We measured the concentration of diatom frustules and amorphous silicon per gram of dry sediment at 5-m intervals in the upper water column (0–40 m), in sediment traps placed at 37 and 60 m below the surface, and in a sediment core. The average concentration of frustules per gram of dry suspended sediment in the water column was 3.14 × 108. The 37 and 60 m trap sample averages were 1.16 × 108 and 5.03 × 107 frustules/g dry sediment, respectively. Subsamples from the sediment core averaged 6.31 × 106 frustules/g dry surficial sediment. The average proportion of amorphous silicon/g dry sediment was 8.6% in the water column, 7.7% in the 37 m traps, 6.0% in the 60 m traps, and < 2.0% in subsamples from the sediment core. The major fraction of amorphous silicon produced annually as diatom frustules was decomposed before incorporation in the permanent sediment. A comparison of the annual silicon requirement for diatom production and silicon inputs showed that the watershed contributes < 5.0% of the dissolved reactive silicon required for annual diatom production. These observations suggest that recycling of biogenic silicon provides the major source of soluble reactive silicon required for diatom blooms in Lake Michigan. Key words: Lake Michigan, diatoms, sedimentation, sediment traps, dissolution, deposition, silicon, sources, sinks, recycling

CoNwry, H. L., J' I. penrrn, E. M. yecucHr, eNo D. L. MrlLrNcrn. 1977. Biological utilization and regeneration of silicon in Lake Michigan. J. Fish. Res. Board Can-. 34: 537_544.Depth profiles of dissolved reactive silicon and amorphous pafticulate silicon were made at monthly intervals (April-November 1975) at stations along a iouthwest transect tiom Grand Haven, Michigan, to a point approximately in the center of the southern basin of Lake Michigan. Biological utilization of reactive silicon occurred prior to stratification in Iate May or early Jine. A shift from a 100vo diatom-dominated phytoplaniton community in rpiing to -12%diatoms in August was attributed to the low dissolved iilicon values obser-ved ln tf,e surfuce water (<2 pmolll) during summer. The total amount of biologically active silicon (TBAS) for the lake was found to be approximately 19 p.mol l(. Winter values were -.-2 pmol/l amorphous silicon and -l7 pmoll( reactive silicon. During the period June-August 807o of TBAS had been utilized by the diato-m community, with only 20%o rimaining as ,euciiue silicon. Greater than S|Voof TBAS was lost from the water column during spring and early summer, which was attributed to settling of diatom fru stules and the.sinking of zooplankton fecal pellets containing fil stu les. Th is silicon was subsequently retumed, in.a soluble foim, to the deepwater during thJfall. The amount of TBAS that was recycled was estimated to be g0_100%.

Diatom biomass maxima occurred in spring and fall and produced a bimodal bloom sequence at an offshore Lake Michigan station. The maximum in May was preceded by rapid growth as indicated by increased values of primary productivity and pigment concentration. As the spring bloom progressed, decreasing nutrient levels apparently slowed diatom growth. The diatom-biomass accumulation rate declined, assimilation quotients were minimal, and soluble reactive silicon was reduced from 13 to 6.8 μmol/ℓ. In summer, after the bloom, diatom biomass and silicon reached seasonal minima of < 250 mg C/m2 and ~ 1.0 μmol/ℓ, respectively. Diatom biomass increased again in October when silicon supplies were replenished and the concentration exceeded 6.6 μmol/ℓ. A critical silicon concentration of ~ 6.5 μmol/ℓ, may control the development and timing of offshore diatom populations in Lake Michigan. Key words: Lake Michigan, diatoms, growth, biomass, periodicity, silicon, uptake, limitation

No abstract

2^9The radioisotope v Pu, an alpha emitter with a half-life of 24 000 yr, is a small component of fallout from atmospheric weapons testing and at present can be measured in environmental samples by using large samples and performing a rigorous radiochemical separation procedure. The toxicity of plutonium has been of concern since its production was initiated in 1943, with the result that more is known about the toxicology of plutoniurn than about that of most other hazardous elements (Bair and Thompson, 1974). However, its biogeochemical behavior, especially in aquatic ecosystems, remains very poorly known. A recent review (Noshkin, 1972) of plutonium dissemination | in aquatic environments emphasizes the relatively limited knowledge of plutonium I behavior in the oceans and the virtual absence of data for freshwater ecosystems.Because of projected increasing production of plutonium and its use in presentday reactors, future breeder reactors, and other applications, we initiated \ 239 ] studies of the behavior of fallout Pu in a large freshwater ecosystem, Lake I