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MAURI ORA, 1976, 4: 121-131 121 AN ELEMENT SURVEY OF TH E AQUATIC MACROPHYT E S, WATER AND PLANKTON FROM THE WAIKATO RIVER, NORTH ISLAND, NEW' ZEALAND DAVID J. RAWLENCE Department of Extension Studies, University of Canterbury, Christchurch, New Zealand and J.S. WHITTON Soil Bureau, D.S.I.R., Private Bag, Lower Hutt, New Zealand ABSTRACT Element levels were determined in aquatic macrophytes, water, and plankton from six hydro-electric storage lakes on the Waikato River, North Island, Ne\~ Zealand. Lakes Ohakuri, Atiamuri, Whakamaru, Maraetai, Arapuni and Karapiro, sampled on 14·and 15 March 1973, yielded the following species: Cerat:ophy11um demersum L., Elodea canadensis Michx., Dagarosiphon major (Ridley) Moss, Egeria densa Planch., pot:amoget:on crispus L., Pot:amoget:on cheesemanii A. Bennett, and Nymphaea alba L. The plankton and macrophytes were analysed for Si, K, P, Na, Mg, Ca, S, Al, Cl, Fe, Zn, Cu, Mn, Co, Mo, Ni, V, Cr, Sr, Ba, Pb, Ti, Ga, and Zr. Water samples were analysed for the same elements, except for P. Ceratophy11um from Lake Atiamuri contained the highest levels of 4 of the 5 macronutrients determined: K, 54 000 ~9/gi P, 5 100 ~g/g; Mg 5 000 ~g/gi S, 6 500 ~g/g. The Lagarosiphon sample from Lake Karapiro contained the highest Ca level: 14 800 ~g/g. The highest levels of the six micro nutrient elements determined were: Cl, 21 000 ~g/g, Mn, 4 700 ~g/g (both-in Ceratophyllum); Fe, 1 130 )Jg/g, Co, 10.4 ~g/g, and lY",o, 5.6 ~g/g (in Potamoget:on crispus) i and Cu, 5 ~g/g (in Egeria densa). These maxima were more distributed th~oughout the 6 lakes than the maximum values for the macronutrient elements. Two adventive species, Cerat:ophy11um demersum and Potamoget:on crispus, were clearly able to absorb large quantities of several essential elements. The influence of these plants on the nutrient balance of the lake system is considered, and the possible relationship between competitive success and enhanced capacity for nutrient absorption is also considered. INTRODUCTION Aquatic macrophytes have, with rare exception, rated scientific interest in New Zealand only as weeds to be controlled or eradicated. Various species which have thrived under local conditions include Elodea canadensis Michx. in Lake Rotorua, Dagarosiphon major (Ridley) Moss in Lake Rotorua and the upper lakes of the Waikato River hydro-electric storage system, Egeria densa ?lanch. in Dakes Karapiro and Maraetai, and Ceratophy11um demersum L. in most lakes of the Waikato River. Macrophytes 122 MAURI ORA, 1976, Vol. 4 make a significant contribution to annual production in fresh wate.r and, therefore, play an important role in the nutrient budget of many lakes and rivers. Much of the scanty literature dealing with the chemical composition of macrophytes has been reviewed by Caines (1965). More recently, the effects of nutrient enrichment on macrophyte chemical composition have received more attention (Boyd 1969, Boyd and Vickers 1971, Gossett and Norris 1971, Ryan et a1. 1972). An extensive body of literature on the protein, fibre, dry matter, ash content and carotenoid values of macrophytes was reviewed by Little (1968). Other studies have dealt with the relationship between water chemistry and weed distribution (Moyle 1945), the nutritive value of weed (Nelson et a1. 1939), the role of macrophytes as limnological indicators (Seddon 1972), and the relationship between macrophyte morphology and nutrient levels (Adams et a1. 1971). The formulation of a nutrient budget for any body of water requires a comprehensive set of hydrological data, as well as chemical analyses of the water, plankton, and macrophytes. Although some values have been published for a few elements in macrophytes collected in New.Zealand (Fish and Will 1966, Lancaster et a1. 1971, Reay 1972) there is no comprehensive data even for the species from a single site. '1'0 provide information toward a nutrient budget for the Waikato River hydro-electric storage lakes, element levels were determined in macrophyte samples from Lakes Ohakuri to Karapiro. Because of the restricted shallow or other areas suitable for macrophyte growth in many of these lakes, phytoplankton probably account for a considerable amount of the total lake production. Element levels were therefore determined in a plankton sample from each lake, and in a representative water sample. DESCRIPTION OF THE AREA The Waikato River and its sources, Lakes Taupo and Rotoaira, are shown in Fig. 1. Between LaRes Aratiatia and Ohakuri the Waikato River receives geothermal waters from the Broadlands, waiotapu, andOrakeikorako thermal areas. Lake Ohakuri is also subject to runoff from the extensively farmed Whirinaki and Waikite Valleys. Vegetation on land surrounding Lakes Atiamuri, Whakamaru, and Maraetai is predominantly exotic forest, while the catchments of