Annual changes in the mixed depth and critical depth for photosynthesis by Aphanizomenon ovalisporum that allow growth of the cyanobacterium in Lake Kinneret, Israel

Abstract

A limnological database for Lake Kinneret was used to determine the period each year when the interaction of mixed depth and light availability would permit growth of the planktonic cyanobacterium Aphanizomenon ovalisporum. The organism, recorded for 56-211 days in each of the 7-years, 1994-2000, was usually distributed through the mixed epilimnion and reached a maximum in late October. The mixed depth (Zm) was calculated from the temperature profile, wind speed and wind direction. Changes in the total population were related to changes in the daily integrals of photosynthesis, calculated from vertical profiles of water temperature (Θ) and light attenuation (Kd), and hourly changes in photon irradiance (E0) each day, using temperature-compensated algorithms based on measurements of photosynthesis at different irradiances. Net photosynthesis was calculated for different ratios of respiration (R) and maximum photosynthesis (Pm). In a modelling analysis, the daily integral of net photosynthesis fell below zero on the day of the population maximum, and remained negative during the period of population decline, when R was set at -0.08 Pm. Similar modelling, based on the 7-year mean values of Θ, Kd and E0 for each day of the year, showed the critical depth for photosynthesis by Aphanizomenon would exceed the mixed depth in Lake Kinneret from mid-March to late October. Direct measurements of -R/Pm ratios could be used with information in the database to analyse the periods of photosynthetic production by different phytoplankton species in Lake Kinneret.