The turnover of aluminium, iron and phosphorus, organic carbon and nitrogen has been studied in humic lakes in northern Sweden. Most of the studies were performed in the large, low-productive, Lake Ortraesket. Sedimentation, mineralisation and utilisation were investigated in field studies and nutrient limitation in plankton was investigated in field experiments.

The major regulating factor for sedimentation of the investigated elements in Lake Ortraesket was the flow of water, with high input and sedimentation of particles at high discharge and low input and sedimentation at low discharge. Organic carbon and nitrogen sedimentation were closely associated with the sedimentation of amorphous iron oxides, which in turn, could be adsorbed to clay particles. The sedimentation rates of all elements were high, even though retention was small for organic carbon and nitrogen. Most of the sedimentation took place during spring and there was almost no sedimentation during winter. No substantial sediment mineralisation and release of iron and phosphorus were found. Organic carbon and nitrogen were largely mineralised and released from the sediment. A significant mineralisation of organic carbon in the hypolimnion water from bacterial respiration was also found. Bacterial utilisation of the organic carbon pool in the water was low compared with the much higher bacterial utilisation of the sediment organic carbon pool. Nutrient limitation was demonstrated for the planktonic community in Lake Ortraesket. Bacteria and obligate autotrophic plankton were phosphorus-limited. The dominant group of autotrophic plankton (mixotrophic flagellates) was nitrogen limited. Mixotrophic plankton graze and ingest phosphorus-rich bacteria and thereby become nitrogen-limited. The mixotrophs are probably an important link for transfer of energy and nutrients from bacteria to higher organisms in Lake Ortraesket.