Health warnings have traditionally been issued based on cell concentration thresholds; for example, over 15,000 cells/mL. However, cell concentrations do not account for the variability in size of cyanobacteria (see Figure A4.1). This is particularly relevant when there are high concentrations of cyanobacteria that are very small. Also, toxin concentration per cell is more closely related to cyanobacteria biovolume than to total cell number. Thus, simply relying on cell concentrations as an indicator of health risk may give biased results on the cyanobacterial taxa that are abundant in the water body. In the last three years there has been an increase in reports of pico-cyanobacteria (< 2 µm; eg, Aphanothece sp. and Aphanocapsa sp.) in some regions of New Zealand (eg, the Rotorua lakes), and basing health warning solely on cell counts has in some instances resulted in the unnecessary issuing of health warnings.
Figure A4.1: Light photomicrographs demonstrating the difference in cell size among A. Microcystis sp.; B. Aphanocapsa holsatica; and C. Anabaena planktonica (arrow points towards Ap holsatica)
Light photomicrographs demonstrating the difference in cell size among three cyanobacteria species. Microcystis sp. and Anabaena planktonica have much larger cell sizes than Aphanocapsa holsatica.
It is time consuming and impractical to measure and calculate a biovolume for every individual in routine counting, so it is recommended that standardised species lists with fixed biovolumes be used. Where possible these should be specific to the water bodies being monitored. In conjunction with the development of these guidelines, biovolumes for 22 of the most problematic species in New Zealand lakes have been established (Table A4.1).
However, there are several caveats that need to be considered when using biovolumes.
- In taxa that contain specialised cells such as akinetes and heterocytes, volume measurements are of vegetative cells only. Specialised cells usually make up a very small proportion of all cells, and this is unlikely to have a significant effect on overall biovolume.
- Hawkins et al, (2005), showed that preservation of samples with Lugol’s iodine (a preservative commonly used in New Zealand) causes shrinkage rates of up to 40 per cent, depending on the Lugol’s concentration, the species and the length of time in Lugol’s iodine. Using a low concentration of Lugol’s iodine and analysing samples within 24−48 hours of collection will minimise shrinkage. The cell biovolumes produced for this document were obtained on Lugol’s-preserved samples that had been stored for several months.
The biovolume (BV) in mm3/L of each species in a sample can be calculated using the following formula:
BV = (n x vol) / 1 x 106
n = number cells in a sample (cells/mL)
vol = volume of each cell (µm3)
1 x 106 is a units conversion from µm3/mL to mm3/L.
The total biovolume (TBV) of each sample is calculated by combining the individual totals for each species. For example, the total biovolume in a sample containing 10,200 cells/mL of Anabaena planktonica and 5600 cells/mL of Microcystis wesenbergii is calculated as follows:
BV (A. planktonica) = (10,200 x 399*) / 1 x 106 = 4.07 mm3/L
BV (M. wesenbergii) = 5600 x 182* / 1 x 106 = 1.02 mm3/L
Total BV = 4.07 + 1.02 = 5.09 mm3/L
* Using values from Table A4.1.
Table A4.1: Volumes of common cyanobacteria in New Zealand
|Average volume (μm3)||Length (μm)||Width (μm)||Diameter (μm)||Source||Shape||Count (n)|
|Anabaena circinalis||208||5.9 (4, 8.2)||8.2 (6.1,10.9)||Kainui, Maraetai||OVO||39|
|Anabaena lemmermannii||116||5.5 (3.1, 8.5)||6.3 (4,8.5)||Karapiro, Okareka, Rotoehu, Rotoiti, Rotorua, Tarawera,||OVO||50|
|Anabaena planktonica||399||6.8 (3.9, 10.2)||10.5 (7.3,13.3)||Kaituna River, Karapiro, Ngaroto, Okaro, Rotoiti, Rotorua, Tarawera||OVO||75|
|Aphanocapsa holsatica||1.7|| |
|1.4 (0.8,2.3)||Kaituna River, Okareka, Okaro, Rotoiti, Waahi||OVOR||48|
|Aphanizomenon gracile||32||4.4 (2, 11.3)||2.8 (1,5)||Rotoiti, Tarawera, Waikare||CYL||50|
|Aphanizomenon issatschenkoi||89||10.7 (6.5, 264.1)||3.2 (1.6, 4.7)||Kainui||CYL||30|
|Aphanothece clathrata||2.1||2.3 (1.8, 3.2)||1 (0.7,1.4)||Okareka, Okaro, Tarawera||CYL||30|
|Chroococcus cf. minutus||35||2.7 (2.1, 3.4)||4.9 (3.6, 6.1)||Waahi||OVO||30|
|Coelosphaerium kuetzingianum||8.9||2.0 (1.2, 2.9)||2.7 (2, 4)||Kaituna River, Rotoiti||OVO||32|
|Cylindrospermnopsis raciborskii||15||6.5 (3.6, 9.9)||1.7 (1, 3.9)||Whangapehe||CYL||30|
|Leptolyngbya cf. subtilis||8.6||3.2 (2, 5)||1.8 (1.2, 2.6)||Kainui||CYL||30|
|Merismopedia punctata||6.4||2.8 (2, 3.8)||2 (1.5, 2.8)||Forsyth||OVO||30|
|Microcystis sp. (small)||19|| |
|3.2 (2.4, 4.2)||Ngaroto, Okareka, Okaro, Rotoehu, Rotoiti, Tarawera,||OVOR||60|
|Microcystis sp. (large)||93|| |
|5.5 (4.1,7.4)||Kaituna River, Rotoehu, Rotoiti, Rotorua, Tarawera||OVOR||54|
|Microcystis wesenbergii||182|| |
|6.9 (4.6,9)||Ngaroto, Rotoiti, Rotorua,||OVOR||60|
|Nodularia spumigena||355||5.2 (3.2, 8.3)||9.3 (7.3, 10.7)||Lake Forsyth||CYL||30|
|Planktolyngbya cf tallingii*||1||3 (2, 4.4)||0.6 (0.5, 0.8)||Waikare||CYL||30|
|Planktolyngbya subtilis||3||3 (2, 4.5)||1.1 (0.8, 1.6)||Waahi, Waikare||CYL||42|
|Planktothrix cf agardhi||28||3 (2, 4.8)||3.4 (2.9, 4.1)||Oxidation pond (Horowhenua)|| |
|Pseudanabaena limnetica||8.3||3.7 (1.9, 6.8)||1.6 (1.1, 2.2)||Karapiro, Okareka, Rotoehu, Rotoiti,||CYL||30|
|Snowella lacustris||99||5 (3.5, 7.7)||6.0 (4.2, 8.6)||Rotoiti, Rotorua||OVO||48|
|Trichodesmium iwanoffiana||102||4.3 (3.4, 5.3)||5.4 (4.4, 7.1)||Okareka, Okaro, Rotoiti, Tarawera||CYL||30|
Notes: Equations given by the United States Environmental Protection Agency (2007) were used to calculate volumes
CYL = cylinder; OVOR = ovoid (round); OVO = ovoid. Minimum and maximum dimensions are given in brackets.
* This species is commonly identified as Planktolyngbya cf contorta in New Zealand.
Table A4.2: Volume equations for common cyanobacteria cell shapes
|Ovoid (round)||V = ((4 / 3)** (diam / 2)3)|
|Ovoid||V = (4 / 3) * * (width / 2)2 * (length / 2)|
|Cylinder||V = ( * (width / 2)2 * (length))|