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A PRELIMINARY INVESTIGATION OF THE LOCATION AND G E N E R A L ECOLOGY OF MAT PLANTS IN THE

ROTORUA L A K E DISTRICT

by D . A . Carter*

S U M M A R Y

Mat plants were found on sheltered, gently sloping lakeshores with a sandy substrate. They grow to a maximum depth of approximately 2 metres and from this depth other water weeds such as Lagarosiphon provide the bottom cover. The limit is possibly caused by wave action and competition on the young germinating seeds. The method of colonisation is uncertain (cf. Chapman et al. 1971).

I N T R O D U C T I O N

Surveys of Lake Rotoi t i by Chapman et al. (1971) consisted of a determination of the vegetation types of the lake so that comparisons could be made later in case of changes caused by further eutrophication. They noted the occurrence of mounds of vegetation (the " low community") in water up to a depth of 1.5 m. Later investigation of these mounds was carried out to determine whether or not they showed a phenomenon of Pattern and Process of Cycl ic Change as described by Watt (1933) with other plants. This work is reviewed in a paper by Chapman et al. (in press 1972).

In this project, lakes Rotoma, Rotoehu, Blue Lake, Okareka, Tarawera, Rerewhakaaitu were surveyed to locate more of these mats and note their distribution and general ecology with reference to the species present, the type of substrate and the depth to which they grew. Most of these plants have a creeping, slender rhizome system and produce a pair of green leaves (phyllodes) at the nodes; here adventitious roots are also formed. It is because of this rhizome system that these plants are able to form a stable mat community on the substrate and tend to hold it together. When a mat becomes-old or damaged, it tends to be swept away as wave action undermines the rhizome system and the mat is removed in toto. The number of species present (see list) in these mats seems to be fairly constant.

M E T H O D

The lakes were all surveyed from the road and the mat plants located. When an area with an even cover of these plants was located a representative sample was taken from a mat. A n aluminium quadrat measuring 5 x 5cm. was laid on the vegetation and cut around. The plants were then removed from a depth to

*Department of Botany, University of Auckland.

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which rhizomes penetrated and placed in a plastic bag. From the same position a soil sample was removed to a depth of 10cm.

One end of a nylon line marked off from this position a measure of the distance and depth to the point beyond which the mat plants ceased to grow. A sample of the vegetation that replaced the mat plants was collected for identification.

The mat plants were taken back to the laboratory and separated out into species types and weighed after drying at 1 0 5 ° ; their dry weights were then expressed as a percentage of the total plant cover for that quadrat.

Soil Sample Analysis Soil samples were treated and analysed as follows: After breaking up lumps,

the soils were laid out on newsprint and allowed to air dry. The soil was then put through a series of sieves to obtain a range of particle size. The fractions were then dried in an oven at 1 0 5 ° , collected and weighed. Each fraction was expressed as a percentage of the total soil sample (Table 1).

R E S U L T S

Areas marked with '+' and '—' (Figs. 1 - 6) represent the areas surveyed. '+' indicates that mat plants were growing in the location and '—' indicates none were present. 'S ' denotes the position where the sample was taken.

Lake Rotoma (Fig. 1) Mats were found on the southern shore of the north-western arm of the lake,

at the bottom of a clifff some 5 m high, growing on a gently sloping sandy beach. It was noted that the mat plants formed a wide band about 1 m out from the water's edge and could extend for a distance of about 2 m before their place was taken by a dense growth of other lake weeds. A l l these distances are approximate as it was impossible to reach this beach. Further around the lake on the northern end more plants were found and here a sample was taken.

Table 1. Soil sample analysis

Lake Soil Fraction (% by Dry Weight)

Sieve No. >10 >30 >60 >120 <120

Rotoma 20.0 65.0 5.5 8.3 0.9 Okareka (1) 28.5 35.1 18.6 15.2 2.8 Okareka(2) 29.7 21.3 18.7 23.3 7.0 Blue Lake 26.3 22.4 22.2 20.4 8.7 Tarawera 12.3 21.6 41.3 21.8 3.0 Rerewhakaaitu 40.4 21.3 20.4 12.8 5.1

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Species Present % Cover (dry wt.) Elatine gratioloides 25.0% Glossostigma submersion 62.3% Limosella lineata 12.7%

Depth limit to which the mat plants extended was 1.27 m at 15.0 m out from the shore. At this point there was a 'wall ' o f very thickly growing Lagarosiphon major (Ridler) Moss that broke the surface.