Lakes Arapuni and Karapiro are dominated by pastoral farming. The catchments of all the Waikato River lakes are rhyolitic in nature, and include lahar deposits, pumice breccia and tuffs, massive ignimbrite sheets, and current-bedded alluvial sands and gravels derived from pumice and ignimbrite. Consequently, the river is subject to similar geochemical influence from Lakes Aratiatia to Karapiro, maximum geothermal influence at Lake Ohakuri, and the influ-ence of pastoral farming from the catch ments of Lakes Ohakuri, Arapuni and Karapiro. RriiVLENCE & fYHITTON - WAIKATO RIVER ELEMENT LEVELS 123 -- dam Lake Karapiro Lake Arapuni I I Whirinaki Arm I Waiotapul Lake ~ o I Lake I Lake I I o I Wairakei 0 " I Broadlands \ Lake Aratiatia Lake Taupo Lake Rotoaira o :> km Fig. 1. The Waikato River hydroelectric storage lake syste~ and Lakes Rotoaira and Taupo. 124 MAURI ORA, 1976, Vol. 4 METHODS Lakes Ohakuri I Atiamuri I Whakamaru, Ma.raetai, Arapuni and Karapiro (Fig. 1) were sampled on 14 and 15 March 1973. The physical features of each are summarised in Table 1. Because of its dominance by Lake Taupo, Lake Aratiatia was included with the former in a separate survey (in preparation) . TABLE 1. PHYSICAL FEATURES OF THE SIX WAIKATO RIVER HYDRO-ELECTRIC STORAGE LAKES SAMPLED IN THE STUDY. Maximum height above Lake Year filled (ha) Area sea level (rn) Ohakuri 1961 1295 287 Atiarnuri 1958 220 252 Whakarnaru 1951 712 226 Maraetai 1952 505 188 Arapuni 1929 932 111 Karapiro 1947 777 54 All sampling was carried out from a boat. Ten to twelve complete plants of each macrophyte species were recovered by grapple, washed, and processed as a single sample. Two hundred grams wet weight of weed was collected wherever possible. A 10 1 water sample, and a tow with a 30 lIm mesh net were taken near the centre of each lake. Plant samples were oven dried at 110°C, finely ground in a Krups coffee mill, and bottled. Plankton and water samples were evaporated to near dryness in platinum basins on a hot plate under infra-red lamps, dried in an oven at 110°C, finely ground in a mortar, and bottled. The following elements were analysed by emission spectro scopy: Na, Mg, Al, Si, Ti, V, Cr, Fe, Co, Ni, Cu, Ga, Sr, Zr, Mo, Ba and Pb. S, P, Cl, K, Ca and Zn were analysed by x-ray fluorescence spectroscopy. Because of the high levels encountered in these samples, Mn was determined by atomic absorption. As the amounts of several macrophyte samples were insufficient for Kjeldahl analysis, the incomplete data for N is omitted from this contribution. For emission spectroscopic analysis, all samples were ignited in platinum basins at 500°C for 2 h. A 10 mg sample of the ash was mixed with carbon powder containing an Ag internal s·tandard, in a 1:3 ratio of ash to carbon powder. Sample electrodes were burnt in a d.c. arc as cathode at 10 amps, using a Hilger Large Quartz Spectrograph, until the sample was consumed. Exposures were made on Ilford G.30 chromatic plates, which were marked for the elements of interest, using a Hilger projection comparitor. Element line densities were measured with a zeiss Microdensitometer, and concentration was assessed from graphs of line density against element concentration. For x-ray fluorescence analysis, 2 g of sample in a mylar based holder was irradiated, using a Philips PW 1540 spectro graph. A Cr target tube and PE analysing crystal were used for S, Cl, K and Ca, and a W target tube and LiF analysing crystal were used for Zn. A counting procedure was used for the estimation of all elements' except Zn, where a graphic procedure was followed. RAWLENCE & WHITTON - WAIKATO RIVER ELEMENT LEVELS 125 Standard mixtures of trace elements were prepared following the procedures of Mitchel (1964), and calibration graphs were prepared for each element. These were checked for accuracy against various standard reference materials (Flanagan 1969, 1973, Roubalt et al. 1966, Bowen 1967). Throughout this paper plankton and macrophyte element values are expressed as ~g/g of the element in oven dry material. and values for water are expressed as mg/l or ~g/l. RESULTS WATER The level of dissolved solids, represented by the ash values (Table 3), decreased steadily between Lakes Ohakuri and Arapuni and increased slightly in Lake Karapiro. The range of dissolved solids values (116 to 148 mg/l) for the Wakato lakes is significantly higher than the values for Lake Taupo (58 to 64 mg/l, in preparation). The variation downstream in amounts of K, Mg, S, Na, Mo, Cr, Sr, Ba and Ti showed a trend similar to that for dissolved solids. The levels of Si, Al and Fe decreased only as far as Lake Maraetai, and then increased through Lake Arapuni to Lake Karapiro. Cobalt and B decreased steadily downstream from Lake Ohakuri to Lake Karapiro. The high levels of K, S, Na, Cl, Si and B in Lake Ohakuri largely relate to the various geothermal inflows to the lake.