Variation in temporal [14C]plankton photosynthesis among warm monomictic lakes of coastal British Columbia

Seasonal patterns of [C-14]phytoplankton photosynthesis (PP) were examined in six warm monomictic lakes of coastal British Columbia. Four of our study lakes followed typical lake patterns with maximum PP occurring in the spring and minimal rates occurring during the winter. However, the spring maximum occurred several weeks earlier than lakes in other climatic regions. In addition, maximum rates of daily photosynthesis were observed to occur during the winter months in Maxwell Lake, rather than during the standard growing season. All study lakes except Maxwell Lake had large Daphnia in the plankton community. Maxwell was dominated by small crustacean zooplankton implying the importance of trophic structure in mediating seasonal patterns of productivity. The four oligotrophic lakes in our study also exhibited P-deficiency during winter, as indicated by P-debt bioassays and rapid (PO43-)-P-32 turnover rates. Our data suggest that these coastal oligotrophic lakes were co-limited by nutrients and light during winter. The importance of winter (November-March) photosynthetic production to the total annual carbon budget in the six lakes studied here is greater than that typically reported for other temperate zone lakes. If plankton community respiration decreases more than photosynthetic production with wintertime temperatures, then >50% of annual net pelagic carbon fixation could occur in winter in some coastal lakes.