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Seasonal studies in freshwater lakes (1)
Lakes of Northland, Waikato and Rotorua, New Zealand
by Dr J Floor Anthoni, 2005
www.seafriends.org.nz/dda/fresh01.htm


Although freshwater lakes were not of direct concern to us, we hoped that seen through the 'eyes' of the Dark Decay Assay, they could further our understanding. Freshwater planktonic ecosystems are much simpler than their marine equivalents, and this could help us make a number of new discoveries (see method.htm). This document follows a number of freshwater lakes through their seasons, recording how various parameters change.
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For comments and suggestions, e-mail the author, Dr Floor Anthoni.
-- Seafriends home -- DDA index -- site map -- Rev 20050610,20050805,
General
Because freshwater lakes are not intimately interconnected like seas, their DDA curves show high variability, and are indeed specific to each lake. The minerals and acids of a lake depend largely on the geology and land use of its surrounding catchment area and also on the presence of volcanic vents. Freshwater lacks the salt of the sea which makes water much less solvent. As a consequence, fresh water can store higher concentrations of nutrients and minerals and produce much higher plankton biodensity than seawater. All this shows clearly in DDA tests.
For seasonal studies we chose a number of lakes and rivers on the North Island of New Zealand for the following reasons:
 
Rate of Attack
When the rate of attack (over 48 hours, green) is plotted versus biodensity, an almost straight line results (green), implying that bacterial attack rate in 48 hours is a constant part (27% at 27ºC) of the measured biodensity but less for low biodensities. (Note that the black dash-dot line gives precise linearity) 
We also plotted the 24 hour rate of attack (RoA24, red) because this is more representative in fresh water where decomposition proceeds rapidly. For high biodensities, the curve follows the straight linearity, implying that RoA24 is also a fixed part (10% at 27ºC) of total biodensity. However, for low values of biodensity, the RoA decreases rapidly, signifying plankton health. In this area a doubling of biodensity results in a five-fold increase in bacterial attack. It suggests that true healthy lakes have biodensities very similar to the sea, less than 100 hion with much reduced bacterial activity and RoA48 less than 20 hion. Lakes Pupuke, Tarawera, Rotoma (crater) and Quarry Lake (next to Pupuke) are the healthiest with low initial rates of decay and biodensity. Note that the winter results obtained with alcohol enhancement, do not corroborate the previous statement marked in blue. Do bacteria become more aggressive in winter? Is new growth insufficient? Is the effect an artifact?
 
Rate of attack and biodensity in freshwater

 
 
Biodensity vs natural pH
The maximum biodensity a lake can carry when it is fully eutrophied, is a function of its natural pH, following the dash-dot line in the graph. Note that the winter data, enhanced by alcohol, show larger biodensities but the maximum biodensity line remains at the same position. Due to the technique of alcohol enhancement, this grap has now become less relevant.

Natural pH vs biodensity

 
Visibility vs biodensity
In the visibility versus biodensity plot phytoplankton has two regions, depending on whether it is limited by nutrients or by light. The thin chevrons show biodensities of 1,2, 5 and 10 times that estimated for phytoplankton. For instance, Blue Lake has a biodensity about 8 times that of the phytoplankton, most of which made up of decomposers. Note that because alcohol enhancement was not used, biodensities shown have not reached their ultimate values.

biodensity vs visibility in lakes

Autumn 2005

DDA in freshwater lakesIn this single graph 24 New Zealand lakes have been brought together in autumn of 2005. At the conclusion of each curve, the vials were opened (while still in darkness) to ventilate in order to establish the natural pH of each lake. For some curves this has been drawn as the upward segment ending in a large dot. 
For completeness we also measured the crystal-clear spring water of Hamurana Springs, that flows out into Lake Rotorua. It had a very slight hint of peat (we tasted it) and measured a biodensity of 93 hion of what is almost certainly non-living organic matter. We cultivated this water to estimate its mineral/nutrient content and the DDA curve of this aquarium is shown in Ea, measuring 405 hion. Thus even very clear streams can contribute large to the nutrient budget of a lake. Lake Rotorua had biodensities of 578 (north) and 1030 (city)! The Blue Lake due to its volcanic vents supports nearly 2000 hion biodensity! It is envisioned that seasonal variations tell us more about these lakes.

