RELAXATION OF PHOSPHORUS LIMITATION DUE TO TYPHOON-INDUCED MIXING IN 2 MORPHOLOGICALLY DISTINCT BASINS OF LAKE BIWA, JAPAN

We measured changes in hydrodynamics and phosphorus cycling due to a t yphoon in two morphologically distinct basins of Lake Biwa, Japan. The typhoon, with wind speeds up to 20 m s(-1), led to physical, chemical , and physiological responses that differed dramatically in the shallo w (mean depth of 3.5 m), eutrophic south basin compared to the deep (m ean depth of 45.5 m), oligo-mesotrophic north basin. prior to the typh oon, PO4-P concentrations were <1 mu g liter(-1) in both basins. A sui te of physiological tests (protein:carbohydrate ratio, sestonic C:N:P ratios, nutrient-enrichment bioassays, alkaline phosphatase activity, P-32 uptake) all indicated that the plankton was P-deficient. Bacteria dominated (avg of 65%) the uptake of (PO43-)-P-32. As a result of the typhoon, P concentrations increased to similar to 2.5 mu g liter(-1) and suspended solids increased from 4.5 to similar to 17 mg liter(-1) in the south basin. The day after the typhoon, all physiological tests indicated that the south basin plankton was P-sufficient and that phy toplankton dominated (65%) P-32-uptake. A week after the typhoon, P co ncentrations had dropped to near pre-typhoon levels and the physiologi cal tests indicated that the plankton was again P-deficient. Only on t he day of the typhoon were wave-induced shear stresses great enough to cause considerable entrainment of sediment and associated pore waters in the South Basin. Because interstitial porewater P concentrations w ere greater than or equal to 2 orders of magnitude greater than the ov erlying water concentration, we concluded that the increase in P was c aused by entrainment of nutrient-rich pore water. In the north basin t he typhoon-induced upwelling and mixing in the thermocline did not pen etrate to depths where P concentrations were elevated. Consequently, p lankton were as P-deficient immediately after the typhoon as they were prior to it.