The purpose of this study was to examine effects of seasonal and climatic factors on diatom succession and interannual variability in Saanich Inlet, British Columbia. Frozen sediment cores were obtained and the laminae sampled to represent spring and summer seasons from light layers, and fall/winter seasons from dark layers for the years 1900 to 1991. Diatom stratigraphies were related to historical records of sea surface temperature, salinity, sea level, and the Pacific North American Index (PNA) using canonical correspondence analysis (CCA). Results indicate that laminae represent annual depositions and seasons could be defined by distinct species patterns. THALASSIOSIRA species were characteristic of early spring deposition. SKELETONEMA COSTATUM peaks confirm that summer samples follow the early spring bloom. RHIZOSOLENIA sp. was most abundant in fall/winter, dark samples. CCA showed that the environmental variables explained much of the variation in the diatom data and species of a particular season generally had optima for temperature and salinity characteristic of that time.

The role of environmental factors in seasonal succession was also examined by testing effects of various combinations of temperature and photoperiod on resting stage germination and cell division rates. Resting stages were induced in batch cultures using low nutrient media and a cold, dark period. A 3-factor ANOVA showed that species, temperature, photoperiod and their interactions significantly influenced germination time and division rate. Resting stages of CHAETOCEROS SIMILIS and ODONTELLA AURITA were most successful on warm, long days (20 degrees C/16 hours). CHAETOCEROS DIDYMUS and C. CINCTUS were most successful under more moderate conditions (15 degrees C/14 h),and DITYLUM BRIGHTWELLII germinated and divided rapidly under all conditions. The results were compared with environmental conditions prevailing during the presence of these species in Saanich Inlet. Favorable growth conditions determined for C. DIDYMUS, D. BRIGHTWELLII, and T. ROTULA were similar to conditions present when these species were commonly found in the fjord.

Several interannual changes were evident from the core data. The most notable is the cyclic pattern of variation in S. COSTATUM. This species showed highest peaks during periods of cold temperatures, such as cool ENSO cycles. The CCA indicates that changes in sea level and winter weather patterns may also play a role in diatom variations. Several littoral species had optima high on the PNA gradient, suggesting that they are more abundant in the sediments during times of heavy winter rains. The relationship between diatoms and sea level could be related to variations in nutrient supply to surface waters. Diatoms were also used to infer past water conditions in Saanich Inlet with weighted averaging regression and calibration. Results suggest that the diatom community underwent the greatest change in species composition during the 1930šs and 1940šs. Survival of diatom resting stages in cold/dark conditions supports the hypothesis that this life stage can provide a means of long-term survival for some species and may also represent an important component of the ecological memory of aquatic systems.