 
DDA data of 24 New Zealand lakes
12-13 March 2005, autumn
site ref date.time viz ipH +/- fpH npH bio RoA48 RoA24
Pupuke Quarry Lake X 20050312.0845 1.5 8.70 +3 6.99 8.33 100 16.9 3.4
Lake Pupuke Y 20050312.0900 4.6 9.05 +2 6.76 8.55 172 15.8 2.6
Waikato River @ Mercer W 20050312.1050 1.0 7.70 -12 6.53 8.12 275 85 28
Lake Waikare V 20050312.1040 0.15 8.24 -7 6.34 7.90 452 115 57
Lake Waahi U 20050312.1110 0.05 9.10 +8 6.56 8.07 274 70 24
Lake Rotoroa Hamilton Lake T 20050312.1230 1.4 7.39 -25 6.33 7.79 426 100 57
Lake Karapiro S 20050312.1320 2.2 8.28 +5 6.66 7.99 214 65.5 34.5
Lake Rotoma (road side) R 20050312.1515 10.3 6.80 -145 6.51 7.87 150 19 7.5
Lake Rotoehu Z 20050312.1600 2.5* 6.64 -225 6.89 8.41 - - -
Lake Rotoma (crater) A 20050313.0940 12.5 7.25 -42 6.29 7.85 457 85 29
Lake Rotoiti East B 20050313.1020 6.0 7.94 +14 6.22 7.60 591 154 58
Lake Okataina C 20050313.1050 10.8 8.24 +8 6.38 7.86 411 65 28
Lake Rotoiti West outlet end D 20050313.1130 2.4 7.42 -12 6.23 7.59 550 143 74
Hamurana Springs E 20050313.1200 >30 6.48 -320 6.36 7.96 105 - -
Hamurana Springs aquarium culture Ea 20050521.1630 - 7.95 -0 6.37 7.96 405 55 45
Lake Rotorua north F 20050313.1225 3.0 6.64 -212 6.09 7.77 578 137 -
Lake Rotorua City end G 20050313.1410 2.6* 6.15 -676 5.76 7.50 1013 - -
Lake Okareka H 20050313.1440 6.1 8.13 +5 6.38 7.91 409 116 46
Blue Lake Tikitapu I 20050313.1500 4.1 7.09 -17 5.69 7.19 1960 468 188
Lake Tarawera J 20050313.1550 8.4 8.23 -2 6.97 8.47 101 22 5
Green Lake K 20050313.1620 5.7 8.78 +10 6.43 7.94 369 87.5 23
Lake Tomarata L 20050329.1000 2.6 7.50 -13 6.55 7.74 250 68 21
Lake Taharoa (Kai Iwi lakes) M 20050329.1250 11.0 6.46 -304 5.95 7.37 775 284 -
Lower Nihotupu Dam drinkwater N 20050305.1430 - 7.78 -8 6.45 8.06 339 118 58
Legends to the tables above and below
ref: single character reference as used in the DDA curves, also corresponding to the vial names of the experiment.
date.time: date and time as yyyymmdd.hhmm, denoting the moment the vials were confined to darkness.
viz: underwater visibility measured vertically where possible and where not, horizontally. In most cases by entering the water with dive mask and measuring tape and swimming out till the water clears.
ipH: initial pH, measured at the time of collection
+/-: initial pH less natural pH, in hion. This indicates producer gain (+) or bacterial gain (-). = ALOG(-npH) - ALOG(-ipH) ppb
fpH: final pH, the lowest pH measured on day 9 or 13 when the decomposition curves have flattened or begin rising again.
apH: the final pH after alcohol enhancement, when done after final pH is reached.
npH: natural pH measured after one week of ventilation with the air. The highest value measured.
bio: the biodensity calculated from initial and ending pH in hions without alcohol enhancement.
abio: the biodensity with alcohol enhancement, in this experiment after day 9.
RoA48: the biodensity (hion) decomposed in the first 48 hours, calculated from the curves and initial pH.
RoA24: the biodensity (hion) decomposed in the first 24 hours.
Notes: *= measured later on a calm day

Winter 2005
Samples were collected on a weekend with fine weather and low winds. There has been rainfall before, some of it quite high.

June results of Waikato & Rotorua lakesThe graph shows the DDA curves of the June expedition. A very small quantity of alcohol was added after day nine since we discovered that decomposition in freshwater lakes does not predictably complete within 13 days. As one can see, it had drastic results on most lakes, making the previous expedition rather irrelevant as far as total biodensity is concerned. A slight irregularity happened in the night between day two and three when the incubator failed and the samples cooled to 18ºC. Fortunately this did not affect the measurements of rates of attack or that of total biodensity. Interestingly, many curves veered upward. Why? 
Lakes Rotorua and Rotoehu again showed behaviour comparable to that of the clear Hamurana Springs water, as the pH rises before coming down. We surmise that this indicates the synthesis of bacteria, which is certainly true for Hamurana Springs, and suggests that planktonic bacteria do not dominate in the other two lakes.

