WHY DOES THE RELATIONSHIP BETWEEN SINKING FLUX AND PLANKTONIC PRIMARY PRODUCTION DIFFER BETWEEN LAKES AND OCEANS

The fraction of primary production lost to sinking (the export ratio) increases with productivity in the ocean and decreases slightly with productivity in lakes. To explore why this distinction exists, we compared marine and freshwater regressions relating chlorophyll concentrations in the euphotic zone to each of the three variables that define the export ratio: primary productivity, carbon sinking fluxes, and euphotic zone depth. Chlorophyll was found to predict these three variables well (r2 = 0.54-0.90) in both lakes and the ocean. The differences between the marine and freshwater export ratio-productivity relationships stem primarily from a discrepancy in the primary productivity-Chl relationships. On average, a > 10-fold difference in Chl-specific productivity exists between the most oligotrophic lakes (Chl = 0.2 mg-3) and oceanic regions with similar Chl concentrations. This difference disappears at higher concentrations of Chl because primary productivity: Chl ratios increase with productivity in lakes. In addition, carbon sinking rates average 2-3-fold higher in the oceans than in lakes with similar concentrations of Chl. The trends of marine and freshwater export ratio-production can be qualitatively reproduced with Chl-based predictions of euphotic zone primary productivity, depth, and carbon sinking losses from regressions. Marine and freshwater ecosystems may differ systematically in the efficiency of nutrient recycling processes in the water column and in the nature of settling material.