Phytoplankton photosynthesis and its relation to standing crop and nutrients in two warm‐monomictic South Island lakes

Seasonal and vertical changes in phytoplankton photosynthesis, chlorophyll, NO 3 ‐N, NH 4 ‐N, PO 4 ‐P, pH, and major ions are described for a 2‐year period in Lakes Hayes and Johnson, and related to previously published results for phytoplankton species composition and standing crop, zooplankton, and oxygen for the same period. A preliminary assessment of nutrient sources was made. Accumulation of PO 4 ‐P and NH 4 ‐N in the anoxic hypolimnia were major features of the annual cycles. Surface concentrations at the autumn circulation reached 40–50 mg.m ‐3 of PO 4 ‐P and 150–400 mg.m ‐3 of NH 4 ‐N. Surface maxima of NO 3 ‐N were 26 mg.m ‐3 in Lake Johnson and 60 mg.m ‐3 in Lake Hayes. Both forms of nitrogen were undetectable in the epilimnion for several months during stratification. Secondary NO 3 maxima in the hypolimnia may have resulted from nitrification of ammonia. Assimilation by phytoplankton accounted for 2.1–4.0 times the NH 4 ‐N lost after autumn circulation, corrected for nitrification. Complexities in the vertical distribution of N and P in Lake Johnson arose from development of anoxia in the metalimnion while the upper hypolimnion remained oxygenated. There was a large accumulation of chlorophyll degradation products in the anoxic hypolimnion of Lake Johnson in 1970, and metalimnetic chlorophyll maxima were recorded occasionally. Annual minima of photosynthesis and chlorophyll generally occurred during stagnation, with secondary minima in winter. Autumn increases in photosynthesis and total chlorophyll content of the lakes were closely related to nutrient entrainment as the thermocline sank. Short‐term changes in photosynthesis at optimal light intensity were predictable from changes in inorganic nitrogen, except when nitrogen‐fixing algae were dominant and immediately after autumn circulation. Algal blooms in some summers were related to persistence of high epilimnetic PO 4 ‐P concentrations till early summer. Nitrate stimulated photosynthesis in 5‐day in situ bioassay experiments in both lakes during stagnation. Phosphorus alone had little effect, but the response to nitrate was enhanced by phosphorus addition in Lake Hayes. Replacement times for the phytoplankton crop ranged from 0.3 to 7.9 days, and were generally higher in Lake Johnson than in Lake Hayes. Specific loss rates by the phytoplankton were generally low during isothermal periods and highest when large flagellated cells declined. Specific loss rates were low while an Anabaena bloom developed, but increased at the end of the bloom. Approximate figures calculated for specific zooplankton grazing rates suggest that grazing may be a significant factor in specific phytoplankton losses.