Plankton Observation Discussion Examples | Classification, Population Density, Dominance, and Sources of Error

Water samples were collected from Site A, a flowing stream; Site B, a park pond; and Site C, an agricultural canal, and plankton were observed under a microscope.

At Site A, 158 organisms from eight taxonomic groups were observed, with diatoms being the most abundant at 82 organisms.

At Site B, 450 organisms from eight taxonomic groups were observed. Green algae (118), cyanobacteria (94), and rotifers (64) were abundant.

At Site C, 292 organisms from eight taxonomic groups were observed, with green algae (73) and diatoms (68) being abundant.

The mean count in 0.10 mL at Site B was 150 organisms, corresponding to a population density of 1500 organisms/mL in the concentrated sample.

The relative dominance of green algae at Site B was 26.2%.

The main groups were diatoms at Site A, green algae and cyanobacteria at Site B, and green algae and diatoms at Site C.

In this experiment, water samples were collected from three sites with different environmental conditions, and the taxonomic composition and abundance of phytoplankton and zooplankton were compared.

At Site A, diatoms were the most abundant group, with 82 organisms, accounting for approximately 51.9% of the total 158 organisms.

Site A was a flowing stream with relatively low water temperature, high transparency, and low COD and phosphate phosphorus concentrations.

Some diatoms are commonly found in relatively clean flowing waters. The dominance of diatoms at Site A may therefore be related to the flowing environment and relatively low nutrient loading.

However, some diatom species prefer eutrophic waters, so water quality cannot be determined solely from the abundance of diatoms.

At Site B, green algae (118) and cyanobacteria (94) were abundant, and the total count of 450 organisms was the highest among the three sites.

Site B was a low-flow pond with a water temperature of 24.2°C, pH 8.4, COD 6.8 mg/L, and phosphate phosphorus 0.145 mg/L.

In low-flow waters, plankton are less likely to be carried downstream and can accumulate. Under conditions of high water temperature and nutrient concentrations, green algae and cyanobacteria can proliferate readily.

The high pH at Site B may have been influenced by carbon dioxide consumption through phytoplankton photosynthesis.

In contrast, dissolved oxygen at Site B was 6.0 mg/L, lower than at Site A. Photosynthesis can increase oxygen during the day, whereas respiration by plankton and microorganisms consumes oxygen at night.

In ponds with abundant phytoplankton, dissolved oxygen may fall substantially at night or early in the morning, so measurements at different times of day are needed.

Zooplankton were also abundant at Site B, including 64 rotifers, 27 cladocerans, and 21 copepods.

The abundance of phytoplankton may have supported an increase in zooplankton that feed on them.

However, zooplankton abundance is also affected by fish predation, water temperature, the size of available phytoplankton, diel vertical migration, and other factors.

At Site C, green algae (73) and diatoms (68) were abundant, with a total count of 292 organisms.

Site C was an agricultural canal with higher nutrient concentrations than Site A but more current than Site B.

Nitrogen and phosphorus runoff from surrounding farmland may have promoted phytoplankton growth. At the same time, flowing water may have transported plankton away, preventing them from accumulating to densities as high as those at Site B.

Across the three sites, the total abundance was highest at low-flow Site B and was approximately 2.85 times that at Site A.

This difference likely reflects the combined effects of water temperature, nutrients, current, residence time, light conditions, and other factors.

The relative dominance of green algae at Site B was 26.2%, and no single taxonomic group accounted for more than half of the organisms.

This suggests that multiple groups of phytoplankton and zooplankton coexisted at Site B.

However, communities with the same number of observed taxonomic groups can differ in diversity if one group dominates strongly in one community while abundance is distributed more evenly in another.

Therefore, detailed comparisons of diversity should use not only taxonomic richness but also indices such as the Shannon index.

In replicate counts at Site B, the mean was 150 organisms, the standard deviation was approximately 3.6 organisms, and the coefficient of variation was approximately 2.4%.

Because variation among replicates was relatively small, the sample on the slide was probably reasonably uniform and counting reproducibility was good.

Nevertheless, plankton can sink, float, or swim within a container, so counts can change depending on how the sample is mixed immediately before subsampling.

Large cladocerans and colonial algae tend to be unevenly distributed, making gentle mixing and repeated counting important.

When a plankton net is used, organisms smaller than the mesh pass through it, so larger species may be overrepresented relative to the actual community.

Direct water sampling includes smaller species but may make rare large zooplankton harder to detect.

Thus, differences in sampling method must be considered when interpreting community composition.

Errors may also occur during species identification. In particular, morphologically similar green algae and diatoms may not be identifiable to species level by brief light-microscope observation alone.

Uncertain organisms should be recorded at the genus or higher taxonomic level rather than being assigned a species name without sufficient evidence.

In the example seasonal values, both phytoplankton and zooplankton were most abundant in summer.

Higher water temperatures and greater solar radiation in summer can promote photosynthesis and growth, leading to increased phytoplankton abundance.

As phytoplankton increase, zooplankton that feed on them may increase after a time lag.

In winter, low water temperatures and shorter daylight hours can reduce growth rates and lead to lower abundance.

However, some diatoms dominate during cold periods, so seasonal changes cannot be explained by water temperature alone.

Overall, Site A had abundant diatoms, reflecting a relatively low-nutrient flowing environment, whereas green algae and cyanobacteria increased at Site B under conditions of high water temperature, high nutrient availability, and long residence time.

At Site C, both nutrient supply from farmland and water flow likely influenced the community, resulting in abundant green algae and diatoms.

Accurate assessment of plankton communities requires seasonal and time-of-day surveys at the same site, sampling at multiple depths, simultaneous water-quality measurements, and standardized sampling methods.

Possible causes of abundant cyanobacteria include high water temperature, stagnant water, and increased nitrogen and phosphorus. Water discoloration, odor, pH, and nutrient concentrations should be examined together, and seasonal and sampling-time changes should also be considered.

Possible causes of low zooplankton abundance include fish predation, insufficient phytoplankton food, plankton-net mesh size, sampling time, and uneven distribution caused by swimming behavior. Sampling at multiple times and locations is necessary.

Large variation among replicate counts may result from insufficient mixing, settling, swimming, colony formation, or selection of microscope fields. The sample should be gently homogenized and many randomly selected fields should be observed.

Even if sites contain the same number of taxonomic groups, their community structures are not necessarily the same if the relative abundances of those groups differ. Comparisons should therefore include relative dominance and diversity indices, not only taxonomic richness.

With a light microscope, subtle morphological differences among closely related species may not be visible. Therefore, this observation recorded uncertain organisms at the genus or broader taxonomic level rather than forcing a species identification. More accurate identification may require higher-magnification observation, specialized identification keys, electron microscopy, or genetic analysis.