Colman, J. A.. U.S. Geological Survey, firstname.lastname@example.org
, . A.. ,
RELATION OF HYPOLIMNETIC OXYGEN DEFICIT, TROPHIC STATE, AND OXYGEN FLUXES TO NITELLA GROWTH IN WALDEN POND, CONCORD, MASSACHUSETTS, USA
Walden Pond is a glacial kettle-hole lake (surface area 25 ha and maximum depth 31 m) without surface-water inputs or outputs and a forested ground-water contributing area. The dilute (0.09 mS/cm) water in the pond would be expected to be oligotrophic but for intense visitor and swimmer attendance that reaches a maximum of 100,000 people per month in July. Measurements (1996-1998) of Secchi disk transparency (6-8 m) and chlorophyll-a concentrations (0.5-3 micrograms/L), which indicate an oligotrophic state, contrasted with a mesotrophic state indicated by a large relative areal hypolimnetic oxygen deficit (HOD) of about 0.40 mg O2/cm2/d. Fluctuations in visitor attendance were correlated with HOD for three years of record. Metalimnetic primary production, largely by the macroalga Nitella, which grows on littoral sediments between 8.5 and 13 m depth, resulted in pronounced metalimnetic oxygen maximums during summer stratification. Results of eddy-flux computations of vertical oxygen transport rates indicated that the thermocline blocked upward transport, but substantial amounts of oxygen were transported downward. Nitella likely contributed to water clarity by competing with planktonic algae for nutrients and by maintaining aerobic conditions over metalimnetic sediments.
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