The lake level was also up approximately 1.2 m from 1971 (local information and a scale placed on a jetty). This was shown by a willow tree growing in the water, and the entire area where the mat plants were growing was beach the previous year.

The beach sand was predominately pumice and many mats on this beach were partially eroded off their substrate, possibly due to wave action.

Further round this bay to the east was a large area of reeds and to the west was a cliff and deep water with much weed growth. About 300 m either side of the sample area was surveyed by boat. From local information it appears that Myriophyllum propinquum has appeared since last season.

Lake Okareka(Fig. 2) On the western side of the lake there were large piles o f Lagarosiphon thrown

up onto the beach while on the southern and eastern shores there were no mats to be found. On the eastern end of the lake the farm land was under water and no mat plants were growing. The only plants found were on the end of the peninsula (station 1) and across the other side of the lake near a skiing club's rooms (station 2).

Station 1 Plants in this area were scattered and patchy, possibly due to the local

population using the area for swimming.

Species Present % Cover (Dry Wt.) Limosella lineata 46.8% Glossostigma submersum 46.4%

Depth limit of plants was 23 cm, 9.4 m out from the shore.

Station 2 Here the cover by mat plants was very extensive and had the occasional

'island' consisting of Lagarosiphon major and Myriophyllum propinquum interdispersed.

Species Present % Cover (Dry Wt.) Limosella lineata 21.8% Lilaeopsis lacustris 72.3% Elatine gratioloides 5.8% One plant of Glossostigma elatinoides and also some Nitella hookerii.

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Depth limit o f these plants was 1.6 m, 20 m out from the shore line. Beyond this point there was a dense and extensive bottom flora of Lagarosiphon, Elodea canadensis, and Myriophyllum.

Blue Lake (Fig. 3) A few isolated plants were found at the northern end of this lake, but most

specimens were located in the south-eastern corner. These mats tended to be small in size and close to large rocks. They were not close in to the shore at the northern end, probably because of skiing activities. Vegetation in this lake is sparse and no dominant weed species was seen here as in the other lakes examined, although individual plants were present. Much of the lake's shore had reeds growing in the shallows and mats do not form in such positions.

Species Present % Cover (Dry Wt.) ^Myriophyllum sp. (Juv.) 65.0% Glossostigma elatinoides 23.2%

Lilaeopsis lacustris 11.8%

++ possibly M. propinquum

Depth limit - no plants were found below 0.73 m and further than 7 m from the shore.

Lake Tarawera (Fig. 4) The area surveyed on this lake was confined to the western arm. A sample

was collected from near the area commonly called "The Orchard", the site of a now non-existent homestead.

This bay has a very sheltered situation. The mats are extremely well developed and their height is some 3-4 cm above the surrounding substrate.

Species Present Glossostigma elatinoides Lilaeopsis lacustris Limosella lineata Glossostigma submersum

Other plants also present:-Algae Nitella hookerii Musci - Drepanocladus fontinaliopsis

Also growing on the major plants were diatoms and a species of Tolypothrix.

Depth limit o f the plants was 0.89 meters at 14.7 meters from the shore line. The mat plants were delimited by a heavy growth of Myriophyllum

elatinoides amongst which grew Enteromorpha nana var. rivularis.

% Cover (Dry Wt.) 3.2%

28.2% 34.4% 34.2%

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Lake Rerewhakaaitu (Fig. 5) A l l bays along the western side of the lake had large areas of reeds present.

Mat plants were located on a small peninsula in the north-western corner of the lake. Access was by a road used for launching small pleasure craft. The substrate of this beach consisted o f granules (2 mm) of dense pumice resting on hard siliceous rock. Where the mat plants were found, they had their own soil in large channels in the rock parallel to the shore. The lake level was above normal as indicated by many Leptospermum plants that were standing in water.

Species Present % Cover (Dry Wt.) Myriophyllum sp. (juv.) 35.0% Elatine gratioloides 37.2% Lilaeopsis lacustris 2.5% Glossostigma submersum 1.5% Scirpus sp. 24.8%

Depth limit was 0.80 m, 18.5 meters out from the shore. The mat plants were delimited beyond this by a massive growth of Nitella hookeri and Nitella. sp.

Lake Rotoehu (Fig. 6) The level of this lake has risen a great deal and is almost 1.5 meters above

normal. A survey was carried out around the southern end of the lake and no mats at all were found. A sample was taken of some of the weed. Then the area near Soda Springs was surveyed, with the same result. A survey was then carried out by boat from Otautu Bay around to the east and west (see map) but no mat plants were to be found. In the shallow areas of the lake there were large communities o f reeds and Lagarosiphon.

Species Present Lagarosiphon major Myriophyllum elatinoides

Algae present amongst the two species included Spirogyra sp., Rivularia sp., and Oedogonium sp. Many epiphytic green algae and diatoms were also present.

F rom the depth limits of the mat plant species, gradients for the sample areas of each lake can be determined (Fig. 7).

Recently (1972) another visit has been paid to this lake and the following mat plants were found:-

Omahota Bay (western side of lake) Elodea canadensis Ranunculus fluitans Chara sp. Glossostigma elatinoides (submerged and emergent) Lilaeopsis lacustris Ludwigia palustris

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Bay on eastern side of lake above Otautu Bay Lagarosiphon major Elodea canadensis Potamogeton cheesemanii Myriophyllum ?elatinoides

Otautu Bay Lagarosiphon major Elodea canadensis

Boat launch area beside road south of Otautu Bay Glossostigma elatinoides G. submersum Elatine sp. ined. Elodea canadensis Myriophyllum Potamogeton crispus

Shore slopes, as calculated from mat plant distribution arc given in Fig . 7.

| Sho re Lake sur face

M e t r e s Fig. 7. Shore slopes of lakes. (Data from mat-plant depth limits)

a. Lake Okareka (Station 1) b. Lake Tarawera c. Lake Rotoma and Blue Lake d. Lake Okareka (Station 2) e. Lake Rerewhakaaitu.

SPECIES LIST List o f plant species found forming mats in the various lakes.

Chlorophyceae

Cyanophyceae

Musci Angiosperms

Enteromorpha nana var. rivularis (Somerf.) Bliding Nitella hookeri (Collins) comb. Braun Nitella sp. Spirogyra sp. Tolypothrix sp. Rivularia sp. Drepanocladus fontinaliopsis (CM) Dix Elodea canadensis Muchy. Lagarosiphon major (Ridler) Moss Lilaeopsis lacustris Hill Limosella lineata Gluck

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Elatine gratioloides A. Cunn. Myriophyllum elatinoides Gaud. M. propinquum A. Cunn. Glossostigma elatinoides Benth. Glossostigma submersum Petrie Scirpus sp.

D I S C U S S I O N

In all o f the lakes studied, the mat plants have been found on gently sloping, sandy, sheltered areas around the lakes, and an idea has been gained of what species can be expected to be found.

Depending on the slope, these plants seem to occupy the substrate to a maximum depth of about 2 meters. Below this depth there is a predominance of other plant species that may form a dense cover e.g. Lagarosiphon major. About a quarter was removed from one side of the mat on Lake Tarawera and three weeks later much more was found to have been eroded. This suggests that initial colonisation o f a shore is dependent on the effect of wave action on establishment of the seeds or seedlings at this depth, and may determine chiefly why particular (mat) plants colonise specific areas and not other lake areas.

With many of the lakes surveyed, it was noticed that the water levels were much above normal. Indications of this were the fact that many normally terrestrial plants were growing in the water, i.e. I^eptospermum and many other trees (round Lake Rotoma) were dead or moribund. This raising of the lake level appears to have caused the Lagarosiphon and Myriophyllum to extend higher up the beach. Patches of dead mat plants were found on Rotoma among the Lagarosiphon. Any mat plants that are present appear to be relatively new colonists.

A C K N O W L E D G E M E N T S

I would like to thank my supervisor, Professor V . J . Chapman, for his assistance, and also Dr. J . M . A . Brown. I also wish to thank my fellow students for valuable discussion.

REFERENCES

Chapman, V.J . 1966: The lakeweed problem in the North Island of New Zealand Phycos 5: 72-82.

Chapman, V . J ; Brown, J.M.A.; Dromgoole, F.I.; Coffey, B.T. 1971: The submerged vegetation of the Rotorua and Waikato Lakes. 1. Lake Rotoiti. N.Z. Journ. Mar. Freshw. Res. 5: 259-79.

Chapman, V.J . ; Coffey, B.T.; Brown, J.M.A. 1972: The submerged vegetation of the Rotorua and Waikato Lakes. 2. Pattern and Process or Cyclic Change Ibid. (In press)

Mason, R. 1970: Aquatic Weeds. In 'Identification of Weeds and Clovers' A.J . Healy. Editorial Services Ltd. Wellington, N.Z.

Watt, A.S. 1947: Patterns and Process in the Plant Community. J. Ecol. 35: 1-22.

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Fig. 1. Lake Rotoma

L A K E O K A R E K A

Fig. 2. Lake Okareka