Effects of diurnal vertical mixing and stratification on phytoplankton productivity in geothermal Lake Rotowhero, New Zealand

Mixing processes in lakes are key factors controlling light availability for phytoplankton growth, but understanding the contribution of mixing is often confounded by other factors such as nutrient availability and species dynamics. Our study examined this problem in a low pH, geothermally heated lake dominated by one phytoplankton genus and lacking the complexity of nutrient limitation, phytoplankton species interactions, or grazing pressure. We hypothesized that the continuous strong convectively driven circulation resulting from atmospheric instability and sediment heating would negate any tendency of thermal stratification, entraining phytoplankton and transporting them away from high surface irradiance that could induce photoinhibition. During our study, water temperatures were considerably warmer than air temperatures, with a diurnal maximum surface temperature of 37.5 °C and minimum of 35.5 °C. Surface heating induced stratification, with a temperature difference of 1–2 °C evident during the day, but there was sufficient heat loss and mixing during the night to erode the stratification and create isothermal conditions. The vertical entrainment velocity driven by convective circulation was on the order of 0.1 mm s−1, but when there was strong solar heating, phytoplankton within the top 0.5 m of the water column still showed depressed photosynthetic quantum efficiencies, as determined with a Pulse Amplitude Modulated fluorometer (PHYTOPAM); however, this depression was less than for phytoplankton cells maintained throughout the day in surface waters with bottle incubations. At other times mixing generated by continuous heating and atmospheric instability meant that phytoplankton did not show photoinhibition; therefore, despite the geothermally driven mixing in Rotowhero, the intensity of solar radiation is still the key mechanism determining the stratification response and resultant photoinhibition of the phytoplankton. Lake Rotowhero provides an excellent natural laboratory to examine the relative time scales of mixing and phytoplankton photoinhibition responses because small changes in solar radiation have such marked impacts on the diurnal stratification and radiation experienced by cells located above the diurnal thermocline.