
DATASET TITLE
Zooplankton abundance across habitats of the Lower Oder River system (November 2023 – December 2025)

DESCRIPTION
This dataset contains abundance data of rotifers, cladocerans, and copepods collected from multiple habitats within the Lower Oder River system, including the main channel, groyne fields, shore zones, mid-channel areas, backwaters, and floodplain waters. Samples were collected between November 2023 and January 2025. Abundances were calculated from microscope counts using measured sample volumes and dilution factors.

OBJECTIVE
The dataset documents spatial and seasonal variation of zooplankton communities across different habitats of Lower Oder River system.

GEOGRAPHIC COVERAGE
Sampling sites were distributed along the river between approximately 52.067°N and 53.206°N latitude and 14.126°E and 14.758°E longitude.
System: Lower Oder River
Habitats: main channel (includes groynes, shore, mid-channel) , backwaters, and floodplain waters
At the beginning of the study, 20 sampling sites were established in the main channel of the Lower Oder River. 
During the study, the number of main-channel sites was reduced to 15 in order to include additional habitats (backwaters and floodplain waters). 
The sites OSO40, OSO70, OS100, and OS140 were discontinued, and data for these sites are only available from November 2023 to April 2024. 
In addition, site OS500 was discontinued, as sampling was conducted in parallel with Subproject TP3, which also stopped sampling at this site.
Number of sites:
- Main channel: 15
- Backwaters: 5
- Floodplain waters: 9

TEMPORAL COVERAGE
Start: November 2023
End: December 2025
Sampling frequency: approximately monthly for the main channel and only during the summer season of 2024 for the backwaters and floodplain waters

SAMPLING DESIGN
Zooplankton were sampled from surface water (0–50 cm depth) at different habitats within the Lower Oder River system. Surface water was collected into a large field bucket, from which a measured subsample volume was taken and filtered through a plankton net. This approach ensured consistent sampling of known water volumes under variable field conditions.
Different mesh sizes were used for rotifers and cladocerans and copepods:
• Rotifers: 30 µm mesh plankton net
• Cladocerans and copepods: 55 µm mesh plankton net
Because rotifers are generally more abundant than cladocerans and copepods, smaller filtered volumes were sufficient for rotifers, while larger filtered volumes were required for cladocerans and copepods to obtain representative counts. 
In addition, filtered volumes were reduced in backwaters and floodplain habitats where suspended material can increase clogging of plankton nets. 


Filtered volumes applied in abundance calculations:
Rotifers
• Main channel, groynes, shore, mid-channel: 20 L
• Backwaters and floodplain waters: 5 L
Cladocerans and copepods
• Main channel, groynes, shore, mid-channel: 100 L
• Backwaters and floodplain waters: 40 L
After filtration, the retained material was rinsed into a 50 mL laboratory bottle and preserved immediately in the field with 4 % buffered formaldehyde solution and stored in sealed 50 mL sample bottles until laboratory processing. 


LABORATORY PROCESSING
Prior to counting, preservative was removed by gently rinsing the sample through an appropriate mesh net (30 µm for rotifers, 50 µm for crustaceans) using deionized water. The washed concentrate was transferred to a measuring cylinder and homogenized to ensure an even distribution of organisms.
Subsamples (Counted_volume) were taken from the well-mixed concentrate using a graduated pipette and transferred to a Sedgewick–Rafter counting chamber. When necessary, a small amount of surfactant was added to reduce surface tension and prevent organisms from adhering to container surfaces. Multiple subsamples were examined to obtain representative counts.

ABUNDANCE CALCULATION
Abundance (individuals per litre) was calculated using:

Abundance = (Concentrate_volume × Count) / (Counted_volume × Filtered_volume) × Dilution

where
Filtered_volume = water volume filtered in the field (L)
Concentrate_volume = total concentrated sample volume (mL)
Counted_volume = subsample volume examined microscopically (mL)
Dilution = additional dilution factor (1 = no dilution;10= tenfold dilution)
If required sample volumes were unavailable (e.g., sampling not performed), abundance values are reported as NA. All volumes were converted to litres (L) prior to abundance calculation to ensure unit consistency.

DATA ORGANIZATION
Rotifers, cladocerans, and copepods were initially processed in separate tables due to differences in sampling volumes for each taxonomic group. Raw counts were subsequently converted to abundance values (individuals per litre, ind/L) for each taxon.
The datasets were then merged into a single table using shared sample identifiers (SAMPLE_CODE). In the final dataset, each row represents a single sample, and each taxon is represented as a separate column containing abundance values (ind/L)

Missing data:
NA indicates that no data are available (e.g., sample not collected).
A value of 0 indicates absence of the taxon in the sample.

File:
data/SP08a_Zooplankton_Abundance_2023-2025.csv – calculated zooplankton abundances (ind/L.)

VARIABLES
Detailed descriptions of all variables, including units and data types, are provided in metadata/Data_Dictionary.docx.

TAXONOMIC COVERAGE
Rotifera,Cladocera, and Copepoda
Identification level: species or genus based on light microscopy (Olympus SZX2-ILLTQ) using standard freshwater zooplankton keys.


QUALITY CONTROL
Volume measurements and dilution factors were checked for consistency prior to abundance calculations. Abundance values calculated using the provided R scripts were cross-checked against independent spreadsheet calculations. Samples with missing or zero volume measurements were assigned NA abundance values.

SOFTWARE
Abundance was calculated in Excel and verified using R.. 

DATA LICENSE
This dataset is released under the Creative Commons Attribution 4.0 International (CC BY 4.0) license.
CONTACT
Name: Abrehet Kahsay Mehari
Email: abrehetkahsay66@gmail.com


