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The in situ photodegradation of aquatic humic substances by underwater UV light (300–400 nm) in peaty lake water was studied. The photodegradation of aquatic humic substances (HS) occurred at depths where UV light with wavelengths <320 nm can penetrate. The daily photodegradation rate of aquatic HS under a cloudless summer sky in the northern Netherlands appeared to be of the same order of magnitude as that of the mean daily pelagic photosynthetic fixation of dissolved inorganic C in typically oligotrophic humic lakes. However, the measured UV degradation of aquatic HS on a yearly basis is small compared to the approximate annual primary production of temperate lake phytoplankton (8%) and submerged macrophytes (3%) under fertile conditions. The measured UV‐light‐driven HS degradation implies a higher turnover of organic C in humic lakes than believed so far.

SUMMARY. 1. Sephadex gel filtration of filtered water from small, Finnish forest lakes demonstrated abiotic movement of 33P from added PO4 to two higher molecular weight fractions. This movement was most pronounced in waters with high humic content which also had high iron content. The two fractions which took up 13P had nominal molecular weights of > 100,000 and 10,000‐20,000. 2. An equilibrium existed between free PO4 and the two fractions. However, one fraction, at least, appeared to exist in two phases, with one phase in rapid equilibrium with free PO4 but the other in only slow equilibrium. 3. Additions of ferric iron up to 1 mg Fe l−1 to the filtered lake water stimulated movement from free PO4, provided high concentrations of humic materials were present. In the absence of humic materials even 0.1 mg Fe 1−1 would precipitate all added 33PO4. 4. The high molecular weight P was only partially reactive with standard molybdate reagents. Exposure of the high molecular weight P to sunlight caused a small release of PO4 under the experimental conditions employed. 5. Possible implications for biological phosphorus demand of such sequestration of free PO4 by humic materials in combination with iron are discussed.

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Abstract-Abiotic transformations of Fe and PO,)-in humic water from Finnish forest lakes were studied by Sephadex G-100 gel filtration following incubation of filtered (0.2 pm) epilimnetic water samples to which both 5SFeCl,.6H20 and 32P043-had been added.The simultaneous movement of 5sFe and 32P to higher molecularweight fractions (lO,OOO-20,000 MW) depended on the presence of dissolved humic substances (DHS). In the absence of DHS almost all 55Fe, probably as inorganic hydrolyzed iron particles, sorbed to the Sephadex. The binding of ssFe by DHS was not affected by increasing the natural ionic strength (I = -3 x lO-4 M) by a factor of 10'. In the absence of quasi-equilibrium between free and DHS-bound 32P0,3-, the DHS-Fe-PO,'-complex readily released 3ZP0,'m-P. Compared to the 3zP binding, ssFe binding was rapid. More than 20% of the Fe had been bound after only 1 min, whereas for labeled PO,'-this percentage was reached only after 24 h.