No abstract

Recoveries of planktonic algae from sediment traps placed near the bottom of Windermere are compared with areal expressions of the corresponding standing crops. Good, order-ofmagnitude agreement was found between the annual fluxes and maximal standing crops of five species of diatoms and two genera of desmids. Trap recoveries of three colonial chlorophytes, three blue-green algae, and a dinoflagellate were l-3 orders smaller than the crop potential. The differences are attributed to specific differences in sinking behavior and in susceptibility to decomposition and consumption by animals. Algal remains recruited to the permanent sediments are dominated by diatoms, four species of which may together contribute between 0.6 and 10.5 mg dry wt*crn2.yrl. Potentially, the heavy metals transported by these algae may account for the annual flux of Zn and Pb to the deep water sediments of Winder-mere.

The spring to summer transition in a productive English lake is considered with respect to phytoplankton and its environmental conditions. Salient environmental changes include the onset of temperature/density stratification that is usually accompanied by a clear-water phase associated with a maximum of grazing Daphnia and a minimum of phytoplankton in the 0-5 m zone. Below this zone, as thermal stratification progresses, a deep maximum of phytoplankton can develop under strong thermal/density gradients and enhanced light penetration. Examples are resolved by estimations of chlorophyll-a, beam attenuance in situ and cell counts. Attributed origins are by sedimentation of diatoms, migration of flagellates, and depthadjusted buoyancy of a gas-vacuolate cyanophyte. The transition period involves a decline of springassociated diatom populations and a rise of summer-associated species. The generally low algal abundance within the transition phase has at least four origins -prior nutrient (Si) depletion, sedimentation, grazing, and low 'inoculum' levels of successor species. It can be augmented by the re-growth of species abundant in spring, by early extensions of normally summer species, by seasonally characteristic colonial chrysophytes, and by other phytoflagellates of small size that are seasonally less specific ('opportunistic') and probably critical for Daphnia grazing with consequent generation of the clear-water phase.