 
DDA data of 24 New Zealand lakes
5-6 June 2005, winter
site ref date.time viz ipH +/- fpH apH npH bio abio RoA48 RoA24
Pupuke Quarry Lake X 20050605.0900 2.5 8.04 +6.7 7.10 6.24 7.80 70 566 29 12.3
Lake Pupuke Y 20050605.0910 8.3 8.28 +16 7.21 6.70 7.68 56 194 21.7 9.2
Waikato River @ Mercer W 20050605.1000 0.5 7.14 -2 6.41 5.61 7.15 317 2382 196 97
Lake Waikare V 20050605.1040 0.10 8.57 +46 6.52 6.02 7.31 299 952 118 40
Lake Waahi U 20050605.1125 0.30 7.98 +39 6.54 6.22 7.31 278 592 69 25
Lake Rotoroa Hamilton Lake T 20050605.1215 1.4 7.54 +100 6.46 5.79 6.89 318 1593 170 78
Lake Karapiro S 20050605.1330 1.9 7.84 +61 6.63 6.08 7.12 220 817 98 39
Lake Rotoma (road side) R 20050605.1510 10.0 7.23 +132 6.52 5.86 6.72 243 1322 127 59
Lake Rotoehu Z 20050605.1535 2.5 6.68 -174 6.80 6.32 7.46 - 320? - -
Lake Rotoma (crater) A 20050605.1615 12.5 7.71 +48 6.67 6.40 7.17 194 379 88 51
Lake Rotoiti East B 20050605.1650 6.4 7.14 +62 6.21 5.68 6.87 544 2017 210 75
Lake Okataina C 20050606.1300 10.8 7.78 +118 6.39 6.10 7.01 391 778 79 22
Lake Rotoiti West D 20050606.1340 3.2 7.21 +143 6.03 5.55 6.69 872 2756 285 138
Hamurana Springs E 20050606.1400 >30 6.65 -175 6.45 6.46 7.31 131 123 - -
Hamurana Springs aq culture Ea 2005 - 7.74 +328 - 6.46 - - 723 123 32
Lake Rotorua north F 20050606.1350 3.2 7.49 +99 6.32 5.92 6.88 446 1170 219 150
Lake Rotorua City end G 20050606.1200 2.6 6.12 -360 5.81 5.66 6.40 790 1429 - -
Lake Okareka H 20050606.0940 7.6 7.41 +59 6.47 5.90 7.01 300 1220 93 32
Blue Lake Tikitapu I 20050606.1140 4.7 6.67 +208 5.81 5.74 6.38 1335 1606 265 125
Lake Tarawera J 20050606.1020 8.6 8.02 +13 6.96 6.59 7.64 100 247 33 13.4
Green Lake K 20050606.0955 4.2 7.46 +37 6.48 6.35 7.15 296 412 72 28
Lake Tomarata L 20050623.1845 2.6 7.34 - - 6.14 - - 1185 113 50
Lake Taharoa (Kai Iwi lakes) M 20050 11.0 6.46 -
Lower Nihotupu Dam drinkwater N 20050 - 7.78

 

RoA vs biodensity, alcohol-enhancedAs a result of more complete decomposition, the relationship between Rate of Attack and biodensity gave quite a different picture, although the strict linear relationship remains. Instead of Blue Lake, both Rotoiti East and West and the Waikato River had highest biodensities, followed by Blue Lake and  (surprisingly) Lake Rotoma at the road side. Apparently a large amount of sewage enters here or the small quantity that enters, is not easily diffused into the lake. However, Rotoma-crater showed higher than normal rates of attack, together with Green Lake and Rotorua-north. On the other side of the line the lakes Quarry, Okataina and Okareka had substantially lower rates of attack. We think that the results obtained with alcohol enhancement paint a better picture of the comparative health of these lakes. Note that the 'healthy' lakes Pupuke and Tarawera now show the RoA expected from their biodensities, 11% and 4.5% respectively.
The lakes located to the right of the line have aggressive decomposers (Green Lake, Rotoma-crater, Rotorua, Blue Lake) whereas those to the left more benign ones (Quarry, Okataina, Okareka)

 
biodensity vs natural pHFor unknown reasons the natural pH in this batch of samples showed high variability, which required averaging over two days. The resulting scatter diagram shows that most lakes have veered off the maximum biodensity line towards a lower biodensity with lower natural pH. We suspect that these shifts are seasonal and that both a low temperature and low light levels have caused it. Because alcohol enhancement was applied to this batch, it cannot be compared with the first one. Further seasonal monitoring may show how these lakes behave.

 
 

visibility vs biodensity in freshwater lakesThe picture emerging from DDA with alcohol enhancement looks quite different from that of March as hidden biomass is now revealed. It leaves Tarawera at the top but places the Waikato River and Lake Rotoiti at the bottom, suggesting that these three have the largest bacterial load of all. Blue Lake remains in its position but Okareka and Okataina have joined as lakes with very high bacterial biodensity compared to their phytoplankton biodensity. A lake like Okataina for instance, with winter visibility of 7-8m, supports over 10 times the biomass estimated for its phytoplankton.
We think that this scatter diagram is more representative of the health of these lakes but more work needs to be done.

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