Functional Groups and phytoplankton development in two deep lakes (Lake Garda, Italy and Lake Stechlin, Germany). Hydrobiologia 578, 97–112. DOI: 10.1007/s10750-006-0437-0
PhytoVictoriaFactors.csv: contains the analysed surface area per subsample: this needs to be used to correct numbers and calculate densities
Analysis_Victoria3_Phyto_final2.R: this is the script file (in R, version 4.1.0). This script produces all density, size, and composition calculations, as well as statistics on differences between samples.

Explanation of variables:
Chlorophyll: chlorophyll a content (in µg/L)
Class: taxonomic class
ColoniesMucus: morpho-functional group is colony-forming with mucus (1 indicates the taxon belongs to this group; 0 indicates it does not)
ColoniesNoMucus: morpho-functional group is colony-forming, but without mucus (1 indicates the taxon belongs to this group; 0 indicates it does not)
Family: taxonomic family
FilamentsHeterocytes: morpho-functional group is filamentous with heterocytes (1 indicates the taxon belongs to this group; 0 indicates it does not)
FilamentsNoHeterocytes: morpho-functional group is filamentous, but without heterocytes (1 indicates the taxon belongs to this group; 0 indicates it does not)
Genus: taxonomic genus
ID_NR: Identity number of taxon
Length_mmu: largest dimension (length) of an individual phytoplankton organism (expressed in mmu - micrometer unit - which equals 2.93 µm)
Order: taxonomic order
Phylum: taxonomic phylum
Season: (Dry, Rainy): indicates seasonality around Mwanza Gulf (Tanzania)
SingleCells: morpho-functional group is single cells (1 indicates the taxon belongs to this group; 0 indicates it does not)
Species: species name (short notation)
Species_long: species name including authorities
Station: (S1, S2, S3): S1 is the most inland station; S3 is situated at the mouth of Mwanza Gulf in the open lake
Subsamp: Number of subsample (1, 2, 3, 4). Each subsample had a different volume and needed to be corrected for this separately
Surface_mm2: the surface area analysed for each subsample (in mm^2)
Width_mmu: smallest dimension (width) of an individual phytoplankton organism (expressed in mmu - micrometer unit - which equals 2.93 µm)

Methods, materials and software:
Sampling
Seston was sampled between 10:00 h and 14:00 h during five days in the period 31 August - 7 September 2010 (towards the end of the dry season, with ca. 6 mm rainfall in September), and on four days in the period 2 – 10 April 2011 (during the wet season, with ca. 130 mm rainfall in April) at each of the three stations (Fig. 1), resulting in a total of 27 samples. Each of these samples consisted of subsamples taken at three separate locations per station, each ca. 500 m apart. Each subsample was taken with a volume sampler (Offenberg) at three different depths: just below the surface, at the euphotic depth and at intermediate depth. Since stations had different depths also sampling depths differed. Water from all depths and locations per station were pooled. Immediately after return to the laboratory the chlorophyll a content was measured with a Hydrolab DS5 multiprobe (OTT Messtechnik GmbH and Co, Kempten, Germany). For the analysis of phytoplankton composition, the pooled samples from each station, taken on 4 April and 1 September were used (total of six pooled samples).

Phytoplankton analysis
For phytoplankton analysis, a well-mixed, 1-litre subsample of lake water was taken from each of the abovementioned six pooled samples. Each subsample was placed in a glass bottle and fixed with 5 ml Lugol's iodine solution. After 1 week the upper layer was siphoned off with a pipette, the lower layer (ca. 200 ml) containing the sedimented algae was stirred and transferred to a 250 ml bottle. After 1 week the upper layer was again siphoned off and the lower layer (ca. 45 ml) with the sedimented algae was then transferred to a 50 ml polyethylene tube and preserved with 2.3 ml 40%-formaldehyde solution. Finally distilled water was added to a volume of exactly 50 ml. Samples were stored in a refrigerator or cool box until microscopic analysis.
A fixed volume of concentrated sample was used for microscopic identification, counting, and measuring of phytoplankton individuals. After careful homogenization, one drop (0.05336 ml) of the sample was transferred to a microscope slide and covered by a standard cover slip with a surface area of 400 mm2. To prevent evaporation, the edges of the cover slip were sealed with nail polish. After drying, the prepared mount was observed top-down with an inverted microscope (Leitz diavert) at 400-times magnification. The surface area in which the phytoplankton individuals were counted was measured with an ocular square grid, which was divided in 100 squares of 0.02935 mm2. A total of 210-305 individuals were counted in different grids.
Phytoplankton species were identified using the species descriptions of Huber-Pestalozzi (1938), Komárek and Kling (1991), Komárek and Anagnostidis (1999), Komárek et al. (2002), Komárek and Anagnostidis (2005), and Komárek (2013) for Cyanobacteria, those of Huber-Pestalozzi (1942), Krammer and Lange-Bertalot (1986), Krammer and Lange-Bertalot (1988) and Krammer and Lange-Bertalot (1991) for Bacillariophyceae, and those of  Coesel and Meesters (2007) and Komárek and Fott (1983) for Chlorophyta. In addition, the keys of Prescott (1962) as well as Talling (1987) on Lake Victoria were consulted. Nomenclatural authorities and abbreviations for species are given in Table 1. Higher taxa were checked in AlgaeBase (Guiry & Guiry, 2022)
Phytoplankton species densities (numbers·mL-1) were estimated by counting all individuals, whether these were single cells, colonies, or filaments. We defined colonies as individuals consisting of two or more cells. Thus, also Chlorococcales species, such as Scenedesmus, where individuals mostly contain only a few cells, were categorized as colonies. Spherical, elliptical and short cylindrical individuals (< 4.5 µm) were counted from diameter 2 µm and elongated individuals with diameter < 2 µm from a length of 4.5 µm. 
Within Cyanobacteria and Chlorophyta we distinguished five morpho-functional groups (MFG), an approach based on morphological and structural traits (Salmaso and Padisák, 2007). These MFG’s were: 1) colonies without mucus; 2) colonies with mucus; 3) filaments without heterocytes; 4) filaments of taxa which generally developed heterocytes; and 5) single cells. 
We also characterised the whole phytoplankton community at the three stations in the two seasons by their size structure. Size was estimated by the largest dimension of phytoplankton individuals.
Statistical analyses were performed in R, version 4.1.0 (R Core Team, 2021).

This dataset is published under the CC BY (Attribution) license.
This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator.