Report 84. This Report Is Dív
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The phytoplankton, macrophytes, zooplankÈon and macrobenthos of Lake Rotoaira D.J. FORSYTHI, L. MACKENZTE2, I. D. McCALLIIM3 and E . J. CUDBY4 1. Taupo Research Laboratory, Division of Marine and Freshwater Science DSIR, P.O. Box 415, Taupo 2. Cawthron Institute, P.O. Box 175, Nelson 3. P.O. Box 203, Chernside, Brísbane, Australia 4. Fisheries Research Division, Minístry of Agricul-ture & Fisheries, Turangi DIVISION OF MARINE AND FRESITI^IATER SCIENCE Decenber 1985 Taupo Research Laboratory Taupo Research T,aboratory Report 84. This report is Dív. Marine & Freshwater provisional and should not Science, DSIR, Box 415, be quoted r¡ithout consultíng Taupo the authors or Director INTRODUCTION Lake Rot.oaíra lies at an altitude of.564 metres a.s.l. between Mts Tongaríro and Pihanga south-west of Lake Taupo on the CenËral Volcanic Plateau of the North Island. The lake is a val1ey b1-ocked by andesitic larva from volcanic eruptions which formed the nearby mountains. It has an area of 1530 ha, a mean depth of 9 rn and a maximum depth of 14.6 m (Gregg, 1960). The Tongariro Power Development Scheme which began Ln 1964 changed the natural flor,¡ pattern of the Lake Rotoaira catchment to include some of the headwaters of the Moawhango, Tongariro and trüanganui catchments, a total area of about 2500 km2. The naËural catchment of Lake RotoaÍra includes the northern face of Mt Tongariro, the south síde of Mt Pihanga and some low-lyíng land to the r^rest. tr{ater diverted from the souËh-west, and south-east of the lake reduced the hydraulic load on Lake Rotoaira from the natural flow raËe of 2 2 6.9 m's'to-1 41.1 m's -'l- by L975-76 (White, 1977) and decreased the average residence time of the lake waters from about 247 d to about 30 d. At completion of the Scheme v/ater from the diversions into Ëhe Moawhango darn and pohrer Ëo about 57 t3 Rangipo station further increased the hydraulíc load "-1 and reduced the residence Ëime to about 28 d. Apart from a sma1l compensation release of water down the Poutu Stream, the original outlet, wíth the compleÈion of the Scheme in 1982 rnrater from the lake ís now discharged through the Tokaanu pohrer statíon ínto Lake Taupo. The nutrient loads of phosphorus and nitrogen on the naËural ínputs to Lake RoÈoaira are unknown but were probably at least 0,3 g y-L of. ^-2 2 phosphorus and 3.0 g * y-L of nírrogen in vier¿ of its eutrophíc state at thaË time (Freshwater SecËion, Ecology Division, DSIR report, Apríl 1-975). were predicted to be 2.42 g y-I of. At cornpletion of the diversions loads ^-2 phosphorus and 4.86 g *-' ,-t of nitrogen. Ilowever, the reduced residence time of the lake watelwas expected to ameliorate the effect of increased nutrienÈ load. Indeed, according Ëo 2 I{hite (Lg77) by 1975-76 the increased. hydraulic load to 4L m3 s-1 produced an íncreased nutrient load but no apparent change in trophic staÈus illustrated by reduced depletion of díssolved oxygen in the boÈtom waters, Thís suggests a move from mesotrophy (Fish & Cudby, pers. conm.) to oligotrophy. Early research on the lake was carried out by MAF between 1968 and L972. Dissolved oxygen, r.rater temperature, transparency, chlorophyll a and dÍssolved reactíve phosphorus were sampled at 10-14 d intervals from 2 October 1968 to 9 February 1972 by Fish and Cudby (pers. comm.) for baseline data to be used in the future assessment of the trophic sÈatus of the lake. They considered rhe lake to be mesotrophic. The aim of this study r¡ras to further assess prospectíve change in the trophic state of Lake Rotoaira following the Tongariro Power Scheme and specíal atËention was paid to benthic macroinvertebrates, phytoplankton zooplankton. Methods InËegrated wh-ole rvater column samples for phytoplankton and pigment analysis were taken using a weighted plastic hose (20 mm diam.) at six sites (Fig. 1) monÈh1y from 12 September L975 to 16 Septernber 1-976. Additional samples v/ere occasíonally taken from selecËed depths at site B (Fíg. 1) for primary producÈíon, pigment and biomass profiles, using a Van Dorn bottle. Phytoplankton pigments ïrere exÈracted in cold 9O7. acetone and analysed colorimetrically according to the method of Golterman (1969). Samples for ATP det,erminatíon from siÈe B were prefiltered through 200 prn mesh, filtered through GF/C fílters which \^rere extracÈed in boí1ing Tris buffer. ATP r¡ras converted to carbon biomass using a C:ATP rat.io of 250 (Ho1m-Hansen 1970). To measure primary produetion líght and dark 125 ml bottles containing 14HCOr- v/at,er from various depths wíth r (10 uci) addition, were incubated ín situ for 2.5-3.0 hours. IncorporaËed radioactivity was measured wÍÈh a geiger counter. AlkalíniÊy was determíned Ínmediately upon return to Ëhe laboratory. To enumerate phyÈoplankton and determine biomass, samples \^7ere preserved with Lugols iodine immediately after collection and examined after settling under an inverted microscope. !Íhere numbers and cell size made it possible total counts of all cel1s (of a particular species) in 10 ml were made. I^Ihere large numbers or small ce11 size made this impracÈicaL 25-50 random fields lsere counted at different nagnifications (200-400x) Phytoplankton bíomass values for all species r¿ere obtained by estimating ce1l volume assuming regular volumes (cylínders, spheres etc.) and combinations of these. The carbon biomass equivalents r^/ere calculated using the regression of Mullin et al. (L966). The taxonomy followed that of Prescott (1970). Data used ín the calculations are shown in Table 1. Five benthic sediment samples hTere taken by means of a lJisconsin hand net 30 cm square and a mesh of 700 Um (lüelch, 1948) at 1m depth i-ntervals f.rom 2 m to 10 m and at L2 m from a transect perpendicular to the shore ín Onepoto Bay (Fig. 1) using SCUBA on 7 and 13 November 1968 and 8 November L978. 0n I January L973 two samples each were taken at depths of 3 m, 9 rn and L2 m from the same transect. T¡¿o sediment samples r^rere t.aken with an Ekman grab 18 cm square at monthly intervals from ApriL 1976 to March 7977 at each of six siÈes A-F (Fig. 1) at depths of 11 m to 14 m. The samples r^rere washed through a 500 ¡.rm mesh sieve and the anímals removed manually, counËed and ídentifíed. Two Ekman grab samples were taken aË site B at a similar depth to Elne L2 m transect, tr'Iisconsin neË samples on B Novenber 1978 for a comparison of the results obtained from the two samplers, Zooplankton Írere sampled by means of tr¿o vertical net hauls from 12.5 rn at each of sÍtes B and E at monthly íntervals from September L975 to August 1976. The cone-shaped net r^ras 115 crn long with a mouth of. L7 cm diameter 4 and a mesh size of 55 Um. The catch was immediately preserved in 47" formalin. At least 3 subsamples were counted until reasonable agreement was reached between consecutive counts. The values of each net haul r^7ere averaged and the result expressed as numbers per litre. Díssolved oxygen concentration and \.rater temperature hTere measured aË sites A-F with a Inleston and Stack analyser and transparency with a 20 cm diameter white Secchi disc. Differences in numbers of benthic animals between years and beËween depths lrere tested by an analysis of variance and were signíficant at l-east at p <0.05. Results Dissolved oxygen and temperature The waÈer column was always mixed, seasonal t.emperaËures ranged from 6.9 to 17.5"C and dissolved oxygen concentrations did not change from the surface to the bottorn (12 rn) (Fíg. 2). The lake was isothermal in August and differed on1ry 2.5oC between the surface and the bottom in January. Secchí dísc Ëransparency values (Fig. 3) are the mean of readings from each of siÈes A-F. They correlate closely with the chlorophyll a concentrations (Fig. 5) indicating little turbidity frorn suspended inorganíc nateri-al. Flora Phytoplankton. Forty-one genera of phytoplanktonic algae (Table 1) were observed throughouË the survey. The only generâ observed on all occasions were NosËoc, Ch1orel1a, Melosira granulata, Cryptomonas and Chroomonas. Several other genera, although sometimes abundant were raie aË oÈher Èimes. The two major groups, Èhe Chlorophyta and Chrysophyta appeared to be almost equally diverse in the numbers of genera represented, however, a few genera were always most cotnmon in each group. The seasonal changes in Èotal phytoplankton cell numbers and biomass are shown Ín Fíg. 4. ATP values whích reflect totâl microbíal mass are 5 shown in Fig. 2. Chlorophyll concenËrations and prímary produetion esËimates are shown ín Fig. 3. Peaks of chlorophyll a occurred ín the spríng of. L975 and L976 wíËh minor peaks in sununer and auËumn of L976. Although at any one time there hras considerable vari.ation in chlorophyll concentratíons between sarnpling sites, overall there rras no significant difference betr¿een the mean annual concentrations at each site. The mean ratios of chlorophyll to carbon biomass are sholrn in Table 2. The overall rnean was 1:32.L t f3.g. Figs. 6-8 illustrate the changes in the major taxonomic groups of phytoplankton throughout Èhe year and Fig. 9 shows Ëhe percentage contribution to the total biomass and numbers by each group. Although the Chlorophyta rüere numerícally dominant throughout much of the year (cf. Fig. 7), the biomass was usually overwhelmingly dominated by the Chrysophyta, particularly the díatoms.