FOSSIL PIGMENT RECORDS OF PHYTOPLANKTON IN TROUT-STOCKED ALPINE LAKES

Paleolimnology, bioenergetics modelling, and mesocosm experiments were used to quantify changes in phytoplankton following introduction of t rout into fishless alpine lakes in the Canadian Rocky Mountains. Durin g the 1960s, Snowflake and Pipit lakes were stocked with brook (Salvel inus fontinalis), cutthroat (Oncorhynchus clarkii) and rainbow trout ( O. mykiss) either singly or in combination. Stocked trout eliminated l arge invertebrates (Daphnia spp., Hesperodiaptomus arcticus, Gammarus lacustris), but the fish died within 15 yr. High performance liquid ch romatographic analysis of carotenoids and chlorophylls in sediments in ferred that algal abundance increased 4- to 10-fold shortly after fish stocking. In contrast, phytoplankton composition and biomass were con stant in nearby, unstocked Harrison Lake, as inferred from fossils. Pi gment analysis of mesocosms showed that phytoplankton were sensitive t o moderate fertilization: 11 mu g P . L(-1) resulted in four- to six-f old increases in algal biomass. Bioenergetics modelling was used to es timate phosphorus (P) excretion from trout. The flux of excreted P was highly correlated (r(2) = 0.76, p < 0.0001, N = 12) to changes in alg al biomass, as estimated from fossil pheophytin b. Consequently, we in fer that nutrient recycling by stocked trout was one of several mechan isms that contributed to increased algal biomass.