BIOGEOCHEMICAL CYCLES AND BUDGETS 



The effects of biological uptake and sedimentation on available phosphorus, 

 nitrate, and silica in Maine lakes are described by Davis and coworkers 

 (1978a). Ordinarily nitrogen rarely is a factor limiting production in lakes 

 because it is readily available from the atmosphere, and is fixed by some 

 aquatic algae. However, in summer low levels of dissolved, fixed nitrogen, 

 the result of biological uptake, favor certain phytoplankton forms, especially 

 the nitrogen-fixing, bloom-forming, blue-green algae. Silica rarely affects 

 lake productivity but its scarcity or abundance may have a considerable effect 

 on the abundance of some organisms. For example, low silica favors 

 nonsiliceous algae instead of silica-demanding diatoms. 



The availability of phosphorus probably has the greatest single effect on lake 

 productivity (Wetzel 1975), although the availability of certain trace 

 elements (e.g., molybdenum) can have an effect on a short-term basis (Thurlow 

 et al. 1975). Phosphorus budgets or cycles have been estimated for only two 

 of the coastal zone lakes. Pleasant Pond in region 2 (Gordon 1977) and Little 

 Pond in region 3 (Mower 1978) . The closest approaches to complete phosphorus 

 budgets (including in-lake cycling) for Maine lakes are those developed for 

 Haley Pond (Scott et al. 1977) and for Lake Sebasticook (Hannula 1978), both 

 outside the coastal zone. The major sources of phosphorus input in lakes are 

 (1) biological input: for example, bird and insect droppings and animal and 

 plant detritus; (2) dry and wet atmospheric fallout; (3) surface runoff and 

 inflowing streams; (4) ground water; and (5) pollution (human and industrial 

 wastes). Losses of phosphorus from lakes may be caused by (1) emerging 

 insects, and food consumed by birds and animals; (2) outlet streams; (3) 

 ground water loss, and (4) binding of phosphorus to sediment. In internal 

 cycling of phosphorus in a lake system phosphorus may be transferred by (1) 

 biological and chemical decomposition in the water column and sediment-water 

 interface, and by animal excretions; (2) uptake from water and sediment by 

 algae and macrophytes; or (3) consumption by herbivores and carnivores. In 

 the open water epilimnion (warm, shallow water) the internal cycling of 

 phosphorus is extremely rapid. Complete turnover between the phytoplanktonic 

 algae and water is measured in minutes, but for the epilimnion as a whole 

 turnover is measured in days or weeks (Wetzel 1975) . Large quantities of 

 phosphorus are replaced in lakes by residential and municipal sewage. Sewage 

 may significantly alter the concentrations and flux of phosphorus in lakes 

 and, thereby, increase productivity to the point of causing obnoxious algal 

 blooms and related detrimental phenomena. 



BIOTA 



How organisms live and interact is important in understanding the significance 

 of the biology of lakes. The following is a general description of some of 

 the major forms of biota and the methods of classifying them. 



Benthic organisms live on the bottom, in bottom sediments, or attached to 

 surfaces protruding from the bottom (e.g., macrophytes). Planktonic organisms 

 live in the water column and possess limited or no power of locomotion. 

 Neustonic organisms live at the air-water interface (e.g., water striders) . 

 Organisms in Maine lakes sometimes are sorted on the basis of their littoral, 

 profundal, and pelagial distribution (figure 7-3). Lacustrine organisms can 

 also be grouped on the basis of their nutritional habits: primary producers 



7-22 



