EFFECTS OF WATER DEPTH, TYPE OF PRIMARY PRODUCER, AND VARIATION AMONG LAKES ON DECOMPOSITION RATES IN SHALLOW LAKES

Shallow lakes play an important role in the global carbon cycle, and can bury substantial quantities of organic carbon in their sediments. However, burial rates vary greatly among shallow lakes, and the variability could be partially due to differences in decomposition rates, with decomposition driven by differences in lake depth, dissolved oxygen levels, temperature, and dominant primary producer. We examined the influence of water depth, type of primary producer, and variation among lakes on decomposition rates by deploying litter bags with four types of primary producers at two depths (0.75m and 1.5m) from October through August 2010 in 11 shallow lakes in western MN, USA. The four types of primary producers were the filamentous algae Cladophora sp., the submerged angiosperm Stuckenia pectinatus (sago), the submerged macroalgae Chara sp., and the terrestrial grass Panicum virgatum. Results showed significant differences in decomposition rates among species, depths, and among lakes. Surprisingly, sago showed the slowest decomposition rate, grass and Cladophora were moderate, while Chara decomposed the fastest, with percent dry-mass remaining after 293 days of 25%, 10%, 6% and 2%, respectively. Remaining mass averaged across species was twice as high at the shallow depth (10%) compared to the deep depth (5%) despite a depth difference of just 0.75m. Differences among lakes were large, as lakes ranged from 19% to 3% remaining mass averaged across depths and species. However, we failed to find any factors associated with differences among lakes. Our results indicate yearly decomposition rates are influenced by water depth and type of primary producer, and also that rates differ among lakes regardless of depth or type of organic material. Factors responsible for differences among lakes remain elusive.