Climate warming decreased primary productivity in Lake Tanganyika, inferred from accumulation of dissolved silica and increased transparency

SARVALA et al. (2006) has suggested that O'REILLY et al. (2003) and VERBURG et al. (2003) did not provide sufficient evidence for a strong causality between lake warming and decreased prirnary productivity in Lake Tanganyika. SARVALA et al. (2006) agreed that clirnate warrning has increased the density gradient between shallow and deep water, but they are not convinced that the increase in density gradient has significantly affected the nutrient renewal rate from deep water to the shallow euphotic zone to reduce primary productivity over the past century. This reply to SARVALA et al. (2006) reiterates why we think VERBURG et al. (2003) provided convincing evidence for both a reduction in nutrient renewal from deep water and for a decrease in primary productivity over the past century. SARVALA et al. (2006) appeared to suggest that productivity in Tanganyika may not be related to nutrient availability. This seems highly unlikely because of the oligotrophic status of the lake and the stimulation that seasonal upwelling and seasonal rnixing provide to the primary productivity of surface water (HECKY et al. 1991). In particular, dissolved inorganic phosphorus (P) concentrations are extremely low (VERBURG et al. 2003), suggesting that P supply is likely controling primary productivity. SARVALA et al. (2006) stated that our evidence for a decrease in phytoplankton biomass since 1975 is weak. In their words, "time series for phytop1ankton were short and inconclusive". VERBURG et al. (2003) showed that phytop1ankton biomass in 2000-2001 was about 70% lower than in 1975 for two comparable seasons and reported that the difference was significant (P < 0.05). Nevertheless, SARVALA et al. (2006) suggested that the degree of accuracy was low and that only 1arge differences can be regarded as significant. While agreeing that tropica11akes are undersamp1ed over long periods of time because of the laek of systematic monitoring programs, we note that our results were corroborated by those of Vuoruo et al. (2003) and SARVALA et al. (2003), who found low phytoplankton biomass in 1998-2001 similar to our findings in 2000-2001. The recent data showed low interannual variability suggesting that two points in time might be compared, or it is necessary to explain why 1975 may not be characteristic of the lake in that decade. Moreover, we are fortunate that analysis o f the phytop1ankton biomass in 197 5 and in 2000-2001 in VERBURG et al. (2003) was done by the same person (HJK) using the same method, removing the possibility that differences in methods caused the observed decrease. Second, SARVALA et al. (2006) did not mention that VERBURG et al. (2003) provided several other pieces of evidence for the decrease in primary productivity that extend over the past century. Both the accumu1ation of disso1ved silica in surface waters and increased transparency support our conclusions. Dissolved si1ica in the upper 30 m was significantly higher in 2000 and 200 l (both wet and dry seasons) by about three-fold compared with all previous published data (collected in 1938, 1947 and 1975).