Rd. Robarts et al., RELAXATION OF PHOSPHORUS LIMITATION DUE TO TYPHOON-INDUCED MIXING IN 2 MORPHOLOGICALLY DISTINCT BASINS OF LAKE BIWA, JAPAN, Limnology and oceanography, 43(6), 1998, pp. 1023-1036
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.