|dc.description.abstract||This study continued long-term limnological monitoring of Lake Mahinerangi, on which measurements have been made since 1965. Since this time, 35% of the catchment has been developed for agriculture.
Light-optimal 14C photosynthetic (P-max) rates have increased approximately eight-fold since 1965. The lake is now mesotrophic. Long-term changes in percentage oxygen saturation and DRP concentrations are also consistent with eutrophication. Anabaena flos-aquae has been found in the lake since 1979-80. Melosira distans and M. granulata, not previously seen in this lake, were commonly found throughout the present study period. Crustacean zooplankton abundance during winter has increased as have numbers of Daphnia carinata, the largest zooplankter. In spite of these changes, average chlorophyll a concentrations increased by only 50% and this increase was not statistically significant. No significant long-term reduction in Secchi disc transparency was evident perhaps because of the effects of cell size on light penetration in this lake. Commonly observed correlations between light attenuation, Secchi disc and chlorophyll a were reversed in Lake Mahinerangi. Even at the low chlorophyll levels found in this lake, seasonal fluctuations in phytoplankton cell size affected light attenuation and reduced self-shading effects at times of high biomass.
Size fractionation studies indicated that netplankton contributed significantly to the overall form of the annual productivity curve. The nannoplankton contribution was seasonally more constant. Data collected from different basins indicated the presence of a slight gradient of increasing trophy down Lake Mahinerangi.
Routine laboratory 14C batch culture, ammonium enhancement and luxury uptake bioassays during 1981-82 revealed that the phytoplankton were primarily nitrogen limited for most of the year while phosphorus was most limiting during summer. Secondary limitation by either nutrient was rapidly attained, however; photosynthetic responses in batch culture experiments were much larger in the presence of both nutrients than when either nutrient was added alone. Two replicated, 5 m3 in situ enclosure experiments were carried out to eliminate artifacts imposed by small volume laboratory incubations. Although both experiments were prematurely destroyed by wind, results were consistent with the routine assays.
To calculate N and P loads to the lake, concentrations in twelve inflowing streams were monitored. Average concentrations were significantly correlated to percentage agricultural development in the seven major catchments. When export rates were calculated, only TP and N03-N were correlated with agricultural development. Load TN:TP ratios showed a highly significant decrease with agricultural development. Agricultural development, therefore, has not only increased the level of eutrophication, but has also shifted Lake Mahinerangi towards increasing nitrogen limitation. Baseflow concentrations of N and P forms were not highly correlated with flow rates in one stream, in which flow rates were continously monitored.
Loads from two small agriculturally developed catchments were the highest found in this study. High losses of N and P, in particular N03-N, were found in two clearfelled exotic forest catchments compared to a forested one. , Export rates of dissolved N and P forms in this study were low when compared to rates from other New Zealand catchments of similar land uses, but of differing geological origin.
Past and present N and P loads to Lake Mahinerangi were estimated from stream concentration and export rates based on historical land development and hydrological data. TP loads have increased approximately four-fold since 1965, and TN by only 50%. Several empirical loading models, developed primarily on data from north temperate lakes, were tested on the Lake Mahinerangi data. Predictions from phosphorus retention and phosphorus loading models were generally in good agreement with observed TP retention and TP and DRP concentrations. Models aimed at predicting TN concentrations were less accurate when applied to Lake Mahinerangi.
Nutrient-trophic state models were also tested for Lake Mahinerangi. The lack of change in long-term chlorophyll concentrations and a large contribution of phaeopigments to total chlorophyll values made estimations of chlorophyll from published TP-Chl a models difficult. However, at TN:TP < 15, TN appeared to be useful to predict corrected chlorophyll a both regionally and annually in this lake. Published models which predict zooplankton biomass in north temperate lakes greatly overestimated biomass in New Zealand lakes. Strong relationships between mean annual TN and TP concentrations and P-max were found for New Zealand lakes, and served as the basis for prediction of future trophic state in Lake Mahinerangifrom estimated TP concentrations. P-max rates are expected to reach eutrophic levels in Lake Mahinerangi once projected land development is completed.||en_NZ