Does climate change influence phosphorus sources in lakes (internal versus external) and are they fuelling algal blooms?

English Abstract:

One of this millennium’s limnological paradigms is why recurrent cyanobacteria (CYA) blooms arise in lakes with low phosphorus (P) concentration in epilimnion. Recent studies suggest that climate change may play an important role, whereby increasing temperatures can indirectly affect P availability to algae. Hot summers influence water column stability, thus increasing anoxia in the hypolimnion and augmenting sedimentary P release. In addition, climate change has also lead to an increase in storm events frequency, severity and gust winds. We hypothesize that these meteorological events induce turbulent vertical instabilities in the thermocline bringing deep water P to the epilimnion. In this talk, we present the P-sources and their distribution in a lake, the relationship between the frequency and magnitude of storm events, thermocline stability and pulses of P-upsurge and burst CYA biomass over lake surface. This study was performed in a small stratified Canadian lake, over a 5 year period. In this lake, recurrent surface blooms are frequent and are dominated by noxious CYA, growing in the epilimnion. We measured the P budget annually and in the lake’s open season, in situ thermistor chains registered temperature. Solar irradiance, precipitation, wind speed and direction were recorded by a nearby weather station. On a weekly basis, oxygen, pH (YSI) and algal distribution and composition (fluoroprobe) were measured. Water was collected at discrete depths for dissolved and total phosphorus and CYA species composition. We found that, the internal/external P ratio varies from 1.2 to 19 among years, as a function of the length of the anoxic period. Summer metalimnetic standing stock of CYA varies from 900 to 1900 kg and is highly correlated to hypolimnetic P concentrations. Upsurges of metalimnetic P occur during storm events and as a consequence, metalimnetic cyanobacteria are upwelled and bloom at the surface.