SPATIAL, SEASONAL AND LONG-TERM VARIABILITY OF PHYTOPLANKTON PHOTOSYNTHESIS IN LAKES

A cross-system, worldwide approach has been used to ascertain the spat ial, seasonal and long-term variability of areal phytoplankton photosy nthesis (PP) in lakes using published data sets. Also, the average fra ction of annual PP occurring under ice is calculated. The lakes consid ered embrace a range of properties (depth. mixing, flushing rate, lati tude and trophic status). The overall yearly PP distribution is skewed to the left, suggesting the dominance of low PP rates in the data set . When comparing lake types, no differences in average PP have been fo und among them. In particular, there are no clear areal PP differences among lakes of different trophic status on yearly, averaged basis, su ggesting that environmental limitations to PP also exist in lakes of h igher trophic status. Volumetric-based PP can be better used to outlin e PP-based trophic differences, but some degree of overlap is also app arent. Across ail lake types (except in tropical lakes), the PP season al course experiences only one peak in the year, but its timing is cle arly different for each lake type. The seasonal variability of PP is l ower in tropical lakes, as previously reported, but the variability of the other lake types is roughly the same. Therefore, the effects of d epth, mixing regime, flushing rate and nutrient status on PP seasonali ty are difficult to ascertain since they appear to be counterbalanced by other more pervasive. local effects. Particularly, there is no incr ease in temporal variability with the trophic status of lakes, suggest ing that PP seasonal control by physical variables overrides that of n utrients. Also, no significant relationship between average PP and lat itude has been found. Seasonal variability increases as the yearly PP increases. On a relative basis, there is a spatial gradient of seasona l variability of PP, which is weaker when seasonal variability of PP i s considered in interyear comparisons. Long-term (i.e. interannual) va riability of PP is clearly related to increasing yearly averaged PP. S pecifically, in temperate, stratifying lakes the seasonal time course of PP is clearly different from that of phytoplankton biomass, suggest ing an uncoupling of both variables as a result of differing P-max and losses throughout the year. On an average basis, environmental variab les are poor predictors of areal daily PP, thereby implying that the i nterplay of factors is complex and changing throughout the year. PP un der ice averages 10% of yearly PP, but its variability is high enough to make its measurement advisable.